Geology of the country around Flint: Memoir for 1:50 000 Geological Sheet 108 (England and Wales)

By J.R. Davies, D. Wilson, I.T. Williamson

Bibliographical reference: Davies, J R, Wilson, D, and Williamson, I T. 2004. Geology of the country around Flint. Memoir of the British Geological Survey, Sheet 108 (England and Wales)

Geology of the country around Flint: Memoir for 1:50 000 Geological Sheet 108 (England and Wales)

By J.R. Davies, D. Wilson, I.T. Williamson

British Geological Survey. Keyworth, Nottingham: British Geological Survey 2004. © NERC copyright 2004 First published 2004. Printed in the UK for the British Geological Survey by Halstan & Co Ltd., Amersham.

The grid used on the figures is the National Grid taken from the Ordnance Survey maps. (Figure 2) is based on material from Ordnance Survey 1:50 000 scale maps, numbers 162 and 163. © Crown copyright reserved. Ordnance Survey Licence No. GD272191/2004 ISBN 0 85272 487 X

Notes

Welsh words

The following list gives a translation of some of the most common Welsh words used in this memoir:

Acknowledgements

The Silurian Chapter was written by R A Waters and I T Williamson. The Dinantian and Silesian chapters were written by J R Davies and D Wilson. The Permo–Triassic, Tertiary and Quaternary chapters were written by I T Williamson. The Structural Chapter was written by J R Davies and D Wilson based partly on contributions by R Addison and S Holloway; the concealed geology section of the chapter was provided by J D Cornwell and A J Gibberd, and S Holloway supplied seismic interpretations for the eastern part of the district. Fossil identifications and comments on biostratigraphy were provided by S P Tunnicliff, D E White and M Williams (Silurian); and by N J Riley, A M McNestry and N Turner (Carboniferous). R J Merriman has contributed data on the low-grade metamorphism of the Silurian rocks. The Economic Geology chapter was compiled from contributions by D E Highley (minerals, including coal) and M A Lewis (hydro­geology and water supply). The memoir was compiled by J R Davies and R Addison, and edited by A A Jackson, R D Lake and R A Waters. The page setting is by J Norman, and figures were drawn by R J Demaine, P Lappage and G Tuggey, BGS Cartog­raphy, Keyworth.

We thank the quarry and gravel pit operators, and the farmers and landowners throughout the district who allowed access to their excavations and land during the course of the survey. Thanks are also due to the former Clwyd County Council and former Alyn and Deeside, Delyn, Glyndwr and Wrexham district councils, and also to the Welsh Development Agency, and the former British Coal for providing details of boreholes, shafts and mine plans, and to the Welsh and Northwest Regions of the former National Rivers Authority for providing licensed abstraction data. The resurvey was supported in part by the Department of the Environment on behalf of the Welsh Office.

Preface

The wealth of the United Kingdom lies in its resources, be they material, human or aesthetic. The optimal use of these resources, for the benefit of the population is the aim of all government. The British Geological Survey is funded by central Government to provide the geological information to allow the efficient identification, evaluation and use of mineral, water and land resources. Such information also allows decisions on the planning and prioritisation of development to be based on the best possible evaluation of the resource, its uniqueness and the amenities and landscape of the surrounding area. In addition planners may use the information to avoid areas with geological hazards or high developmental costs. In order to fulfil this requirement, the British Geological Survey maintains a programme of data collection, collation, interpretation, publication and archiving, that has as one of its principal aims the production of a national coverage of 1:50 000 scale geological maps, most with accompanying memoirs or explanatory booklets. This memoir is part of that series.

During the 19th and 20th centuries, the economic development of the Flint district was based on coal mining and the production of iron and steel. The mining and quarrying of brick clay, lead and zinc ores, chert, limestone and aggregates served to underpin that process. Although only limestone and aggregate working are of significance today, the decline of coal and lead-zinc mining and clay extraction has left an enormous legacy of derelict sites and contaminated land. The redevelopment of such sites requires detailed earth science information that is best provided by reference to modern geological maps.

The provision of thematic maps and explanatory reports was the purpose of the commissioning by the Department of the Environment, on behalf of the Welsh Office, of a geological resurvey initially of the Deeside area, and sub­sequently of the Wrexham region. These areas embraced much of the Flintshire and Denbighshire coalfields, and the Halkyn–Minera lead-zinc mining province, where the impact of industrial development and mineral extraction had been the most severe. The resurvey was later extended to cover the whole of the Flint district.

This memoir presents the results of detailed mapping by the British Geological Survey integrated with subsurface information from the coal, hydrocarbon and water industries and from site investigations. It is intended to be of practical value to a wide range of users including earth scientists and related disciplines, planning authorities concerned with land-use planning and development, and the mineral extraction industry.

This volume contains much hitherto unpublished material, and the many specialist contributions highlight the increasing emphasis placed by the Survey on a multidisciplinary approach to geological mapping. This memoir represents a major advance in the understanding of the geological history of the district, and I am confident it will play its part for many years in serving the needs of the scientific, planning and commercial communities.

David A Falvey, PhD Director, British Geological Survey, Keyworth, Nottingham, NG12 5GG

Geology of the country around Flint—summary

The district described in this memoir comprises both upland and lowland areas in the north-east of the former county of Clwyd (now included in parts of the unitary authorities of Denbighshire, Flintshire and Wrexham), and western parts of Cheshire. The Clwydian Range constitutes the highest part of the district and is cored by Silurian rocks. The range is fringed by Carboniferous strata including Dinantian limestones and the Silesian sequences of both the Flintshire and the northern part of the Denbighshire coalfields. Permo–Triassic strata underlie the Cheshire Plain to the east, and the Vale of Clwyd to the west. The lowland areas of the district are covered by extensive Quaternary deposits of variable thickness.

The decline in traditional heavy industries and sub­sequent redevelopment of parts of the district has high­lighted the need for up-to-date geological information, especially for planners and civil engineers. This memoir, in addition to earlier thematic reports, seeks to address this need. It details the results of a resurvey which, supported by numerous specialist investigations, has enabled a major re-evaluation of the area’s stratigraphy and geological structure.

The cleaved and folded Silurian (Ludlow) rocks are com­posed of mudstone and sandstone turbidites, slumped and destratified strata, and of distinctive laminated deposits which record the background sedimentation. They were deposited in a deep water basin, the Denbigh Trough, part of the larger Lower Palaeozoic Welsh Basin.

The memoir provides a detailed account of the local Dinantian rocks and of the palaeogeography of north-east Wales during this period. The earliest Carboniferous rocks comprise reddened fluvial and coastal plain conglomerates and mudstones. These are overlain by thick sequences of ramp and platform limestones, locally with intercalations of shallow-marine sandstone. Early Carboniferous deep-water deposits occur at depth in the north-east of the district. Southerly and northerly derived fluviodeltaic sandstones, together with sequences of cherts, and marine and nonmarine mudstones form the components of a complex Namurian succession which was influenced by movements in contemporary sea level. The overlying Westphalian sequence includes the productive Coal Measures and barren Red Measures of the Flintshire Coalfield and the northern part of the Denbighshire Coalfield. The memoir rationalises the nomenclature and correlation of both coal seams and overlying red-bed divisions.

The Permo–Triassic rocks of the Cheshire Plain and Vale of Clwyd comprise thick sequences of aeolian and fluvial sandstones preserved in rift basins created by contemporary extensional tectonics.

Tertiary ‘pocket’ deposits (clays and silts), preserved in solution hollows in the Carboniferous limestones, reflect an interval of fluviolacustrine sedimentation which followed a period of prolonged uplift, weathering and erosion.

Glacial (Drift) deposits, the products of the Devensian glaciation, cover much of the district and erratics on the highest ground of the Clwydian Range indicate that ice-cover was total. Two opposing and contrasting ice masses, a Welsh ice sheet travelling eastwards and an Irish Sea ice sheet moving south to south-eastwards, met and interacted: this is reflected in the distribution, complexity and variety of resultant deposits.

The geological structure of the district demonstrates the effects of the late Caledonian (Acadian), Variscan and Alpine orogenic episodes. This resurvey has also elucidated the role of syndepositional faulting in the Carboniferous, as a control on the distribution and thickness of sediments.

Today the principal mineral resources are limestone and sands and gravels for aggregate. There has been limited investigation of the hydrocarbon potential of selected areas. Data on local aquifers, including the regionally important Permo–Triassic sandstones, are assessed.

(Front cover) View of Moel Fammau, the highest summit of the Clwydian Range, formed of Silurian turbidites and disturbed beds [SJ 200 580] (GS1050). Photographer: I T Williamson

(Frontispiece) View looking westwards from Eryrys across the upper Alyn valley towards the Clwydian Range. Dinantian limestone crops out in the middle and foreground. Beyond the limestone escarpment, the fault-guided Alyn valley separates the Dinantian from the Silurian strata of the Clwydian Range, which forms the more rounded hills in the middle distance (GS1051).

(Table 1) Summary of the geological sequence in the Flint district.

Chapter 1 Introduction

The Flint district, situated between the Denbigh Moors to the west and the Cheshire plain to the east, is an area of marked physical and economic contrasts which reflect a diverse geology (Figure 1). It lies largely in the former county of Clwyd (now parts of Denbighshire, Flintshire and Wrexham County Borough) and partly in Cheshire. The oldest rocks that crop out are Silurian mudstone, ­siltstone and sandstone. These are unconformably overlain by a Carboniferous succession, comprising Dinantian lime­stone, Namurian sandstone, mudstone and chert, and Westphalian Coal Measures and Red Measures. Carbonif­erous rocks underlie the greater part of the district; in the east and in the Vale of Clwyd, they are unconformably overlain by Permo–Triassic sandstone. Solution pipes in the Dinantian limestones preserve pockets of Tertiary clay and silt. Much of the low ground within the district, especially in the valleys and the gentler slopes near the Dee estuary, is underlain by Quaternary Drift deposits.

In the south-western corner of the district where the ground rises gently to 200 m above OD, the Denbigh Moors are underlain by Silurian strata. The Vale of Clwyd (Figure 2), ranging from about 30 to 80 m OD, is floored by Permo–Triassic rocks, concealed beneath Quaternary deposits, which form gently undulating low ground. Minor outcrops of Carboniferous strata occur along the eastern and western flanks and the Vale of Clwyd Fault, along the eastern margin, delineates the extent of the Permo–Triassic crop.

Eastwards, the Clwydian Range forms the highest ground in the district. Formed of Silurian strata, the scenery owes much to the effects of ice-action during the Pleistocene. The range forms a broadly north-north-west-trending cuesta, the western escarpment of which rises abruptly from the floor of the Vale of Clwyd. In detail, it comprises a series of smooth rounded hills, ‘moel’, separated by incised valleys and minor ridges (Figure 2). The highest, central part of the range lies around Moel Fammau (554 m above OD), elevations falling progressively to the north and south.

East of the Clwydian Range, the Dinantian limestone forms a succession of prominent west-facing escarpments, rising to over 250 m above OD on Halkyn Mountain in the north-eastern part of the district, and 350 m OD around Eryrys and Moel Findeg in the south. Farther east, in the Flintshire Coalfield, minor north to north-west-trending escarpments are largely drift covered, and form a subdued topography, which rarely exceeds 150 m OD. The land falls gently north-eastwards towards sea level along the Dee estuary.

In the south-east of the district, gently undulating low ground lies at the western margin of the Cheshire plain, an area covered by thick Quaternary deposits which conceal Permo–Triassic strata. The latter are exposed mainly around Kinnerton and on the Wirral peninsula, the broad, gently undulating ridge that separates the Dee and Mersey estuaries.

The drainage of the district is divided into two catchments, separated by a watershed that runs along Halkyn Mountain in the north, and the crest of the Clwydian Range in the south (Figure 2). Ground to the east of the watershed is drained by the River Alyn and its tributaries, which join the River Dee. In this area, the Dee is tidal and confined within man-made embankments. To the west of the watershed, the River Clwyd follows a gently meandering course north-north-westwards through the Vale of Clwyd, reaching the sea at Rhyl to the north-west of the district. The River Wheeler flows westwards, dissecting the Clwydian Range, to join the Clwyd close to the northern edge of the district.

Economic and social aspects

The district has a mixed economy with both industry and agriculture playing important roles. Parts of the district are designated as development areas.

In the past, coal and lead-ore mining were the principal extractive industries. Coal was mined in the district at least since the Middle Ages and production reached its peak during the 19th century. The last colliery in the district closed in 1965; two licensed mines operated briefly in the late 1980s. Fireclay was formerly an important resource but, apart from a single pit at Buckley, it is no longer worked. There remains, however, some potential for continuing opencast coal development and for exploitation of coalbed methane. Mining of lead and zinc ore, centred on veins in the Carboniferous Limestone, dates back to Roman times but reached its peak in the 19th and early 20th centuries when the Halkyn–Minera area was one of the world’s leading orefields. Tertiary pocket deposits were formerly worked as a source of pipeclay and refractory sand.

Today the main extractive industry is limestone quarrying, for aggregate and cement, but also for other industrial and agricultural uses. Production is currently from seven sites in the Carboniferous Limestone. Glacio-fluvial sands and gravels are another source of aggregate and they are quarried in the Alyn and Wheeler valleys. Weathered Permo–Triassic sandstones are worked for moulding sand at Kinnerton and silica sand is worked in the south of the district. Building stone has been quarried locally, in particular, the Permo–Triassic red sandstones in the Vale of Clwyd and at Burton Point, and Carboniferous, mainly Namurian, sandstones throughout their crop.

Geological history

The oldest exposed rocks in the district (Table 1) are Silurian mudstones, 420 to 415 million years old. They were deposited in a deep marine environment in a small sedimentary basin, the Denbigh Trough, which formed part of the larger Lower Palaeozoic Welsh Basin. This larger basin was flanked to the south-east by the Midland Platform and to the north-west by an Irish Sea Platform. The Silurian rocks were folded, cleaved and faulted during the Acadian (Late Caledonian) orogeny in the Early Devonian.

Uplift associated with the orogeny was followed by a long period of erosion. Later, the oldest Carboniferous rocks in the district were deposited in a fluviatile environment and these Dinantian sediments rest unconformably on Silurian rocks. The succeeding limestones are shallow-water marine deposits which accumulated on a carbonate platform. Palaeosol horizons, present in the upper part of this sequence, reflect periods of subaerial exposure. Syndepositional tectonic activity, mainly faulting, gave rise to localised variations in the rates of sediment accumulation revealed by pronounced thickness variations.

During the late Dinantian, southerly derived calcareous and quartzose sands of fluviodeltaic origin were deposited and their source deltas prograded into the district during the ensuing Namurian. The deposition of spicular chert was a feature of early Namurian times. Periodic marine incursions flooded the deltas, but by the late Namurian, marine influences began to wane as sedimentation became dominated by a northerly sourced delta system, which introduced sand rich in feldspar. The succeeding Westphalian strata were formed mainly in delta-top environments and include cyclic sequences of mudstone, siltstone, sandstone, seatearth, and coal that comprise the productive Coal Measures. The youngest Carboniferous strata in the district are barren red mudstones deposited in alluvial plain, delta-top and lacustrine environments.

In the late Carboniferous and early Permian, faulting and uplift related to the Variscan orogeny was followed by a long period of erosion. The climate became warmer and drier. The Permo–Triassic rocks of the district include fluvial pebble beds and aeolian sandstones laid down on eroded Carboniferous rocks during a subsequent period of rifting. The lead/zinc vein mineralisation in the Dinantian limestones dates from this period.

Younger Triassic and Mesozoic strata were probably deposited across much of the district but were subsequently removed by erosion during the early Tertiary (Palaeogene), prior to the deposition of probable Neogene sands and clays, now preserved in solution pipes in the Carboniferous limestones.

A number of major faults which cross the district have exercised a controlling influence on sedimentation at various times. These include the Vale of Clwyd, Alyn Valley, Nercwys-Nant-figillt, Great Ewloe and Neston faults and fault zones, but these were all subordinate to the regionally important and long-lived north-north-east-trending Bala Lineament. The latter is a complex zone of faulting and folding which traverses the south of district (Figure 1). Major facies and/or thickness changes in both Carboniferous and Permo–Triassic sequences occur across this zone.

During the Pleistocene, several glaciations affected the region, but only evidence of the last one (Devensian) has been preserved. Two contemporaneous ice sheets occupied the area, one originating from the Arenig Mountains and Snowdonia in the west, and the other entering from the Irish Sea to the north. The zone of convergence of these Welsh and Irish Sea ice sheets was in the central part of the district. Both ice sheets laid down large volumes of till and outwash sands and gravels, and subglacial erosion accentuated pre-existing valleys, such as that of the Dee.

After the retreat of the ice, the unconsolidated glacial deposits were extensively eroded and redeposited as head, scree and alluvial deposits. The postglacial rise in sea level during the Holocene, drowned the valley of the River Dee creating the modern estuary in which extensive, largely intertidal marine deposits have since accumulated.

History of research

The early history of geological investigation in the district is documented in detail in the memoir accounts which followed each of the earlier surveys of the district (Strahan, 1890; Wedd and King, 1924), and in the special report on the lead and zinc mining by Smith (1921). ­Subsequent studies on the Silurian rocks of the district include those by Woods and Crossfield (1925), Boswell (1932) and Simpson (1940).

Following the work of Hind and Stobbs (1906), significant advances in the understanding of the Dinantian rocks of the district were made only when Somerville (for example 1977, 1979a), and later Somerville and Strank (1984a, b, c) published the results of their stratigraphical and palaeontological investigations. Subsequently, early results of this resurvey were presented by Davies et al. (1989).

A summary of important inter-war investigations of the Namurian succession was given by Jones and Lloyd (1942), and the results of limited later work were reviewed by Ramsbottom (1974). The implications for the Flintshire succession, of regional stratigraphical models for the Namurian developed by Ramsbottom (for example 1979), are presented in Ramsbottom et al. (1978). A detailed study of the Namurian chert facies was undertaken by Oldershaw (1968). The results of limited studies of the Westphalian rocks were reviewed by Calver and Smith (1974) and Ramsbottom et al. (1978).

The Permo–Triassic strata in the district have been extensively examined, notably by Poole and Whiteman (1966), Thompson (1989, and references therein), Warrington et al. (1980), Macchi and Meadows (1987) and Macchi (1991).

The most recent study of the Tertiary deposits preserved in solution pipes in the Dinantian limestones (Walsh and Brown, 1971) also provides a summary of the earlier investigations.

Little structural work has been undertaken since the synthesis by Wedd and King (1924) but relevant regional studies include those by Fitches and Campbell (1987) and Woodcock et al. (1988). The deep structure of the two Permo–Triassic basins present within the district has been studied by Powell (1956), Wilson (1959) and Collar (1974), and most recently by Smith et al. (in press).

An extensive literature on the Quaternary deposits of the district was reviewed by Embleton (1970), Worsley (1970, 1991), Bowen (1974), Earp and Taylor (1986), Addison (1990), and Campbell and Bowen (1989). The results of more recent work on deposits in the Vale of Clwyd were provided by Livingston (1986).

Geological field guides which include itineraries within the district are provided by Bathurst et al. (1965) and Somerville et al. (1986b).

BGS reports commissioned by the Department of the Environment (Campbell and Hains, 1988; Hains, 1991), highlight the geological factors which are relevant to land use planning in the district. These followed earlier reports (Dunkley, 1981; Ball and Adlam, 1982), also funded by the Department of the Environment, which assessed the sand and gravel resources of parts of the district. Other survey publications relevant to the district are listed in the section in Information Sources (p.174).

Chapter 2 Silurian

Silurian strata crop out on either side of the Vale of Clwyd. The larger eastern outcrop forms much of the Clwydian Range; the smaller outcrop, to the south-west, is part of that of the Denbigh Moors. These rocks underlie the Carboniferous and Permo–Triassic rocks of the Vale of Clwyd and are thought to occur at depth elsewhere in the district.

Throughout much of the late Precambrian and early Palaeozoic, Wales largely occupied an area of enhanced crustal subsidence and sediment accumulation, known as the Welsh Basin. This north-east to south-west elongated, intracratonic basin, together with the rest of southern Britain, was located on continental crust which, until earliest Ordovician times, formed part of the supercontinent of Gondwana, located on the southern side of the Iapetus Ocean (Cocks and Fortey, 1982). In response to early Ordovician rifting, southern Britain, as part of a microcontinent known as Eastern Avalonia, broke away from Gondwana. During the remainder of the Ordovician and early Silurian, Eastern Avalonia drifted northwards towards the continent of Laurentia. The oceanic crust underlying the Iapetus Ocean was subducted both southwards beneath Avalonia and northwards beneath Laurentia. Late in the early Silurian (late Llandovery), Eastern Avalonia began to collide obliquely with Laurentia (Soper and Woodcock, 1990). Underthrusting of the leading edge of the Eastern Avalonia beneath Laurentia during the remainder of the Silurian and early Devonian gave rise to the period of compressional deformation known as the Acadian (late Caledonian) orogeny.

An estimated maximum thickness of about 1000 m of late Silurian rocks, referable to the Ludlow Stage, are present in the district. They were deposited in the northern part of the Welsh Basin, in the Denbigh Trough (Cummins, 1959b), an east–west-orientated sub-basin, partitioned from the main basin to the south by the Derwen Ridge. They comprise a basinal sequence, dominated by turbidites and hemipelagites, as well as slumped and destratified strata, grouped together as disturbed beds. These recycled sediments appear to have been derived principally from a contemporary shelf region, the Irish Sea Platform, sited to the west and north-west, possibly extending over present-day Snowdonia, and delimited in part by the Conwy Valley Fault.

The sequence comprises the Nantglyn Flags Formation and the overlying Elwy Formation. The former is a sequence of rhythmically interbedded turbiditic and hemi­pelagic mudstones with scattered thin turbidite siltstones. The Elwy Formation is dominated by thinly bedded turbidite siltstones, sandstones and mudstones with very subordinate hemipelagites. Discrete packets of sandstone turbidites and disturbed beds occur in both formations but are most abundant in the Elwy Formation.

Previous research

Early accounts of the Silurian rocks of north-east Wales were provided by Bowman (1838, 1841, 1842), Sedgwick (1845), Sharpe (1846) and Ramsay (1866, 1881). The first six-inch scale (1:10 560) survey of the district led to the earliest systematic account of the Silurian rocks (Strahan, 1890). Apart from mapping local sandstones, the surveyors did not subdivide the sequence. Although the later resurvey did not extend to the Silurian rocks, it provided new biostratigraphical data (Wedd and King, 1924).

Attempts to subdivide the Silurian rocks of north-east Wales include those by Hughes (1894), Edmunds (1923), Woods and Crosfield (1925), Blackie (1931), Boswell (1931) and Simpson (1940) for the Clwydian Range, and by Wills and Smith (1922) for the Llangollen area to the south. However, it was the survey of the adjacent Denbigh district (Warren et al., 1984) that established a definitive lithostratigraphical and biostratigraphical framework for the Wenlock and Ludlow sequences of the region. Previous stratigraphical classifications of the Ludlow strata of the district and adjacent areas, together with that used in this account, are shown in (Table 2) . Other pertinent regional assessments of the stratigraphy and sedimentology of the Silurian succession in north Wales include those by Boswell (1926, 1932, 1943, 1953), Jones (1937, 1940, 1943), Cummins (1957, 1959a, b), Dimberline et al. (1990), Bassett (1992) and Eva and Maltman (1994).

Lithology and facies

For reasons of continuity, this account is based largely on the facies terminology of Warren et al. (1984) erected in the adjacent Denbigh district, but these facies are interpreted here in terms of the more recent classifications of resedimented deposits and hemipelagites currently used in the Welsh Basin (for example Cave, 1979; Davies et al., 1997). The turbidite classification used by Davies et al. (1997) in mid Wales is shown in (Figure 3). It incorporates elements of the widely used schemes devised by Bouma (1962), Lowe (1982), Stow and Piper (1984) and Pickering et al. (1986).

Ribbon-banded mudstones (Warren et al., 1984)

This facies consists mainly of rhythmically interbedded silty mudstones and laminated muddy siltstones, with sub­­ordinate, thin calcareous siltstones. The silty mudstones are blue-grey, structureless and unfossiliferous. Individual beds range in thickness from 1 to 10 cm and are sharp-based. Thin beds or laminae of calcareous siltstone are present at the bases of some of the silty mudstone beds, so together they form siltstone/mudstone couplets. The siltstones have sharp, locally erosional bases and exhibit parallel and cross-lamination. They are commonly graded and range in thickness from 1 to 10 mm.

These silty mudstones and calcareous siltstones were regarded by Warren et al. (1984) as turbiditic in origin, a view confirmed by Dimberline et al. (1990). They represent mud-dominated Bouma turbidites in which the structureless silty mudstone represents the Te division and, where present, the underlying calcareous siltstones constitutes the parallel-laminated Td and cross-laminated Tc divisions of such turbidites. In terms of the classification of Davies et al. (1997), individual mudstone turbidites lacking a basal siltstone represent Type Dii turbidites, whilst the siltstone/mudstone couplets represent Type Di or Type Cii turbidites. These turbidites were deposited by low concentration turbidity currents.

The laminated muddy siltstones range generally from 1 to 4 cm in thickness, but beds up to 10 cm thick are present. They comprise delicate but laterally impersistent alternations of dark grey, carbonaceous laminae and paler grey silt laminae. Graptolites are common, as well as orthoconic ­nautiloids in places.

Although Warren et al. (1984) regarded the laminated muddy siltstones as the deposits of very low concen­tration turbidites, such beds are widespread in the Silurian of the Welsh Basin and are now regarded as hemipelagic in origin (Cave, 1979; Dimberline et al., 1990; Davies et al., 1997). They record the slow, but constant rain of sediment out of suspension on to the sea floor and thus represent the muddy background sediment of the basin. Each pair of pale and dark laminae is thought to represent an annual varve reflecting seasonal variations in sedimentation rates (Dimberline et al., 1990). The absence of bioturbation and the preservation of organic debris demonstrates that they accumulated under anoxic (anaerobic) bottom ­conditions (Cave, 1979).

Striped silty mudstones (Warren et al, 1984)

This facies comprises irregular alternations of thinly bedded silty mudstone, siltstone and fine-grained sandstone with very subordinate beds of laminated muddy siltstone (hemipelagite) (Plate 1). The thickness of individual beds of all the component lithologies is variable, ranging from a few millimetres to several centi­metres. The proportion of each lithology is also very variable, ranging from mudstone dominant to siltstone and sandstone dominant. This facies contrasts with the ribbon-banded mudstone facies in its much higher sand and silt content, the irregular alternation of its com­ponent lithologies, and its much lower proportion of hemipelagites.

Apart from the hemipelagites, Warren et al (1984) interpreted all the component lithologies of the facies as distal turbidites. Most of the turbiditic elements of the facies are arranged as sandstone/mudstone and siltstone/mudstone couplets, either of which can display Tcde or Tde Bouma sequences and are equivalent to ­turbidites types Cii and Di of Davies et al. (1997). Some exclusively mudstone Type Dii turbidites are present locally. Type C and D turbidites are the product of low concentration turbidity currents. The siltstones and sandstones are parallel-sided, locally with sharp erosive bases. Basal shelly lags of derived fossils are present in some of the thicker beds and commonly weather to rottenstone. Some units of silt-striped mudstones up to 10 cm thick may represent fine-grained turbidites (Type E of Davies et al., 1997), deposited by very low concentration turbidity currents (Stow and Piper, 1984).

The beds of laminated muddy siltstone (laminated hemipelagite) are identical in lithology and origin to those in the ribbon-banded mudstone facies. In sections in siltstone-rich sequences, they are commonly difficult to distinguish from turbidite siltstones, with tractional parallel-lamination.

Sandstones

Both, a thin-bedded facies and a thick-bedded facies of sandstone have been recognised in the district.

Thin-bedded sandstone facies

This facies comprises interbedded, very thin- to medium-bedded sandstones and mudstones, which form sequences up to 30 m thick (Plate 2a). Scattered thin packets of ribbon-banded or striped silty mudstone and rare thick beds of sandstone occur interbedded within such sequences.

The sandstones and mudstones occur as sandstone/ mudstone turbidite couplets equivalent to Types Ci and Cii of Davies et al. (1997), in which either the sandstone or mudstone divisions may predominate. In sandstone-dominated couplets, the medium to fine-grained sandstones form thin to medium, normally graded beds up to 0.4 m thick. The bases of these sandstones are sharp and commonly erosional, exhibiting flutes and grooves. Basal shelly lags are developed locally. The mudstone component of these couplets is generally less than 0.15 m thick. The couplets may exhibit all the internal divisions of a Bouma turbidite, but Tbcde units are the most common. In mudstone- dominated couplets, the sandstones are fine grained and occur as very thin to thin beds between 1 and 10 cm thick. The sandstones are sharp based, but generally lack sole structures. The overlying mudstones are normally thicker. Typically, these couplets comprise Tcde Bouma sequences.

Both types of couplet are the result of deposition from medium to low concentration turbidity currents. Thin beds of laminated hemipelagite commonly intervene between couplets.

Thick-bedded sandstone facies

This facies comprises sequences, up to 10 m thick, of thick bedded, clean to argillaceous, medium-grained sandstones (Plate 2b). The mappable units may also contain thin intercalations of ribbon-banded mudstones, striped silty mudstones or thin-bedded sandstone facies. Individual sandstone beds range from 0.5 to 2.5 m in thickness but amalgamation is common. Their bases are sharp and commonly erosional, but sole structures are sparsely developed. Internally they are mainly structureless, apart from parallel and cross-lamination in the top few centimetres of some beds.

The sandstones do not display the typical internal sequences of Bouma turbidites and are best regarded as part of the suite of coarse-grained turbidites described by Lowe (1982) and Pickering et al. (1986). They represent the Type B turbidites of Davies et al. (1997) and are interpreted as the product of high concentration turbidity currents.

Disturbed beds

In this district, the use of the term disturbed beds follows that adopted by Warren et al., (1984) in the adjacent Denbigh district. They include deposits ranging from those which display complex slump folds, internal slide planes and disrupted bedding, to those in which the primary layering has been lost, or is preserved only in isolated and commonly contorted clasts. The most common type comprises structureless mudstone or siltstone in laterally impersistent units up 30 m thick. Many such units contain scattered angular rafts and clasts of mudstone, siltstone and sandstone, or diffuse wisps of sandstone. Some contain scattered derived shelly fossils including brachiopods, trilobites, crinoid ossicles, bryozoa and bivalves and locally these may be concentrated near the base of a unit. Basal contacts are sharp and commonly crosscutting; upper contacts may be planar or irregular. In contrast to the undisturbed parts of the sequence, these structureless rocks exhibit a characteristic irregular and anastomosing Acadian cleavage.

Many of these deposits reflect slope-failure and downslope mass movement, possibly over considerable distances, but in others the folding and destratification may be due to load or seismicly induced diapirism and liquefaction with negligible lateral displacement (Eva and Maltman, 1994).

Pebbly mudstones and conglomeratic debrites, the products of cohesive debris-flows, are also included within the spectrum of distrubed beds (Pickering et al., 1986). Where poorly exposed, clast-poor, mudstone-dominated debrites are commonly difficult to distinguish from, and can be viewed as gradational with, disturbed mudstones in which the bedding has been destroyed by slumping or by in situ liquefaction.

Biostratigraphy

Graptolites provide the principal means of biostratigraphical subdivision of the Silurian sequences in north Wales and their biozonation in the Denbigh district was described in detail by Warren et al. (1984). Derived shelly fossils, contained within turbidites and disturbed beds, provide an additional means of dating some sections. A reassessment of the graptolites and shelly fossils collected from the Clwydian Range (BGS collection) by Woods and Crosfield (1925) showed that much of the material is poorly preserved and hence the new identifications are commonly more conservative. Much of the material collected during this resurvey is also poorly preserved. Details of the biostratigraphy are given under the descriptions of the respective formations. The stages and graptolite biozones of the Ludlow Series are shown in (Table 2) . In the Denbigh district, Warren et al. (1984) divided the nilssoni Biozone into a ‘Lower’ and ‘Upper’ part. However, this subdivision should be treated with caution (see discussion in Rickards, 1976 p.170).

Nantglyn Flags Formation

This formation is composed mainly of ribbon-banded mudstone facies comprising regularly interbedded turbidite mudstones and hemipelagites with very subordinate thin turbidite siltstones and sandstones. The term ‘Nantglyn Flags’ was first used by Hughes (1894). The type locality is in the Denbigh district at quarries [SJ 9790 5980] south of Nantglyn. Hereabouts, Warren et al. (1984) subdivided these strata biostratigraphically, recognising a Lower Nantglyn Flags Group of Wenlock age, and an Upper Nantglyn Flags Group of Ludlow age. However, these divisions are lithologically identical, and have since been included in a single Nantglyn Flags Formation (British Geological Survey, 1993, 1994).

Only the upper (Ludlow) part of the formation is thought to crop out in the district, where it is confined to the Clwydian Range. Two crops occur in the north-east, flanking the eastern margin of the range, around Bryngwyn Hall [SJ 1035 7385], and between Ddol [SJ 1400 7100] and Cilcain [SJ 1780 6520]; and two occur in the south-west between the Vale of Clwyd Fault and the Range Fault, around Llangynhafal [SJ 1350 6360], and between Moel y Gaer [SJ 1470 6170] and Bathafarn Hall [SJ 1550 5750]. Thicknesses are difficult to determine because most contacts are faulted, but an estimated 180 m are present in the north-east, whereas as much as 400 m may be present in the south-west. The ‘Lower Nantglyn Flags’ of the Denbigh district range up to 660 m in thickness, and the ‘Upper Nantglyn Flags’ up to 600 m.

Impersistent but locally mappable units of thin-bedded sandstone facies up to 20 m thick are present in the upper part of the formation in the south-western crop. The sandstone content is rarely greater than 40 per cent. Rare disturbed beds are also present in the upper part of the formation in this area. Derived, commonly fragmentary shelly fossils, including brachiopods, bryozoa, ostracodes and crinoids, occur mainly in the siltstone and sandstone turbidites and disturbed beds of the formation.

In the uppermost part of the formation, there is a decrease in the presence of laminated hemipelagite and an increase in the content of siltstone/mudstone and sandstone/mudstone couplets (type Di and Cii ­turbidites), forming an upward transition into the succeeding Elwy Formation.

In this district, the formation has yielded only grap­tolites referable to the basal Ludlow nilssoni Biozone. Critical taxa from the Clwydian Range area include ?Neodiversograptus nilssoni from [SJ 1515 5980] and [SJ 1493 6031], and Monograptus uncinatus orbatus [from [SJ 154 597] and [SJ 1559 6057]. However, assemblages containing taxa referable to the latenilssoni Biozone are present in the overlying Elwy Formation (see below), and since such taxa have not been found in the Nantglyn Flags, the possibility remains that the latter is of early nilssoni Biozone age, in common with the upper part of the formation in the Denbigh district (Warren et al., 1984). A derived brachi­opod fauna from a locality [SJ 1483 6851] near Henfaes low in the local sequence, includes Dicoelosia biloba, Howellella elegans and Jonesia (Aegiria) grayi, indicating an horizon close to the Ludlow/Wenlock boundary.

Details

South-western crop

Typical ribbon-banded mudstones were noted in a roadside quarry [SJ 1595 5887] on the A494, east of Llanbedr–Dyffryn–Clwyd. Similar lithologies were seen in the quarries [SJ 1515 5980] at Rhiw-las, 1 km to the north-west. A poorly preserved fauna from this locality, in the Woods and Crosfield (1925) collection, includes Lobograptus progenitor (or ?Neodiverso­graptus nilssoni), ?N. nilssoni, Pristiograptus vicinus, Saetograptus c. colonus, S. cf. varians and ?S. chimaera s.l., indicating the nilssoni Biozone.

A fauna obtained by Woods and Crosfield (1925) from a locality [SJ 154 597] on the Old Mold Road contains Monograptus uncinatus orbatus and Saetograptus c. colonus indicative of the nilssoni Biozone. North-west of the Clwyd Gate, a unit of thin-bedded sandstone facies is exposed in a road section [SJ 1615 5835] on the A494. The Woods and Crosfield (1925) collection from this locality contains Saetograptus colonus compactus, ?N. nilssoni and ?Pristiograptus sp., possibly indicating the nilssoni Biozone.

Good crag exposures occur on the hillsides between Bwlch Penbarras [SJ 161 605] and Teiran [SJ 146 604]. In a quarry [SJ 1559 6057] on the north side of the valley in ribbon-banded mudstones, turbidite mudstones 7 to 10 cm thick, are interbedded with regularly spaced hemipelagites 2 to 4 cm thick, and rare beds of turbidite siltstone 1 to 2 cm thick. In addition to material in the Woods and Crosfield (1925) collection, these beds have yielded M. uncinatus orbatus and ?S. cf. chimaera s.l., indicative of the nilssoni Biozone. Derived shelly fossils include a lingulid, Chonetes sp., ?Skenidioides sp., Leptodesma sp., beyrichiid ostracodes, crinoid columnals, bryozoans and Trachyderma cf. squamosa. A roadside quarry [SJ 1559 6012] on the south side of the valley yielded M. uncinatus orbatus? and S. colonus compactus to Woods and Crosfield (1925), indicating a possible nilssoni Biozone age; this locality has also provided orthocone fragments, crinoid columnals and shelly debris. A fauna collected by Woods and Crosfield (1925) from ribbon-banded mudstones in Coed Ceunant [SJ 1493 6031] includes ‘Orthoceras’ cf. recticinctum, ?N. nilssoni, ?Pristiograptussp. and S. colonus compactus, indicating a possible nilssoni Biozone age. A quarry [SJ 1462 6228] in ribbon-banded mudstones, beside Nant-y-Ne, contains numerous, 10 to 20 mm-thick turbidite siltstone beds exhibiting sharp bases and loaded ripple lenses. These rocks have yielded Saetograptus sp. and abundant M. uncinatus orbatus indicative of the nilssoni Biozone.

South-west of Ceunant [SJ 1512 6010], a unit of thin-bedded sandstone facies is traceable northwards for a short distance. Disturbed beds also occur in the sequence at much the same level.

North-eastern crop

Near Henfaes, a small quarry [SJ 1483 6851] and road section [SJ 1482 6841] expose ribbon-banded mudstones with deeply weathered, thin calcareous, turbidite siltstones. One of these localities is probably ‘Tan-yr-allt near Gwrych-Bedw’ from which Woods and Crosfield (1925) obtained a fauna that includes the brachiopods Dicoelosia biloba, Howellella elegans and Jonesia (Aegiria) grayi, in addition to beyrichiid ostracodes and a possible pristiograptid. The brachiopods indicate an age close to the Wenlock–Ludlow boundary.

Elwy Formation

The Elwy Formation is synonymous with the Elwy Group of the Denbigh district (Warren et al., 1984). It consists mainly of striped silty mudstone facies (Plate 1) comprising thinly bedded turbidite mudstone, siltstone and sandstone with very subordinate laminated hemi­pelagites. Scattered, laterally impersistent units of thin- and thick-bedded sandstone facies and disturbed beds occur in the formation. The principal outcrop of the formation extends along the length of the Clwydian Range. Along much of its western margin it is faulted against the Permo–Triassic rocks of the Vale of Clwyd, whereas in the east it is either faulted against, or is overlain unconformably by Carboniferous strata. A small outcrop of the formation occurs on the western side of the Vale of Clwyd, near Ruthin. The top of the formation is not seen in the district, but the maximum exposed thickness is estimated to be close to 600 m. In the adjacent Denbigh district, Warren et al. (1984) recorded a maximum exposed thickness of 1750 m, but again the top is not seen.

The base of the formation was defined in the Denbigh district by Warren et al. (1984) at the facies change from ribbon-banded mudstones (Nantglyn Flags) to overlying, sandier, striped silty mudstones that characterise this formation. This facies transition is associated with the base of the ‘Upper’ nilssoni Biozone of Warren et al. (1984). In part of the Denbigh district, thick lenticular disturbed beds are present at this level and serve to separate the two formations. However, in undisturbed sequences, the facies change is very gradational, and as a mapping practicality, Warren et al. (1984) commonly used the incoming of ‘Upper’ nilssoni Biozone faunas to establish the base of the Elwy Formation. In the Flint district, the change from ribbon-banded mudstones to striped silty mudstones facies is also very gradational and may occur at about the same biostratigraphical horizon. Thick units of disturbed beds are largely absent. Due to extensive faulting in the Clwydian Range, the base of the formation crops out only around Penyfelin [SJ 155 690] and Cilcain [SJ 165 650] in the east, and around Moel Arthur [SJ 146 615] in the west.

Sequences of thin-bedded and thick-bedded sandstone facies, up to 25 m and 10 m thick respectively, are present in the formation in the Clwydian Range. Both facies may contain thin packets of the dominant, striped, silty mudstone facies, or of the other sandstone facies. Units of disturbed beds up to 30 m thick occur in the formation. Although the sandstone sequences, together with promi­nent disturbed beds, are scattered throughout the forma­tion, they are clustered locally around two levels. The lower cluster occurs just above the base and is restricted to Moel y Gaer [SJ 1490 6174], west of the Range Fault and near Penbedw [SJ 163 682]. The upper one, typified by the sandstone units that outcrop around Penycloddiau [SJ 130 675], occurs in the middle part of the formation and possibly extends north-west and south-east for several kilometres. Palaeocurrent vectors, obtained principally from bottom structures in the thin-bedded sandstone facies, reveal flow directions predominantly from the north-west, but with subordinate westerly and south-westerly sourced flows, broadly consistent with data obtained from the Denbigh district (Warren et al. 1985).

West and south-west of Ruthin, the Elwy Formation sequence belongs to the upper part of the formation, but the thickness present is unknown. It consists predominantly of striped silty mudstone (Plate 1), but laminated hemipelagites form a significant proportion of the sequence. Units of sandstone facies and disturbed beds are rare or absent.

Graptolite faunas show that the formation probably ranges in age from the late nilssoni Biozone to the incipiens Biozone. Other fossils present include brachiopods, bryozoans, ostracodes and crinoids: these occur mainly in turbidite sandstones and siltstones and in disturbed beds.

In the northern part of the Clwydian Range, east of the Range Fault, possible late nilssoni Biozone graptolite assemblages occur at two localities to the west of Caerwys. At one locality [SJ 0992 7173], the critical taxa are Monograptus (cf. uncinatus group), Saetograptus chimaera chimaera and S. chimaera salweyi; at the other [SJ 1117 7247] they comprise M. micropoma?, M. uncinatus orbatus and S. cf. chimaera semispinosus (see also below). The middle part of the scanicus Biozone is indicated by Pristiograptus cf. welchaefrom localities [SJ 1586 5713] to [SJ 1632 5693] in the south of the range. Graptolites referable to the incipiens Biozone have not been found in the Clwydian Range, but derived brachiopod faunas from localities [SJ 1578 6252 and [SJ 1570 6300] west of Moel Fammau suggest a possible late Gorstian Stage level, thought to approximate to that biozone.

In the Clwydian Range, west of the Range Fault, possible late nilssoni or scanicus Biozone graptolites including Lobograptus scanicus and Monoclimacis aff. micropoma were obtained from a locality [SJ 1460 6130] in the packet of sandstone facies and debrites just above the base of the formation on Moel y Gaer.

The incipiens Biozone has been identified only west of the Vale of Clwyd, where it is indicated by the presence of abundant S. leintwardinensis incipiens in a stream section [SJ 1110 5668] south-west of Galchog and in exposures [SJ 0967 5917] to [SJ 1020 5952] along the River Clywedog.

Details

Clwydian Range–northern part

A small quarry [SJ 0992 7173] in Sodom Covert, exposes ribbon-banded mudstone facies with scattered thin sandstones. Warren et al. (1984) assigned this locality to the Elwy Formation because it yielded an ‘Upper’ nilssoni Biozone fauna. The assemblage comprised Scyphocrinites? pulcher, ‘Monograptus’ (cf. Saetograptus colonus s.l.), M.? (cf. uncinatus group), S. chimaera chimaera, S. chimaera salweyi and Saetograptus sp. (cf. varians). At Coed Cochion, a track section [SJ 1117 7247] in striped silty mudstone facies with thin beds of laminated sandstone also yielded an assemblage suggestive of the late nilssoni Biozone including Monoclimacis micropoma?, M. uncinatus orbatus, cf. Neodiversograptus nilssoni, S. cf. chimaera semispinosus, and S. varians.

In a disused quarry [SJ 1185 7246] at Coed Maes-mynan, 1 km west-south-west of Caerwys, striped silty mudstone facies yielded Pristiograptus cf. tumescens, P. cf. vicinus and Saetograptus varians suggesting an age within the range of the mid scanicus to early incipiens biozones.

Clwydian Range–central part

On the south-eastern slopes [SJ 135 669] of Penycloddiau, numerous crags of striped silty mudstone facies contain scattered thin sandstone turbidites which increase in abundance upwards. A forestry track section [SJ 1338 6684] yielded Saetograptus chimaera s.l. and S. incipiens? suggesting the nilssoni to scanicus biozonal interval. The most complete and extensive exposures of the upper sequence of sandstone facies occur on the south-eastern slopes [SJ 1320 6730] of Penycloddiau, extending north-westwards towards Nant Coed-y-mynydd [SJ 1225 6900]. The lower part of the sequence is dominated by thin-bedded sandstone facies (Plate 2a), but contains a thick, laterally variable, disturbed bed, which can be traced from a point [SJ 1284 6818] 300 m north-north-east of the summit cairn, along the Offa’s Dyke footpath, north-westwards along the main spine of Penycloddiau, to the pass at Nant Coed-y-mynydd [SJ 1207 6896]. At its northernmost point the disturbed bed is at least 10 m thick, and comprises irregularly cleaved silty mudstone with scattered ‘clasts’ and ill-defined lenses of contorted sandstone and siltstone. In a forestry track [SJ 1312 6702] south of Penycloddiau, the most southerly outcrop of the disturbed bed is represented by fine-grained, fossiliferous, silty sandstone. Outcrops are also seen on the comparatively better exposed south-eastern flanks of Penycloddiau [SJ 1315 6747], which expose up to 6 m of a muddy and silty disturbed bed with ‘clasts’ of medium-grained, feldspathic sandstone. Above the disturbed bed, units of thin-bedded sandstone facies alternate with units of striped silty mudstone facies with scattered thin sandstones. The thin-bedded sandstone facies is well exposed in crags [SJ 1318 6734] on the south-eastern flanks of Penycloddiau (Plate 2a). In these exposures, packets of very thin turbidite sandstone/mudstone couplets alternate with packets of thicker couplets on a 1 to 3.5 m scale. The sandstone/mudstone couplets display mainly Tbcde or Tcde motifs. Thin basal lags, less than 10 mm thick, of brown-weathering, medium-grained, calcareous sandstone, containing shell debris, are developed locally. Bottom structures, including flutes and grooves, are preserved in places. The upper part of the upper sequence of sandstones on Penycloddiau, comprises units of thick-bedded facies and argillaceous disturbed beds, and is exposed on the west-facing slopes — for example [SJ 1277 6752], [SJ 1271 6769] and [SJ 1230 6772].

A unit of thin-bedded sandstone facies is seen in several exposures on the south-eastern flanks [SJ 1435 6900] of Bryn Goleu, where it forms an ill-defined coarsening-upwards sequence over 70 m thick. Striped silty mudstones, exposed below [SJ 1462 6926] yielded Monoclimacis micropoma?, Pristiograptus cf. tumescens, P. vicinus, Saetograptus cf. chimaera semispinosus and S. chimaera s.l. indicating a probable mid- to late scanicus Biozone age.

A thick sequence of thin-bedded sandstone facies is exposed on the south-eastern slopes [SJ 1475 6585] of Moel Arthur. The sandstone/mudstone couplets in the lowest part of this sequence [exposed at [SJ 1482 6600] are relatively thick. In the upper third of the sequence, exposed in crags [SJ 1479 6597] to [SJ 1481 6592], the couplets are much thinner and are interbedded with striped silty mudstones which become more common upwards. A 2 m-thick sandstone occurs in a quarry [SJ 1460 6605], 75 m east of the summit of Moel Arthur, where it is overlain by a disturbed bed. Units of thin-bedded sandstone facies interbedded with striped silty mudstones are exposed farther to the west [SJ 1457 6601]. Quarries [SJ 1463 6591] to [SJ 1467 6584] on the south-eastern flanks of the hill provide sections in a higher unit of thick-bedded sandstone facies (Plate 2b). To the north-west, amalgamated, massive sandstones, included in a 10 m-thick sequence of thick-bedded sandstone facies, are exposed in the quarry [SJ 1428 6608] west of the hilltop.

Between Fron Haul and Moel Arthur, south-west of Moel Llys-y-coed, rocks of thick-bedded sandstone facies crop out on the crest of the Clwydian escarpment. Penmachno quarry [SJ 1473 6507] reveals sandstones in beds, 0.5 to 2.0 m thick, interbedded with striped silty mudstone facies, overlying a sequence of thin-bedded sandstone facies and disturbed beds. A collection made by Woods and Crosfield (1925) contains cf. Lobograptus scanicus indicating a probable age range within the late nilssoni to scanicus biozones.

Thin-bedded sandstone units near the base of the formation are exposed on the hillslopes of Moel y Gaer [SJ 1490 6174]. At the base [SJ 1485 6156], a few metres of striped silty mudstones interbedded with thin sandstones are overlain [SJ 1457 6126] by up to 45 m of interbedded fine-grained sandstones, in beds up to 0.3 m thick, cross-laminated siltstones, and mudstones. The sequence generally coarsens upwards. Traced laterally, this sequence appears to be cut-out by an overlying disturbed bed up to 15 m thick. The disturbed bed forms an extensive, westerly inclined dip-slope on the western flanks of the hill, extending from Pen-y-fron [SJ 1459 6133] to a point [SJ 1486 6170] 50 m south-west of the summit. It is composed primarily of grey silty mudstone with scattered, rounded sandstone clasts. However, 200 m west-south-west [SJ 1480 6156] of the summit, it contains contorted slabs of sandstone, siltstone and mudstone and a raft of feldspathic sandstone, measuring 0.3 m by 3 m, occurs near the top of the bed. These rafts resemble the beds underlying the disturbed bed. From a sequence of sandy facies and disturbed beds [exposed around 1460 6130] Woods and Crosfield (1925) obtained a graptolite fauna comprising Lobograptus scanicus, Monoclimacis aff. micropoma and Pristio­graptus sp., suggestive of the latenilssoni to scanicus biozone. Immediately south-west of the summit of Moel y Gaer, the disturbed bed is overlain by an outlier of thin-bedded sandstone facies exposed in two places [SJ 1478 6171] to [SJ 1478 6155].

West of Moel Fammau, the gully [SJ 1578 6252] at the head of Nant-y-Ne exposes a sequence of thin-bedded sandstone facies interbedded with striped silty mudstones. The sandstone beds are generally about 0.2 m thick, but range up to 0.5 m thick. In addition to Pristiograptus sp., the exposure, collected by Woods and Crosfield (1925), has yielded the brachiopods Micro­sphaeridiorhynchus nucula, Atrypa reticularis, Isorthis orbicularis, Leptostrophia filosa, Protochonetes? sp. and Sphaerirhynchia wilsoni. The brachiopod assemblage indicates a possible late Gorstian age. Woods and Crosfield (1925) obtained another derived brachiopod assemblage from a locality [SJ 1570 6300] just to the north. The presence of Howellella elegans, L. filosa and Proto­chonetes? suggests a similar age.

A sequence of thin-bedded sandstone facies near the base of the formation is exposed between the eastern slopes of Moel Evan [SJ 1565 6750], Firwood farm [SJ 1585 6775] and the hillsides [SJ 1630 6820] west of Penbedw. The lowermost beds are seen in a quarry [SJ 1594 6746]. In exposures [SJ 1595 6738] lower on the same slopes, striped silty mudstones coarsen upwards into the sandstone facies.

Exposures in striped silty mudstones occur around the summit of Foel Fenli and one locality [SJ 1650 6005] has yielded ?Lobograptus scanicus and an indeterminate ?Saetograptid, suggesting the latenilssoni or scanicus biozones.

Clwydian Range-southern part

Thin-bedded sandstone facies and disturbed beds crop out on the hill slopes [for example [SJ 1585 5710, [SJ 1585 5685], south-east of Bacheirig. A 9 m-thick sequence of thin-bedded sandstone facies is overlain by a muddy disturbed bed at least 2.5 m thick. Exposures [SJ 1586 5713] to [SJ 1632 5693] in these facies provided Pristiograptus cf. welchae, Saetograptus cf. clunensis and S. cf. variansindicating a probable mid scanicus Biozone age. Sections [SJ 1556 5687] to [SJ 1550 5693] in small disused quarries to the west yielded Pristiograptus cf. vicinus and Saetograptus chimaera chimaeraindicative of an age range within the late nilssoni to mid scanicus biozones.

A quarry at Plymog [SJ 1852 1582], south-east of Fron Hen, exposes rocks of sandy, striped silty mudstones facies with thin sandstones up to 5 cm thick.

West of the Vale of Clwyd

The River Clywedog [SJ 0967 5917] to [SJ 1020 5952] and the track known as Lady Bagott’s Drive, in the north-west of the crop, offer some of the best exposures in striped silty mudstones facies in the district (Plate 1). Many localities along the River Clywedog have provided graptolite faunas. Though many of these yielded only long ranging taxa, in others the abundance of S. leintwardinensis incipiens supports an incipiens Biozone age. A small quarry [SJ 0984 5917] near Coed-y-Fforest yielded Pristiograptus sp., and S. cf. colonus, indicative of an age range within the nilssoni to mid-scanicus biozones, as well as the derived brachipods Dayia sp., Glassia sp., Microsphaeridiorhynchus nucula and Sphaerirhynchia wilsoni.

Striped silty mudstones in a stream section [SJ 1110 5668], south-west of Galchog, yielded the graptolite S. leintwardinensis incipiens, suggestive of the incipiens Biozone, and a chitinozoan fauna (Lister, 1970) that compares with that in low Upper Elton Beds of the Ludlow district. An argillaceous disturbed bed is inter­bedded with similar lithologies in a section [SJ 0991 5609] west-south-west of Tai-isa.

Conditions of deposition of the Nantglyn Flags and Elwy formations

The early Ludlow ribbon-banded mudstones facies of the Nantglyn Flags compare with the synonymous Wenlock facies of central Wales (Davies et al., 1997). The facies reflects anoxic deposition on a mud-dominated slope-apron across which hemipelagic deposition was periodically interrupted by the deposition of mud, silt and sand introduced by low concentration turbidity currents. Local disturbed beds record the periodic emplacement of submarine slump sheets and of material carried by debris flows (debrites).

The increase in turbidite siltstones and sandstones in the upper part of the Nantglyn Flags, together with the presence of a sequence of thin-bedded sandstone facies, heralded a change in sedimentation which culminated in the deposition of the Elwy Formation. The striped silty mudstone facies of the Elwy Formation, with its lower proportion of hemipelagites and associated sequences of thin- and thick-bedded sandstone facies and disturbed beds, records a major increase in the grade and volume of sediment supplied to the slope-apron. However, the continued presence of thin beds of laminated hemi­pelagite confirms that anoxic bottom conditions still prevailed. This pulse of westerly derived turbidites can be linked to the uplift of siliciclastic source areas consequent on the diachronous collision of Eastern Avalonia with Laurentia (Soper and Woodcock, 1990). King (1994) and Kneller et al. (1993) have suggested that the thick, early to mid Ludlow sequences of north Wales record a period of rapid, flexurally controlled subsidence, possibly related to the southward migration of a foreland basin, generated by continental collision to the north.

The sequences of thin- and thick-bedded sandstone facies, some displaying coarsening-upwards motifs, appear to represent discrete, small-scale, sandy turbidite lobes. The sediment supply to each lobe appears to have been terminated fairly quickly and may reflect feeder channel avulsion and lobe switching (see for example Davies and Waters, 1994; Normark et al., 1979).

Current vector data from the Nantglyn Flags and Elwy formations in the Denbigh Moors and the Clwydian Range (Cummins, 1959a, b; Warren et al., 1984) demonstrate that turbidity currents flowed from west to east along the axis of the Denbigh Trough. The source areas of the turbidites probably lay west of the Conwy Valley Fault and possibly also to the north of the Menai Straits Lineament suggesting that these structures were active in defining the basin margin at this time. The orientations of slump folds in disturbed beds in the Elwy Formation of the Denbigh district, many of which provide evidence of mass movement in a southerly direction, have been cited as evidence of a southward facing slope associated with the northern margin of the Denbigh Trough (Eva and Maltman, 1994).

Chapter 3 Dinantian

The Dinantian succession of the district, traditionally termed the Carboniferous Limestone, comprises a thick sequence of shallow marine ramp and platform carbonates, locally underlain by red beds. It is up to 900 m thick, and rests with marked unconformity on folded Silurian rocks. The succession crops out in three main areas; a western crop along the western side of the Vale of Clwyd; a central crop occupying a broad north-south tract flanking the eastern side of the Clwydian Range and small south-eastern crop, north of the Bala Lineament, around Hope Mountain. Further small crops occur along the western side of the Clwydian Range in the Vale of Clwyd Fault Zone.

Locally, but principally in the Vale of Clwyd, the basal part of the succession comprises up to 75 m of reddened alluvial breccia, conglomerate, sandstone, siltstone and mudstone known as the Basement Beds. The overlying succession consists of a wide range of peritidal, lagoonal, shoal and open marine limestones, commonly richly ­fossiliferous, with shelly benthic assemblages dominated by corals and brachiopods. In this account, the limestones have been classified according to the textural schemes of Dunham (1962) and of Embry and Klovan (1971). Marine and palaeosol mudstones form a small but ­characteristic part of some of the limestone formations. In the uppermost part of the succession, thick sandstones are interbedded with the limestones; they signal a gradation into the overlying, dominantly siliciclastic Silesian sequence. In the east of the district, concealed Dinantian strata, proved in the Blacon East Borehole, comprise deeper water, basinal facies and include limestone turbidites and hemipelagic mudstones.

Although the limestone succession is entirely Viséan in age, ranging from late Chadian to Brigantian, the age of the Basement Beds is uncertain and may range down into the Tournaisian. The complete Dinantian succession of the district is present only in the south of the central crop (Figure 4). Here the base of the Silesian (Namurian) may lie within the lower part of the conformably overlying Cefn-y-fedw Sandstone (Ramsbottom, 1974) described in Chapter Four. Locally, in the north of the central crop, the youngest part of the Dinantian sequence is missing due to a sub-Namurian disconformity. In the western crop, Brigantian and late Asbian strata are not preserved beneath unconformable Westphalian or Permian strata.

Classification and correlation

The first stratigraphical subdivision of the Dinantian succession of the district was made by Morton (1870, 1883–6, 1897–8) (Table 3) using his classification developed in the Llangollen area to the south (Morton, 1878). These subdivisions were not considered ‘sufficiently definite’ to be mapped during the first six-inch survey of the district, but were employed however, in the contemporary memoir account (Strahan, 1890). Modern studies have largely substantiated Morton’s four-fold division of the succession in the Llangollen area (Somerville, 1977, 1979a, b; British Geological Survey, 1994). However, the much thicker Dinantian succession of the Flint district is now known to include formations not present and considerably older than those near Llangollen (Table 3).

The coral-brachiopod biozonal scheme of Vaughan (1905) was first applied in this district by Hind and Stobbs (1906), who concluded that the entire limestone succession, here and throughout north Wales, lay within the latest Dinantian Dibunophyllum (or D) Zone, with both the D1 and D2 subzones represented (Table 3). This view was accepted during the resurvey of the district (Wedd and King, 1924) and by surveyors in adjacent districts (Wedd et al., 1927; Warren et al., 1984), as well as other workers (for example Neaverson, 1929, 1946; George, 1974; Somerville, 1977, 1979c). North Wales, and specifically this district, was cited by George (1958) as displaying the thickest development of the D Zone in Britain.

Following the subdivision of the British Dinantian into stages (George et al., 1976), broadly based on Vaughan’s biozonal scheme, the north Wales sequence was thought to span only the upper two of the newly proposed stages, that is the Asbian and Brigantian which corresponded to the D1 and D2 subzones, respectively (Table 3). However, earlier Chadian, Arundian and Holkerian strata were subsequently discovered by Somerville and Strank (1984a) in the thick succession of the central crop and, following this, Holkerian and Arundian rocks were shown to be widespread in north Wales (Somerville and Strank, 1984b, 1992; Somerville et al., 1986a). Although Chadian strata were thought to be restricted to the Dyserth area of north Clwyd (Somerville and Strank, 1984a, 1992), early results of this resurvey showed that late Chadian rocks underlie the whole of the central crop of the district (Davies et al., 1989).

The various biostratigraphical and event-based methods currently employed in Dinantian subdivision and correlation have been critically reviewed by Riley (1993). Corals and brachiopods, although abundant and widely used by earlier workers (for example Hind and Stobbs, 1906; Wedd and King, 1924; Neaverson, 1929, 1945; George, 1958, 1974; Somerville, 1979c; Somerville and Strank, 1984b and c; Somerville et al., 1986a), have not been systematically used for detailed correlation during the resurvey, due to the patchy distribution of critical taxa and their susceptibility to facies control (Riley, 1993). Application of the ammonoid zonation in the district is limited to the Brigantian, where goniatites and posidoniid bivalves occur in both platform and basinal facies.

As foraminifera are common throughout much of the limestone succession, they have been used as the main biostratigraphical tool during the resurvey. This account provides the first systematic application of the Belgian foraminiferal biozonal scheme (Conil et al., 1980; Riley, 1993) in north Wales and follows earlier limited studies by Somerville and Strank (1984b, c) and Davies et al. (1989). All the Visean biozones and subzones are recognised in the district ((Figure 4); see (Table 4), (Table 5), (Table 6), (Table 7), (Table 8), (Table 9).

Conodonts have proved rare and of limited biostratigraphical value in the Dinantian of north Wales. They have been reported from the district by Aldridge (1993) and also from the Prestatyn and Dyserth area to the north-west (Aldridge et al., 1968; Reynolds, 1970; Austin and Aldridge, 1973; Somerville et al., 1989). Miospore assemblages have been recovered from the district, notably from the Alyn Valley and Blacon East boreholes. They augment occurrences cited in earlier studies elsewhere in north Wales (for example Hibbert and Lacey, 1969; Somerville et al., 1989) and have enabled the recognition of some Dinantian miospore biozones (Figure 4).

The subdivision of British Dinantian successions using event stratigraphy has centred on the relative importance of eustatic versus tectonic influences. Ramsbottom (1973, 1977, 1979) argued that the cyclical facies changes (mesothems) and associated faunal variations, which formed the basis of Dinantian biozonation were the result of eustatic transgressive/regressive pulses that could be used for widespread correlation. The mesothem scheme has been applied in north Wales by several workers (Somerville and Strank, 1984c; Davies et al., 1989) (Figure 4).

The contrary view that local tectonism, clearly reflected in the pronounced thickness and facies variations between individual blocks and basins, over-rode the effects of small scale eustasy to be the dominant influence on sequence development, has been argued by George (1958, 1978) and Bott and Johnson (1967). Furthermore, the pronounced wedge-shaped geometry displayed by many Dinantian successions suggests accumulation across a framework of tilted fault-blocks; thin and condensed shallow-water successions formed over uplifted footwalls, whilst thicker, more complete, deeper water sequences characterised adjacent half-grabens (Miller and Grayson, 1982; Ebdon et al., 1990). More recently Gawthorpe et al. (1989), Frazer et al. (1990) and Frazer and Gawthorpe (1990) have suggested that many of the cyclical and widespread facies changes reflect discrete periods of rifting and subsidence, stimulated by plate collision events to the south, alternating with episodes of tectonic quiescence. The proposed sequence stratigraphy for Dinantian successions in northern Britain comprises six tectonic/ seismic divisions (sequences EC1 to 6) within a syn-rift megasequence (Frazer and Gawthorpe, 1990; Corfield et al., 1996). Only sequences EC2 to 6 are represented in this district (Figure 4).

The fact that carbonate platforms are not dependent on an external source for their sediment supply, and therefore aggrade rapidly during major transgressions, is frequently overlooked. Commonly, they display a predictable sequence of evolutionary changes (Kendall and Schlager, 1981; Read, 1985). Initially, ramp-like structures with relatively steep gradients display rapid lateral facies transitions; these evolve into broad flat platforms which are characterised by abrupt and widespread vertical changes in facies and biota (for example Wright, 1984; Somerville et al., 1989). As a result, correlatable facies changes within separate Dinantian block successions may, in part at least, reflect parallel evolutionary trends operating largely independently of small-scale eustatic and tectonic events.

Palaeogeography

Palaeomagnetic evidence suggests that, in Dinantian times, Britain was situated in tropical latitudes (Scotese and McKerrow, 1990). Worldwide facies distributions indicated to Witzke (1990) that during the late Viséan, Britain occupied an arid climatic zone situated to the south of a humid equatorial belt. It lay to the north of the Variscan subduction zones of southern Europe, occupying a region of north-south extension, which had resulted from backarc spreading (for example Leeder, 1987) or from transtensional shear (for example Dewey, 1982) (see Chapter 8). Prevailing stresses, influenced by pre-existing crustal fractures and by the distribution of Acadian intrusions, created the framework of blocks and basins, which is recognised in the Dinantian of the Britain (George, 1958; Leeder, 1982).

In north Wales and throughout northern Britain, the Dinantian saw the onset of a marine transgression that ended the protracted period of subaerial denudation which followed late Caledonian (Acadian; Middle Devonian) deformation and uplift. The dominantly shallow-water limestone succession reflects the initiation and growth of a carbonate platform along the northern edge of the Wales–Brabant Massif (St George’s Land), a residual upland of deformed Lower Palaeozoic rocks (George, 1958, 1974; Somerville and Strank, 1992). The timing of the inundation, the facies types and the rates of deposition all reveal strong tectonic influence (Figure 5). To the north lay the Craven/Irish Sea Basin, a region of more rapid subsidence characterised by deeper water carbonate and siliciclastic facies (Riley, 1990).

The initial marine inundation of north Wales occurred during the late Chadian. As a result, peritidal and lagoonal facies (Foel Formation) were established along the eastern flank of the Clwydian Horst, an upfaulted region defined by precursor Alyn Valley and Vale of Clwyd faults. Contemporary movements on these faults is reflected in the marked thickness variations of these facies and in the restriction of coeval and perhaps earlier alluvial red beds (Basement Beds) to the Vale of Clwyd region (Figure 5b), (Figure 6). This fault activity broadly coincided with a rifting event documented in other Dinantian successions (Leeder et al., 1989; Frazer and Gawthorpe, 1990). During the early to mid-Arundian, a rapid and widespread transgression introduced shallow-water carbonate ramp facies (lower part of Llanarmon Limestone) to a broad sector of north Wales ((Figure 5)c). Late Arundian shoal facies progradation and aggradation established a low-gradient platform during an interval of steadily rising sea level.

Preservation of Arundian strata within a tract now defined by the Bala Lineament, to the south-east, and the Menai Straits Lineament, to the north-west, may reflect the form of an original, structurally defined, marine embayment (Somerville and Strank, 1984b). However, the original distribution of these facies may have been greater in extent. Intraclasts of Arundian grainstone, ­associated with terrigenous detritus, deposited within the embayment during the latest Arundian (Davies et al., 1989) provide evidence for a period of uplift, leading to erosion both along the embayment margins and possibly beyond. This was coeval with a rifting event recognised elsewhere (Riley, 1990; Barclay et al., 1994) and favours the subdivision of the EC3 sequence proposed by Frazer et al. (1990) (ECa and ECb on (Figure 3)). Renewed transgression, sustained throughout the Holkerian and early Asbian, introduced widespread changes to the pattern of carbonate deposition. Contemporaneous facies reconfigurations are recorded in south Wales (Wilson et al., 1988). Earlier structural barriers were inundated; shoal facies withdrew to the outer margins of the platform (Llanarmon Limestone; upper part in north) where they sheltered an embayed platform interior which became the site of rhythmic, peritidal, carbonate deposition (Leete Limestone) ((Figure 5)d).

The onset of cyclical, glacio-eustatic changes in sea level in late Asbian times resulted in frequent emergence of the platform. This is demonstrated by the common karstic and pedogenic features which cap abundant, open marine, shoaling-upwards sequences of platform carbonates (Loggerheads Limestone). During this period, the platform margin was marked by knoll reefs, now exposed on the Little Orme, at Dyserth and at Axton in the adjacent Rhyl district ((Figure 5)e). To the north lay an apron of limestone turbidites (Prestatyn Limestone). Renewed rifting in latest Asbian and early Brigantian times is recognised in other areas (Frazer and Gawthorpe, 1990), and is reflected in north Wales by marked thickness changes across the Bala Lineament and the contiguous Nercwys–Nant-figillt Fault Zone. From the early Brigantian onwards, regional (thermal) subsidence (Leeder, 1982), superimposed on continuing eustatic oscillations in sea level, caused transgessions which were individually more extensive and more rapid than those which occurred during the Asbian. Cyclical platform facies of deeper-water aspect (Cefn Mawr Limestone) were deposited during these inundations. During this period, the marginal knoll reefs were abandoned and the platform margin retreated south-westwards. The surviving Berwyn and Anglesey peninsulas were largely transgressed as platform carbonate facies in north Wales achieved its greatest geographical extent ((Figure 5)f).

Uplift to the south, in the late Brigantian, may have coincided with climatic changes promoted by plate collision in southern Europe as well as the Gondwanan glaciation (Leeder, 1988). The net result was to increase the volume and grade of terrigenous material supplied to the north Wales carbonate platform (Minera Formation). The sub­sequent encroachment of deltaic facies (Cefn-y-fedw Sandstone), from latest Brigantian times onwards, brought widespread carbonate deposition in the district to an end.

Brigantian turbiditic and hemipelagic facies (Teilia Formation), present at Prestatyn in the adjacent Rhyl district and at depth in Blacon East Borehole in the east of this district, are of basinal aspect (Ramsbottom, 1978). The borehole demonstrates that the margin of the north Wales platform, contrary to recent models (Warren et al., 1984; Somerville and Strank, 1992; Frazer et al., 1990; Frazer and Gawthorp, 1990), must have curved southwards, parallel to the central crop, along a line running approximately through Flint and Hawarden as originally suggested by George (1958) ((Figure 5)e, f). It is not known whether the location of the edge of the Dinantian shelf was structurally controlled, though it appears to coincide broadly with the course of the Hawarden Fault and a contiguous zone of north-west–south-east trending faulting in the adjacent Liverpool district (Institute of Geological Sciences, 1974). Asbian knoll reef facies overlain by a Brigantian sequence of proximal turbidite limestones in the Croxteth Borehole of the Liverpool district (Riley, 1988; Magraw and Ramsbottom, 1956), formerly taken to indicate the eastward extension of the north Wales platform margin (Ramsbottom, 1969), may relate to a quite separate platform formed over the Rossendale or Central Lancashire High (Frazer and Gawthorpe, 1990; Riley, 1990) ((Figure 5)e).

Basement Beds

Basement Beds is the informal formational name given to siliciclastic deposits present at the base of the Dinantian sequence throughout north Wales (for example Strahan, 1885, 1890; Warren et al., 1984). In this district, such facies occur principally in the Vale of Clwyd, but are also present locally in the central crop. In both areas, they rest unconformably on Silurian rocks and are overlain by the limestone sequence with a sharp erosive contact. They were first included in the Carboniferous sequence by Strahan and Walker (1879), and, with the exception of Ramsey (1881) who regarded them as Old Red Sandstone, subsequent authors (Morton, 1878; Neaverson, 1945; George, 1958; George et al., 1976; Warren et al., 1984; Somerville et al., 1989) have supported this view. The Basement Beds comprise unfossiliferous, reddened and variegated breccia, conglomerate, sandstone and silty mudstone, with scattered thin calcretes. They are up to 75 m thick in the Vale of Clwyd, west of Llanfwrog, but, as reported from the northern parts of the Vale (Warren et al., 1984), they exhibit marked lateral thickness variations (Figure 6). In the River Clywedog, less than 2 km to the north, only about 15 m are present. In the central crop, the Basement Beds appear to be very thin or absent and have been mapped only south of Nannerch, where red clays are exposed in streams to the north [SJ 1642 6877] and south [SJ 1695 6780] of Penbedw Hall. In the southern part of the central crop, the formation is 3.5 m thick in the Alyn Valley Borehole (Figure 7), (Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9) and comprises mottled red and green breccia (Plate 3a), conglomerate and sandstone.

The angular unconformity between the Basement Beds and Silurian rocks in the Vale of Clwyd is well displayed in the bed of the River Clywedog [SJ 1023 5955], south-west of Rhewl (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13). Shallow, north-easterly dipping, red and green mottled breccias rest on steep, westerly inclined mudstones of the Elwy Formation. The clasts in the breccias are all of local Silurian lithologies, and comprise angular to subrounded fragments of mudstone and micaceous silt­stone up to 0.15 m across.

The range of local Basement Beds facies is well seen in Galltegfa Dingle [SJ 1049 5782] to [SJ 1097 5780], west of Llanfwrog (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13) where the formation comprises a variegated sequence, dominated by micaceous, striped and mottled, purple, red, green, grey and fawn siltstones and silty mudstones. Subordinate, coarser grained lithologies include channel-fill conglomerates and pink and red, parallel-laminated calcareous sandstones, locally in thin, fining-upwards sequences. The conglomerates contain abundant vein quartz, siltstone and reworked calcrete pebbles and elsewhere, in the adjacent Rhyl district, also include clasts of fossiliferous Ludlow lithologies (Strahan and Walker, 1879, p. 271).

Replacive calcareous nodules (‘cornstones’ of earlier accounts), and thin beds of variegated, red and green, argillaceous, nodular limestone or dolomite are present throughout the formation and reflect varying stages in the development of glaebular calcrete profiles.

The beds are unfossiliferous but early Arundian foraminifera in the overlying Llanarmon Limestone in the Vale of Clwyd ((Table 7) , locality 1), and late Chadian microfossils in the overlying Foel Formation in the central crop at Penbedw Hall (Davies et al., 1989), in the Alyn Valley Borehole (Figure 7) and south of the district, provide an upper age limit for the formation in these two areas and demonstrate the diachronous, eastward younging nature of its top (Figure 6). Palynological samples from the formation have yielded only woody carbonaceous debris (melanogen), material which is most resistant to microbiological and chemical degradation (Cope, 1981; Davies et al., 1989).

Conditions of deposition

The association of red beds and calcretes, the absence of marine fossils and the preservation only of woody plant material show that the formation accumulated under strongly oxidising, terrestrial conditions and comprises a suite of alluvial facies. The thick sequence in the Vale of Clwyd probably includes the earliest Dinantian deposits in north Wales. It records the infilling of an incised, but possibly fault-influenced topography, formed during the preceding interval of subaerial denudation. Strahan and Walker (1879) suggested that the detritus which makes up these beds in the Vale of Clwyd was derived from the north. However, there seems little doubt that extensive Welsh upland areas to the immediate west and south were the principal sediment source ((Figure 5)b) (Neaverson, 1946, p.124).

The basal breccias, composed of fragments of local Silurian strata, represent an initial accumulation of scree or alluvial cone deposits. The succeeding striped and ­variegated facies were deposited on a low gradient alluvial/coastal plain; the preponderance of mud- and silt-grade detritus reflects the importance of overbank sedimentation. Abundant glaebular calcrete profiles testify to the early and widespread pedogenic alteration of these deposits, whereas thin, conglomerate-based, fining-upwards sequences reflect deposition within associated meandering stream channels (Figure 6).

The distribution of Basement Beds, in this district and adjoining areas, suggests that deposition was largely confined to the west of a subdued, but effective, physiographic barrier, centred on the present Clwyd Range. Although this barrier may have been a passive topographical feature (such as an interfluve), formed by preceding subaerial erosion, the thickness variations of the Foel Formation (see below) indicate it was an upfaulted horst created by Chadian movements on the Vale of Clwyd and Alyn Valley faults (compare with Davies et al., 1989), and possibly linked to contemporary activity on the Bala and Menai Straits lineaments (Figure 5). In contrast, the region occupied by the present-day Vale of Clwyd was an area of relative sub­sidence which preferentially received the detritus of hinter­land drainage systems ((Figure 5)b). Possibly the thickest sequences of Basement Beds accumulated and remain preserved beneath the eastern parts of the Vale, adjacent to the Vale of Clwyd Fault (Figure 6).

Details

Vale of Clwyd

The basal unconformity and overlying breccias exposed in the River Clywedog [SJ 1023 5955], west of Rhewl ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), locality 13), are described above. Higher in this thin sequence, and exposed for 30 m downstream, are purple and green mottled, muddy siltstones with thin beds of red sandstone, conglomerate and calcrete. The upper 5 m of the formation is seen in a landslip scar [SJ 1029 5975] ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), locality 1) to the north of the river. There, laminated and striped, purple, grey and fawn siltstone with calcrete glaebules are erosively overlain by a fining-upwards sequence. The latter comprises a lenticular conglomerate with pebbles of vein quartz, purple siltstone and reworked calcrete, which passes up into soft purple and green micaceous sandstone. The sequence is sharply overlain by the Llanarmon Limestone.

Galltegfa Dingle [SJ 1049 5782] to [SJ 1097 5780] exposes the most extensive section through the Basement Beds in the district ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), locality 14). A bed of glaebular calcrete, up to 0.4 m thick, unconformably overlies cleaved mudstone of the Elwy Formation and is in turn overlain by the basal breccias. In a tributary [SJ 1034 5790] of the main stream, purple, thin-bedded siltstones and sandstones are exposed, faulted against Silurian rocks ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), locality 15).

Foel Formation

The Foel Formation was named by Warren et al. (1984) and has its type section in the Dyserth area, north of the district. It comprises a lithologically varied, thin- to medium-bedded sequence which consists of porcellaneous calcite mudstone and wackestone, argillaceous and foetid packstone, peloidal and locally ooidal grainstone, cryptalgal laminite and oncolitic floatstone, together with inter­calated mudstone, plant-bearing siltstone and calcareous sandstone (Figure 7), (Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8). In this district, the formation occurs only in the central crop, where it forms the basal division of the Dinantian sequence in the absence of the Basement Beds (Davies et al., 1989).

West of the Alyn Valley Fault and its splays, the formation is estimated to range up to 20 m in thickness. Although there are few thickness data, sections to the south of the district indicate that it thins southwards, dying out north of the Llanelidan Fault (Davies et al., 1989). However, to the east of the Alyn Valley Fault, 88 m of Foel Formation were encountered in the Alyn Valley Borehole (Figure 7), (Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9).

The limestones of the Foel Formation display a wide range of allochems and textures. Terrigenous impurities are ubiquitous and dolomitisation is common. Calcite mudstones and calcisphaere wackestones with commonly articulated ostracode valves (Plate 3e), together comprise the porcellaneous, white-weathering ‘chinastones’ of earlier workers (for example Strahan, 1890). They occur in units up to 0.35 m thick and locally form up to 25 per cent of the formation, but are rare in the type area to the north of the district (Somerville et al., 1989). Spar-filled, laminar and tubular fenestrae (‘bird’s-eyes’) are commonly developed (Plate 4e). Packstone beds in the formation are typically thin, dark, argillaceous and fissile; they issue a strong foetid smell when broken, which suggests a high organic content. Grainstones are commonly parallel- or cross-laminated and are paler in colour. They are composed mainly of micritic peloids and intraclasts with subordinate, skeletal grains derived from foraminifera, brachi­opods and echinoderms (Plate 4c). Quartz sand and silt grains are ubiquitous. Rare ooidal grainstones are also present (Plate 4a). The coarsest limestones in the formation are oncolitic floatstones, in which individual oncolites are up to 20 mm across (Plate 3e); Davies et al., 1989, fig. 6). Limestones extensively altered to micrite, and with rhizoliths (fossil not-related structures) (Plate 4d), or displaying micrite cement bridges (Plate 4b) testify to the effects of synsedimentary calcretisations and early diagenesis in the vadose zone (Davies, 1990).

The most abundant siliciclastic components of the formation are fissile, argillaceous and variably calcareous siltstones and silty mudstones, characteristically rich in carbonaceous debris. Plant remains are common; diverse and well preserved floral assemblages have been described from the type area, north of the district (Hind and Stobbs, 1906; Walton, 1928; Lacey, 1962; Warren et al., 1984). Variegated red and green mudstones, com­parable to those in the Basement Beds are also present.

The Alyn Valley Borehole (Figure 7) proved facies in proportions not seen at outcrops to the west of the Alyn Valley Fault. For example, cryptalgal laminites (stromatolites), form a distinctive component in the borehole (Plate 3b) and (Plate 3c), but only a minor facies in surface exposures (Somerville et al., 1989). They occur throughout the upper part of the borehole sequence in units up to 6 m thick. They alternate with units of thickly bedded, parallel- and cross-bedded, sandy and dolomitised grainstone and crinoidal packstone up to 17 m thick. The latter units contain levels rich in the tabulate colonial coral Syringopora (Plate 3d). Variegated silty mudstones and sandstones form most of the lower part of the formation. A basal 0.25 m-thick bed of skeletal grainstone (Figure 7) rich in angular lithoclasts, sharply overlies the Basement Beds.

The biostratigraphy of the type section has been fully described by Somerville et al. (1989). Microfossil assemblages from the district have been assessed by Davies et al. (1989). Formerly thought to be early Asbian in age (Warren et al., 1984), the Foel Formation is now known to be entirely late Chadian (early Viséan) containing foraminifera and algae of the Cf42 Subzone ((Figure 7); (Table 6) ). Diagnostic forms include the foraminifera Bessiella sp., B. legrandi, Biorobis duplex, Dainella sp., D. cf. cognata, Eoparastaffella, Endothyra cf. laxa, Florenella sp., F. stricta, Lysella and Palaeospiroplectammina cravenensis and the dasycladacean alga Koninckopora spp. with single and double-layered walls. Critically, archaediscid foraminifera are absent. Rare abraded corals (Somerville et al., 1989) are principally solitary rugosan genera, and, in the absence of lithostrotionids, are consistent with a late Chadian age. The smooth-shelled brachiopod Composita cf. ficoidea dominates in situ shelly macrofossil assemblages in the type area. Linoprotonia cf. hemispherica, Athyris expansa and Megachonetes papilionaceus are also reported, in addition to the gastropod Straparollus, thin-shelled bivalves and Spirorbis (Neaverson 1930; Somerville et al., 1989). Stomatolitic and oncolitic lithologies preserve the remains of the blue-green algae Garwoodia gregaria, Ortonella and Girvanella. Mudstones in the Foel Formation, in the Alyn Valley Borehole (Figure 7), provided well preserved miospore assemblages of the Lycospora pusilla (Pu) Biozone.

Conditions of deposition

The presence of scattered beds of calcite mudstone and ostracode calcisphaere wackestone, displaying abundant desiccation and root fenestrae (Shinn, 1968; 1983), together with the associated stromatolitic and oncolitic facies suggest that the Foel Formation formed in a peritidal setting (Davies et al., 1989). Locally developed calcrete textures and vadose zone cements point to periods of prolonged subaerial exposure.

In contrast, peritidal porcellaneous limestones are rare in the type area near Dyserth, and accordingly Somerville et al. (1989, p.56) have argued that there the Foel For­mation accumulated mainly in a ‘shallow water sub­tidal shelf’ setting, supporting a stenohaline open marine benthos. However, direct evidence of an open marine origin for the Foel Formation is lacking. Much of the ­stenohaline benthic assemblage recorded by Somerville et al. (1989) is transported. The abundance of the spiriferid brachi­opod Composita is, empirically, diagnostic of lagoonal facies (George, 1933, 1958, 1974; Neaverson, 1946; Ramsbottom, 1973) and, with other elements of the autochthonous macrofauna (for example Linoprotonia, Straparollus and Spirorbis), con­stitutes the specialised, hypersaline Composita Community of Ramsbottom (1978). The diverse and delicate plant remains in the mudstones and siltstones at Dyserth, combined with the absence of shelly benthos, suggests that these lithologies also accumulated in a quiet, restricted setting, close to plant source areas, where reducing conditions in a water-logged or subaqueous environment favoured plant preservation.

Nevertheless, the presence of thick units of locally cross-bedded packstones and grainstones in the type area (Somerville et al., 1989) indicates proximity to a more energetic environment, and may record the invasion of landward lagoons by sediment derived from sheltering shoals or barriers. High energy grainstones, including ooidal units are also observed in sections south of Dyserth, but in contrast, the greater abundance of fenestral, porcellaneous limestones reflects the more frequent progradation of intertidal and supratidal carbonate mud flats in this region. These types of flats may develop on the leeward side of barrier systems (Inden and Moore, 1983), but the southerly attenuation and overstep of the Foel Formation (Davies et al., 1989) strongly suggests that they were a feature of the landward side of the postulated lagoon in which the formation accumulated ((Figure 5)b).

The thick sequence encountered in the Alyn Valley Borehole strongly indicates syndepositional movement on the Alyn Valley Fault. The thin sequences lying to the west of the fault (Dyserth, Caerwys, Penbedw and Gelli-gynan) appear to be condensed, probably having formed along the margins of the Clwydian Horst. The thick packets of cross-bedded grainstone and crinoidal packstone in the borehole show that contemporary high-energy and open marine facies interdigitated with peritidal facies; the westward spread of the former was possibly restricted by the topographical influence of the upward propagating fault (Figure 6).

Details

The northernmost section is a disused quarry [SJ 1212 7258] in Coed Maes-mynan, west of Caerwys, where the upper 5.25 m of the formation and its contact with the Llanarmon Limestone is exposed ((Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8); (Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12), locality 2). Fenestral calcite mudstones and ostracode calcisphaere wackestones make up a quarter of the sequence (Plate 4d). Also present are dark, argillaceous packstones, calcareous siltstones rich in plant debris and a thin ooidal grainstone bed. Microfossil assemblages recovered indicate the Cf4α2 Subzone ((Table 6) , locality 2).

In a stream [SJ 1698 6783], south of Penbedw Hall, dolomitised and sandy peloidal packstones and grainstones with scattered oncolites overlie red clays (Basement Beds). In the Aber Eilun [SJ 1908 6284], west of Pwll-y-blaw, 2 m of interbedded porcellaneous wackestones and peloidal packstones with carbonaceous laminae and scattered plant remains are exposed. Cf4α2 Subzone microfossils are present at both these localities (Davies et al., 1989).

Exposures in the thick sequence of Foel Formation present on the eastern side of the Alyn Valley Fault are confined to crags [SJ 1884 5864] to [SJ 1896 5885] to the north and south of Llwyn-y-frân Farm ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), localities 8 and 9). These outcrops, close to the top of the formation, show dark, peloidal grainstones containing scattered oncoids and a rich microfossil assemblage (Davies et al., 1989). Details of the Foel Formation in the Alyn Valley Borehole and of the macrofaunal, foraminiferal, algal and miospore taxa present are provided in (Figure 7).

Llanarmon Limestone Formation

The Llanarmon Limestone Formation comprises pale, structureless or cross-bedded, thick-bedded, peloidal and skeletal grainstone, with subordinate dark, thinner bedded packstone. The formation was originally described by Somerville and Strank (1984b) with a type section at Pistyll Gwyn Quarry [SJ 1900 5750] (Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), near Llanarmon-yn-Ial. Its definition was subsequently modified by Davies et al. (1989), and is further amended here. The formation is the equivalent of the Llandudno Pier Dolomite, Llysfaen Limestone and the Ochr-y-foel Limestone (Warren et al., 1984), of the Dyserth Quarry, Moel Hiraddug, the Gop Hill limestones (Somerville et al., 1986a) ((Table 3) ) and of the Llanymynechs Member of Grey (1981).

The formation sharply overlies the Foel Formation in the central crop but succeeds the Basement Beds in the Vale of Clwyd (Figure 6), (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13). In the north of the central crop, a thick sequence of Llanarmon Limestone is overlain by the Loggerheads Limestone Formation. Traced southwards the upper part of the Llanarmon Limestone intertongues with, and is progressively replaced by the Leete Limestone Formation (Figure 4), (Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), (Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11) (Davies et al, 1989). In addition to the normal range of Llanarmon Limestone facies, these upper parts also include lithologies rich in micritic intraclasts, oncoids and coquinas of Composita. Poor exposure has precluded the recognition, in this expanded northern sequence, of an horizon equivalent to the base of the Leete Limestone in the south, even though this level marks a stratigraphical event of widespread significance (see p.17). The Llanarmon Limestone is 75 m thick in the Vale of Clwyd (where it is also overlain by the Leete Limestone) and varies from 140 m in the south of the central crop, to a maximum of about 280 m in the north.

The Llwyn-y-frân Sandstone Member (Davies et al., 1989) is present near the base of the formation (Figure 6), (Figure 7), (Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), in the central crop, east of the Alyn Valley Fault. It comprises a 20 to 30 m-thick sequence of dolomitised, locally cross-bedded, sandy grainstones and calcareous sandstones. In the Alyn Valley Borehole it occurs 13 m above the base of the formation, overlying dark crinoidal packstones with scattered coral colonies of Syringopora and Dorlodotia (Figure 7). Comparable, though less sandy grainstones seen at the base of the formation to the west of the Alyn Valley Fault, at Coed Maes-mynan [SJ 1212 7258] and, south of the district at Gelli-gynan Farm [SJ 1812 5465] (Davies et al., 1989), may represent a coeval facies (Figure 6).

In the central crop, above the Llwyn-y-frân Sandstone, dark brown-grey, medium- to thin-bedded, fine-grained skeletal packstones, locally crinoidal, comprise much of the lower third of the formation (Figure 6). In contrast, pale grey and brown, thick-bedded and structureless, skeletal peloidal grainstones and packstone-grainstones predominate throughout the remainder of the formation and form the bulk of the sequence in the Vale of Clwyd (Figure 6). The component grains in these limestones are typically well rounded. The skeletal elements are abraded, commonly micritised and dominated by foraminifera, echinoderms, brachiopods and calcareous algae (principally the dasycladaceans Anthra­coporella, Kamaenella and Koninckopora) (Plate 5a), (Plate 5b), (Plate 5c), (Plate 5d). Micritic peloids are abundant and commonly dominant. Grainstone intraclasts are widely present (Plate 5a) and though typically under 2 mm in diameter, in places attain pebble-size. Superficial ooids are also widespread, and well developed radial-fibrous ooids are present locally. Low-angle, cross-bedded, superficial ooid and skeletal grainstones form the upper part of the formation in the south (for example Spring Quarry [SJ 1916 5968]: (Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 11) and the coeval middle part in the north of the district (as in a quarry [SJ 1230 7254], near Caerwys: (Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12), locality 11). A rudstone, up to 7 m thick, is widely recognised and caps the formation in the south of the central crop (Somerville and Strank, 1984b; Davies et al., 1989) ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), localities 12 and 13). It is rich in rounded pebbles of vein quartz, angular fragments of green mudstone and grainstone intraclasts. In the north of the central crop, those parts of the formation that are laterally equivalent to, and intertongue with the Leete Limestone, include packstones and grainstones with a high proportion of micritic intraclasts and small oncoliths. Dasycladacean grainstones, composed dominantly of the micritised plates of Koninckopora inflata (Plate 5c), are also common at this level.

Evidence of syndepositional calcretisation and karstification in the Llanarmon Limestone has been observed only in the district at the type locality, 75 m above the base of the formation ((Figure 7)). There, a unit of cross-bedded grainstones displaying rhizoliths and laminar calcretes is capped by an irregular palaeokarstic surface and overlain by a 0.6 m-thick, mottled and carbonaceous clay palaeosol. A comparable and possibly equivalent horizon was noted 25 m above the base of the Llysfaen Limestone (equivalent to the Llanarmon Limestone) at Tan-y-graig Quarry [SH 889 773] in the Rhyl district.

In situ macrofossils are rare in the formation. Reworked brachiopod taxa occur mainly as coquinas of disarticulated valves. Commonly, they include Lino­protonia hemispherica, Megachonetes sp. and coarse-ribbed rhynchonellids; Athyris expansa and Overtonia frimbriata have also been recorded. Rare coquinas, composed exclusively of Compositavalves, are confined to horizons laterally equivalent to the Leete Limestone. Commonly rolled and abraded rugose corals recovered mainly from the south of the central crop include the colonial taxa Dorlodotia briarti, Lithostrotion sociale and L. martini, and the solitary forms Axophyllum sp., Haplolasma cf. densa, Koninckophyllum cf. vesiculosum, Palaeosmilia murchisoni, and Siphonophyllia cf. garwoodi. This coral assemblage is indicative of an Arundian age (Somerville and Strank, 1984b).

The basal few metres of the formation in the central crop, on both sides of the Alyn Valley Fault, contain foraminiferal assemblages, similar to those in the Foel Formation, referable to the Cf4α2 Subzone. The chronostratigraphical implications of these assemblages are discussed by Davies et al. (1989) and Riley (1993), who regarded them as earliest Arundian in age. The presence of the Cf4β Subzone is indicated by assemblages containing the archaediscid Uralodiscus sp. from 8.2 m above the base of the formation in the Alyn Valley Borehole (Figure 7), from less than 5 m above the base at Gelli-gynan Farm [SJ 1812 5465] (Davies et al., 1989) and, possibly, from 2.2 m above the base in a quarry [SJ 1212 7258] near Caerwys ((Table 6) , locality 3b). In the south of the central crop, high diversity archaediscid assemblages indicative of the Cf4γ and Cf4δ subzones characterise the upper two thirds of the formation. Low-diversity assemblages however, are present at intervals throughout the formation. The upper levels of the thicker northern sequence are considerably younger, containing Cf5 Biozone and Cf6α Subzone assemblages, which confirm the presence of Holkerian and early Asbian strata, respectively (Table 4)(Table 5)(Table 6).

In the Vale of Clwyd, low diversity archaediscid assemblages from the basal bed, exposed in the River Clywedog [SJ 1026 5975] to [SJ 1029 5933] ((Table 7) , locality 1), indicate that the base of the formation there may lie within the Cf4β Subzone. However, just 0.7 m above the base of the formation, diverse assemblages of the Cf4γ Subzone, including the archaediscid association Glomodiscus, Paraarchaediscus and Uralodiscus, but without Kasachstanodiscus are present ((Table 7) , locality 2). The base of the formation is therefore significantly younger than in the central crop. Cf5 Biozone taxa indicative of the Holkerian Stage are confined to the overlying Leete Limestone and show that the Llanarmon Limestone in the Vale of Clwyd is entirely Arundian in age.

Miospores obtained from mudstone partings in the Alyn Valley Borehole comprise only poorly preserved, long-ranging Dinantian taxa (Figure 7).

Conditions of deposition

Abundant, locally cross-bedded grainstones, composed of well rounded and sorted but diverse carbonate grain assemblages including ooids, indicate that much of the Llanarmon Limestone accumulated under relatively high-energy, warm, shallow marine conditions. Diverse, but commonly abraded and micritised skeletal components testify to sustained, yet intermittent, reworking prior to final deposition. The abundance of crinoids, brachiopods and foraminifera in some grainstones points to normal marine salinities; those dominated by the plates of dasycladacean algae, suggest that elsewhere or at other times, hypersaline conditions prevailed. Common grainstone intraclasts relate to the reworking of crusts or nodules formed by early cementation. In contrast, subordinate, thinner bedded, commonly crinoidal and coral-bearing packstones record accumulation in less energetic, open marine settings where interstitial fines were preserved. Common packstones and grainstones lacking tractional structures probably reflect the effects of intense bioturbation.

The macrofauna of the grainstone-dominated parts of the formation, including the large solitary corals Koninckophyllum and Palaeosmilia and the brachiopods Linoprotonia and Megachonetes, compares with the Oolitic Community of Ramsbottom (1978). The taxa of this faunal community were adapted to a mobile sea floor and typically exhibit evidence of post-mortem reworking and abrasion.

The sharp base to the formation records a major marine transgression. This transgression eventually established open marine carbonate facies across a broad sector of north Wales, certainly between the Bala and Menai Straits lineaments (Somerville and Strank, 1984b, 1989; Somerville et al., 1989; Davies et al., 1989) ((Figure 5)c) and possibly beyond. In this district, the transgression advanced from east to west, inundating the area of the central crop during the Cf4α2 Subzone, and the Vale of Clwyd area, late in the succeeding Cf4β Subzone. The basal, dark packstones with Dorlodotia and Syringopora, below the Llwyn-y-frân Sandstone Member, indicate that a low-energy, open marine environment was initially established in the area of the central crop. High-energy, nearshore shoals or barriers were possibly present to the west, seeded on a residual Clwydian Horst, still defined by the Alyn Valley Fault. The Llwyn-y-frân Sandstone Member marked an initial eastward progradation of that facies at a time when quartz sand and silt, derived from still active alluvial tracts to the west and south, were abundant (Figure 6).

The inundation of the Vale of Clwyd enabled grainstone shoal facies to migrate to that area and dark, offshore packstones to replace the Llwyn-y-frân Sandstone in the central crop (Figure 6). The facies distribution during this Cf4β-γ subzone period was therefore consistent with an eastward facing carbonate ramp (Somerville et al., 1989) ((Figure 5)c). This east–west differentiation diminished as grainstone facies were re-established in the central area during the Cf4γ-δ subzones. The thick, late Arundian, grainstone sequences, which accumulated throughout the district, marked the onset of a blanket platform facies (Figure 6) and of aggradation in response to a generally rising sea level. The palaeokarstic surface and palaeosol at Pistyll Gwyn Quarry record a period of temporary emergence which, if equivalent to similar horizons present near Llysfaen and in the High Tor Limestone of south Wales (Spalton, 1982; Ramsay, 1987), may record a minor, yet widespread marine regression.

The pebbly rudstones which cap the Llanarmon Limestone in the south of the central crop reflect the brief rejuvenation of a nearby siliciclastic source area, possibly the uplifted footwall region of the Bala Lineament. Abundant grainstone intraclasts in the rudstones testify to the simultaneous reworking of late Arundian platform facies. They offer evidence for a period of uplift and erosion along the southern margin of the Clwyd Embayment broadly coincident with a rifting episode documented in other regions (Riley, 1990; Barclay et al., 1994). This event heralded major changes in platform conditions and configuration at the onset of Holkerian times (Cf5 Biozone). The grainstone shoals of the Llanarmon Limestone now steadily withdrew to the outer margins of the platform so that, at the close of early Asbian times, the shoals lay across the north and possibly to the east of the central crop ((Figure 5)d). Their interdigitation with the hypersaline, lagoonal facies of the Leete Limestone form­ing across the restricted platform interior is reflected in grain assemblages which are locally rich in ­dasycladacean algae, oncolites and micritic intraclasts and in brachiopod coquinas composed of Composita Community taxa.

Details

Central crop–southern part

Pistyll Gwyn Quarry [SJ 1900 5750], the type section of the formation, and the Alyn Valley Borehole, sited in the quarry floor, together provide a continuous section through the lower 85 m of the Llanarmon Limestone including the Llwyn-y-frân Sandstone Member. A log of the sequence, and details of the macrofauna and microfossil assemblages are presented in (Figure 7). To the south of the quarry, the Llwyn-y-frân Sandstone is exposed in crags [SJ 1867 5662] east of Rhiw-ia Farm, and at a section [SJ 1852 5596] south of Maes Llan. North of Pistyll Gwyn Quarry, the member is well exposed at its type locality, the prominent scarp [SJ 1894 5863] overlooking Llwyn-y-frân Farm ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 10), and in crags [SJ 1900 5885] to the north, which form its northernmost section.

Coarse-grained, peloidal and ooidal grainstones with Dorlodotia briarti and coquinas of Linoprotonia hemisphaerica, exposed in crags [SJ 1948 5617], east of Llanarmon-yn-Ial, lie in the upper part of the formation. Exposures [SJ 1961 5655] to the north, in com­parable lithologies, contain the foraminifera cf. Paraarchaediscus stilus st. involutus indicative of the Cf4γ-δ subzone interval. Spring Quarry [SJ 1916 5968] ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 11) and the contiguous crags to the east afford an extensive section through the uppermost parts of the formation. The cross-stratified, superficial ooid and peloidal grainstones seen in the quarry were previously believed to occupy a much lower stratigraphical horizon (Spring Quarry Limestone of Somerville and Strank, 1984b), but were subsequently shown to lie near the top of the Llanarmon Limestone by Davies et al., (1989). Arundian fossils reported by Somerville and Strank (1984b) include the corals Dorlodotia briarti, Lithostrotion martini, Koninckophyllum spp., Palaeosmilia murchisoni and Siphonophyllia cf. garwoodi, and the foraminifera Archaediscus spp. (now Paraarchaediscus), Eoparastaffella simplex, Rectodiscus sp. (now Uralodiscus) and Tubispirodiscus settlensis (now Kasachstanodiscus). A comparable foraminiferal assemblage, but with Eblanaia michoti is present in the crags [SJ 1923 5962] to the east. Intraclast rudstones with abundant pebbles and granules of vein quartz, are exposed at the top of the Llanarmon Limestone along the nearby track [SJ 1927 5959] ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 12). Foraminifera present in the grainstone intraclasts include Nodosarchaediscus sp. and Uralodiscus sp., an association indicative of the late Arundian (Cf4-δ subzones). Further exposures of these rudstones include crags [SJ 1918 5800] at Bryn-y-glôch ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 13) and a 7 m-thick section in the River Alyn [SJ 1932 6130] at Cascade Wood (Somerville and Strank, 1984b, p. 90). Upstream in the River Alyn [SJ 1936 6118], peloidal and skeletal grainstones overlying a thin sequence with porcellaneous limestones are included in the Leete Limestone, but may provide the southernmost indication of the lateral passage between the two formations (Davies et al., 1989, fig.2).

The limestone ridge trending east-north-east between Maes-mawr [SJ 1765 6442] and Maes-y-groes [SJ 1850 6480] ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10)) provides scattered exposures in peloidal and skeletal packstones and well sorted, locally cross-bedded and coquinoid grainstones. Details of the microfauna from these strata are given in (Table 4) (localities 3 to 6). Loose blocks of calcite mudstone associated with exposures of dark, fine-grained packstones [for example 1831 6459] ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), locality 11) provides evidence of a thin tongue of Leete Limestone. The presence of Pojarkovella nibelis in an underlying exposure of Llanarmon Limestone [SJ 1810 6471], in strata also rich in Koninckopora and micrite intraclasts (Plate 5c), confirms that the lower portion of the formation in this area ranges at least into the Cf5 Biozone (Holkerian) ((Table 4) , locality 6). Microfossil assemblages obtained from an upper tongue of the Llanarmon Limestone exposed in Nant Gain [SJ 1844 6509] to [SJ 1864 6511] ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10)), are given in (Table 4) (localities 8 and 9). The presence of Archaediscus st. angulatus? in the westernmost sample from the Nant Gain section (locality 9) suggests that the base of the Cf6α-β subzone (equivalent to early Asbian) lies within the upper tongue of Llanarmon Limestone.

Central crop–northern part

Quarries [SJ 1729 6556] to [SJ 1720 6596] north-west of Cilcain, expose coarse-grained, thick-bedded, locally crinoidal grainstones which Somerville and Strank (1984b) included in their ‘Spring Quarry Limestone’. Details of microfossils present at these localities, are given in (Table 4) (localities 1 and 2); the diverse archaediscid assemblages confirm a mid to late Arundian age (Cf4γ-δ subzones). Comparable facies and faunas are present in quarries [SJ 1701 6720] to [SJ 1712 6732], west of Siamber Wen. To the east, a quarry [SJ 1784 6580] exposes 0.8 m of porcellaneous calcite mudstone, part of a tongue of Leete Limestone. The calcite mudstone contains abundant calcisphaeres, spar-filled fenestrae and euhedral gypsum pseudomorphs (Plate 6a). It is overlain by 2 m of Llanarmon Limestone, comprising peloidal, skeletal and superficial ooid packstone-grainstones which contain the Holkerian to early Asbian foraminifer cf. Pojarkovella nibelis ((Table 4) , locality 7). A nearby quarry [SJ 1800 6563] exposes up to 12 m of packstone-grainstones with coquinas of Linoprotonia hemisphaerica ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), locality 13).

Crags [SJ 1722 6769] to [SJ 1720 6763] to the west of Tardd-y-dwr, expose coarse-grained dolomitised packstones and sandy, ooidal grainstones which have yielded Cf4γ-δ foraminifera and algae including Paraarchaediscus st. involutus. East of Nannerch, a quarry [SJ 1712 6943] and disused railway cutting [SJ 1689 6983] provide sections in higher tongues of the formation. The former section exposes 6 m of peloidal, dasycladacean, intraclast grainstones and packstones with a 0.6 m-thick unit of coquinoid, productid rudstone. Abundant Koninckopora inflata, associated with Nodosarchaediscus sp. are present in the quarry and in the railway cutting. Together with Cribrospira denticulata from the railway cutting they indicate a Cf6α-β Subzone age (equivalent to early Asbian).

Extensive sections along Pantgwyn [SJ 1460 7168] to [SJ 1561 7270], the deep valley north of Ysceifiog, illustrate well the intertonguing of Llanarmon Limestone and subordinate Leete Limestone lithologies (Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11). Exposures low in the local sequence in foraminiferal and Anthracoporella grainstones, on the south side of the valley [SJ 1481 7144], at the head of Ysceifiog Lake [SJ 1495 7180] and to the south-east of Ysceifiog [SJ 1547 7112], all contain diverse microfossil assemblages indicative of the Cf4δ Subzone (late Arundian), including Kasachstanodiscus settlensis and Paraarchae­discus st. involutus ((Table 5) , localities 1 to 3). A stream section [SJ 1560 7269] in the uppermost tongue of Llanarmon Limestone, close to the base of the Loggerheads Limestone, exposes coarse packstone-grainstones with micritic intraclasts and abundant small oncoids. The association of cf. Bribadya sp., Dainella holkeriana, Florenella llangollensis and Pojarkovella sp. confirm a Cf6α-β Subzone age (Asbian) ((Table 5) , locality 7).

Coed Maes-mynan, the wooded dingle west of Caerwys, provides an extensive section through the lower part of the thick northern sequence of Llanarmon Limestone. Beds overlying the Foel Formation are exposed in a quarry [SJ 1212 7258] (see Foel Formation) and adjacent crags to the north (Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12). To the north, nearly 100 m of strata are exposed in the floor and sides of the dingle [SJ 1221 7282] to [SJ 1220 7370]. Medium to thick-bedded, skeletal, locally crinoidal, and peloidal grainstones and packstone-grainstones dominate the lower part of the section; the upper part comprises cross-stratified, dasycladacean and superficial ooid grainstones. Scattered thin coquinas of disarticulated chonetid, rhynchonellid and productid brachiopod valves, including Linoprotonia hemisphaerica, occur throughout. Microfossils show that the sequence ranges into the Cf4δ Subzone (Table 6). The plateau to the north and east of Croes-wian includes localities [such as 11257481] which yield Cf5 Subzone microfossils ((Table 6) , locality 10), whereas assemblages with Florennella llangollensis and Groessenseilla moldensis from crags [SJ 1370 7382] and with Holkeria sp., Dainella holkeriana, Porjarkovella nibelis, cf. Spinobrunsiina goweri, together with Choma­tomediocris sp. from roadside exposures [SJ 1400 7410] confirm that the formation ranges into the Cf6α Subzone. Micritic intraclasts, small oncoids, dasycladacean algae and superficial ooids are common in the poorly sorted grainstones exposed at these locations. Leete Limestone lithologies are absent from this region, but intraclasts of fenestral calcite mudstone in crags [SJ 1323 7376] of fine-grained rudstones associated with coquinas of Composita and Linoprotonia reflect a former proximity.

Vale of Clwyd

The River Clywedog [SJ 1029 5961] to [SJ 1079 6000] south-west of Rhewl provides an important, but faulted section through the formation, in which contacts with the Basement Beds, below, and the Leete Limestone, above, are exposed (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13). Microfossils obtained from the section are of the Cf4β-δ Subzonal interval (Table 7). In the lowermost part of the formation in a river bank [SJ 1029 5933], a suite of atypical lithologies occur within a 0.7 m-deep, channel-like feature, incised into the basal, sandy, peloidal grainstone bed of the formation ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), locality 2). The channel fill comprises very thinly bedded, partially dolomitised and recrystallised calcisiltites with reddened, silty mudstone partings separating beds of intraclast, lithoclast rudstone. The rudstone displays abun­dant fragments and pisoids of mamillated, cryptalgal (Girvanella) bindstone laminite (Plate 5e), in addition to abraded coral debris, quartz pebbles and hematite-plugged cavities. The underlying basal bed of the formation, and a similar horizon, exposed in a landslip [SJ 1026 5975] to the north ((Table 7) , locality 1), both contain restricted archaediscid assemblages suggestive of the Cf4β Subzone (Plate 5a). However, diverse assemblages recovered both from the channel fill and overlying pebbly and sandy grainstones are of the Cf4γ Subzone ((Table 7) , locality 2). Comparable arenaceous and pebbly limestones are exposed downstream [SJ 1071 5985] to [SJ 1074 5992] but yield low diversity foraminiferal assemblages, locally devoid of archaediscids ((Table 7) , localities 3 to 6).

Intraclasts of peloid grainstone with Para­archae­discus st. involutus and Uralodiscus sp. associated with pebbles of quartz, siltstone and red chert, occur in an exposure [SJ 1023 5870] of sandy grainstones, north-east of Ferm ((Table 7) , locality 7). The reworked archaediscid assemblage is suggestive of the Cf4γ Subzone. A nearby quarry [SJ 1040 5863], higher in the sequence, exposes 5 m of thick-bedded, locally cross-stratified, peloidal and skeletal grainstones with productid coquinas and yields a stunted and low diversity archaediscid assemblage ((Table 7) , locality 8). Quarries [SJ 1058 5837] to [SJ 1067 5823] to [SJ 1085 5804] along the escarpment to the south-east afford further sections in comparable horizons within the formation ((Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13), localities 16 to 18).

Localities in the commonly veined and dolomitised grainstones present between splays of the Vale of Clwyd Fault are listed with details of their microfossil assemblages in (Table 8) .

Leete Limestone Formation

The Leete Limestone Formation is a thin to thick-bedded, heterolithic formation characterised by abundant ­porcellaneous limestones (fenestral calcite mudstones and wackestones) in addition to dark, argillaceous skeletal packstones, characteristically containing the thick-shelled brachiopod Daviesiella llangollensis, and paler grainstones, rich in oncolites, intraclasts and calcareous algae. The formation, named after Leete Country Park [SJ 1985 6280] at Loggerheads in the central crop, was first defined by Somerville (1977, 1979c), who included in it the whole of the Dinantian limestone sequence below the Loggerheads Limestone. Following the recognition of earlier Arundian formations, the base was subsequently redefined by Somerville and Strank (1984b) and again by Davies et al. (1989). It is now recognised that, in the central crop, the formation intertongues with the Llanarmon Limestone. In the south, where such tongues are absent, the base of the formation is taken at the base of the lowest porcellaneous limestone and commonly overlies the widespread pebbly rudstone unit at the top of the Llanarmon Limestone. The formation is broadly synonymous with the Tandinas or Careg-onen Limestone of Anglesey (Power, 1977; Davies, 1983), the Dulas Limestone of the Rhyl and Denbigh districts (Warren et al., 1984), the Ty-nant Limestone of the Wrexham district (Somerville, 1977, 1979b) and the Sychtyn Member of the Oswestry region (Grey, 1981).

The formation is up to 140 m thick in the south, both in the central crop and in the Vale of Clwyd, but as a consequence of its lateral passage into the Llanarmon Limestone is absent from northern parts of the central crop. A comparable facies change has been described from the Oswestry district by Grey (1981).

The principal lithologies of the Leete Limestone display a rhythmic distribution. Each complete rhythm overlies a basal undulatory erosion surface and typically comprises a lower packstone and/or grainstone division and an upper porcellaneous limestone division (Figure 14). Each lithology may predominate locally to the exclusion of the others. Individual rhythms range from less than 0.5 m to several metres in thickness. They compare with those described from the Ty-nant Limestone of the Llangollen area (Somerville, 1979b; Gray, 1981; Somerville and Gray, 1984).

The dark grey, argillaceous, skeletal packstones are variably dolomitised and commonly weather to a brown colour. The skeletal grains are typically dominated by those derived from calcareous algae, principally dasycladaceans, but also include echinoderm (principally crinoid), brachiopod, foraminiferal and molluscan remains. The macrofossil assemblage is of low diversity, dominated by the distinctive, thick-shelled, giganto-chonetidDaviesiella llangollensis sensu lato, commonly preserved in life-position. Other common and in situ taxa include the brachiopods Composita and Linoprotonia hemisphaerica, the sclerosponge Chaetetes and the colonial tabulate coral Syringopora. Rhythms dominated by such packstones are common in the lower part of the formation in the south of the district, where they form thin-bedded sequences, several metres thick.

Pale grey and brown, peloidal, skeletal and intraclast grainstones, interbedded with, or in place of the darker packstones, are a feature of rhythms in the upper part of the formation; also of its northern tongues, where associated grainstone sequences, tens of metres thick, are mapped as tongues of the Llanarmon Limestone (Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), (Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11). The skeletal grains are well rounded, micritised and commonly dominated by the abraded plates of dasycladacean algae, notably of Koninckopora. Grainstone units are commonly graded, overlying basal rudstone and floatstone lags comprised of abundant large oncolites, intraclasts and reworked macrofossils. Both grainstone and micritic intraclasts abound in both the coarser and finer-grained lithologies. The transported macrofossil assemblage includes abundant gastro­pods, in addition to those taxa present in the packstones. Oncolitic coatings on these various remains are common.

The characteristically white-weathering, porcellaneous limestones grade from calcite mudstone to very fine-grained, ostracode calcisphaere wackestone (Plate 6a)(Plate 6b). They vary in colour from dark grey to light brown or cream. In the packstone-dominated lower part of the formation, they occur as thin and widely spaced components. In contrast in the upper part of the formation, they are more abundant and locally form sequences several metres thick. The calcite mudstones and fine-grained wackestones are rarely structureless; commonly they display abundant spar-filled fenestrae, including tubular, irregular and laminoid varieties (Logan et al., 1974; Somerville and Grey, 1984). Rhizoliths, cryptalgal lamination and, in fenestrae-poor varieties, mottling and peloidal textures are also preserved locally. Skeletal grains, principally in the wackestones, are dominated by calcisphaeres (Plate 6a)(Plate 6b), including both smooth-walled and spinose taxa (Gray 1981), and ostracodes preserved in both articulated and fragmented form. Sparse, micritised foraminifera and dasycladacean algal grains are also recorded. Gastropods are the dominant macrofossils and include turbinate and turreted varieties. Articulated specimens of Composita are locally abundant in non-fenestral units and typically exhibit body cavities filled by geopetal micrite overlain by sparry calcite and/or dolomite. Calcite pseudomorphs of euhedral gypsum crystals are locally present (Plate 6a). Carbonaceous mudstones containing terrestrial plant remains and stringers of coal locally cap these porcellaneous litholgies in places.

The brachiopod Daviesiella llangollensis sensu lato is the only biostratigraphically significant macrofossil in the formation. It is regarded as being restricted to the Holkerian and early Asbian (Cope, 1940; Somerville and Strank, 1984b), but may be subject to strong facies control.

Microfossils from the basal beds [SJ 1930 5957] of the formation, east of Spring Quarry, include the foraminifera Holkeria sp. which, in the absence of later biozonal indicators, is indicative of the Cf5 Biozone (Holkerian) ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 14). The lower part of the formation in the south of the district contains assemblages with abundant Holkeria avonensis and Pojarkovella nibelis, also indicating an Holkerian age. The upper part of the formation and its northern tongues contain foraminifera of the Cf6α-β Subzone (early Asbian). In addition to common Pojarkovella nibelis, both Gigasbia gigas and bilaminar palaeotextulariids appear in these assemblages. Sparse Euxinita sp., a taxon normally confined to the Cf6γ Subzone, are also present [as at [SJ 1974 5872]. However, many individual assemblages recovered from the formation are of low diversity and comprise long ranging taxa.

Conditions of deposition

This interpretation of the depositional environment draws on the work of Somerville and Gray (1984 and references therein) on the synonymous Tynant Limestone of the Llangollen area. The Leete Limestone reflects deposition in restricted, probably hypersaline, lagoonal and peritidal settings, sheltered by grainstone shoals and barriers (represented by the coeval parts of the Llanarmon Limestone) ((Figure 5)d). Complete rhythms represent progradational or aggradational sequences initiated by marine inundations which gave rise to basal erosion surfaces. The dark and argillaceous packstones forming the lower part of the rhythms were deposited below fair-weather wave-base, in a relatively low-energy, subtidal environment, where there was incomplete winnowing of interstitial carbonate and terrigenous fines. Dasycladacean algae, which today thrive in warm, shallow waters and can tolerate varying degrees of hypersalinity (Wilson, 1975), were the dominant contributors to the sediment. However, skeletal grains, derived from stenohaline taxa (crinoids and some brachiopods and foraminifera), confirm that salinities were not everywhere, or at all times, greatly above normal. The low diversity, macrofossil suite of the packstones compares with the hypersaline Composita Community assemblages of the Foel Formation.

The calcite mudstones and wackestones which form the upper part of the rhythms were formed on tidal mud-flats and record the effects of repeated and varying degrees of emergence (Shinn 1983). Units rich in tubular fenestrae or displaying mottling and pelleted fabrics indicate burrowing in frequently submerged, low intertidal regions. Pervasive, irregular and laminoid, spar-filled fenestrae record the effects of desiccation and shrinkage in high intertidal and supratidal settings subject to long periods of exposure. Associated rhizoliths and cryptalgal lamination testify to the colonisation of these regions by plants and by mats of blue-green algae (stromatolites) respectively. Rare gypsum pseudomorphs record the action of evaporitic processes. Evidence for syndepositional dolomitisation of the sediments and early vadose cements in the sparry fills of the fenestrae have been reported by Grey (1981). The abundant turbinate gastro­pods preserved in these lithologies compare closely with the grazing cerithids typical of modern carbonate tidal flats (Bathurst, 1975), while the nests of Composita have analogues in modern intertidal mussel banks (Raup and Stanley, 1971). Grey (1981) has argued that the thicknesses of the peritidal units and the distribution of fenestral fabrics suggests that the prevailing tidal range, in the Llangollen Embayment, was generally less than 2 m.

Peloidal, skeletal grainstones in the rhythms reflect deposition in a variety of associated settings, all of which ­facilitated the complete winnowing of interstitial fines. Many may indicate a subtidal environment above fair-weather wave-base, and therefore compare with the mobile or unstable carbonate sands of modern platform interiors (Bathurst, 1975). Other graded grainstone units with oncoid and intraclast-rich rudstone basal lags, overlying sharp erosion surfaces, may have formed either as transgressive shoreface deposits or within laterally migrating intertidal channels (Shinn, 1973). Thinner units, interbedded with packstones, may have been emplaced during storms.

The thick sequence of Leete Limestone records a period of sustained, shallow, subtidal and peritidal deposition in which sediment accumulation broadly kept pace with a protracted and slow marine transgression. In response to this sea-level rise, the Leete Limestone facies gradually extended south-westwards to cover a much wider area of the north Wales shelf. Holkerian to early Asbian peritidal facies closely comparable with the Leete Limestone are widely developed in other Dinantian block areas (Burgess, 1985; Wilson and Cornwell, 1982; Stevenson and Gaunt, 1971; Wilson et al., 1988, 1990; Kellaway and Welsh, 1993). This suggests that the sea-level rise was widespread and possibly eustatic in origin (Somerville, 1979b), but also demonstrates that different block areas had independently evolved low-gradient carbonate platforms at this time. However, marked thickness variations in north Wales (George, 1974; Grey, 1981; Somerville and Strank, 1989) also demonstrate the importance of regional subsidence and of major faults on both facies distribution and rates of accumulation. The Leete Limestone and its broad correlatives appear to occupy embayments between structurally controlled headlands ((Figure 5)d).

The interpretation of the rhythms as representing repeated periods of inundation, progradation/aggradation and emergence, suggests that the ambient transgression was, in detail, accomplished as a series of consecutive small-scale, perhaps orbitally driven elevations in sea level. However, aggradational rhythms are a common feature of ancient carbonate peritidal sequences prompting the ­suggestion that, under the influence of a sustained and steady transgression, peritidal facies generate such rhythms automatically (Wright, 1984). Thus, as intertidal facies fill the subtidal areas, which supplied them with sediment, aggradation of an individual rhythm gradually ceases. The rising sea level then leads to the re-inundation of the abandoned flats and to deepening; only following the re-establishment of an extensive subtidal region will sufficient sediment be generated and supplied to intertidal areas to stimulate the next phase of progradation (Bosellini and Hardie, 1973).

Details

Central crop

Crags [SJ 1931 5617] at Tomen y Faerdre, east of Llanarmon-yn-Ial, expose 16 m of interbedded packstones and grainstones and two porcellaneous limestone beds. Abundant Pojarkovella nibelisand cf. Holkeria sp. indicate, in the absence of younger taxa, a Cf5 Biozone age. Just to the north, crags [SJ 1926 5631] above Plâs isaf, show white-weathering, fenestral calcite mudstones and dark packstones with Daviesiella llangollensis sensu lato. The packstones also contain Cf5 Biozone foraminifera including cf. Eostaffella parastruvei, cf. Eblanaia michoti, Holkeria cf. avonensis, Millerella excavata and Pojarkovella nibelis.

Crags [SJ 1971 5875] to [SJ 1978 5876] north-west of Eryrys expose the upper 23 m of the formation ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 3; (Figure 14)). A 4 m-thick sequence of porcellaneous limestones caps the formation beneath well bedded packstones of the Loggerheads Limestone. Microfossil determinations from these strata and from nearby localities in both the Leete Limestone and Loggerheads Limestone are given in (Table 9) . Upper levels of the Leete Limestone in this vicinity contain the foraminifera Euxinita sp., suggestive of the Cf6γ Subzone.

The basal beds of the formation, comprising ostracode calcisphaere wackestones, occur in a pit [SJ 1930 5957] ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 14) east of Spring Quarry. They yielded a Cf5 Biozone assemblage including the foraminifera Earlandia sp., Holkeria sp., Mediocris mediocris, cf. Mitcheldeania sp., Parathurammina sp. and Pseudo­ammodiscus sp. and the algae Girvanella sp., double-walled Konincko­pora and indeterminate kamaenids. To the east, Somerville and Strank (1984b, pp.95–96) reported abundant Pojarkovella nibelis and Klubonibelia sp., also characteristic of the Cf5 Biozone, in addition to Brunsia spirillinoides, Eostaffella parastruvei, Septabrunsiina sp. and Spinoendothyra spp. from crags [SJ 1940 5964] of grainstones ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), locality 15). Somerville and Strank included these strata in the Llanarmon Limestone and, though not distinguished on the map, they may indeed represent the southern extension of one of the intertonguing units of that formation that are a feature farther north. Brown, argillaceous packstones exposed in nearby track cuttings [SJ 1942 5952] yielded Daviesiella llangollensis sensu lato (Somerville and Strank, 1984b, p.97).

Cliffs [SJ 1978 6284] to [SJ 1985 6273] in Leete Country Park, constitute the type section of the formation. They expose the upper 65 m of the formation, which mainly comprises oncolitic and intraclast grainstones, and fenestral porcellaneous limestones with turbinate gastropods and cryptalgal lamination. Daviesiella llangollensis sensu lato is common, both in situ and as transported and disarticulated valves. Porcellaneous calcite mudstones and wackestones, forming a unit up to 3 m thick, cap the sequence and pass up into massive and rubbly skeletal packstones of the Loggerheads Limestone. Somerville and Strank (1984b, p.97) reported the presence of Bibradya inflata, abundant Pojarkovella spp. and Septabrunsiina tynanti indicative of a Cf6α-β subzonal age.

To the north, the steep eastern side [SJ 1960 6295] to [SJ 1970 6547] of the Alyn valley exposes extensive sections through the formation in both normal and faulted contact with the Loggerheads Limestone. Cliffs [SJ 1892 6482] ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), locality 12) expose 10 m of fenestral calcite mudstones and packstones overlying 15 m of skeletal, superficial ooid packstones and grainstones, the latter included in a tongue of the Llanarmon Limestone. Microfossils from the Llanarmon Limestone in a comparable succession in Nant Gain suggest an early Asbian age ((Table 4) , locality 9); porcellaneous limestones in the succeeding tongue of Leete Limestone, exposed in crags [SJ 1861 6511] ((Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10), locality 10) to the east, contain abundant calcisphaeres and cf. Renalcis sp. (Plate 6b).

Sections in the northern tongues of the formation are seen in quarries [SJ 1714 6867] to [SJ 1699 6932] south-east of Nannerch. The brachi­opods Daviesiella llangollensis sensu lato and Linoprotonia hemisphaerica are associated with Holkerian or early Asbian microfossil assemblages including abundant Koninckopora. In a disused quarry [SJ 1677 6903] west of the former Nannerch station, a thin coal seam, up to 40 mm thick, associated with black mudstones with plant remains is present in a rhythmic sequence of porcellaneous limestones, packstones and locally ooidal grainstones (Strahan, 1890, p.19). The section contains an Holkerian or early Asbian assemblage including Archaediscus sp., Brunsia spirillinoides, Endospiroplectamminia sp., cf. Eostaffella sp., cf. Millerella sp., Paraarchaediscus sp, Pseudoammodiscus sp., Septabrunsiina sp. and the dasycladacean algae Koninckopora inflata and K. minuta, both in abundance. An extensive section to the north of Nannerch is exposed in a disused quarry [SJ 1640 7036]. Peloidal packstones yield non-diagnostic, low diversity microfossil assemblages. The northernmost exposures of the formation in the district are sections along Pant-gwyn where thin sequences of Leete Limestone (see p.38) intertongue with Llanarmon Limestone (Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11).

Vale of Clwyd

The River Clywedog [SJ 1051 5982] to [SJ 1045 5977], exposes the lowest part of the Leete Limestone which here comprises porcellaneous and oncolitic lithologies. Microfossils indicate the Cf5 Biozone ((Table 7) , locality 9). The nearby Craig-y-ddywart Quarry [SJ 1100 5930] (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13) exposes 40 m of massive packstones and grainstones in which porcellaneous calcite mudstones and ostracode, calcisphaere wackestones, locally rich in turreted and turbinate gastropods, are rhythmically intercalated in the upper part. The presence of Pojarkovella in the lower beds of the quarry ((Table 7) , locality 10) and Holkeria avonensis in a sample from less than 2 m below the top ((Table 7) , locality 11) demonstrates that most of the section lies within the Cf5 Biozone.

The prominent scarp of Graig Lom [SJ 1146 5602] to [SJ 1133 5669], north-east of Efenechtyd, exposes up to 15 m of well-bedded and massive, skeletal packstones with graded grainstones and rare porcellaneous wackestones. Foraminifera indicative of the Cf5 Biozone, recovered from the northern end of the section, confirm that these strata lie at a level low in the local Leete Limestone sequence. Higher parts of the formation are exposed extensively in the well featured ground to the east. There, microfossil assemblages from a scarp [SJ 1142 5620] and disused quarry [SJ 1149 5605] indicate the Cf6α-β Subzone.

Localities of the Leete Limestone present between splays of the Vale of Clwyd Fault are listed with details of their micro­fossil assemblages in (Table 8) (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15).

Loggerheads Limestone Formation

This formation consists mainly of pale, thick-bedded, skeletal and peloidal packstone which may be massive, rubbly, mottled or pseudo-brecciated. The packstone, together with minor grainstones, is arranged in a series of laterally persistent cyclic sequences (cycles), the tops of which are commonly defined by irregular or pitted palaeokarstic surfaces overlain by mudstone palaeosols. The type section of the formation is the prominent scarp [SJ 1995 6328] overlooking Loggerheads (Somerville, 1977, 1979c) (Plate 7). The Loggerheads Limestone is broadly similar in lithology and age to the Penmon Limestone of Anglesey (British Geological Survey, 1989), the Great Orme Limestone of Llandudno (Warren et al., 1984), the Llanddulas Limestone of Abergele (Warren et al., 1984), the Eglwseg Limestone of Llangollen (Somerville, 1979a) and the Llynclys Formation of the Oswestry area (Grey, 1981).

The formation is completely preserved only in the central crop of this district, where it thickens southwards from around 150 to 175 m (Figure 4). In the west of the Vale of Clwyd, about 100 m of the formation are preserved beneath unconformable Westphalian or Permian rocks. In the south of the central crop, the base of the formation is generally taken above a thick packet of porcellaneous limestones which is commonly present at the top of the Leete Limestone, though in places, grainstones also typical of the latter formation intervene. In the north, as a consequence of the major lateral facies changes in the underlying units, the Loggerheads Limestone succeeds the distinctive, early Asbian grainstone and packstone facies of the Llanarmon Limestone. Detailed correlation of the cyclic sequences in the formation and of those in the overlying Cefn Mawr Limestone demonstrates that the top of the former is diachronous in the central crop; cycles recognised in the upper part of the Loggerheads Limestone in the north form part of the Cefn Mawr Limestone in the south (Figure 16). However, anticipated thickness variations consequent on these lateral facies changes appear partly to have been offset by southward thickening of the lower part of the formation (Figure 4).

The individual cycles range from under 0.5 to 21 m in thickness. Up to twenty five cycles are recognised in the type area, but lateral amalgamation and passage into the Cefn Mawr Limestone cause fewer to be present to the north and south respectively (Figure 16). The cycles are constructed principally from a range of skeletal packstones including rubbly, mottled, pseudo-brecciated and massive varieties. Cycles constructed entirely of mottled or massive packstones are common (Figure 16). In more complex cycles, rubbly or mottled packstones occupy the lower part of each cycle, irrespective of cycle thickness. The rubbly limestones comprise irregular shaped lenticles and cylindrical burrow casts of recrystallised packstone, separated by pale grey, anastomosing, mudstone laminae. They occur in units up to 1.5 m thick and are most common and thickest in the lower cycles of the formation. Mottled, pale grey packstones are more abundant. The dark mottles are pervasive and include both narrow (up to 10 mm) and wide (up to 30 mm) varieties; either type may dominate individual horizons. A petrographical study (Solomon, 1989) showed that the mottles contain an opaque, pale brown, inclusion-rich, calcite cement. This contrasts with the late, pore-filling, clear, calcite cement present in the surrounding paler areas. The mottle cements are thought to have formed during early diagenesis, possibly along burrows or around buried biota. The pale grey to cream, massive and thick-bedded, peloidal and skeletal packstones and packstone-grainstones of the upper parts of the cycles (Plate 8) locally exhibit a diagenetic fabric termed pseudobreccia (Dixon and Vaughan, 1911). Here the ‘clasts’ are the product of pressure solution and are defined by pervasive, randomly orientated and cross-cutting stylolitic surfaces. Thin units of cross-bedded grainstone cap some of the cycles: these are peloidal and dasycladacean with coated grains. Locally they contain coquinas of disarticulated brachiopod valves and rolled corals.

The skeletal grain assemblages of the various packstones are typically micritised and poorly sorted. They are dominated by the remains of foraminifera, echinoderms (principally crinoids), pseudopunctate brachiopods and calcareous algae together with trilobites, bryozoa and corals. Coarsely crinoidal limestones are common in cycles in the upper parts of the formation. Packstones with abundant oncolites up to 30 mm in diameter also occur. Thin porcellaneous limestones, reminiscent of the underlying Leete Limestone, are present in the basal cycles.

The uppermost parts of the cycles commonly display the effects of syndepositional calcretisation which have been reviewed in Dinantian sequences elsewhere in north Wales (Davies, 1991). The most common and pervasive macroscopic features are rhizoliths: ramifying, spar-filled, tubular structures, 1 to 3 mm in diameter, with brown, tan and cream micritic rims. Larger examples, up to 30 mm across, display strong concentric lamination and extend down below the tops of individual cycles for up to 2 m. Vadose cement fabrics are common.

Coatings of dark brown and black, laminated micrite up to 100 mm thick are widely developed at the tops of the cycles and can be compared with modern laminar calcretes. They line former fissures, envelope intraclasts and form crust-like veneers on palaeokarstic surfaces. These surfaces, which define the cycle tops, vary greatly in relief and form. Isolated pits up to 2 m deep with steep and, in places, overhanging sides occur locally (Plate 8), but hummocky to gently undulating surfaces with relief of less than 0.3 m are the most common. The surfaces are commonly overlain by impersistent palaeosols, that comprise soft, blocky, listric and pyritic mudstones locally containing rootlets and lenticles of coal. Where fresh, they vary in colour from grey to green to locally red, and are commonly mottled. The weathering of the disseminated pyrite gives rise to ochreous hues which stains the adjacent limestones and renders the palaeosols conspicuous in quarries. The paleosols are thickest where they fill deep palaeokarstic pits (Plate 8), but are typically less than 0.1 m thick and commonly pinch out above palaeokarstic hummocks. Geochemical and X-ray diffraction analysis of the palaeosols (Somerville, 1979a) show that they are composed of illite/smectite mixed layer clays, display high K, Zr and Rb values and contain the heavy minerals zircon, apatite and anatase. They compare with potassium-rich bentonites and are thought similarly to include the distal, wind-blown products of contemporary volcanism, sub­sequently altered during diagenesis (Walkden, 1977).

Where palaeosols are absent and the limestones of one cycle rest directly on those of another, individual cycles may be difficult to distinguish, because the defining palaeokarstic surfaces have commonly been modified or destroyed by pressure solution. In such cases, rhizoliths preserved beneath stylolitic contacts, may provide the only objective criteria for recognising a cycle top.

The limited range of lithologies in the formation and the absence of distinctive marker horizons renders cycle correlation necessarily subjective and this is based ­principally on counting-down and thickness criteria. A putative correlation of the cycles of the central crop is shown in (Figure 16). Across the northern parts of the crop, the marked lithological change which defines the base of the succeeding Cefn Mawr Limestone serves as a datum for cycle correlation. In Burley Hill [SJ 204 600] and Graig [SJ 205 565] quarries in the southern part of the crop, the equivalent cycle boundary is readily distinguished (Plate 15). However, at both localities, cycles displaying the lithological features of the Cefn Mawr Limestone, underlie this horizon (Figure 16). Moreover a greater number of Cefn Mawr Limestone cycles are present below the datum at Graig quarry than to the north, ­suggesting that the diachronous Loggerheads Limestone/Cefn Mawr Limestone contact descends southwards to underlie successively older cycles.

Cycles in the lower part of the formation tend to be thicker than those in the upper part. South of the Nercwys-Nant-figillt Fault Zone, those in the upper part thicken and split southwards as they pass laterally into the Cefn Mawr Limestone. Cycles to the north of the Nercwys-Nant-figillt Fault Zone in Pant [SJ 200 703] and Pant-y-pwll-dwr [SJ 190 718] quarries are thicker than their assumed correlatives in the nearest sections to the south (Hendre [SJ 193 680] and Trimm Rock [SJ 192 660] quarries). Comparisons with sequences of Loggerheads Limestone on the south side of the Bala Lineament at Minera, suggest a pronounced thinning and amalgamation of cycles (Figure 16).

The Loggerheads Limestone displays a sparse, but diverse coral assemblage. Siphonodendron junceum is the most common colonial form, locally forming monospecific bands up to 0.2 m thick. Other common species include S. intermedium, S. martini and S. pauciradiale and the solitary rugosans Dibunophyllum bourtonense and Palaeosmilia murchisoni. These forms are consistent with the late Asbian D1 Subzone, as are brachiopod assemblages which, in addition to Linoprotonia hemisphaerica and other long-ranging productids, include the late Asbian taxon Davidsonina septosa (George et al., 1976; but see Chisholm et al., 1983). The occurrence of D. septosa in the uppermost parts of the formation together with the colonial corals Diphyphyllum furcatum and D. lateseptatum, species traditionally indicative of the D2 Subzone (Brigantian Stage) (Nudds, 1981), has cast doubt on the biostratigraphical value of these corals as Brigantian markers (Somerville and Strank, 1984c).

The presence together of the foraminifera Euxinita sp. and Pojarkovella sp. in assemblages from the lower part of the formation [as at [SJ 1986 5698] suggests a horizon around the Cf6α-β/Cf6γ subzonal boundary. The appearance, at higher levels, of the foraminifera Bibradya sp., Cribrospira panderi, Koskinobiogenerina sp., Howchinia sp. and species of Neoarchaediscus, in assemblages rich in the algae Koninckopora inflata, but devoid of Pojarkovella, confirm the Cf6γ Subzone (Table 8). Such assemblages persist into the uppermost part of the formation. Broadly, they indicate a late Asbian age, though com­parable assemblages are known to range into the early Brigantian (Somerville and Strank, 1984c).

Conditions of deposition

The cyclic sequences of the Loggerheads Limestone compare with the shoaling-upwards rhythms of the Leete Limestone and are interpreted, similarly, as the product of consecutive marine inundations. However, the karstic surfaces, bentonitic soils and well developed calcrete profiles demonstrate that the emergent intervals were not, as in the Leete Limestone, the product of simple progradation, but record long periods of vadose diagenesis, dissolution and volcanogenic soil accumulation in response to marine regressions (Walkden, 1974, 1977; Somerville, 1979a; Davies, 1991).

The diverse skeletal grain assemblages, derived principally from stenohaline taxa, indicate that the packstones record open marine deposition. They compare with the foraminiferal/molluscan muddy sands of modern carbonate shelves (Ginsburg and James, 1974) that characterise settings below fair-weather wave base, where there is incompete winnowing of mud-grade carbonate. The rubbly and mottled varieties of packstone are thought to have been deposited during the acme of transgressive events (Somerville, 1979a). The preservation of discrete burrow-casts and burrow-related mottles suggests that both facies record accumulation in settings where there was only limited bioturbation. Solomon (1989) suggested that the distinctive brown cements present may have formed when fluctuating marsh/freshwater mixing zones developed during periods of emergence. Mixing zone diagenesis was evidently not an important influence on the massive and pseudobrecciated types of packstone present in the upper levels of the cycles. In these, absence either of recognisable burrow-forms or of tractional structures suggests homogenisation of the sediment by bioturbation. The locally cross-bedded grainstones which cap some of the cycles record accumulation under conditions of semi-continuous agitation, probably above fair-weather wave base, in shoal settings where the dominance of peloids and dasycladacean grains indicates that elevated salinities prevailed. The absence of these shallow-water grainstones from many of the cycles may reflect their subsequent removal by dissolution during periods of emergence.

The Loggerheads Limestone compares with similar late Dinantian cyclic shelf successions widely developed throughout Britain, Europe and North America. Their geographical and stratigraphical range suggests that the cyclicity had a common and widespread cause. The differing types and distribution of cycles displayed by British Dinantian successions have been reviewed by Walkden (1987) and computer-modelled by Walkden and Walkden (1990). These authors conclude that small-scale, glacio-eustatic, sea-level oscillations were the principal cause of late Dinantian cyclicity but that the incremental accumulation of the cyclic sequences was accommodated by regional subsidence. Whether Milankovich-type orbital factors also played a part is difficult to prove. Differing amounts and rates of sea-level movement, subsidence and sediment accumulation account for differing cycle motifs. Dominated by subtidal packstones, the cycles of the Loggerheads Limestone are typical of other late Asbian successions and reflect relatively small, but relatively frequent sea-level fluctuations in which rapid transgressions and regressions precluded the formation or preservation of peritidal facies (Walkden and Walkden, 1990).

In such cyclic successions, the lateral splitting and thickening of cycles are sensitive indicators of local tectonic influences. In the Loggerheads Limestone such trends demonstrate that higher subsidence rates were in operation to the north of the Bala Lineament and north-east of the contiguous Nercwys-Nant-figillt Fault Zone and point to syndepositional movements on these fracture belts (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15).

Details

Central crop

The lithological logs of the quarries which provide the principal sections in the formation are presented in (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15). Microfossil assemblages present in the lower part of the formation, in the Eryrys area, are given in (Table 9) .

The junction with the Leete Limestone is exposed in crags [SJ 1978 5876] north-west of Eryrys (Figure 14), in cliffs [SJ 1985 6273] at the base of the type section ((Figure 16), column 6) and in a section [SJ 1888 6469] to the north. The upper 51 m of the Loggerheads Limestone were proved in the Gwernymynydd Borehole [SJ 2111 6221] (Figure 17). Much of the northern part of the central crop is drift-covered but exposures are provided by crags [such as 1680 7072 and 1688 7100] at Lixwm, a disused quarry [SJ 1550 7308] along the upper reaches of Pantgwyn ((Table 5) , locality 8) and a quarry [SJ 1645 7293] south of Prysau; at the last locality an undulating palaeokarstic surface overlies packstone/grainstones with rhizoliths. Grainstones exposed in a disused quarry [SJ 1239 7510] at the northern edge of the district yielded a diverse assemblage of foraminifera including Archaediscus sp., Bibradya inflata, Bogushella ziganensis, Endothyra acantha, E. ex gr. phrissa, E. ex gr. spira, Endothyranopsis crassa, Endostaffella sp., Eostaffella sp., cf. Gigasbia gigas, Globoendothyra sp., Mediocris sp., cf. Mikhailovella sp., Neoarchaediscus sp., Palaeo­textularia sp.,Paraarchaediscus sp. stage angulatus, Plano­archaediscus concinnus, Plectogyranopsis cf. ampla, Priscella prisca, Septabrunsiina sp. and Valvulinella sp., in addition to the dasycladacean alga Konickopora sp. (bilaminar) and indeterminate calcisphaeres, kamaeniids and stacheinids. The assemblage indicates the Cf6γ Subzone.

Vale of Clwyd

In the central part of the Vale, south-west of Ruthin, the formation gives rise to well featured slopes. Pink-stained, fine- to coarse-grained, massive, rubbly, locally crinoidal packstones (as seen in crags [SJ 1210 5697] west of the A494 road) are typical of the numerous, low scarp sections. They consistently yield Cf6γ Subzone microfossils. A track cutting [SJ 1125 5720] exposes a monospecific coral bafflestone bed composed of intermeshed colonies of Siphonodendron junceum.

Details of microfossil assemblages from outcrops of the formation present between splays of the Vale of Clwyd Fault are given in (Table 8) (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15).

Cefn Mawr Limestone Formation

The Cefn Mawr Limestone Formation comprises wackestones, packstones and grainstones arranged in cyclic sequences, the boundaries of which are defined by ­correlatable penecontemporaneous pedogenic and karstic features. The lower parts, and commonly much of the cycles comprise dark grey-brown, thin-bedded, highly fossiliferous, argillaceous wackestone–packstone with interbedded mudstone (Plate 9a). Distinctive cross-bedded crinoidal and brachiopod rudstones are commonly interbedded with these lithologies (Plate 10a)(Plate 10b). Thick-bedded packstones and grainstones similar to those in the Loggerheads Limestone are typical of the upper part of the cycles.

The formation was first described by Somerville (1977, 1979c) from the type section at Cefn Mawr Quarry [SJ 200 634] ((Figure 18), column 5). It is present in the central crop, the Hope Mountain area and between splays of the Vale of Clwyd Fault. The presence of inliers of Cefn Mawr Limestone on the north-east side of the Nant-figillt Fault, around Rhosesmor [SJ 213 685], has been determined mainly on data from underground mineral workings and surface trials reported by Strahan (1890, p. 29). The formation compares closely with the Traeth Bychan Limestone of Anglesey (Davies, 1983; British Geological Survey, 1989), the Bishop’s Quarry and Summit limestones of the Great Orme (Warren et al., 1984) and the Trefor Limestone of the Llangollen and Oswestry areas (Somerville, 1977, 1979a; Grey, 1981).

Units of the thin-bedded facies within the formation include the ‘Aberdo Limestone’ and ‘Black Limestone’ of early workers (Morton, 1886; Strahan, 1890). In the north of the district, the latter name was given to an extensively quarried limestone used in making cement and, at that time, thought to occupy a discrete stratigraphical horizon. However, the term ‘Black Limestone’ was also applied by Strahan (1885) to rocks of rather different aspect in the Prestatyn area which now consititute the Teilia Formation of Warren et al. (1984; see p.24). These misapplications caused stratigraphical confusion to later workers (for example Hind and Stobbs, 1906; Sargent, 1927; Jones and Lloyd, 1942).

The formation varies in thickness from 40 to 275 m. The base of the formation is taken at the first appearance of dark, thin-bedded wackestones/packstones and mudstones above the pale packstone sequences of the Loggerheads Limestone (Figure 16), (Figure 17), (Figure 18). Across the northern part of the central crop the contact coincides with a single cycle boundary. However, in the south of the central crop, there is clear evidence that the base of the formation is markedly diachronous, becoming older southwards (Figure 16), (Figure 18). However, the pronounced southward thickening of the formation in this tract, from 100 m in the vicinity of Hendre Quarry [SJ 193 680] to at least 275 m around Graig Quarry [SJ 205 565], is due both to this lateral passage and the expansion of individual cycles.

Throughout the southern part of the district, the Cefn Mawr Limestone is overlain by the Minera Formation. However, to the north-east of the Nercwys–Nant-figillt Fault Zone the latter is absent and the Pentre Chert Formation rests disconformably above (see p.63). Thicker sequences of Cefn Mawr Limestone as in the vicinity of Pant Quarry [SJ 200 703] (130 m) and, farther north, at Brynford [SJ 180 745] (140 m) reflect, at least in part, a lateral passage from the Minera Formation (Figure 18), although individual cycles also show a degree of incremental expansion. In contrast, between these two latter localities to the west of Pentre Halkyn [SJ 190 725], as little as 40 m of Cefn Mawr Limestone is preserved beneath the sub-Pentre Chert Formation disconformity. The formation was found to be 138 m thick (uncorrected for dip) in the Gwernymynydd Borehole [SJ 2111 6221] (Figure 17).

The thin-bedded wackestone–packstones exhibit nodular, irregular and tabular pseudo-bedding styles which are the result of pressure solution (Wanless, 1979) and cannot readily be related to primary depositional events (but see Bathurst, 1987). They vary from very fine- to coarse-grained and are locally coarsely crinoidal. The diverse skeletal grain assemblage includes crinoids, brachiopods, calcareous algae (mainly the dasycladacean Coelosporella), fenestellid bryozoa, corals, molluscs, trilobites and foraminifera, including the large benthic form Saccamminopsis. Locally abundant, intact spiriferid and spinose productid and gigantoproductid brachiopods as well as solitary and colonial corals are present. Much of this benthic macrofauna appears to be preserved in life-position (Plate 9a). Locally, the upper bedding surfaces of the limestones display the trace fossil Zoophycus (Plate 9b). Black, replacive chert nodules are common (Plate 9a). The dark grey mudstone interbeds are mainly less than 0.05 m thick. Although most represent pressure solution seams (Wanless, 1979), local units, up to 2 m thick, are primary beds and of value in cycle correlation. The fossils in the mudstones are crushed, but com­parable to these in the limestones. These sequences also include richly fossiliferous coral biostromes (bafflestones), also useful as local marker beds.

Distinctive units of large-scale, low-angle and foreset, cross-bedded, crinoidal and brachiopod rudstone occur commonly in this thin-bedded facies (Plate 10a)(Plate 10b). Sections in Pant [SJ 200 703], Aberduna [SJ 204 615] and Graig [SJ 205 565] quarries show that they commonly form low-relief, wedge-shaped bodies which display flat, typically erosional bases and sharp, asymmetric, gently curving, convex-upward tops (Plate 10a). These bodies are up to 6 m high and display minimum widths of less than 200 m. The cross-bedding is typically tangential to the base of the body, but discordant to the tops and, in places, displays internal, cross-cutting, reactivation surfaces (Plate 10b). Cross-bedding orientations from several sections consistently show an eastward trend. Such units rarely occur at the top of the cycles.

Locally, sequences of this thin-bedded facies exhibit the effects of internal sliding where packets of inclined strata are bounded by curved trunction surfaces. Such features are well seen at Waen-brodlas Quarry [SJ 1875 7310] (Plate 11a), where they were previously mistaken for cross-bedding (Oldershaw, 1969), and at Graig Quarry [SJ 205 565]. A series of synsedimentary faults previously exposed at Pant Quarry [SJ 200 703] (Plate 11b) offer further evidence of instability.

The upper parts of thicker cycles comprise pale grey, mottled, massive and pseudobrecciated skeletal packstones and peloidal, coated-grain and dasycladacean grainstones, in units up to 7 m thick. Some thinner cycles are constructed entirely of such lithologies and, therefore, compare with cycles in the Loggerheads Limestone. The calcretes, karstic surfaces and palaeosols, commonly developed at the tops of these packstone and grainstone units, also compare with those described from the Loggerheads Limestone. In the south of the district (for example Aberduna [SJ 204 615], Graig [SJ 205 565] and Penrhiw [SJ 284 560] quarries), beds of calcite mudstone and ostracod, calcisphaere wackestone, locally up to a metre thick, cap several of the cycles. These lithologies display abundant rhizoliths and jigsaw brecciation, but evidence for karstification is commonly lacking and mudstone palaeosols are thin or absent.

Individual cycles in this formation range from 0.5 to 75 m in thickness. On average, they are over twice as thick as those in the Loggerheads Limestone and display a greater diversity of lithologies and motifs together with distinctive marker beds. The correlation of cycles in the central crop can be made with greater confidence than in the Loggerheads Limestone (Figure 18). The argillaceous, foetid wackestone-packstones of the lower parts of the cycles, form units up to 49 m thick. In general, cycle thicknesses are proportional to the amounts of this thin-bedded facies that are present. Cycles typically thicken southwards. This is also true for those cycles in the lower part of the formation at Graig [SJ 205 565] and Burley Hill [SJ 204 600] quarries, which equate with the upper parts of the Loggerheads Limestone farther north. Southwards from the transition zone with the Loggerheads Limestone, dark, thin-bedded and foetid wackestones, crinoidal floatstones, cross-bedded grainstones and coral biostromes appear within the cycles. Despite this lateral transition, the cycle boundary which marks the base of the formation in the north, is still readily distinguished ((Figure 18), columns 9 and 10; Plate 15) where it defines the base of the thickest cycle (up to 75 m) in the formation. This cycle contains the thickest unit (49 m) of the thin-bedded facies. It appears to be laterally equivalent to several thinner cycles farther north and thus their capping packstones and grainstones, and pedogenic and karstic features appear to fail southwards. The expansion of this (early Brigantian) part of the sequence accounts for much of the southward thickening displayed by the formation.

In the succeeding cycle, a mudstone unit with thin nodular limestones, the ‘Thick Shale’ of Campbell and Hains (1988), is the lowest of the widespread marker beds recognised in the formation. It ranges from 2 to 6 m in thickness, and is conspicuous in the main quarries of the central crop (Somerville, 1979c, p.76) (Figure 18). Important marker beds in the upper part of the formation are the ‘Main Shale’ and the ‘Coral Bed’, both of which occur in the same cycle. The former, another dominantly mudstone unit with nodular limestone horizons, is up to 7 m thick and was widely recognised in mines in the south of the central crop (Earp, 1958). At the surface, it gives rise to a prominent slack and serves to limit the workings of several of the main quarries. In general, the ‘Coral Bed’ (Strahan, 1890; Somerville, 1979c, p.78) lies between 15 and 20 m above the top of the ‘Main Shale’ and is well seen at Hendre [SJ 193 680] and Graig [SJ 205 565] quarries; at the latter it is up to 6 m thick. In the central crop it is about 8 m below the top of the cycle in which it occurs and up to 34 m below the top of the formation. The bed contains, commonly silicified, branching and massive coral colonies, dominantly of species of Corwenia, Diphyphyllum, Lithostrotion, Lonsdaleia, Orionstraea and Siphonodendron that are in many cases over a metre in diameter.

The cycles identified in the Hope Mountain area appear comparable in both form and thickness and are readily correlated with those of Graig Quarry ((Figure 18), columns 10 and 11). However, cycle-capping packstones and grainstones are commonly sandy; calcretes and karstic surfaces are less well developed; fissile mudstones and siltstones with abundant plant debris replace blocky palaeosols. Both the Main Shale and the Coral Bed are recognised. A floatstone lens, rich in abraded goniatite conchs, occurs in a section [SJ 2861 5595] at the top of the cycle containing these markers (Plate 9c). At least three complete cycles and the lower part of a fourth are present between this level and the base of the succeeding Minera Formation.

The correlation of cycles recognised to the north and south of the Bala Lineament ((Figure 18), columns 11 and 12) uses data from quarries at Minera (Grey, 1981), just south of the district. Here, those cycles which pass southwards from the Loggerheads Limestone into the Cefn Mawr Limestone in the south of the central crop, have changed thickness and facies once again and are re-included in the Loggerheads Limestone. Moreover, the lower cycles of the Cefn Mawr Limestone at Minera are at least seven times thinner than their correlatives north of the fault (for example at Graig [SJ 205 565] and Penrhiw [SJ 284 560] quarries).

The cycle boundary which marks the base of the formation in the northern part of the central crop (Figure 18) was taken by Somerville and Strank (1984c) to represent the local Asbian/Brigantian boundary. The rich coral faunas occurring above this level (Somerville, 1979c) are typical of the D2 Subzone of the Dibuno­phyl­lum Biozone and also of the Brigantian Stage. Diagnosic elements include the colonial forms Actinocyathus floriformis, Lonsdaleia duplicata, Palaeosmilia regia, Corwenia rugosa and Orionastraea phillipsi and subspecies of the solitary taxon Dibunophyllum bipartitum. Other common colonial corals include Siphonodendron junceum, S. martini, S. pauciradiale and species of Diphyphyllum (see below), and the solitary forms Caninia juddi and C. cf. cornucopiae. Diverse brachiopod assemblages include Eomarginifera setosa, Rugosochonetes hardrensis, Semiplanus latissimus, Rhipidomella michelini, spirifers, rhynchonellids and abundant gigantoproductids. Somerville and Strank (1984c, p.233) acknowledged that diagnostic Brigantian foraminifera of the Cf6δ Subzone are absent from basal parts of the formation, at Cefn Mawr Quarry [SJ 200 635]; definitive Cf6δ Subzone taxa such as Asterarchaediscus spp. enter some 8 m above the base and the large ovoid and spar-filled tests of Saccamminopsis, up to 2 mm across, appear at a higher level in the same cycle. Coelosporella replaces Koninckopora as the dominant dasycladacean alga in these Brigantian rocks.

The position of the local base of the Brigantian proposed by Somerville and Strank (1984c) rests on the assumption that the onset of widespread Cefn Mawr Limestone deposition in the district equates with similar facies changes in many Dinantian platform sequences, including the stage stratotype (Walkden, 1987) and that these changes were all the result of a single widespread transgressive event (Ramsbottom, 1973, 1977). By implication, the cycles in the south of the central crop which predate this event, but are included in the Cefn Mawr Limestone (Figure 18), are of late Asbian age. They contain abundant colonial corals, including S. junceum, and the traditional Brigantian taxa Diphyphyllum furcatum and D. lateseptatum. The distribution and abundance of the last two species in strata that is by current definition pre-Brigantian (Somerville and Strank, 1984c) suggests that they are subject to facies control.

Ammonoids from the floatstone lens in a crag on Hope Mountain [SJ 2861 5595], 31 m below the top of the formation (see above), include the forms Arnsbergites sphaeri­costriatus, Hibernicoceras carraunense and Paraglyphio­ceras bisati, indicative of the mid-Brigantian P1c Biozone. In the north of the central crop, in Bryn Mawr Quarry [SJ 1880 7340], a single specimen of Sudeticeras sp. ex gr. splendens/stolbergi, strongly suggestive of the P2b Biozone, was recovered less than 10 m below the top of the formation, here defined by the sub-Pentre Chert disconformity. This dated horizon, lying some 90 to 100 m above the base of the formation, appears equivalent to a level within the highest Cefn Mawr Limestone cycle recognised further south (Figure 18). Upper parts of the thicker formational sequence preserved to the north, around Brynford, where the effects of the disconformity are reduced, logically include still younger strata probably equivalent to the Minera Formation farther south (p.56) and which possibly range into the uppermost Brigantian P2c Biozone.

Conditions of deposition

The cycles of the Cefn Mawr Limestone, as with those in the preceding formation, represent shoaling-upwards sequences formed in reponse to repeated transgressive and regressive movements in sea level. Each regressive interval culminated in the formation of widespread calcrete and karstic features, locally preserved beneath a volcanogenic palaeosol. The thicker cycles reflect rises in sea level, which were less frequent, but, on average, over twice as large as those suggested for the Loggerheads Limestone (Davies, 1984; Walkden, 1987; Walkden and Walkden, 1990). These more extensive transgressions are reflected in the thick sequences of thin-bedded facies that characterise the lower portions of many of the cycles. Evidence that this facies formed under deeper platform conditions than had prevailed previously includes:

However, in contrast, the associated cross-bedded grainstone and rudstone bodies indicate effective winnowing and the tractional transport of coarse skeletal debris under high-energy conditions. The geometry and internal structure of these bodies compares with modern offshore sand waves or bars, which migrate under the influence of tidal and storm-generated currents (Johnson and Baldwin, 1986). Although these structures may have displayed a relief of several metres, it seems unlikely that relief alone accounted for the marked contrast in hydraulic regime with that of the enveloping thin-bedded facies. The difference in sediment mobility may be due to the distribution of binding and baffling organisms on the sea floor. The wackestone textures of the thin-bedded facies may record the efficient entrapment of fines on a sea bed extensively colonised by ­vegetation and crinoids. In contrast, the grainstone and rudstone bodies may have formed on non-vegetated areas of the sea bed in the wake of severe storm erosion or migrated into vegetated regions of the platform from higher energy, nearshore or platform margin settings. Once established they would have been sustained by currents and their shifting substrates would have precluded effective colonisation by indigenous baffling or bioturbating organisms. Abandonment and burial of these bodies followed successful colonisation and perhaps followed periods of infrequent storm activity or renewed deepening.

The packstone and grainstone facies which typify the upper parts of the cycles reflect deposition in shallow-water settings as suggested for comparable lithologies in the Loggerheads Limestone.

Southward changes displayed by the Cefn Mawr Limestone (Figure 18), including the thickening of individual cycles, the splitting of cycles, the failure of pedogenic and karstic horizons and the preservation of peritidal facies beneath cycle boundaries, are all consistent with enhanced subsidence adjacent to the northern side of the Bala Lineament (see Chapter 8). High rates of subsidence in this tract are also reflected in the lateral passage from Loggerheads Limestone to Cefn Mawr Limestone in the south of central crop. Evidently in this area, late Asbian transgressions created the deeper water conditions under which the argillaceous facies diagnostic of the latter formation accumulated. Furthermore, the relative thicknesses of the cycles indicate that subsidence adjacent to the fault was greatest in latest Asbian and early Brigantian times. The thickening of cycles and absence of well formed karstic features in sequences east of the Nercwys–Nant-figillt Fault Zone, including those on Hope Mountain, suggest this fracture was also active.

The onset of widespread Cefn Mawr Limestone deposition in the district correlates with comparable changes in cycle style at around the Asbian/Brigantian boundary on other Dinantian platforms (Somerville and Strank, 1984c; Walkden, 1987), and suggests that regional effects were also operating. The strongly asymmetric form of these Brigantian cycles, dominated by thick sequences of deeper water platform facies, indicates transgressive inundations which were not just larger than the Asbian ones, but were more rapid and protracted (Walkden, 1987). Possibly the eustatic sea-level rises were now enhanced by regional downwarping (Ramsbottom, 1981; Walkden, 1987; Walkden and Walkden, 1990) heralding the onset of thermal subsidence, which later dominated Silesian basin development (Leeder, 1982).

Details

Central crop

Logs of the principal quarries in the district are shown in (Figure 18). Details of the fossils in the basal 20 m of the formation at Cefn Mawr Quarry [SJ 200 634] are given by Somerville and Strank (1984c, fig. 2), and of the coral and brachi­opod taxa in Hendre Quarry [SJ 193 681] by Somerville et al. (1986b). Additional exposures in the Coral Bed are seen at the following sections [SJ 2051 6001] to [SJ 2065 6169] to [SJ 2041 6284] to [SJ 2009 6523] to [SJ 2068 5832] to [SJ 2049 5985] to [SJ 2054 5894]. The Coral Bed and Main Shale were both recognised in the Gwernymynydd Borehole [SJ 2111 6221] which provided a rich coral and brachiopod fauna (Figure 17).

Waen-brodlas Quarry [SJ 1875 7313] exposes the upper part of the formation in the northern part of the central crop. The dark, tabular-bedded, fine-grained wackestones with mudstone partings, which form the lower part of the section, constitute the ‘Black Limestone’ of the previous survey (Strahan, 1890). They are overlain by coarser-grained wackestones and ­packstones with abundant, commonly nested, gigantoproductid brachi­opods. Units of inclined bedding bounded by curving truncation surfaces are spectacularly displayed (Plate 11a). Previous interpretions ascribing these features to large-scale, tractional bed forms (Oldershaw, 1969) are at odds with the textural aspects of the facies. They may instead represent mass movement phenomena, involving several generations of internally bedded slides, each transported on spoon-shapped glide surfaces (compare Clari and Ghibano, 1979; see also p.70). Movement was probably towards the putative platform margin to the north-east ((Figure 5)f). The nearby Bryn Mawr [SJ 1880 7340] and Pen yr Henblas [SJ 1900 7300] quarries both expose sections through the uppermost part of the local Cefn Mawr Limestone sequence and its contact with the Pentre Cherts Formation. The irregular nature of the contact owes its form, at least in part, to karstic relief and cavity infilling, but elsewhere may also represent a plane of slippage.

Hope Mountain area

Logs of the quarries at Pen rhiw [SJ 2843 5596] to [SJ 2845 5612], north of Ffrith, are included in (Figure 18). A quarry [SJ 2850 5560] to the south exposes the Main Shale, downfaulted in its eastern face.

Vale of Clwyd

Details of the microfossils recovered from outcrops of the formation present between splays of the Vale of Clwyd Fault are given in (Table 8) (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15).

Minera Formation

This formation comprises wackestones, packstones and grainstones arranged in cyclic sequences, closely comparable with those of the Cefn Mawr Limestone, but is distinguished from the latter by thick, calcareous sandstones commonly present at the tops of the cycles. The term Minera Formation was first used by W W M Brown (1960), and has been used here and in the Llangollen and Oswestry districts, for strata previously termed the Sandy Passage Beds or the Sandy or Arenaceous Limestone (Morton, 1876–9; Strahan, 1890; Wedd et al., 1927; Somerville, 1979a; Grey, 1981; British Geological Survey, 1994). The base of the formation is taken at the base of the lowest mapped sandstone and appears to lie at the same level across the district (Figure 19). Resulting from this definition, the base of the formation lies within a cycle, the lower components of which are included in the underlying Cefn Mawr Limestone.

The formation is absent west of the Vale of Clwyd due to intra-Westphalian and Permian erosion. However, it is present along the eastern side of the Vale within narrow fault slices associated with the Vale of Clwyd Fault. In its type area, south of the district and south of the Bala Lineament, the Minera Formation is around 60 m thick (Hains, 1991). In the south of the district, and here north of the Bala Lineament, the formation is much thicker, ranging from about 140 m in the southern part of the central crop to up to 180 m on Hope Mountain (Figure 19). In these areas, the junction with the overlying Cefn-y-fedw Sandstone is conformable, taken at the top of the uppermost, laterally persistent and mappable limestone unit in the Minera Formation (but see p.76). However, cycle correlations suggest that the uppermost limestone on Hope Mountain equates with strata included in the Cefn-y-fedw Sandstone in the south of the central crop (Figure 19). In the northern part of the central crop, north of Gwernymynydd, the formation thins markedly and is overlain by Namurian rocks, probably disconformably (see below). About 60 m are preserved to the west of the Nercwys Fault at Hendre [SJ 199 676] and east of Babell [SJ 165 735]; on the eastern side of the fault, at Moel-y-crio [SJ 202 694], less than 30 m are present. The formation is absent on Moel y Gaer [SJ 212 690] and from north-eastern parts of the central crop.

The lower parts of thicker cycles in the formation commonly comprise dark grey, thinly interbedded, foetid, argillaceous wackestones and mudstones. This thin-bedded facies commonly contains abundant chert in the form of nodules and, at some levels, as distinctive irregular and digitate sheets. Corals and brachiopods are commonly replaced by, and their body cavities filled by silica, locally in the form of agate or beekite. Silicified coral biostromes are present locally and form marker beds. The mudstones are also extensively silicified and may show a brittle and platy fabric, intercalated with sheets of impure chert. Lenticular bodies of cross-bedded, crinoidal rudstone, similar to those in the Cefn Mawr Limestone, are also present in lower parts of some cycles.

Pale grey, medium to thick-bedded, burrow-mottled, pseudobrecciated, massive, skeletal, commonly crinoidal packstones and peloidal, dasycladacean grainstones form the middle parts of many cycles. However, some thinner cycles are largely constructed of these lithologies which are commonly sandy and grade both laterally and vertically into the calcareous sandstone units. Sandy ooidal grainstones, similar to those seen at Trefor Rocks, in the adjacent Wrexham district (Wedd et al., 1927), are also locally developed. They comprise well sorted, radial fibrous ooids, each cored with a quartz sand grain.

Although the sandstones in the formation occur mainly at the tops of cycles, some are developed at the base, and others comprise whole cycles. Individual sandstones range up to 15 m in thickness: the thicker ones give rise to marked features. They are white, brown, red, purple or variegated, and composed dominantly of calcite-cemented, fine to coarse, quartz sand grains and variable proportions of skeletal and peloidal carbonate particles. Quartz and lithic granules and pebbles occur scattered throughout some sandstones or concentrated in thin, lenticular conglomerates overlying undulating erosion surfaces. In weathered exposures, where the carbonate has been leached, the sandstones are typically soft and friable. The common, ribbed or honeycombed, weathering effects reflect an original segregation of carbonate and siliciclastic grains by tractional or burrowing processes respectively. Trough or tabular cross-bedded sets, directed typically either towards the east or west, are common and range up to 4 m in thickness. Parallel and cross-lamination and hummocky and low-angle cross-bedding are also widely developed. Other sandstones are massive or burrow-mottled.

As the sandstones typically form the upper part of the Minera Formation cycles, karst and calcrete features dependent on a carbonate host are poorly developed. The recognition of cycle boundaries is therefore mainly based on motif and correlations are more subjective and, hence, less reliable than in the preceeding Cefn Mawr Limestone. A putative correlation of cycles between Hope Mountain and the central crop is shown in (Figure 18) and appears to demonstrate the lateral persistence of some of the main sandstones in the formation. It also shows that the cycles, in common with those in the Cefn Mawr Limestone, thin northwards away from the Bala Lineament and appear to reflect the continued influence of this structural zone on sedimentation. However, the rate of northward thinning, over twice that recorded in the underlying division, suggests that internal attenuation may not solely account for the marked thinning apparent to the north of Gwernymynydd (Figure 19). Though Strahan (1890) argued that this thinning reflected the northward failure of the lower sandstones, cycle correlations in the underlying Cefn Mawr Limestone mitigate against this option in this area (Figure 18). However, the possibility that an erosional disconformity associated with the base of the succeeding Cefn-y-fedw Sandstone contributes to this attenuation cannot be discounted (see p.76). Certainly farther north, the evidence for a disconformity at the base of the Pentre Cherts Formation appears unequivocal (see p.63) and may explain, in part, the absence of the Minera Formation from the north-east of the central crop. However, at the northern edge of the district in the central crop, around Brynford, the upper parts of an unusually thick sequence of Cefn Mawr Limestone are likely to postdate the level with a probable P2b Biozone goniatite in Bryn Mawr Quarry (p.54). These strata possibly include cycles equivalent to those in the lower parts of the Minera Formation farther south, but here lacking in the diagnostic sandstone components and hence included in the Cefn Mawr Limestone.

The Minera Formation is considered to be entirely late Brigantian in age, yielding colonial rugose corals, including Lonsdaleia duplicata, of the D2 Biozone and foraminifera, including Asterarchaediscus, of the Cf6δ Subzone. It postdates the Plc ammonoid assemblage recovered from the upper part of the Cefn Mawr Limestone on Hope Mountain. Thickness considerations and cycle correlations in the north-east of the central crop, where the Cefn Mawr Limestone ranges at least into into the P2b Biozone, suggest that much of the expanded southern sequence of the Minera Formation lies within the P2b to P2c biozones. However, the possibility that uppermost parts of the formation in this area may range into the Namurian cannot be ­discounted.

Conditions of deposition

The cyclic sequences of the formation represent shoaling-upwards sequences initiated by marine transgressions and terminated by episodes of falling sea level. The thin-bedded argillaceous limestone facies and associated crinoidal rudstones are similar to the facies in the Cefn Mawr Limestone. They were deposited on a possibly vegetated sea floor, close to storm-wave base and traversed by migrating crinoidal bars. The burrow-mottled, pseudobrecciated and massive packstones, as in both the Cefn Mawr and Loggerheads limestones, reflect deposition in shallower settings around fair-weather wave base. The sandstones of the Minera Formation represent the deposits of shallowest waters, taking the place of the cycle-capping packstones and grainstones present in the preceding formations. Well rounded and sorted quartz grains, abundant skeletal grains, local ooidal coatings and the preservation of both large- and small-scale tractional sedimentary structures all indicate marine deposition under conditions of frequent agitation and reworking above fair-weather wave base. The suite of sedimentary structures including bimodal cross-bedding directions are consistent with modern, upper shoreface and beach deposits (Clifton et al., 1971; Clifton, 1973; Davidson-Arnot and Greenwood, 1976). In these, large-scale trough and tabular cross-stratification is produced by both seaward and landward migrating, curved- and straight-crested megaripples present on the crests of near-shore bars and in high subtidal settings. Migrating beach ridges generate low-angle cross-bedding, whereas parallel lamination is formed in surf and swash zones. Hummocky cross-stratification is attributed to storm wave action (Harms et al., 1975). Scour surfaces overlain by pebble and granule lags are typical of such upper shoreface settings (Clifton, 1973). Thus these sandstones record the entrainment of sand and coarser grade siliciclastic detritus within upper shoreface bars and beaches, and the progradation of these facies across the deeper, more offshore, carbonate phases of the cycles. The sandstones present at the bases of cycles represent transgressive sheet sands.

The sandstones mark a resumption in the supply of terrigenous detritus, which had been largely excluded from this sector of the north Wales Dinantian platform since the deposition of the Basement Beds (though not from other sectors, see Walkden and Davies, 1983; Davies, 1991). As with increases in sand supply to other Dinantian platforms (Walkden, 1987) at this time, it has been suggested that the cause was the onset of more humid conditions and an attendant increase in fluvial supply to coastal regions. These effects may reflect late Brigantian climatic changes, consequent on the glaciation of Gondwana (Veevers and Powell, 1987). Tectonic rejuvenation of the landmass to the south has been suggested as an alternative or additional factor and, in north Wales, may record uplift along the southern margin of the developing ‘sag’ basin in Northern England, (Leeder, 1982, p.484). The shelly and oolitic nature of the Minera Formation sandstones record deposition some distance from the point(s) of southerly sourced fluvial supply.

The overall distribution and thickness variations of the Minera Formation demonstrate the continuing influence of the Bala Lineament and of the Nercwys-Nant-figillt Fault Zone on sedimentation. Together these structures now established a depositional pattern for coarser grade siliciclastic sediment which was broadly followed by the southerly supplied fluvio-deltaic systems of the Namurian. Subsidence to the north of the Bala Lineament served to accommodate thickened sequences and focus terrigenous sand deposition. The Dinantian precursor of the Nercwys-Nant-figillt Fault Zone possibly provided a footwall barrier, and appears to have largely prevented the spread of distal sand to the north-east allowing dominantly carbonate facies to persist. Uplift, accom­panied by erosion along the crest of this footwall high during the late Dinantian to early Namurian may account for the marked attenuation of the formation to the north of Gwernymynydd.

Details

Central crop

Sandstones give rise to marked features on Maes-y-droell [SJ 214 566], to the north of Grainrhyd. Limestones in a section [SJ 2119 5645] above the lowest sandstone yielded Cf6δ Subzone microfossils. The basal parts of the formation are also exposed on Craig yr Wolf [SJ 2103 5795]. A quarry [SJ 2084 6023] exposes packstones with abundant silicified gigantoproductids and colonies of litho­strotionid corals and Syringopora, overlain by honeycombed calcareous sandstones and quartz-cored ooid grainstones with rhizoliths. Farther north, the prominent scarp of Moel Findeg [SJ 2097 6119] and the ridge beyond exposes much of the formation (Figure 19). The Gwernymynydd Borehole [SJ 2111 6221] com­menced in the lowest sandstone of the formation (Figure 17). The nearby, disused Cae Cymro Quarry [SJ 2112 6207] and Rainbow Quarries [SJ 2133 6229] to [SJ 2155 6219] worked horizons ­comparable with those encountered on Moel Findeg [SJ 207 612] (Figure 19). Quarries [SJ 2100 6290] to [SJ 2113 6318] to [SJ 2070 6480] between Gwernymynydd and Gwernaffield provide sections in the uppermost coarse-grained crinoidal packstones of the formation.

Crags [SJ 1962 6609] on the steep valley sides of the River Alyn at Nant Alyn expose the lower sandstones of the formation. At Rhydymwyn, a disused quarry [SJ 2015 6668] south of the river exposes cross-bedded sandstones with intercalated crinoidal rudstones near the top of the formation. There, cross-bedding is directed towards the east, but in a nearby quarry [SJ 2010 6680] north of the river, a comparable sequence includes westerly inclined cross-beds.

In Hendre Gorge, the access road [SJ 1992 6764] to Hendre Quarry exposes much of the attenuated Minera Formation (Figure 19). The section can be related to that of Strahan (1890, fig. 7), who described the now poorly exposed uppermost parts of the formation and its presumed disconformable contact with succeeding Namurian rocks (see p.77). Farther north, cross-bedded crinoidal grainstones and rudstones underlying the Cefn-y-Fedw Sandstone are exposed in crags [SJ 1920 6929] west of Bryn Gwiog. Quarries [SJ 1909 6988] west of Moel-y-crio provide further sections in these strata lying within the Nercwys–Nant-figillt Fault Zone and crags [SJ 1892 6996] to the west expose 5 m of the underlying cross-bedded and conglomeratic sandstone. Exposure in the only sandstone of the Minera Formation discovered to the east of the fault is seen in a small quarry [SJ 2024 6943]. The succeeding limestones are exposed in mineral workings [SJ 2010 6914], where 1.5 m of crinoidal rudstones are overlain by pebbly quartzitic sandstones of the Cefn-y-fedw Sandstone.

Crags [SJ 1645 7345] at Hafod-dew, to the east of Babell, expose a sequence of pebbly sandstones and sandy grainstones with micritised allochems and quartz-cored ooids, at the base of the formation. The limestones contain diverse microfossil assemblages which include the foraminifera Archaediscus karreri, Asterarchaediscus sp., Betpakodiscus sp., Earlandia trans. Gigasbia sp., Endostaffella sp., Endothyra ex gr. bowmani, E. ex gr. phrissa, Endothyranopsis ex gr. crassa, Eostaffella sp., Koskinotextularia sp., Neoarchaediscus incertis, Paraarchaediscus sp., cf. Planospirodiscus sp., Plectogyranopsis convexa, Priscella sp., Pseudoammodiscus sp., Pseudoendothyra sp., Pseudotaxis sp., cf. Rhodesina sp., Spinoendothyra sp. and Tetrataxis sp., and the algae Cabrieropora porkornyi, Draffania biloba, Girvanella sp., together with indeterminate calcisphaeres, kamaeniids and stacheiinids. The presence of abundant Asterarchaediscus sp. indicates the Cf6δ Subzone.

The upper 55 m of the formation were encountered, beneath Namurian strata, in a borehole [SJ 2636 6180] at Leeswood.

Hope Mountain area

The principal sections in this area are shown in (Figure 19).

Vale of Clwyd

Crags [SJ 1456 5626] of crinoidal grainstone lying to the west of Pentre Coch Manor occur in an inlier of Minera Formation and have yielded Cf6δ Subzone microfossils ((Table 8) , locality 17; (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15)).

Dinantian rocks beneath the Cheshire Plain

A sequence of Asbian and Brigantian strata, at least 402 m thick, was penetrated in the Blacon East Borehole beneath Silesian and Permo–Triassic rocks in the east of the district (see Evans et al., in press). Bilaminar Palaeotextularia sp. associated with the dasycladacean alga Koninckopora minuta in argillaceous limestones close to the base of the borehole sequence (2261.53 m depth) indicate the Asbian Cf6α-γ subzones. The proven Brigantian strata comprise interbedded limestone turbidites and hemipelagic mudstones (Riley, 1988). Ammonoids and bivalves from between 1868.73 and 1872.67 m depth confirm the presence of the mid Brigantian P1/P2 biozonal boundary. The bivalves Posidonia membranacea and P. trapezoedra, associated with the ammonoid Hibernioceras waldeckensis at 1872.67 m, are indicative of the P1d Subzone. Posidonia membranacea, together with the ammonoid Sudeticeras crenistriatus from 1870.53 m and 1868.73 m, indicate the lower part of the P2a Subzone. The geophysical logs of the borehole indicate that marked lithological boundaries occur at 1866.5 and 1920.15 m, above and below the dated horizons, suggesting that this Brigantian turbiditic division is up to 53.6 m thick. It appears comparable in facies and age to the lower part of the Teilia Formation of the Prestatyn area (Warren, 1984), suggesting that the underlying sequence of Asbian limestones in the borehole may equate, at least in part, with the late Asbian Prestatyn Limestone of that area. The position of the Dinantian/Namurian boundary in the borehole is poorly constrained, but probably lies within a thick, geophysically bland sequence of silty mudstones with thin limestones present above 1866.5 m depth (p.83). In the context of this district, it is convenient to view these strata as part of an expanded Holywell Shales sequence (see Chapter 4), but their stratigraphical relationships and age also invite comparison with the turbiditic Bowland Shales in the Craven Basin (Earp et al., 1961; Arthurton et al., 1988; Aitkenhead et al., 1992; Brandon et al., 1998).

Chapter 4 Silesian: introduction and Namurian

Introduction

Silesian strata, ranging from Namurian to Westphalian in age, crop out over most of the central part of the Flint district, and underlie the Permo–Triassic strata of the Cheshire Basin in the east. In the west of the district, strata of probable late Westphalian age crop out in small areas north-west of Ruthin and along the Vale of Clwyd Fault Zone. They have also been proved by boreholes to lie beneath the Permo­–Triassic rocks of the Vale of Clwyd.

The Silesian succession of the district, up to some 1.6 km thick, comprises the Millstone Grit Group, of largely Namurian age, and the Coal Measures and Red Measures groups of Westphalian age. They consist mainly of alternations of sandstone and mudstone, but include chert horizons in the lower part of the Millstone Grit Group, and subordinate coal seams and fireclays, particularly in the Coal Measures Group. Red beds and a paucity of coal seams characterise the Red Measures Group. These siliciclastic sequences record a history of cyclic sedimentation in a variety of alluvial plain, deltaic and marine environments. Horizons with ammonoids (marine bands) punctuate the Silesian sequence, except in the Red Measures Group (Ramsbottom, 1969; Holdsworth and Collinson, 1988).

Silesian cyclicity and correlation

The concept of cyclical sedimentation underpins the stratigraphical framework of the Carboniferous System, and minor cyclicity is a widely recognised feature of these rocks (for example Wright et al., 1927; Bott and Johnson, 1967; Ramsbottom, 1977). Silesian cyclicity in central Britain was controlled by repeated, widespread marine transgressions which are generally considered to be glacioeustatic in origin (Martinsen et al., 1995), a theory increasingly supported by evidence of glaciation in Gondwana during the Carboniferous (Heckel, 1986; Veevers and Powell, 1987). In an ideal cycle a basal, transgressive marine band is overlain by a progradational deltaic sequence, capped by a seatearth and coal. Where fully developed, the lower part of a marine band displays a sequence of macrofaunas, which is thought to reflect increasing salinity during the passage of the transgression, when basins that had formerly accommodated significant freshwater discharges became connected to a wider ocean (Holdsworth and Collinson, 1988; Martinsen et al., 1995). This faunal sequence comprises in upward succession: fish—Planolites–Lingula—bivalve spat—thin shelled ammonoids–thick shelled ammonoids. The thick shelled ammonoids mark the establishment of normal marine salinities and are the principal means of correlation of Silesian sequences. The faunal sequence is reversed in the upper part of such a marine band and records the progressive reduction in salinity, due to freshwater influxes into the basin as the transgression waned. However, they are replaced laterally towards the palaeo-shoreline by benthonic faunas (brachiopods and shallow burrowing bivalves). In these cases, marine band correlation is more difficult and depends on information from the surrounding succession, such as the relative position to other dated marine bands, and additional biostratigraphical data, including miospores and conodonts. This is exemplified in the Westphalian, where a gradual decline in the geographical extent and diversity of marine horizons occurred as delta plain and alluvial conditions became widely established (Calver, 1968). In areas marginal to the main basin, such as Flintshire, only the most extensive and prolonged transgressions are represented by ammonoid faunas, and correlation therefore relies on the presence of widely developed coal seams.

In the basinal areas and on sediment-starved platforms, these faunal repetitions generally occur in condensed mudstone sequences. In areas marginal to the basin, the bulk of strata within any cycle is generally non-marine, predominantly fluviodeltaic or lacustrine in origin. It includes deposits formed during highstand periods, when deltaic sequences prograded across platform areas, as well as regressive sequences produced during eustatic lowstands. Brandon et al. (1995) and Martinsen et al. (1995) presented models showing the relationship of these sediments and facies with respect to the Namurian in northern Britain. These authors have also demonstrated that basement and local tectonic influences were important in modifying the facies of both the marine bands and intervening strata.

Ramsbottom (1973, 1977) suggested that larger scale cyclical repetitions of strata can be recognised in ­Carboniferous sequences, by the appearence of new faunas coincident with widespread transgressive-regressive episodes. The boundaries of each major cycle (‘mesothem’) are marked by widespread disconformities in shelf areas. The events governing this large-scale cyclicity were inferred to be eustatic sea-level fluctuations. Eleven mesothemic cycles, linked to the appearance of new ammonoid genera, were proposed for the British Namurian (Table 10). By defining mesothems in this way, Ramsbottom regarded them as chronostratigraphical rock units, correlating with the standard British Carboniferous stages (George et al., 1976; Ramsbottom et al., 1978). However, the proposal that Namurian stage boundaries should be adjusted to coincide with mesothem boundaries was never adopted (Ramsbottom, 1977; Ramsbottom et al., 1978). Although the mesothem model for the British Namurian has been critically reviewed (Holdsworth and Collinson, 1988; Martinsen, 1990), the underlying concept of eustasy as an influence on sedimentation, particularly with respect to widespread marine transgressions, has never been ­successfully challenged.

Palaeogeography and depositional history

Palaeomagnetic and facies evidence show that, during the Silesian, the British part of Laurentia lay within humid equatorial latitudes (Scotese and McKerrow, 1990; Witzke, 1990). The rifting phase that established the blocks and basins of the Dinantian (Frazer and Gawthorpe, 1990; Frazer et al., 1990) gave way to a period of more uniform regional subsidence during the Silesian, in what is now regarded as the thermal sag phase of extensional basin development (Leeder, 1982; Leeder and McMahon, 1988). The early rift basins of northern England and north Wales were gradually filled and overtopped by Namurian sediments, as they merged and became subsumed within the much larger Pennine Basin (Guion and Fielding, 1988; Collinson, 1988), part of the Central Province of Ramsbottom (1969). The Silesian rocks of this district were deposited on the southern margin of this basin, which was centred on south-west Lancashire and was bordered to the south by the older rocks of the Wales–Brabant Massif (also referred to as the Midland Landmass or ‘St George’s Land’). All of the Namurian strata and the bulk of Westphalian strata deposited during this period are included in the post-rift megasequence of Frazer and Gawthorpe (1990).

During the Namurian, the Wales–Brabant Massif supplied sediment to small deltas fringing the southern part of the Pennine Basin, but was progressively onlapped by larger delta systems which had spread across the basin from the north (Collinson, 1988; Guion and Fielding, 1988). The first appearance, in this district, of abundant feldspar in the Millstone Grit Group sandstones, during the late Namurian (Yeadonian), marks the arrival of coarser detritus derived from the northerly sourced deltas and enables these facies to be distinguished from the dominantly quartzose sandstones of southerly derivation.

The late Namurian pattern of sedimentation continued into the Westphalian, with the progressive establishment of delta plain environments and diminished marine influences. Sediment supply from the Wales–Brabant Massif was generally subordinate to that from the north, but periodic influxes of southerly sourced clastic material possibly record local tectonic movements (compare Waters et al., 1997). The early Westphalian delta plains were suitable environments for coal formation. However, the coals of the Flint Coalfield, in contrast to those of Lancashire and North Staffordshire, are commonly thin, relatively uneconomic and subject to ‘washouts’, reflecting the basin marginal situation of the district (Frazer and Gawthorpe, 1990). During the mid to late Westphalian, in this district, as in other areas bordering the Wales–Brabant Massif, coal-bearing sequences gradually gave way to red beds containing thick seatearths (fireclays), marking the demise of the deltaic environments and the onset of alluvial and coastal plain conditions. The reasons for the change are complex, involving a combination of local and regional factors including tectonic inversion associated with the Variscan Orogeny, and the change to an increasingly arid climate to the north of the developing Variscan mountain belt (Besly, 1987, 1988). These reddened Westphalian rocks are included in the inversion megasequence of Frazer and Gawthorpe (1990), as used by Corfield et al. (1996).

Namurian

Rocks of Namurian age flank the Flintshire Coalfield and crop out extensively south of Hawarden [SJ 315 657]. Smaller fault-bounded crops occur west of Brynford [SJ 177 745], and as inliers in the coalfield (Figure 20a). They comprise the Millstone Grit Group, a 700 m-thick succession of sandstones and mudstones, with cherts developed locally in the lower part. In places, the lowest parts of the group may include strata of late Dinantian age, whereas the uppermost part ranges into the early Westphalian. It is convenient to describe these pre- and post-Namurian parts of the group in this chapter. The Millstone Grit Group is broadly equivalent to the ‘Halkyn Formation’ of Campbell and Hains (1988) and Hains (1991).

A summary of the earlier work on the Millstone Grit Group in north Wales was provided by Ramsbottom (1974). Subsequent to the faunal studies of Jones and Lloyd (1942), little original work on these strata had been undertaken prior to this resurvey. In this district the group comprises four formations which exhibit complex intertonguing relationships (Figure 20a), (Figure 20b), (Figure 21). The lower­most beds in the north of the district are the Pentre Chert Formation, a succession of cherts and cherty mudstones. The formation rests with locally pronounced disconformity on the Cefn Mawr Limestone, and is overlain by the Holywell Shales Formation, comprising mainly marine, brackish and nonmarine mudstones. The Pentre Chert and Holywell Shales intertongue with, and pass laterally southwards into the Cefn-y-fedw Sandstone Formation comprising quartzose sandstone of fluvio­deltaic aspect. To the south-west of a major intra-Carboni­ferous structure, the Nercwys–Nant-figillt Fault Zone, the Cefn-y-fedw Sandstone succeeds the Dinantian Minera Formation and is overlain by, and laterally transitional with, the Holywell Shales. Across the entire district, the upper part of the Holywell Shales interdigitates and is succeeded by the Gwespyr Sandstone Formation, comprising interbedded feldspathic sandstones and mudstones, of late Namurian to early Westphalian age. Upper parts of the Gwespyr Sandstone are laterally equivalent to and interdigitate with the Lower Coal Measures. Recently, workers in northern England have sought to restrict the usage of the term Millstone Grit Group to Namurian strata dominated by feldspathic sandstones (for example Rees and Wilson, 1998). Using this definition, only the Gwespyr Sandstone of this district would form part of the group.

Biostratigraphy

The division of the Namurian into biozones and stages by means of ammonoid marine bands is well established as the primary means of biostratigraphical correlation (Ramsbottom et al., 1978) (Table 10). In addition to ammonoids collected during the resurvey, previously collected material has been re-examined, and published faunal lists, including those in Wood (1936), Jones and Lloyd (1942) and Ramsbottom (1974) have been re-assessed. Re-examination of several of the second authors’ localities has confirmed the accuracy of their determinations. All seven Namurian stages are represented in the district (Table 10). The majority were recorded previously in a series of boreholes at Abbey Mills, Holywell, immediately north of the district (Ramsbottom, 1974) but, during the resurvey, the presence of Chokierian strata was also proved for the first time in north Wales.

In the district, 12 out of the 19 Namurian ammonoid biozones currently recognised, have been proved. Although only 15 of the 49 marine bands listed from the basins of the Central Province are present, several of these are particularly widely developed. The latter appear to represent transgressive periods of unusual magnitude, which interrupted the progradation of the local Millstone Grit deltas. They are the Arnsbergian Eumorphoceras ferrimontanum (E2a2) and Cravenoceratoides nitidus (E2b3) marine bands in the lower part, an unidentified marine band within the mid-Kinderscoutian Reticuloceras nodosum Biozone (R1b), as well as the Marsdenian Bilinguites gracilis (R2a1) and B. superbilinguis (R2c1) marine bands, and Yeadonian Cancelloceras cancellatum (G1a1) and C. cumbriense (G1b1) marine bands in the upper part. A list of the critical localities for these and other less widespread marine bands, with their respective faunas is given in (Table 11) .

Pentre Chert Formation

The Pentre Chert Formation, previously defined by Warren et al. (1984) in the Rhyl district, comprises colour banded, glassy to impure, granular-textured chert, interbedded with siliceous (cherty) mudstone and siltstone. Subordinate beds, up to 0.1 m thick, of variably silicified, white and buff, fine- to medium-grained, locally pebbly, quartzose sandstone, and thin beds of crinoidal chert (silicified former crinoidal limestones) also occur in places. The formation crops out in the Halkyn area, north of the Nercwys–Nant-figillt Fault Zone, between Milwr [SJ 192 748] and Rhosesmor [SJ 2148 6873]. South of this fault zone, packets of chert and cherty mudstones interbedded in the lower part of the Cefn-y-fedw Sandstone (see below) are assumed to be broadly synchronous and possibly contiguous with the Pentre Chert Formation. The formation equates largely with the ‘Cherty Sandstone’ division of Morton (1882–4), and with the ‘Chert Beds’ of Strahan (1885). In its type area, the Pentre Chert Formation overlies ammonoid-bearing, late Dinantian (Brigantian) basinal facies (Teilia Formation). In this district, it rests largely disconformably on coeval platform facies (Cefn Mawr Limestone), except in the Rhosesmor area where a thin sandstone included in the Cefn-y-fedw Sandstone intervenes.

The formation exhibits marked lateral thickness variations which in part complement thickness changes in the underlying Cefn Mawr Limestone (see p.50), and also reflect a lateral, southwards passage into the Cefn-y-fedw Sandstone. At the northern margin of the district, the formation is about 40 m thick and the basal disconformity is probably minimal. Traced south, it thickens to 180 m between Pentre Halkyn and Halkyn Mountain. There the Pentre Chert Formation rests with marked disconformity on the Cefn Mawr Limestone, of which as much as 100 m is absent (see Chapter 3, (Figure 4)). Farther south, the chert sequence thins again as the underlying Cefn Mawr Limestone thickens, as seen in the vicinity of Pant Quarry [SJ 200 203] (Figure 4), (Figure 21). Field relationships in this region also suggest a thinning of the uppermost part of the chert succession, complementary to the incoming and southward expansion of the Cefn-y-fedw Sandstone Formation. Thin mudstone units which appear to extend across the chert/sandstone interface seem to provide evidence of a lateral passage rather than an onlapping relationship between the two formations (but see below).

A largely petrographic study of the cherts was undertaken by Oldershaw (1968). He recognised that the various chert lithologies are arranged in cyclic sequences which, individually, can exceed 35 m in thickness. The dominant glassy and granular cherts occupy the upper parts of these cycles. These cherts include blue-grey, black and brown varieties which weather to white and orange-brown hues. They occur in tabular beds, which range from 0.01 to 0.05 m, and locally up to 0.5 m in thickness and commonly exhibit regular colour lamination and banding (Plate 12a). Bed contacts are com­monly stylolitic. Early chert nodules, preserving some of the pre-compaction thickness and spacing of the lamination and banding, are present locally (Plate 12a). Both planar and low-angle cross-lamination are recorded and evidence of scour at the contacts between some colour bands has been observed. Grey and brown, cherty mudstones and argillaceous cherts occur as subordinate thin beds intercalated with the glassy and granular cherts towards the middle of the cycles, but dominate their lower parts, forming sequences several metres thick. In these basal units, the cherts occur as thin, platy beds, each rarely greater than 20 mm in thickness. Thin seams and partings of fissile, carbonaceous mudstone commonly separate the more siliceous beds.

Within the cycles, individual bed thick­nesses increase upwards (Oldershaw, 1968, plate 1b) and the amount of terrigenous material, both in the form of shale interbeds and as a contaminant in the cherty lithologies, decreases. Geochemical analyses confirm that the banded, glassy and granular cherts in the upper parts of the cycles, are largely devoid of both clay and organic impurities; the colour banding is attributed to minor variations in iron oxide content. However, beds of black ‘sapropelic’ cherts containing up to 6 per cent organic matter and high levels of clay impurity, principally in the form of illite, may occur at any level within the cycles. SEM examination of surface textures suggests that the form of the silica in the cherts also varies according to the levels of terrigenous and organic impurity. In the impure cherts and cherty mudstones, the silica occurs principally in the form of spongy or pitted, cryptocrystalline chalcedony. With decreasing levels of impurity, microcrystalline quartz becomes increasingly important.

A conspicuous feature of the for­mation is the structural complexity of many exposures. The structures range from widespread low-angle bedding discordances, to small-scale, tight, disharmonic chevron folds, boudinaged bedding and high-angle faults (Plate 12c). The discontinuous nature of some faults and secondary silicification of structures, notably slump or collapse folds, suggests that much of the deformation occurred syndepositionally or during lithification. The effects of this syn­sedimentary disturbance appear to be preferentially developed in the expanded sequence north of Halkyn Mountain and are best seen in both Bryn Mawr and Pen-yr-henblas quarries [SJ 1881 7326] to [SJ 1900 7290].

On a local scale, the disconformable nature of the base of the Pentre Chert Formation can be demonstrated in the Pentre Halkyn area, in Bryn Mawr Quarry [SJ 1881 7326], where it comprises an irregular, low-angle surface against which the underlying beds of the Cefn Mawr Limestone are cut out. Pockets of reddish brown silt, indicative of dissolution of the limestone, occur along the contact. Irregular pods of chert, several metres in diameter, are present in the limestone and display distorted lamination, consistent with the collapse of partly lithified (silicified) sediment at the same time as the surrounding limestone was being dissolved (Plate 12b). How these features relate to the disconformity which accounts for the overstep of the Minera Formation by the Namurian rocks (Cefn-y-fedw Sandstone) across the Nercwys–Nant-figillt Fault Zone (Chapter 3; (Figure 20), (Figure 21) is unclear. The thickness changes evident in the Cefn Mawr Limestone, to the north of this structure, between Pant Quarry and Pentre Halkyn, appear to reflect a continuation of this overstep process. However, the pattern of thickness changes which reveal the highly incised nature of the contact (Figure 4), (Figure 21), and the nature of the contact itself suggest that other processes were at work in this tract (see below).

The formation contains a sparse macrofauna including crinoidal debris, indeterminate productid and rhynchonellid brachiopod fragments, the bivalve Posidonia membranacea, and thin-shelled ammonoids. Foraminifera, bryozoa and plant remains have also been reported. Well preserved carapaces of dithyrocarid (shrimp-like) crustaceans have also been recovered principally from the more argillaceous facies. Petrographic studies of the glassy cherts reveal common spherical structures resembling former radiolaria tests, and simple rod-shaped sponge spicules are particularly abundant in some samples (Morton, 1887a and b; Hinde, 1887; Strahan, 1890; Hind and Stobbs, 1906; Jones, 1921; Sargent, 1923, 1929; Oldershaw, 1968).

There has been considerable debate about the age and lithostratigraphical affinity and position of the Pentre Chert Formation (see Jones and Lloyd, 1942). It has either been placed in the upper part of the ‘Carboniferous Limestone Series’ (Hind and Stobbs, 1906; Sargent, 1923, 1927; Neaverson, 1930), or in the ‘Millstone Grit Series’ (Strahan, 1885; Wedd and King, 1924). In its type area, Warren et al. (1984) included the Pentre Chert Formation in the Dinantian, suggesting that it may be partly equivalent to the chert-rich Summit Limestone (of probable P2 age) of the Great Orme. Comparable chert horizons have been recorded from the earliest Namurian strata of northern England (Wells, 1955; Dunham and Wilson, 1985), but also from the latest Dinantian (Chisholm et al., 1988). In south Wales, similar cherty facies range across the Dinantian/Namurian boundary (Woodlands and Evans, 1964).

A lower limit to the age of the Pentre Chert Formation in the district is provided by a single specimen of ­Sudeticeras sp. (splendens/stolbergi group), indicative of the late Brigantian P2b Subzone, from near to the top of the local Cefn Mawr Limestone sequence in Bryn Mawr Quarry [SJ 1880 7340] (p.54). However, thickness comparisons with adjacent areas (Pant Quarry and Brynford) suggest that as much as 50 m of younger limestone strata may be missing due to disconformity at this locality. A P2 fauna has also been obtained from limestones ­immediately underlying the cherts in the Gronant Bore­hole, in the adjacent Rhyl district (Warren et al., 1984). The presence of Posidonia membranaceain the lower part of the Pentre Chert Formation in Pen-yr-henblas Quarry [SJ 1900 7290] ((Table 11) , locality 1), indicates an age no older than the late Brigantian (P1d Subzone), but no younger than the late Pendleian (E1c Subzone). Confirmation that the entire formation is no younger than the E1b Subzone rests on the identification of the Cr. malhamense Marine Band (E1c1), in the Holywell Shales, 11.5 m above a conformable contact with the cherts in Abbey Mills No. 4 Borehole (Ramsbottom, 1974). The presence of a major disconformity at the base of the Pentre Chert Formation in the Halkyn area, and transitional relationships with the Cefn-y-fedw Sandstone support its lithostratigraphical inclusion in the local Millstone Grit Group, with the balance of biostratigraphical evidence favouring a Pendleian (E1a–E1b)age.

Conditions of deposition

Bedded chert sequences are commonly related to major oceanographic and climatic changes resulting from plate convergence (for example Lowe, 1975). The deposition of bedded cherts that occur widely in late Dinantian to early Namurian sequences throughout the British Isles, are probably related to Variscan plate movements which introduced a more humid climate across the British Isles. The resulting influx of terrigenous detritus, coincident with the onset of regional thermal subsidence (Leeder, 1988), curtailed and eventually terminated carbonate production on former shelf areas, as in this district.

The formation succeeds basinal Dinantian facies in the Rhyl district, and this suggests that the cherts represent a Namurian basinal facies there, which onlapped the abandoned Dinantian plat­form margin in this district, in response to regional subsidence. Basinal chert ­formations with comparable onlapping re­lationships were described by Lowe (1975), Price (1977), Iijima et al. (1985) and notably by Jones and Murchey (1986).

Although rare cross-lamination and scour features record the effects of infrequent tractional processes, the well laminated and banded structure of much of the formation is consistent with accumulation ­predominantly from suspension in moderately deep water, largely below the effects of storm wave-base. The thin beds of sand­stone and silicified crinoidal lime­stone indicate infrequent high energy events, and were possibly emplaced by storms and/or as ­turbidites. These events carried coarser detritus to a depositional site which for long periods appears to have escaped all but the finest terrigenous contamination. The crinoidal beds suggest that else­where in shallower waters there survived niches in which steno­haline shelly benthos still flourished, though the possibility that these beds are composed of debris derived from previously deposited limestones cannot be discounted. The well preserved dithyrocarid carapaces point to accumulation in a low-energy, marine environment, but these shrimp-like crustaceans are thought to have been tolerant of both reduced salinity and dissolved oxygen levels. Well preserved depositional lamination and banding in the vitreous cherts reflect the absence of burrowing benthos. However, sponges evi­dently thrived at or close to the sites of deposition and, with the decline in skeletal carbonate production, their siliceous remains, possibly combined with those of planktonic radiolaria, then formed a significant component of the sediment.

The Pentre Chert cycles are superficially comparable with the shoaling-upwards sequences which characterise both the late Dinantian carbonate succession and Silesian deltaic formations of the district. They can similarly be inferred to record sedimentary responses to contemporary move­ments in sea level. However, the chert cycles appear to record an upwards decrease in terrigenous input, which seems inconsistent with deltaic progradation or up­ward shoaling. Furthermore, the well laminated glassy cherts which form the upper parts of the cycles do not appear to record deposition under more energetic conditions than the underlying cherty mud­stones and so, in marked contrast to the cycles in the Cefn Mawr Limestone, winnowing cannot be used to explain the reduction in siliciclastic component. It is therefore possible that the cherty mudstones, which mark periods of maximum terrigenous input, may form the regressive parts of the cycles, and that the intervening transgressions caused a reduction in the levels of terrigenous mud reaching the basin and promoted the formation of the purer, glassy cherts.

Origin of the silica

The origin of the silica in bedded chert sequences remains poorly understood. Recent theories and proceses were reviewed by Hesse (1989) and by Raymond (1995). Debate has focused on the relative importance of primary silica, produced by either organic or inorganic means, and of replacement diagenesis (silicification) of precursor lithologies, especially limestones, to form secondary cherts. Though the effects of remobilisation and reprecipitation during diagenesis are widely acknowledged to overprint primary textures completely (Lowe, 1975), the large volume of silica needed to produce thick sequences of bedded chert has persuaded most workers that the substantial deposition of primary silica is indicated.

Early studies of the Pentre Chert Formation, summarised by Sargent (1923), reflect the same primary versus replacement debate. Many early workers stressed the importance of sponge spicules in these rocks (Morton, 1887 b; Hind and Stobbs, 1906; Jones, 1921), leading to the suggestion that they rep­resent spicular cherts (Hinde, 1887; Wedd and King, 1924). However, Strahan (1890) and subsequently Neaverson (1946) emphasised the impor­t­ance of secondary replacement, both of pre-existing carbonate and siliciclastic lithologies; Strahan suggested that much of the chert was composed of extremely fine-grained, detrital quartz cemented by silica. Subsequent petrographic studies showed that the detrital quartz com­ponent is generally very small (Sargent, 1923; Oldershaw, 1968). Sargent (1923) pointed out that because the thin silicified limestone beds in the chert sequence were easily recognisable, thick limestone units were not part of the primary lithological suite. It is unlikely, therefore, that the formation records the wholesale replacement of limestone (of Cefn Mawr Limestone or Minera Formation type). Warren et al. (1984), agreeing with Sargent, rejected both the spicular and detrital origin and concluded that much of the chert was precipitated as a primary, inorganic silica gel (see also Neaverson, 1946; Wells, 1955).

Primary inorganic silica is known to form in ephemeral lakes and around submarine volcanoes where local conditions lead to silica supersaturation. In other situations, however, direct precipitation of inorganic silica from sea water is unlikely to have contributed significantly to Phanerozoic bedded chert sequences (Heath, 1974). Consequently, primary silica in bedded cherts is now thought to be almost exclusively biogenic in origin and to have been supplied principally by sponges and radiolaria (but see Park and Croneis, 1969). It is likely that sponge spicules were the source of much of the silica in the Pentre Cherts. However, the potential contibution of radiolaria should not be discounted. Bedded radiolarian cherts are geologically widespread and derive from either direct accumulation of radiolaria tests on the sea floor, or their resedimentation as turbidites (Iijima et al., 1985; Price, 1977). Although traditionally thought of as very deep water deposits, it is now acknowledged that radiolaria-rich sediments also form in much shallower settings, in restricted gulfs and embayments adjacent to upwelling oceanic currents rich in nutrients (Jones and Murchey, 1986). In their ‘subsidence’ and ‘continental margin’ chert associations, Jones and Murchey (1986) described upward transitions from platform carbonates, via spicular cherts into radiolarian cherts. They related these transitions to increasing water depth associated with the drowning and foundering of former platforms. These same chert types can also form part of a transitional sequence in which coastal terrigenous facies pass laterally into offshore spicular and then radiolarian cherts, and, in this context, are similar in form and setting to the Pentre Chert.

Regional factors may also have played a part. Active Namurian fluvial systems resulting from the more humid climate, drained a hinterland of deeply weathered Lower Palaeozoic rocks which included extensive volcanic tracts. The waters they discharged may have been significantly enriched in silica (Neaverson, 1946; Wells, 1955; Hesse, 1989). The large amount of available organic material, from flourishing plant communities, could also have been important in determining levels of water acidity, and could also have provided a molecular base for silica mineralisation (Hesse, 1989). Such acidic, organic-rich fluids may also have been important agents during diagenesis. Freshwater plumes, mixing with marine waters, either directly or within the diagenetic zone are known to promote silica remobilisation and reprecipitation (Knauth, 1979).

Although the source of the silica is considered to be organic, and therefore primary, all of the Pentre Chert lithologies display the effects of variable, but pervasive, post-depositional silicification. Indeed, the presence locally of early, pre-compactional chert nodules (Plate 12a) provides evidence for more than one period of silicification. Evidence that silicification (leading to lithification) was initiated soon after deposition was first suggested by Strahan (1890, p.49) and is confirmed at localities in the Cefn-y-fedw Sandstone, where angular rip-up clasts, derived from already lithified cherts and cherty mudstones occur. The Pentre Chert Formation represents a group of primary sediments (principally spicular and possibly radiolaria-rich ‘ooze’, but also terrigenous sand, silt and mud as well as beds of crinoid debris) which have suffered a similar diagenetic history. The mapped boundaries of the formation, particularly at the base, may mark the position of a silicification front which may not everywhere coincide with primary (pre-silicification) lithological contacts. Abundant chert nodules offer ample evidence of selective or incomplete replacement in the limestones below the contact. The lateral transition of the Pentre Chert into the Cefn-y-fedw Sandstone may, similarly, represent a boundary between strongly and weakly silicified lithologies which obscures the primary facies geometry.

Origin of the basal disconformity

The locally pronounced disconformity at the base of the Pentre Chert Formation may owe its form to two separate, but not mutually exclusive processes:

• tectonic uplift leading to subaerial erosion

• localised dissolution and foundering of the former Dinantian platform margin

The deeply incised nature of the disconformity on Halkyn Mountain is inconsistent with simple subaerial bevelling. The local geometry, if explained by subaerial erosion alone, requires a fall in local base level far more severe than those envisaged during the Dinantian. Evidence for a major eustatic regression at this time is lacking (for example Ramsbottom, 1979), suggesting that the disconformity may record, instead, a brief episode of tectonic inversion leading to uplift along the southern margin of the developing Pennine Basin. Such an event is in accord with the observations of Williams and Eaton (1993) and could be coeval with a similar event in south Wales (George, 1956; Wilson et al., 1989). Additional factors however, appear to have contributed to the irregular form of the disconformity in the Halkyn Mountain region, which may reduce, or possibly negate the need for such tectonism.

On Halkyn Mountain, the pillow-like bodies of chert seen intruding the Cefn Mawr Limestone at Bryn Mawr Quarry [SJ 1880 7340] provide unequivocal evidence of intrastratal dissolution at the chert/limestone contact. Syn- or early post-depositional collapse of the cherts into developing cavities within the underlying limestones provides an explanation for the random pattern of faulting and folding hereabouts. The marked thickening of the chert sequence and complementary thinning of the Cefn Mawr Limestone, is consistent with chert accumulation within a broad, actively growing, doline-like, karstic depression. Such karstic dissolution was evidently not accomplished by downward migrating vadose groundwaters but, more probably, by aggressive formation waters expelled from compacting chert and adjacent basinal mudstones. The same fluids were possibly responsible for the secondary silicification.

The Dinantian sequence subjacent to the disconformity on Halkyn Mountain provides evidence of further complexity. Features indicative of synsedimentary faulting and sliding, seen in Waen Brodlas Quarry and formerly in Pant Quarry (Plate 11), suggest that the Halkyn Mountain area was the site of instability during the late Dinantian. These structures may record slope failures along the carbonate platform margin, thought to lie close to the north-west, perhaps related to movements on the adjacent Nercwys–Nant-figillt Fault Zone. Thus, the form of the sub-Pentre Chert disconformity on Halkyn Mountain may owe little to subaerial processes, but represents a relict, late Dinantian, mega-slide or slump scar, subsequently draped and gradually filled by the chert, which then acted as a focus for intrastratal dissolution. The disturbances within the local chert sequence may reflect continuing seismically induced, slope instability at the site during the early Namurian.

The topography created by movement along the Nercwys–Nant-figillt Fault Zone (see p.57) may have served also to obstruct the northward spread of coeval deltaic facies, and the exclusion of such terrigenous contamination was possibly an important factor in promoting chert deposition in this area and to the north-west. The contact between the Pentre Chert and Cefn-y-fedw Sandstone formations appears to record the subsequent northward encroachment of terrigenous facies into the chert belt, not from the west, across the fault zone, but from the south-east, throughout the Pendleian (Figure 20) and (Figure 21). The possibility emerges that the disconformity associated with the base of the Namurian succession between Gwernafield and Pentre Halkyn may represent a compound feature formed by different processes in different places. Contemporary activity on the Nercwys–Nant-figillt Fault Zone was perhaps the common factor, accounting for the apparent juxtaposition of a subaerial erosion surface, formed across the fracture belt, with a slide-generated hiatus (but which was subsequently modified by dissolution) in the Halkyn Mountain area.

Details

The eastern face of Bryn Mawr Quarry [SJ 1877 7340] exposes the low-angle, irregular, disconformable base of the Pentre Chert Formation, overlying the Cefn Mawr Limestone. The cherts, forming the upper 20 m of the quarry, are well laminated, white, grey and black, in tabular units up to 0.4 m thick; nodular chert overprints bedding in places. Silicified beds (up to 2 m thick) of cross-laminated, quartzose, pebbly sandstone and sandy, crinoidal limestone occur locally. The formation is cut by several low-angle discordances and displays numerous open to tight, disharmonic, small-scale folds.

In the floor of Pen-yr-henblas Quarry [SJ 1900 7290], an irregu­lar, possible solution surface marks the boundary between the Pentre Chert Formation and Cefn Mawr Limestone. The lowermost 11 m of the Pentre Chert Formation comprise pale grey, silicified siltstones and fine-grained, thinly laminated, quartzose sandstones with pebbly lenses and, near the base, brachi­opod-rich units. Rare dithyrocarid carapaces, telsons and mandibles occur in the lowermost 2 m of the sequence. A prominent angular discontinuity truncates these lower beds and is overlain by up to 10 m of white, grey and black, glassy cherts, with interbedded shaly, siliceous mudstones and siltstones, and rare, thin beds of silicified crinoidal limestone. The cherts are well bedded, in units ranging from 5 to 100 mm thick and, in places, laminated. They are pervasively folded and display numerous fractures, ranging from low-angle discordances (thrusts or slides) to high-angle reverse faults (Plate 12c).

A quarry [SJ 1932 7335] 500 m east of Bryn Mawr, exposes a 3 m-thick sequence of well bedded and laminated, white, grey and black cherts. At several horizons, bedding has been deformed by compaction around early formed silica nodules (Plate 12a).

A small quarry [SJ 2054 7032] and adjacent exposures, near the radio transmitter on Halkyn Mountain, reveal a sequence, up to 5 m thick, of cherts with subordinate, variably silicified, shaly mudstone and siltstone. The cherts are pale grey, well bedded, in units of 10 to 50 mm thicknesses, thinly laminated, locally cross-laminated and, in places, pseudobrecciated; a massive chert horizon up to 0.3 m thick forms the base of the sequence. Several stages of early silicification and compaction have resulted in a ‘pinch and swell’ appearance to the bedding.

Holywell Shales Formation

The term Holywell Shales was applied by Strahan (1885, 1890) to the sequence of marine and non-marine mudstones lying between his Gwespyr Sandstone and a local division of the ‘Millstone Grit’ (broadly equivalent to the Cefn-y-fedw Sandstone and Pentre Chert Formation of this account). At that time, they were generally regarded as part of the ‘Lower Coal Measures’ despite their lack of coal seams. The controversy that subsequently surrounded their age (for example Hind and Stobbs, 1906; and reviewed by Ramsbottom, 1974) was only resolved by ammonoid biozonation (Jackson, 1925; Sargent, 1927; Wood, 1936; Jones and Lloyd, 1942).

The Holywell Shales are here given formational status and the boundaries have been re-defined. The type sections of the formation are the Abbey Mills No. 1 and No. 4 boreholes, immediately to the north of the district, which together proved a complete 152 m-thick sequence between the Pentre Chert Formation and the Gwespyr Sandstone Formation, including a number of marine bands (Ramsbottom, 1974). Important reference sections are also provided by the Point of Ayr Colliery No. 3 Shaft [SJ 1250 8371] (Ramsbottom, 1974) and the Coed Pen-y-maes stream section [SJ 1939 7650] to [SJ 1962 7687] in Holywell, both in the adjacent Liverpool district.

The formation is largely drift covered and exposures are confined to stream and river sections. The main crop extends from Milwr [SJ 195 745], in the north of the district, to Llanfynydd [SJ 2794 5666] in the south. An eastern crop extends between Hawarden [SJ 316 658] and Gwern Estyn [SJ 320 575]. Smaller, fault-bounded crops occur within the Flintshire Coalfield, in the central part of the district. The westernmost outcrop of Holywell Shales in the district, the Calcoed Outlier of Strahan (1890, p.62) situated to the west of Brynford [SJ 160 745], is down-faulted on all sides against Dinantian rocks.

In the north, the formation succeeds the Pentre Chert Formation, whereas south of Pentre Halkyn it overlies the lower Cefn-y-fedw Sandstone (Figure 20a), (Figure 20b), (Figure 21); see below). South and east of the Halkyn area, the formation exhibits complex intertonguing with the Cefn-y-fedw Sandstone (Figure 21), but two widespread tongues are recognised. The lower tongue broadly correlates with the ‘Lower Shale’ of Morton (1876–78). At its maximum, in the north and west, it is up to 120 m thick, and separates the lower and upper Cefn-y-fedw Sandstone. However, it thins and splits south-eastwards to accommodate the entry of the middle Cefn-y-fedw Sandstone (see below; (Figure 21)). The upper tongue, which equates with Morton’s ‘Upper Shale’, is up to 50 m thick. It intertongues with, and overlies, the upper Cefn-y-fedw Sandstone. In its upper part, it intertongues with and is everywhere succeeded by units of the Gwespyr Sandstone Formation (Figure 21).

The Holywell Shales comprise mudstone with subordinate thin sandstones, impure limestones and impersistent coal seams. Patchy silicification occurs close to the boundary with the underlying Pentre Chert Formation. The mudstone is typically dark grey or olive-brown, weathering to buff or reddish brown hues, finely micaceous and fissile to blocky. Marine bands, com­prising highly fossiliferous, black, fissile mudstone, up to several metres thick, punctuate the sequence. Commonly, they contain thin beds of argillaceous limestone, up to 0.3 m thick. The strata between the marine bands are generally unfossiliferous apart from scattered fish remains and plant debris. Sideritic bands and nodules are sporadically developed and small phosphate nodules have been recorded in places. Grey, blocky mudstones with sideritised rootlets, locally developed beneath rare thin coal seams, represent seatearths.

The sandstones are fine to medium grained, quartzose and generally 0.01 to 0.05 m thick, although beds up to 1.5 m occur in places. They are locally micaceous and contain small amounts of carbonaceous material, disseminated throughout the bed, or concentrated in thin laminae. Cross-lamination is common, with flaser bedding developed in places, and bioturbation structures are preserved locally.

The age of the Holywell Shales has been established mainly from the ammonoid-bearing marine bands encountered in the type area, and proved principally in the Abbey Mills boreholes (Ramsbottom, 1974). They demonstrate that all 7 Namurian stages and 12 of their component ammonoid biozones are present (Figure 20). The Pendleian Cr. malhamense Marine Band (E1c), 11.5 m above the base of the formation in the Abbey Mills No. 4 Borehole, is the lowest recognised ammonoid horizon. This suggests that the base of the formation in the north of the district is probably mid-Pendleian (E1b) in age. The basal Arnsbergian Cr. cowlingense Marine Band (E2a1) was encountered around 27 m above the base of the formation, and the widespread E. ferrimontanum band (E2a2) at 53 m above the base, in the same borehole. The latter is the earliest marine band recorded from the lower tongue of Holywell Shales in the south of the district. However, it appears to overlie at least 70 m of unfossiliferous Holywell Shales in the River Terrig [SJ 2363 5725] (Jones and Lloyd, 1942). These unfossiliferous mudstones possibly include the correlatives of the Cr. malhamense and Cr. cowlingense marine bands recorded in the Abbey Mills No. 4 Borehole. Farther south in the Flint district, the lower tongue of Holywell Shales thins, splits into two leaves (separated by the middle Cefn-y-fedw Sandstone) and eventually wedges out in the vicinity of the Bala Lineament. On Hope Mountain and in the northern part of the Wrexham district, the lower of these leaves comprises a thin sequence of Arnsbergian mudstone containing the E. ferrimontanum (E2a2) and Cr. nitidus (E2b3) marine bands (Wood, 1936; Jackson, 1946; Jones and Lloyd, 1942). The upper leaf contains a Kinderscoutian, R. nodosum Biozone (R1b) fauna on the eastern side of Hope Mountain [SJ 2967 5835]. The top of the lower tongue of Holywell Shales ranges into the late Kinderscoutian, as shown by a fauna of the R. reticulatum Biozone (R1c), recovered from mudstone underlying the upper Cefn-y-fedw Sandstone in a borehole [SJ 3202 6572] south of Hawarden.

The upper tongue of Holywell Shales overlies the upper Cefn-y-fedw Sandstone. The contact is diachronous ((Figure 20a)(Figure 20b) ; see below). In the west, mudstones immediately succeeding the sandstone, exposed in the River Terrig, contain faunas of the basal Marsdenian B. gracilis Marine Band (R2a1). Farther east, in faulted crops in the Hawarden area, both the mid-Marsdenian B. bilinguis Marine Band (R2b2) and the late Marsdenian B. superbilinguis Marine Band (R2c1) are recorded close to the base of the upper tongue of Holywell Shales, but, to the south, in Warren Dingle, mudstones bearing the R2c1 marine horizon are interleaved with an expanded upper Cefn-y-fedw Sandstone sequence. Here the highest Cefn-y-fedw Sandstone unit is overlain by later Marsdenian mudstones which closely underlie the C. cancellatum Marine Band (G1a). The boundary between the Holywell Shales and the lowest tongue of the feldspathic Gwespyr Sandstone Formation is also diachronous. Throughout the northern and central parts of the district, feldspathic sandstones appear above the widely identified basal Yeadonian C. cancellatum Marine Band (G1a) (Table 11). In contrast, in sections along the rivers Terrig and Cegidog, in the south of the district, the lowest Gwespyr Sandstone unit succeeds the younger, late Yeadonian C. cumbriense Marine Band (G1b) (Jones and Lloyd, 1942).

In the Nant-figillt and Rhydymwyn areas, in the central part of the main crop, localities yielding basal Yeadonian C. cancellatum Marine Band faunas occur close to the mapped top of the local Dinantian succession. This has been cited as evidence that much of the pre-Yeadonian part of the Namurian sequence is missing in this area above a major non-sequence at the base of the Holywell Shales (Strahan, 1890; Jones and Lloyd, 1942; Ramsbottom, 1974). However, faulting is now seen to provide another explanation, for, with the exception of specimens of doubtful origin recovered from spoil tips [SJ 2054 6634] at Rhydymwyn, all the Yeadonian fossil localities in this region are now known to lie to the east of the easterly downthrowing Nercwys Fault. The poorly exposed mudstones to the west of the fault, which were previously thought to rest disconformably on a thin development of the ‘Millstone Grit’ (Cefn-y-fedw Sandstone) in a road cutting [SJ 2018 6764] near Hendre, and by inferrence to be late Marsdenian in age (Jones and Lloyd, 1942, p.256), are now thought to be Pendleian and to intertongue with the Cefn-y-fedw Sandstone sequence (see Smith, 1921, fig. 10). Therefore, it is now doubtful that much, if any of the post-Pendleian part of the sequence is missing in this area, and this negates the need for the complex and regionally inconsistent palaeogeography suggested by Ramsbottom (1974). An Yeadonian fauna recovered from a temporary exposure [SJ 1545 7588] of Holywell Shales in the Calcoed Outlier occurs within a similar down-faulted tract within the Nercwys–Nant-figillt Fault Zone.

Conditions of deposition

The intertonguing relationship of the Holywell Shales with the fluviodeltaic Cefn-y-fedw Sandstone suggests that across much of the district they represent a prodelta mudstone facies. However, sequences in the north and east in which sandstones are rare, but marine bands abundant may, in their lower (pre-Marsdenian) parts at least, also include deeper, offshore ‘basinal’ facies contiguous with those of the Pentre Chert and the Dinantian at Prestatyn. The relationship between the largely unfossiliferous mudstones and the marine bands is well established (Ramsbottom et al., 1962; Ramsbottom, 1969. The latter represent transgressive pulses and record periods of elevated sea level when fully marine conditions prevailed within the basin, and deltaic sand supply to the basin was reduced. The impure limestone beds and calcareous nodules associated with the marine bands are largely of diagenetic origin and probably reflect the higher levels of skeletal carbonate available within the marine facies for dissolution and reprecipitation during burial. In contrast, the mudstones between the marine bands record periods of lowered salinity, due to high levels of freshwater discharge to the basin, when delta progradation was at a maximum. The link between lowered salinity levels and delta-advance was probably eustatic sea level fall, which would radically alter the basinal water volume, thus allowing fluvial discharge to reduce the salinity levels in the basin over a very short period of time (Holdsworth and Collinson, 1988).

The intertonguing sequences of Cefn-y-fedw Sandstone record periods of progradation of delta sand facies across the prodelta mud apron. Isolated sandstone bodies within the Holywell Shales record the re-working and redistribution of sand by intertidal and subtidal processes acting on the delta front, but may also include bodies formed by sedimentary bypass of the delta during periods of low stand (compare Brandon et al., 1995). Seatearths with rhizoliths, locally underlying thin coals, provide evidence of periodic emergence, soil formation and plant colonisation.

Details

Calcoed Outlier

Details of sections in the Calcoed Outlier of Holywell Shales and their structure were provided by Strahan (1890). During the resurvey, a C. cancellatum Marine Band assemblage including B. superbilinguis, C. cf. cancellatum, Caneyella multirugata, Dunbarella ex gr. elegans and Posidonia sp. was obtained from temporary exposures [SJ 1545 7588] in mudstones to the north of the district, during the construction of the Holywell Bypass.

Main crop

In the extreme north of the district, a small, degraded quarry [SJ 1928 7520] in Coed Llwybr-y-bi, north of Milwr ((Table 11) , locality 3) exposes mudstones with crushed impressions of Isohomoceras subglobosum indicating an early Chokierian (H1a) age. This is the first recorded occurrence of Chokierian strata in north Wales. About 140 m to the south, spoil tips [SJ 1935 7509] from an old adit ((Table 11) , locality 6) provided a mid Kinderscoutian fauna, ascribed to the R. nodosum Biozone (R1b). Dark brownish grey, shaly mudstones are exposed in places south-eastwards along the stream as far as its source [SJ 1925 7458], east of Milwr.

South-east of Milwr, exposures [SJ 1997 7421] along a minor road and in an adjacent field ((Table 11) , locality 9), revealed up to 2.7 m of grey mudstones and interbedded fine-grained, micaceous sandstones with plant fragments and a fauna indicative of the youngest Kinderscoutian, R. coreticulatum Marine Band (R1c4); a comparable fauna, containing R. reticulatum and Vallites striolatuswas previously recovered from these and nearby exposures (localities 4 and 6 of Jones and Lloyd, 1942).

A series of deeply incised streams, draining north-eastwards off Halkyn Mountain into the Afon Nant-y-Flint, expose several sections through the Holywell Shales. Indeterminate ammonoid fragments and Dunbarella have been obtained from 8 m of dark grey shaly mudstone in a stream section [SJ 2077 7324] to [SJ 2084 7326] north-east of Pentre Halkyn. The mudstone is overlain by 7 m of interbedded mudstone and fine-grained, cross-laminated, quartzose sandstone.

Several important ammonoid horizons crop out in stream sections through Coed-y-felin and Coed-y-cra east of Halkyn (Figure 22a). Homoceras undulatum, indicative of a mid Alportian (H2b1) age, has been found at two localities [SJ 2268 7113] to [SJ 2254 7098] in a stream flowing north-eastwards from Coed-y-cra Farm ((Table 11) , localities 4a and b) (probably localities 11 and 12 of Jones and Lloyd, 1942). In the lower (downstream) locality, the fauna occurs in a 6 m-thick sequence of dark grey mudstones overlying a quartzose sandstone, a correlative of the middle Cefn-y-fedw Sandstone.

The section in Coed-y-felin north-east of Cae Glaision exposes an incomplete sequence of mudstone with subordinate quartzose sandstone, complicated by folding and faulting (Figure 22a). The sequence lies in an anticlinal structure, with the oldest strata exposed immediately above the confluence [SJ 2220 7130] of two streams. Three marine bands, yielding faunas ranging from Kinderscoutian to Marsdenian in age, crop out downstream [SJ 2220 7126] to [SJ 2228 7142] to [SJ 2237 7150]. The oldest of these bands ((Table 11) , locality 5) contains a fauna consistent with a Kinderscoutian (R1) age. In an abandoned stream bank 200 m downstream (Locality 11), up to a metre of fossiliferous, fissile, black mudstones, overlie a thin sequence which in upward sequence comprises ferruginous mudstone with plant remains, a thin coal and a seatearth. The black mudstones have yielded an early form of B. gracilis. Jones and Lloyd (1942) previously recorded R. reticulatum(probably a misidentification) from this locality indicating an upper Kinderscoutian age, but the former discovery suggests the upper part of the B. gracilis Marine Band and, therefore, an earliest Marsdenian (R2a1) age. A Kinderscoutian sandstone exposed [SJ 2201 7114] to [SJ 2224 7135] 15 m below this marine band is a correlative of the upper Cefn-y-fedw Sandstone (Figure 22a). The third marine band (Locality 12) yields a Marsdenian fauna, possibly indicative of the bivalve phase of the B. bilinguis Marine Band (R2b). Although the equivalent strata should crop out to the south-east, on the south-eastern limb of the local anticline, the equivalent marine horizons have not been recognised. However, Jones and Lloyd (1942, their locality 13) obtained the late Yeadonian ammonoid C. cumbriense (G1b) from this part of the section, from black mudstones with argillaceous limestone which underlie a prominent sandstone included in the Gwespyr Sandstone Formation (Figure 22a).

Between Coed-y-cra and Gwysaney Hall [SJ 2277 6646], the Holywell Shales are poorly exposed and partly faulted out, but a fauna ascribed to the basal Yeadonian C. cancellatum Marine Band (G1a) has been recorded from mudstone in a stream section [SJ 2301 7007] west of Bryn-y-garreg.

The higher parts of the Holywell Shales, comprising laminated, micaceous siltstone, black shale, and local, thin, quartzose sandstone, are well exposed in Nant-figillt [SJ 1997 6897] to [SJ 2032 6829] between Wern-y-gaer and Coed Lygan-uchaf. In places along the stream [SJ 1991 6890] to [SJ 1999 6866] to [SJ 2000 6875] to [SJ 2002 6859] to [SJ 2008 6853], the black shale yields a fauna diagnostic of the C. cancellatum Marine Band ((Table 11), locality 16); Jones and Lloyd (1942, their localities 16–18) recorded a comparable fauna from these exposures. The section straddles the Nercwys–Nant-figillt Fault Zone and there is repetition by faulting. The same strata were exposed to the east, in the disused Ruby Brickworks quarries [SJ 2060 6785] to [SJ 2063 6782], where degraded sections revealed up to 20 m of brownish grey, laminated, silty mudstone and siltstone with thin beds of quartzose sandstone and, near the top of the section, grey mudstones with thin coal partings. An overlying 10 m-thick sequence of black mudstones reported to contain B. superbilinguis and C. cancellatum (Hind and Stobbs, 1906; Bathurst et al., 1965) is no longer visible. However, a section in these beds in a stream [SJ 2089 6798] north-east of the brickworks, yielded a fauna ascribed to the C. cancellatum Marine Band ((Table 11), locality 17). A comparable fauna was also recovered during previous surveys, from spoil associated with the Tyddyn-y-gwynt Shaft [SJ 211 671] and from tips in Rhydymwyn [SJ 2054 6634] ((Table 11), localities 18 and 19). Apart from the last locality, all the C. cancellatum faunas in the area lie on the eastern side of the Nercwys Fault.

The River Terrig, south of Nercwys, exposes sections through the lower and upper tongues of the Holywell Shales. Exposures [SJ 2336 5693] to [SJ 2338 5697] in mudstone overlying a sand­stone at the junction with a small tributary stream, yielded a fauna as­signed to the mid-Arnsbergian Ct. nititoides Marine Band (E2b3) ((Table 11), locality 2), and appear to include localities 37 and 38 of Jones and Lloyd (1942). Faulting repeats the sequence downstream, where the early Arnsbergian E. ferrimontanum Marine Band (E2a2) was recorded in the river [SJ 2363 5725] overlying a thick sequence of unfossiliferous mudstone (Jones and Lloyd, 1942, locality 36). Farther downstream, steep riverside banks [SJ 2338 5780] expose strata, overlying the upper Cefn-y-fedw Sandstone, which comprise thinly laminated siltstones and mudstones with thin black fissile shales, interbedded with cross-lami­nated sandstones; it was from these exposures that Jones and Lloyd (1942) obtained specimens of ‘R. reticulatum late mut. α and early mut. β’, indicating the presence of both the B. gracilis (R2a) and the succeeding B. bilinguis (R2b1) marine bands. The C. cancellatum and C. cumbriense marine bands have also been recorded from the upper tongue of the Holywell Shales in the River Terrig, downstream of Cae Gwydd [SJ 2305 5767] (Jones and Lloyd, 1942, localities 33–35).

Exposures of mudstone with argillaceous limestones in the River Cegidog [SJ 2568 5677] and at Hafod Abley, near Bryn Common, have previously yielded a fauna assigned to the E. ferrimontanum horizon (Jones and Lloyd, 1942, localities 40 and 41). A further section [SJ 2622 5694] in the River Cegidog revealed the C. cumbriense Marine Band (Jones and Lloyd, 1942, locality 39).

Hope Mountain

Thin, but stratigraphically significant units of Holywell Shales intercalated within the local sequence of Cefn-y-fedw Sandstone were previously exposed on Hope Mountain. Strahan (unpublished field maps) recorded a section in ‘laminated shales’ in the road [SJ 2773 5875] near Tri-thy, and exposures of fossiliferous mudstones with large Lingula in the road [SJ 2840 5765] leading to Horeb. Jones and Lloyd (1942) assigned a fauna from this latter locality to the E. ferrimontanum Marine Band (E2a2). To the south-east, along the road [SJ 3012 5775] on the eastern side of Hope Mountain, a comparable early Arnsbergian marine fauna was recovered by Jones and Lloyd (1942), from a sandy mudstone sequence, less than 10 m thick. A 20 m-thick sequence of Holywell Shales overlying thinly bedded and argillaceous sandstones, taken to be the top of the middle Cefn-y-fedw Sandstone, is exposed in a stream section south of Talwrn. A fauna of mid-Kinderscoutian (R1b) age was recovered form the highest beds exposed at the eastern limit of this exposure [SJ 2967 5835] ((Table 11), locality 7).

Flint Coalfield

The main exposures in this area occur in the vicinity of Leeswood Old Hall [SJ 2595 6169]. In a stream section [SJ 2583 6122] to [SJ 2577 6177] west of the hall, grey and brown mudstone containing the C. cumbriense Marine Band (Jones and Lloyd, 1942, localities 31 and 32) were previously exposed beneath glacial deposits. The same horizon was revealed in old diggings [SJ 2641 6130] south-east of the hall ((Table 11), locality 23).

Higher Kinnerton area

The principal section in this area is Warren Dingle [SJ 3139 6246] to [SJ 3187 6236], where a sequence through the upper tongue of the formation is exposed (Figure 22b). At the base of the section is the B. superbilinguis Marine Band (R2c1), which is exposed in the stream [SJ 3139 6246] below the A5104 road bridge ((Table 10); locality 14), at a site corresponding to locality 24 of Jones and Lloyd (1942). The overlying sequence, exposed downstream, comprises dark grey, silty mudstone with two thick units of quartzose sandstone representing the highest beds of the upper Cefn-y-fedw Sandstone. Black shales overlain by the lowest unit of the Gwespyr Sandstone are exposed farther downstream and repeated by faulting. The C. cancellatum Marine Band is exposed at three sites [SJ 3179 6234] to [SJ 3201 6231] to [SJ 3209 6232] to [SJ 3215 6234] ((Table 11), localities 21a–c), corresponding to localities 26 and 27 of Jones and Lloyd (1942). The C. cumbriense Marine Band ((Table 11); locality 22) is exposed 200 m downstream from the ­easternmost C. cancellatum exposure. It is succeeded by a sequence of mudstones and feldspathic sandstones. Mappable units of the latter are included in the Gwespyr Sandstone (see below).

The C. cancellatum Marine Band was also recorded from a small exposure [SJ 3260 6431] of dark grey, shaly mudstone with plant fragments and ironstone nodules, in a tributary of Broughton Brook ((Table 11), locality 20). To the north, in Hawarden Park, black mudstones, overlying a sequence of sandstones at a waterfall [SJ 3197 6519] in Broughton Brook (locality 13), have yielded a fauna ascribed to the mid ­Marsdenian B. bilinguis Marine Band (R2b2). The sandstones are inclined south-eastwards, and are bounded on the west by a fault; a small section upstream [SJ 3164 6541], in black mudstones, contains the younger, late Marsdenian, B. superbilinguis Marine Band (R2c1) ((Table 11), locality 15).

A borehole [SJ 3202 6572] north-west of Hawarden Castle, recorded a section through the upper Cefn-y-fedw Sandstone and a substantial part of the underlying lower tongue of Holywell Shales. A fauna ((Table 11), locality 8) recovered from the latter has been assigned a late Kinderscoutian R1c age.

Cefn-y-fedw Sandstone Formation

This formation comprises quartzose sandstone, pebbly sandstone and thin beds of quartz conglomerate, inter­bedded with subordinate mudstone, siltstone, cherty mudstone and glassy chert. It conformably overlies the Dinantian Minera Formation in the south of the district and rests locally on the Cefn Mawr Limestone Formation in the north. It intertongues with and onlaps the Pentre Chert Formation, and both intertongues with, and is conformably succeeded by, the Holywell Shales Formation (Figure 21). The incoming of the Cefn-y-fedw Sandstone marks the completion of a transition from the marine-influenced, carbonate-dominated cyclicity which prevailed during the late Dinantian to the fluviodeltaic, siliciclastic cyclicity of the Namurian. The formation is thickest in the south of the district, where it is up to 600 m thick and flanks the southern margin of the Flint coalfield. Its main crop extends northwards from the moorland east of Graianrhyd [SJ 2179 5612] to Pentre Halkyn [SJ 2023 7223]. Borehole data from the Little Mountain area indicate that the Cefn-y-fedw Sandstone crops out extensively beneath the thick drift deposits of the Alyn valley. This sub-drift crop links the well-exposed Cefn-y-fedw Sandstone sequences that crop out along the northern and eastern slopes of Hope Mountain [SJ 282 582] and the area around Caergwrle [SJ 3042 5758] with those on the eastern margin of the coalfield in the area south of Hawarden.

The term ‘Cefn-y-fedw Sandstone’ was introduced by Morton (1874–8) for the sequence of strata in the ­Llangollen area, lying between his ‘Upper Grey Limestone’ (Cefn Mawr Limestone) and the Coal Measures. It included a number of lithological divisions, notably a basal unit of ‘Sandy Limestone’ (equivalent to the Minera Formation of this account) and, at the top, the ‘Aqueduct Grit’, a local correlative of the Gwespyr Sandstone (see below). The Sandy Limestone was subsequently included within the ‘Carboniferous Limestone Series’ (Morton, 1882–4; Strahan, 1890; Wedd and King, 1924). The remaining part of the Cefn-y-fedw Sandstone was generally regarded as a local correlative of the ‘Millstone Grit Series’ of northern England (Strahan, 1890; Wedd and King, 1924; Wedd et al., 1927; Wood, 1936), despite the obvious compositional differences. Controversy surrounded its stratigraphical relationship with the Holywell Shales (reviewed by Jones and Lloyd, 1942), until ammonoid bio­stratigraphy demonstrated their equivalence (Wood, 1936). In this memoir, the Cefn-y-fedw Sandstone is given formational status and its boundaries are redefined. The new definition includes Morton’s ‘Lower and Middle Sandstone’ and ‘Dee Bridge Sandstone’ divisions, but excludes most of the upper part of his Cefn-y-fedw Sandstone sequence, which is here equated with the Holywell Shales and Gwespyr Sandstone formations. The type area of the Cefn-y-fedw Sandstone is Ruabon Mountain within the Wrexham district (Wedd et al., 1927).

The sandstones of the formation are white or pale grey, weathering to shades of yellowish or reddish brown, and are generally fine to medium grained and well sorted. The thickness of individual sandstone beds ranges from 0.02 to 2.5 m. Planar lamination, and low-angle, hummocky and trough cross-stratification, on a variety of scales, are a conspicuous feature of many beds, although others are poorly laminated and massive. Bedding contacts vary from planar to undulose, and are commonly marked by micaceous or carbonaceous partings. The bases of some beds are sharp and erosional. Locally, coarse-grained sandstones contain scattered quartz pebbles and angular clasts of mudstone and chert. Conglomeratic units occur either as thin lenses lining scour-and-fill structure, or as beds, up to 0.7 m thick, commonly with sharp erosive bases. Although they are dominantly quartzose, the sandstones contain minor amounts of feldspar, comminuted plant debris, and carbonate skeletal debris; locally, where the proportion of carbonate material increases, beds of sandstone pass laterally into sandy limestones (packstones and grainstones). The sandstones are variably cemented by carbonate and leaching of carbonate gives rise to weak and friable beds; moulds of leached bivalves and brachiopods occur in places. Bioturbation occurs at intervals throughout the sequence, and bedding surfaces locally display abundant Zoophycus and Rhizocorallium trace fossils.

The mudstones of the Cefn-y-fedw Sandstone are medium to dark grey, brownish grey or, in places, purplish grey and weather to shades of yellowish brown. They range from thin partings within sandstone beds to units up to several metres thick. Lithologically, they are similar to those of the Holywell Shales Formation (thicker mudstone sequences which are mappable are included in the latter formation) and also locally contain ammonoid-bearing marine bands. Sequences of cherty mudstone and subordinate glassy chert, up to 20 m thick, are widespread in the lower part of the formation. The lithological features of these units were described in detail by Wedd et al. (1927). They represent the ‘Cherty Shales’ of Morton (1882–4), though it is now clear that they occur at several discrete levels.

The lithologies of the formation are commonly arranged in minor cycles. At the base of a typical cycle is unfossiliferous mudstone (or cherty mudstone and chert), which coarsens upwards through a sequence of thinly bedded siltstone, hummocky and low-angle cross-laminated sandstone, into coarse-grained, pebbly, trough cross-bedded sandstone. Some cycles are capped by a channel-fill conglomerate, and in rare instances this is overlain by a seatearth and thin coal. Black shales with ammonoids are present only rarely in the basal part of the cycles. Many cycles in the Cefn-y-fedw Sandstone are dominated by sandstone. A number of cycles in the lower part of the formation (as in Rainbow Quarries [SJ 215 622]) contain a basal bed of pebbly, crinoidal sandstone or, locally, a sandy limestone akin to a Yoredale-type sequence (Duff et al., 1967). The cycle thicknesses range from around 20 m probably up to many tens of metres in thickness in areas of highest subsidence and sediment accumulation (see below).

In the south of the district, the Cefn-y-fedw Sandstone conformably overlies the Minera Formation. This boundary is taken at the top of the highest widely mappable limestone in the latter formation in any particular area. However, higher, impersistent limestone units, included in the Cefn-y-fedw Sandstone, demonstrate that the boundary is an arbitrary one in an essentially transitional sequence. This appears to be confirmed by cycle correlations in the Minera Formation, which suggest that the base may be taken above a younger sequence of limestones in the Hope Mountain area than along the central crop (Figure 19). Northwards from Gwernafield, the Cefn-y-fedw Sandstone rests on a progressively thinner Minera Formation sequence (Figure 18) and, in the vicinity of Moel y Gaer [SJ 2110 6903], comes to rest directly on the Cefn Mawr Limestone (Figure 20). The relationships strongly suggest the presence of a disconformity (Figure 20) and (Figure 21), and of overstep at the base of the Cefn-y-fedw Sandstone (compare Strahan, 1890), although internal attenuation and lateral facies changes within both the Dinantian and Namurian sequences may also play a part (see p.56 and p.63). North of Moel y Gaer, where the basal sandstone units die out, the formation passes into the Pentre Chert Formation.

The Dinantian/Namurian boundary has not been recognised in the district. The traditional correlation of the cherts and cherty mudstones in the lower part of the Cefn-y-fedw Sandstone in the south of the district with the Pentre Chert Formation to the north (Morton, 1882–4; Strahan, 1890) persuaded Ramsbottom (1974) that a considerable thickness of the sandstone sequence was Dinantian in age. However, as the Pentre Chert Formation is now considered to be largely or wholly of early Namurian (Pendleian) age it follows that the bulk of the Cefn-y-fedw Sandstone may also be Namurian. Only the lowest sandstones of the main crop, occurring below the thickest cherty horizons, may be Dinantian, and there is no biostratigraphical evidence to show that the base of the Namurian does not, in fact, lie within the underlying Minera Formation (Chapter 3).

The Cefn-y-fedw Sandstone is succeeded everywhere by the Holywell Shales but, in detail, the two formations intertongue and the boundary is highly diachronous. In the south, where the formation is thickest, dated sequences of Holywell Shales define at least three separate sandstone sequences, informally designated lower, middle and upper (tongues of) Cefn-y-fedw Sandstone (Figure 20a), (Figure 20b), (Figure 21). On Hope Mountain [SJ 282 582] in the extreme south of the district, the intervening tongues of Holywell Shales thin dramatically, and in the northern part of the adjacent Wrexham district, the three sandstone sequences merge into a single, thick sandstone succession (Hains, 1991; British Geological Survey, 1994). Here the formation is shown as undivided. The formation is also shown as undivided in this district, in places where poor exposure prevents the recognition of intervening Holywell Shales horizons.

Lower Cefn-y-fedw Sandstone

The lower Cefn-y-fedw Sandstone forms the bulk of the main crop of the formation bordering the Dinantian crop and includes all those parts in which mapped chert and limestone units occur. It ranges from Pendleian to earliest Arnsbergian in age. The division is up to 425 m thick in the south of the district, where it appears broadly to equate with Morton’s (1876–8) ‘Lower Sandstone and Conglomerate’, ‘Cherty Shale’ and the lower part of his ‘Middle Sandstone’ (part of the ‘Upper Sandstone’ of Morton, 1882–4). It thins eastwards to around 150 m on Hope Mountain. It also thins and splits northwards mirroring the attenuation in the underlying Minera Formation ((Figure 21); see below). A lower Cefn-y-fedw Sandstone sequence around 100 m thick crops out to the east of the Nercwys–Nant-figillt Fault Zone, in the Halkyn area, where it intertongues with, and appears to onlap, the Pentre Chert Formation. Representatives of the lower Cefn-y-fedw Sandstone were not proved in either the Abbey Mills boreholes or the Blacon East Borehole and it is probably absent from the northernmost and eastern parts of the district.

In the Rhydymwyn area [SJ 205 665], near the northern limit of the lower tongue, Strahan (1890, p.53) interpreted anecdotal data from underground mine workings as evidence of a local disconformity, in which the Holywell Shales overlapped the Cefn-y-fedw Sandstone and came to rest on the Minera Formation. However, the mudstones encountered in these workings, as illustrated by Smith (1921, fig. 10), are here interpreted as tongues of Holywell Shales interbedded with the lower Cefn-y-fedw Sandstone sequence. Near Hendre, some 30 m of Holywell Shales intertongue with the local sandstone sequence and succeed a basal calcareous sandstone bed which is less than 2.5 m thick (Strahan, 1890; Jones and Lloyd, 1942). Although recognised by Strahan (1890) as ‘Millstone Grit’, and still included here in the Cefn-y-fedw Sandstone, this bed could equally represent an earlier sandstone of the Minera Formation revealed by erosion beneath the sub-Namurian disconformity. At Moel-y-crio [SJ 1955 6979] the basal sandstone appears to have thickened to around 55 m. Here, its cherty nature persuaded Strahan (1890) of its equivalence to the nearby Pentre Cherts, but it is also locally calcareous and crinoidal, and includes a mappable limestone level; its distinction from the Minera Formation again remains uncertain.

In the north of the district, near Halkyn Mountain, the top of the lower Cefn-y-fedw Sandstone appears to equate approximately to the top of the Pentre Chert Formation, suggesting that it also predates the latest Pendleian. The top of the thick sequence of sandstones in the south of the district probably also lies within the Pendleian for it is succeeded, in the River Terrig, by a substantial thickness of unfossiliferous Holywell Shales lying below the early ­Arnsbergian, E. ferrimontanum (E2a2) Marine Band ((Figure 21); see p.74). East of the River Terrig section, the top of the lower Cefn-y-fedw Sandstone is defined by a thin unit of Arnsbergian Holywell Shales exposed on both the western [SJ 2840 5765] and the eastern [SJ 3012 5775] slopes of Hope Mountain (see p.74). The upper parts of the lower Cefn-y-fedw Sandstone in this area appear, at least in part, to be laterally equivalent to the unfossiliferous mudstone sequence seen in the River Terrig, 5 km to the west.

Middle Cefn-y-fedw Sandstone

A thick sequence of sandstones which overlies a thin, Arnsbergian tongue of Holywell Shales on Hope Mountain represents the middle Cefn-y-fedw Sandstone. It appears equivalent only to the upper part of Morton’s (1876–8) ‘Middle Sandstone’ and is overlain by a sequence of Holywell Shales of mid-Kinderscoutian (R1b) age (Figure 20), (Figure 21). The sandstone, therefore, appears to span the whole of the Chokerian and Alportian stages. The geographical distribution of the middle Cefn-y-fedw Sandstone is poorly known because, in practice, its distinction from the lower tongue of the formation rests heavily on the recognition of the intervening Arnsbergian mudstones. It forms much of the well exposed Cefn-y-fedw Sandstone sequence on the northern part of Hope Mountain, possibly exceeding 60 m in thickness; it may underlie much of the ground to the north and east. It is conjectured to overlie the dated Arnsbergian sequences of Holywell Shales at Hafod Abley and in the River Cegidog (p.74). Farther west, in the River Terrig, around 20 m of pebbly sandstones faulted against the Arnsbergian levels of the Holywell Shales may represent the middle Cefn-y-fedw Sandstone (Jones and Lloyd, 1942). In the north of the district, a thin sandstone exposed in a stream west of Coed-y-cra ((Figure 21)a), succeeded by mudstones containing an Alportian (H2b1) fauna (p.73; (Table 11)), is a distal correlative of the middle Cefn-y-fedw Sandstone. No equivalent sandstones are recorded in either the Abbey Mills boreholes or in the Blacon East Borehole.

Upper Cefn-y-fedw Sandstone

Sandstone sequences included in the upper Cefn-y-fedw Sandstone range from early Kinderscoutian to late ­Marsdenian in age. This division crops out in faulted ground between Nercwys [SJ 2332 6061] and Llanfynydd [SJ 2794 5660], and in the area between Hawarden [SJ 315 630] and Penyffordd [SJ 312 614]. It is also thought to underlie much of the broad, drift covered ridge of Lower Mountain, extending between Penyffordd and Hope (in association with the middle Cefn-y-fedw Sandstone). Thin correlative sandstones occur interbedded with the Holywell Shales in the northern part of the district, and in the Abbey Mills No. 4 Borehole. A sandstone sequence equivalent to the upper Cefn-y-fedw Sandstone was also proved in the Blacon East Borehole. The division appears to correlate with part of the ‘Upper Sandstone’ of Morton’s (1882–4) Graianrhyd section, and also appears broadly equivalent to his ‘Dee Bridge Sandstone’ (Morton, 1874–8). The sandstones of the upper Cefn-y-fedw Sandstone are finer grained, but otherwise comparable to those of the lower and middle Cefn-y-fedw Sandstone. Units of Holywell Shales, intertonguing with the upper Cefn-y-fedw Sandstone, appear to be a feature of the eastern outcrops of the division. They are particularly well developed in the Penyfordd area where locally they serve to subdivide the upper Cefn-y-fedw Sandstone into at least five separate mappable sandstone units. The presence of these mudstone intercalations may account for the lower relief of the upper Cefn-y-fedw Sandstone outcrops in the east, compared with those of the lower and middle divisions to the south and west.

In the Llangollen area of the Wrexham district, the base of Morton’s ‘Dee Bridge Sandstone’ lies around 6 m above black mudstones containing a lower Kinder­scoutian (R1a) fauna. The sandstone underlies mud­stones containing the early Yeadonian C. cancellatum Marine Band (Morton, 1878; Ramsbottom, 1974). In the Flint district, the upper Cefn-y-fedw Sandstone overlies Holywell Shales of upper Kinderscoutian (R1c) age in the Hawarden Borehole (Figure 20), (Figure 21); p.75). Its base may be slightly older in the south of the district, where it is conjectured to overlie the R1b faunas which succeed the middle Cefn-y-fedw Sandstone on Hope Mountain (see above). At its maximum development in the south of the district, the division is at least 100 m thick. Its upper levels intertongue with the Holywell Shales and the boundary is strongly diachronous. In the south-west of the district, in the River Terrig, the upper Cefn-y-fedw Sandstone is overlain by the basal Marsdenian B. gracilis Marine Band (R2a1) (Wood, 1936; Jones and Lloyd, 1942). Around Hawarden, in the east of the district, the top of the local upper Cefn-y-fedw Sandstone sequence is associated with either the later Marsdenian B. bilinguis (R2b2) or B. superbilinguis (R2c1) marine band (p.75). Farther south, in Warren Dingle, the highest levels of the upper Cefn-y-fedw Sandstone occur above the marine band. Here, the top of the uppermost sandstone lies some 20m below the basal Yeadonian C. cancellatum Marine Band (G1a).

Conditions of deposition

Sandstones capping the cycles of the Minera Formation reflect the marine reworking of the earliest delta sands into shoreface bars and beach deposits (Chapter 3). The Cefn-y-fedw Sandstone records the northward progradation of a fluviodeltaic facies over the entire former Dinantian carbonate platform, as a result of an ­increasing supply of terrigenous sediment to the region. Correlation of the lower beds of the Cefn-y-fedw Sandstone in the main crop, with the highest units of the Minera Formation on Hope Mountain, demonstrates that, in the latest Brigantian, fluviodeltaic and open marine carbonate facies co-existed.

The sedimentary structures in the upward-coarsening cycles of the Cefn-y-fedw Sandstone indicate the progradation and accretion of a complex of distributary channels and mouth bars over a sequence of interdistributary and prodelta deposits. The prodeltaic facies is represented by the unfossiliferous mudstones that commonly form the base of each cycle. Shallower water deposition, within a range of delta-front and marine shoreface environments with evidence of wave and current action, is indicated by tractional sedimentary structures in the succeeding thin sandstones and siltstones. The local presence of hummocky cross-stratification in these beds reflects periodic storm activity which reworked the delta front (Harms et al., 1975). The coarse sandstones in the upper parts of the cycles are interpreted as nearshore and distributary mouth bar sand bodies. Large-scale, tabular and trough cross-bedding in this facies show deposition by straight- and sinuous-crested megaripples. The coarse conglomerates were deposited by delta distributary channels. The seatearths and thin coals at the top of some cycles preserve evidence of delta-top swamp deposition.

The regional thickness variations in the Cefn-y-fedw Sandstone suggest that its deposition was largely centred on the Bala Lineament and that this complex fracture belt effectively controlled the course of the major fluvial system which discharged on to the Cefn-y-fedw Sandstone delta. Thickness variations across the lineament indicate that a depositional trough, inherited from the Dinantian, along the northern side of this structural belt, was the site of greatest sediment accumulation (Figure 21). The separate tongues of the Cefn-y-fedw Sandstone reflect periods of expansion of the delta northwards into the Flint district. Their distribution and geometry suggests that progradation was also influenced by fractures operating in concert with the Bala Lineament, as well as by the contemporary depositional relief. Evidence of activity on the Nercwys–Nant-figillt Fault Zone during the deposition of the lower Cefn-y-fedw Sandstone is shown by marked thickness and facies changes which occur across the structure (Figure 20), (Figure 21). The disconformity associated with this fracture belt to the north of Gwernafield (Figure 20) and (Figure 21) offers evidence of an upfaulted ridge or footwall, initially emergent, which impeded the northward spread of deltaic sand bodies throughout the Pendleian (see p.70). Early delta advance into the north-eastern part of the district was possibly restricted by the steepness and instability of submarine slopes associated with the former margin of the Dinantian carbonate platform (Chapter 3); the coeval facies in Blacon East Borehole (see below) reflect turbiditic prodelta ­deposition.

The focus of lower Cefn-y-fedw Sandstone deltaic deposition in the district evidently shifted south-eastwards during the late Pendleian to early Arnsbergian (E1b–2a) and was confined to a broad zone adjacent to the Bala Lineament (Figure 21). The sea floor relief established by the first phase of progradation may have influenced this change, which also appears related to a regional transgression that introduced relatively deeper water, marine to brackish conditions throughout most of the district (see above). The culmination of this event is marked by the Arnsbergian marine bands present within thin mudstone units on Hope Mountain and in the northern part of the Wrexham district which mark the most extensive marine incursions on to the delta. This contraction of deltaic deposition is not mirrored in other areas north of the Wales–Brabant Massif (Trewin and Holdsworth, 1973; Aitkenhead et al., 1992; Brandon et al., 1995), where the late Pendleian (E1c) was a period of major, northerly sourced, delta advance. This event in north Wales possibly marks a period of regional subsidence along the southern margin of the developing Pennine Basin complementary to sediment loading farther north.

The distribution of the middle tongue of Cefn-y-fedw Sandstone is poorly known. It appears to mark a period of re-advance of the delta from the south-west into the central parts of the district during late Arnsbergian to early Kinderscoutian times. Though its apparent westwards and eastwards thinning suggests that both the Nercwys–Nant-figillt Fault Zone and the Great Ewloe Fault (and its associated fractures) could have influenced its location, the sea floor relief generated in the west, during the earlier phase of progradation, was possibly the dominant factor. Marine transgression during the mid-Kinderscoutian appears to have terminated this phase of delta sand deposition, at least in the Hope Mountain area, where the Holywell Shales facies encroaches close to the Bala Lineament.

The upper Cefn-y-fedw Sandstone records the subsequent and final re-advance of the southerly sourced delta into the Flint district during the late Kinderscoutian and Marsdenian (R1c–2c). Net subsidence probably remained greatest in the south of the district, where the sandstone division is thickest; the Bala Lineament continued to influence the delta distributary system. The sandstones of the upper Cefn-y-fedw Sandstone, overlying basinal sequences in the Blacon East Borehole, indicate that earlier progradations had effectively nullified the bathymetric distinction between former platform and basin. Marine influences remained strong in the north of the district, where the Holywell Shales and its contained ammonoid horizons continued to accumulate (see above). Periodic, eustatically driven, marine inundations of the delta are reflected by marine bands within the upper sandstone sequence in the Leeswood No. 1 Borehole [SJ 2636 6180] and in Warren Dingle.

The final delta retreat began during the Marsdenian and, by the close of the stage, the influx of arenaceous material from the south had largely ceased. The diachronous nature of the top of the Cefn-y-fedw Sandstone points to a gradual southward retreat of the delta system, consistent with a steadily rising base level as the effects of thermal subsidence (Leeder, 1988), coupled with eustatic influences, exceeded the rate of sediment supply. It is likely that clastic input was reduced, as prolonged erosion lowered the relief of the Wales–Brabant Massif and rising base level further reduced the extent of the source area. The transgressions which introduced the B. gracilis (R2a1) and B. bilinguis (R2b2) marine bands contributed to this contraction of the delta, bringing extensive deltaic sand deposition to a close on the western and north-eastern sides of the system respectively. The B. superbilinguis (R2c1) Marine Band, widely recognised in both the Flint and Wrexham districts (Hains, 1991), records a subsequent transgression which probably drowned much of the delta top. However, the occurrence of younger Cefn-y-fedw Sandstone units south of Hawarden, and in the Wrexham district (Hains, 1991), suggests that a limited clastic supply was available prior to the widespread C. cancellatum Marine Band transgression (G1a), which appears finally to have terminated southern-sourced deltaic sedimentation in the district (Figure 20), (Figure 21); see below).

Details

Main crop

The lowermost beds of the Cefn-y-fedw Sandstone are exposed in crags on the hillside south and east of Graianrhyd [SJ 2179 5612], where they comprise flaggy and massive beds of pinkish brown weathering, pale grey, quartzose sandstones, with lenses and beds of pebbly sandstone and conglomerate. In Maes y Droell Silica Quarry [SJ 2183 5655], 400 m north of Graianrhyd, white and buff, cross-laminated, fine-grained, quartzose sandstones and siltstones are interbedded with cherty mudstones and white, vitreous cherts. The degree of silicification of the sandstones is variable; many horizons are soft and friable.

Beds in the lower Cefn-y-fedw Sandstone are exposed sporadically along the road from Graianrhyd to Rhyd-y-ceirw Bridge [SJ 2315 5621]. Small roadside quarries [SJ 2243 5616] to [SJ 2252 5619] and a contiguous stream section to the east display sequences of massive, coarse-grained, pebbly sandstones, thinly bedded cross-laminated and ripple marked, silicified sandstones with rare shell moulds, cherty siltstones and shales. Stream and roadside exposures [SJ 2270 5612] to [SJ 2282 5619] reveal cyclic alternations of these lithologies.

Crags on Nercwys Mountain, 2 km north of Graianrhyd, and disused quarries [SJ 2150 5920] to [SJ 2190 5905] within the forestry plantation provide numerous exposures through the lower Cefn-y-fedw Sandstone. The sections reveal well bedded, ­moderately well sorted, fine- to medium-grained, quartzose sandstones with horizons of pebbly sandstone and conglomerate, interbedded with siltstones and sandy mudstones; a few beds contain scattered shell debris. The sandstones commonly display planar lamination, low-angle and trough cross-lamination and, in places, ripple marks and graded bedding. Bedding surfaces are commonly intensely bioturbated.

The sequence of cross-laminated, predominantly fine-grained, quartzose sandstones exposed in the River Terrig, for a distance of 500 m upstream of the bridge [SJ 2342 5766] at Tryddyn-fechan, 2 km north-east of Graianrhyd, lies in the upper Cefn-y-fedw Sandstone The fault-bounded sequence of pebbly sandstones exposed in the river farther south, associated with Arnsbergian levels of the Holywell Shales (see p.74) may represent the middle Cefn-y-fedw Sandstone (Jones and Lloyd, 1942).

The Leeswood No. 1 Borehole [SJ 2636 6180] near Leeswood Old Hall proved the upper and lower (or middle) Cefn-y-fedw sandstones in faulted contact, overlying the Minera Formation. A fauna ((Table 11), locality 10), from a thin black mudstone within the upper Cefn-y-fedw Sandstone sequence, at a depth of about 29 m, is indicative of the B. gracilis Marine Band (R2a).

The lower Cefn-y-fedw Sandstone, represented by cross-stratified, coarse-grained, pebbly, quartzose sandstones, is further exposed in small quarries [SJ 2055 6101] to [SJ 2088 6057] on the south side of Moel Findeg [SJ 2077 6119]. Small crags and quarries along the crop from Moel Findeg to Gwernymynydd [SJ 2144 6241], provide more exposures of the lowermost beds, which comprise thick-bedded, bioturbated sandstones with leached shell moulds.

Immediately south of Gwernymynydd, up to 45 m of lower Cefn-y-fedw Sandstone overlying the Minera Formation are displayed in Rainbow Quarries [SJ 215 622]. A section in the eastern upper level revealed the highest beds of the Minera Formation overlain by the lowermost unit of the Cefn-y-fedw Sandstone. These strata are in turn overlain by the earliest chert horizons (Figure 19). Succeeding strata, exposed in crags and small pits [SJ 2167 6226] to [SJ 2205 6250] to [SJ 2212 6243] to [SJ 2229 6223] east of the village, include sequences of cross-bedded, pebbly sandstones, locally with erosional bases, interbedded siliceous shales and laminated, glassy cherts.

The basal sandstones are exposed between Gwernymynydd [SJ 2144 6241] and Gwernaffield [SJ 2065 6447], in a line of crags [SJ 2102 6281] to [SJ 2120 6320] and in disused quarries [SJ 2089 6384]. Sections in the quarries expose up to 7 m of variably cemented, massive and cross-bedded, granule-rich sandstones, fine-grained, well-cemented, ganister-like sandstones and interbedded thin, bluish green mudstones. Small quarries [SJ 2159 6288] to [SJ 2149 6307] to [SJ 2137 6311] to [SJ 2151 6365] and contiguous natural exposures north of Gwernymynydd display bioturbated, medium- to coarse-grained, pebbly sandstones, interbedded cherty shales and glassy cherts, in the higher parts of the division. Evidence for an early phase of silicification is supported by the occurrence, in places, of laminated chert and cherty mudstone clasts within thin beds of granule-rich sandstone. Thin coal horizons within these higher strata were previously recorded in the Tom and Jerry Shaft [SJ 2156 6251], and from an exposure [SJ 2161 6349] near Fron-heulog (Strahan, 1890).

Quarries [SJ 2067 6469] to [SJ 2071 6480] in Gwernaffield afford further exposures of the junction of the Cefn-y-fedw Sandstone and Minera Formation; small stone pits [SJ 2059 6506] to [SJ 2068 6503] north of the village reveal additional sections in the lowermost sandstones. The overlying strata are mostly drift-covered, but are exposed in a stream section [SJ 2138 6444] to [SJ 2149 6452] to the east of Gwernaffield.

Northwards along crop, the lower Cefn-y-fedw Sandstone is obscured by drift but it reappears, much reduced in thickness, on the limbs of a faulted, southward-plunging syncline in the vicinity of Moel-y-crio [SJ 1955 6979]. Here, the lowermost sandstones, dipping eastward, are exposed in roadside sections [SJ 1946 6890] to [SJ 1960 6910], in Bryn Gwyiog Quarries [SJ 1931 6929], and in numerous small pits [SJ 1918 6989] to [SJ 1913 6981] to [SJ 1913 6987] west of the village. South-east of Moel-y-crio, on the eastern limb of the syncline, a series of small quarries and crags [SJ 1973 6943] to [SJ 1992 6931] in the upper part of the succession reveal 7 m of silicified quartzose sandstones and interbedded siltstones.

Small quarries and pits around Moel y Gaer [SJ 2110 6906] and Rhosesmor [SJ 2148 6873] show numerous sections in the lower Cefn-y-fedw Sandstone and demonstrate overstep at its base. Up to 3.5 m of massive, thickly bedded, pebbly, quartzose sandstones overlie Minera Formation limestones in a quarry [SJ 2011 6913] west of Moel y Gaer. In quarries [SJ 2098 6910, 2124 6896], close to the summit of Moel y Gaer, the basal sandstones overlie crinoidal packstones and mudstones of the Cefn Mawr Limestone. The basal sandstones are interbedded with skeletal, peloidal packstone-grainstones, up to 2 m thick, in places wholly altered to chert; these are also well exposed in a quarry [SJ 2047 6916] west of the summit. The overlying strata, in small pits [SJ 2094 6897] to [SJ 2098 6893] between Moel y Gaer and Rhosesmor, include pervasively silicified sandstones and siltstones, with relict quartz, peloidal and skeletal grains. South of Rhosesmor, the lower division is mostly drift-covered, but sandstones from a high level within the sequence are exposed in a pit [SJ 2194 6697] east of Coed Bryn-gelli.

Although the upper Cefn-y-fedw Sandstone is generally absent from the area around Rhosesmor, it may be locally represented by a sandstone, exposed in a stream section [SJ 2095 6809] to [SJ 2091 6800] south-west of the village, underlying mudstones containing the C. cancellatum Marine Band ((Table 11), locality 17).

From Moel-y-Crio to Halkyn [SJ 2085 7075], the lower Cefn-y-fedw Sandstone is poorly exposed, but a quarry [SJ 2156 6980] by Pen-y-parc reveals 9 m of massive and well laminated, fine- to medium-grained, quartzose, locally pebbly sandstones, interbedded thin, laminated siltstones and carbonaceous mudstones.

Hope Mountain and Lower Mountain

Massive, low-angle, cross-stratified, pebbly sandstones and thinly bedded, bioturbated sandstones, close to the base of the Cefn-y-fedw Sandstone, are exposed in small crags near the summit of Hope Mountain [SJ 2896 5707] and at Fron Farm [SJ 2838 5710]. On the eastern side of the mountain, the base of the formation is represented by 3.5 m of silicified sandstones and cherty shales, with digitate chert layers, exposed in small pits [SJ 2956 5722] north-east of Coed-mawr-ucha Farm. The overlying pebbly sandstones form strong features and crags on the slopes above Caergwrle [SJ 305 575].

Exposures in the middle Cefn-y-fedw Sandstone are provided by crags and quarries [SJ 2844 5792] to [SJ 2807 5853] in the Country Park north-west of Horeb [SJ 2887 5788]. Sections reveal sequences, up to 10 m thick, of trough cross-bedded, quartzose sandstones with horizons of channel-fill conglomerate; large fragments of Lepidodendron occur in the sandstones, and chert nodules are developed locally. Further exposures in these sandstones occur in a large quarry [SJ 2976 5780] north-east of Hope Mountain, and Crowndale Quarry [SJ 2807 5600].

Quartzitic sandstones encountered below thick drift deposits in boreholes [SJ 3075 5957] to [SJ 3115 5960] to [SJ 3159 5955] on Lower Mountain are probably within the upper Cefn-y-fedw Sandstone. They provide evidence that a substantial, drift-concealed outcrop of the formation is present in this tract.

Hawarden to Higher Kinnerton

The principal sections of the upper Cefn-y-fedw Sandstone, in this area, are those of the A55(T) Hawarden By-pass. The base of this division occurs in weathered sections [SJ 3090 6388], [SJ 3102 6376] at Stony Hill, where purplish red mottled, dark grey, locally sulphurous mudstones are overlain by parallel- and cross-laminated, fine-grained sandstones. The main part of the division, exposed in a cutting [SJ 3120 6359] to [SJ 3170 6339], comprises up to 55 m of interbedded, fine-grained, quartzose sandstones and grey mudstones with purplish red mottling. The sandstones range from massive, thick beds to thinner bedded with common parallel lamination and local ripple cross-lamination. Local channels and rootlets affect the upper surfaces of some beds. A coarse-grained sandstone, underlying a sequence of Holywell Shales with the B. superbilinguis Marine Band, is exposed in Warren Dingle [SJ 3131 6248] south of the by-pass (see p.75). Younger sandstones, underlying the C. cancellatum Marine Band, are exposed downstream (Figure 22b).

Other exposures in strata close to the base of the upper Cefn-y-fedw Sandstone are provided by quarries [SJ 3056 6296] to [SJ 3060 6312] to [SJ 3093 6354] to [SJ 3111 6385] and contiguous crags along the scarp between Penyffordd and Stony Hill. Quarries [SJ 3124 6493] to [SJ 3133 6493] in Tinkers Dale reveal sequences of fine-grained, quartzose sandstones, cross-bedded and convolute-bedded in places, and with mudstone partings, representative of the middle part of the division. The highest sandstones are also exposed in Broughton Brook [SJ 3157 6547], where they underlie mudstones containing the B. superbilinguis Marine Band.

Gwespyr Sandstone Formation

The Gwespyr Sandstone Formation comprises a succession of thickly bedded feldspathic sandstones, with subordinate argillaceous sandstones, sequences of thinly interbedded sandstone and mudstone, and, in places, thick units of mudstone. It is of late Namurian (Yeadonian) to early Westphalian (early Langsettian) age and crops out on the margins of, and within, the Flintshire Coalfield.

The name was first applied by Strahan (1885) to the thick sequence of sandstones in the former Gwespyr Quarries, in the adjacent Liverpool district. The sandstones were proved subsequently in the Abbey Mills No. 1 Borehole, overlying a sequence of Holywell Shales (Strahan, 1919; Wedd et al., 1923), but considerable confusion has arisen over their precise stratigraphical position. Previous workers employed definitions based on biostratigraphical, rather than lithostratigraphical criteria (Wood, 1936; Jones and Lloyd, 1942; Ramsbottom, 1974); thus, Jones and Lloyd identified an Upper and Lower Gwespyr Sandstone, separated by the basal Westphalian Subcrenatum Marine Band (G2a), but included similar sandstones, underlying the C. cumbriense Marine Band, within the Holywell Shales (Figure 20a),(Figure 20b). In this memoir, the term Gwespyr Sandstone has been retained and given formational status, but its boundaries have been substantially revised. The type section for the formation is the Gwespyr Quarries [SJ 1093 8307] (Strahan, 1885), but important reference sections through the upper and lower parts of the formation are provided by the Nant Felin Blwm stream section [SJ 1384 8090] to [SJ 1397 8175] near Mostyn (Shanklin, 1956) and the Abbey Mills No. 1 Borehole [SJ 1949 7757], also in the Liverpool district.

The base of the Gwespyr Sandstone is here defined at the first appearance of feldspathic sandstones in the district. Over much of the district, the boundary is gradational and characterised by alternations of feldspathic sandstone, sandy shale and mudstone. In the northern and central parts of the district, it overlies Holywell Shales containing the C. cancellatum Marine Band, and was recorded 20 m above the B. superbilinguis Marine Band in Abbey Mills No. 1 Borehole (Figure 20), (Figure 21). In sections [SJ 2330 5798] to [SJ 2622 5694] in the rivers Terrig and Cegidog, in the south of the district, the formational boundary overlies the C. cumbriense Marine Band. A similar relationship occurs in the Wrexham district, where this marine band is succeeded by the feldspathic Aqueduct Grit (Morton, 1874–8), the local correlative of the Gwespyr Sandstone (Figure 20a)(Figure 20b); Wood, 1936).

The top of the Gwespyr Sandstone was formerly taken below a thin coal, variously called the Little or Chwarelau Seam, above which the Listeri Marine Band (G2b) is found (Shanklin, 1956; Calver and Smith, 1974). Unfortunately, the Chwarelau Seam is poorly developed in the Flint district, and Westphalian marine bands are only sporadically recorded; consequently, the recognition of an upper formational boundary based on these criteria is difficult. Moreover, the lower Westphalian is characterised by thick sequences of variably feldspathic sandstone. For con­venience, the formational boundary is here taken below the Llwyneinion Half Yard Seam (see below), the lowermost coal that can be traced with any confidence across the district. More precisely, it is taken below the few metres of mudstone which underlie this seam. An artefact of this new definition is that mudstone sequences intercalated and locally mapped within the crop of the Gwespyr Sandstone, that predate the Subcrenatum Marine Band, are regarded as intertonguing units of Holywell Shales, whereas those which occur above the marine band are included in the Lower Coal Measures (Figure 20), (Figure 21).

The main outcrops of the Gwespyr Sandstone are severely disrupted by faulting and extend from near Bagillt [SJ 225 750] in the north of the district to Llanfynydd in the south, and from Connah’s Quay [SJ 294 697] to Penyffordd [SJ 300 620]. Large crops also occur between Oakenholt [SJ 263 716] and Ewloe Green [SJ 2871 6649], north of New Brighton [SJ 2540 6545], and from Prenbrigog [SJ 2660 6396] to Leeswood Hall [SJ 2526 6132]. Drift deposits cover much of the crop and the district lacks a complete section through the formation. In the north of the district, boreholes at Abbey Mills and Oakenholt Paper Mill, which together proved the succession above and below the Subcrenatum Marine Band, indicate a thickness of up to 260 m. In the south, thickness estimates based on outcrop measurements between the C. cumbriense Marine Band and the Llwyneinion Half Yard Seam in the Cegidog valley, range from 55 to 75 m.

The sandstones are generally brown and buff, fine-grained, variably feldspathic and micaceous, in alternating flaggy and massive beds, locally up to 5 m thick. They are interbedded with pale grey and buff, thinly laminated siltstones. Carbonaceous detritus is scattered throughout many beds and, in places, well preserved plant fragments are found. Bedding surfaces are commonly defined by carbonaceous or micaceous partings and are locally intensely bioturbated. A common feature of the formation is an abundance of small-scale cross-lamination directed towards the south. Low-angle and hummocky cross-stratification occurs locally, and some units are characterised by large-scale tabular and trough cross-bedding. Flaser-bedded units are recorded in places throughout the formation and a few beds display convolute lamination. Scour-and-fill structures occur locally, lined with coarse, pebbly sandstone or conglomerate containing quartz, igneous and intraformational clasts. Larger-scale channels are recognised in places, by erosional surfaces with bedding cut-outs.

The mudstones range from thin partings between sandstone beds to units up to 15 m thick. They are generally grey, shaly, silty, finely micaceous and, locally, rich in comminuted plant detritus. Coarsely micaceous and sandy shales, transitional with argillaceous sandstones, occur at intervals. The mudstones are generally unfossiliferous, but black carbonaceous shales containing the Subcrenatum Marine Band, with a mixed fauna of ammonoids, bivalves, brachiopods, orthocones and fish remains, was proved 170 m above the base of the formation in Abbey Mills No. 1 Borehole, and at 80 m depth in the Oakenholt Paper Mill Borehole ((Table 11), locality 24). The Listeri Marine Band, containing a comparable fauna, approximately 27 m above the Subcrenatum horizon was proved in a trial pit [SJ 2817 6916] near Connah’s Quay ((Table 11), locality 25) (Shanklin, 1956).

Thin seatearths with sideritised rootlets and horizons of siderite nodules are developed in places within the Gwespyr Sandstone. Coal seams are generally thin and impersistent, and represented in places by little more than a carbonaceous parting. However, the Chwarelau Seam, up to 0.9 m thick in Abbey Mills No. 1 Borehole, has been recognised locally in this district, and a thin coal is present in places beneath the Subcrenatum Marine Band.

Conditions of deposition

The Gwespyr Sandstone marks the first major incursion of northerly sourced deltaic sediments into north Wales, at a time of waning detrital input from the southerly sourced Cefn-y-fedw Sandstone delta system. The feldspathic sandstones are indicative of an igneous or metamorphic source, probably of Archaean and Caledonian rocks (Collinson, 1988; Drewery et al., 1987). Their finer grain size and lithological variation compared with the typical ‘Millstone Grit’ of the Central Province reflect a relatively greater distance from source. The sandstones represent stacked sequences of mouth bar and distributary channel complexes, developed in lower delta plain and delta front environments and, like the Cefn-y-fedw Sandstone, contain a variety of tractional sedimentary structures recording evidence of wave and tidal current activity. The intertonguing mudstones (included in either the Holywell Shales or Lower Coal Measures) represent the associated delta plain overbank, lagoonal and interdistributary bay muds, as well as prodelta deposits.

At this time, a delta plain was rapidly established in the district. Marine influences were apparently restricted because the Subcrenatum and Listeri marine bands preserve the only evidence of widespread marine inundation. Thin coals such as the Chwarelau Seam formed in delta-top swamps. Deeper bathymetry, allied to the relatively high compaction rates of the underlying Holywell Shales, confined the early advance of the delta lobes to the northern part of the district (Figure 20), (Figure 21). Progressive southward advance of the delta continued into early Westphalian times.

Details

West of the Flintshire Coalfield

The most northerly exposures of Gwespyr Sandstone occur between Pentre Halkyn [SJ 205 725] and Flint [SJ 240 726]. In a road cutting [SJ 2157 7256] to [SJ 2168 7260] east of Pentre Halkyn, the base of the formation is exposed, overlying a 34 m-thick sequence of Holywell Shales. The Gwespyr Sandstone comprises 18 m of feldspathic, ripple cross-laminated, fine-grained sandstones in beds up to 5 m thick and interbedded with laminated silty mudstones. Up to 2 m of coarser grained sandstones with vein quartz and igneous pebbles are exposed in old quarries [SJ 2194 7224] south of the road. Further sections through the basal sandstones and mudstones are afforded by crags [SJ 2294 7118] and a roadside exposure [SJ 2300 7089] near Coed-y-cra Farm (Figure 22a).

To the south, the lower part of the formation crops out between Plâs-newydd [SJ 2261 6981] and Gwysaney Hall [SJ 227 667]. Good examples of large-scale, tabular and trough cross-bedded sandstones rich in plant detritus, and interbedded with massive sandstones, are exposed in a series of small pits and quarries [SJ 2264 6908] to [SJ 2283 6844] to [SJ 2288 6845] to [SJ 2307 6718] and stream sections [SJ 2298 6873] to [SJ 2295 6725].

Between Rhosesmor and Rhydymwyn the formation is poorly exposed, but a quarry face [SJ 2070 6778] at the disused Ruby Brickworks provided a 6.5 m-thick section of beds near the base of the formation including conglomeratic units with intraclasts of siltstone and sandstone. Other exposures at this level, in excavations [SJ 2093 6572] behind buildings and in a quarry [SJ 2134 6549] south of Rhydymwyn, showed up to 3 m of thinly bedded, feldspathic sandstones with channel structures and abundant plant debris.

Between Rhydymwyn and Nercwys, the formation is mostly drift-covered. However, the River Terrig [SJ 2438 5940] to [SJ 2444 6076] south and east of Nercwys, affords extensive exposures of yellowish brown, cross-bedded, coarse-grained and locally argillaceous sandstone with partings of mudstone. Similar sequences are exposed in a section [SJ 2625 5696] to [SJ 2664 5716] in the River Cegidog.

Flintshire Coalfield

The upper part of the Gwespyr Sandstone crops out south and east of Oakenholt and comprises cross-bedded and ripple marked, fine-grained sandstone and channelised, pebbly sandstone, with thick sequences of interbedded mudstones. Similar strata were proved in the Oakenholt Paper Mills Borehole down to a level below the Subcrenatum Marine Band. Stream sections [SJ 2643 7069] to [SJ 2678 7096] south of Oakenholt Hall, in Top-y-fron Dingle [SJ 2720 6966] to [SJ 2725 6994], and Kelsterton Brook [SJ 2752 6921] to [SJ 2763 6944] near Wern Ddu provide exposures through these higher strata. Flaser and convolute-laminated sandstones occur in a small dingle [SJ 2741 6966] to [SJ 2744 6971] south-east of Top-y-fron Farm. Other sections, in cliffs [SJ 2722 7130] to [SJ 2902 7010] along the former shoreline of the Dee estuary, reveal sequences of feldspathic sandstones up to 10 m thick.

A section through a faulted sequence of very feldspathic, thickly bedded and massive sandstones, with evidence of channelling, is exposed in faulted contact with Holywell Shales in Wepre Brook and contiguous cliff sections [SJ 2880 6759] to [SJ 2832 6752] by Northop Hall. Further exposures, of up to 4 m of tabular cross-bedded feldspathic sandstones occur in old quarries and a track section [SJ 2508 6714] to [SJ 2512 6707] south of Northop.

Between Buckley and Leeswood, the Gwespyr Sandstone is poorly exposed, but a now degraded quarry [SJ 2601 6276] north of Llong previously exposed 6.5 m of fine-grained, channelled and ripple cross-laminated feldspathic sandstones affected by a major north-west-trending fault.

Hawarden to Higher Kinnerton

The most northerly exposures of the Gwespyr Sandstone Formation in this area occur in small quarries [SJ 3141 6536] to [SJ 3131 6533] to [SJ 3129 6520] south of Hawarden. These reveal up to 4 m of cross-laminated, micaceous and fine-grained, feldspathic ­sandstones. A section in Broughton Brook [SJ 3071 6449] to [SJ 3062 6443] displays 3.7 m of thickly bedded sandstones, with southward-directed cross-lamination.

Other sections in Broughton Brook [SJ 3244 6486] and its tributaries [SJ 3212 6433] to [SJ 3281 6433] south-east of Hawarden provide small exposures through the lowermost beds of the Gwespyr Sandstone, interbedded with thick mudstones. A more complete section through this part of the sequence, including the Holywell Shales and highest beds of the upper Cefn-y-fedw Sandstone, is afforded by Warren Dingle [SJ 3280 6214] to [SJ 3139 6246]. Here, a 70 m-thick sequence of interbedded mudstones (Holywell Shales) and thin feldspathic sandstones (Gwespyr Sandstone) succeeds fault-repeated exposures of the C. cancellatum Marine Band ((Figure 22b); see above). The C. cumbriense Marine Band occurs [SJ 3227 6232] immediately beneath a thin horizon of argillaceous limestone ((Table 11), locality 22).

Namurian rocks beneath the Cheshire Basin

Deep boreholes on the Cheshire plain have proved a Namurian succession underlying variable thicknesses of Permo–Triassic and Westphalian rocks (Earp and Taylor, 1986). Geophysical logs of the Blacon East and Blacon West boreholes, together with limited lithostratigraphical and biostratigraphical data, enable broad lithological ­correlations to be made with the sequence at crop (see Evans et al., in press). The Gwespyr Sandstone, Holywell Shales and upper Cefn-y-fedw Sandstone are represented in a sequence underlying the Llwyneinion Half Yard Coal. The Subcrenatum and C. cumbriense marine bands are recognised by typical signatures on the geophysical (gamma-ray) logs. A thick, upward-coarsening sandstone, with a base at 1036.5 m depth in Blacon East Borehole, represents the lowermost unit of the upper Cefn-y-fedw Sandstone.

In the deeper Blacon East Borehole, a thick, relatively uniform sequence of silty mudstones with thin limestones and sandstones is recorded beneath the upper Cefn-y-fedw Sandstone between 1036.5 and 1866.5 m depth. The lower and middle Cefn-y-fedw sandstones are absent, as is the Pentre Chert Formation. The thickness (830 m) and general uniformity of the sequence on geophysical logs suggest that a basinal facies is present, similar to the underlying Dinantian rocks (Chapter 3). Strata dated as mid-Brigantian (P2a) (late Dinantian) occur below 1866.5 m, so that the Dinantian/Namurian boundary is likely to lie within this uniform mudstone succession. In the local context the sequence may be regarded as a distal facies of the Holywell Shales, but it perhaps compares more closely with early Namurian prodeltaic turbiditic sequences developed elsewhere along the southern margin of the Pennine Basin (Trewin and Holdsworth, 1973; Aitkenhead, 1977) and with the Bowland Shales of the basin centre (for example Aitkenhead et al., 1992). High rates of sedimentation in a prodeltaic setting may account for the absence of Pentre Chert facies from the borehole sequence.

Chapter 5 Silesian: Westphalian

Westphalian rocks, comprising both coal-bearing strata and red beds, form the major part of the Flintshire Coalfield and, south of the Bala Lineament, the northernmost part of the Denbighshire Coalfield (Figure 23). They also crop out in the Vale of Clwyd, and to the north of the Dee estuary, in the southern part of the Neston (Wirral) Coalfield, an outcrop contiguous with the Flintshire Coalfield. They range in age from Langsettian (Westphalian A) to Westphalian D ((Table 12)). A brief account of the history and economic aspects of coal mining in the district is provided in Chapter 10.

Since the 19th century, ‘coal measure’ successions in the British Isles have been traditionally subdivided into Lower, Middle and Upper Coal Measures (Hull, 1860). Recognition of these divisions in separate coalfields was based on variations in gross lithology and on attempted correlations of individual coal seams, or groups of seams. Early workers in the Flintshire and Denbighshire coalfields (Strahan, 1890; Wedd et al., 1923; Wedd and King, 1924; Wedd et al., 1928) sought correlation with the then well documented Lancashire and Staffordshire successions, but the lack of biostratigraphical control led to major correlation problems; these were reviewed by Jones and Lloyd (1942). The Holywell Shales and Gwespyr Sandstone were erroneously included as ‘Lower Coal Measures’ (see Chapter Four), and the ‘Buckley Fireclay Group’ which included barren red measures, was incorporated in the otherwise grey, productive ‘Middle Coal Measures’; the remaining red measures of the Flint district were assigned to the ‘Upper Coal Measures’ ((Table 12)). Since the war, the subdivision and correlation of the British ‘Coal Measures’ has relied more on the identification of marine band faunas (Stubblefield and Trotter, 1957; Calver, 1969; Ramsbottom et al., 1978). Some of the most widespread marine bands are now used as de facto marker beds to define a revised lower, middle and upper formational scheme for the Coal Measures which, though only loosely lithostratigraphical, enables accurate correlations and comparisons to be made between coalfields.

The Westphalian succession of the district, about 1000 m thick, has been subdivided into the Coal Measures Group and an overlying Red Measures Group (Table 12). The Coal Measures Group, previously known as the Grey Measures (Calver and Smith, 1974), Productive Coal Measures (Campbell and Hains, 1988) or Bettisfield Formation (Hains 1991), comprises a fluviodeltaic sequence of predominantly grey mudstone with subordinate sandstone, and includes locally thick coal seams and associated seatearths. Although marine bands are rare in the Flintshire and Denbighshire coalfields, the critical ones have been identified (Calver and Smith, 1974), enabling the Coal Measures Group to be ­subdivided into the Lower Coal Measures and the Middle Coal Measures, in line with other coalfields surveyed in the late 20th century (Table 12). The Red Measures Group comprises red mudstone with subordinate sandstone and generally lacks coals. This is equivalent to the upper part of the Middle Coal Measures and to the Upper Coal Measures of other coalfields. It is divided into two red bed formations, the Ruabon Marl and Erbistock formations, separated by the Coed-yr-allt Formation, a unit of mainly grey measures with inferior coals. The formational names are derived from the Wrexham district (Wedd et al., 1928; Hains, 1991) (Table 12).

The base of the Westphalian is generally defined by the base of the Subcrenatum Marine Band and, in many British coalfields, this is also taken as the junction between the Millstone Grit and Coal Measures groups (for example Woodlands and Evans, 1964). However, across much of the Flint district, this marine band is either absent or has not been recognised, and the equivalent stratigraphical level lies within undivided sequences of the Gwespyr Sandstone Formation, in the upper part of the local Millstone Grit Group (Chapter 4). Therefore, across much of the district, the local base of the Coal Measures Group is taken instead at the base of a thin (up to 10 m) sequence of mudstones that overlies the Gwespyr Sandstone and underlies the widely recognised Llwyneinion Half Yard coal seam. The Subcrenatum Marine Band was proved in the Oakenholt Paper Mills ‘A’ Borehole and the mapped units of mudstone, within the local Gwespyr Sandstone crop that are inferred to overlie this marine marker, are depicted on the map as intertonguing sequences of Coal Measures (Table 12). However, for convenience, these strata are described together with the enveloping sandstone formation in Chapter 4 (see p.81–82).

The Westphalian outcrop is extensively drift-covered, and details of the succession are based mainly on information provided by colliery shaft sinkings, supplemented by opencast coal exploration and site investigations; opencast workings and mine abandonment plans provided valuable information on the position of coal seams. Much of the mining took place during the 18th and 19th centuries, and had largely ceased by the time of the 1890 geological survey. Thus, for certain areas of the coalfields, records of mined seams are scarce and, of those that exist, many are of doubtful accuracy. Invariably, marine bands were not recorded and their position can be inferred only from their associated coals. Unfortunately, correlation of coal seams is hindered by the fact that seams commonly have different names in different pits or areas, and even seams with similar names do not necessarily correlate across the coalfields. In order to correlate the coals, the former British Coal attempted to standardise the names of the local seams, and it is that scheme, updated and with some modifications, which is followed in this account (Table 13). Marine band nomenclature is that of the standard British classi­fication (Ramsbottom et al., 1978). Local names for both coal seams and marine bands are given in brackets where appropriate.

In the following account and in (Figure 24), (Figure 25), (Figure 26), (Figure 27), (Figure 28), (Figure 29), (Figure 30), shaft positions have been derived from a variety of sources including Coal Authority records and both published and unpublished BGS documents.

Biostratigraphy

Marine bands are the principal means of correlation in the Westphalian, allowing precise chronostratigraphical subdivision of the series into four stages (Ramsbottom et al., 1978; Riley et al., 1983) (Table 12). The diagnostic ammonoid species of these marine bands are generally of more restricted distribution than those of the Namurian, and the main marine bands are commonly recognised by biofacies representing only part of the transgressive pulse (Calver, 1968, 1969) (see p.84). In many cases, the marine horizon contains assemblages belong­ing to the Lingula biofacies, recording very shallow water environments at the limits of transgression, and reduced salinities due to dilution by fresh water. Nonmarine bivalves, adapted to brackish water conditions, become increasingly abundant and widespread from the upper Namurian onwards, complementary to the gradual decline in ammonoid biofacies. They are also utilised for correlation, commonly occurring as ‘mussel bands’ in the roof measures of coal seams (Calver, 1969; Ramsbottom et al., 1978).

The biostratigraphy of the north Wales coalfields was summarised by Calver and Smith (1974). The only marine horizons recorded in this part of the Flintshire Coalfield are the Subcrenatum and Listeri marine bands, and the Maltby (Powell) Marine Band, represented by a mixed marine/nonmarine fauna in the roof measures of the Powell Coal at Tan Lan opencast site [SJ 262 577] (Magraw and Calver, 1960). The Vanderbeckei (Llay) Marine Band, which defines the base of the Middle Coal Measures, has also been recorded in the south of the district, in the Denbighshire Coalfield, in the roof measures of the Upper Red (Durbog; Red) seam (Magraw and Calver, 1960). It was also recognised in Point of Ayr Colliery in the Liverpool district together with additional marine horizons that have not been proved in the Flint district, but were recorded to the south, in the Wrexham district. The Aegiranum (Warras) Marine Band, defining the base of the Bolsovian Stage, occurs in the roof of the Warras Coal in the Wrexham district (Magraw and Calver, 1960), but the laterally equivalent strata in the Flint district are commonly represented by barren facies of the Red Measures Group (Ruabon Marl Formation). Other marine bands only recorded in the adjacent districts are the Clown(e) (Gardden Lodge) and Haughton (Lower Stinking) horizons of Duckmantian age, overlying the Drowsell and Lower Stinking coals respectively, and the Edmondia (Ty-cerrig) horizon of Bolsovian age, overlying the Upper Stinking seam (Magraw and Calver, 1960; Calver and Smith, 1974; Ramsbottom et al, 1978). ‘Mussel bands’ present in the group, recorded at Point of Ayr and in the Denbighshire and Neston coalfields, have provided assemblages diagnostic of the Carbonicola communis, Anthraconauta modiolaris and Lower similis-pulchrabiozones ((Table 12); Wood, 1937; Calver and Smith, 1974). The Aegiranum (Warras) Marine Band is conventionally taken as the base of the Upper similis-pulchra Biozone.

Nonmarine bivalves, ostracodes and fish, recovered from the Coed-yr-allt Formation in the Wrexham district, indicate that this division and, by association, the overlying Erbistock Formation, are Westphalian D in age (Calver and Smith, 1974). Bivalves, crustacea and miospores recorded in strata comparable to the Coed-y-allt Formation in boreholes in the Vale of Clwyd, in the Denbigh district, suggest a similar age (Warren et al., 1986).

Coal Measures Group

The Coal Measures of the district include the sequence of productive coal-bearing strata which lie between the Subcrenatum Marine Band (or its equivalent level) and the overlying Red Measures (but see p.81). The group is divided into two formations, the Lower Coal Measures of Langsettian age, and the Middle Coal Measures, of Duckmantian to lower Bolsovian age; the base of the latter is taken at the base of the Vanderbeckei Marine Band or its equivalent horizon (Figure 24).

The group comprises mudstones with subordinate sandstones, siltstones and coal seams, the last commonly overlying seatearths of either mudstone or ganisteroid sandstone. Impersistent beds of sideritic ironstone are also present. In contrast with the reddened and variegated strata of the overlying Red Measures, the Coal Measures strata are mainly grey or brownish grey in colour. Finer grained and more carbonaceous lithologies are commonly dark grey or black; sandstones are weathered yellowish brown and buff.

The mudstones are typically silty and finely micace­ous, but sandy shales occur in places and thick sequences of thinly interbedded mudstone, siltstone and sandstone have been recorded. The terms ‘striped’, ‘rock binds’, ‘faikes’, ‘linn and wool’ or ‘linstry’ were commonly used for these mixed lithologies in shaft sections, and it is usually impractical to differentiate between them. Highly carbonaceous mud­stones also occur in the succession, usually in association with coal, or as partings within seams. Thinly laminated mudstone alternates with homogeneous, blocky mudstone throughout the sequence; the latter may take the form of seatearths, within which the rootlets (rhizoliths) are locally preserved as siderite. Larger nodules or tabular beds of siderite have been recorded at certain horizons, but they represent a minor component of the group.

The sandstones are mainly fine to medium grained, variably feldspathic and micaceous, but locally quartz­ose. They occur in lenticular, massive or cross-bedded and laminated units up to several metres in thickness; coarse-grained and pebbly beds are only rarely recorded. ­Comminuted plant debris is scattered throughout many beds, and is locally concentrated along bedding surfaces. A number of sandstones display ripple-drift lamination.

Several important, widely correlatable sandstone units are recognised. They are thickly developed in a tract occupying the central part of the Flintshire Coalfield, from Coed-talon [SJ 2688 5895] northwards to the Northop Hall and Ewloe areas [SJ 2695 6780] to [SJ 2944 6656] where, in places, they ‘wash out’ several of the important coal seams. They include the Yard Rock and Main Rock, respectively overlying the Ruabon Yard and Main coals, and the Hollin Rock which locally washes out the Drowsell, Powell and Hollin seams. The Yard Rock provides the most striking example of an individual ‘wash out’, locally replacing its underlying coal for a distance of 8 km between the Leeswood and Buckley areas and for a width of up to 550 m at Coed-talon (Wedd and King, 1924). Other sandstones, which are thinner and less extensive, are commonly named after the coals which underlie them (as for the Crank Rock). The Cefn Rock is a major sandstone in the Denbighshire Coalfield.

Coal seams occur throughout the group (Figure 24). The lowermost, widely recognised seam is the Llwyneinion Half Yard. Above this, the principal seams and groups of seams included within the Lower Coal Measures are, in ascending order, the Premier, Ruabon Yard, Nant, Fireclay Group (various named seams), Lower Red and the Upper Red. The Middle Coal Measures include the Main Coal, the thickest and most extensively worked seam in the district and an important marker bed. Important Middle Coal Measure seams below the Main Coal include the Crown and Lower Bench. Above the Main Coal, the Black Bed, Quaker, Crank, Hollin and the Powell Group (Powell and Drowsell coals) have all been worked extensively in the Flintshire Coalfield. A series of younger seams are developed in the uppermost part of the Middle Coal Measures sequence in the Leeswood area. They include the Tryddyn Half Yard, the Pontybodkin Divided, and the possibly Bolsovian Pontybodkin Mountain and Upper Main seams. These seams occur in strata transitional with, and locally included in the succeeding Red Measures (Ruabon Marl Formation). However, immediately south of the Bala Lineament, in the Denbighshire Coalfield, upper Duckmantian coals including the Smith’s, Lower Stinking, John O’ Gate and Warras, as well as the lower Bolsovian Cannel, Upper Stinkingand Gwersyllt Little seams and the Bersham Yard Group (Alpha to Delta seams) are present within a greatly expanded Middle Coal Measures sequence (Figure 24).

Regional thickness variations

The Coal Measures above the Gwespyr Sandstone are between 300 and 350 m thick in the Flintshire Coalfield and up to 820 m thick in the northern part of the Denbighshire Coalfield. These variations are due mainly to the diachronous and locally unconformable nature of the base of the overlying Red Measures, and are particularly marked across the major fault zones. There are also significant thickness variations within the group, both in the coal seams and the interseam measures, which are due to irregular subsidence patterns within the coalfield. These differences are super­imposed on an overall pattern of regional subsidence increasing towards the centre of the main Pennine Basin in south Lancashire. This regional trend is reflected in the general northward increase in the overall thickness of strata between the Gwespyr Sandstone and the Main Coal in the Flintshire Coalfield. They range from approximately 120 m in the south of the district, to 170 m around Buckley and Ewloe, and over 200 m in the vicinity of Bettisfield Colliery in the Liverpool district (Figure 24). Local interseam variations in the Lower Coal Measures are attributed to differential compaction of the sequence, which contains sand bodies of variable thickness and extent and the distribution of which may be tectonically controlled (see below). In general, the seams in the lower part of the formation do not readily conform to a predicted pattern of regional, northward, ­subsidence-induced splitting and decline in quality. A possible exception is the Premier Coal, which divides and becomes inferior northwards between the Flintshire and Neston coalfields. The Crown and Lower Bench seams in the lower part of the Middle Coal Measures also fail north-eastwards into the Queensferry area.

In the upper part of the Middle Coal Measures in particular, tectonic influences on sequence thickness and coal seam distribution obscure the effects of regional subsidence. In the Flintshire Coalfield between Leeswood and Buckley, the top of the formation, as defined by the appearance of red beds, lies between 100 and 130 m above the Main Coal. To the west however, in the area around Mold, red beds appear lower in the succession, occurring only 30 to 60 m above the Main coal (Figure 24), (Figure 26). It has been suggested that reddening of comparable Westphalian sequences resulted from lowering of the contemporary water table, influenced by factors such as seasonal evaporation and differential subsidence (Besly and Turner, 1983). The poorly drained alluvial swamp conditions, which sustained the plant growth that contributed to the substantial thickness of grey measures in the area between Leeswood and Buckley, are attributed to relative subsidence along this belt following deposition of the Main Coal. The thickening of the upper part of the Middle Coal Measures is due mainly to a thicker Hollin Rock, Main Rock and similar sandstones within the sequence in this area; below the Main Coal, the Yard Rock is also at its thickest between Buckley and Leeswood. It is significant that the sandstones wash out a number of seams in places along this tract, indicating that major distributary channels were aligned along it. It has been suggested that these drainage systems were determined by structural features (Wedd and King, 1924) and the superposition of several major sandstones in this area is consistent with their deposition on the subsiding, downthrow side of one or more boundary faults that restricted lateral migration of the facies.

The eastern margin of this belt is broadly defined by the Great Ewloe Fault and associated north-trending extensional faults in the Leeswood area (Figure 23), which progressively upthrow the principal coals to the east. East of the Great Ewloe Fault, in the Queensferry area, the succession above the Main Coal is relatively attenuated, ranging between 55 and 90 m around Big Mancot [SJ 3178 6700]; the Hollin Rock (14 m) is also comparatively thin in this area, and the Main Coal Rock is absent. The western margin of the Leeswood–Buckley tract is less well defined, but was probably confined by a zone of major north-trending faults, extending from the area between Treuddyn [SJ 2516 5816] and Coed-talon to Flint [SJ 2446 7313], separating the central part of the coalfield from that around Mold [SJ 238 639] and Bagillt [SJ 225 747] (Figure 23), (Figure 35). It is considered that this fault plexus, together with the Great Ewloe Fault and its associated structures, were important controls on subsidence and sedimentation in the central part of the coalfield, particularly during the period following deposition of the Main Coal.

Between the Flintshire and Denbighshire coalfields there are no significant differences in thickness or facies in the Lower and Middle Coal Measures below the Main Coal (Wedd and King, 1924; Wedd et al., 1928; Hains, 1991). Therefore, it is unlikely that the component faults of the Bala Lineament directly influenced sedimentation during this period. However, in the upper part of the Middle Coal Measures, there are significant differences between the coalfields (Wedd et al., 1928; Hains, 1991), in the number of coal seams and in the interseam thicknesses, both of which increase markedly in the Denbighshire Coalfield; in the latter the onset of red bed sedimentation was also correspondingly later. The change begins in strata above the Main Coal and becomes marked above the Powell seam (Figure 24). It indicates that contemporaneous movements on the Bala Lineament and its associated fractures were important from the time of Main Coal deposition onwards. Relatively greater subsidence to the south of the lineament sustained alluvial swamp conditions, and allowed grey measures with coals to accumulate here for a longer period than to the north (see Chapter 8). Movement and associated onlap of the Westphalian succession may also have occurred at this time over major, pre-existing, north-trending structures farther west, such as the Nercwys–Nant-figillt, Alyn Valley and Vale of Clwyd faults (Williams and Eaton, 1993). This may explain the absence of both Coal Measures and Millstone Grit from much of the Vale of Clwyd, where Red Measures rest unconformably on Dinantian limestones (Calver and Smith, 1974; Warren et al., 1986).

Coalfield structure

Folding of the Westphalian strata, into a series of open peri­clinal anticlines and synclines, with further disruption by faulting, has created the irregular ‘basin and dome’ structure of the Flintshire Coalfield in which a series of coal-bearing ‘troughs’ are separated by areas of older strata. This provides a structural framework for the description of the succession. These main coal-producing areas of the district, in which selective working of several seams has occurred, are broadly centred on Leeswood, Mold, Buckley, Queensferry and Bagillt and described below. Descriptions of the measures underlying the Dee estuary and contiguous with the Neston Coalfield are included within the Queensferry area. The Lower and Middle Coal Measures of the Denbighshire Coalfield, around Llay [SJ 3310 5594], which include sequences lying within and to the south of the Bala Lineament are described separately.

Conditions of deposition

The lithologies of the Coal Measures occur in coarsening-upward cycles, consistent with the deposits of a fluvial-dominated delta plain (Fielding, 1985; Guion and Fielding, 1988). The lower parts of a typical cycle are alternations of mudstones and siltstones, representing suspension deposits of interdistributary lagoons and fresh water lakes, the coarser lithologies relating to periods of increased fluvial discharge. The upper parts of the cycles are characterised by sandstones, interpreted as the deposits of distributary mouth bars and channels, with associated levee and crevasse splay deposits.

The highest part of a typical cycle tends to fine upwards into siltstone and mudstone, commonly succeeded by a siliciclastic palaeosol (seathearth) and coal. It records the progressive colonisation of the interdistributary bays and lagoons by vegetation, and the development of ombrogenous peat mires on the delta tops. Subsequent burial and compaction of the peat resulted in coal formation.

Lower Coal Measures

Flintshire Coalfield

Over much of the district, the base of the formation has been taken for convenience at the base of the thin (5 to 10 m) sequence of mudstones that occur between the top of the Gwespyr Sandstone (Millstone Grit Group) and the Llwyneinion Half Yard Coal. The following account describes the strata above this level. Lower parts of the formation which occur as intercalations with the Gwespyr Sandstone are described in Chapter Four. They crop out to the west of Flint and in the Oakenholt to Northop Hall area and locally include the crop of the impersistent Chwarelau (Little) seam (see p.81). This coal has been identified in both the Flintshire and Denbighshire coalfields underlying local developments of the Listeri Marine Band (Shanklin, 1956; Calver and Smith, 1974). The main seams of the local Lower Coal Measures sequence and their correlation across the district are presented in (Table 13) and (Figure 24).

South of Leeswood, the Lower Coal Measures is predominantly argillaceous, but, elsewhere, it contains a variable proportions of sandstone. The thickness of the formation overlying the Gwespyr Sandstone ranges from about 75 m in the Mold area to at least 90 m in the Leeswood area, to around 130 m in the Buckley, Queensferry and Bagillt areas (Figure 24). Representative sections through the Lower Coal Measures for each of the principal mining areas of the Flintshire Coalfield, are given in (Figure 25), (Figure 26), (Figure 27), (Figure 28), (Figure 29); further details of specific sections can be found in earlier memoirs (Strahan, 1890; Wedd and King, 1924).

The strata between the Gwespyr Sandstone and the Llwyneinion Half Yard Coal include seatearths with sideritic ironstone nodules. The coal itself is up to 1 m thick and was worked at a few collieries in the Leeswood and Bagillt areas. It is notable for the abundance of fish remains in its roof measures.

Up to 36 m of strata, separate the Llwyneinion Half Yard from the Premier seam. This was a widely worked coal, of consistently good quality over all but the north of the district. It is up to 1.7 m thick in the Leeswood area, where it locally splits, developing a thin basal coal and seatearth parting. In the Neston Coalfield, where it is known as the Seven Foot seam, it is up to 2.2 m thick and also split, with numerous thin dirt partings. Its roof measures, consisting of black, bituminous shales, were exploited locally as oil shale. They contain fish remains and rare Planolites ophthalmoides underlying a widely developed mussel band which, in the Denbighshire Coalfield, has yielded Carbonicola communis, C. pseudorobusta and other forms referred to the C. bipennis to C. antiqua group (Wood, 1937; Calver and Smith, 1974). In the Bagillt area, and northwards into the Liverpool district, a thin coal known as the Englefield Two Foot lies about 5 m above the Premier seam, with a bed ­containing Lingula in its roof. Although not proved, it is possible that this seam has also split from the Premier, with the same attenuated marine horizon being represented by either the Lingula or Planolites faunas; it has been suggested that this horizon equates with the Langley Marine Band of the Midlands (Ramsbottom et al., 1978).

The Ruabon Yard Coal is generally separated from the Premier Coal by 15 to 25 m of strata. In the north of the district, it is a seam of reasonable quality house coal, but it passes into a cannel coal around Leeswood and Coed-talon in the south, where it was extensively worked with its bituminous roof measures as a source of oil and coal gas. Sections through the seam around Leeswood typically record ‘Smooth Cannel’ (which breaks with a large curvilinear fracture) overlying ‘Curly Cannel’ (breaking with a small curvilinear fracture). In the south, it attains its maximum thickness of 1.8 m with, in places, a thin bituminous coal at its base, separated from the cannel by a mudstone parting. The bituminous roof measures of this coal are also thicker in the south, reaching up to 3.7 m locally. At varying distances above the seam, the mudstone is succeeded by a sandstone, known as the Yard Rock, which thickens in places to form most of the sequence between the Ruabon Yard and the next higher seam. In the tract between Coed-talon Banks [SJ 266 581] and Leeswood, the sandstone cuts out an extensive area of the Ruabon Yard (Strahan, 1890, plate 1), and washouts of this seam occur sporadically to the north of Buckley.

The coals between the Ruabon Yard and Upper Red (Table 13) are of variable thickness and distribution, subject to splitting and commonly of inferior quality, with partings of cannel. There are some local difficulties in the identification of these seams (see below). The Nant Coal is a multiple seam, with mudstone partings of variable thickness in all areas except the northern part of the Flintshire Coalfield. It is up to 1.9 m thick in the Mold area, and generally lies about 7 m above the Ruabon Yard Coal, but this separation increases eastward and northward, to 15 to 20 m in the Leeswood area and up to 35 m around Buckley. In the Bagillt area, the Nant Coal is up to 1.8 m thick and lies about 23 m above the Ruabon Yard Coal. Although it was locally regarded as a workable seam in the Mold and Bagillt areas, it thins and deteriorates in quality towards the central part of the coalfield. In the Buckley area, it is either unrecorded or represented in shaft sections by 0.7 to 1.8 m of inferior coal with dirt and shale partings, overlying a variable thickness of carbonaceous mudstone. The thickness of measures below the Nant Coal decreases markedly between the Buckley and Queensferry areas, and the seam correspondingly thickens. In the vicinity of Queensferry it is represented by a seam between 1 and 3 m thick, formerly worked as the Upper Four Foot Coal, and separated from the Ruabon Yard Coal by 10 to 20m of mudstone. The beds immediately underlying the seam are comparable to those near Buckley, comprising carbonaceous shales with coal partings.

A sequence of mudstones, ranging from 4 to 12 m in thickness, with thick seatearths, intervenes between the Nant and the Nant Rider coals, the next higher seam in the Leeswood and Mold areas. This seam was widely recognised in the Denbighshire Coalfield (Table 13), but in places was erroneously equated with the Nine Foot Coal of the Flintshire Coalfield (Hains, 1991; see below). The Nant Rider Coal is an inferior coal, up to 0.9 m thick in the Mold area, splitting into two or three thin leaves around Leeswood and Coed-talon. It thins north-westwards in common with its underlying measures, and is represented by a coal parting 2 m above the Nant Coal around Buckley; it may be represented by the upper leaf of that coal in the Queensferry area and in the Neston Coalfield (Figure 27), (Figure 28). Nonmarine bivalves from above this coal at Neston Colliery [SJ 2899 7632] mark the local base of the A. modiolaris Biozone (Calver and Smith, 1974).

In most areas, the Nant or Nant Rider coals are followed by a representative of the Fireclay Group of coals. The intervening strata are about 10 m thick in the south of the Flintshire Coalfield, and also near Queensferry, increasing to 17 m in the area between Flint and Bagillt. However, they increase markedly in thickness towards the central part of the coalfield, reaching a maximum of 35 m around Buckley, where they consist dominantly of sandstones. In places these sandstones have been mistakenly identified as the ‘Yard Rock’, with mining records commonly applying the name ‘Yard Coal’ to the underlying Nant seam. The geometry of the Fireclay Group is complex and, over most of the Flintshire Coalfield, its seams are susceptible to multiple splitting (Figure 24), (Figure 28). The principal seam, the Stone Coal occurs throughout the coalfield and generally represents the lowermost coal of the group, except in the extreme south. In the Queensferry area, a seam known locally as the Nine Foot Coal, was hitherto considered to occur below the Fireclay Group (Campbell and Hains, 1988), but is here recognised as the local equivalent of the Stone Coal. At Queensferry Colliery [SJ 3181 6778], where it is known as the Jointy seam, it is amalgamated with the higher seams of the Fireclay Group, including the Dirty seam, to form a composite seam of low-quality coal (Figure 28), up to 6.75 m thick, with many thin partings of carbonaceous mudstone and, locally, pyrite. The merged Fireclay Group splits southwards and westwards into a double or, in places, a triple seam of bituminous coal and cannel with interbedded seatearths, the individual coals becoming thinner. In the central part of the coalfield, the group comprises three leaves of coal. The lower two leaves, being closely spaced, were commonly worked together as a composite seam up to 2.1 m thick, known locally as the Wall-and-Bench (also Mount Pleasant; Five Foot). Where the two leaves are shown as separate seams on the published map, as to the east of Northop, the names Wall for the upper and Bench for the lower are given; elsewhere the composite seam is shown as the Stone Coal. The topmost leaf of the group, locally named the Cannel, is commonly represented in the Buckley and Queensferry areas by about 0.5m of inferior coal with partings of cannel, separated from the Stone (Wall-and-Bench) Coal by about 4 m of strata (Figure 27), (Figure 28). In the Leeswood area, two thin, inferior coals, not directly correlating with the Wall-and-Bench, but probably representatives of the Fire Damp and Half Yard seams of the Denbighshire Coalfield, are split from the base of the Stone Coal, with intervening measures mainly of seatearth (Figure 25).

The coals of the Fireclay Group achieve their maxi­mum separation in the Bagillt area, where the sequence is characterised by two main seams, separated by up to 10m of mudstones and seatearths. The lower seam, locally known as the Hard Five Quarter (Two Yard, Nine Foot), is a good quality coal, locally 1.8 m thick that probably represents the Stone (Wall-and-Bench) Coal of the Buckley area, although Campbell and Hains (1988) considered it to be a seam below the Fireclay Group. The higher coal, a split seam known as the Double, which is up to 1.5 m thick with cannel in its upper part, is equated with the Cannel Coal of the Buckley area.

The Red (Dirty; Stinking) Coal, a maximum of 1.3 m thick, is a single seam in the Leeswood and Mold areas, but splits northwards and eastwards into the Lower Red and Upper Red coals. The Lower and Upper Red are separated by up to 10 m of strata, with the maximum separation of the seams occurring in the Queensferry area. The Lower Red (Yard) seam, up to 1 m thick, lying between 4 and 30 m above the Fireclay Group, is generally a sulphurous or cannel coal of little value, having been worked only sporadically; it splits into two leaves in the north of the district. The Upper Red (Durbog; King) Coal, locally 0.7 m thick, is also an inferior coal, with roof measures of sideritic black shale. In adjacent districts these black shales contain the Vander­beckei Marine Band (Calver and Smith, 1974); in its absence from recorded sections in the Flintshire Coalfield, the base of the Middle Coal Measures and the Duckmantian Stage is taken at the top of the Upper Red Coal. For convenience, the details of these roof measures are included below, with those of the Upper Red Coal.

Details

Leeswood area

The coal seams of the Leeswood area are disposed in a series of broad, east to south-east-trending synclines, disrupted by several large northward-trending faults. Major faults define the eastern and western margins of the area. The important Lower Coal Measures shaft sections are given in (Figure 25).

In this, and subsequent sections, the standardised name of the local seam is emboldened, and local names (where different) are given in brackets. Seams with similar local names in different parts of the coalfield do not necessarily correlate, nor can they always be equated with the standardised coalfield name.

Thicknesses abstracted from old mining records have been metricated. The naming of collieries, shafts and boreholes is given as it appears on the original records, and may differ from the name that appears on current OS maps.

Boreholes [SJ 2739 5987]; [SJ 2742 5985]; [SJ 2741 5984] near Pontybodkin proved old workings in the Llwyneinion Half Yard (Queen; Lower Queen) seam at shallow depth. The workings are underlain by a seatearth and fine-grained, cross-bedded ­argillaceous sandstone, the latter representing the uppermost beds of the Gwespyr Sandstone. Dark grey shale with thin ironstones forms the roof measures of the seam.

The Premier (Wall-and-Bench; King) Coal is generally 0.9 to 1.4 m thick in this area. The Coed Talon No. 3 Cannel Pits [SJ 2660 5942] (Figure 25) revealed the following section of the seam:

Thickness m
Black shale 1.73
Coal 0.31
Dirt 0.15
Coal 0.94
Seatearth 0.46
Sandstone 0.31
Coal 0.31
Seatearth 1.68

The Ruabon Yard (Cannel) seam, 0.9 to 1.8 m thick, was worked extensively in the Leeswood area. Coppa Colliery [SJ 275 613] (Wedd and King, 1924) provided a typical section of the seam:

Thickness m
Shale –
Smooth cannel 0.66
Curly cannel 0.41
Shale 0.25
Bad cannel 0.13
Ironstone 0.05
Coal 0.08

At Coed Talon Colliery Deep Pit [SJ 2700 5880] (Figure 25), the roof measures of the Ruabon Yard Coal contain sandstones and a thin conglomerate: the section comprised:

Thickness m
Dark shale 0.97
Sandstone 0.84
Black shale 0.46
Conglomerate 0.16
Smooth cannel 0.16
Curly cannel 0.33
Shale 0.25
Coal 0.13
Ironstone 0.23
Grey seatearth 4.04

The Nant (Yard) Coal, 0.6 to 1.3 m thick in the Leeswood area, was formerly exposed in an excavation at Cae-bleiddyn ­Brickworks [SJ 2628 5968] (Strahan, 1890), where the section revealed:

Thickness m
Clay –
Coal 0.61
Dirt 0.05
Coal 0.31
Dirt 0.05
Coal 0.31
Fireclay and shale 2.44

The Nant Rider Coal is unworked in the Leeswood area, where it is represented by two or three thin seams of inferior coal. A section through these coals at Coed Talon Colliery Wood Pits [SJ 2697 5897] (Figure 25) encountered:

Thickness m
Blue shale 6.40
Poor coal 0.31
Carbonaceous shale 0.31
Poor coal 0.31
Seatearth 6.40

The Stone (Wall-and-Bench) Coal, 0.6 to 1.5 m thick, is the only seam of the Fireclay Group of coals to have been worked in the Leeswood area although the group locally includes two inferior seams, the Half Yard and Fire Damp. A section of the Fireclay Group at the Wood Pits [SJ 2647 5897] is as follows:

Thickness m
Blue shale 2.75
Good coal (Stone) 1.52
Fireclay 1.52
Poor coal (Half Yard) 0.61
Seatearth, dark 0.25
Poor coal (Fire Damp) 0.76
Seatearth, dark 0.91

At the Deep Pit [SJ 2700 5880], the Fireclay Group comprises:

Thickness m
Black shale 0.10
Coal 0.69
Carbonaceous shale (Stone) 0.08
Coal 0.36
Fireclay 1.52
Coal (Half Yard) 0.46
Seatearth 1.22

The Red (Two-Foot-Six) seam, 0.5 to 1.2 m thick, is generally described in shaft sections as a ‘dirty’ or ‘stinking’ coal, and is recorded in the following section, provided by Coed Talon Colliery Wood Pits [SJ 2697 5879].

Thickness m
Sandstone 0.61
Dark grey shales with thin ironstones 8.03
Dirty coal 0.91
Seatearth 0.91
Mold area

The seams of the Mold area crop out on the eastern limb of a major north-west-trending syncline, closing to the south at Nercwys, and faulted on its western side by the Nercwys Fault. Large north-trending faults, transecting this ‘Mold Trough’, progressively downfault the succession eastwards. The principal colliery sections through the Lower Coal Measures are given in (Figure 26).

The Llwyneinion Half Yard Coal, 0.9 to 1.0 m thick, was intersected in the Llong Colliery Cannel Pits [SJ 2555 6183], where it overlies the ‘Top Rock’, the upper part of the Gwespyr Sandstone (see above). The seam was also proved at 18.75 m depth in the Plas Isaf Estate No. 2 Borehole.

The Premier (King) seam is 0.9 to 1.2 m thick in the Mold area. A shaft [SJ 2382 6283] at Broncoed Colliery (No. 1 Pit) (Figure 25) provided a typical section through the seam:

Thickness m
Black shale 1.53
Coal 0.36
Parting 0.31
Coal 0.39
Seatearth 1.76

Shallow shafts and crop workings in Coed Uchaf [SJ 2317 6852] are believed to be those of the Premier seam; black shales with fish fragments, in spoil from the workings, are considered to be from strata immediately overlying the seam (Wedd and King, 1924).

The Ruabon Yard (Cannel) Coal is 0.5 to 1.8 m thick, being thickest in the area adjoining the Leeswood tract, but thinning towards the central part of the ‘Mold Trough’, where its workings are sporadic. Its distinctive divisions of ‘smooth’ and ‘curly’ cannel are generally not recorded in the Mold area.

The Nant (Yard; Four Foot) Coal is between 0.9 and 1.9 m thick in the Mold area, being generally thicker in the central part of the ‘trough’ where it was regarded as a good quality coal. A section of the seam at Bromfield Colliery No. 2 Shaft [SJ 2422 6328] revealed:

Thickness m
Dark metal 1.83
Coal 0.08
Shaly parting 0.07
Four Feet Coal 1.37
Shaly parting 0.31
Coal 0.10
Seatearth (dark) 0.81

The Nant Rider Coal is generally represented by an unnamed single seam, 0.4 to 0.9 m thick in the Mold area. It is directly overlain by a thick sequence of sandstones, probably mistaken for the Yard Rock, at Bromfield and Pentre collieries (Figure 26). At John Pit (Waen and Nerquis Colliery) [SJ 2421 6425] the sandstones appear to have cut down to a level immediately overlying the Ruabon Yard Coal, removing both the Nant Rider and Nant seams.

The Fireclay Group generally comprises two thin coals in the Mold area. The unnamed lower seam, probably the equivalent of the Half Yard and/or Fire Damp coals of Leeswood, is 0.9 m thick in the south-east but thins and disappears north-westwards, towards the central part of the area (Figure 26). The higher seam, correlating with the Stone Coal of Leeswood, generally thins in a northward direction, from a maximum 1.4 m at Llong Colliery to 0.2 m at Bronwhylfa Colliery [SJ 2507 6344]. A section of the Fireclay Group at Llong Colliery is as follows:

Thickness m
Coal (Stone) 0.61
Parting (Stone) 0.31
Coal (Stone) 0.46
Measures 1.75
Coal (Half Yard and/or Fire Damp) 0.91

In the Mold area, the Red (King; Two-Foot-Six) Coal ranges from 0.2 to 1.3 m in thickness, being generally thicker in the south-east of the area.

Buckley area

The area defining the ‘Buckley Trough’ extends from Padeswood [SJ 2751 6238] northwards to include the coal measures underlying the alluvial deposits of the Dee estuary, worked locally from the Neston Coalfield on the Wirral Peninsula. Also included is the succession of coal measures lying to the south-east of Connah’s Quay. The eastern margin of the trough is delineated by the Great Ewloe Fault, and the western margin by a plexus of south-trending faults extending from Flint to Mynydd Isa [SJ 2595 6381]; the latter represent, in part, structures developed along the major anticlinal hinge which separates the Mold and Buckley troughs. The seams of the Buckley area are disposed in a major, westward-closing, synclinal fold, broadly centred on Buckley, but cut by numerous north-trending faults which repeat the structure westwards. The important colliery sections in the Lower Coal Measures are given in (Figure 27).

The Llwyneinion Half Yard Coal was penetrated in a boring from a shaft at Aston Hall Colliery (Conqueror Pit) [SJ 2986 6608] (Figure 27), where the section revealed:

Thickness m
Sandstone (hard) 1.57
Shale, black 0.30
Coal 0.13
Mudstone/ironstone bands 2.01
Mudstone 2.13
Shale, black 1.68
Coal 0.30
Seatearth 0.62

At Latchcroft Colliery [SJ 2995 6790], the Llwyneinion Half Yard is 0.46 m thick, with roof measures of indurated bituminous shale.

The Premier (Five Quarters; Four Foot; Lower Four Foot; Seven Foot of Neston) Coal is generally between 0.6 and 1.5 m thick in this area. The following section of the seam was recorded in Aston Hall Colliery No. 2 Shaft [SJ 2937 6593] (Wedd and King, 1924):

Thickness m
Black slag (roof) –
Jibbing coal 0.08
Top coal 0.46
Dirt parting 0.15
Bottom coal 0.61
Fireclay –

A further section of the Premier Coal at the Aston Hall Conqueror Pit [SJ 2986 6608] revealed: oil-yielding black shale (roof), 4.9 m; Premier Coal, 1.52 m; seatearth, 0.36 m; coal, 0.43 m; seatearth, 2.13 m.

The Ruabon Yard (Yard; Arley; Five Foot of Neston) Coal, 0.5 to 0.9 m thick, overlies a well developed seatearth and is locally canneloid in its upper part, revealing the typical roof measures of bituminous shale. A representative section of the seam is that of Aston Hall Colliery (Conqueror Pit) [SJ 2986 6608]:

Thickness m
Shale, black, oil-yielding 1.37
Arley Coal (superior) 1.0
Fireclay 2.6
Shale, dark 1.22
Coal 0.13
Sandstone/mudstone 5.1

The seam thins and splits northwards, and is represented at Neston Colliery [SJ 2899 7632] by two closely spaced seams, 0.3 m thick, interbedded with mudstones.

The Nant (Yard; Dirty; Upper Four Foot; Upper Latchcroft; Six Feet Nine of Neston) Coal is generally an inferior coal, 0.7 to 1.8 m thick in the ‘Buckley Trough’. It may have been locally mined as the ‘Yard Coal’, a seam name usually used for the lower, Ruabon Yard seam (Wedd and King, 1924). A thick sandstone overlying the seam in this area, and previously ­considered to be the ‘Yard Rock’ (Wedd and King, 1924), is probably the sandstone which overlies the Nant (Yard) in shaft sections of the Mold and Queensferry areas (Figure 26), (Figure 27), (Figure 28).

A section of the Nant Coal at Aston Hall Colliery (Conqueror Pit) [SJ 2986 6608] revealed:

Thickness m
Mudstone (roof) 0.41
Coal 0.66
Parting 0.15
Coal (inferior) 0.33
Mudstone, carbonaceous 3.0
Seatearth 0.61

A nonmarine bivalve assemblage in the roof measures of the upper seam in Neston Colliery (Wood, 1937) was described by Calver and Smith (1974) as ‘consistent with a position near the base of’ the A. modiolaris Biozone.

The Nant Rider Coal is represented by a 0.08 m-thick coal, lying 2.1 m above the Nant seam at Aston Hall Colliery, and by carbonaceous mudstone in the Bryn-y-bâl Isaf Borehole (Figure 27), but is otherwise unrecorded in the area.

The lowest seam of the Fireclay Group of coals is the Stone (Wall-and-Bench; Mount Pleasant; Strong Boney of Neston) Coal. It is a composite seam, 1.3 to 2.1 m thick (including parting), generally regarded as a low quality coal, although extensively worked in this area. Locally, to the east of Northop, its upper and lower leaves are mapped separately as the Wall and Bench seams respectively. A typical section of the seam was provided by Mount Pleasant Colliery (Old Engine or Wood Pit) [SJ 2891 6499]:

Thickness m
Top coal 0.51
Parting 0.01
Coal 0.18
Mudstone 0.56
Bottom coal 0.56

At Aston Hall Colliery the seam comprises: coal, 0.61 m; seatearth 0.30 m; cannel, 0.10 m; coal, 0.61 m; seatearth, 1.93 m.

The Cannel Coal, representing the higher seam of the Fireclay Group, is an inferior coal, locally up to 0.7 m thick. A section of the seam at Aston Hall Colliery revealed: cannel, 0.20 m; coal, 0.46 m; seatearth, 1.60 m.

The Lower Red Coal is a cannel coal, up to 0.2 m thick, usually unrecorded in shaft sections.

The Upper Red (King) Coal, an inferior coal, 0.4 to 0.7 m thick, lies 1 to 2 m above the Lower Red seam, the section at Aston Hall Colliery (Figure 27) recording:

Thickness m
King Coal (Upper Red) 0.71
Seatearth 0.91
Mudstone, carbonaceous 0.43
Sandstone 0.15
Cannel (Lower Red) 0.15
Seatearth 1.22
Queensferry area

The Queensferry area is situated to the east of the Great Ewloe Fault and its southern margin is here defined by a major north-west-trending fault passing through Hawarden. The area is characterised by eastward to north-eastward dipping measures, repeated by a series of major north-trending faults. Sections in the Lower Coal Measures encountered in the principal ­collieries are shown in (Figure 28).

The Llwyneinion Half Yard Coal was not worked in the area, but is probably the seam proved in boreholes for Hawarden Castle Colliery (Figure 28), where it is up to 0.9 m thick and overlain by bituminous mudstones. The coal overlies at least 90 m of interbedded sandstone, mudstone and carbonaceous shale with ironstone, largely devoid of major coal-bearing horizons. These beds probably correlate with the Gwespyr Sandstone and upper part of the Holywell Shales and include the horizon of the Subcrenatum Marine Band. Thin coals from the upper part of the Gwespyr Sandstone in Hawarden Castle Colliery No. 3 and No. 11 boreholes are probably correlatives of the Chwarelau seam.

The Premier (Lower Four Foot) Coal averages 1.2 m in thickness at Eleanor and Queensferry collieries (Figure 28). It is generally underlain by a thick seatearth, locally containing a 0.15 m-thick coal, and overlain by black (possibly carbonaceous) shale. However, it disappears southwards and, in the Hawarden Castle Colliery boreholes, appears to be represented by a sequence of interbedded bituminous shales and seatearths.

The Ruabon Yard (Yard) Coal is generally an inferior coal throughout the Queensferry area and ranges from 0.5 to 1.4 m in thickness, comprising bituminous coal with an upper parting of cannel, commonly overlying a thick seatearth containing many ironstone nodules. Its roof measures are typically of black, carbonaceous shale.

A section of the Ruabon Yard at the Engine Pit of Queensferry Colliery [SJ 3181 6778] revealed:

Thickness m
Black shale 0.76
Cannel 0.13
Coal 1.01
Seatearth 3.66

The Nant (Upper Four Foot) Coal comprises two or three closely spaced lower seams of coal, interbedded with mudstones and seatearths, and a well developed higher seam (Figure 28), in places up to 1.7 m thick (arguably a local development of the Nant Rider Coal). The lower seams, although amalgamated in some sections, are generally thinner, ranging from 0.2 to 0.5 m in thickness, impersistent, and closely interbedded with carbonaceous mudstones. The sequence, as in the Mold and Buckley troughs, is commonly overlain by a sandstone confused by miners with the Yard Rock. A typical section is provided by the Rector’s Meadow No. 1 Borehole at Hawarden (Figure 28):

Thickness m
Shale and fireclay 0.69
Coal 1.30
Coarse seatearth 1.19
Coal and carbonaceous shale 1.40
Mudstones 2.13

The Stone (Wall-and-Bench; Nine Foot; Jointy/Dirty) Coal the main representative of the Fireclay Group of coals, is a composite seam of variable quality in the Queensferry area. In the north, and in the Neston Coalfield, it comprises a single seam up to 3.7 m thick, with partings of carbonaceous mudstone. At Eleanor Colliery No. 2 Pit [SJ 3113 6845], two thick coals are represented at this level (Figure 28), the lower of which, known here as the Nine Foot seam, is probably the Stone Coal. A thick sandstone intervenes between this and a higher seam which is probably the Cannel Coal, but the succession may be repeated by a fault undetected in the shaft section.

At Queensferry Colliery, the Stone Coal splits into two main horizons with many partings of carbonaceous shale, the ­separation increasing southwards towards Mancot Bank and Hawarden (Figure 28). Shaft and borehole sections reveal a succession typical of the central part of the coalfield, comprising the Cannel Coal, up to 1.2 m thick, overlying a split seam up to 2.8 m thick, equivalent to the Wall-and-Bench Coal. A detailed section of these seams in the Rector’s Meadow No. 1 Borehole (Figure 28) revealed:

Thickness m
Blocky fireclay with ironstone nodules 0.91
Cannel, coarse 0.15
Coal, good 0.84
Seatearth 0.53
Shale, soft, dark 0.30
Shale with coal ribs 1.98
Coal, good 1.65
Seatearth, dark 0.20
Coal, good and hard 1.07
Seatearth 0.30

The Lower Red seam, 0.3 to 0.8 m thick, is an unworked, inferior coal with partings of carbonaceous mudstone. A thick seatearth containing ironstone nodules commonly underlies the seam.

The Upper Red Coal is between 0.2 and 0.7 m thick around Queensferry and, as in other areas, underlies sequences rich in ironstones. The following section, from Great Mancot Main Colliery [SJ 3167 6636], is representative of the succession around Queensferry:

Thickness m
Measures with ironstones 10.36
Mudstones and fireclay 1.37
Shale, black, carbonaceous 0.61
Coal 0.23
Seatearth 0.10
Mudstone and sandstone, thinly interbedded 0.91
Sandstone 3.35
Bagillt area

The Bagillt area is separated from the central part of the Flintshire Coalfield by a series of major faults extending southwards from Flint. This major fault plexus downthrows the Ruabon Marl Formation in the vicinity of Flint, and in a narrow crop as far south as Soughton [SJ 2432 6650]. Lower Coal Measures crop out to the north-west, inclined north-eastwards towards the Dee estuary, and gently folded. Details of the succession are generally lacking for much of the area, and correlations are largely based on a section proved in Bettisfield Colliery No. 2 Shaft [SJ 2158 7603] (Figure 29), immediately north of the Flint district; workings from this colliery extend into the northern part of the district.

The Llwyneinion Half Yard (Queen; Three Quarter) Coal was previously described as a ‘good coal’ around Bagillt (Wedd et al., 1923; Wedd and King, 1924). In Bettisfield Colliery No. 2 Shaft (Figure 29), the seam is 0.78 m thick, overlying ­interbedded sandstones and mudstones of the Gwespyr Sandstone and, typically, succeeded by black mudstones.

The Premier (Bychton Two Yard; Four Foot) Coal is 0.6 to 1.3 m thick in this area, a section of the seam at Bettisfield Colliery No. 2 Shaft revealing:

Thickness m
Mudstones, dark, with plant remains 3.07
Mudstones, dark 0.20
Coal (Four Foot) 0.76
Seatearth parting 0.15
Coal 0.40
Seatearth, soft 1.60

The Englefield Two Foot Coal is represented by an unnamed seam 4.9 m above the Premier Coal at Bettisfield Colliery, where the section is as follows: dark grey mudstone, 2.74 m; black mudstone with fossil remains (‘Lingula Bed’), 0.91 m; coal, 0.32 m; soft seatearth, 1.62 m. The ‘Lingula Bed’ may correlate with the Langley Marine Band of the Midlands (Ramsbottom et al., 1978).

The Ruabon Yard (Soft Five-Quarter; Five Foot; Four Foot; Lower) Coal is a reasonable quality coal, 1.0 to 1.6 m thick. A section at Bettisfield Colliery revealed:

Thickness m
Mudstones, bluish grey 2.18
Mudstones, hard, sandy 0.40
Mudstones, bluish grey 0.32
Coal (Five Foot) 1.57
Mudstone, carbonaceous with coal streaks 0.68
Mudstone, seatearth 0.76
Ganister 0.78

The Nant (Brassy; Five Feet) Coal, 1.0 to 1.8 m thick, is generally an impure, sulphurous coal (Wedd and King, 1924), but improves eastwards towards Dee Green Colliery, where it was known as the Best Coal.

The Stone Coal, generally known in this area as the Hard Five-Quarter (Two Yard; Main; Six Foot; New; New Three Yard) is a good quality house and steam coal, being represented by a single seam, 1.0 to 1.8 m in thickness around Bagillt, but splitting south-eastwards into two or three closely spaced seams, interbedded with fireclays, at Dee Green Colliery (Figure 29).

The Cannel (Double; Dirty; Brassy) Coal is an inferior seam of two leaves. The section at Bettisfield Colliery is as follows:

Thickness m
Mudstone, bluish grey, with ironstones 5.03
Mudstones, dark grey 0.33
Coal (Double) 0.92
Seatearth 0.91
Mudstone, carbonaceous, and coal 0.28
Coal 0.66
Sandy seatearth 1.77
Mudstone, dark grey 1.09
Coal 0.18
Seatearth, sandy 0.58

A section of the seam in workings at Coleshill Colliery revealed: coal (upper), 0.71 m; parting, 0.3 m; coal (lower), 0.46 m.

The Lower Red Coal is a split seam of two leaves, with intervening mudstone, including seatearth and carbonaceous shale; as in other areas, a thick seatearth with ironstone nodules commonly underlies the seam. The upper leaf, usually called the Yard, One Yard or Two Foot, is typically between 0.3 and 1.1 m in thickness. The lower leaf is an inferior coal, about 0.4 m thick, possibly worked to a limited extent from Bagillt Colliery [SJ 2168 7560], as the Gloin Bydur Coal. The separation of the lower and upper leaves increases south-eastwards, being at its maximum of 10 m in workings at Flint Colliery [SJ 2352 7321].

The Upper Red Coal is an unworked seam of inferior coal, 0.4 to 0.5 m thick, its roof measures comprising characteristic black, carbonaceous shales with abundant ironstones and nodules (Figure 29). At Bagillt Colliery [SJ 2168 7560] the seam is split, the section revealing:

Thickness m
Shale, black with ironstone bands 9.40
Coal 0.46
Seatearth 0.91
Coal 0.16
Seatearth 1.21

Middle Coal Measures

Flintshire Coalfield

The base of the Middle Coal Measures is taken at the base of the Vanderbeckei (Llay) Marine Band located in the roof measures of the Upper Red seam (for mapping convenience, the top of the coal has been taken as the formational contact). Although the marine faunas have not been widely recorded, the distinctive sequence of black marine mudstones overlain by sideritic ironstones, described in detail from the section at Point of Ayr Colliery (Calver and Smith, 1974), is widely recognised throughout the district (Figure 24)(Figure 25)(Figure 26)(Figure 27)(Figure 28)(Figure 29). Only in the Mold area are these marine strata difficult to recognise (Figure 24) and (Figure 26) suggesting that this area, sited closer to the local margin of the Coal Measures basin, lay beyond the limit of the marine incursion. The top of the formation is taken at the incoming of reddened measures of the Ruabon Marl Formation. However, the transition from predominantly grey to red measures is commonly gradational over several tens of metres in the Flintshire Coalfield, and the sequence includes several thin coals (Figure 24). These include the Pontybodkin Mountain Coal, a seam worked in part of the Leeswood area, lying within the lowest part of the Ruabon Marl Formation but described in this section for convenience. The boundary between the Middle Coal Measures and Ruabon Marl is thus markedly diachronous ranging in the Flintshire Coalfield from mid- to latest Duckmantian in age (see below).

The Middle Coal Measures include many of the most extensively mined coal seams in the district. The principal seams within the sequence and their correlation across the district are presented in (Table 13) and (Figure 24).

The formation comprises roughly equal proportions of sandstone and mudstone, and includes several major sandstones including the widespread Hollin Rock, which ­substantially increase the thickness in the Buckley and Leeswood areas. Sideritic ironstones are widely developed. Apart from those associated with the Vanderbeckei Marine Band, they also occur in association with the Black Bed seam, and commonly occur in strata between this coal and the Quaker seam.

Formational thicknesses vary greatly and partly reflect the different levels at which the red bed facies enters the sequence in each area of the coalfield. The formation is represented by as little as 55 m in parts of the Mold area, increasing to as much as 180 m in the Buckley area; intermediate thicknesses are recorded in the Leeswood (140 m) and Queensferry (90 to 120 m) areas. Up to 160 m are present in the Bagillt area, but near Flint, the Middle Coal Measures are cut out by a local unconformity at the base of the red measures (Figure 24). Representative sections through the Middle Coal Measures for each of the principal mining areas of the Flintshire Coalfield, are given in (Figure 25), (Figure 26), (Figure 27), (Figure 28), (Figure 29); further details of specific sections can be found in earlier memoirs (Strahan, 1890; Wedd and King, 1924).

The lowest widely worked seam in the Middle Coal Measures is the Crown (Mostyn Two Yard; Diamond) Coal. This is a generally inferior coal, represented by a double seam around Bagillt, and a single coal up to 1.5 m thick in the Leeswood area, although it thins and becomes mainly unworkable eastwards. It lies 15 to 20 m above the Upper Red seam. The intervening measures typically contain much sandstone, and a thick seatearth commonly underlies the coal. In the Neston Coalfield, the Crown Coal is doubtfully represented by the Four Feet Mine, and locally attains a thickness of 1.3 m (Figure 27).

The Lower Bench (Three Yard) Coal was formerly worked for steam coal around Leeswood and in the Bagillt area, where it is locally up to 2.3 m thick. However, it thins to 0.6 m and splits into two leaves around Mold, and thins further towards the eastern part of the coalfield. The seam lies 2 to 5 m above the Crown in the southern part of the Flintshire Coalfield, but this increases to 10 m in the north; the thickness of measures between the Lower Bench and Main similarly increases northwards, from about 2.5 m near Leeswood to 13 m in the Bagillt area. The measures above and below the Lower Bench are dominantly of mudstone, and a thick seatearth commonly underlies the seam.

The Main (Five Yard) Coal is consistently the thickest seam in this coalfield, where it was extensively worked as a high quality steam and house coal; in the Buckley area, it was also worked as a coking coal. Over most of the coalfield it ranges from 2.7 to 3.4 m in thickness, but it is generally thinner in the Mold area, where it rarely exceeds 2.2 m; however, in the vicinity of Queensferry it locally attains a thickness of 5 m. Thickness variations indicate an overall increase towards the eastern and northern parts of the coalfield.

The Main Coal is not generally susceptible to splitting, but thin partings from the top and bottom of the seam have been recorded in certain areas. The lower parting, known as the Main Bench (Finger) Coal, is 0.1 to 0.2 m thick and occurs mainly in the vicinity of Mold, where it is separated from the Main Coal by up to 1 m of mudstone. The Main Bind(Foul) Coal, which splits from the top of the Main Coal, is about 0.2 m thick in the Mold area, but up to 0.6 m around Queensferry. The intervening measures are mainly of mudstone, with a maximum thickness of 8.5 m at Great Mancot Colliery [SJ 3205 6644]. The Main Bind Coal is also locally split and, in places, passes into carbonaceous mudstone.

The measures between the Main Coal and overlying Black Bed seam are up to 28 m thick in the Leeswood area, generally thinning northwards, and decreasing markedly westwards, to between 2 and 8 m in the Mold area (Figure 25), (Figure 26). They are dominated by sandstones and sandy mudstone in a belt between Leeswood and Buckley, representing the maximum development of the ‘Main Rock’, resting on and, in places, cutting down into the Main Coal (Wedd and King, 1924). Westwards and eastwards the sandstones pass into, and become subordinate to mudstones, although vestiges of the ‘Main Rock’ locally occur in the Mold and Queensferry areas. Above the Main Rock, a group of ironstones, known locally as the New Mine Ironstone, were worked in the neighbourhood of Leeswood.

The Black Bed seam (Black Band; Black Stone; Rough) is a good steam coal, up to 1 m thick in the vicinity of Buckley and Ewloe, but thins and deteriorates in quality away from this area, and is unrecorded in the north-western part of the coalfield. A thick seatearth usually underlies the seam, and a thick bed of sideritic ironstone commonly forms the roof, locally containing shells (possibly mussels), as at Coed Talon Ironworks [SJ 2662 5832]; the seam name is derived from the occurrence of this (‘black band’) ironstone. Nonmarine bivalves from this same level in the Denbighshire Coalfield mark the base of the Lower similis–pulchra Biozone (Calver and Smith, 1974).

The strata between the Black Bed and the overlying Quaker seam are about 8 m thick in the south of the Flintshire Coalfield and around Queensferry, increasing to a maximum of 19 m in the Buckley area, but thinning markedly to 2.8 m at Ty-n Twll Colliery [SJ 2449 6563], north of Mold. In the Leeswood area, they comprise black carbonaceous mudstones with abundant sideritic bands and nodules, up to 0.3 m thick, which were previously worked as the White Mine Ironstone (Figure 25). Ironstones are common at this level throughout the Flintshire Coalfield, but sandstones and sandy mudstones become increasingly abundant in the intervening measures north of Leeswood, and account for the thickening of this part of the sequence in the Buckley area.

The Quaker (Brassey) seam is another good quality coal, formerly exploited as a house coal throughout most of the coalfield, and as a steam and coking coal around Buckley and Queensferry, although generally absent in the Bagillt area. Its thickness ranges from 0.6 to 1.9 m, revealing a general increase towards the eastern part of the coalfield, where a thin basal leaf locally splits from the main seam. An extensive, well developed seatearth, in places over 4 m thick, underlies the Quaker seam. The measures overlying the seam comprise black shale with sideritic ironstones, which pass upwards into a sequence dominated by sandstone and sandy mudstone, ­representing the ‘Brassy Rock’. The Brassy Rock is well developed in the tract between Leeswood and Buckley, but thins westwards into the Mold area and fails around Northop Hall. Its distribution accounts for thickness ­variations in the sequence between the Quaker and next higher seam, ranging from about 13 m around Leeswood and Buckley, to 5 m in the Mold area.

The Crank (Foul) Coal is an inferior coal, locally up to 0.65 m thick, although often unrecorded in shaft sections; it was worked to a very limited extent in the Buckley area. The overlying measures are arenaceous, with a thick sandstone, the ‘Crank Rock’, in places forming the roof of the seam, commonly succeeded by ironstone-bearing mudstone. The thickness of measures between the Crank and overlying Hollin seams largely reflects the distribution of the Crank Rock, being a maximum of 15 m in the Leeswood area, but thinning northwards and westwards to less than 5 m in the vicinity of Mold.

The Hollin (Two Yard) Coal was one of the principal coals of the district, ranking second only in importance to the Main Coal. It is generally over 2 m thick, and locally up to 2.75 m thick in the central part of the Flintshire Coalfield, thinning southwards to about 1.7 m in the Leeswood area, and to 0.5 m, with a concomitant reduction in quality, around Coed-talon. It also thins and deteriorates in quality towards the north of the district, where it is probably represented by a 0.9 m-thick seam, lying up to 45 m above the Main Coal (Figure 24), (Figure 29).

An horizon of cannel, up to 0.5 m thick, forms the basal part of the Hollin Coal in the vicinity of Mold. It has not been recorded elsewhere in the district, where it is apparently replaced by bituminous coal. However, it may be represented by the Hollin Bench Coal, up to 0.9 m thick, a split from the base of the Hollin seam in the Leeswood area, and also recorded in borings in the Dee estuary (Figure 28). The Hollin Bench Coal lies 0.5 to 0.9 m below the Hollin Coal, the intervening measures being mainly seatearth; both seams were commonly worked together. In places around Mold, a thin leaf of coal also splits from the top of the Hollin Coal, separated from the latter by up to 2 m of sandy mudstone.

Bituminous shales with ironstone beds and nodules, overlie the Hollin Coal in the south of the district, but give way northwards to sandstones or striped sandstones and mudstones. Around Leeswood, the shales contain the Powell Group of coals (Magraw and Calver, 1960), comprising the Powell and Drowsell seams, which lie at an increasing height above the Hollin Coal as the sequence is traced north-eastwards (see below). The Powell (Bind) Coal is usually between 0.6 and 0.9 m thick, decreasing to 0.3 m in places around Queensferry. It is a seam of no great importance, having been worked in places with the Hollin Coal, but is notable for the presence of the Maltby (Powell) Marine Bandin its roof (Magraw and Calver, 1960; Calver and Smith, 1974). In the south of the district, it lies 1 to 2 m above the Hollin Coal, increasing to between 12 and 16 m north of Mold and Buckley, and up to 22 m in the Bagillt area. The Drowsell (Bind; Massey) Coal, is an inferior seam lying between 2 and 12 m above the Powell, with the Clown(e) (Gardden Lodge) Marine Band in its roof (Magraw and Calver, 1960). It comprises up to three, thin, closely spaced leaves of bituminous coal and cannel, interbedded with mudstone and seatearth, although in many sections it is unrecorded. It is likely that the seam is more persistent than records suggest, but in some areas it is undoubtedly removed by wash out together with the Powell Coal, by the sandstones of the Hollin Rock.

The Hollin Rock is the thickest and most widely developed sandstone, or group of sandstones, in the Flintshire Coalfield (Figure 24). It generally succeeds the Powell Group of seams, but the base is locally erosional down to the level of the Hollin Coal, as at Elm Colliery [SJ 2754 6624], west of Buckley. At its maximum development in the neighbourhood of Buckley, it is over 75 m thick. However, it rapidly thins northward and westward: it is absent locally in the Mold area and is represented by only 10 to 15 m of sandstone in the north of the district. It is generally absent from the Denbighshire Coalfield.

In places, in the central and western parts of the Flintshire Coalfield, the upper 20 to 30 m of the Hollin Rock are reddened, and gradational with the overlying Red Measures. However, in the southern part of the coalfield, these rock units are separated locally by up to 70 m of grey mudstones with subordinate sandstones and a number of workable coals. The seams are, in ascending order, the Tryddyn Half Yard, Pontybodkin Divided and Pontybodkin Mountain coals. The Pontybodkin Divided Coal is a split seam which, with the Tryddyn Half Yard Coal, is thought to correlate with the John o’Gate Coal of the Denbighshire Coalfield (Hains, 1991). The Pontybodkin Mountain Coal lies within the lowest part of the local Ruabon Marl Formation. A further seam, the Upper Main Coal, which overlies the Hollin Rock in the area east of Buckley, may be the local correlative of the Tryddyn Half Yard or Pontybodkin Divided coals, or may be an altogether higher seam.

The Tryddyn Half Yard Coal is 0.3 m thick around Leeswood, and lies about 30 to 40 m above the Hollin Rock. The intervening measures are mainly black shales, locally grading upwards into sandstones, and the coal is usually underlain by a thick seatearth. The seam generally thins northwards and westwards together with the underlying measures such that, at Tryddyn Lodge Colliery [SJ 2627 5826] south-west of Coed-talon, it is 0.15 m thick and separated from the Hollin Rock by only 4 m of mudstone. The measures overlying the coal also thin markedly in this direction and, to the west of Coed-talon, reddened mudstones of the Ruabon Marl Formation lie about 7 m above the seam, with no evidence of higher coals (Figure 25). The Tryddyn Half Yard Coal is mostly unrecorded outside the Leeswood area but, at Broncoed Colliery [SJ 2382 6283] (Figure 26) near Mold, it is doubtfully represented by a thin coal about 42 m above the Powell seam, included in the Ruabon Marl Formation.

The Pontybodkin Divided seam lies 3 m above the Tryddyn Half Yard Coal, and is separated from it, in the Leeswood area, by a sequence of pale grey mudstone, containing several ironstone beds. The seam is 1.8 m thick, comprising two leaves of bituminous coal, with a 0.4 m-thick dirt parting, and a roof of black, carbonaceous shale. Although it was exploited to a limited extent in the area east of Coed-talon, it is generally absent from the rest of the Flintshire Coalfield; however, it may be represented in the Mold area by a thin seam within the Red Measures at Broncoed Colliery (Figure 26).

In the Leeswood area, the measures between the Pontybodkin Divided and Pontybodkin Mountain seams comprise 25 m of mudstone, with subordinate sandstones, seatearths, carbonaceous shales and thin coals. The local base of the Ruabon Marl Formation is defined by the reddening of the mudstones that occurs in places towards the top of this sequence. The Pontybodkin Mountain Coal, a 0.86 m-thick seam worked in a small area east of Coed-talon, lies within these lowermost beds of the Ruabon Marl Formation.

The Upper Main (Massey) Coal is a single seam of bituminous coal, up to 1 m thick in the area east of Buckley, where it was worked; it has not been recorded elsewhere in the Flintshire Coalfield. It may equate with the Tryddyn Half Yard or the Pontybodkin Divided seams, being the first coal of significance above the Hollin Rock, but it has also been correlated with the Upper Stinking seam of the Denbighshire Coalfield (Campbell and Hains, 1988; Hains, 1991), a correlation which would place it, and its immediately underlying measures in the Bolsovian Stage.

Details

Leeswood area

The important colliery sections in the Middle Coal Measures in this area are given in (Figure 25).

The Crown (Diamond; Four Foot) Coal is generally 1.2 to 1.5 m thick, with dirt or ironstone partings, and overlies a thick seatearth. A section of the seam at the No. 3 Cannel Pits of Coed Talon Colliery revealed:

Thickness m
Coal 0.74
Black shale 0.18
Mudstone 1.22
Coal 0.15
Black shale 0.08
Coal 0.18
Shale 1.22

A further section in a shaft at Coed Talon Ironworks recorded: coal, 0.66 m; ironstone, 0.20 m; coal, 0.41 m; seatearth, 0.38 m.

The Lower Bench (Two Foot) Coal is 0.6 to 0.9 m thick, and was worked to a limited extent in the Leeswood area.

The Main Coal, ranging from 2.7 to 3.7 m in thickness, was exploited over most of this area. In the Coed Talon Ironworks Shaft [SJ 2662 5832] (Figure 25), the seam comprises: Top Coal, 0.91 m; Furnace Coal, 1.07 m; Bottom Coal, 1.22 m.

There are no records of underground workings in the Black Bed (Black; Black Vein/Stone/Band) seam, but it was probably exploited along with the adjacent New Mine and White Mine Ironstone horizons in the neighbourhood of Coed-talon and Leeswood. It ranges from 0.2 to 0.7 m in thickness, the section of the coal at Tryddyn Lodge Colliery No. 2 Shaft [SJ 2627 5826] recording:

Thickness m
Shales with ironstone 6.40
Coal 0.30
Mudstone, carbonaceous 0.15
Coal 0.15
Seatearth 1.50

The Quaker (Brass Vein; Brassey) Coal is 1.2 to 1.9 m thick and the section at Tryddyn Lodge Colliery revealed:

Thickness m
Shale about 1.52
Mudstone, carbonaceous about 0.30
Coal 1.37
Seatearth 0.15
Coal 0.38
Seatearth 1.22

The Crank Coal is between 0.1 and 0.5 m thick, but is generally unworked in this area.

The Hollin Bench and Hollin (Two Yard) coals were generally worked as a single seam, with a total thickness exceeding 2.5 m. However, in the immediate vicinity of Coed-talon, records show a single seam, 0.5 to 0.9 m thick, at this level (Figure 25). An unsited shaft for Leeswood Ironworks (Strahan, 1890; possibly the Flue Pit [SJ 2683 5984] of Leeswood Green Colliery) showed the following section:

Thickness m
Shale 0.61
Coal, Powell 0.64
Seatearth, soft blue 0.91
Coal, Two Yard (Hollin) 1.83
Seatearth 0.46
Little Coal (Hollin Bench) 0.69
Hard seatearth 3.51
Coal (unnamed) 0.46
Seatearth 0.91

The Powell (Bind) Coal, which may locally include the Drowsell seam, is commonly 0.6 to 0.9 m thick in the Leeswood area. At the Tan Llan opencast site, about 6 m of grey mudstone with thin ironstone beds overlie the seam, and yielded a fauna diagnostic of the Maltby Marine Band (Magraw and Calver, 1960). A shaft at Coppa Colliery No. 3 Pit [SJ 2772 6145] revealed:

Thickness m
Shale, dark 4.34
Mudstone, carbonaceous and cannel 0.30
Coal (Drowsell) 0.30
Mudstone, carbonaceous 1.42
Coal (Powell) 0.91
Mudstone, carbonaceous and coal 1.37
Coal (Hollin) 1.78
Shale 0.23
Coal (Hollin Bench) 0.66
Seatearth, pale 0.30

There are no records of workings in the Tryddyn Half Yard or Pontybodkin Divided (Mountain) seams, but the higher, Pontybodkin Mountain (Top) Coal (in the lowermost Ruabon Marl Formation) was worked to a limited extent south-east of Pontybodkin.

Mold area

The principal shaft sections in Middle Coal Measures for this area are given in (Figure 26).

The Crown (King) Coal commonly is between 0.2 to 0.5 m thick, generally thinning northwards and eastwards across the area (Figure 26). It typically comprises a single seam of coal with an associated thick seatearth, but the section at Broncoed Colliery No. 1 Pit [SJ 2382 6283] revealed:

Thickness m
Very hard fireclay 1.73
Mudstone 0.73
Coal (with parting) 0.77
Dark fireclay 1.97
Coal 0.54
Dark fireclay 2.36

The Lower Bench Coal is of inferior quality and apparently unworked in the Mold area. It ranges between 0.2 and 0.9 m in thickness, usually in two leaves, with a parting of bituminous shale or fireclay. The section of Bromfield Colliery No. 2 Shaft [SJ 2422 6328] (Figure 26) revealed:

Thickness m
Mudstone, grey 4.42
Coal 0.15
Mudstone, grey and bituminous shale 0.46
Coal 0.20
Fireclay, dark 1.02

The Main Coal is 1.8 to 2.2 m thick in this area. A borehole [SJ 2450 6240] for Waen and Nerquis collieries proved the follow­ing section of the seam, together with the horizon of the Main Bind Coal.

Thickness m
Mudstone, brown 0.30
Shale, black with coal (Main Bind) 0.45
Mudstones, pale and dark blue-grey 3.23
Coal (Main) 2.05

There are no records of workings in the Black Bed (Rough) seam which is between 0.1 and 0.9 m thick. The associated ironstone bands, which characterise the roof measures in the Leeswood area, are generally absent hereabouts, although the thick seatearth which underlies the coal is recorded in a number of shafts (Figure 26). At Broncoed Colliery No. 1 Pit [SJ 2382 6283], the seam is split, with an intermediate parting of mudstone, and in Bronwhylfa Colliery No. 2 Shaft [SJ 2507 6344] the section records:

Thickness m
Sandstone, hard brown and white 7.0
Coal 0.9
Seatearth 0.9
Inferior coal 0.3
Mudstone 2.1

The Quaker (Brassey) Coal is 0.6 to 1.0 m thick hereabouts and the thick seatearth which commonly underlies the seam was widely recorded in shaft sections. The following section was proved in Pentre Colliery No. 2 Shaft [SJ 2430 6375]:

Thickness m
Drift deposits 19.71
Mudstones, bluish grey 2.43
Coal 0.84
Seatearth 3.59

The Crank Coal is between 0.4 and 0.6 m thick, generally lying a few metres above the Quaker seam, but nowhere worked. A thin representative of the Crank Rock overlies the seam in places, as at Broncoed and Bronwhylfa collieries (Figure 26).

The Hollin Coal ranges from 1.9 to 2.7 m in thickness, locally with a thin split from the top of the seam (Figure 26). A section referable to ‘Broncoed Oak Pits’, possibly an alternative name for Broncoed Colliery No. 1 Pit (Figure 26) records:

Thickness m
Coal (Drowsell ?) 0.26
Seatearth, pale 2.43
Coal (Powell) 0.84
Seatearth, dark 1.22
Mudstone, dark 1.92
Coal 0.07
Seatearth, pale 0.46
Coal (Hollin) 1.83
Seatearth, hard 2.34

Shaft sections of Rhyd Galed, Ty’n-twll and Alltami collieries identify a bed of cannel at the base of the Hollin seam. The section at Rhyd Galed Colliery [SJ 2449 6464] (Figure 26) shows: strong blue mudstone (roof), 3.65 m; Hollin Coal (in three beds), 1.98 m; cannel, 0.45 m; strong blue mudstone, 2.43 m.

The Powell (Bind) Coal is about 0.8 m thick, lying a few metres above the Hollin Coal in the south of the area, but with increasing separation between the seams northwards from Mold; comparable sections of the seam occur in the northern part of the Buckley area (Figure 26), (Figure 27).

The Drowsell (Massey; Four Foot) Coal is unrecorded in the south of the area, but is possibly equivalent to the upper part of the Powell seam. In the north however, it is apparently represented by one or more coals, up to 1.2 m thick, lying a few metres above the Powell; a comparable sequence is recorded in the northern part of the Buckley area (Figure 26), (Figure 27). The horizon of the Drowsell seam at Rhyd Galed Colliery, Mold, comprises 1.21 m of coal and cannel.

Sandstones equivalent to the Hollin Rock are recognised in several shaft sections in the Mold area (Figure 26). In others however, where the succession above the Hollin Coal is largely reddened, there is little to distinguish the Hollin Rock from sandstones normally occurring within the Ruabon Marl Formation. The Tryddyn Half Yard and Pontybodkin Divided seams are largely unrecorded in this area, but may be represented by two thin coals encountered in the upper part of Broncoed Colliery No. 1 Pit sequence, above the local base of the Ruabon Marl Formation (Figure 26).

Buckley area

The main shaft sections in Middle Coal Measures for this area are given in (Figure 27).

The Crown Coal is 0.6 m thick in workings at Aston Hall Colliery, but unrecorded elsewhere; a thick seatearth underlies the seam, as in other areas. It is tentatively correlated with the Four Foot Mine of the Neston Coalfield. The Lower Bench (Small) Coal lies about 2 m above the Crown and is 0.4 to 0.6 m thick; it also has a well developed seatearth. The Main Coal is usually 3.4 to 3.5 m thick, and was extensively worked in this area; however, the quality of coal varies throughout the seam. It was recorded in a shaft section of Mare Hay Colliery [SJ 2928 6703] (Figure 27) as follows: Flat Coal, 0.53 m; Bon (Bone) Coal, 0.41 m (best quality); Second Coal, 1.15 m (second best quality); Bottom Coal 1.27 m.

The Black Bed (Rough) Coal is a bituminous coal between 0.6 and 1.1 m thick, and locally considered to be a good steam coal.

The Quaker (Brassey) Coal, 0.8 to 1.4 m thick, was generally regarded as a good house coal and also used as a steam and coking coal, but is pyritic in places. The well developed seatearth which underlies the seam is recorded in several sections; at Little Mountain Colliery No. 4 Pit [SJ 2951 6395] (Figure 27) the section is:

Thickness m
Sandstone (Brassey Coal Rock) 2.13
Sandstone and mudstone, interbedded 3.19
Mudstone with sandstone partings and ironstone 2.38
Mudstone, carbonaceous 0.17
Coal 1.04
Shale, dark grey 0.22
Seatearth 1.37

The Crank (Little) Coal is 0.3 to 0.6 m thick in this area and, as elsewhere in the Flintshire Coalfield, is of poor quality.

The Hollin Coal was worked extensively in the Buckley area. It is up to 2.5 m thick east of Buckley, but thins north-westwards to 1.2 m in the vicinity of Northop Hall. Bituminous shale overlies the seam at Little Mountain Colliery No. 4 Shaft (Figure 27), giving way northwards to mudstones with ironstones and striped sandstones and mudstones. Little Mountain Colliery provides a typical section of the seam in the south of the area:

Thickness m
Mudstone, thinly bedded, and sandstones 2.79
Mudstone, bluish grey 1.82
Coal (Powell) 0.91
Mudstone, carbonaceous and seatearth 1.45
Coal (Hollin) 2.44
Seatearth 1.83

The Powell (Bind) Coal is up to 1.1 m thick, thinning north-westwards to 0.5 m in the vicinity of Northop Hall. The thickness of strata between this seam and the underlying Hollin Coal also increases from about 1 to 15 m (Figure 26). The coal correlates with the seam recorded in shaft sections of Rhyd Galed and Alltami collieries, in the northern part of the Mold area (Figure 26).

The Drowsell (Massey) Coal is an inferior seam, about 0.3 m thick, lying 8 m above the Powell Coal. At Little Mountain Colliery (Figure 27) it comprises a seam of carbonaceous mudstone and coal. At Dublin Main Colliery [SJ 2726 6757] it splits into two thin leaves, as the following section shows:

Thickness m
Mudstone, soft, pale grey 2.61
Coal (Small) 0.07
Seatearth and mudstone 0.45
Coal (Drowsell; Massey) 0.25
Seatearth and mudstone 1.98

The maximum thickness of 75 m for the Hollin Rock occurs in the vicinity of Northop Hall. At this locality (Figure 27), ­variegated red and white mudstones, interbedded with the sandstones in the upper 20 m, demonstrate that the upper part of the Hollin Rock is transitional with the Ruabon Marl Formation. The base of the Hollin Rock represents an erosional contact, cutting down progressively south-eastwards (Figure 27) such that, in the area around Elm Colliery [SJ 2754 6624], the Drowsell, Powell and Hollin seams are washed out (Wedd and King, 1924).

At Lane End Colliery, the Upper Main (Massey) Coal comprises two closely spaced leaves of coal, 0.77 m and 1.52 m thick respectively, with an intervening seatearth mudstone (Figure 27).

Queensferry area

Sections through the Middle Coal Measures in the principal collieries of this area are given in (Figure 28).

The Crown is a thin, inferior coal, up to 0.3 m thick where recorded in shaft sections, but underlain by a well developed seatearth, 1.8 m thick in places, with abundant ironstone nodules (Figure 28).

The Lower Bench Coal, 0.3 m thick, lies 11 m above the Crown at Queensferry Colliery, but is otherwise unrecorded in the area, being represented only by its thick seatearth.

The Main Coal is commonly between 2.7 and 3.4 m thick. At Hawarden Castle Colliery Rake Pit [SJ 3323 6566] (Figure 28) the seam is split, the upper leaf probably correlating with the Main Bind Coal.

The Black Bed (Rough) Coal is about 0.6 m thick and is generally unworked. Shaft sections commonly record the thick seatearth underlying the seam and the sideritic ironstone horizon forming the roof. The section of Queensferry (Aston) Colliery No. 5 Pit [SJ 3165 6470] revealed:

Thickness m
Mudstone, carbonaceous about 1.8
Ironstone 0.15
Coal (Rough) 0.61
Seatearth about 2.68

The Quaker (Brassey) Coal is 0.8 to 1.2 m thick and is present in boreholes beneath the alluvium of the Dee estuary (Wedd and King, 1924); a borehole [SJ 3328 6813] proved:

Thickness m
Fireclay, grey and white, sandstone and shale 12.88
Coal (Quaker) 0.91
Fireclay 0.94
Coal (Quaker: Lower Leaf) 0.25
Fireclay and shale 5.23

The Crank (Foul) Coal is about 0.5 m thick. A thick sandstone, overlying the seam at several localities (Figure 28), is probably the local equivalent of the Crank Rock; at Great Mancot Colliery [SJ 3192 6720] the base appears to be erosional down to the level of the coal.

The Hollin Coal is about 2.4 m thick in workings at Mancot Bank Colliery, but is either thin or absent in most sections.

The Powell (Bind) Coal is a maximum of 0.8 m thick at Mancot Bank Colliery, although there is no evidence that it has been worked in the Queensferry area. A sandstone, which may be the local representative of the Hollin Rock, reddened in its upper part, is recorded near the top of the Mancot Bank shaft section. Red mudstones indicative of the Ruabon Marl Formation occur a few metres above the Powell seam in Great Mancot Colliery.

Bagillt area

Within the fault zone which defines the eastern side of the area, much of the upper part of the Middle Coal Measures appears to be absent. Down faulted, red measures are reported locally to rest, apparently unconformably, on strata below the horizon of the Main Coal (Wedd and King, 1924; (Figure 24) and (Figure 29); see below). More complete sequences are preserved to the north-west. The principal shaft sections in the Middle Coal Measures of this area are given in (Figure 29).

The Crown (Mostyn Two Yard; Double) seam comprises two to four closely spaced leaves of inferior coal, interbedded with seatearths (Figure 29). The lower leaves are generally thin, but the upper two leaves are each up to 0.7 m in thickness. However, the seam deteriorates south-eastwards as the separation of the coals increases.

The Lower Bench (Three Yard; Brassy) seam, a good quality steam coal, is 1.2 to 2.3 m thick, with mudstone partings locally near the base. A sandstone commonly underlies the seam; a section of (New Flint) Deeside No. 1 Shaft [SJ 2352 7321] showed:

Thickness m
Shale, black 0.91
Coal (Brassy) 1.68
Dirt 0.23
Coal 0.38
Dirt 0.30
Sandstone, hard, grey 5.11

The Main (Five Yard) Coal is up to 3.7 m thick and was worked from numerous collieries between Bagillt and Flint. However, in the eastern part of the area the seam is absent through unconformity at the base of the Ruabon Marl Formation (Wedd and King, 1924). There are few seams of any importance in the succession above the Main Coal, although the Quaker and Hollinseams may possibly have been worked in the vicinity of Bagillt. The most complete section through the upper part of the Middle Coal Measures in this area was provided by Bettisfield Colliery No. 2 shaft [SJ 2158 7603] immediately north of the Flint district (Figure 29).

Lower and Middle Coal Measures

Denbighshire Coalfield

The small area of the Denbighshire Coalfield in the district includes some Coal Measures which are stratigraphically higher than in the Flintshire Coalfield. The succession generally dips eastwards, passing beneath the Red Measures. Deep mining took place from Llay Main Colliery, and it is from the detailed section of the No. 1 Shaft [SJ 3279 5648] (Figure 30) that the following description is taken. Coal Measures are also present between major splays of the Bala Lineament, where a series of coals ranging from the Hollin Coal down to the Fireclay Group of seams were worked from Hope Colliery [SJ 3115 5650].

The thickness of the Coal Measures in the Llay Main No. 1 Shaft, from the base of the Ruabon Marl Formation to the base of the Nant seam, is about 437 m. The Lower Coal Measures of this succession is comparable to that of the Leeswood area, with interseam measures of similar thicknesses and close correlation between the coal seams (Figure 24). The Middle Coal Measures are generally thicker and more arenaceous around Llay [SJ 3310 5594], and in places, include a thick sandstone, the Crank Rock between the Crank and Hollin seams.

Significant variations in the succession occur above the Hollin Coal in the north of the Denbighshire Coalfield, where a considerably thicker succession of grey measures (up to 260 m), ranging into the Bolsovian Stage, contains several thick coals and marine bands not recorded in the Flintshire Coalfield. These include the Smith’s seam, a good-quality house coal about 0.7 m thick, lying about 5 m above the Drowsell Coal, and the Lower Stinking (Lower Droughy) Coal, a 1.4 m-thick seam in the area around Llay, situated about 11 m above the Smith’s Coal. At several localities in the adjacent Wrexham district, the roof measures of the Lower Stinking seam reveal a rich marine fauna, correlating with the Haughton Marine Band of the Lancashire Coalfield (Magraw and Calver, 1960; Calver and Smith, 1974).

An unworked, thin seam of cannel, a few metres above the Lower Stinking Coal, may be the equivalent of the John o’ Gate of the Wrexham district, because the next higher seam at Llay Main Colliery is reputedly the Warras (New) Coal. The latter is a clean, bright, bituminous coal, 0.5 m thick, lying 20 m above the Lower Stinking seam and also apparently confined to the Denbighshire Coalfield. At localities in the Wrexham district, its roof contains a thin marine band, notable for its rich fauna of calcareous brachiopods. This correlates with the Aegiranum Marine Band of the Pennine coalfields which marks the base of the Bolsovian Stage (Magraw and Calver, 1960; Calver and Smith, 1974).

A group of thin coals, about 26 m above the Warras, includes a seam known hereabouts as the Cannel Coal (Figure 30). At a higher stratigraphical level, two thick leaves (1.4 and 0.7 m) of second-quality, pyritic coal, with intervening measures of coaly mudstone, comprise the Upper Stinking (Top Droughy) seam, the supposed correlative of the Upper Main of the Flintshire Coalfield (Hains, 1991). At the Ty Cerryg opencast site [SJ 293 530] in the adjacent Wrexham district, a marine horizon comparable to the Edmondia Marine Band of the Pennine region was recorded in the measures overlying the upper leaf (Magraw and Calver, 1960).

The Gwersyllt Little, a 0.5 m-thick seam of bituminous coal, present about 14 m above the Upper Stinking Coal at Llay Main Colliery (Figure 30), has no correlative in the Flintshire Coalfield. The succeeding measures are up to 60 m thick, and comprise mudstone with ironstones, coarsening upwards into thinly interbedded sandstones and mudstones. The arenaceous lithologies are considered to be the correlatives of the Cefn Rock (Wedd et al., 1928; Hains, 1991), 25 m thick in this area, but increasing to at least 40 m in parts of the Wrexham district. The Cefn Rock has not been recognised in the Flintshire Coalfield, although it may correspond to one of several sandstone sequences within the Ruabon Marl Formation. The coal overlying the Cefn Rock in the Denbighshire Coalfield is probably referable to the Five Foot seam of Wynnstay Colliery, near Ruabon (Wedd et al., 1928).

A sequence of four important seams, the Bersham Yard Group of coals, is present in the upper part of the Middle Coal Measures in the Denbighshire Coalfield (Figure 24), (Figure 30). The seams are conventionally designated Alpha to Delta in descending order and they represent the highest worked coals of the district. The Delta seam (1.4 m thick) is situated 30 m above the Wynnstay Five Foot Coal in Llay Main Colliery, and is a good quality coal. The Gamma seam (0.5 m thick) lies about 7 m above the Delta Coal and is of comparable quality. The Beta seam, 0.7 m thick, lies about 6 m above the Gamma Coal. An upper leaf, the Beta Rider Coal, commonly splits from the top of the main seam; in Llay Main Colliery, it may be represented by a thin inferior coal, about 0.5 m above the Beta seam. The Alpha seam lies about 10 m above the Beta Coal in this area. It comprises up to 0.8 m of good quality coal in two leaves; the upper leaf is known as the Alpha Rider Coal. Around 20 m of measures overlie the Alpha seam and these represent the highest beds of the Middle Coal Measures in the district (Figure 30). They comprise mainly arenaceous shales and shaly sandstones, interbedded with thick seatearths and coal partings, and they pass upwards into reddened mudstones of the Ruabon Marl Formation. Ramsbottom et al. (1978) implied on their correlation chart that these highest coal measures of the Denbighshire Coalfield equated with strata in nearby coalfields (Lancashire, North Staffordshire) which are included in the Upper Coal Measures. However, there is no fossil evidence to support this (Calver and Smith, 1974), and inter-marine band thickness comparisons with these coalfields suggest that such a correlation is in fact unlikely.

Red Measures Group

The term Red Measures was introduced by Calver and Smith (1974) for all the predominantly red Westphalian strata in the Flintshire and Denbighshire coalfields that lie between a lower, diachronous contact with grey, productive Coal Measures and the sub-Permo–Triassic unconformity. The term is not entirely satisfactory, because it includes some grey measures with inferior coals and restricted marine and brackish faunas. Nevertheless, the group comprises a succession in which oxidised facies of alluvial character predominate and in which no significant worked coals occur. The group includes the Ruabon Marl, Coed-yr-allt and Erbistock formations. The first and last of these divisions comprise both variegated and pervasively reddened strata; the middle division is characterised by grey strata transitional in facies with the underlying Coal Measures.

In the Flintshire Coalfield, only the mid to late Duckmantian Ruabon Marl Formation is present. In the Denbighshire Coalfield, where all three formations are present, the base of the group lies within the younger Bolsovian Stage. The Erbistock Formation also crops out in the Vale of Clwyd, and strata equivalent to the Coed-yr-allt Formation are present at depth to the north in the adjacent Denbigh district (Warren et al., 1984). However, strata equivalent to the Ruabon Marl Formation, and to the whole of the Coal Measures and Millstone Grit groups appear to be absent from much of the Vale of Clwyd (Calver and Smith, 1974; Warren et al., 1984). Any former representatives of these divisions in this area, albeit present only in an attenuated form, were possibly removed during periods of inversion of the local Pennine Basin margin. Tectonism was possibly a consequence of Variscan events farther south (see below; Waters et al., 1994). However, in a more radical interpretation, Williams and Eaton (1993) suggested that much of the north Wales region, west of the present coalfields, was inverted at the close of Dinantian times and remained beyond the limits of Namurian and much of Westphalian deposition.

Powell et al. (1999) proposed a standard lithostratigraphical scheme for the Westphalian red measure sequences throughout the Pennine Basin which, if adopted, will replace the nomenclature used here. Their scheme formalises the acknowledged correlation of the red measure divisions in north Wales with those of the Staffordshire Coalfield (for example Wedd et al., 1928). The new scheme, with the current names used in the Flint district shown in brackets, comprises the Warwickshire Group (Red Measures) composed of, in ascending order, the Etruria Formation (Ruabon Marl Formation), Halesowen Formation (Coed-yr-allt Formation) and the Salop Formation (Erbistock Formation) (Table 12).

Ruabon Marl Formation

The Ruabon Marl Formation, defined in the Wrexham district (Hains, 1991), comprises variegated mudstones with subordinate siltstones and sandstones, some breccias and conglomerates, rare thin Spirorbis limestones and a few thin coals. In this account, the name is extended to include similar strata in the Flintshire Coalfield previously known as the ‘Buckley Fireclay’ (Strahan, 1890; Wedd and King, 1924), ‘Buckley Fireclay Group’ (Calver and Smith, 1974) and ‘Buckley Formation’ (Campbell and Hains, 1991).

In the Flintshire Coalfield, the formation crops out in a series of mainly fault-bounded inliers, most extensively around Mold, Buckley, and in a narrow tract between Flint and Soughton. Many of the crops are drift-covered, but the formation is well exposed in the central part of the Flintshire Coalfield, where the mudstones have long been worked for refractory clays (the ‘Buckley Fireclays’; Chapter 10). In the Denbighshire Coalfield, the formation crops out in a small area in the extreme south of the district, between fractures of the Bala Lineament, south-east of Caergwrle [SJ 3089 5740].

In the Flintshire Coalfield, the Ruabon Marl is over 200 m thick, but the red-bed sequence concealed beneath the Permo–Triassic rocks of the Cheshire Basin exceeds 300 m. The formation consists predominantly of variegated mudstones and siltstones, locally with sideritic concretions, but also includes thick sandstones and rare, impersistent, thin coals. The interbedded mudstones and siltstones are commonly banded or mottled in various colours including red, white, maroon, yellowish brown and greenish grey. Most are poorly bedded seatearths, displaying evidence of widespread rootlet penetration and pedogenesis. Thick units of brick-red mudstone locally display features typical of calcretes, including calcified rootlets (rhizoliths) and glaebules. Dark grey and buff mudstones occur throughout the sequence and locally contain comminuted plant detritus and, in places, develop into thin beds of carbonaceous shale and coal.

The sandstones are white, pinkish or yellowish brown and grey, mainly fine to medium grained, feldspathic and micaceous. They occur in massive, or trough cross-bedded and parallel-laminated units, locally lenticular and channelled, and arranged in fining-upward sequences. The bases of some sandstones contain thin conglomeratic lags of intraformational red mudstone clasts or ironstone pebbles.

Over much of the district, the base of the Ruabon Marl is conformable and gradational, being taken at the first appearance of variegated mudstones. In the Flintshire Coalfield, the base of the formation is Duckmantian in age, but, by reference to correlatable coal seams, can be shown to be markedly diachronous (Figure 24)(Figure 25)(Figure 26)(Figure 27)(Figure 28)(Figure 29) and, in the extreme south, includes a worked seam, the Pontybodkin Mountain Coal (p.105). The formational boundary falls from about 50 m above the Upper Main Coal near Lane End Colliery [SJ 2866 6403] in the Buckley area to within a few metres of the Powell Coal at Broncoed Colliery [SJ 2382 6283] near Mold. Similarly, around Mancot Bank and Great Mancot collieries [SJ 3205 6644] to [SJ 3192 6720] near Queensferry, the boundary also lies several metres above the Powell Coal (Figure 28).

However, locally in the north of the Flintshire Coalfield, the Ruabon Marl appears to rest sharply and with marked unconformity on older parts of the Westphalian sequence. The unconformity has been recorded in strata underlying the Dee estuary, where red measures with marl breccias and Spirorbis limestones abruptly overlie Duckmantian strata only slightly younger than the Vanderbeckei (Llay) Marine Band (Calver and Smith, 1974). The unconformity was also recorded in mine workings in the vicinity of Flint town, where the upper part of the local Middle Coal Measures sequence is missing and the Ruabon Marl locally rests on a horizon below that of the Lower Bench (Three Yard) Coal. Workings in the Flint and Dee Green collieries [SJ 2352 7321] to [SJ 2412 7164] demonstrated a low-angle surface, which truncated successively lower horizons eastward (Wedd and King, 1924), and along which subsequent tectonic movement may have occurred. Other, nearby shaft sections illustrate the localised nature of this cross-cutting relationship, and suggest it is confined to the graben-like tract which encloses the local red measures crop. In contrast, the shafts of Bettisfield and Bagillt collieries [SJ 2158 7603] to [SJ 2168 7560] immediately to the north-west record a Middle Coal Measures sequence up to a horizon above the Powell seam (Figure 29). Even within the graben, but several kilometres farther south around Soughton, Ruabon Marl mapped as contiguous with that around Flint town overlies high Middle Coal Measures in normal succession.

The unconformity undoubtedly postdates the onset of red bed sedimentation in the Flintshire Coalfield, but its exact age is not known. It may be more widespread than currently known, because a comparable erosion surface (or surfaces) may be present, but unrecognised, within the red bed sequences of Ruabon Marl in other parts of the district. Calver and Smith (1974) suggested that the red measures overlying the unconformity should be regarded as equivalent to the younger Erbistock Formation (see below), the intervening Coed-yr-allt Formation having been overstepped. However, in the absence of any faunal or other dating evidence, there is little to distinguish the two red measure formations.

In the northern part of the Denbighshire Coalfield the base of the formation generally occurs a few metres above the highest coals of the Bersham Yard Group, in strata of Bolsovian age. Although its crop is mostly drift covered in this area, a complete section was proved in Llay Main Colliery No. 1 Shaft (Figure 30), where the formation is up to 120 m thick and appears to conformably overlie Middle Coal Measures.

The age of the top of the Ruabon Marl is poorly constrained, due to the lack of diagnostic fauna. A sparse fauna of ostracodes, annelids (Spirorbids), fish and rare mussels has been recorded in places (Calver and Smith, 1974). Specimens of Anthraconauta (including A. cf. phillipsii), from the Castle Clay Pit [SJ 2760 6642] (Wood, 1937; Ramsbottom et al., 1978) are indicative of an upper Bolsovian age. Faunas from the overlying Coed-yr-allt Formation, and equivalent formations elsewhere (see below), indicate that the top of the Ruabon Marl approximates to the base of Westphalian D (Calver and Smith, 1974; Butterworth and Smith, 1976). Calver and Smith (1974) have suggested that much younger Westphalian D strata (the Erbistock Formation) are present in the region of the Dee estuary, but there is no direct evidence that red measures of this age occur within the Flintshire Coalfield.

Conditions of deposition

The Ruabon Marl Formation and the broadly comparable Etruria Marl Formation of Staffordshire, record the widespread accumulation of late Westphalian alluvial floodplain deposits, north of the Wales–Brabant Massif (Edwards, 1951; Besly, 1988; Glover et al., 1993). The gradational base of the formation over much of the Flintshire Coalfield and the northern part of the Denbighshire Coalfield, records the gradual transition from poorly drained delta swamps of the Coal Measures into a low-gradient alluvial plain. The thick, variegated mudstone seatearths aggraded in settings which, for most of the time, were situated above the water table. The degree of mottling and reddening in these lithologies relates to porosity differences resulting from root disturbance and preferential oxidation of iron compounds along root traces. In contrast, the associated dark grey and buff mudstones reflect periodic accumulation under anaerobic, waterlogged conditions and demonstrate the proximity of the main facies to contemporary marine base level.

In the totally reddened Ruabon Marl, the units of structureless, red silty mudstones capped by calcrete-bearing levels record phases of floodplain deposition; each mudstone unit represents one phase of aggradation followed by pedogenesis. Lack of facies diversity, and the association of red beds with rhizoliths, suggests that accumulation took place largely under emergent, oxidising conditions associated with improved drainage. The channel-fill sandstones generally demonstrate fining-upward and lateral accretion, characteristics of meandering fluvial channel deposits. The thin freshwater Spirorbis limestones represent the deposits of ephemeral lakes.

It is generally recognised that the northward progradation of red beds in the Pennine Basin during the late Westphalian marks a switch in the pattern of sediment derivation, from a northerly to a southerly source, associated with tectonic uplift of the Wales–Brabant Massif, possibly linked to events in the Variscan orogenic belt (Besly, 1988; Waters et al., 1994). In this district, the earliest appearance of red beds occurs above the Powell Coal, the roof measures of which are thought to include the Maltby Marine Band; this coincides with the earliest of several phases of red bed formation in the English Midlands, thought to mark the onset of block uplift along pre-existing lineaments (Williams and Chapman, 1986; Besly, 1988).

Differential subsidence across major fault lines during Duckmantian times was probably sufficient to separate well drained areas of red bed sedimentation from poorly drained, coal-forming swamp lands and is likely to account for the diachroneity of red beds within the Flintshire Coalfield. Thus, the local basin between Leeswood and Buckley, formed initially by downwarp against the Great Ewloe Fault during the early Westphalian, continued to accumulate grey measures of the Middle Coal Measures, including coals from the Tryddyn Half Yard to Upper Main seams, at the same time as red beds were accumulating in the adjoining Mold and Queensferry areas; these last named areas, with their comparatively thin sequences of productive coal measures (see above), appear to represent structural highs on the footwalls of major faults, maintaining relatively low subsidence rates throughout the Westphalian.

Continued uplift along the main faults produced a block and basin topography. Elevation of some block areas was evidently sufficient to initiate localised erosion rather than deposition, and led to the development of local unconformities at the base of the Ruabon Marl, as in the Dee estuary and near Flint, and possibly at levels unrecognised within the body of the formation. Although the presence of a regional unconformity at this level has previously been discounted (Calver and Smith, 1974), one was suggested to be present in the Denbighshire Coalfield (Wedd et al., 1928) and in Shropshire (Davies, 1872). A comparable disconformity (the Symon Unconformity) also occurs beneath the equivalent Etruria Formation of the Coalbrookdale Coalfield (Hamblin, 1993; Hamblin and Coppack, 1995), and one has been recorded below supposed Westphalian red beds in north-west Wales (Greenly, 1919, 1938). Taken together, this evidence appears to indicate an episode of uplift and erosion around the margins of the Pennine Basin during Duckmantian to Bolsovian times. Waters et al. (1994) considered this to be a local tectonic readjustment of the basin margins to continued thermal sub­sidence and, within the Flintshire Coalfield, as in the area around Flint, the effects of this uplift also appear to be localised along major fractures. The precise timing of this uplift is difficult to constrain in the Flint district, and therefore there is uncertainty as to its relationship with other events along the southern margin of the basin.

Details

The main shaft sections through the Ruabon Marl are provided by (Figure 25), (Figure 26), (Figure 27), (Figure 28), (Figure 29), (Figure 30). The principal section through the formation in the Denbighshire Coalfield was provided by Llay Main Colliery No. 1 Shaft (Figure 30).

Purple sandstones and mudstones observed by Strahan (1890), once exposed in a quarry [SJ 2232 6396] south-east of Maes-garmon, are of historical importance. Strahan included these strata in his ‘Lower Coal Measures’ (Holywell Shales and Gwespyr Sandstone of this account), but was unable to account for their colour; a problem resolved by their inclusion here in the tract of Ruabon Marl downfaulted to the east of the Nercwys Fault. Their similarity to feldspathic purple beds in the Vale of Clwyd, led Strahan to cite this as evidence that the latter were also Upper Carboniferous in age. This was disputed (Morton, 1883–6; Neaverson, 1945) but it has been shown to be the case (Warren et al., 1984. However, the Red Measures present in the Vale of Clwyd are here included in the Erbistock Formation (see below).

The major sections in the Ruabon Marl occur in extensive workings around Buckley where it was locally known as the ‘Buckley Fireclay’. The main working face of Lane End Clay Pit [SJ 2872 6442] to [SJ 2879 6438] proved up to 21 m of variegated purple, maroon and yellow, structureless mudstones with abundant rhizoliths, interbedded with fine- to medium-grained, white and grey, locally channel-form sandstones.

At the former Ewloe (Trap) Clay Pit [SJ 2791 6543] to [SJ 2795 6512], a comparable sequence of up to 10 m of interbedded sandstones and mudstones is exposed. There the sandstones are locally 4 m thick, in channelled units displaying well developed cross-bedding.

The southern part of Brookhill Clay Pit [SJ 2783 6546] to [SJ 2790 6557] reveals the junction of the Ruabon Marl with the underlying Middle Coal Measures. The main part of the section comprises up to 30 m of purple, yellow and grey mottled, silty mudstones with rhizoliths, interbedded with fine- to medium-grained and locally coarse-grained, feldspathic, channel-form sandstones; a thicker sandstone forms the upper 6 m of the section. Beds of predominantly grey mudstone are interbedded in the lower part of the sequence, and a 0.12 m-thick seam of coal with a mudstone seatearth occurs at the base of the section. The base of the formation is taken at the lowest variegated mudstone horizon, which is situated about 2 m above the coal. The seam is probably the Upper Main which, at Lane End Colliery [SJ 2866 6403] 1.5 km to the south, lies about 49 m below the base of the red measures. These relationships reinforce the fact that locally, the base of the Ruabon Marl is markedly diachronous.

The former Ewloe Barn Clay Pit revealed a section [SJ 2779 6580] to [SJ 2786 6583], comprising purple, yellow-brown and grey mottled mudstones with rhizoliths, and interbedded white to pale grey, fine-grained sandstones. At the top of the section, a 7 m-thick, feldspathic sandstone with well developed cross-bedding was exposed; it appears to be analogous to that at the top of the previous described section.

A comparable sequence is exposed in the Castle Clay Pits [SJ 2755 6650], where the 7 m-thick sandstone forms the top of the section in the north-east face [SJ 2758 6673] to [SJ 2760 6655]; the large-scale cross-bedding in this unit is particularly well displayed here. The remainder of the section comprises up to 15 m of purple, greenish grey and white variegated silty mudstones, with plant remains and rhizoliths, and interbedded fine-grained sandstones. In places, the mudstones are colour laminated and contain thin tabular sideritic ironstones.

An old quarry and crags [SJ 2902 6765] to [SJ 2902 6775] near Ewloe Castle show up to 12 m of sandstone, considered to be the upper part of the Hollin Rock. It is greyish brown, with reddening along some bedding planes and joints, medium- to coarse-grained and feldspathic. Intraclasts of red mudstone are concentrated in a number of beds, and large- and small-scale cross-bedding is well developed.

Small exposures of yellowish to pinkish brown, feldspathic sandstones were noted in an old quarry [SJ 2741 7073] and nearby stream [SJ 2735 7072] south-west of Kelsterton Farm. To the north, a degraded cliff section [SJ 2722 7130] to [SJ 2726 7128] east of Oakenholt, exposed up to 3.3 m of purple, reddish brown and greyish green, mottled mudstones, and interbedded yellowish brown, feldspathic sandstones, locally containing clasts of reddened sideritic nodules.

Coed-yr-allt Formation

The Coed-yr-allt Formation (Hains, 1991; Coed-yr-allt Beds of previous authors) consists predominantly of grey mudstone, sandy mudstone and sandstone, with thin coals and seatearths, local ironstone beds and nodules, and scattered limestone beds. The lithologies are commonly arranged in cyclical sequences, com­parable to those of the Coal Measures. Although the formation is mainly grey, parts of it display red and purple banding and mottling. The lower and upper boundaries are gradational over several metres with the red and variegated Ruabon Marl and Erbistock ­formations.

The formation is absent from the Flintshire Coalfield but it is present in the extreme south of the Flint district around Cefn-y-bedd [SJ 320 560] where it is completely drift covered. A section through the formation was proved in the Llay Main Colliery No. 1 Shaft [SJ 3279 5648] (Figure 30), where it is about 140 m thick and apparently conformable on the Ruabon Marl Formation (but see Erbistock Formation below). It may also be present in parts of the Vale of Clwyd, where boreholes have locally proved substantial thicknesses of upper Westphalian strata (Calver and Smith, 1974). Up to 40 m of cyclical red and grey measures, comparable to the Coed-yr-allt Formation, were encountered in the lower part of the St Asaph Borehole [SJ 0366 7312] in the adjacent Denbigh district; these strata are thought to rest unconformably on Dinantian rocks (Warren et al., 1984).

The nonmarine bivalves A. phillipsii and A. tenuis, as well as ostracodes, Spirorbis, fish fragments and plant remains have been recorded from the formation in the Wrexham district. On the basis of the faunal evidence in the area, Calver and Smith (1974) suggested that the base of the formation approximates to the Bolsovian–Westphalian D boundary. Miospores from the equivalent Newcastle Formation of the Staffordshire area corroborate a Westphalian D age for the lower formational boundary (Butterworth and Smith, 1976). Faunas and miospores recovered from the equivalent strata in the St Asaph Borehole, in the Vale of Clwyd, suggest a consistent Upper Coal Measures age, near to the Bolsovian-Westphalian D boundary (Calver and Smith, 1974, Warren et al., 1984).

Conditions of deposition

The Coed-yr-allt Formation and its correlatives in the Midlands coalfields (Besly, 1988; Powell et al., 1999), comprise a lacustrine association of freshwater limestones, shales and deltaic lake-fill sequences capped by palaeosols and thin coals. The widespread occurrence of this facies suggests that a fairly uniform pattern of sub­sidence was established to the north of the Wales–Brabant Massif during Westphalian D times (Waters et al., 1994). The causes of the change from red to grey measures are unknown, but appear to reflect fluctuations in sub­sidence rate in response to Variscan events to the south.

Erbistock Formation

The Erbistock Formation of this district (Hains, 1991; Erbistock Beds of previous authors) comprises at least 200 m of red and purple mudstone, locally with green mottling or banding, interbedded with grey, purple and variegated feldspathic, pebbly sandstone, subordinate grey mudstone with thin coals and scattered thin limestones. The dominant mudstone facies exhibits the effects of extensive pedogenesis and includes levels with abundant calcrete glaebules and rhizoliths.

The formation succeeds the Coed-yr-allt Formation in the Denbighshire Coalfield, in the south of the district, where a section through its lower part is provided by the Llay Main Colliery No. 1 Shaft (Figure 30).

Purplish red sandstones and mudstones rest unconformably on Dinantian limestones in the Vale of Clwyd, west [SJ 1237 5825] and north-west [SJ 100 610] of Ruthin, south-east of Gellifor [SJ 140 620], and between splays of the Vale of Clwyd Fault, near Llangynhafal [SJ 130 643]. They have been previously interpreted as ‘Lower Coal Measures’ (Holywell Shales/Gwespyr Sandstone; Strahan, 1890), Cefn-y-fedw Sandstone (Wedd et al., 1928), or a late Dinantian clastic sequence (Morton, 1898; Neaverson, 1945). However, comparable facies, from boreholes in the northern part of the Vale of Clwyd, were considered by Calver and Smith (1974) to equate with the Erbistock Formation. Up to 90 m of these strata were proved in the St Asaph Borehole [SJ 0366 7312], overlying Bolsovian–Westphalian D grey measures comparable to the Coed-yr-allt Formation (Warren et al., 1984; see above). Therefore, it is tentatively suggested that the red measures in the southern part of the Vale of Clwyd are likewise of late Westphalian age, probably correlating with the Erbistock Formation in other parts of the district.

The suggestion of a regional unconformity at the base of the Erbistock Formation, to explain its apparent overstep relationships in north Wales (Wedd et al., 1929; Calver and Smith, 1974) is not supported by current regional ­lithostratigraphical correlations (Waters et al., 1995; Besly and Cleal, 1997). It now appears that the Erbistock and Coed-yr-allt formations are part of the widely recognised conformable sequence of grey to red measures, that extended across much of the southern part of the Pennine Basin, bordering the Wales–Brabant Massif. At the margins of the basin, a widespread unconformity, the pre-Halesowen unconformity of Waters et al. (1994), is developed below strata equivalent to the Coed-y-allt Formation. In this context, the unconformable relationships at the base of the Erbistock Formation in the south of the Vale of Clwyd (and, arguably in the Dee estuary; see above) can be interpreted as local overlap along the partly fault-controlled basin margin in north Wales.

In reality, however, it is not known if the stratigraphical relationships of the Erbistock, Coed-yr-allt and Ruabon Marl formations were maintained throughout the district, due to poor exposure and the lack of precise dating of the red measures in the southern part of the Vale of Clwyd. It is possible that the sequences preserved here could locally include Westphalian red measures of differing ages, resting on erosional surfaces of variable magnitude and extent, that formed during one or more periods of intra-Silesian uplift of the basin margin.

The only recorded fossils from the formation in the Denbighshire Coalfield are plants, Spirorbis and tetrapod footprints (Calver and Smith, 1974). Here the Erbistock Formation is probably entirely Westphalian D in age, but thicker sequences recorded below the Permo–Triassic rocks of the Cheshire Basin (Earp and Taylor, 1986), and laterally equivalent sequences in the Midlands, may range into the Stephanian or even early Permian (Waters et al., 1995). In the Vale of Clwyd, palynomorphs including Crassispora kosankei, Lycospora, Densosporities, Calamospora, ?Florinites and the freshwater alga Botryococcus, collected from exposures [SJ 1411 6187] in a stream section south-east of Gellifor, confirm a Silesian age.

Conditions of deposition

The Erbistock Formation and the equivalent Keele Formation of the English Midlands (Powell et al., 1999) record a return to well drained, oxidising, alluvial floodplain conditions throughout the area north of the Wales–Brabant Massif, during the latest Westphalian. Depositional environments were broadly comparable to those of the Ruabon Marl Formation, but evidence of increasing aridity is indicated by the presence of widespread calcrete features including calcified roots (rhizo­liths) and glaebules. It has been suggested that this climatic change was due to the growth of a rain-shadow north of the Variscides during the latest Westphalian (Besly, 1988).

Details

The Llay Main No. 1 Colliery shaft [SJ 3279 5648] provided the best section through the Erbistock Formation in the northern part of the Denbighshire Coalfield.

Details of strata in the Vale of Clwyd here assigned to the Erbistock Formation were given by Strahan (1890, pp.11–13). The main extant section [SJ 1388 6179] to [SJ 1415 6185] occurs in a stream north-east of Llanbedr Farm [SJ 1360 6170] which exposes an unconformable contact with Dinantian limestones of Brigantian age ((Table 8), locality 16). The section shows an incomplete sequence through purplish brown, micaceous, locally pebbly, feldspathic sandstones, with subordinate conglomerate beds up to 0.4 m thick, interbedded with purple silty mudstones containing rhizoliths and plant debris. At the eastern end of the section Strahan reported the presence of ‘about 20 feet of black shales with a small seam of thin coal, on which some shallow shafts have been sunk’ (no longer visible), above purple sandstones and a basal conglomerate. An assemblage of long ranging Silesian palynomorphs (see above) was obtained from exposures near [SJ 1411 6187] about 25 m downstream from the contact with the limestones.

Westphalian rocks beneath the Cheshire Basin

Subsequent to the publication of the 1:50 000 sheet, the geophysical logs obtained from Westphalian sequences proved in the Blacon East and Blacon West boreholes have been reassessed by Smith et al. (in press). In both boreholes, the Westphalian rocks underlie the Permo–Triassic Kinnerton Sandstone. In the Blacon West Borehole, a 350 m-thick Langsettian to Duckmantian, predominantly Coal Measures sequence, is overlain by around 385 m of Red Measures. On the map cross-section, all these Red Measures were included in the Ruabon Marl Formation, but the new interpretation now suggests that all three Red Measure Group divisions are present, the Ruabon Marl comprising only the lowest 120 m. The 100 m-thick Coed-yr-allt Formation is succeeded by 160 m of strata assigned to the Erbistock Formation. The Kinnerton Sandstone rests on the latter unconformably.

In the Blacon East Borehole, less than 30 m of Ruabon Marl are preserved beneath a faulted contact with the Kinnerton Sandstone.

Chapter 6 Permo-Triassic

The Permo–Triassic rocks of the district comprise about 800 m of aeolian and fluvial red beds, mainly sandstones and conglomeratic sandstones, forming part of the Sherwood Sandstone Group. They crop out in the west in the Vale of Clwyd, and in the east in the Cheshire Plain (Figure 31), (Figure 32). Both crops are situated at the margins of major Permo–Triassic basins which formed part of a series of linked, post-Carboniferous rift basins extending south-eastwards from the Clyde Belt in Scotland (McLean, 1978) though the Worcester graben to the Paris Basin in France (Jackson and Mulholland, 1993). The Vale of Clwyd crop lies within a southern extension of the East Irish Sea Basin which is now separated from the Permo–Triassic outcrops of the latter following subsequent faulting and erosion. The larger crop, in the east of the district, lies along the western margin of the Cheshire Basin. Both the Vale of Clwyd and Cheshire basins are asymmetric, half-graben structures with partly fault-defined margins (Figure 31). Their development during the Permo–Triassic was a response to the onset of east–west extensional tectonism associated with the break up of Pangaea and the opening of the North Atlantic Ocean (Chadwick et al., 1990; Coward, 1993).

Early accounts of the Permo–Triassic rocks (‘New Red Sandstone’) of the Flint district and adjacent areas were given by Hull (1860, 1869), Ramsay (1866, 1881) and Hughes (1885). Further descriptions were given in Geological Survey memoirs for the district (Strahan, 1890; Wedd and King, 1924), and in Reade (1891). Relevant studies of the Permo–Triassic rocks of the district and adjacent areas included those by Thompson (1969, 1970 a, b, 1985, 1989), Poole and Whiteman (1966), Hains and Horton (1969), Earp and Taylor (1986) and Evans et al. (1993). The deep structure of the two Permo–Triassic basins was studied by Powell (1956), Wilson (1959) and Collar (1974). Earp and Taylor (1986) give details of deep boreholes penetrating the Permo–Triassic sequences principally in the adjacent Chester district, but also in the east of the Flint district. The stratigraphy, structure and regional geological framework of the Cheshire Basin have been described by Colter and Barr (1975), Jackson et al. (1987), Evans et al. (1993) and Smith et al., (in press). Jackson et al. (1995) assessed the sequences in north Wales in the regional context of the offshore East Irish Sea Basin.

The stratigraphical nomenclature used in this account follows Warrington et al. (1980). This, together with earlier nomenclature and that used in adjacent areas are shown in (Table 14).

Sherwood Sandstone Group

In the district, the Sherwood Sandstone Group everywhere rests unconformably on Carboniferous rocks. It is up to 800 m thick and incompletely preserved in the district. To the east, in Cheshire, the group dia­chron­ously succeeds the early Permian to early Triassic, ­Manchester Marl Formation (Warrington et al., 1980). The group is divided into two formations. The lower, Kinnerton Sandstone Formation is distinguished from the overlying Chester Pebble Beds Formation mainly by the absence of pebbly facies (Table 14).

Kinnerton Sandstone Formation

The Kinnerton Sandstone Formation consists mainly of red, brown and yellow friable sandstones. The type section in Brad Brook [SJ 329 604], near Higher Kinnerton (p.120), is located in east of the district, where the formation has an extensive crop on the western margin of the Cheshire Basin. The formation also crops out in the Vale of Clwyd where it is the only Permo–Triassic division preserved (Figure 32). Borehole and seismic data indicate that the formation is over 600 m thick at the western margin of the Cheshire Basin. In the Vale of Clwyd, borehole and geophysical data suggest a maximum preserved thickness of more than 525 m (Collar, 1974; Warren et al., 1984).

The age of the Kinnerton Sandstone sequence along the eastern margin of the Flint district is uncertain, but it probably spans the Permian–Triassic boundary. It has been suggested that the lower part is the lateral ­equivalent of the predominantly Permian Collyhurst Sandstone and Manchester Marl formations of central Cheshire (Smith et al., 1974; Warrington et al., 1980; Colter and Barr, 1975; Evans et al., 1993). In contrast, Jackson et al. (1995) considered the whole of the Kinnerton Sandstone to be pre-Triassic in age, and excluded it from the Sherwood Sandstone Group. They further suggested that the sequence preserved in the Vale of Clwyd Basin is exclusively early Permian in age and, hence, a direct correlative of the Collyhurst Sandstone. The formation as a whole is the lateral equivalent of the Collyhurst Sandstone, St Bees Evaporite and St Bees Shale formations, and arguably the lower part of the St Bees Sandstone Formation of the East Irish Sea Basin (compare Smith et al., 1974; Warrington et al., 1980; Jackson et al., 1995).

The formation rests unconformably on Carboniferous rocks both in the Vale of Clwyd (Strahan, 1890) and along the western margin of the Cheshire Basin. The junction of the Kinnerton Sandstone with the Namurian Gwespyr Sandstone was previously well exposed in the type section in Brad Brook, but the section is now degraded. Hull (1869), and subsequently Strahan (1890) and Wedd and King (1924) interpreted the junction as an angular unconformity, but Thompson (1985, 1989) suggested that the contact may be faulted.

The sandstones are dominantly fine to medium grained and predominantly dune cross-bedded (Plate 13; (Figure 33)). Individual cross-sets range, exceptionally, to over 10m in thickness. Planar-laminated sandstones are also common. Rare thin beds of red mudstone, pebble conglomerate and mud-flake conglomerate are also present in the formation. Typically, the sandstones comprise well rounded and generally well sorted grains of quartz, with subordinate feldspar and lithic grains. Harris (1924), Double (1926) and Warren et al. (1984) showed that the heavy mineral assemblages contained significant proportions of ilmenite, zircon, rutile and tourmaline and lesser amounts of monazite, apatite, brookite and garnet.

From sections no longer visible along the eastern side of the Vale of Clwyd, Strahan (1899) and Wedd et al. (1928) reported fragments of local Carboniferous and possibly Silurian rock types contained within the local Permo–Triassic sandstones. The presence of Silurian clasts, if confirmed, would imply that early Permian erosion had locally stripped these rocks of their Carboniferous cover.

Conditions of deposition

The Kinnerton Sandstone was deposited mainly by aeolian processes, though there is evidence of localised fluvial and lacustrine deposition. Aeolian deposition is indicated by the presence of dune cross-bedding associated with typical interdune planar laminated units and well rounded ‘millet seed’ sand grains. Thompson (1985) interpreted many of the structures in the exposures at Kinnerton [SJ 330 605] as typical of transverse-barchanoid ridge-dunes or draa. A similar interpretation was favoured by Macchi and Meadows (1987) and Macchi (1991) for the Burton Point section [SJ 3023 7356] (Figure 33). They suggested that the dune-bedded sandstones accumulated as migrating transverse ridges and that interbedded, planar- to low-angle cross-stratified sandstones were a flash flood, sheet-sand facies deposited in interdune areas. Although much of the formation in the Vale of Clwyd is aeolian in character, pebble conglomerates, mud-flake breccias and red mudstone and siltstone beds indicate that ephemeral lacustrine and flash flood fluvial conditions operated periodically.

On the evidence of heavy minerals, Double (1926) suggested a northerly, possibly Scottish, source for the Kinnerton Sandstone. However, palaeocurrent measure­ments from dune cross-bedded strata in both the Vale of Clwyd and the western Cheshire Basin (Figure 32) indicate that the prevailing palaeowinds were from the east (Shotton, 1956; Thompson, 1985; Macchi, 1991).

Details

Western Vale of Clwyd

Plas-yr-Esgob No. 1 Borehole penetrated 54.56 m of cross-bedded sandstone.

Numerous exposures in Ruthin show up to 4 m of reddish brown, dune cross-bedded sandstones with well rounded ‘millet seed’ grains as in Llanrhydd Street [SJ 1304 5798] to [SJ 1317 5796]: similar sandstones are present near the junction of Rhos Street and Mold Road [SJ 1302 5835] to [SJ 1286 5826] and beneath Ruthin Castle [SJ 1230 5806].

Reddish, parallel to low-angle cross-bedded sandstones, with well rounded grains, are seen in sections along the A525 road at Pen-y-Maes [SJ 1290 5766] and Cantaba Farm [SJ 1294 5704]. Reddish, fine- to medium-grained, dune cross-bedded sandstones, with some polished grains occur in numerous exposures [SJ 1118 5867]; [SJ 1117 5854]; [SJ 1116 5851]; [SJ 1112, 5837]; [SJ 1117 5838] along farm tracks and dry valleys between Ty’n-y-caeau and Llanfwrog, to the west of Ruthin. In a disused quarry at Penstryt [SJ 1112 5780], cross-bedded sandstones are overlain by an inversely graded sandstone breccia; a bed composed of angular, grey siltstone clasts within a red sandstone matrix occurs at the eastern side of the quarry.

Eastern Vale of Clwyd

The most northerly exposures occur in the east–west valley at Tan-y-Glyn [SJ 1223 6520] to [SJ 1215 6520] to [SJ 1205 6515], 1.5 km east of Llandyrnog. At the first locality, around 2.5 m of friable, pink to reddish brown, well sorted, medium-grained sandstone with subrounded grains, dips steeply to the west. Similar lithologies occur in the stream section to the west. The Waen Borehole proved 13.71 m of alternating sandstone and mudstone beds, overlying 45.12 m of sandstone. Sporadic exposures [SJ 1272 6428] to [SJ 1257 6421] to [SJ 1171 6375] to [SJ 1125 6365] in the stream between Pentre Farm and Llawog reveal red, cross-bedded sandstones with westward dipping foresets. Similar strata occur in roadside exposures at Seler [SJ 1256 6400] and Ty-coch [SJ 1292 6376].

Minor streams and roadside cuttings around Llangynhafal provide exposures [SJ 1285 6360]; [SJ 1294 6357]; [SJ 1329 6349]; [SJ 1314 6331; [SJ 1320 6335]; [SJ 1319 6334] in reddish brown, medium-grained sandstone and pinkish, cross-bedded, coarse-grained, feldspathic sandstone. In a farm track [SJ 1341 6314]; [SJ 1353 6319] at Bryn-bedw, cross-bedded sandstones occur close to the faulted contact with the Erbistock Formation.

A farm track [SJ 1368 6240]; [SJ 1375 6241] between Tyn-y-Celyn and Plas Draw exposes westerly dipping, reddish brown ­sandstones; steep north-easterly inclined shear planes demonstrate the proximity of the Vale of Clwyd Fault. Cross-bedded sandstones also occur in the stream to the south [SJ 1375 6232] to [SJ 1384 6232] to [SJ 1373 6232]. At Plas Draw, the section in the lane north of the farm referred to by Strahan (1890, fig. 2) is no longer visible, though the junction here between the Kinnerton Sandstone and Erbistock formations can be located accurately [SJ 1385 6233] and traced southwards towards Llanbedr Farm [SJ 1370 6175].

Red sandstone is exposed in the stream [SJ 1343 6164] north-west of Hirwaen and in sporadic roadside exposures [SJ 1365 6148]; [SJ 1369 6143]; [SJ 1380 6126] to [SJ 1380 6124] both in and to the south of the village. At the last locality [SJ 1380 6124], a ‘spotted’ ironstone unit, up to 0.1 m thick, cross-cuts the cross-lamination in westward dipping red sandstones.

Track and valley-side exposures [SJ 1404 6133] to [SJ 1406 6132] to [SJ 1405 6130] to [SJ 1434 6136] and a former building stone quarry [SJ 1407 6130] to [SJ 1418 6135], between Hirwaen and Fron Ganol all expose typical, red to reddish brown, cross-bedded, medium-grained sandstones. Bedding and cross-bedding in these sections are both inclined, almost exclusively to the west; dune cross-bedding foresets dip at between 25° and 28°. The stone quarry provides a 35 m-thick section in which predominantly medium-grained, locally feldspathic sandstones are cut by numerous small normal faults.

Locally cross-bedded, red sandstones occur along farm tracks [SJ 1406 6076] to [SJ 1413 6081] to [SJ 1446 6048] near Teiran Farm, south-east of Hirwaen, and in stream courses [SJ 1442 6027] to [SJ 1453 6002] to [SJ 1456 6035] to [SJ 1420 6014] to the south. A dingle [SJ 1472 5953] to [SJ 1466 5952] to [SJ 1463 5949] north of Llanbedr-Dyffryn-Clwyd exposes reddish brown, cross-bedded, medium-grained sandstones with a high proportion of well rounded quartz grains.

Head of the Vale of Clwyd

The Oaklands Bridge Borehole, sited 400 m to the south of the district, provided an important section in the Kinnerton Sandstone, abridged as follows:

Thickness m Depth m
Quaternary
No record 18.58 18.58
Permo–Triassic
Kinnerton Sandstone Formation Sandstone, red, soft, fine to medium-grained, massive to laminated, with rare cross- laminations; sporadic coarser grained, graded beds to 15 mm; sporadic black beds and silty partings 82.00 100.58
Pebble conglomerate, reddish brown; clasts to 5 mm across; marl laminae 0.26 100.84
Marl, red, micaceous, laminated 0.70 101.54
Sandstone, red, medium-grained, poorly bedded, with irregular laminae and flakes of slickensided mudstone between 103.59 and 104.39 m 4.53 106.07
Breccia, mudflake 0.30 106.37
Sandstone, red, fine-grained, with sporadic coarser grained beds to 50 mm, some mud-flakes; below 114.25 m fine and coarse laminae become abundant 15.51 121.88
Burton area

The abandoned sea cliffs at Burton Point [SJ 3023 7356] to [SJ 3041 7361] expose the uppermost beds of the Kinnerton Sandstone and their contact with the Chester Pebble Beds Formation ((Figure 33), (Plate 13), Plate 14). The section was described by Hull (1869), Strahan (1899) and Wedd and King (1924) and assessed sedimentologically by Macchi and Meadows (1987) and Macchi (1991) (Figure 33). The lowermost beds, east of the main cliff section, include sandstones with channel-like bedforms and abundant evidence of soft-sediment deformation. The lower sequence also contains beds of irregular, parallel- and low-angle cross-stratified sandstone, which is locally bioturbated. In the upper 30 m, reddish brown to yellowish brown, well sorted, fine- to medium-grained sandstones exhibit large-scale dune cross-bedding.

Small exposures of comparable Kinnerton Sandstone were noted in the foundations of houses and walls along the main road in Burton [SJ 3120 7427] to [SJ 3187 7441]. Coarser grained sandstones with some pebbles, overlying the most easterly of these exposures, are included in the Chester Pebble Beds. Two metres of red, fine-grained sandstone, with dune cross-bedding, are exposed by the side of the Burton to Puddington Road [SJ 3175 7415].

Higher Kinnerton

In its type section in Brad Brook [SJ 329 604], the basal 3 m of the formation comprise dark purple, coarse- to medium-grained sandstones and breccias with abundant angular to subangular fragments from the underlying Carboniferous sandstone, as well as ironstone clasts and white, well rounded quartz pebbles. In the succeeding sandstones, scattered thin beds and lenses of conglomerate become progressively finer grained upwards and well rounded pebbles and granules of quartz increasingly dominate the clast assemblage.

Nearby, at Kinnerton Sand Pit [SJ 330 605], higher beds comprise predominantly pinkish red, fine- to coarse-grained sandstones, with yellow mottling. Trough cross-bedding displays considerable variation in style and orientation; individual cross-sets range from 2 m to over 10 m in thickness (Thompson, 1985).

Chester Pebble Beds Formation

This formation crops out in the north-east corner of the district where it consists predominantly of pebbly and medium-grained sandstone. The top of the formation is not preserved in the district, but over 200 m is present above the Kinnerton Sandstone. Limited palaeontological evidence from formations stratigraphically above and below it elsewhere, favours an early Triassic, probably lower Scythian age (Pattison, 1970; Pattison et al., 1973; Warrington et al., 1980). Jackson et al. (1995) followed Smith et al. (1974) in equating the base of the Chester Pebble Beds with the Permian–Triassic boundary. In contrast, Warrington et al. (1980) suggested that the base is diachronous, and that parts of the underlying Kinnerton Sandstone are Triassic in age.

The Chester Pebble Beds Formation passes laterally northwards into a more distal sandstone facies largely indistinguishable from the Wilmslow Sandstone Formation which succeeds it elsewhere in the Cheshire Basin (Warrington et al., 1980). Together, these divisions are laterally equivalent to much of the St Bees Sandstone Formation of the East Irish Sea Basin (Jackson et al., 1987, 1995).

The formation overlies the Kinnerton Sandstone with a sharp and probably erosional base, as seen at Burton Point [SJ 3023 7356] to [SJ 3014 7369] ((Figure 33), Plate 14). The basal 25 m here comprise units, up to 5 m thick, of reddish brown, cross-bedded, conglomeratic and pebbly sandstone separated by thinner units of parallel-laminated, medium-grained sandstone. The sand-grade component of these lithologies is generally poorly sorted and ranges from ­subangular to subrounded, with rarer well rounded grains. In the conglomeratic and pebbly lithologies, the well rounded pebble-grade clasts are dominantly of vein quartz and quartzite, with scattered igneous material. The conglomeratic and pebbly units comprise a series of superposed tabular and trough cross-bedded sets. Some cross-sets exhibit strongly erosional bases with overlying lenticular pebble and intraclast lags. Many beds exhibit upward-fining motifs. Both borehole and outcrop data suggest that the pebbly facies dominates only in the lower part of the formation, but becomes less common within a sandstone-dominated upper part. However, the sandstones of the Chester Pebble Beds are distinguished from those of the Kinnerton Sandstone by coarseness, grain angularity and poor sorting, and by the dominance of tabular, rather than dune cross-bedding.

Conditions of deposition

The sedimentary structures displayed by the Chester Pebble Beds indicate deposition by low-sinuosity rivers flowing across the surface of a large alluvial fan complex (Macchi and Meadows, 1987; Macchi, 1991), an inter­pretation consistent with studies elsewhere in the Cheshire Basin (for example Fitch et al., 1966). Palaeocurrent data indicate sediment transport predominantly from the south-east ((Figure 32); Thompson, 1985).

Details

Burton

The best section is in the cliffs at Burton Point [SJ 3023 7356] where the junction with the underlying Kinnerton Sandstone is exposed ((Figure 33), Plate 14).

Coarse-grained sandstone with scattered pebbles occurs in small roadside sections [SJ 3114 7437] to [SJ 3111 7446] in Burton and in a track section [SJ 3107 7463] south of Red Bank Farm. The railway cutting west of Burton [SJ 3044 7380] to [SJ 3027 7500] and continuing northwards to the edge of the district provides an extensive, though now largely overgrown section, described by Wedd and King (1924).

A discontinuous, 4 m-thick section in reddish brown, coarse-grained sandstone with small rounded cobbles and pebbles of quartz and quartzite is exposed in a quarry [SJ 3269 7487] north-east of Burton. In Shotwick Dale, up to 6 m of similar sandstone is exposed in a quarry [SJ 3490 7287].

Chapter 7 Tertiary

Small, widely separated ‘pockets’ of sand, silt, clay and breccia occur in the Dinantian limestones in the district ((Figure 34); (Table 15)). They are thought to represent remnants of a formerly continuous sheet of late Tertiary (?Neogene) fluviolacustrine sediments preserved in solution pipes. The deposition of these sediments is presumed to post-date a period of pronounced early Tertiary uplift, during which the previously more extensive Mesozoic and Upper Palaeozoic sequences were deeply eroded in common with large areas of northern Britain and the Irish Sea (Brown, 1960; George, 1974; Tappin et al., 1994; Jackson et al., 1995). Several events may have contributed to this phase of uplift. It has been attributed both to thermal uplift associated with the igneous activity of the Tertiary Volcanic Province (related to major rifting events in the North Atlantic), and to tectonic inversion consequent on Alpine collision events in southern Europe (Murdock et al., 1995; Jackson et al. 1996). Eustatic regression may also have played a part (Haq et al., 1987). However, Cope (1994) suggested that uplift was more localised and resulted from a late Cretaceous to early Tertiary ‘hot-spot’ centred beneath the Irish Sea and formed above an upwelling mantle plume. Decay of this plume, during the Oligocene (late Palaeogene), led to subsidence of the region. This subsidence, locally accommodated by movements on pre-existing fractures, established the landscape in north Wales on which the late Tertiary deposits of the district accumulated. The associated marine flooding of the collapsed centre of the dome led to the creation of the present, partly fault-defined Irish Sea Basin (Cope, 1994, 1995, 1997).

During the early 19th century, the ‘pocket’ deposits of the district were worked commercially as sources of both pipe clay and of fine siliceous sand, used in the manufacture of porcelain. The deposits are poorly exposed today, and much of the following account is based on earlier observations made at the former workings. The earliest accounts by Traill (1821) and Bishop (1822) were concerned mainly with the commercial aspects of the deposits. The first geological descriptions were given by Maw (1867), and the deposits were subsequently assessed by Mackintosh (1874a), Strahan (1890), Boswell (1918) and Wedd and King (1924). The most recent and comprehensive review was provided by Walsh and Brown (1971) (Table 15).

The deposits are commonly sited along or adjacent to worked mineral veins, and this may account for their discovery. They are located mainly beneath steep-sided surface depressions formed over solution pipes in Dinantian limestones. Some of the deposits however, may represent passive infills of subterranean fissures and solution channels within the limestones. The surface depressions are probably due to the combined effects of earlier extraction and continuing, though intermittent, dissolution-related collapse. The hollows range from a few tens of metres to nearly 300 m across. They are typically ovoid in form, though some of the larger examples have more irregular outlines. The thicknesses of the deposits preserved beneath the depressions are highly variable: they may extend from a few metres to considerable depths below the surface. The best documented sequence, at Rhes-y-cae [SJ 194 709], is reported to extend to a depth of over 27.4 m; in the Berth Ddu [SJ 2039 6954] pocket, in excess of 73.2 m were recorded, and up to 137.2 m of sediment were noted at Maeshafn [SJ 203 610] (Table 15).

Exposures today are confined to degraded sections in weathered materials located around the commonly slipped margins of the hollows. Previous accounts described mainly white, mottled and variegated clays with subordinate fine-grained sands and silts (Strahan, 1890; Walsh and Brown, 1971) (Table 15). Structureless, white clay (pipeclay), largely composed of kaolinite, is the dominant lithology, but red, purple, blue and yellow clays with local mottling and lamination are also reported. Less common are dark laminated and carbonaceous (lignitic) clays, a sample of which from the Rhes-y-cae pocket contained up to 30 per cent organic carbon (Walsh and Brown, 1971). Several of the pockets also contain collapse-breccias composed of angular blocks and fragments of Namurian chert and sandstone. Apart from bedding-parallel lamination, no sedimentary structures have been reported from the sediments. Bedding in the clays and sands is commonly steeply inclined towards the centre of the hollows; local vertical and highly contorted strata have also been reported (Maw, 1867; Strahan, 1890; Walsh and Brown, 1971).

The ‘pocket’ deposits are locally overlain by late ­Quat­er­nary (Devensian) glacial deposits, and thus predate them. Maw (1867) was the first to suggest a Tertiary age for the former. Samples of carbonaceous clay from Rhes-y-cae [SJ 1946 7102], collected by Walsh and Brown (1971), were barren of microflora, but regional considerations persuaded them that the pocket deposits were probably of Neogene (late Tertiary) age (misquoted by George, 1974, as Palaeogene).

Conditions of deposition

Similar deposits occur elsewhere in northern England and Wales, for example in Derbyshire (Ford and King, 1969; Boulter and Chaloner, 1970) and Pembrokeshire (Dixon, 1921; Allen, 1981). An intact sequence of Tertiary deposits occupying the offshore Cardigan Bay Basin was proved in the Mochras Borehole (Wood and Woodland, 1968; Woodland, 1971; Dobson and Whittington, 1987). These various occurrences indicate that there were once extensive sequences of late Tertiary, freshwater, fluviolacustrine deposits in England and Wales. The Tertiary ‘pocket’ deposits of the district, located on elevated regions like Halkyn Mountain, clearly represent remnants of a previously more extensive cover which, if preserved thickness is indicative, may locally have exceeded 70 m in thickness. All but the higher ground of the Clwydian Range were probably overlain by these sediments which are thought to comprise materials reworked from deep, early Tertiary (Palaeogene) subtropical weathering profiles previously developed throughout upland Wales (Walsh and Brown, 1971).

The attitudes of bedding in these deposits suggest that the foundering of the sediments into the solution pipes occurred some time after deposition. However, it is possible that dissolution occurred at the base of a local blanket of Tertiary terrestrial sediments while deposition continued at the surface. Though associated breccia deposits are consistent with localised or periodic catastrophic collapse into voids, the commonly bedded nature of the silts and clays suggests that they were let down gradually as the solution pipes were enlarged beneath them (Walsh and Brown, 1971). The subsequent erosion of much of the Tertiary sequence was largely by fluvial incision during periods of lowered base level, and by later glacial agencies.

Chapter 8 Structure

The main structural elements of the Flint district are shown in (Figure 35). The most important feature is the Bala Lineament, one of a series of major, long-lived, north-east-trending fracture zones that dominate the structural ‘grain’ of Wales. At the surface, these lineaments occur as subparallel zones of faulting, folding and relatively strong deformation. They are thought to record polyphase deformation in cover sequences above steeply inclined, deep-seated basement faults or shear zones (for example Woodcock, 1984 a, b, 1987, 1990; Woodcock and Gibbons, 1988; Woodcock et al., 1988; Owen and Weaver, 1983; Wilson et al., 1988). Much of this district occupies an area between the Bala Lineament and the postulated extension of the Menai Straits Lineament, situated in north-west Wales. Much of its sedimentological and tectonic history reflects movements on these lineaments and associated structures. The region south of the Bala Lineament, part of which lies in the south-east corner of the district, occupies an area between the lineament and the Welsh Borderland Fault System (Woodcock and Gibbons, 1988).

Tectonic setting

The oldest rocks exposed in the district, the deep-water facies of Ludlovian (late Silurian) age, were deposited in the northern part of the Lower Palaeozoic Welsh Basin. The structural evolution of these strata forms part of the broader picture of the late Caledonian (Acadian) transpressive deformation that affected the Welsh Basin from late Silurian until early Devonian times. During this period, the rocks were deformed and underwent low grade metamorphism, followed by uplift during the late stages of the orogeny. The role of the major lineaments during this period was assessed by authors including Fitches and Campbell (1987), Gibbons (1983), Woodcock et al. (1988) and Woodcock and Gibbons (1988). In north Wales, the Acadian orogeny was followed by a long period of subaerial erosion which, in the Flint district, did not end until marine transgression during the early Dinantian.

In Dinantian times, the Flint district lay at the southern margin of the Craven Basin, a major area of subsidence, north of the Wales–Brabant Massif, and a precursor to the larger, Silesian Pennine Basin. Rapid crustal extension (or transtension) with rifting of the margins was the dominant mechanism for basin development, resulting in discrete episodes of pronounced syndepositional faulting, alternating with periods of tectonic quiescence. It is generally accepted that many pre-existing structures, including the Bala and Menai Straits lineaments, were reactivated during this period, possibly as oblique-slip faults (for example Wilson et al 1988; Undershill et al., 1988), and movement continued into the Namurian (Read, 1989). Movement appears to have been a response to plate subduction and north–south back-arc spreading (Leeder, 1982, 1987; Gawthorpe et al., 1989) rather than crustal dextral shear (Badham, 1982; Badham and Halls, 1975; Arthaud and Matte, 1977; Dewey, 1982; Arthurton, 1984), and it is unlikely that it involved exceptionally large amounts of strike-slip on any of these structures.

In the late Dinantian, probably during the Brigantian stage, crustal extension and rifting gradually gave way to widespread thermally induced subsidence as the domi­nant mechanism of basin development. Subsidence affected the Craven Basin and its adjacent platforms, as it evolved into the major Pennine Basin throughout the Namurian and early Westphalian. Movements on faults continued periodically as the basin margins readjusted to the changing pattern of basin evolution.

The first effects of the Variscan (Hercynian) orogeny north of the Wales–Brabant Massif, occurred in mid-Westphalian times. Local uplift on fault blocks, accompanied by the diachronous spread of red bed facies (Besly, 1988), was followed by inversion of the Pennine Basin with tilting, gentle folding and faulting, including reversals on many of the major fractures (Corfield et al., 1996). At the end of the Carboniferous, there was widespread uplift of the region.

Evidence from surrounding areas (Evans et al., 1993; Chadwick and Evans, 1995; Chadwick, 1997; Jackson et al., 1995) shows that episodic east–west extension, related to the break up of Pangaea, eventually leading to the opening of the Atlantic Ocean, probably started in the early Permian and continued into the Triassic. It produced a series of mostly fault-bounded basins, of which the Cheshire Basin and Vale of Clwyd Half-graben lie partly within the Flint district. Further phases of extension, possibly of Jurassic and early Cretaceous age, produced faults which cut the Permo–Triassic strata, and probably reactivated some of the earlier structures.

The post-Mesozoic erosion, which established the broad pattern of outcrop seen today, probably relates, as elsewhere in the UK, to regional uplift associated with the Alpine orogeny. However, it has been suggested that thermal uplift above upwelling mantle plumes was an additional, and perhaps dominant factor in the Irish Sea region (Chapter 7; Cope, 1994, 1995, 1997).

Structures of the district

For much of its geological history, the Flint district has remained in a marginal tectonic setting, and periods of orogeny or intracratonic subsidence have never entirely removed the evidence of earlier tectonic episodes. As each event introduced an array of new structures, those created during earlier episodes were commonly modified or reactivated. The earliest recorded structures, formed during the end-Caledonian (Acadian) orogeny, are a series of minor folds and a weak cleavage that affect Silurian rocks. However, the main structural grain of the district is inherited from an array of generally north to north-north-west trending faults that were mainly initiated during the period of Dinantian rifting, and continued to develop throughout the Silesian. They bounded a series of horsts and grabens (or half-grabens) that were a major influence on sedimentation, and later controlled the ‘basin and dome’ pattern of folding that developed in the Flintshire Coalfield during the Variscan orogeny. The faulting also influenced the form of later Triassic basins that developed in the region. A subsidiary, though very important group of large east-north-east-trending faults crosses the south-eastern corner of the district. They are component structures of the Bala Lineament, a deep crustal fracture with a history of movement dating from Precambrian times, and a major influence on Carboniferous sedimentation in the district.

The distribution of the main faults, folds, uplifted blocks (horsts) and local basins (grabens or troughs) is shown in (Figure 35), and a description of the important basin-defining structures, and their relationship to the Bala Lineament is given below.

Bala Lineament

This important lineament comprises a series of generally north-east-trending subparallel faults, folds and associated minor structures, extending from Cardigan Bay, through north Wales into the Permo–Triassic outcrop of the Cheshire Basin (Figure 37). Its long history of movement includes components of both strike-slip and normal faulting (Fitches and Campbell, 1987; Pratt, 1992) which, along much of its length, have resulted in an apparent northward downthrow and a sinistral offset of up to 7 km in outcrop patterns. In the Wrexham district it is represented by a major en echelon splay, the Bryneglwys Fault, which is truncated by the easterly trending Llanelidan Fault at the southern end of the Vale of Clwyd. At Four Crosses [SJ 251 530], the Llanelidan Fault intersects the Minera Fault and, from this point north-eastwards, the lineament comprises a plexus of east-north-east-trending faults with variable displacements; in this sector its cumulative downthrow appears to be to the south. This fault zone crosses the south-eastern part of the Flint district, where individual structures include the Caergwrle Fault, with an apparent northward downthrow, the Paper Mill Fault, downthrowing south (Hains, 1991) and its splay, the Trevalyn Fault, ostensibly downthrowing northwards. The major fault that juxtaposes Coal Measures against the Namurian strata of Caergwrle may represent an extension (or accommodate the throw) of the Bwlchgwyn Fault, recognised in the Wrexham district (Wedd et. al., 1928).

The complex history of the Bala Lineament was reviewed by Bassett (1969) and by Fitches and Campbell (1987). It probably represents a late Precambrian frac­ture within basement rocks (Gibbons, 1983). It ­influenced ­sedimentation and volcanism during the Lower Palaeozoic, and was a site of strong Acadian deformation (Pratt, 1992). Deflection of the regional Acadian cleavage and development of a secondary cleavage indicate that dextral strike-slip movements along the lineament continued after the culmination of the Caledonian orogeny (Fitches and Campbell, 1987; Pratt, 1992).

Its role during the Upper Palaeozoic was assessed by Wills and Smith (1922), Wedd et al. (1927, 1928), George (1958, 1974), Owen (1974), Somerville and Strank (1984b), and Gawthorpe et al. (1989) (see below). Several authors have commented on the sinistral offset of up to 7 km in Upper Palaeozoic outcrops and structures across the lineament and its splays, and have produced strike-slip models to explain these displacements as the products of Variscan compressional deformation (Wedd et al., 1927, 1928; Smith and George, 1961). However, these do not take into account the stratigraphical and thickness contrasts that occur in Dinantian rocks across the lineament, which would require sinistral displacements of nearer 20 km to conform with outcrop patterns. George (1974) regarded this amount of post-Carboniferous strike-slip movement as unlikely, and suggested that a component of synsedimentary dip-slip displacement may account for some of the facies and thickness contrasts. This resurvey gener­ally confirms this view and enables the timing of dip and strike-slip movement to be assessed.

Vale of Clwyd Fault and half-graben

The Permo–Triassic deposits of the Vale of Clwyd are confined within a half-graben bounded on its eastern side by the Vale of Clwyd Fault. It closely resembles other Permian half-grabens developed in and around the Irish Sea (Jackson et al., 1995), the present form of the half-graben and the displacement on the fault mainly relating to an early Permian to Triassic period of east–west extension. The Vale of Clwyd Fault accommodates an asymmetric, eastward thickening fill of sandstones of probable Permian age, estimated locally to exceed 525 m (Collar, 1974). The fault follows an arcuate trend from the Wheeler valley [SJ 098 698] in the north, to Pentre Coch [SJ 150 560] in the south. It is composed of several anastomosing strands that enclose inliers of various Westphalian and Dinantian formations. It is interpreted as a dextral oblique-slip fault, which separates the Permo–Triassic fill of the half-graben from the adjacent Silurian rocks of the Clwydian Horst to the east (Wilson 1959; (Figure 35)). Near Tremeirchon, in the Denbigh district, the fault is considered to have a westerly downthrow of at least 450 m (Warren et al., 1984). Its throw decreases southwards and, in the Wrexham district, it is not represented as a single structure, but as a series of parallel fractures.

The western margin of the Vale of Clwyd Half-graben is defined by a series of en echelon, north–south trending faults which downthrow east and cut Silurian, Carbonifer-ous and Permo–Triassic outcrops (Figure 36). These structures were interpreted by Wilson (1959) as normal faults, antithetic to the main Vale of Clwyd Fault. Collar (1974) showed that they have variable displacements along their length, and argued that they represented dip-slip faults generated in response to a component of dextral strike-slip movement on the Vale of Clwyd Fault, an interpretation consistent with east–west Permian extension (Jackson et al., 1995).

Contact relationships of rocks along both margins of the half-graben indicate that the Vale of Clwyd Fault and ­associated structures have undergone repeated move­ments, some of which may predate the Permian.

Range Fault

A major, north-north-west-trending, normal fault, downthrowing to the east, is present on the western side of the Clwydian Range between Bacherig [SJ 1600 5750] and Siglen Uchaf [SJ 1340 6520]. The fracture is repeatedly offset by the numerous east-north-east to west-south-west-trending cross-faults that traverse the Clwydian Range, and two such faults terminate its crop to the north and to the south. The fault juxtaposes the Elwy Formation that forms the high ground of the Clwydian Range against the westerly younging Nantglyn Flags immediately east of the Vale of Clwyd Fault. It appears to represent the Range Fault of Woods and Crosfield (1925), although its course differs in detail from theirs. Although the Range Fault is parallel to the Vale of Clwyd Fault, it has an opposite sense of throw.

Alyn Valley Fault

Between Nannerch [SJ 162 697] and the southern margin of the district at Llanarmon-yn-Ial [SJ 185 563], the newly recognised, easterly downthrowing Alyn Valley Fault, and its splays, define the eastern margin of the uplifted Silurian rocks that form the Clwydian Horst (Figure 35). North of Nannerch, the fault cuts the Dinantian outcrop and may connect with a major easterly downthrowing fault trending through Llanasa [SJ 107 815], in the adjacent Liverpool district (the Axton Fault of Wedd et al., 1923) (Figure 5). To the south, in the Wrexham district, the Alyn Valley Fault terminates against the Llanelidan Fault. The trend of the fault varies from north–south to north-north-east–south-south-east. It gives rise to a series of typically north–south-trending splays which also commonly downthrow east. These splays, several of which represent the cross-courses of the mining field (see below), link the Alyn Valley Fault with the Nercwys-Nant-figillt Fault Zone. Although the fault has a proven history of movement only during the mid-Dinantian, it is likely to have moved subsequently.

Nercwys–Nant-figillt Fault Zone

The Nant-figillt Fault of Strahan (1890), is now recognised as one of several structures within a plexus of fractures, here termed the Nercwys–Nant-figillt Fault Zone. This zone trends in a north-north-westerly direction from near Treuddyn [SJ 255 585] in the south, through Mold, to near Babell [SJ 160 740] in the north. In the southern and central parts of the district, the fault zone encloses the elongate periclinal Nercwys Syncline (Figure 35). The newly recognised Nercwys Fault forms the western margin of this syncline and defines the western boundary of the Flintshire Coal­field. Its downthrow is to the east, locally juxtaposing Westphalian Red Measures against Holywell Shales. The Nant-figillt Fault displaces the eastern limb of the syncline and downthrows west. The eastern limb of the syncline is further displaced by an array of similar faults, including the Soughton Fault, to the east of the Nant-figillt Fault. The Nercwys and Nant-figillt structures are recognisable as normal faults in the underground coal workings around Nercwys. In the south of the district, both structures merge, and a single fault with an easterly downthrow continues southwards to its junction against a component fault of the Bala Lineament in the Wrexham district.

In the north, in the vicinity of Moel-y-crio [SJ 190 700], where it cuts Namurian and high Dinantian rocks, the fault zone comprises a complex belt of fractures, with the net throw taken up by several fault strands. Farther to the north-west, within the crop of the Loggerheads Limestone, the fault zone is represented on the published map by a single fracture. However, close to the northern margin of the district, near Babell, the fault zone comprises a plexus of fractures, where two faults enclose the Calcoed outlier of Holywell Shales. Data from underground workings (Strahan, 1890) confirm that a component fracture, the Pant Celyn Fault, bounding the east of the outlier, is a steep, westerly dipping, normal fault. The Pant–Wacco Fault (also known as the Garreg Fault, the Spar Vein and the Vein-uchaf; Strahan, 1890) which bounds the western side of the outlier, also has a normal geometry (Smith, 1921; BGS, 1974). In the Liverpool district, to the north, these faults merge with the Axton Fault ((Figure 5); see above), and together separate Dinantian rocks from Coal Measures.

Soughton, Flint and Leeswood faults

The easterly downthrowing Soughton Fault and westerly downthrowing Flint and Leeswood faults are the principal structures defining a narrow, elongate graben which can be traced southwards across the district from Flint to Leeswood. In the north, a tract of Westphalian Red Measures is preserved within the graben. In the south, the graben encloses the previously productive ‘Leeswood Coalfield’ and here the Leeswood Fault equates with the ‘Great Boundary Fault’ of Strahan (1890).

Great Ewloe Fault

This westerly downthrowing fault is the largest and most persistent of the suite of north–south fractures in the central and eastern parts of the Flintshire Coalfield. It extends across almost the whole district, from the Dee estuary in the north to Hope Mountain in the south, where it intersects faults of the Bala Lineament. It forms the eastern margin of the productive Coal Measures of the Buckley–Connah’s Quay area (see Chapter 5), juxtaposing them against Namurian rocks along much of its length. The throw in this region may locally exceed 500 m, but diminishes southwards into the Namurian and Dinantian crop, on Hope Mountain, where it may have been transferred to subparallel, adjacent fractures such as the Penymynydd and Leeswood faults.

Hawarden Fault

The Hawarden Fault is a newly recognised west-north-west-trending major fracture affecting Silesian and Permo–Triassic strata. It is truncated by the Blacon–Dodleston Fault (see below) in the area east of Kinnerton [SJ 371 633], and has been traced to the area south of Oakenholt [SJ 260 700]; it may continue as a belt of similarly orientated fractures in the Bagillt area. The position of the fault is ill-defined, having been displaced by several of the major north–south fractures, and it is mainly recognised from borehole and mining data. It restricts coal workings to its nor­thern (hanging wall) side in the Queensferry area, and juxtaposes Coal Measure and Namurian successions around Connah’s Quay. Surface relationships indicate that the Hawarden Fault predates the north–south-trending faults of the Flintshire Coalfield, and geo­physical evidence shows that it coincides with a weak magnetic anomaly (see below). It may therefore represent the surface expression of an early, deep-seated basement structure, possibly a Carboniferous basin-bounding fault, that separated the Dinantian carbonate platform succession to the south-east from the basinal succession, as proved in the Blacon East Borehole, to the north-west.

Blacon–Dodleston Fault

The newly recognised, westerly downthrowing Blacon–Dodleston Fault is a major, north–south normal fault which cuts the Permo–Triassic outcrop in the western part of the Cheshire Basin. Its position has been largely determined from geophysical data (see below). It probably represents one of a series of large, basin-form­ing structures which were active during the east–west Mesozoic extension that initiated the Cheshire Basin.

Structural history of the district

The influence of four distinct periods of tectonic activity can be recognised in the Flint district, each characterised by a suite of structures. They can be broadly grouped into Caledonian, intra-Carboniferous, Variscan and post-Variscan structures, although the boundaries between these alternating periods of crustal compression and extension are not always clear-cut, and there are regional structures such as the Bala Lineament, with a history of repeated movement that spans these events.

Caledonian structures

During the late Caledonian (Acadian) orogeny, the Silurian rocks of the district were folded and cleaved. The dominant west-north-west trend of these structures, in common with that of the adjacent Denbigh and Wrexham districts, forms the northern part of the major structural arcuation of the Welsh Basin (Woodcock et al, 1988). White mica crystallinity studies of the cleaved mudrocks indicate that they were subjected to anchizonal, and locally epizonal grade metamorphism, largely through deep burial beneath younger strata, including Devonian rocks no longer preserved.

Folding

The scale of folding within the Clwydian Range is such that minor folds (other than slump folds) are rarely seen at outcrop. Medium and large-scale folds, although relatively few in number, are generally open, symmetrical, periclinal structures, with wavelengths ranging from several metres to a few hundred metres. Fold limbs are commonly inclined at low angles to the north and south, and gently plunging fold axes trend approximately west- north-west to south-south-east, as in the adjacent Denbigh district (Warren et al., 1984). These folds produce the complex outcrop pattern of sandstone turbidite sequences in the Silurian of the Clwydian Range.

Cleavage

Cleavage domains vary from closely spaced and continuous in higher grade (upper anchizone to epizone) mudstones to weakly developed or anastomosing (‘rough cleavage’ of Grey, 1978; see also Soper, 1986) in lower grade (diagenetic to low anchizone) mudstones. Sandstones and disturbed beds generally display a widely spaced cleavage. The cleavage trends north-west to west-north-west within the district. Across the Clwydian Range, there is a systematic change in the inclination of cleavage planes which Boswell (1928) described as a ‘cleavage fan’, but which appears to represent part of a ‘vergence divide’ (Figure 36), similar to those described elsewhere in the Welsh Basin (Cave and Hains, 1986; Craig, 1987; Woodcock et al., 1988; Wilson et al., 1992), possibly resulting from transpressional deformation. In the northern part of the Clwydian Range, cleavage planes dip to the south-south-west, whereas in the south they dip to the north-north-east. These two areas are separated by a linear zone of near-vertical cleavage, trending north-west to west-north-west, and apparently displaced by later faulting (Figure 36). The divide is ill-defined in places, due to poor exposure, but it can be traced intermittently from Moel-y-Parc [SJ 119 700] to the area north of Frith Mountain [SJ 180 635].

Regional metamorphism

Studies of the regional low-grade metamorphism of the Lower Palaeozoic rocks of the Welsh Basin have shown that grades range from zeolite to greenschist facies (Roberts and Merriman, 1985). A white mica crystallinity study of the Silurian rocks of the Flint district was undertaken (75 typical mudrock lithologies), using a sampling density of approximately 1 sample per km2. The mudrocks commonly consist of silt and fine-grained quartzose sand, albite and detrital white mica, with minor anatase, rutile, tourmaline, zircon and rare chlorite-mica stacks, in a phyllosilicate matrix of intergrown chlorite and white mica. Considerable textural modification of the sedimentary fabric has resulted from low-grade metamorphism and slaty cleavage development. Kubler indices of white mica crystallinity and the mineralogy of the fractions greater than 2 mm were determined by X-ray diffraction analysis using the techniques described by Roberts et al. (1991).

The results of the survey, summarised in (Figure 36), show two trends. The first is an increase in metamorphic grade with age, which probably relates to maximum depth of burial. The second is a pattern of regional ­metamorphism which reflects the influence of post-metamorphic uplift and faulting. Epizonal strata are exposed in the south-west of the Clwydian Range and a few narrow tracts of low anchizone strata crop out close to the Vale of Clwyd Fault; areas of low anchizone rocks occur in a few places on the eastern side of the Clwydian Range. The widespread occurrence of high anchizonal and epizonal rocks indicates that temperatures of at least 250°C and up to 300°C were reached prior to late Acadian uplift (Soper et al., 1987). Given the estimated thickness of approximately 1 km for the Silurian succession in the district (see Chapter 2), the inferred temperature range of 50°C indicates a geothermal gradient in excess of the range 36–44°C/km predicted elsewhere in the Welsh Basin (Bevins and Merriman, 1988; Roberts et al., 1991). A geothermal gradient of 50°C/km indicates that the Silurian rocks were buried beneath a pre-tectonic overburden 5 to 6 km thick (a higher geothermal gradient would reduce the overburden thickness required). This figure may not be excessive however; similar overburden thicknesses have been inferred for the concealed rocks of eastern England (Merriman et al., 1993), and a thicker overburden is inferred for the Lower Ludlow strata of the Pennines (Merriman et al., 1995). Woodcock (1990) has suggested that the cover over north Wales may have been comprised largely of early Devonian (Old Red Sandstone) strata.

Intra-Carboniferous structures

Apart from the Bala Lineament, the principal structural features of the Carboniferous outcrops are north–south to north-west-trending faults and a series of gentle, open folds. Although the folds were formed during the Variscan orogeny, there is clear evidence that at least some of the movement on some of the faults was syndepositional. Many of the faults which cut the Dinantian crop are mineralised, although the mineralisation is considered to be a later (Triassic) effect (see below).

Intra-Dinantian fault movements

The Bala Lineament was the dominant influence on Dinantian sedimentation in the district, although contiguous movements also occurred on associated north–south and north-north-west-trending faults that developed or were regenerated at this time; these include the Vale of Clwyd Fault, the Alyn Valley Fault, the Nercwys–Nant-figillt Fault Zone and, possibly, the Great Ewloe and Hawarden faults. It is possible that, as with other Dinantian basin margin faults, these north to north-west-trending structures were components of a listric fault array, including antithetic faults, that allowed thicker Dinantian sequences to accumulate locally.

Detailed structural data relating to the kinetics of the Upper Palaeozoic displacements on the Bala Lineament is lacking, but stratigraphical evidence of intra-Carboniferous movement has been given in chapters 3 to 5. In particular, the patterns of cyclic deposition in the late Asbian and Brigantian, provide unequivocal evidence of higher rates of subsidence and sediment accumulation on its northern side, and also account for earlier Dinantian, and later Namurian, facies and thickness contrasts that occur across the structure. The overall distribution of sediment is asymmetric with the thickest sequences, including a considerable accumulation of deltaic sandstone (the Cefn-y-fedw Sandstone), situated immediately adjacent to the lineament. These high sub­sidence rates can be related to the opening of a half-graben on the northern flank of the lineament during the Dinantian and Namurian (the ‘Mold Gulf’ of Gawthorpe et. al., 1989).

It is widely accepted that development of asymmetric grabens and half-grabens is a characteristic structural feature of Dinantian and early Namurian basins, and results from north–south extension, related to regional back-arc spreading (Leeder, 1982, 1988). In north Wales this is likely to have been partly accommodated by oblique-slip, dextral transtensional reactivation of north-east-trending Caledonoid fractures such as the Bala Lineament (Gawthorpe et al., 1989). The development of a half-graben is associated with folding (‘reverse drag’; Hamblin, 1965), generally regarded as a consequence of listric faulting. Volumetric considerations suggest that a listric fault geometry (as opposed to a planar geometry) is necessary to accommodate the high subsidence rates adjacent to such a boundary fault (Gibbs, 1984). However, there is no evidence that the Bala Lineament or any of its component faults are relatively shallow, low-angle, north-westerly inclined listric structures. Its long history of strike-slip movement (Fitches and Campbell, 1987) suggests that the lineament is seeded on a deep crustal detachment, with its active fault strands maintaining steep attitudes to considerable depths. Although its overall geometry during Dinantian and Namurian transtension may have been comparable to a ‘negative flower structure’ (Harding, 1983, 1985; Harding et. al. 1985), there is evidence that other structures also influenced subsidence and sedimentation in the Mold Gulf (see below).

Subsidence and stratigraphical thickening is thought to have been caused, to a considerable extent, by downthrow on the north–south to north-north-west-trending, predominantly dip-slip faults, orientated at a high angle to the Bala Lineament. Syndepositional movement is locally represented by growth faulting with thickened hanging wall sequences or erosion and non-sequence across the footwall of such faults. It is recorded on the Alyn Valley Fault and perhaps the Vale of Clwyd Fault in the late Chadian, the Nercwys–Nant-Figillt Fault Zone during the Brigantian and early Namurian, and the Great Ewloe Fault during the Westphalian (compare Campbell and Fitches, 1987). The relatively large dip-slip displacements on this north-north-west-trending fault set mask the evidence of any oblique-slip movement, although the latter may occur to a limited extent. The relative timing and importance of activity on these faults may be related to differing slip rates on fault segments of the Bala Lineament, as well as regional extension. It is possible that one or other component faults of the lineament acted as transfer structures between these dip-slip faults in a complex linked fault system during regional north–south extension. High subsidence rates, confined to a small area adjacent to the lineament, could be accommodated by a combination of oblique transfer on one or more of its component faults, coupled with roll-over (‘reverse drag’) on north-north-west-trending dip-slip faults, with or without the development of a small strike-slip basin along the lineament. Such a narrow downwarped zone would provide a conduit for clastic sediment influx, as seen in the thick development of the Cefn-y-fedw Sandstone between the Alyn Valley Fault and Bala Lineament. The north-westerly trending Minera and Aqueduct faults of the Wrexham district may represent comparable structures on the southern side of the lineament (Figure 37), with footwall uplift responsible for the attenuation of the Dinantian sequence at Minera.

Intra-Silesian fault movements

The thicker sequences of the deltaic Cefn-y-fedw Sand­stone on the northern side of the Bala Lineament indicate that its syndepositional sense of downthrow remained the same in the Namurian as in the Dinantian (Figure 21). Both the Nercwys–Nant-figillt Fault Zone and the Great Ewloe Fault appear to have influenced the form and extent of delta lobe progradation on the north side of the lineament.

Coal Measures deposition in the Flintshire Coalfield was related largely to the broad regional pattern of thermal subsidence in the Pennine Basin, although marked differential subsidence occurred locally through syndepositional movements on the principal north–south and north-north-west-trending fractures (see Chapter 5). Movement on the Great Ewloe Fault in particular appears to have influenced the position of successive delta distributary sandstones and related washouts which occur along its western (hanging-wall) side. This local depocentre continued to accumulate substantial thicknesses of grey measures at a time when red measures were being deposited across the elevated (footwall) area to the east, around Queensferry. The thinner succession of Coal Measures to the west, around Mold, together with the earlier onset of red measures in this area, may mark the crest of a corresponding, anticlinal roll-over that accommodated downthrow against the Great Ewloe Fault; however, it may, alternatively, record uplift along antithetic fractures and/or the precursor of the Nercwys–Nant-figillt Fault Zone (see below). Localised uplift along north–south faults in the north of the district, may also be responsible for the unconformity at the base of the Red Measures within the Flint–Leeswood Graben around Flint, and in the Dee estuary.

The early onset of Red Measures deposition around Mold and in the Nercwys Syncline, during the mid-Duckmantian, suggests that the Nercwys–Nant-figillt Fault Zone, possibly in conjunction with the Alyn Valley Fault and other structures, maintained a region of relative uplift along the western margin of the coalfield.

The northern extension of the Alyn Valley Fault, passing through Llanasa in the Liverpool district, defines the known western limit of the Flintshire Coalfield on the north Wales mainland. Possibly in association with other structures, it appears to represent the margin of the Pennine Basin in this area, separating a region of net downwarp with enhanced deposition and preservation in the east, from a region of relative uplift to the west.

The Flintshire Coalfield therefore compares with coalfields situated elsewhere along the southern margin of the Pennine Basin, such as Coalbrookdale (Hamblin and Coppack, 1995) and parts of South Staffordshire (Waters et al., 1994). In these marginal settings, local tectonic (allocyclic) effects predominated over regional sub­sidence throughout the Westphalian. Local tectonism influenced subsidence and sedimentation, and thereby, the contemporary water table (Besly and Turner, 1983). This, in turn, controlled the distribution of red measures and the thickness and quality of coals, which, particularly in the Flintshire Coalfield, are commonly thin, and of poor quality; local tectonism may also have restricted successive marine transgressions and the full development of marine bands.

There are no marked thickness or facies differences between the Westphalian sequences of the Flintshire and Denbighshire coalfields below the Main Coal. Therefore, it is unlikely that the Bala Lineament was an active control on sedimentation during this period, even though it may have continued to move. However, above the Main Coal there are significant differences between the two coalfields. The number of seams and the interseam thicknesses both increase in the Denbighshire Coalfield, and the onset of red bed sedimentation was significantly later. These relationships appear to record a period of tectonic reconfiguration in which the main site of subsidence and deposition switched to the south of the lineament. During this period, the lineament clearly defined the northern margin of this subsiding coalfield basin. The form of its southern and western margins are largely unknown, but fractures, such as the Minera and Wrexham faults in the Wrexham district, may have been important boundary structures. These faults, hitherto regarded as predominantly strike-slip structures (Wedd et al., 1927; Smith and George, 1961) can, alternatively, be considered as dip-slip faults, that operated in the same way as similarly orientated faults north of the lineament during the earlier Carboniferous (see below).

The change in the sense of displacement across the lineament, and the occurrence of Duckmantian red measures within the Flintshire Coalfield, could be considered as evidence of early Variscan inversion of the region. Wedd et al. (1927, 1928) suggested that inversion began even earlier during the Westphalian, with the growth of a series of anticlines (his ‘Horseshoe’ and ‘Interior’ anticlines) that funnelled sediment into a synclinal trough between Leeswood and Buckley, superimposing washouts in the underlying coal measures (Chapter 5). However, the scale of such basin inversion does not appear to have been of sufficient magnitude to completely reverse the earlier Carboniferous downthrow on the northern side of the Bala Lineament. Furthermore, the detailed pattern of red bed deposition in the Flintshire Coalfield suggests that faults there continued to behave as extensional structures, as they had done throughout the preceding Dinantian to early Westphalian, and there is no evidence that they experienced reversals of movement at this stage. Comparable distributions of Duckmantian sand bodies and red measures adjacent to extensional faults have been recorded in south Staffordshire (Hamblin and Glover, 1991; Glover, et al., 1993; Waters et al., 1994), where they are regarded as local tectonic (allocyclic) phenomena that developed from differential subsidence in marginal coalfield areas.

The general Westphalian succession in the Denbighshire Coalfield, south of the Bala Lineament, and the widespread Bolsovian onset of red measures sedimentation is comparable to that of the North Staffordshire Coalfield, where subsidence rates actually increased during the Upper Carboniferous (Waters et al., 1994). It has been suggested that rapid expansion of red measures sedimentation in the central parts of the subsiding Pennine Basin resulted from uplift and erosion of its southern margin, and development of the widely recognised pre-Halesowen unconformity (Besly, 1988). There is no evidence of this unconformity in the Denbighshire Coalfield beneath the equivalent Coed-yr-allt Formation, suggesting that at this time the area, along with north Staffordshire, was situated some distance from the tectonically active margin of the Pennine Basin.

The question of how the predominantly basinal Westphalian succession of the Denbighshire Coalfield became juxtaposed against the Flintshire Coalfield, with its more marginal affinities, depends on postulated displacements on the Bala Lineament and associated structures. It is complicated by the fact that the later depositional history and setting of the Flintshire Coalfield are unknown, as the youngest strata (of Duckmantian age) probably predate any widespread (pre-Halesowen) unconformity. One explanation is that considerable dextral movement on the Bala Lineament has brought the two coalfields together; however, this model conflicts with the observed Dinantian and Namurian thickness and facies distributions (see above). It is possible that, as in Dinantian sequences to the north, differential subsidence of the Denbighshire Coalfield in the Westphalian was achieved by downthrow on north–south faults and subordinate oblique transfer of movement along component structures of the Bala Lineament. This would open a small graben south of the lineament and obviate the need for substantial amounts of strike-slip movement on it. However, the relationship of such a structure to the wider palaeogeography of the Pennine Basin is unknown until such time as the intra-Carboniferous movements on the Bala Lineament can be quantified.

Significant movements along faults on both sides of the Vale of Clwyd occurred before the deposition of the Erbistock Formation (and possibly the Coed-yr-Allt Formation) in late Westphalian times, because this rests unconformably on a block-faulted surface of Dinantian strata. Along the eastern side of the Vale, and also at its southern end (in the Wrexham district), the Erbistock Formation overlies the late Dinantian Minera Formation. Along the western side of the Vale, it oversteps, probably across concealed faults, to rest on the late Asbian Loggerheads Limestone at Llanfwrog [SJ 115 577]. At the western margin of the district [SJ 100 605] near Rewl, it overlies the Arundian Llanarmon Limestone. This pattern of overstep suggests that at least 160 m of late Dinantian strata, absent in the west, are preserved in the eastern and central parts of the Vale within a downwarp against the Vale of Clwyd Fault. The erosional event may record early Variscan inversion of the half-graben and relate to the formation of the pre-Halesowen unconformity elsewhere. Any Namurian or Coal Measures strata previously deposited in the Vale were possibly removed at this time.

Variscan structures

In contrast to areas south of the Wales–Brabant Massif, the culmination of the Variscan orogeny in north Wales was probably muted (Smith and George, 1961), although much of the evidence from the marginal areas of the coalfield basin is lacking. A general review of the Variscan inversion tectonics of the region was given by Corfield et al. (1996).

Faults

Many of the faults which operated syndepositionally during the Carboniferous, were probably reactivated during the later part of the Variscan orogeny and some probably experienced reversals of throw. The complex pattern of faulting in the Carboniferous rocks was probably largely imposed during the Variscan phase of movement, although the current horst and graben structure of the coalfield was considerably influenced by subsequent Mesozoic extension (see below).

North–south compression or transpression along the Bala Lineament during the culmination of the Variscan orogeny probably reversed its displacement. The amount of displacement is unknown, but its component faults are likely to have been reactivated in either an orthogonal (reverse) or oblique sinistral sense. The divergent throws on faults, that enclose and juxtapose small Dinantian inliers against Silesian strata along the lineament in the Flint and Wrexham districts, suggest that transpression may have produced a ‘positive flower structure’ of upward-diverging fault splays (Harding, 1985). The effects of Variscan inversion of the Bala Lineament in latest Carboniferous or early Permian times, were recorded by Earp and Taylor (1986) in the Cheshire Basin. They recognised a marked difference in the thickness of Red Measures preserved beneath the Permo–Triassic unconformity on either side of the linea­ment, with thinner successions on its uplifted northern side.

A series of mineralised, north-west-trending faults (veins) cut the main Dinantian and Namurian crop in the south of the district ((Plate 15) ; (Figure 35)). Comparable faults in central parts of the Dinantian crop and farther north on Halkyn Mountain, have an east–west orientation. There is no evidence of syndepositional movement on these dominantly northward downthrowing structures, suggesting that they formed during or after the Variscan deformation. Although the mineralisation is probably associated with subsequent Triassic extension (see below), the orientation of the faults suggests that they are oblique-slip fractures of Variscan age, possibly synthetic to the Bala Lineament.

Laterally persistent north–south-trending, generally eastward downthrowing faults within the Dinantian crop represent the so called ‘cross-courses’ of the mining field (Strahan, 1890; Smith, 1921) (Figure 35). These fractures typically offset the veins and some are locally mineralised. Several of these structures are splays of the Alyn Valley and Nercwys–Nant-figillt faults, but they are considered to be much later fractures. In contrast to the veins, and despite their favourable orientation in relation to Mesozoic east–west extension, it is clear that, other than on Halkyn Mountain, these north–south-trending faults did not provide a ready conduit for mineralisation, and some may even post­date this event.

Folds

During the final stages of the Variscan orogeny, the Carboniferous strata were folded into a series of anticlines and synclines. Previous workers recognised a series of such folds to explain the distribution and dip of the Carboniferous rocks (Strahan, 1890; Wedd and King, 1924; Wedd et al., 1927, 1928). The attitude of the Dinantian rocks along both flanks of the Vale of Clwyd Half-graben was regarded as evidence of a major synclinal fold in the Carboniferous rocks beneath the Permo–Triassic fill, with the Silurian rocks of the Clwydian Range occupying the denuded core of a Clwydian Anticline, and the main Dinantian crop rocks forming its eastern limb. The broad tract of Namurian rocks between Hope Mountain and Hawarden Bridge was regarded as the continuation of the ‘Horse-shoe Anticline’ of the Wrexham district (Wedd and King, 1924; Smith and Jones, 1961). The development of these folds was considered to result from east–west compression that mainly took place at the end of the Carboniferous (Wedd et al., 1927).

The recent survey has shown that north–south faulting largely controls the outcrop pattern in the dis­trict, and the major folds of earlier workers are a series of fault-bounded horsts and grabens (Figure 35); tilting of adjacent fault blocks and localised thickness and facies changes in Carboniferous rocks are also responsible for some apparent ‘fold’ configurations. However, smaller scale folding is evident in the district, mainly from the distribution of coal seams and red measures within the Flintshire Coalfield. The folds are mostly open periclinal structures, with a wavelength of 2 to 3 km, but are severely disrupted by faulting; fold axes are generally aligned northwards or north-westwards, parallel to the main fault trend. Periclinal culminations and depressions give rise to the distinctive ‘basin and dome’ outcrop pattern that is well displayed in the Leeswood area.

The axial trace of the Nercwys Syncline, a large-scale periclinal fold in the Mold area, runs parallel to the Nercwys Fault that displaces its western limb. The fold can be traced from near Pentre [SJ 230 589] northwards to Moel-y-crio [SJ 190 700] where it culminates in Dinantian rocks. A complementary en echelon fold, the Halkyn Anticline (Figure 35), has been traced in Dinantian strata north-westwards from Rhosesmor [SJ 214 687] to Brynford [SJ 180 745]. Around Moel-y-crio, the Nercwys and Nant-figillt faults displace the common limb of these folds. The faulted eastern limb of the Nercwys Syncline is shared with the Mynydd Isa Anticline, a fold occupying the area between Mold and Buckley (Figure 35). The anticline is a highly faulted structure with a trace somewhat oblique to the Flint–Leeswood Graben, although approximating to the trend of the Leeswood Fault in the south of the district. Its closure can be mapped in Coal Measures near Soughton [SJ 243 665]; elsewhere its axis is faulted. The fold culmination lies around Leeswood Old Hall [SJ 260 616] within a faulted Namurian sequence.

A common relationship between folding and faulting in the Flintshire Coalfield is suggested by the north-west trend of both fold axes and major faults. It is generally recognised that fold nucleation and modulation may be strongly influenced by pre-existing fault controls and the original depositional geometry, particularly the disposition of sand bodies, and the presence and frequency of incompetent horizons such as black shales and coal seams (Arthurton, 1984; Corfield et al., 1996; Woodcock, 1987). Thus, the development and orientation of folds within the Flintshire Coalfield were probably strongly influenced by reactivation on the principal faults, facilitated by the variable distribution of coals and the presence of stacked sequences of distributary sandstones in the Leeswood–Buckley area.

Post-Variscan structures

The effects of Mesozoic east–west extension, are most clearly evident in the Permo–Triassic rocks. However, knowledge of their structure is derived mainly from borehole seismic reflection data. In the two major crops, the Vale of Clwyd and Cheshire Basin, Permo–Triassic strata rest unconformably on Carboniferous rocks, and are cut by a series of north–south-trending faults. It is possible that some of the faults within the coalfields also reacted in response to Mesozoic extensional episodes.

The Cheshire Basin was initiated by syndepositional movement on the north-east-trending Wem–Red Rock Fault, which lies south-east of the district (see for example Evans et al., 1993), in latest Carboniferous to earliest Permian times (Figure 31). In this district, fractures associated with the western margin of the basin, such as the Neston and Blacon–Dodleston faults, may also have been active. Seismic and gravity data interpretations (see below) reveal the path of the Bala Lineament beneath the Permo–Triassic rocks of the Cheshire Basin and suggest that it was also active during the early Permian.

In the Flint district during this period, net displacement was to the north, because it accommodates a thickened sequence of Kinnerton Sandstone on its northern side. However, there was probably a significant component of sinistral movement in response to the predominant east–west Permian extension (compare Chadwick, 1997). The presence of Kinnerton Sandstone of probable Lower Permian age in the Vale of Clwyd suggests that the Vale of Clwyd Fault, and the en echelon, north–south-trending faults along the western margin of the half-graben, also responded to Permian extension.

There is little evidence for the timing or magnitude of post-early Triassic tectonic activity in the Flint district, although faulting probably occurred in the Chester district to the east (Earp and Taylor, 1986). Such faulting may be represented by the significant downthrow on the northern side of the Trevalyn Fault, a strand of the Bala Lineament which juxtaposes the Erbistock Formation with the Kinnerton Sandstone in the south-east of the district. It may also have juxtaposed Coal Measures against Chester Pebble Beds Formation along the Neston Fault in the north of the district.

The steep dip on the basal Permo–Triassic unconformity in the east of the district indicates that there has been uplift and tilting, probably related to the Alpine orogeny, which culminated in mid-Miocene times, and/or thermal uplift associated with a late Cretaceous to early Tertiary mantle ‘hot-spot’ (Chapter 7). Seismic activity related to movement on the Bala Lineament continues to this day (Blenkinsop et al., 1986).

Deep structure

In the absence of deep boreholes, knowledge of the concealed geology and structure of the district is dependent on geophysical investigations. Much of the district and its surrounding areas were included in a regional gravity survey of north Wales (Powell, 1956). The eastern part was included in surveys of the Cheshire Basin by the Anglo-American Oil Company (White, 1949). Data from subsequent British Geological Survey gravity surveys have been augmented by detailed surveys undertaken to define anomalies associated with the Cefn-y-Fedw Sandstone (Cornwell, 1987) and in the Dodleston area (Allsop, 1974 personal communication). All these data have been integrated to produce the Bouguer gravity anomaly map (Figure 37).

The aeromagnetic coverage forms part of the British Geological Survey National Aeromagnetic Databank and the data (Figure 38) include anomalies due to man-made sources, notably at Ellesmere Port [SJ 410 760] and Shotton [SJ 310 700]. Other available geophysical data include induced polarisation and resistivity surveys connected with mineral exploration in the Llandegla area (Cornwell and Kimbell, 1985) and seismic reflection surveys at Dodleston (Andrew and Collar, 1976). Commercial seismic reflection surveys have been carried out in the Cheshire Basin to the east.

Data on physical properties are available from various sources, including Powell (1956), Wilson (1959), Collar (1974), Cornwell and Kimbell (1985), Smith (1986) and Cornwell (1987). These comprise mainly densities, apparent resistivities and sonic velocities. Average densities for the main rock types of the district are listed in (Table 16) (compare with Cornwell in Warren et al., 1984). Notably, in a study of the Cefn-y-Fedw Sandstone, Cornwell (1987) reported a wide range of densities between 2.2 and 2.6Mg/m3. Magnetic susceptibility data are not available for the district, but values for the rock types in the near-surface zone are typically low.

The main features determined from interpretation of the regional geophysical data, including the locations of lineaments, are summarised in (Figure 39). Several profiles across the area were selected for interpretation by model­ling the gravity and magnetic anomalies. Profile AA′ (Figure 40) illustrates a model for the deep magnetic basement. Profiles BB’ and CC′ (Figure 41), (Figure 42) were located to examine gravity anomalies associated with the Namurian sandstones of the Llandegla Moor area [SJ 225 537] and the Permo–Triassic rocks around Dodleston respectively.

The Bouguer gravity anomaly data reflect largely the variations in thickness of the Permo–Triassic and Silesian rocks. They provide thickness values for the Triassic rocks in the Vale of Clwyd Basin and reveal a local basin, probably largely of Triassic age, on the north side of the Bala Lineament at Dodleston.

Silurian and older rocks

Over much of north Wales, the Bouguer gravity data shows a systematic increase north-westwards with average gradients of about 0.5 mGal/km. This is apparent over the Silurian rocks of the Clwydian Range and the Denbigh moors area, west of the Vale of Clwyd (Figure 37). This long-wavelength feature was first described by Powell (1956) who ascribed it to an increase, towards the south-east, in the depth to a mid- or base-crustal layer. Later evidence from a segment of the long seismic refraction profile LISPB (Bamford et al., 1976) indicated that the gravity gradient was due to a change in the depth to a mid-crustal layer from about 13 to 17 km (Nunn, 1978; ­Manchester, 1983). The regional gravity gradient appears to be interrupted along the line of the Bala Lineament by local increases in values to the south. This is most apparent over the Ordovician rocks of the Cyrn-y-Brain area [SJ 211 492] (Figure 37), (Figure 39), and is interpreted as due to the higher density of these rocks and a reduction in the depth to the underlying basement.

Aeromagnetic data provide evidence for a well defined zone of magnetic rocks crossing the Flint district with an overall north-east to south-west trend (Figure 38). This zone extends from south-west of the Berwyn Dome to near the Dee estuary where it increases in width and swings into a north-north-easterly trend towards its termination around Ormskirk in Lancashire, a total distance of about 100 km. Its origin is not known but in the Berwyn Dome area it coincides with an area of upper Ordovician sedimentary and volcanic rocks. The anomaly is, however, separated from that associated with the Ordovician rocks of Snowdonia and its amplitude is considerably less. To the south-west of the district, the northern margin of this magnetic zone (Lineament M1 in (Figure 39)) broadly coincides with the course of the Bala Lineament. However, in the northern part of the Wrexham district, as the Bala Lineament swings into a more east-north-easterly orientation, the anomaly traverses this structural zone. Within the Flint district, the whole of the anomaly lies to the north of the Bala Lineament suggesting that the magnetic zone is only partially influenced by this structure. A model for the form of the magnetic basement across the south-western corner of the district (Figure 40) shows the shallowest part just to the north of the Bala Lineament and a secondary peak beneath the Carboniferous rocks to the south-east.

In the Broughton–Dodleston area, at the point where it changes trend and width, this main magnetic zone intersects a weaker magnetic lineament which trends in a west-north-westerly direction, parallel to the southern shore of the Dee estuary ((Figure 37); M2 in (Figure 39)). This feature approximately coincides with the surface course of the Hawarden Fault, as well as along the northern margin of the Dinantian crop in the Liverpool district. It appears to reflect a deep-seated structure which may have controlled the local edge of the Dinantian carbonate platform.

Carboniferous rocks

The Dinantian limestones have a density contrast of only about –0.05 Mg/m3 with the underlying Lower Palaeozoic rocks and the regional gravity data are therefore unlikely to provide evidence of thickness variations within these strata. The decrease in Bouguer gravity values to the east of the main Dinantian outcrop appears to relate to the presence of low density sandstones, both intercalated with limestones in the late Dinantian Minera Formation, and as the dominant facies in the succeeding, early Namurian, Cefn-y-Fedw Sandstone. Within and to the south of the district, the crop of these sandstone-bearing divisions coincides with a particularly well-defined gravity gradient (G3 in (Figure 39)) which was investigated by Cornwell (1987). The gradient here is partly due to a high porosity sandstone facies in this area. However, lithological changes may be insufficient on their own to account for the rate of decrease of Bouguer anomaly values, which appear also to imply a rate of eastward thickening in Namurian rocks greater than expected from the regional easterly dip. In explanation, Cornwell (1987) suggested the presence of an eastward downthrowing north–south fault and/or a syncline within the Namurian crop. The gravity feature is most clearly seen by the presence of a gravity low in the Llandegla moor area [SJ 270 530] (Figure 37). The low coincides with an extensive outcrop of Cefn-y-Fedw Sandstone where the thickest sequences of this formation are known to occur (Chapter 4; Wedd et al., 1928). Profile BB’ ((Figure 37), (Figure 41) shows a steep gradient on the south-eastern side of the low (G2 in (Figure 39)), that represents the Bala Lineament. The gravity data are therefore consistent with the stratigraphical evidence, which suggests that the Bala Lineament was active in controlling sedimentation during Namurian times. The gravity gradient associated with the western margin of the main Namurian crop weakens northwards as the sandstone facies interleave with, and give way to the higher density mudstones of the Holywell Shales. The general decrease in Bouguer anomaly values eastwards across the district (Figure 37) reflects the increasing thickness of lower density Westphalian rocks in the Flintshire Coalfield and of Permo–Triassic rocks in the Cheshire Basin.

Permo–Triassic rocks

The local rocks of Permo–Triassic age have densities that are lower than the Carboniferous and older rocks (Table 16) and give rise to gravity anomaly lows.

Vale of Clwyd

The distinctive gravity anomaly associated with the Vale of Clwyd Half-graben is obvious from (Figure 37). The broadened form of the anomaly largely reflects the lack of detailed gravity coverage in places and local changes in the background field.

Estimates of the shape of the Vale of Clwyd Half-graben and of the thicknesses of strata present have been based on geophysical surveys (Powell, 1956; Wilson, 1959; Collar, 1974). The Bouguer anomaly low associated with the Vale of Clwyd Basin is well defined by steep gradients along the Vale of Clwyd Fault ((Figure 37); G1 in (Figure 39)). The gravity anomaly will include contributions from any underlying Carboniferous rocks that have lower densities than the surrounding Silurian rocks, and it is not possible to estimate the thickness of the Permo–Triassic rocks from the gravity data alone. In the southern part of the basin, Collar (1974) used resistivity soundings to estimate the extent and thickness of the concealed Westphalian rocks, and was then able to derive an isopachyte map for the Permo–Triassic sequence (Collar, 1974, fig.4). The latter indicated that thicknesses locally exceeded 525 m. The gravity data also provided evidence that the north-trending faults on the western side of the vale extend across the half-graben to the Vale of Clwyd Fault.

Western Cheshire Basin

In the western part of the Cheshire Basin, borehole, seismic reflection and gravity data have enabled the structural framework to be refined (Evans et al., 1993). However, identification of the basal Permo–Triassic reflec­tor is generally unreliable. Contours show the unconformity dipping eastwards over most of the area (Figure 44). A major normal fault, the Blacon–Dodleston Fault, is interpreted to have a maximum throw to the west of about 350 m. It may be en echelon with the Great Ewloe Fault and eastward throwing faults in the north-east of the district around Puddington [SJ 328 734]. The north-west-trending Hawarden Fault appears to be truncated by the Blacon–Dodleston Fault. Normal faults trending approximately east-north-east cut the Permo–Triassic unconformity east of the Blacon–Dodleston Fault to the north of Pulford [SJ 375 588].

The low Bouguer anomaly values in the east form part of a major gravity low over the Cheshire Basin (White 1949, Smith 1986). Smaller gravity anomalies and zones of increased gradient (Figure 37), (Figure 39) superposed on the main anomaly indicate local features, the three most prominent being the Dodleston gravity low and gravity highs at Blacon [SJ 360 660] and Burton [SJ 290 750]. The well defined Dodleston feature (Figure 37), (Figure 42) occurs at the western margin of the Permo–Triassic rocks [SJ 335 360], and its southern margin lies at the projected course of the Bala Lineament (G4 in (Figure 39)). Detailed gravity surveys suggested the presence of a thick sequence of low density Permo–Triassic rocks. A seismic survey (Andrew and Collar, 1976) indicated about 200 m of low velocity rocks (1.73 km/s) overlying a seismic refractor interpreted as the Westphalian, although the presence of an additional seismically ‘hidden’ layer was suspected. Interpretation of subsequent seismic surveys in the area suggests that the local basin of Permo–Triassic rocks may extend down to almost 600 m below OD.

The form of this gravity low was interpreted (Figure 43) by dividing the basin fill into an upper layer of low density, 200 m thick, indicative of high porosity Triassic sandstones or younger, less consolidated rocks, and a lower layer extending to 600 m comprising denser Permo–Triassic rocks. Interpretation of the gravity data in this area is complicated by the presence of the thick fill of the palaeo-Dee channel. Farther to the north-east, the course of the concealed Bala Lineament can be inferred where it forms the northern margin of the gravity high associated with the Westphalian inlier at Milton Green, just east of the Flint district.

The Blacon gravity high corresponds to a concealed ridge of Carboniferous rocks revealed by seismic reflection surveys and proved in the Blacon East Borehole.

The Burton gravity high has steep gradients to the east that suggest marked thickening of the Permo–Triassic rocks; however, the gravity data coverage is insufficient to allow a reliable, quantitative interpretation. The aeromagnetic anomaly apparently coincident with this gravity feature is also poorly constrained by data points.

Mineralisation

Metalliferous mineralisation occurs sporadically throughout the Clwydian Range, in the western part of the district, but is widespread in the Dinantian limestones. For a time during the early 20th century, these rocks constituted one of the world’s richest lead-zinc orefields, centred on Halkyn Mountain, and extending south to Minera in the Wrexham district. Other metals formerly exploited include iron, gold and copper in small amounts. Evidence from mapping, mine plans and other published reports indicates that the bulk of the mineralisation occurs along fault planes or master joints associated with such fractures. A brief account of the historical and economic aspects of metal mining in the district is provided in Chapter 10.

Lead and zinc

The broad pattern of lead-zinc mineralisation in the district was established by Strahan (1890), refined by Smith (1921), and was subsequently re-examined by Earp (1958). They showed that the bulk of this mineralisation occurs along fault planes in ‘veins’, with some also occurring along ‘cross-courses’ (Figure 35). Some mineralisation also occurs as zones of replacement, parallel to bedding. These mineralised structures are almost wholly sited in Dinantian limestones and occur mostly in the upper part of the Loggerheads Limestone and Cefn Mawr Limestone. Only locally does economically important mineralisation extend into the Namurian Cefn-y-Fedw Sandstone and Pentre Chert.

The principal ore minerals are galena and sphalerite with lesser amounts of chalcopyrite, and oxidation products such as cerussite and smithsonite (calamine); the galena is generally silver-bearing. Fluorite is only a very minor component of the mineralisation as a whole but may be locally important in the south of the district; baryte is also rare. The main associated gangue minerals are calcite and, to a lesser extent, quartz. In the wider context of the north Wales orefield, which includes the Minera district and areas on the western flanks of the Vale of Clwyd, a broad sulphide zonation is present in which zinc/lead ratios gradually increase with distance from the Silurian rocks of the Clwydian Range (Smith, 1973).

In the north of the district, most veins are oriented in an east–west direction, but the regional trend varies and becomes more north-west-oriented in the southern area between Loggerheads [SJ 190 630] and Llanarmon-yn-Ial [SJ 190 560] ((Figure 35); (Plate 15) ). Most mineralised faults terminate upwards in the Cefn-y-Fedw Sandstone or in the Holywell Shales. The mineralisation does not extend down to the Lower Palaeozoic (Silurian) sequences; proven terminations occur in the uppermost parts of the Leete Limestone or the Loggerheads Limestone. The ‘cross-courses’ are oriented roughly north–south and typically offset the veins. Major north–south to north-north-west-trending faults, such as the Nercwys–Nant-figillt and Alyn Valley faults (Figure 35), do not appear to be mineralised.

Lead-isotope dates obtained from galena samples from the Halkyn area yielded an improbable 170 Ma (± 8 Ma) age (Moorbath, 1962), but more recently Fletcher et al. (1993) reported an age of 240 Ma (early Triassic) from the ‘Clwyd Mining District’. Bituminous hydrocarbons occur in association with the lead-zinc mineralisation in north-east Wales (Parnell, 1983) and solid hydrocarbon inclusions are found in some fluorite crystals (Smith, 1973). The hydrocarbons may have been genetically involved in the mineralisation. Mineralogical and lead-isotope studies by Parnell (1988a) and Parnell and Swainbank (1990) have shown that the solid hydrocarbons are uranium enriched and were most likely deposited synchronously with mineralisation, some 248 Ma (± 21 Ma) ago, rather than by petroleum fluids migrating through fracture porosity at a later date.

The mineralisation is similar to that of the Pennine Orefields (Dunham and Wilson, 1985; Plant and Jones, 1989). Both the Pennine and north Wales deposits are thought to be of the Mississippi Valley type of epigenetic ore deposit. They were probably emplaced from hot brines that were expelled from Carboniferous basinal argillaceous rocks by tectonism. Thick Namurian and Westphalian sequences in the East Irish Sea and Cheshire basins have been suggested as potential sources (Dunham, 1970). Fluid-inclusion studies indicated that the mineralising solutions were connate sodium chloride brines and that mineral deposition occurred at temperatures of 105 to 130°C (Smith, 1973).

Nickel, cobalt and manganese

A small amount of nickel-cobalt-manganese mineralisation, hosted mainly in basal Carboniferous sequences, has been reported from north Wales (Foster, 1882; Strahan, 1890; Warren et al., 1984), the principal area being north of this district. No mineralisation of this type has been recorded from the Halkyn or Minera orefields. Parnell (1988a, 1988b) recorded bismuth-tungsten enrichment in uraniferous hydrocarbons from the Halkyn area.

Barium

Baryte occurs as a rare accessory to the lead-zinc mineralisation of the district. Earp (1958) recorded baryte in the Maes-y-pwll vein and Pant-y-gwlanod veins in the Graianrhyd area. It also occurs in veins and replacements in the small inliers of limestone adjacent to the Vale of Clwyd Fault, for example between Tyddyn-dedwydd [SJ 1370 6305] and Bryn-cirion [SJ 1380 6295] south-east of Llangynhafal, and at Rhiwbebyll [SJ 1260 65665]. Baryte was mined in areas adjacent to the district in the Carboniferous Limestone near Efenechtyd, 2.8 km south of Ruthin, and with witherite from the Nantglyn Flags at the Pennant Mine near Rhuallt.

Iron

Hematite and more rarely limonite occurs in association with quartz, in small quantities in the Caerwys area (Strahan, 1890). The ores were worked in small veins and pockets mainly located along minor north-south-trending faults in the Foel and Llanarmon Limestone formations. The hematite emplacement does not appear to be related to the lead–zinc mineralisation, and may have occurred earlier. The source of the mineralisation is not known, but it was possibly redeposited from ­hydrothermal waters or circulating meteoric groundwater which had leached ferruginous cements present in the Dinantian Base­ment Beds or in late Carboniferous to Permo-Triassic red bed sequences.

Gold and copper

Small amounts of copper and gold were won from veins in the Silurian rocks in the Clwydian Range (Collins, 1975). The source of this mineralisation is also not known.

Chapter 9 Quaternary

About two thirds of the district is underlain by Quaternary (Drift) deposits. Only the high ground of the Clwydian Range and parts of the main Carboniferous limestone escarpment are largely free of drift (Figure 45). Forming a link between the Quaternary successions of the uplands of north Wales and the Cheshire Plain, the district has attracted the attention of many Quaternary workers during the past 150 years. Reviews of these studies were provided by Embleton (1970), Worsley (1970, 1991), Bowen (1974), Earp and Taylor (1986), Addison (1990) and Campbell and Bowen (1989). The Quaternary deposits in the district fall into two major groups: glacial deposits, including till (boulder clay), and water-laid sand and gravel, which formed during the last major glaciation of the Pleistocene stage, and postglacial deposits including head and scree, but principally of alluvial origin, which have accumulated since the retreat of the ice sheets. The latter range from latest Pleistocene to Holocene in age.

Although the district undoubtedly has experienced a complex Pleistocene history, probably being over-ridden by ice several times, the glacial deposits which blanket the region today relate principally to the last major period of ice sheet advance during the Late Devensian (Older Dryas). This event reached its acme some 20 000 years ago, but most of the glacial deposits relate to the subsequent melting and retreat of this ice from the district, which was completed around 15 000 years ago. It is possible that pre-Devensian drift deposits are preserved at depth in parts of the buried palaeovalleys of the River Dee and River Clwyd. To the west of the district, Middle Pleistocene interglacial deposits have been identified in the Pontnewydd Cave [SJ 015 710] in the Elwy valley, and early Late Devensian deposits were recorded from the Tremerchion Caves [SJ 085 724] in the Vale of Clwyd (Campbell and Bowen, 1989). Extensive Middle and Upper Pleistocene sequences, preserved beneath the Irish Sea and locally up to 300 m thick, were detailed by Jackson et al. (1996).

The Devensian glacial deposits and landforms of the district relate to two separate ice sheets, a Welsh ice sheet which entered from the west-south-west and an Irish Sea ice sheet which advanced from the north-north-west. The approximate junction between these ice-masses was first established by Mackintosh (1873, 1874b, 1879) after a systematic survey of the distribution of glacial erratics (see also Strahan, 1890). The most conspicuous erratics are those deposited east of the Vale of Clwyd by the Welsh ice sheet. Here, large boulders of Ordovician volcanic rocks derived from the Snowdonia and Arenig areas occur in a number of localities, for example on Moel y Parc [SJ 1099 7029] (Plate 16), north-east of Moel Arthur [SJ 1418 6646] and at Eryrys [SJ 2048 5801]. In Deeside and western Cheshire, Devensian tills contain abundant marine shells (see for example Shone, 1874, 1878; Strahan, 1886, 1890; Read, 1874; Morton, 1871) as well as erratics from the English Lake District and southern Scotland, indicating deposition from the northerly derived Irish Sea ice sheet. The two ice sheets met and interacted along a line which extends from the northern end of the Vale of Clwyd, around the northern margin of the Clwydian Range to Halkyn Mountain, Mold and along the Alyn valley (Figure 46) (Thomas, 1985). During deglaciation, both ice sheets released large volumes of water and left many stranded ice masses across the district. Meltwater eroded channels in both the basal till sheet and in the rockhead surface, and locally it was impounded by ice margins to form transient proglacial lakes. Large quantities of glaciofluvial sand and gravel were deposited by the meltwaters of both ice sheets. Proglacial lakes and associated deltas appear to have been a particular feature of the ground between the two retreating ice sheets. As the ice retreated, tundra-like periglacial conditions were established across the district, and promoted localised solifluction and head formation on the slopes of the recently formed glacial deposits.

Progressive amelioration of the climate during the mid Late Devensian (late Older Dryas to Allerød) saw periglacial conditions briefly give way to a cool temperate climate during the acme of the so-called Windermere Interstadial, around 13 000 to 11 000 years ago. Thus, the cycle of erosion and deposition that established the present-day landscape of the district was initiated. In many cases, notably in the valleys of the Clwyd, Wheeler and Alyn, rivers enhanced or subtly modified drainage features developed subglacially. Extensive deposition of calcareous tufa may have commenced in the Wheeler valley at this time. A brief return to a colder climate in the latest Late Devensian (or Younger Dryas), 11 000 to 10 000 years ago, saw the reappearance of glacier ice and readvance in the upland regions of northern Britain, including Snowdonia. During this period, known as the Loch Lomond Readvance, permanent snowfields may have formed on the Clwyd hills and Denbigh moors. However, across most of the district, in common with other lowland regions, this period was marked by the return of periglacial ground conditions. Isostatic rebound of the land, caused by the removal of the weight of ice, was active throughout this latest Devensian period and served to offset or diminish the effects of contemporary eustatic rises in sea level.

The final disappearance of permanent ice from the Welsh mountains and throughout the British Isles, coincident with the onset of a major contraction of polar ice caps, marked the start of the Holocene stage, defined by convention at 10 000 years ago. Significant isostatic rebound in Wales is thought to have ended around 9000 years ago, whence Flandrian eustatic transgression raised sea level to its present-day level around 5000 years ago (Tooley, 1974). The major alluvial tracts of the district and their associated river terraces developed mainly during this interval.

In historic times, deforestation and the spread of agriculture have enhanced onshore erosion and have ­con­tributed to the silting-up of the Dee estuary. Such areas have been progressively reclaimed, often through artificial enclosure or ‘warping’, for agricultural and industrial use. There are significant ‘man-made’ deposits in all the main urban and industrial centres of the district.

Glacial landscape

The shape of the subdrift surface (rockhead) can be determined with reasonable accuracy from boreholes and shafts in the coalfield areas, in the central part of the district. Elsewhere, notably in the Vale of Clwyd and across the Cheshire plain, the surface is poorly known.

In the eastern half of the district, thick drift deposits occupy a broad hollow that runs roughly north-west to south-east from the Dee estuary to Rossett [SJ 368 566] and southwards to the Wrexham area (Figure 47). Wills (1912) interpreted this feature as the buried valley of the River Dee (Figure 46), (Figure 48). The valley may originally have been that of a preglacial river, but its present profile suggests that it was greatly modified by Devensian subglacial erosion. The rockhead floor of the valley has an undulating longitudinal profile. It rises from about 80 m below OD in the south, in the Trevalyn area [SJ 377 572], to 35 m below OD at Burton Meadows [SJ 357 590], from where it falls northwards to around 60 m below OD at Bretton [SJ 353 635]. In the present Dee estuary, rockhead rises gradually to about 50 m below OD at Queensferry [SJ 320 680]. The valley floor is between 30 and 50 m below OD beneath the Dee estuary opposite Bagillt [SJ 224 748].

A similar, but less well developed, concealed valley may occur beneath the Vale of Clwyd (Warren et al., 1984) with rockhead at about 50 m below OD (Livingston, 1986). The axis of the valley is probably centred on the maximum development of glaciolacustrine deposits at Pont Glan-y-wern [SJ 090 659] between Denbigh and Llandyrnog and is almost coincident with the present course of the River Clwyd. Other major channels in the district cut into rockhead, including the ‘Flint-Mold trench’ (Figure 47), (Figure 48); those associated with the Alyn and Wheeler valley systems, are thought to have originated as glacial meltwater channels. The crudely U-shaped cross-sections and the undulating longitudinal profiles of buried valleys such as the palaeo-Dee and the Flint–Mold trench may have been initially ice-sculpted, but their present geometry is generally attributed to erosion by subglacial melt-water.

Glacial striae have been recorded, mainly in the upland areas (Morton, 1876; Strahan, 1886, 1890). The extent of glacial overdeepening in the valleys of the Clwydian Range is difficult to assess. Embleton (1970) has interpreted cirque-like forms, such as Cwm-llydan [SJ 165 625] on the slopes of Moel Fammau, and others on the eastern side of the Clwydian Range, as nivation-hollows rather than the products of glacial erosion.

Subglacial or ice-marginal meltwater channels are prominent in the district, occurring principally between the sites occupied by the Irish Sea and Welsh ice sheets, to the south-east of Halkyn Mountain. When the two ice sheets were amalgamated, meltwater drainage was almost exclusively subglacial and formed deeply incised rock gorges throughout the Alyn–Wheeler system, for example between Cilcain [SJ 188 652] and Rhydymwyn [SJ 202 668], at Hendre [SJ 200 676] and at Caergwrle [SJ 309 574]. Subglacial channels also drained meltwaters beneath the Welsh ice sheet in the Terrig and Cegidog valleys to the south-south-east of Mold (Figure 46). At this time, the Alyn drainage system was captured by the Dee at Hendre, leaving the Wheeler valley portion of the preglacial course isolated. As the ice retreated, the subglacial channels were largely replaced by ice-marginal drainage systems. However, these were still supplied subglacially, mainly from the Irish Sea ice sheet (north and east of Mold) via a system of channels preserved on the flanks of Halkyn Mountain, which includes the Sarn Galed and related channels around Rhosesmor [SJ 220 672] and those associated with the ‘Flint–Mold trench’ (Figure 46).

Thomas (1985), reviewing earlier work (Embleton 1956, 1957, 1964, 1970; Peake, 1961; Derbyshire, 1962), established the principal stages of meltwater drainage and channel formation between Flint and Wrexham. Embleton (1964) proposed a subglacial origin for valleys such as that running southwards from Pantasaph [SJ 160 757] and forming a tributary of the Wheeler above Ddol [SJ 141 715], and the east–west valley of Sarn Adda between Plas-y-brain [SJ 248 595] and Pontybodkin [SJ 272 593]. The present-day form of these valleys, the superficial deposits flanking them and the misfit streams within them, suggests they owe their origin to glacial meltwater. A similar origin is suggested for channels in the north-west of the district, especially those developed on the crop of the Dinantian limestones, including the higher portions of the northern tributary valleys to the Wheeler valley, the narrow, largely dry and drift-free valleys on either side of Caerwys [SJ 130 730], and the east–west valley through the Dinantian limestone escarpment at Graianrhyd [SJ 217 561].

In the Clwydian Range, many of the valleys and cols between the Vale of Clwyd and the Alyn–Wheeler valleys may have been deepened by subglacial erosion. Such valleys are narrow, unusually steep-sided, commonly follow faults and typically floored by thin alluvium or head. Examples may be seen on the north-eastern side of Penycloddiau [SJ 1325 6730] and on Moel-y-Parc [SJ 1110 7055], [SJ 118 710].

In the Vale of Clwyd, glacial meltwaters, perhaps initially flowing beneath the Welsh ice sheet, probably formed the rocky gorge of the River Clywedog south-west of Rhewl [SJ 110 604], the valley south-west of Galchog [SJ 114 569] and, in part, the buried valley of the Clwyd.

Glacial deposits

Till is the most widespread type of glacial deposit, and masks the rockhead surface throughout much of the district. Glaciofluvial sand and gravel commonly occurs in landforms which enable those deposits which formed in contact with ice, to be distinguished from those which were laid down as extensive sheets and as glaciolacustrine deltaic bodies in front of the retreating ice sheets. Undifferentiated glaciofluvial sand and gravel deposits, which occur beneath, within or on top of till, are also widespread. A series of schematic cross-sections illustrating the distri­bution and geometry of glacial deposits within the central and southern parts of district, were provided by Thomas (1985) who utilised extensive BGS borehole data (Dunkley, 1981; Ball and Adlam, 1982). Sections for the Dee estuary region (Figure 48) using additional borehole information, illustrate the complex glacial sequences of the palaeovalleys.

Till

Two types of till are recognised in the district, deposited by the Welsh and Irish Sea ice sheets, in the west and east, respectively. Till deposited by the Welsh ice sheet is generally a dark grey to greyish brown, pebbly clay (diamicton), with clasts predominantly of Lower Palaeozoic lithologies derived from the Denbigh moors and the upland regions to the west. In the Vale of Clwyd, where bedrock comprises mostly Permo–Triassic sandstone, the till is rich in red-brown sand and sandstone clasts. The Irish Sea till is compositionally more varied than the Welsh till. It is commonly a reddish brown sandy or silty, pebbly clay (diamicton), with clasts from the Irish Sea Basin, the English Lake District and southern Scotland. Over the hilly ground of Deeside, it contains much locally derived material and may vary in colour, texture and clast content over short distances. On the margins of the Cheshire Plain, it includes Permo–Triassic debris and shelly clay derived from the floor of the Irish Sea basin.

Welsh till

This till occurs from the Vale of Clwyd, through the valleys and passes of the Clwydian Range to beyond the Dinantian limestone escarpments, from Halkyn Mountain in the north to Graianrhyd in the south (Figure 45), (Figure 46). The area in the south of the district, around Treuddyn, is also underlain by Welsh till. Residual deposits and erratics on the highest ground indicate that the ice sheet covered the entire western part of the district. Drumlins occur south-west of Ruthin. The Welsh tills probably include lodgement, ablation and flow tills.

In the south-western parts of the Vale of Clwyd, the till shows little lateral variation and is typically a moderately stiff, grey to greyish brown, silty clay with abundant pebbles and cobbles and rare boulders. Weathered till is friable and olive or yellowish in colour. Clasts comprise mainly angular to subangular, weathered Silurian rocks with some erratics of Ordovician volcanic rock. In the Llanfwrog–Rhewl area, the till also includes blocks of Dinantian limestones and the matrix is more sandy. In the northern parts of the Vale, where Permo–Triassic sandstones occur beneath the drift, two till lithologies are recognised, one sandy and the other gravelly (compare Strahan, 1890). There seems to be no consistent stratigraphical relationship between these till types and they are probably intergradational. A particularly coarse gravelly till occurs between Gellifor [SJ 1240 6255] and Rhydonnen [SJ 1135 6325] and at Plas Gwyn [SJ 1245 6100].

In the Clwydian Range, the till is confined mainly to the valleys. Sections along tracks and minor stream courses show the till to be a yellowish grey clay with abundant clasts of Silurian rocks and a few rounded exotics of limestone, quartz and possible Ordovician volcanic rocks.

In the Alyn valley, extensive till interdigitates with glaciofluvial deposits (Dunkley, 1981; Ball and Adlam, 1982). In the upper Alyn valley, between Llanferres and Cilcain, the till comprises stiff, dark to pale grey or brown clay with abundant subrounded to subangular pebbles and cobbles of siltstone, sandstone and some limestone. Widespread till occurs to the north of the Wheeler valley and west of Halkyn Mountain, where it blankets much of the Dinantian outcrop. The till here is a brownish grey to yellowish brown clay with abundant clasts of Silurian rocks. Limestone and igneous clasts are also present and increase in abundance eastwards.

Welsh till is extensive in the southern part of the area, between Treuddyn and Mold. The thickest deposits are located along the Terrig and Cegidog valleys. Strahan’s (1890) description of ‘a tough yellow and red till, packed with great boulders of limestone, calcareous sandstone, Millstone Grit, and a few Upper Silurian rocks’ remains pertinent.

Details

In the Vale of Clwyd, the Plas-yr-Esgob No. 1 Borehole encountered 3.2 m of clayey till overlying 3.8 m of sandy till. A trench section [SJ 1320 5990] to [SJ 1420 5595] from Wern to west of Pentre Coch Manor provided sections in stiff, brown and grey mottled, sandy, stony clay with beds of sand and gravel.

In the Clwydian Range, a deeply incised stream [SJ 1707 5979] above Fron-heulog exposes 4 m of grey clay with poorly sorted granule- to boulder-sized clasts of siltstone, resting on silty mudstone of the Elwy Formation. The banks of the stream [SJ 1293 7085] draining northwards to Afon-wen expose up to 5 m of very poorly sorted till, overlying an irregular northward-inclined surface of Silurian rocks. Silurian siltstones dominate the angular to subrounded clast assemblage, which also includes Ordovician volcanic rocks, coarse-grained gritty sandstone and rare red sandstones.

A section [SJ 2313 5923] 350 m east of Trefrwd in a deeply incised gully exposes 3 to 4 m of till overlain by head. The till comprises grey and brown stony clay and clayey sand with numerous large cobbles and boulders of sandstone, quartzite and siltstone, as well as rare cobbles of limestone.

Irish Sea till

The clay-rich till covers most of the district east of Halkyn Mountain and Hope Mountain, but includes interbedded glaciofluvial sands and gravels both at the surface and at depth (Figure 48). Variations in bedrock geology strongly influence till composition. Across the Deeside–Cheshire lowlands, the more hilly areas of Deeside, between Halkyn Mountain and Buckley and the southern Wirral, the till consists mainly of reddish brown, massive, calcareous, silty, clay-rich diamict. A sandy till is associated with the crop of the Kinnerton Sandstone. In areas underlain by the Coal Measures, the matrix contains less Permo–Triassic debris and is generally grey. The clasts in this till vary in size from granules to boulders and are commonly rounded to subrounded. Lithologies derived from local Carboniferous and Permo–Triassic bedrock are abundant, but they are accompanied by exotic clasts derived from Permo–Triassic sequences in the Irish Sea Basin, and distinctive igneous rocks from the Lake District and Scotland. Reworked, marine, shelly micro- and macrofossils are locally abundant. In some farmland areas, the till surface is characterised by small, commonly flooded pits formerly worked as a source of agricultural lime.

Cross-sections, demonstrating the variations in thick­ness and composition of the Irish Sea till in the area of the Dee estuary, are shown in (Figure 48). In the east, the till sheet commonly displays a gently undulating upper surface and masks a complex bedrock topography. The thickness of the sheet is therefore very variable. On the Wirral plateau around Ledsham and Capenhurst [SJ 365 745], the maximum proved thickness is some 15 m but around 32 m is present adjacent to a preglacial cliff near Shotwick [SJ 3425 7225]. Postglacial erosion in the Dee estuary, south-west of Shotwick has reduced its thickness to about 12 m and boreholes show that it is locally absent. Over 60 m of till, including intercalations of sand and gravel, are present locally in the buried valley of the River Dee. In the west, in the Flintshire Coalfield and along the Alyn valley, where the surface expression of the till sheet broadly mirrors rockhead, the till is generally less than 20 m thick.

Boreholes through the till, for example along the line of the A483(T) road, commonly show an upper mottled, but otherwise structureless till, overlying a lower, stiffer, streaky or laminated till commonly including lenses and beds of glaciofluvial sand and gravel. The contact is commonly marked by a change in colour. Such sequences, once interpreted as evidence of more than one glaciation, may represent different processes within a single glacial event. Thus, the lower till may be a compact lodgement till deposited beneath the ice during its advance, and the upper one an ablation or flow till deposited from wasting ice during its retreat. Eyles and McCabe (1989) suggested that, in areas peripheral to the main Irish Sea glaciation such as the Cheshire–Shropshire lowlands, the rapid retreat of the ice sheet left large areas of stranded ice from which poorly consolidated ablation till would have been widely deposited. The relatively clast-free nature of some tills in the Wrexham area led Wedd et al. (1928) to suggest that they may have been deposited in a glacial lake formed along the margins of the Irish Sea ice sheet as it retreated northwards.

In the south of the district, around Llay and Rackery, Irish Sea tills locally overlie, or occur interbedded with glaciofluvial deposits of the ‘Wrexham Delta-terrace’ (see below). These relationships have been cited as evidence of a later glacial readvance (Peake, 1961, 1979, 1981). Francis (1978) interpreted some of these deposits as lobate sheets of flow-till derived from the retreating Devensian ice; Thomas (1985) argued that others represent lodgement tills deposited during a temporary and local resurgence of the decaying ice sheet.

Details

Gullies near Padeswood [SJ 2807 6304] to [SJ 2807 6265] sporadically expose up to 10 m of brown to yellowish brown clay or silty clay with cobble and small boulder clasts. Two metres of a similar till with predominantly Coal Measures clasts underlie glaciofluvial deposits in Alltami Brook [SJ 2698 6629] north of Buckley. This section, formerly more extensive, was described by Wedd and King (1924, p.157).

A thick sequence of tills, intercalated with glaciofluvial sands and gravels, fill the palaeovalley of the Dee as proved in one of the Hawarden Castle Colliery boreholes [SJ 3405 6561]:

Thickness m
Till, sandy in upper part 32.9
Sand and gravel, apparently normally graded 12.6
Till 1.8
Sand 3.7
Till and sand 10.0
Sand, loamy in upper part 6.7
Till 1.8
Sand 0.9

A stream [SJ 3804 6962] near Crabwell Hall, to the north of Blacon, exposes over 3 m of reddish brown, very sandy, clayey till with numerous subrounded, granule- to cobble-grade clasts of pale sandstone, pinkish brown sandstone, igneous rocks (including two granites) and rare comminuted shells. The abandoned cliff [SJ 308 738] to [SJ 377 670] on the north side of the Dee estuary running from Burton to Blacon shows numerous exposures of a weathered, reddish brown to brown, variably shelly and sandy till, which is gravelly in places.

Bank Farm (also known Rackery or Astbury’s) Sand Pit [SJ 332 572] exposes till overlying glaciofluvial deposits of the Wrexham Delta Terrace. A 2.5 m-thick bed of diamict, interpreted by Thomas (1985) as lodgement till, is interbedded with sands and gravels of the ‘terrace’ in workings [SJ 344 558] at Singret (see below).

Glaciofluvial deposits

Glaciofluvial sand and gravel deposits are widespread in the district, both at surface and beneath till. Outwash sands and gravels were deposited in the main valleys across the district, notably those of the Wheeler, the Alyn and their tributaries. Three categories of deposit are distinguished in this account: ice-contact, sheet and undifferentiated deposits (Figure 45).

Adjacent to ice sheet margins, sand and gravel accumulated as ice contact deposits (eskers, kames and kame terraces), whereas in front of the ice, sheet deposits formed in braided sandar and deltas. Both are commonly preserved as constructional landforms on the till surface. Locally, they may be interbedded with thin tills. Undifferentiated glaciofluvial deposits include those deposits which are of uncertain origin or in which ice contact and sheet components are difficult to distinguish.

The presence of numerous intercalations of sand and gravel in the Irish Sea till of the Deeside–Cheshire lowland (for example Hawarden Castle Colliery Borehole: see above), led to an early model where a single, widespread ‘Middle Glacial Sands and Gravels’ division intervened between lower and upper units of boulder clay (see for example Hull, 1864; Strahan, 1886). The validity of this tripartite sequence has since been questioned, and it is now acknowledged that the sand and gravel bodies occur at different levels in different places and have a varied origin (Wedd et al., 1923; Boulton and Worsley, 1965). Some of the more extensive sheet-like deposits may be the product of proglacial outwash, whereas smaller lensoid bodies, many included within the complex sequences which fill buried valleys, may have formed in a range of glaciogenic environments. Shoestring sand and gravel bodies directly overlying bedrock are widespread and may represent proglacial or subglacial channel deposits.

Glaciofluvial ice-contact deposits

The Wheeler and Alyn valleys, and their tributaries, are flanked and floored by supraglacial, ice contact sand and gravel complexes which exhibit relict glacial landforms including terraces, ridges, mounds and hollows. Ice contact deposits are also recognised close to the northern margin of the district, north-east of Pant and near Brynford and Pentre Halkyn. Comparable deposits occur in the west of the district, along the western flanks of the Clwydian Range, and on the western side of the Vale of Clwyd, south of Rhewl.

Wheeler valley and its tributaries

Glaciofluvial sands and gravels are widespread throughout the Wheeler valley and its tributaries. These deposits and their associated landforms were interpreted as lacustrine in origin (Embleton, 1956; Peake, 1961, 1964). However, subsequent studies (Derbyshire, 1962; Embleton, 1964; Brown and Cooke, 1977) failed to show the presence of either palaeo-shorelines or lacustrine bottom deposits and concluded that the sediments and landforms are the products of ice-contact and ice-marginal processes, relating to the decay of the Welsh ice sheet.

Numerous small mounds, ridges, terraces and enclosed hollows characterise the slopes of the main valley along its entire length, from near Bodfari [SJ 095 720] to beyond Penbedw [SJ 165 680], a distance of some 10 km. South and east of Nannerch [SJ 166 695], the valley widens and broad, pitted terraces are more common. Similar constructional features are seen in many of the tributary valleys to the south of the main valley, for example the valley south-west of Pen-y-felin [SJ 155 696] and along a northern tributary extending from Ddol [SJ 141 712] to the north-east of Babell [SJ 156 745]. These landforms are constructed dominantly of reddish brown, fine-grained, well sorted sand. The bedding is subhorizontal, or shows large-scale cross-stratification. Sand beds are commonly either ungraded or fine upwards, though rare inverse graded units are recorded. Towards the margins of mounds and terraces, bedding is commonly disturbed and locally overturned. Gravels consisting of rounded to subrounded pebbles and rare boulders of Silurian rocks, limestone, quartz, quartzite and volcanic rocks are common as thin beds and lenses. Thin beds of till and laminated silt are also locally present.

Prominent, but discontinuous terraces occur at several levels on both sides of the main valley. They are more common and typically wider on the northern side. Cross-valley correlation of surface levels is seldom possible. The terraces vary in height from 2 to 10 m above the floodplain and are generally gently inclined down valley to the west. Borehole evidence from the Nannerch area (Ball and Adlam, 1982; Thomas, 1985) suggests that these deposits may locally exceed 30 m in thickness. Brown and Cooke (1977), using seismic soundings, determined a minimum thickness of 37 m for the lower terrace at Nannerch. They considered the more extensive terraces to represent pitted outwash terrain. Fan-like terraces with curvilinear fronts occur where tributary valleys enter the main valley both from the north and south. They have been interpreted as kame terraces associated with tributary alluvial fans and deltas.

Enclosed hollows, thought to be kettleholes, are a feature of the upper surfaces of the terraces, notably around Nannerch [SJ 163 697], where they are over 25 m deep (Plate 17). They are partially filled by late-glacial and postglacial head deposits, peaty clay (see below) and locally, as seen in Tyddyn-onn Borehole, by laminated lacustrine deposits.

Steep-sided, conical mounds, locally over 15 m high, occur throughout the valley, but notably to the north-west of Nannerch. Good examples are at Candy Mill [SJ 104 717], Maes-mynan [SJ 118 725], Ddol [SJ 145 712], and at Bryn Rug [SJ 160 702]. They consist of stratified sands and gravels with some thin, interbedded, red or grey, sandy diamicts. These are interpreted as kames.

The crests of compound ridges of sand and gravel, present between Maes-mynan [SJ 114 722] and Ddol [SJ 142 712] and to the east-south-east of Afon-wen [SJ 1315 7160] to [SJ 1395 7145], are aligned parallel with, or slightly oblique to the axis of the main valley. Brown and Cooke (1977) interpreted these features as eskers suggesting that they originated as supraglacial crevasse-infills which were subsequently let down by melting ice. A similar esker-like ridge of sand and gravel is also present in a tributary valley to the south at Bryn [SJ 169 673].

Pant, Brynford and Pentre Halkyn areas

The glaciofluvial deposits [SJ 155 750] north-east of Pant form a series of low hummocky ridges, mounds and indistinct terraces within a shallow valley which widens to the north and links with the Ddol to Babell valley near Plas-newydd [SJ 155 746]. The deposits, interpreted as degraded kames and kame terraces, probably relate to terraced sands and gravels in the Brynford area [SJ 185 745] and at Pentre Halkyn [SJ 197 727], and together record glaciofluvial deposition between the retreating margins of the Welsh and Irish Sea ice sheets. An undulating, esker-like ridge of fine-grained sand with subordinate gravel occurs at Naid-y-march Farm [SJ 168 751].

Lower Alyn valley

The Alyn valley below Mold is floored mainly by glacio­fluvial sheet deposits and till, but also local deposits of ice contact origin formed in the tract between the Welsh and Irish Sea ice sheets. Low, hummocky kame-like mounds of sand and gravel resting on till characterise the western side of the valley, between Mold [SJ 225 637] and the River Terrig near Pentre [SJ 242 593]. The mounds are up to 10 m high and have oval outlines elongated north–south.

A broad, hummocky and pitted platform in sand and gravel, with associated minor mounds, ridges, crude terraces and kettleholes, extends from near Padeswood [SJ 275 620] to beyond Hartsheath [SJ 287 603]. Wedd and King (1924) suggested that the hummocky nature of the ground may have been accentuated by local mining subsidence, but abundant kettleholes, partly filled with alluvial clay and peat, form steep-sided hollows, up to 10 m deep and 50 m in diameter. Esker-like ridges present near Hartsheath (Wedd and King, 1924) may represent modified kames (see also Embleton, 1970, p.75). The borehole data of Ball and Adlam (1982) were incorporated in cross-sections of this area by Thomas (1985, fig. 3). Clayey gravels which underlie irregular and dissected slopes at Pant Farm [SJ 288 584], on the northern slopes of Hope Mountain, may represent either a degraded kame terrace or an ablation deposit.

Clwyd Range and Vale of Clwyd

Ice contact glaciofluvial deposits fill a narrow valley [SJ 1345 6820] on the north-eastern slopes of Penycloddiau. Comparable deposits also occur at the head of the major valley [SJ 131 682] between Penycloddiau and Colomendy, and on the col [SJ 140 668] between Penycloddiau and Moel Arthur, as well as at Plas-yw [SJ 159 671] east of Moel Arthur. These sites lie between sloping drift-free ground, to the west, and till-filled valleys to the east. They exhibit poorly developed hummocky and terraced featuring. Stream exposures reveal reddish brown, fine-grained sand grading into pebbly, sandy clay diamict. These bodies may represent remnant moraines, ablation deposits or solifluction-modified kames and kame terraces, deposited against stagnant ice lodged in the valleys below.

More extensive ridges, steep-sided mounds and relict terraces with kettleholes occur along the western flanks of the Clwydian Range, notably around Castell [SJ 111 687] and Llangwyfan [SJ 120 662], with numerous smaller patches farther south. The deposits typically abut drift-free ground to the east and pass downslope into glaciofluvial sheet deposits or till. Terrace features dip westwards, but are deeply incised by later channels. Scattered exposures in these deposits reveal reddish, poorly bedded, clayey sand with scattered pebbles of Silurian siltstone, brown sandstone and rare limestone and acid volcanic rock. Intercalated sandy clay diamicts are locally present. The landforms represent degraded kame terraces or deltas formed along the edge of the decaying Welsh ice sheet in the Vale of Clwyd. The associated diamicts may represent flow tills derived from the ablating ice margin.

A dissected terrace with shallow kettlehole-like depressions occupies both sides of the Clywedog gorge [SJ 1115 5940] south of Rhewl. It consists of glaciofluvial ice contact deposits, composed of gravel and gravelly sand. To the north-east and east, these deposits lie upslope from a lower terrace feature which forms the Rhewl alluvial fan (see Older Fan Deposits). The origin of these terrace features at Rhewl has been discussed by Strahan (1890), Wedd et al. (1924) and Rowlands (1955). Most recently, Livingston (1986) interpreted the terraces as remnants of kame-deltas which formed against down-wasting Welsh ice, trapped in the Vale of Clwyd.

Details

Sections in sand and gravel pits in the kames, kame terraces and outwash of the Wheeler valley were described by Brown and Cooke (1977). The results of borehole investigations of these deposits were given by Ball and Adlam (1982). Exposures in the esker ridge east-south-east of Afon-wen [SJ 1315 7160] to [SJ 1395 7145] reveal 15 to 20 m of well bedded sand overlain by up to 2 m of gravel and sandy till. A disused quarry near the eastern end of the same ridge provides a degraded 20 m-high section in reddish brown, well sorted, fine-grained sands in which tabular and trough cross-bedding and ripple cross-lamination is disrupted by small synsedimentary faults. In the upper part of the section, there are lenticular channel-fills of trough cross-bedded gravel, containing reworked clasts of cemented gravel. The tractional structures at this locality show that the current direction was mainly from the west and south-west.

Sections in ice contact deposits, east-north-east of Pant [SJ 1465 7445] and west of Naid-y-march [SJ 157 751], comprise interbedded sands and gravels; associated lenses and beds of pebbly clay diamict may be flow tills.

In the lower Alyn valley, the Nercwys Hall Borehole, sited on the northern flank of one of the mounds south of Mold, proved over 13 m of sand and gravel without reaching bedrock (Ball and Adlam, 1982). The sequence included a 0.5 m-thick bed of till. A nearby pit [SJ 2369 6052] exposes 7 m of cobble gravel including clasts of limestone, chert and igneous rocks. Former sections in the Padeswood and Pant Farm deposits were described by Strahan (1890) and Wedd and King (1924).

Exposures in clay-rich sands and gravels are seen in the stream [SJ 110 690] and sand pit [SJ 107 691] north of Castell, and in the lane [SJ 708 698] 500 m east of The Grove Hall.

Glaciofluvial sheet and glaciolacustrine delta deposits

Several spreads of sand and gravel present within the Alyn valley and its tributaries (Figure 45) were deposited as sheet-like bodies beyond retreating ice sheet margins in the form of braided alluvial fans or sandar, but also as deltas which built out into proglacial lakes (Thomas, 1984, 1985). Subsequently dissected, these bodies now typically occur as a series of linked terraces, commonly with kettleholes in their upper surfaces. They are of considerable thickness and have been extensively worked for sand and gravel. Three main areas of sheet deposits are recognised in the Alyn valley: in the Rhosesmor–Mold area; the Hope area; and the area to the south of Caergwrle and between Rackery and Marford. Sheet deposits are also present in the Vale of Clwyd.

Rhosesmor–Mold area

The deposits hereabouts represent the dissected remnants of a once continuous body, in detail a series of terraces, which occupied the northern margin and floor of the Alyn valley. They were deposited in a complex ice-marginal, proglacial lacustrine environment (Peake, 1961; Embleton, 1964; Thomas 1984, 1985).

Sections reveal laterally variable sequences of interbedded tabular and trough cross-stratified sand and gravel, channel-fill gravels, laminated clays and silts and subordinate diamicts (flow tills). In the pit at Rhosesmor [SJ 216 670], Thomas (1984) identified topset, foreset and bottomset facies in what he interpreted as two stacked progradational deltaic sequences; a lower one up to 17 m thick and an upper one up to 28 m. Deformation by syndepositional glaciotectonic processes produced a series of subparallel folds, thrust planes and normal faults in the more ice-proximal deposits of the lower sequence.

The glaciolacustrine delta sequences overlie tills along the former line of contact of the Welsh and Irish Sea ice sheets. The two deltaic sequences at Rhosesmor are both considered to have built out mainly from the north-east. Initially, sediment was transported subglacially through the Sarn Galed channel [SJ 227 674] and via a marginal outwash fan into a temporary, fluctuating, proglacial lake, the Rhydymwyn Lake (Peake, 1961; Thomas, 1984, 1985). The sheet deposits in the floor of the valley around Mold, may relate to a lower lake, the Mold Lake, developed slightly later in the deglaciation (Peake, 1961; Thomas, 1985). The deltaic deposits are flanked by undifferentiated glaciofluvial deposits, at least partially of ice contact origin, suggesting that the ice margin lay no great distance to the north and east. The relationships between these deposits and with more distal (down-valley) glaciofluvial sheet deposits, such as the Hope Sandur (see below), were shown schematically in cross-sections by Thomas (1985).

Hope area

Glaciofluvial sheet deposits along the floor of the Alyn valley to the north-west and south of Hope [SJ 300 586] were described and interpreted by Thomas (1985). He envisaged deposition as a braided sandur (the Hope Sandur) supplied by meltwater streams flowing from the north-west. The sheet-like body was confined within a narrow ‘trough’ between the margins of the retreating Irish Sea ice sheet to the east, and the exposed valley-side to the west, and in the east, the deposit is underlain by Irish Sea till. To the west and north-west it appears to merge with ice-contact glaciofluvial deposits. Locally, in central parts of the valley, it overlies laminated glacio-lacustrine clays (Ball and Adlam, 1982; Thomas, 1985). The succession above the laminated clays is about 12 m thick. Above a basal unit of parallel- and cross-laminated sand, it coarsens upwards markedly through alternating sets of parallel-laminated sand and planar cross-bedded sand, with thin gravel lenses, into an upper unit comprising stacked sequences of massive, coarse gravel including channel-fills, commonly associated with intercalated stony diamicts; the last may represent debris flows or flow tills derived from an adjacent ice margin.

Area south of Caergwrle, and between Rackery and Marford

These glaciofluvial sheet deposits at the southern margin of the district are the northernmost parts of the most extensive tract within the region, the so-called ‘Wrexham Delta-terrace’ (Wedd et al., 1927). This body, locally in excess of 30 m thick, covers an area of over 40 km2, mainly to the south and east of Wrexham. It is underlain by till and locally, as in the south of the district at Llay [SJ 330 560], it appears to be also overlain by till. The upper surface of the terrace preserves a variety of ablation and ice contact structures, which include eskers, and kame and kettle topography (Campbell and Bowen, 1989). Internally it comprises a complex suite of intercalated sands, gravels and diamicts. The ‘terrace’ was previously interpreted as a glaciolacustrine deltaic body (Peake, 1961), as morainic outwash with the outer escarpment representing an original ice contact slope (Poole and Whiteman, 1961), as an ice marginal lateral terrace (Worsley, 1970), and as a largely braided, subaerial fan or sandur (Francis, 1978; Dunkley, 1981; Wilson et al., 1982). Thomas (1985) concluded that it represents a complex, diachronous body with elements of ice front, debris-flow, alluvial fan, sandur and both proglacial and ice-contact lacustrine facies which formed at the margins of the Irish Sea ice sheet (see also Campbell and Bowen, 1989). Subsequently, Thomas (1989) envisaged this body as one of a series of coalescing ‘fan-deltas’, initially deposited between bedrock and the margins of the Irish Sea ice sheet, but which later prograded into a large proglacial lake (Glacial Lake Bangor) sited between Chester and Ellesmere. Schematic cross-sections through the ‘Wrexham Delta-terrace’, based upon exposures and boreholes (for example Dunkley, 1981), and of the adjoining areas were provided by Thomas (1985).

The portion of the ‘terrace’ that extends into the Flint district is defined on its eastern side by a prominent, north-east-facing escarpment at Marford, and between Singret and Rackery [SJ 332 574]. Its gently inclined upper surface declines from about 100 m above OD at Caergwrle to approximately 70 m above OD at the escarpment edge. Between Marford and Singret it is cut through by the River Alyn. It has been extensively worked within the district as a source of sand and gravel at Marford [SJ 358 558] and Singret [SJ 344 558] (Plate 18).

Vale of Clwyd

A series of distinctive fan- or lobe-shaped sand and gravel bodies are present along the western side of the Vale of Clwyd; commonly located opposite valleys which emerge from the Clwydian Range (Figure 45). The largest individual examples are located at Dre-goch [SJ 105 685], south-west of Llangwyfan [SJ 120 655], and east of Rhos [SJ 123 618]. Several poorly defined possibly composite fans occur along the foot of the Clwydian escarpment south of Hirwaen [SJ 137 613].

The upper surfaces of these bodies typically slope gently westwards, but they are commonly traversed by deeply incised head-filled valleys. The eastern, upper portions of many of these fans are steeper and hummocky and, in the north, merge with the kettled glaciofluvial ice-contact deposits near Castell and Llangwyfan. The fans in the south commonly abut outcrops of the Kinnerton Sandstone. These bodies all appear to overlie, and also to pass downslope on to the main Welsh till sheet of the Vale. Livingston (1986) considered that both the fan-shaped sheet and associated ice contact deposits which characterise this eastern portion of the Vale were elements of supraglacial gravel fan complexes let down on to the till surface following ice decay. However, the geometry of the deposits and absence of kettleholes is consistent with accumulation after (or during) ice retreat, as a series of outwash fans supplied by swollen meltwater streams draining the Clwydian Range (see p.159).

Details

Detailed descriptions of sections in the Rhosesmor area, principally observed in the local sand and gravel pit [SJ 216 670], were provided by Thomas (1984). Sections in the sandur deposits exposed in the Hope sand and gravel pit [SJ 360 587] were described by Thomas (1985).

In sections in the ‘Wrexham Delta-terrace’, provided by pits [SJ 358 558] in the vicinity of Marford, Thomas (1985) noted up to 15 m of closely alternating, lenticular beds of gravel, sand and red clayey diamict. Borehole evidence suggests the deposit here is in excess of 20 m thick. Sand and gravel workings at Singret [SJ 344 558] and Ball’s Wood [SJ 348 563], south of Mount Alyn, expose a highly complex and variable sequence up to 30 m thick described in detail by Francis (1978), Dunkley (1981) and Thomas (1985). In a lower sand and gravel sequence, which overlies a channelled contact with the underlying till, massive cobble-grade gravels, associated with impersistent diamicts pass up into parallel- and cross-laminated, medium-grained sands in a series of upward-fining beds. At a higher level, cross-laminated, finer grained sand, alternating with silt and clay coarsen upwards via parallel-laminated sand, into planar and trough cross-bedded gravels. A massive disconformable bed of reddish brown, stony diamict, up to 2.5 m thick, which underlies the channelled base of an upper sequence of sands and gravels, has been interpreted as either a subaerial debris-flow (Francis, 1978) or a subglacial lodgement till (Thomas, 1985). In the upper sand and gravel sequence, upward-fining gravels alternate with parallel- and cross-laminated sands associated with numerous gravel-filled channels (Plate 18a). This sequence passes north-eastwards into tabular cross-bedded gravel overlain disconformably by a subhorizontal sheet of variably sorted structureless and cross-bedded gravel up to 10 m thick (Plate 18b).

In a 12 m-high section formerly exposed in the sand and gravel workings at Bank Farm (also known Rackery or Astbury’s) Sand Pit [SJ 332 572], well-sorted, parallel- and cross-bedded, fine-grained sand alternated in cyclic sequences with silt and clay (Peake, 1961; Francis, 1978; Dunkley, 1981).

In the Vale of Clwyd, a ditch [SJ 1264 6168], 275 m east of Plas-yn-rhos, exposed 1.55 m of red sandy gravel with an overlying pinkish brown stony clay. A stream exposure [SJ 1400 6014] in the Llanbedr fan shows up to a metre of imbricated gravel, composed of discoidal clasts of Silurian siltstone, resting on an uneven surface of Permo-Triassic sandstone.

Undifferentiated glaciofluvial deposits

These sands and gravels, of mixed or of unknown origin, occur extensively within the upper parts of the Alyn valley and to the south of the Dee estuary, with isolated occurrences on the Cheshire plain.

Upper Alyn valley, south of Cilcain

South of the gorge near Cilcain, hummocky spreads of sand and gravel with numerous depressions, some associated with swallow holes in the Carboniferous limestone, occupy both flanks of the Alyn valley. Borehole data suggest that these deposits seldom exceed 5 m in thickness, and that clayey gravel and sandy gravel commonly underlie a surface veneer of pebbly clay up to 2.2 m thick (Ball and Adlam, 1982). Silurian rocks with less common limestone comprise the bulk of the clasts in the gravels. The overall form of these deposits suggests that they represent a series of low, poorly defined and degraded kame terraces which locally pass into pitted outwash terrain.

Deeside and Cheshire Plain

Hummocky and moundy terraces of sand and gravel occur at varying elevations along the Flint–Mold valley (Figure 45) and extend across the watershed between the rivers Dee and Alyn. Terraces with enclosed hollows, such as that [SJ 237 710] north of Flint Mountain, may have formed as ice contact deposits against the retreating Irish Sea ice sheet, but others may be subglacial in origin and form part of the complex fill of the associated buried channel.

The numerous sand and gravel patches, which occupy the south-western slopes of the Dee estuary to the north-west and south-east of Flint, may relate to a more continuous ridge that extends from Connah’s Quay [SJ 280 690] to Old Warren [SJ 320 640], and possibly to discrete bodies on the Cheshire Plain, to the west and south of Higher Kinnerton [SJ 320 600] (Figure 45). These deposits exhibit marked changes of thickness and lithology and are ­apparently both underlain and overlain by Irish Sea till (Strahan, 1890) (Figure 48). They may represent elements of a dissected kame terrace or outwash fan derived from the Irish Sea ice sheet, but subsequently over-ridden during a temporary re-advance. Thus they are similar in form and origin, if not in age, to the ‘Wrexham Delta-terrace’ (see above) (Wedd and King, 1924; Peake, 1979).

Details

Details of the deposits in the upper Alyn valley were provided by Ball and Adlam (1982).

In the April Rise Farm Borehole, brown, laminated sandy and silty clay of probable glaciolacustrine origin is succeeded by 18.5 m of interbedded clayey sand and gravel, capped by 5.1 m of dark orange-brown, sandy clay with abundant pebbles and gravel lenses ((Figure 48), A–A′).

A disused pit [SJ 3036 6686], north-east of Ewloe, exposed 8 m of pebble and cobble gravel including clasts of local Carboniferous sandstones and siltstones, well rounded erratics of quartz, quartzite and coarse-grained igneous rocks, and shell fragments. De Rance (in Strahan, 1890) observed over a metre of ‘reddish boulder clay with northern erratics’ capping the section.

Boreholes in the linear crop which extends for 1.7 km south-east from Grange Farm [SJ 337 597] to Burton Green [SJ 346 584] show it to contain interbedded till and to dip towards the north-east (Ball and Adlam, 1982). Sand and gravel in excess of 14.7 m in thickness was recorded in the Burton Green Borehole, but evidence from nearby boreholes suggests that the deposit thins markedly to the north, south and east.

Glaciolacustrine deposits (excluding ­glacio-lacustrine delta deposits)

Temporary proglacial lakes are a common feature during deglaciation. The deposits of these lakes interdigitate with glaciofluvial sediments. In this district, late Devensian glacial lakes have previously been invoked, largely on geomorphological grounds, to explain the form and distribution of many of the Quaternary deposits preserved in the Alyn and Wheeler valleys, in the Vale of Clwyd and on the Cheshire plain (Rowlands, 1955; Peake, 1961). Present evidence suggests that such lakes were less extensive, but nevertheless played an important part in the deglaciation history of the district (Derbyshire, 1962; Embleton, 1964; Thomas, 1984, 1985; Livingston, 1986).

In this account, extensive glaciolacustrine delta deposits, principally composed of sand and gravel, have not been distinguished from contiguous glaciofluvial sheet deposits and are described with these above. Thick sequences of contemporary prodeltaic glaciolacustrine deposits that consist mainly of laminated clay, silt, and fine sand with local interbedded stony clays have been widely encountered in boreholes (Dunkley, 1981; Ball and Adlam, 1982) but are seldom seen at surface.

Details

The only mapped crop of glaciolacustrine deposits occupies the floor of a narrow ravine [SJ 3355 5585] near Llay at the southern margin of the district. Here, augering proved the presence of soft, silty sand interbedded with stone-free clays. These sediments relate to similar deposits present beneath the ‘Wrexham Delta-terrace’ and proved in boreholes around Llay and Singret (Dunkley, 1981). A borehole [SJ 3438 5586] in the floor of the Singret Sand Pit proved the following sequence:

Thickness m
Glaciofluvial deposits
Sand and gravel. The gravel is fine to coarse-grained with some rounded cobbles of quartzite, volcanic rock, limestone, sandstone, quartz, siltstone and chert. The sand is mainly medium grained. 5.0
Glaciolucustrine deposits
Clay, stiff, reddish brown, stony, passing down into laminated sands, silts and clays 2.0
Clay, dark greyish green, very hard and stony with angular to rounded clasts of quartzite, limestone, volcanic rocks, shale and sandstone 2.5
Silt, laminated reddish brown clay and sand with pebbly horizons 12.5
Clay, hard, brown, massive and stony, with poorly sorted angular to subrounded clasts of quartzite and coal 3.0
Silt, brownish buff and brownish green, laminated clay and sand 1.5
Silt, clayey, greenish brown, and sand with rare pebbles over 4.3

Boreholes penetrating the glaciofluvial sequences of the Alyn valley below Mold commonly encountered sandy stone-free clay and laminated sand and silt believed to be glaciolacustrine in origin (Ball and Adlam, 1982; Dunkley, 1981). The Tai Bowen Borehole proved over 16.5 m of soft, greyish brown, laminated, silty clay beneath glaciofluvial sands and gravels. Over 3 m of stiff, reddish brown, laminated sandy clay and silt, proved in the Tan y Mynydd Borehole, Caergwrle, underlie the glaciofluvial sheet deposits of the Hope area.

Thin sequences of laminated clay and alternating clay and silt occur intercalated with the glaciolacustrine delta deposits in the Rhosesmor sand and gravel pit [SJ 216 670] (see above), and similar deposits were encountered in boreholes sited to the south and west (Ball and Adlam, 1982; Thomas, 1984, 1985).

Postglacial deposits

As the main ice sheets retreated from the district, their deposits were reworked under the largely periglacial conditions which prevailed during the subsequent intervals (15 000 to 10 000 years ago). The deposits formed include valley head and scree, as well as older alluvial fans in the Vale of Clwyd. Some of the lacustrine deposits and peat found in kettleholes probably date from this period also, as do the earliest calcareous tufa deposits found in the Wheeler valley. During the Holocene, as sea level rose and final isostatic adjustments occurred, marine and estuarine deposits, including raised storm beaches, were deposited in the north-east of the district along the Dee estuary. Meanwhile terraces were developed along many of the major river valleys and later alluvium was deposited on their present floodplains. Tributary streams fed recent alluvial fans, and lacustrine alluvium, peat and blown sand were deposited elsewhere.

Head

Under periglacial conditions, alternating freeze-thaw cycles resulted in the slow downslope movement of material by solifluction to produce sheet-like deposits of head and scree. Such deposits as mapped also include surficial materials contributed by more recent processes such as hillwash, gravitational creep and mudflows. Head deposits are variable in composition and closely reflect local lithologies up slope. They occur on most slopes and valley bottoms throughout the district, but have been mapped only where they form distinct geomorphological features and are of significant thickness (>1 m).

Thin ribbons of head follow the courses of a series of dry valleys along the eastern side of the Vale of Clwyd. The deposits typically comprise reddish brown, clayey and locally gravelly sand. More extensive deposits occupy valleys of the Clwydian Range as between a col [SJ 1465 6570] and Siglen Uchaf [SJ 137 652], and east of Llanbedr–Dyffryn–Clwyd [SJ 160 595]. The deposits there are heterogeneous and include grey-brown, stony clay, clayey silty sand and ill-sorted gravels composed of local Silurian clasts. They locally display solifluction terracing, as seen to the east-north-east of Siglen Uchaf [SJ 1419 6553]. Modern mud and debris-flows also contribute material to the head deposits in these valleys as is evident above Glyn Arthur [SJ 1415 6590] and in the valley [SJ 146 669] west of Moel Plas-yw.

East of the Clwydian Range, head deposits typically represent either soliflucted tills or combined solifluction-hillwash deposits derived from glaciofluvial deposits. The most extensive of these deposits occur in the Wheeler valley south of Nannerch, and in the Alyn valley and its tributaries around Mold. They are highly variable in composition and include soft to stiff, weakly laminated, sandy clay diamicts, sandy clays and clayey sands, as well as fine-grained sands and silts.

Scree

Thin spreads of scree material are widespread across the crops of the Silurian and Dinantian rocks of the district. The largest mapped deposits are located at the bases of cliffs of Dinantian limestone at Pant-du [SJ 202 594]; [SJ 207 595] near Eryrys and at Graig Lom [SJ 113 563], south of Llanfwrog. They also occur below crags of Silurian rocks [SJ 1635 5875] north-west of the summit of Gyrn. Though accumulation continues today, the main periods of scree formation relate to periglacial frost shattering during the Late Devensian.

Older alluvial fan deposits

Gently sloping fan-like spreads of sand and gravel occur where the River Clywedog and Ruthin Brook enter the Vale of Clwyd to the north-west and west of Ruthin. The Clywedog fan, near Rhewl [SJ 110 604], has an average elevation of 45 m above OD, and the southern fan about 65 m above OD. The Clwedog fan consists mainly of coarse gravel in its upper parts, grading downslope into sandier deposits. The fan appears terraced where it is incised by the Clwedog and Clwyd rivers, and is cut by several shallow, subparallel, north-north-east trending dry-valleys.

Strahan (1890) and Wedd and King (1924) viewed both these deposits as postglacial river terraces. In contrast, both Rowlands (1955) and Livingston (1986) interpreted the Clwedog fan, in conjunction with the adjacent ice contact deposits, as part of a complex glaciolacustrine deltaic body (the Rhewl Delta of Rowlands) (see p.156). Their fan-like form clearly shows that both features largely relate to deposition by tributary streams entering the Vale of Clwyd from the west. They are substantially lower than the perched glaciofluvial fan-like deposits in the east of the Vale and appear, in contrast, to relate more closely to the modern river system, yet are deeply incised by it. For these reasons they have been classed as older alluvial fan deposits. However, the distinction may be artificial, for these bodies may simply represent the youngest and lowest of a series of fans, supplied by tributary streams, which built out in into the Vale from both margins, during and immediately following ice retreat. The size and sediment grade of the Clywedog and Ruthin Brook fans suggest that contemporary streams were resurgent possibly following the latest Devensian (Younger Dryas) glacial event.

Sections in the Clywedog fan are seen along its source river [SJ 1080 6000], where up to 3 m of coarse gravels are exposed along the banks. Rowlands (1955) reported up to 9 m of gravel from an excavation near Oakfield Farm [SJ 108 612], 450 m south-south-west of Plas-yr-Esgob and boreholes near here have proved comparable thicknesses of fan gravels overlying till.

A section of the Ruthin Brook fan occurs in the River Clwyd [SJ 1252 5602], 450 m south of Ty’n-y-wern, where up to 2.5 m of reddish brown, poorly stratified, pebbly, silty and clayey sand with subangular to well rounded pebbles rest on a close-packed cobble gravel with a sandy silt matrix.

Alluvial fan deposits

Small, recent alluvial fans are common in the district. They are typically developed where constricted, steep-gradient tributary streams and gulleys discharge on to areas of significantly lower gradient such as the floodplains of the larger rivers. Examples are seen in the Vale of Clwyd at Rhydonnen [SJ 109 633]; in the Wheeler valley at Ty-draw [SJ 108 714], Pandy [SJ 117 717] and Ddol [SJ 140 710]; in the upper Alyn valley at Cyfnant-Uchaf [SJ 185 573]; and lower Alyn valley to the north of Rhydymwyn [SJ 207 676]; [SJ 209 672] and east of Padeswood [SJ 280 621]. They are common along the Nant-y-Flint, to the north of Plas-isaf [SJ 216 721] and [SJ 214 726], and along the northern coast of the Dee estuary, to the west and south-west of Puddington [SJ 319 731]; [SJ 325 726]. These sediments are highly variable in composition reflecting local conditions of supply. Most consist of poorly sorted gravel and sand which may grade downstream into finer grained sand, silt and silty clay.

River terrace deposits

These deposits represent remnants of earlier river floodplains which have survived incision and reworking by the modern river system. Such incision was either in response to a fall in base level, or represents a regrading caused by changes in discharge levels and sediment load. Isostatic rebound caused a relative fall in sea level, and thus may have promoted incision of Late Devensian outwash plains during the latest Pleistocene and early Holocene. Later as sea level began to rise, falling river discharges would have led to regrading. Discrete levels of river terraces indicate that these processes were not gradational, but accomplished in a series of steps, although meandering rivers may form terraces at different levels as a result of lateral migration during a single phase of incision.

An extensive terrace of the River Clwyd lies between the modern floodplains of the rivers Clwyd and Clywedog near Llanynys [SJ 1035 6265], merging upstream with older alluvial fan deposits (see above). The terrace varies in height from around 0.5 to 1.5 m above the adjacent alluvium. Ditch sections reveal sandy gravels and silty, gravelly sand. The gravels contain abundant Silurian siltstone and subordinate Carboniferous limestone and brown Permo–Triassic sandstone clasts. Low, narrow terraces occur at a similar level above alluvium intermittently along both sides of the River Wheeler [SJ 109 718; 115 718]. Remnants of a higher, more gravelly terrace occur in the valley south-west of Candy Mill [SJ 100 706] and south-west of The Grove Hall [SJ 098 692]. Seismic soundings by Brown and Cooke (1977) suggest that the deposits beneath some of the terraces in the Wheeler valley may be up to 10 m thick.

River terraces are rarely developed along the upper reaches of the River Alyn or its tributaries. Small patches, up to 1.5 m above the present floodplain, occur near Cefn-bychan [SJ 189 641], Rhydymwyn [SJ 210 669] and east of Bellan [SJ 2185 6525]. A larger crop of a higher terrace is present south-east of Rhosesmor quarry [SJ 220 662]. Small crops also occur along the Alyn south-east of Hartsheath [SJ 288 600], and along the River Cegidog at Cefn-y-bedd [SJ 311 559] and River Terrig near Ty-newydd [SJ 2495 6155] to [SJ 2475 6117].

An extensive system of terraces is developed in the lower reaches of the Alyn valley, downstream from Marford. Up to four separate levels are recognised extending over an area of nearly 9 km2. The main terrace is a compound, partially dissected feature lying at a height of between 5 and 10 m above the present floodplain. Lower terrace levels occur at Cook’s Bridge [SJ 382 563] and at Lavister [SJ 371 585]. Ditches expose crudely imbricated fine gravel. Borehole data (for example Dunkley, 1981) show that the terrace deposits at Marford locally approach 12 m in thickness, but gradually thin downstream. They comprise sandy or clayey gravel, gravelly clay and rare sand and laminated clay. The gravel is composed typically of subrounded pebbles and cobbles, mainly of quartzite and igneous rocks. Local bedrock lithologies generally contribute less than 25 per cent of the clasts and clearly these deposits were derived mainly from the reworking of Irish Sea glacial deposits.

In the north of the district, a prominent terrace, some 5 to 8 m above the modern alluvium, occurs along the eastern bank of the Nant-y-Flint, south of Mount Pleasant [SJ 2385 7170].

Alluvium

Wide spreads of alluvium underlie the modern floodplains of all the major water courses within the district. The most extensive lie in the valleys of the Clwyd and Clywedog (in the Vale of Clwyd), the Wheeler and the Alyn. Narrower tracts occur along tributary rivers and streams. Boreholes and sections indicate these deposits predominantly consist of fine sand, silt and clay, locally up to 5 m thick, typically overlying gravels (Ball and Adlam, 1982).

Lacustrine deposits

Lacustrine deposits (or alluvium) comprising predominantly peaty silts and clays commonly fill depressions in till throughout the district, for example at Fferm [SJ 1030 5845], north-west of Caerwys [SJ 1110 7315], north of Babel [SJ 1560 7480], near Brynford [SJ 178 750], west of Plas Coch [SJ 1660 7250], and associated with kettled terrain near Padeswood [SJ 2725 6180]; [SJ 284 614].

Comparable deposits, including peat, are associated with areas of foundered Dinantian and Namurian strata to the south of Maeshafn [SJ 205 601]; [SJ 2100 6040], and in the Eryrys [SJ 206 579] and Mynydd Du [SJ 2165 5735] areas. Some of these sites are also the location of Neogene ‘pocket’ deposits, for example at Pwll Helyg [SJ 1949 5868], 1.2 km north-west of Eryrys (see Chapter Six).

The largest expanse of lacustrine alluvium is at Burton Meadows [SJ 350 600], south-west of Dodleston. There, a shallow depression between low undulating till ridges, marks the site of a now reclaimed marsh (or fen), which possibly originated as a proglacial lake or giant kettlehole. The lacustrine alluvium is contiguous with the alluvium of streams entering and exiting the basin, and the boundaries between these deposits are arbitrary. Sited on the lacustrine flats, the Burton Meadows Borehole proved 0.6 m of peaty clay overlying a clay-diamict (Dunkley, 1981). Ditch sections expose up to a metre of organic, silty clay with lenses of gravelly sand.

Raised marine deposits

The only recorded raised marine deposit in the district is situated on the northern shores of the Dee estuary. A narrow strip of sand and gravel runs northwards, from north of Burton Point [SJ 3014 7400] to south-east of Denhall House Farm [SJ 2990 7488], and reaches an elevation of about 7 m above the estuarine deposits. It was described by Simmons (in Wedd and King, 1924, pp. 164–165) as an abandoned storm-beach deposit composed principally of coarse gravel and marine shells.

Marine deposits

Marine deposits, including estuarine and intertidal flat deposits, occupy a 4 km-wide tract of low-lying ground in the lower Dee valley, between the Welsh coast and the southern part of the Wirral ((Figure 42)a). The present channel of the River Dee, below Chester, is artificially straightened and almost entirely confined within man-made banks. Strahan (1882, 1890) observed that, at one time, every high tide ‘overflowed the estuarine flats of the Dee as far as Chester, and maps as late as 1720 show the estuary as an open arm of the sea’. Subsequently, much of the ground adjacent to the river was reclaimed and a detailed account of this process was provided by Strahan (1890). The surface of the reclaimed land lies only slightly above sea level, rising very gently away from the river to reach elevations of between 7 and 8 m above OD in the upper parts of the valley. Faint meandering depressions mark the position of former tidal creeks and there are traces of former embankments built as flood controls and for the purpose of ‘warping’ (a process used to induce the deposition of alluvial silts and muds).

Extensive modern intertidal flats occupy the Wirral side of the estuary, west of Burton Point, and fringe more stable areas of established saltmarsh. Narrower tidal flats occur along the Welsh coastline between Connah’s Quay and Bagillt, and along the margins of the canalised course of the River Dee.

Borehole data reveal that the estuary occupies the course of much earlier, preglacial valley filled mainly with glacial deposits (Figure 48). The sequence of Holocene marine deposits, formed in response to the Flandrian transgression, fill an asymmetric channel eroded in the underlying glacial sediments. The axis of this channel and, therefore, the thickest marine sequences do not everywhere coincide with either the axis of the preglacial valley or the present course of the River Dee. The base of the marine sequence is known locally to extend down to 20 m below OD.

The marine deposits in the estuary are highly variable. Very gravelly sand and sandy gravel are present in places, particularly towards the base of the estuarine sequence and may be difficult to distinguish from underlying glaciofluvial deposits because both may contain marine shells. These deposits commonly grade upwards through fine-grained sand into silts and clays. Strahan (1890) described ‘ice-rafted’ pebbles and cobbles, and what may be slumped and locally transported masses of till within the deposits in the lower part of the valley. Rare, thin, organic-rich silts and clays occur within sand-dominated sequences particularly in the upper parts of the valley between Blacon and Chester.

Modern intertidal deposits exposed along saltmarsh creeks, between Burton Point [SJ 3020 7350], White Sands [SJ 280 720] and the northern margins of the district [SJ 270 751] west of Denhall Farm, comprise mainly laminated silt and mud, with thin beds of sand. Bioturbation is locally intense enough to destroy the lamination, and disturbance due to dewatering and channel collapse is also present. Similar deposits occur along the course of the Dee between Sandycroft [SJ 340 675] and Saltney [SJ 386 654].

Peat

Peat deposits have accumulated in a variety of settings since the end of the Late Devensian. The moorland areas of the Clwydian Range and Nercwys Mountain include extensive, unmapped spreads of hill peat commonly less than a metre thick. Thicker accumulations occupy poorly drained hollows and fill former lake-sites both on bedrock and on till, for example in the Clwydian Range south-east of Penycloddiau [SJ 136 674], south-west of Moel Llys-y-coed [SJ 148 651], north-east of Moel Dywyll [SJ 160 648], south-south-east of the summit of Moel Fammau [SJ 164 621], and on Fron Hen [SJ 1725 6025]; and at Llyn Ochin [SJ 2150 5835] and Pen-y-foel [SJ 225 588] on Nercwys Mountain. Thin beds of peat and organic-rich clay occur as a common but subordinate component in many lacustrine deposits and alluvium. Late Devensian peat beds are associated with the calcareous tufa deposits in the Wheeler valley (see below). Discrete, peat-filled hollows and channels occur on the terrace and modern floodplain of the River Clwyd in the Ruthin area [SJ 115589] and around Llanynys [SJ 102 628].

Some of the oldest peat in the district occurs in kettleholes associated with the glaciofluvial deposits of the Alyn and Wheeler valleys, as near Padeswood [SJ 276 620], Soughton [SJ 240 660] and Rackery [SJ 3343 5736], and along the Flint–Mold valley at Plas-y-mynydd [SJ 2370 7100].

A subsurface sequence of thin peats and peaty clays, encountered in boreholes in the Dee estuary, intervenes between marine and glacial deposits, and appears to lie within a shallow depression coincident with the axis of the postglacial Dee channel. No marine fossils are reported in these organic deposits and they may mark the site of an early Holocene freshwater lake or marsh formed within the channel during the onset of the Flandrian transgression (Strahan, 1898) ((Figure 48), sections B–B’ and E–E’).

Calcareous tufa deposits

The calcareous tufa in the Wheeler valley has long been worked as a source of agricultural lime and ornamental garden stone. It overlies, and has partly infiltrated, glaciofluvial deposits (see above) sited downstream of lime-rich springs, associated with the unconformity between Silurian mudstone and Carboniferous limestone. The most extensive deposits occur at Afon-wen [SJ 129 719] and Ddol [SJ 142 715]. These deposits, amongst the thickest and most complex in Britain, have been studied for over 125 years (Maw, 1866; Hughes, 1885; Strahan, 1890; Jackson, 1922; Wedd and King, 1924; McMillan, 1947; Preece, 1978; Preece et al., 1982; McMillan and Zeissler, 1985; Pedley, 1987). These studies, reviewed by Campbell and Bowen (1989) and Preece and Turner (in Addison et al., 1990), provide the basis for the following account.

Both deposits form gently sloping, lobate bodies on the north side of the Wheeler valley. The Afon-wen tufa has a preserved thickness of around 12 m; the Ddol tufa is less than 6 m thick. The former, exposed by quarrying [SJ 1306 7182], can be traced up-valley for nearly 850 m to opposite Caerwys Castle [SJ 1318 7245]. The Ddol tufa was also extensively quarried in the past, but exposures are now confined to the Afon Pant-gwyn. Both tufas show considerable lateral and vertical lithological variability. According to Preece et al. (1982) and Campbell and Bowen (1989), the succession at Afon-wen shows a broadly three-fold stratigraphy with diffuse to gradational boundaries. The main mass of tufa overlies tufa with peat and soil horizons which, in turn, rests on a sandy tufa containing two thin soil horizons. The deposits rest on glaciofluvial sand and gravel. The tufas at Ddol are richer in carbonaceous beds than those at Afon-wen (Preece, 1978; Preece et al., 1982). Deposits at both sites contain abundant fossils, including a varied flora, ostracodes, insects and terrestrial gastropods.

The basal sandy tufa at Afon-wen contains a probable late-glacial gastropod fauna (Preece, 1978; Preece et al., 1982). A compressed sedge peat observed by Pedley (1987) probably represents one of the peat horizons in the overlying tufa with peat beds division. The pollen and plant remains in this peat also suggest a Late Devensian age. Radiocarbon dating of organic material from two seemingly higher levels within this division provided early Holocene ages of 9780 ± 200 BP and 7880 ± 160 BP (Preece, 1978; Preece et al., 1982). An age of 6260 ± 120 BP was obtained from the main tufa mass.

The Afon-wen deposit is the only tufa formation in Britain known to date from the end of the late glacial, through the gradually ameliorating climatic conditions of the early Holocene, to historic times. The petrography of the tufa and its fauna illustrate a complex history of formation involving the precipitation of calcium carbon­ate on aquatic and semi-aquatic plants including mosses, liverworts and reeds, in association with filamentous cynobacteria. The local depositional environments ranged from braided stream, small pond, marsh, and cascading barrage to marginal wetland, periodically with woodland cover. The environment of the Ddol tufa was dominated by heavily shaded streams.

Laterally extensive tufa deposits, less than 0.3 m thick, also occur in the bed of the stream [SJ 1186 7217] to [SJ 1228 7284] which issues from the base of the Carboniferous escarpment in Caerwys Dingle, upstream of Maes-mynan. The deposits comprise in situ and reworked encrustations of aquatic and semi-aquatic vegetation and form extensive flats and minor cascades. Smaller deposits occur in the River Wheeler, opposite Bryn Rug [SJ 1656 7015], and along a tributary stream near Fron [SJ 1520 7195].

Blown sand

Remanié deposits of pale, shelly, blown sand are located [SJ 300 710] north of the present river Dee and east of the old sea wall that runs between Burton Point and Shotton Steelworks. The deposit was formerly much more extensive (compare with Wedd and King, 1924) but is now mostly removed or buried beneath made ground.

Landslips

Landslips occur widely in the district, mainly on steep valley sides in Quaternary deposits, but also on the Namurian and Westphalian rocks of the coalfield. Most landslips are of the rotational type, but slides involving movement along bedding planes, partings and joints in rock also occur, particularly where the valley sides and the dip of the strata slope in the same direction. The landslips include many which probably formed during periglacial conditions and have now stabilised; others formed or have been reactivated more recently, commonly in response to stream erosion and undercutting. Several slips were active at the time of survey.

Examples in the coalfield include the numerous slips, some up to several hundred metres in length, developed along Nant-y-Flint valley [SJ 194 750] to [SJ 232 711] and its tributaries; the slip [SJ 199 740], 1 km south-east of Milwr has been active recently. A major area of past slippage, approaching a kilometre in length and up to 400 m in width, is sited on the Cefn-y-fedw Sandstone near Pen-y-parc [SJ 2170 7000]. A kilometre-long slip occurs in the Leadbrook valley [SJ 2595 6932] to [SJ 2588 7023] and extends into tributary valleys, south-south-west of Oakenholt. Landslips are also widespread along Wepre Brook and its tributaries; the slip between Northop Hall and Ewloe Castle [SJ 282 677] is over a kilometre long and part [SJ 290 678], north-east of Ewloe Castle, was active at the time of survey. Many of these coalfield slips appear to effect both solid and drift materials, but extensive areas in the Nant-figillt valley [SJ 205 680] are developed exclusively in bedrock, principally the Holywell Shales, and locally show active instability as at [SJ 2065 6790]. A slip [SJ 277 600] to the east at Pontblyddyn is developed in Coal Measures. A small landslip [SJ 3277 6024] in the Kinnerton Sandstone partially obscures the Permo–Triassic unconformity in Brad Brook, south of Higher Kinnerton.

Examples of slips developed principally in Quaternary deposits include a kilometre-long slip along the south-western side of the Alyn valley at Bellan [SJ 2070 6605]. Landslips [SJ 250 599] and [SJ 250 600] on both sides of the glacial channel north-east of Plas-y-brain effect till and head deposits. Large landslips along the Cegidog valley at Cefn-y-bedd [SJ 307 560] are in glaciofluvial sands and gravels which rest on Coal Measures. Large slips in glacial materials are located along the southern side of the Hendre gorge [for example [SJ 188 675], and along the Alyn valley between Cilcain and Loggerheads [for example [SJ 188 637] and [SJ 188 645].

Foundered ground and ground instability

Dissolution of Carboniferous (Dinantian–Namurian) limestone and calcareous sandstone has occurred at depth locally, inducing collapse or foundering of both strata and the superficial overburden. Examples in the central and southern parts of the district are commonly related to major cross-faults and associated lead-zinc mining, for example around Loggerheads [SJ 1900 6305] to [SJ 1885 6260]; [SJ 206 630], Maeshafn [SJ 2035 6050] to [SJ 2100 6040] to [SJ 2050 6010] and Nercwys Mountain [SJ 2135 5875] to [SJ 2215 5730]. Collapse, partly natural but mainly induced by shallow mining, has also occurred within the lead-zinc mining areas around Halkyn Mountain in the north-west of the district.

The problems of ground stability induced by deep-mining, principally for coal, were discussed in detail by Campbell and Hains (1988) and Hains (1991).

Made ground and infilled ground

The mapped areas of made ground and infilled ground depict tracts where the present ground surface is underlain by substantial thicknesses of materials which have been dumped by man. Made ground comprises areas in which these artificial deposits have a constructional form and largely overlie the original ground surface. Infilled ground depicts areas where such materials have been used, either partly or completely, to backfill pre-existing excavations.

The largest areas of made ground in the district are situated along the Deeside coast, at major industrial developments such as Shotton Steelworks [SJ 31 70], and sites at Flint [SJ 245 735], Connah’s Quay [SJ 28 71], Sandycroft [SJ 340 674] and Saltney [SJ 380 652]. Modern industrial sites are commonly landscaped and include variable veneers of made ground, for example Sealand Industrial Estate [SJ 386 663], west of Chester and much of the Shotton–Connah’s Quay area. Both areas of made and infilled ground are recognised in industrial estates near Ruthin [SJ 123 590]. Made ground is common throughout the coalfield areas in the environs of former collieries, and also in areas of former metal mining, notably on Halkyn Mountain (Plate 19). Extensive strips of made ground occur as road, railway and river embankments. The Iron Age hill-forts along the crest of the Clwydian Range are much older examples.

There are a number of backfilled opencast coal sites within the district, notably to the north of Buckley for example [SJ 264 660] and [SJ 290 659], to the north-west of Northop Hall [SJ 268 682], south of Queensferry [SJ 312 674], and near Leeswood [SJ 266 579]. Many of the former clay pits which worked the ‘Buckley Fireclay’ (Ruabon Marl of this account) are also partially backfilled as at [SJ 287 642] and [SJ 278 660].

Much of the made and infilled ground in the district is composed of locally generated waste from adjacent areas of excavation (cut and fill). Many of the older limestone quarries and surface metal vein workings are now either wholly or partially backfilled with local quarrying or mining waste, but are likely to include varying amounts of domestic and industrial refuse. ­Fly-ash, a by-product of coal-burning for power generation, forms part of the made ground in the vicinity of Connah’s Quay Power Station [SJ 288 705]. Former toxic waste disposal sites exist in Rhydymwyn and Gwernymynydd, and at the site of former chemical works at Flint.

Chapter 10 Economic geology

The varied geology of the district is reflected in a long history of mineral exploitation. In the past, the principal worked resources included coal, and the ores of lead, zinc and silver, as well as fireclays and refractory sands. Today, aggregates including limestone (also worked for cement), and sand and gravel, form the basis of an extensive extractive industry. Groundwater also constitutes an important resource because both Carboniferous limestones and Permo–Triassic sandstones represent important regional aquifers. Former mineral resources include oil shales, pipe clays, calcareous tufa, chert and calcite, as well as ores containing gold, iron, copper and manganese. Many of the rocks have been used as a source of local building stone.

Coal

The district contains the larger proportion of the historically important Flintshire coalfield, and includes the northernmost part of the adjacent Denbighshire coalfield, south-east of Caergwrle (Figure 23). Underground mining also took place close to the northern boundary of the district from the long-defunct colliery at Neston on the Cheshire shore of the Dee estuary.

In both coalfields, crop mining commenced in medieval times or possibly earlier. Within the district the oldest documented evidence for coal working appears in a lease of 1322 referring to Ewloe near Buckley. References to the drowning of pits, as early as 1618, show that by this time, coal was being mined below the level of free drainage. In common with many others, the development of these coalfields accelerated with the full impact of the Industrial Revolution in the latter half of the 18th century. Expansion of the industry continued into the following century and, by the 1850s, there were few parts of eastern Clwyd where coal resources had not been worked. A feature of coal mining to the south and south-west of Buckley was the extraction of the Ruabon Yard coal and its overlying bituminous shale for the distillation of oil (see below). In the southern part of the Flintshire coalfield, at least, coal was won mainly from small-scale operations, generally less than 150 m deep. This was because of the difficulties of working in geologically complex ground and the lack of resources to deal with serious drainage problems (Thomas, 1961). Underground mining in this area had almost ceased by the 1920s. Two licensed underground mines at Tan Llan [SJ 266 574] and Coed Talon Colliery Peel Pit [SJ 2700 5880] (and to the south-east) [SJ 2707 5827] were last worked in December 1989 and May 1987 respectively, and are classified by HM Inspectorate of Mines as Discontinued, not Abandoned.

Deep mining ceased more recently in that part of the Denbighshire field which lies within the district. Coal was worked from Llay Main colliery (closed 1965) [SJ 328 565] and this was one of the deeper collieries in north Wales with workings reaching depths of over 1000 m (Thomas, 1961).

Since the inception of opencast coal mining in Britain during the Second World War, Clwyd has been a modest source of opencast coal with total production ranging from 0 to 300 000 tonnes per year, with an average of about 100 000 tonnes per year. Recent production has been small and the only opencast working in 1992 was the licensed Maes y Grug site [SJ 262 664], north-west of Buckley. The main geological constraints on opencast working are the structural complexity of the Coal Measures and substantial overburden. Workable reserves of coal occur as discrete pockets between areas of residential development and most of the potential sites are small with reserves well below the national average (2.2 million tonnes in 1992).

North Wales coals range from high volatile, very strongly caking to weakly caking types, although most are strongly and medium-caking types (Thomas, 1961). The Main Coal was the most important seam by reason of its quality, thickness and persistence. Several other seams were worked but, in general, the presence of the Main Coal appears to have been essential for profitable mining. Discrete mining areas situated on tracts of this coal were located around Mold and Nercwys in the west, in the Buckley and Leeswood areas in the central to southern part of the coalfield, and in the Queensferry area to the east. Within these areas, coal was also won from the older Fireclay Group, Ruabon Yard and Premier seams and from the younger Quaker and Hollin seams. In the later stages of deep mining in the Denbighshire coalfield, other seams matched the Main Coal in importance, notably the Quaker seam.

In several British coalfields, exhaustion of reserves at shallow depth was followed by the progressive migration of deep mining down-dip into areas concealed by younger strata. Coal-bearing strata of the exposed coalfields of Clwyd extend eastward under the Permo–Triassic cover, particularly in the Dee estuary area. The technical problems of mining in the lower Dee valley would not, however, have compared favourably with opportunities available elsewhere.

Oil shales

Bituminous oil shales between 0.10 and 0.25 m thick, associated with the Ruabon Yard Coal, were worked in the areas around Leeswood and Llong from the middle of the 18th century until production ceased in the 1930s. Uses included paraffin and gas production. At its maximum development in the Leeswood area, the Ruabon Yard seam consisted of an upper ‘curly cannel’ and a lower ‘smooth cannel’; these were overlain by the oil shale. Yields quoted by Wedd and King (1924) from earlier accounts by Pringle (in Strahan, 1920) are oil shale 33 gallons/ton (147.2 litres/tonne), curly cannel 80 gallons/ton (356.8 litres/tonne) and smooth cannel 35 gallons/ton (156.1 litres/tonne). An oil shale in the roof of the ‘Premier seam’ (either the Premier or Ruabon Yard seam of this account) was also worked at Aston Hall Colliery [SJ 2937 6593].

Limestone

The extensive Carboniferous (Dinantian) limestone crop is currently the most important mineral resource within the district (Harrison et al., 1983, 1991). It supports a sizeable extractive industry with eight active quarries operating in 1998 (Table 17), mainly producing crushed rock aggregate, but also limestone for cement manufacture and, to a much smaller extent, for agricultural and industrial purposes. In 1991, the total production of limestone in the former county of Clwyd for aggregate use was 7.56 million tonnes, of which 5.55 million tonnes were produced within the district (north Wales Working Party on Aggregates Annual Report, 1991). A further 300 000 tonnes were used in cement manufacture. The mechanical and chemical properties of the limestones were given in Harrison et al. (1983, 1991).

Previously there was minor extraction from the Llanarmon Limestone in the area of Llanarmon-yn-Ial, but presently production is mainly from the Cefn Mawr and Loggerheads limestones, which have a more extensive crop and a combined thickness locally in excess of 290 m. Typical values for the mechanical properties of the local limestone aggregate are shown on (Table 18). The limestones are consistently of good aggregate quality, with sufficient strength for use as concrete aggregate and as roadstone, other than in the wearing course of major highways.

The limestone worked at Cefn Mawr Quarry is transported to a plant at Padeswood, near Buckley, where it is used in the manufacture of cement. Colliery spoil is used to provide the silica, alumina and iron oxides necessary for the manufacture of cement clinker here. The cement clinker capacity of the Padeswood plant in 1990 was 500 000 tonnes, based on three kilns.

The Loggerheads and Llanarmon limestones are generally of high chemical purity (Table 19). However, despite the presence of large resources of high-quality limestone, only modest quantities are produced for industrial applications from the Trimm Rock quarry [SJ 190 660], because of an absence of major markets.

High-purity limestone was formerly mined from large chambers some up to 25 m high, at the Olwyn Goch mine [SJ 2016 6782] near Rhydymwyn for the manufacture of flat and container glass, and for agricultural use. Limestone mining started in 1939 and apart from the war years continued until the end of 1968 when production ceased. Output ranged between 1000 and 2000 tonnes a week with, for example 73 000 tonnes produced in 1966. All the limestone was hoisted from the Olwyn Goch shaft for subsequent crushing, grinding and bagging. The Loggerheads Limestone was worked and this contained 99.3% CaCO3, 0.1% Al2O3, 0.2% MgO, 0.5% SiO2 and 0.022% Fe2O3.

Sand and gravel

Potentially workable deposits of sand and gravel are almost exclusively of glaciofluvial origin. The resources are patchily distributed, being concentrated along the Wheeler and Alyn rivers, to the north of Mold, to the north of Wrexham, and on the south-west side of the Dee estuary. Descriptions of the sand and gravel resources, with associated resource maps, were given by Dunkley (1981), Ball and Adlam (1982), Campbell and Hains (1988) and Hains (1991). The deposits are very heterogeneous in character and vary markedly in thickness. They may be locally over 30 m thick as at the Rhosesmor quarry, near Rhydymwyn, and between Marford and Singret in the south-east of the district. The coarse gravels are mainly confined to irregular channels and occur as lenses within deposits of much finer material. As a result, the deposits exhibit marked changes in lithology and particle size. The nature of the clasts varies, depending on location, but west of Rhydymwyn the gravel and coarse sand fraction consists mainly of locally derived Lower Palaeozoic siltstones and sandstones, with high concentrations of Carboniferous limestone in places. Elsewhere, the gravel is characterised by quartzites and igneous rocks, also with locally high proportions of limestone. Coal may also occur.

In 1991, total production of sand and gravel in Clwyd was 1.43 million tonnes, of which some 950 000 tonnes were extracted in this district (north Wales Working Party on Aggregates Annual Report, 1991). Much of the output was used for concrete aggregate and the remainder was used mainly as building and asphalting sand.

The Triassic Kinnerton Sandstone is worked as a source of fine-grained granular fill at the Kinnerton Bank Quarry [SJ 327 605], near High Kinnerton.

Calcareous tufa

Tufa has been worked extensively, but intermittently, at two locations along the Wheeler valley. Lobate-shaped masses of tufa, varying between 6 and 12 m thick, occur near Caerwys village at Afon-wen [SJ 134 716] and at Ddol [SJ 142 713]. The principal use of tufa is as a source of agricultural lime, but harder layers also have a market as ornamental rockery stone; these tufas have for example been used in the construction of features at the Royal Botanical Gardens, Kew.

Chert

One of the few areas in Britain where bedded chert has been extracted commercially is situated on the eastern side of Halkyn Mountain, within the Pentre Chert Formation. From this thick sequence of black and white glassy cherts, trimmed blocks were produced for lining the floors of pan mills, which were used in the pottery industry for grinding calcined flints. Chert was quarried in the Halkyn area for many years, until pan mills were replaced mostly by modern ball mills. It was worked mainly from two quarries, Bryn Mawr [SJ 188 738] and Pen yr Henblas [SJ 192 730], south of Brynford, where the thin bedding and closely spaced jointing within the chert made extraction easy. It was also quarried to a limited extent south of the Halkyn area, from chert horizons within the Cefn-y-fedw Sandstone, as on Nercwys Mountain [SJ 220 580].

A subsequent attempt to use chert from the Halkyn area as an alternative to flint in the manufacture of ­earthenware was abortive. However, small quantities of selected and washed chert were produced for the manufacture of silica refractory bricks until the late 1970s. Selected chert has also been used for concrete aggregate but, in recent years, the Bryn Mawr and Pen yr Henblas quarries have only been worked on an intermittent basis as a source of construction fill. There was no production in 1991.

Silica and silica sand

The quartzitic sandstones of the Cefn-y-fedw Sandstone were, in the past, the main source of silica rock for the manufacture of silica refractory bricks in north Wales. Beds of silica rock in the Ruabon Marl around Northop were also worked locally for this purpose, as were the cherts on the Halkyn Mountain. With rapid changes in refractory and steelmaking technology and, in particular, the demise of open-hearth steelmaking (the last plant closing at Shotton in 1979), the importance of silica rock as a refractory has declined markedly since the late 1950s. There has been no production of silica rock in north Wales for many years.

The silica rock used in the manufacture of silica bricks contained at least 97 per cent SiO2, less than 1 per cent Al2O3 and 0.2 per cent alkalis. A hard compact rock, in which the individual sand grains are cemented with secondary silica to give a high bulk density and low porosity, was also desirable. The best quality quartzite was found to occur adjacent to the Bala Lineament and immediately south of the district against the Minera Fault (Davies, 1948).

Locally, and unpredictably, the more carbonate-rich Cefn-y-fedw Sandstone near the base of the formation may weather to give unconsolidated silica sand deposits. One such deposit is worked at Maes y Droell quarry [SJ 219 565], east of Llanarmon-yn-Ial, for the production of fine-grained silica sands. Sand and loosely consolidated sandstone are crushed, ground and washed to remove clay impurities. The washed sand is fine grained, with 90 per cent less than 425 μm and up to 80 per cent less than 250 μm.

The Permo–Triassic Kinnerton Sandstone at the Kinnerton Bank Quarry [SJ 327 605] (see above), near Higher Kinnerton, was formerly worked as a source of naturally bonded moulding sand.

Fireclay and brick clay

The most important fireclays occur in the Ruabon Marl of the Buckley area (the former Buckley Fireclay). Grey, red and purple mudstones and fireclays with no significant associated coals were worked in two crops. Only the lowest 12 to 15 m of the local Ruabon Marl succession have been worked.

Historically the fireclays were used to produce acid resistant goods and refractories, including ladle bricks for the former Shotton steelworks nearby. With the decline in demand for these applications, the fireclays, together with associated mudstones and fine-grained sandstones, are blended for the production of a range of high-quality facing bricks, clay pavers and special shaped bricks. Extraction is currently confined to two sites at Lane End [SJ 288 643] and Parry’s quarries [SJ 277 665] near Buckley. With their relatively high alumina content, the clays are of moderate to high quality, requiring firing temperatures of some 1200°C.

Metalliferous mining

Lead, silver and zinc

The North-east Wales Orefield, of which the district forms the largest part, was by far the most important source of lead and zinc in Wales, and only second in national importance to the Northern Pennine Orefield. Lead mining in the district dates back at least to Roman times and continued, at varying levels of activity, until well into the 20th century. Mining activity expanded greatly during the 18th century and peak output was in the middle of the 19th century (Plate 19). In the early part of the 20th century, the industry declined until mining ceased in the 1920s. Activity was revived in 1928 when Halkyn District United Mines Ltd was formed and, during the 1930s, output increased to record levels. After a period of closure during the Second World War, mining continued at a declining rate and finally ceased in 1978.

Within the Halkyn–Llanarmon mining district, the mineralisation occurs mainly as discontinuous, steeply dipping fissure veins, but also in steep pipes (prob­ably filled solution pipes) and along selected bedding planes (Smith, 1921). Mineralisation is confined mainly to extensional faults, which trend approximately east–west in the northern part of the district, near Halkyn, and west-north-west farther south. The mineralisation (see p.140) occurs mainly in the upper parts of the Loggerheads Limestone and in the Cefn Mawr Lime­stone. Favoured environments are on structural highs beneath mudstones, and where faulting has brought impermeable horizons into juxtaposition (Schnellmann, 1959).

Details of named veins and the workings associated with them were comprehensively described by Strahan (1890), Smith (1921), Earp (1958), Foster-Smith (1974) and Williams (1987) (see also Campbell and Hains, 1988). The main ore mineral is galena and some ­sphalerite occurs, which is more common at depth, at the expense of galena. The galena is argentiferous. Wedd and King (1924) cited yields of between five and six ounces of silver per ton (139 to 166 g/t) as typical and, exceptionally, 14 ounces a ton (389 g/t); values of up to 551 g/t per ton of galena have been reported by Ridgeway (1983) mainly from the more southerly and deeper parts of the orefield. Calcite is the main gangue mineral; fluorspar is common in only a few veins in the eastern part of the orefield. Baryte is recorded as a vein mineral but is generally rare. Some secondary ore minerals are present, and disseminated chalcopyrite (CuFeS2) or calamine (ZnCO3) was worked at some mines during the 18th century (Strahan, 1890).

Minerals statistics for the whole of the orefield for the period 1845–1938 record a total output of 668 000 tonnes of lead concentrates containing 75 to 80 per cent Pb and 346 000 tonnes of zinc concentrates containing about 45 per cent Zn (Dunham, 1943–1944). Other estimates of production for the period 1692–1938 have given figures of 1 900 000 tonnes of lead concentrates and 295 000 tonnes of zinc concentrates (Schnellmann, 1938). Burt et al. (1992) gave the following output statistics for Flintshire: lead concentrates (1845–1913) 387 000 tonnes, zinc ­concentrates (1863–1913) 128 000 tonnes, and silver (1851–1913) 1 838 320 ounces. Production statistics for individual mines show that the most important were Halkyn, Rhosesmor, Halkyn Deep Level, Westminster and North Hendre. Individual orebodies were not large, the biggest recorded being the Westminster lode [SJ 208 572], which yielded somewhat over 100 000 tonnes. More recent mining was based on much smaller orebodies with reserves of 5000 to 10 000 tonnes. Typically, these ribbon-shaped oreshoots were up to 400 m long and extended up to 60 m down dip. They were locally up to 1.5 m wide although 0.15 to 0.30 m was more common and, as a result, large amounts of waste had to be removed. In spite of dilution, ore grades were high at about 12 per cent Pb (Schnellmann, 1938).

Water was a continuing problem throughout the history of lead mining, and many of the earlier and smaller mines were abandoned because of drainage problems. However, the topography provided opportunities for driving deep drainage levels, which were not only a means of dewatering existing mines but also facilitate exploration and development. Major drainage schemes started in 1818 when the Halkyn Deep Level was commenced, at an elevation of 5 m above OD, from a point east of Halkyn village [SJ 230 711]. Work continued intermittently until 1875 when the Halkyn District Mines Drainage Company was formed and incorporated by a special Act of Parliament which allowed a levy to be charged on ore produced by the mines that benefitted. Tunnelling continued up until the First World War. Its total length was nearly 6.5 km and it led to some important discoveries, including the Halkyn Lode and the Powell’s Lode, and a consequent revival in output (Schnellmann, 1959). Thus, at a time when base metal mining in Britain was in terminal decline, output from the North-east Wales Orefield was increasing (Burt et al., 1992), forming an increasing proportion of UK production around the turn of the century.

In 1896, the Holywell-Halkyn Mining and Tunnel Co. Ltd was formed to drain the mines to the north. Work began on the Milwr or Sea-Level Tunnel in 1897, at Bagillt [SJ 213 760] on the Dee estuary and continued, at first in a south-westerly direction and then, in the lead ore-bearing strata, in a more southerly or cross-cutting direction. By 1913, the tunnel had reached the area drained by the Halkyn tunnel and the two companies agreed that the deeper Milwr tunnel should continue into the Halkyn area. Despite declining mining activity, work on the tunnel continued intermittently until the late 1920s (Schnellmann, 1959). The positions of the various drainage tunnels are shown on maps included in Campbell and Hains (1988).

In 1928, Halkyn District United Mines Ltd was formed, bringing under unified control the mining and drainage interests of an area of some 65 km2 from Windmill southwards to Llanarmon. The company operated from the Penybryn Shaft at Halkyn from its formation until 1958, with a period of closure during the Second World War. Production was about 16 000 tonnes of lead concentrates a year up to the war and 2500 to 3000 tonnes a year after the war, until 1958. The driving of the Milwr Tunnel southwards from Windmill recommenced in 1928 and, at the time of the cessation of mining in 1958, had reached the Cathole Lode some 15km from the portal. The Halkyn mines were drained 58 m deeper than had been previously possible but no great quantity of ore was found at depth. Mining operations were based on previously unknown lodes intersected during driving the tunnel (Foster-Smith, 1974). Mining activity in the post-Second World War period was associated with the maintenance of the Milwr tunnel, and was based on both the mining of high-grade limestone for agricultural and industrial purposes, as well as lead ore.

Full-time mining operations of Halkyn District United Mines ceased in 1958 because of falling prices and the cost of hauling ore long distances to the shaft. Subsequently, underground mining was confined to the raising of high-grade limestone at the Olwyn Goch shaft [SJ 2016 6782], near Rhydymwyn, an activity which finally ceased in 1968. The Olwyn Goch shaft is 141 m deep to the Milwr Tunnel. The mine was operated by the Holywell-Halkyn Mining and Tunnel Co. Ltd. which, together with Halkyn District United Mines Ltd, became subsidiaries of Courtaulds Ltd. in 1962 for the purposes of maintaining the Milwr Tunnel to supply water to the company’s large factory at Holywell. As a subsidiary activity to the maintenance of the tunnel, small-scale production of lead-zinc ore recommenced at Olwyn Goch in 1964 and continued intermittently until 1978 when all mining ceased. The ore was blasted and hand-picked, and also worked by pneumatic pick from veins 1 to 1.5 m wide. Output towards the end of the operations averaged about 30 to 40 tonnes per week of lead-zinc concentrates, and the last vein to be worked was lode 635. Courtaulds factory closed in 1985 and the Olwyn Goch mine was officially abandoned in 1987.

Iron

Hematitic iron ore occurs in small pods and localised veins in the lower part of the Dinantian limestone succession in the north-east of the district. During the last century, several small veins were exploited from mines at Bryn Sion [SJ 1455 7189], Pant [SJ 1435 7472], Gledlom [SJ 1686 7098] and Llwyni [SJ 1490 7130], in the area between Caerwys [SJ 1280 7290] and Lixwm [SJ 168 714] (Strahan, 1890). Hematite-stained limestone is exposed at the entrance to an adit [SJ 1228 7278] in Caerwys Dingle which is thought to have worked the ore.

Beds and nodules of sideritic ironstone within the Westphalian succession were locally exploited for the iron smelting industry at Coed-talon and Leeswood in the middle of the last century. The main workings were in horizons associated with the Black Bed and Red coal seams.

Copper and manganese

Minor copper mineralisation is reputedly associated with some quartz veining in the Silurian rocks; Strahan (1890) reported occurrences on Moel Dywyll [SJ 152 635] and to the west of Moel Fammau. Manganese oxide is a rare accompaniment (Strahan, 1890, p. 183) to the lead-zinc mineralisation in the Dinantian limestones.

Gold

The earliest reference to the presence of gold in the district was by the Elizabethan chronicler Camben, in his celebrated work ‘Britannia’ (Collins, 1975). Simpson (1940) briefly referred to the excavation of old gold levels into Silurian siltstone between Moel Arthur [SJ 1460 6605] and Penycloddiau [SJ 1270 6760]. These are probably the workings in the area east of Fron-dyffryn [SJ 141 664], which comprise at least eight groups of small shafts, levels and associated trial diggings north and north-east of Moel Fammau [SJ 1610 6260], located near north-east-trending faults and subordinate north-west-trending fractures (Collins, 1975).

Calcite

Calcite is a common gangue mineral in most of the metalliferous veins of north-east Wales. It was a by-product of the washing of lead ore won through local trials on small farms such as those in the vicinity of the Belgrave Vein near Eryrys [SJ 208 584]. Apart from this source, calcite (spar) production was almost entirely confined to the north-south-trending Gallop Vein that outcrops between Hendre and Cilcain near Mold. The vein, with a thickness of up to 12 m, was worked at two mines, Hendre [SJ 188 676] in the north and Cefn Ucha [SJ 187 661] in the south. The Hendre mine last produced in 1982 but was kept on a care and maintenance basis until recently abandoned. Calcite resources at the mine are adequate to support continued extraction if the market for calcite improves (information from M Lloyd, Lloyds Spa Quarries (Mold) Ltd). The Cefn Ucha mine last produced in 1968, but was maintained with the ultimate objective of connecting through to the Hendre Mine. Calcite production was some 5000 tonnes per annum towards the end of the operations, with peak output of about 8000 tonnes/during the 1950s. Coarse calcite was used as an external decorative aggregate in pebble dashing, concrete panels, terrazzo tiles and reflective roof chippings. The fines were used as a filler in reflective paints for white-line marking of roads. Increasing competition from imported material finally caused operations to cease.

Building stone

Almost all the main lithologies have been utilised, at certain times, as a local source of stone in vernacular buildings throughout the district. The Silurian mudstones, with a well developed cleavage, have been used locally as roofing slates, and small slate workings are scattered sporadically throughout the Clwydian Range. The better developed slates occur in areas which are only moderately folded; good examples are common in the more central parts of the Silurian outcrop, between Moel Dywyll [SJ 1500 6370] and Moel Llys-y-coed [SJ 1520 6560], and on Moel y Gaer [SJ 1480 6170]. The more massive Silurian siltstones and sandstones have been used as a source of local roadstone and for dry-stone walling. Long-disused quarries occur on the southern slopes of Moel Arthur [SJ 1465 6585], at Llangwyfan [SJ 1255 6610] and at Fron Haul [SJ 1435 6360].

Dry-stone walling has traditionally relied on a plentiful supply of local material, and both the Carboniferous limestones and Cefn-y-fedw Sandstone have been used extensively for this purpose over much of their outcrop. To a lesser extent, sandstones within the Coal Measures such as Yard Rock and Hollin Rock, have also been used for building purposes.

In the Vale of Clwyd and on the Wirral Peninsula, the sandstones of the Kinnerton Sandstone have been quarried on a small scale as a source of building stone. Ruthin Castle, in the Vale of Clwyd, is the most striking example of its use. Quarries at Burton Point [SJ 3030 7355], on the eastern side of the Dee estuary, have also supplied stone of reasonable quality for building purposes, from both the Kinnerton Sandstone and Chester Pebble Beds formations.

Hydrocarbons

There are two prospective areas for exploitation of coalbed methane within the district. These are the Vale of Clwyd and the areas east of the exposed coalfields. Both tracts are characterised by broadly similar geology, in so far as the target coals within the Lower and Middle Coal Measures are overlain by younger Carboniferous strata and by Permo–Triassic formations (mainly Kinnerton Sandstone). In the Deeside–Cheshire area, the regional dips of these rocks, and of the sub-Permian unconformity, indicate that there is a marked and progressive deepening of the Coal Measures to the east. Despite this, and a degree of structural complexity caused by faulting, the potential may still be considered good. In contrast, the Vale of Clwyd area is at present a poor prospect. This is due mainly to uncertainties concerning the source rocks under much of the vale. In particular, the prospective area falling within this district is considered poorest, because the Westphalian strata are thought to consist mainly of red bed sequences (Calver and Smith, 1974).

The western margin of the Permo-Triassic Cheshire Basin lies in the eastern part of the district, and has been regarded as a potential minor target area for oil and natural gas. Adjacent areas of the East Irish Sea Basin are already established gasfields. Exploration wells have shown that the area is underlain by Permo-Triassic (mainly Kinnerton Sandstone) rocks which unconformably overlie thick Westphalian, Namurian and Dinantian sequences, and that there is considerable structural complexity. Within the Carboniferous sequences elsewhere in northern and central England, the principal source rocks for hydrocarbons are the lower Namurian prodeltaic shales. In this district, these are represented by the Holywell Shales. Despite the presence of these potential source lithologies, Frazer et al. (1990) considered that various factors have adversely affected the area’s hydrocarbon potential. They suggested that the source rocks have only limited potential because of their maturity and restricted distribution.

Macchi and Meadows (1987) showed the presence of minor amounts of hydrocarbon residues in the Permo-Triassic rocks exposed at Burton Point [SJ 3030 7355], perhaps indicative of a ‘former, fairly strong show’.

Water supply and hydrogeology

The district lies predominantly within the catchments of the rivers Clwyd and Dee, and their respective tributaries the Wheeler and the Alyn (hydrometric areas 66 and 67). The water resources are administered by the Welsh Region of the Environment Agency (EA Wales). A small area in the north-east corner of the district is in the Mersey catchment (Hydrometric Area 68) and within the North-West Region EA. The average annual precipitation varies from less than 700 mm in the Dee valley west of Chester to over 1000 mm on the high ground around Graianrhyd [SJ 215 560]; the average annual evapotranspiration is around 530 mm. Annual infiltration rates vary from 50 to over 400 mm, depending on both the bedrock geology and the thickness and permeability of the overlying superficial deposits. The overall drainage direction in the district is northerly. The River Alyn at Rhydymwyn [SJ 206 667] is flashy, with a base flow index of 0.39. The Alyn at Pont-y-capel [SJ 336 541] and the Clwyd at Ruthin weir [SJ 122 592] have base flow indices of 0.56 and 0.58 respectively. The Wheeler at Bodfari [SJ 105 714] has an index of 0.83; the catchment rocks here are Silurian mudstone and Carboniferous limestone, overlain by glaciofluvial sand and gravel.

Historically, the district obtained its water from both groundwater and surface supplies. The former comprised mainly springs and shallow wells in the glacial deposits, and a few deep boreholes that encountered the Permo-Triassic sandstones in the east and south-west of the area (Wedd and King, 1924). (Table 20) shows the current licensed abstractions of groundwater within the district, subdivided by both aquifer and usage. The major aquifer is the Kinnerton Sandstone which provides significant quantities for both public and industrial supplies. On the Silurian strata, in the south-west of the area, only the springs are licensed; wells and boreholes are licence-exempt except along river valleys. Over 80 per cent of the licensed water is abstracted from surface water sources. Much is for industrial purposes and is taken from the River Dee. The public water supply licences total 2203 Ml/a from small streams and reservoirs underlain by Silurian strata. However, to the east in the Chester district, water is abstracted from the River Dee for public supply, some of which is used in this district. The Vale of Clwyd is supplied by the Llyn Alwen aqueduct.

Silurian

The indurated mudstones and subordinate sandstones of the Nantglyn Flags and Elwy formations are relatively impermeable. However small supplies of soft water are obtained locally from springs, but no flow rates have been recorded.

Carboniferous

The Dinantian limestone formations are the second most important aquifer in the district in terms of water usage. However, drilling for water is highly speculative because the limestones have minimal primary porosities and permeabilities, and groundwater movement is restricted to fissures enlarged by solution. Fissures are commonly fault-controlled, but they are neither regularly spaced nor extensively interconnected. During drilling, failure to intersect a water-bearing fissure generally results in a dry hole. Although the fissures are sparse, they tend to be large and groundwater movement may be rapid. Out-flows from fissure systems issue from a limited number of springs where flow rates vary considerably with time. Tunnels through the limestones, resulting from old mineral workings into lead and zinc veins, intersect the fissure systems and have an effect on the hydrogeology of the district. Two of these, the Milwr Sea-Level Tunnel [SJ 213 760] and the Halkyn Deep Level Tunnel [SJ 230 711] (see p.168), are now used for water supply (Campbell and Hains, 1988). Few boreholes have been drilled in the district. A borehole 152 mm in diameter, and 48.8 m deep, at Hendre [SJ 1822 6802] yielded 1.9 litres per second (l/s) for a period of 30 days from the Loggerheads Limestone beneath 20.1 m of superficial deposits.

Where the limestone crops lack a cover of superficial deposits, recharge is almost equal to effective precipitation. In the Vale of Clwyd, the limestones recharge the Kinnerton Sandstone aquifer along its faulted western margin.

Under low flow conditions, water from the limestones is generally hard, but of good quality: the concentration of metal ions may be high. Typical analyses are given in (Table 21). However after heavy rain, water supplies from both boreholes and springs may become turbid and polluted. The water quality, reflecting that of the recharge, may be softer and possibly reveals higher concentrations of suspended solids, organic matter, bacteria and nitrates. Tunnels and adits in the old lead and zinc workings have not been monitored for their groundwater chemistry. However, the groundwater is likely to be of good quality, but reasonably hard. Iron could be a problem where it exceeds 1 milligramme/litre (mg/l), and other metals may be present where the water has been in contact with mineral workings.

On the Namurian strata, springs issue locally at the junction of jointed sandstone with underlying less permeable mudstone. Outcrop samples from the Cefn-y-fedw Sandstone at Pen-y-Foel [SJ 2193 5645] had porosities of 20 per cent and laboratory hydraulic conductivities of 0.4 to 0.7 m/day. Boreholes that fail to intersect water-bearing joints and fissures are generally dry, and only a few boreholes are known as yield water in the district. One borehole into the Cefn-y-fedw Sandstone near Pen-y-Parc [SJ 279 598] is artesian, and licensed for the abstraction of 455 cubic metres/day.

The mudstones and sandstones of Westphalian age (Coal Measures and Red Measures groups) have low primary permeabilities. Outcrop samples from the Hollin Rock at Wepre Dingle [SJ 2902 6765] had porosities of around 11 per cent and laboratory hydraulic conductivities of less than 0.0006 m/day. However, water occurs in the sandstones at depths of a few hundred metres within joints and fractures caused by mining subsidence. The yields of boreholes depend on the number of joints intersected, but 5 l/s is not uncommon and a 123 m-deep, 203 mm-diameter borehole at Mold [SJ 2418 6348] yielded 16.1 l/s from beneath 13.4 m of superficial deposits. Although disused colliery workings have considerable storage, yields are rarely sustained in the long-term. This is because recharge is limited by the separation of the aquifer into fault blocks and by an extensive cover of low permeability superficial deposits of considerable thickness. Springs occur where the more permeable beds crop out; locally, beneath superficial deposits overflowing conditions exist. The quality of the water is variable, but usually nonpotable because of pollution by mine waste. Total dissolved solids may exceed 2000 mg/l, and iron and manganese may be present in solution where the water is associated with coal workings. A typical analysis is given in (Table 21). However, a borehole, 76.2 m deep, at Ferry Bank Farm [SJ 3280 6874], which had an initial yield of 4.4 l/s, probably from both superficial deposits and Westphalian strata, is of sufficiently good quality to be used for spray irrigation.

Permo–Triassic

The Kinnerton Sandstone and Chester Pebble Beds are the major aquifers in the district. The Kinnerton Sandstone forms around 80 per cent of the combined crop but the overlying Chester Pebble Beds occupy an area of about 20 km2 in the north-east of the district (south Wirral).

Porosities of 26 to 27 per cent and laboratory hydraulic conductivities of 1.6 to 3.7 m/day were recorded for outcrop samples of weakly cemented Kinnerton Sandstone near Burton Village [SJ 3123 7426]. Respective values for the Chester Pebble Beds at Burton Point [SJ 3015 7370] were 23 to 24 per cent and 0.5 to 2.9 m/day. In the Vale of Clwyd, the Kinnerton Sandstone Formation is better cemented and porosities are slightly lower. A borehole at Plas-yr-Esgob [SJ 1132 6191] had porosity values of between 19 and 25 per cent and laboratory hydraulic conductivities were 0.0006 to 1.5 m/day. The higher values are significantly less than the hydraulic conductivities encountered in the field. This is because water movement is controlled by fissures which provide nearly all the permeability. Transmissivities and yields therefore vary considerably, depending upon the development of such fissures, but boreholes are rarely total failures, particularly in the eastern crop. In the Vale of Clwyd, faults generally form zones of increased permeability, although locally permeability is lower.

Yields of over 75 l/s have been obtained from 610 mm- to 762 mm-diameter boreholes north of Shotton [SJ 3380 7112] to [SJ 3376 7166] to [SJ 3199 7258], which range in depth from 174 to 183 m. A yield of 53 l/s was obtained from a 457 mm-diameter, 169 m-deep borehole at Gorstella, Lower Kinnerton [SJ 3646 6218]; but elsewhere yields of 30 l/s from large diameter boreholes are more common. High yielding boreholes have also been recorded in the Vale of Clwyd, where a 381 mm-diameter, 122 m-deep borehole at Ruthin [SJ 1195 5807] and a 304 mm, 92 m-deep borehole at Plas-yr-Esgob [SJ 1125 6182] both yielded in excess of 50 l/s. They are both artesian and currently used for river recharge. All aquifer tests conducted were on sites where the aquifer is confined by superficial deposits. Transmissivities range from 20 to 1500 square metres per day (m2/d) and storage coefficients from 0.0001 to 0.0005 in the Vale of Clwyd. In the main eastern crop the values are 25 to 625 m2/d and 0.0003 to 0.002 respectively.

In much of the Vale of Clwyd, the sandstones are overlain by considerable thicknesses of superficial deposits. Recharge is therefore controlled by the distribution of the more permeable drift deposits. There is also a contribution from the Dinantian limestone formations across their mutual boundary in the west. In the Vale of Clwyd, heads of up to 6 m above ground level cause boreholes to overflow in the centre of the basin, north of Ruthin. In the east of the district, the aquifer is also almost entirely overlain by till and estuarine alluvium. The only drift-free areas are around Burton [SJ 315 740] and Ledsham [SJ 357 745] in the north and along the Brad Brook, near Higher Kinnerton [SJ 332 604], in the south. Where the thickness of clay drift exceeds 2 m, it is estimated that there is only 2 per cent infiltration of the potential recharge. Therefore, the industrial boreholes north of Shotton [SJ 33 71] must be supplied by recharge from the north and east. Here, natural seasonal water-level fluctuations are generally low, and often less than a metre.

Typical analyses of the Kinnerton Sandstone waters are shown in (Table 21). The quality of the water is good, but hard: bicarbonate concentrations in excess of 400 mg/l have been recorded. In the Vale of Clwyd, bicarbonate values are less than 250 mg/l. They increase towards the margins of the basin, particularly south of Ruthin, reflecting the influence of hydraulic continuity with the Dinantian limestones. Chloride concentrations are generally less than 50 mg/l, but higher values (up to a few hundred mg/l) occur locally, where mixing with saline water has occurred from beneath the natural zone of active groundwater movement: this occurs when pumping reverses the hydraulic gradient. Iron concentrations are normally less than 0.3 mg/l, but levels in excess of 5 mg/l have been recorded. Nitrate levels are increasing with time in areas of recent recharge, but are gener­ally less than 3 mg/l, due to the confined nature of the aquifer in most of the district.

Superficial deposits

Much of the district is overlain by superficial deposits, ranging from impermeable till, marine and lacustrine deposits, to permeable fluvial and glaciofluvial sands and gravels. They commonly exceed 20 m in thickness and reach 100 m locally, for example in the preglacial Dee valley (Chapter 9). Yields of over 12 l/s have been obtained from a 4.1 m-thick gravel bed at the base of a 9.3 m-deep borehole at Hendre [SJ 1828 6767]. Elsewhere, however, recorded yields are generally less than 5 l/s. Yields may decrease in dry weather, due to limited storage and recharge. A large proportion of boreholes in the superficial deposits are artesian, particularly those in the east of the district (River Dee valley and tributaries), where estuarine alluvium confines water in underlying sands and gravels.

The quality of the water from these deposits varies. Unpolluted sources are chemically similar to, but harder than, any surface water with which they are in continuity. Total hardness values are generally less than 200 mg/l and chloride concentrations up to 50 mg/l. Typical analyses are given in (Table 21). However, the water is susceptible to contamination from agriculture, disposal sites and other surface sources. Exceptions occur where the permeable sands and gravels are overlain by impermeable deposits. Near the coast, the water is liable to saline intrusion.

Vulnerability to pollution

Groundwater is generally less vulnerable to pollution than surface sources because of the filtering and attenuating effects of the saturated zone above the aquifer. Consequently it often receives little treatment before being pumped into the supply. This means that it is important that aquifers be protected from potential pollutants. There are two types of pollution sources, point and diffuse.

Point sources include landfills, other waste disposal sites such as sewage treatment works, and storage tanks for silage, fuels, industrial solvents and other chemicals. There are several landfill sites in the district that hold domestic and industrial wastes. However, generally they represent little risk to groundwater quality because their sites are underlain by impermeable superficial deposits or they are located on bedrock (such as Coal Measures) where any spread of leachate is limited by the multi-layered and faulted nature of the strata.

Where potentially leachate-producing wastes are tipped on aquifers, great care is required with site construction and monitoring to ensure that the leachate does not represent a hazard to surface or groundwater. New sites of this type will normally be lined with an impermeable layer of clay, an artificial membrane, or both. Domestic refuse also produces methane, which requires collecting and burning off. Two sites at Cadole, [SJ 206 633], on the Cefn Mawr Limestone Formation and at Rhes-y-cae [SJ 195 712] on the Loggerheads Limestone, received domestic refuse in the past; they closed in 1964 and 1974 respectively.

Storage tanks, if poorly constructed or in bad repair, can represent a serious risk to groundwater quality, particularly where the aquifer is at outcrop.

Diffuse sources of pollution include nitrate, applied to the ground in the form of fertiliser, and biocides (particularly herbicides). As much of the district is agricultural land, these are a risk to groundwater quality. However, the risk is low with respect to the main aquifer (the Kinnerton Sandstone) because it is generally overlain by a considerable thickness of low permeability superficial deposits. Drift aquifers ((Table 21)) are at greater risk, as are those areas where the Dinantian limestone formations have little superficial cover.

Ground conditions and natural hazards

Ground conditions have a direct bearing on the economy of an area, whether it be through the effects of mining activity, material strengths as related to foundation works or slope stabilities. Two comprehensive BGS reports covering much of Deeside (Campbell and Hains, 1988) and the Wrexham area, which includes the southern margins of the present district (Hains, 1991), contain a discussion of these topics, and the conclusions reached are likely to be widely applicable throughout this district.

Both the coal and metalliferous mineral mining industries have left their legacy by way of extensive underground workings, numerous shafts and adits (not all sealed at the surface) and waste. These have consequences not only for the siting of development or planning projects but also for water supply.

Where groundwater or surface water has drained through colliery spoil, their sulphate content and pH value may present geotechnical problems requiring the use of higher quality concrete and protected steelwork.

Naturally occurring hazards include landslips and other mass movement phenomena, differential subsidence and risk of seismic activity. Landslips, both recent and active, have been identified in several localities (Chapter 9). Although many occur in superficial deposits, others effect bedrock, most commonly along steep valley sides in the Cefn-y-fedw Sandstone, Gwespyr Sandstone and Coal Measures. Differential settlement of foundations may occur where these straddle contrasting lithologies. It can be enhanced by the presence of underground workings and faulting. Subsidence has also occurred as a result of dissolution of limestone, and cavities have developed either as single swallow holes or larger areas of foundered ground.

The area is not particularly prone to seismic activity but the effects of major earthquakes with epicentres in north Wales or the Welsh Borderland have previously caused minor damage. Shallow seismic activity can also be caused by settling of disused underground workings, especially in coalfield areas.

Information sources

Further geological information held by the British Geological Survey relevant to the Flint district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Other information sources include borehole records, mine plans, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in British Geological Survey libraries and on the developing website: http://www.bgs.ac.uk.

Maps

Geology

1:10 000

History of Survey

The district covered by Sheet 108 Flint was originally surveyed by W T Aveline, D H Williams, E Hull, W W Smyth, J B Jukes and A C Ramsay and published in 1850 (revised 1855–56) as old series one-inch quarter Sheets 79SE and SW (part) and 74NE (part). Subsequent revisions of these quarter sheets at the 1:10 560 scale were carried out by A Strahan, C E De Rance and R H Tiddeman for new editions published in 1886, 1892 and 1895. The results of this resurvey were presented in a descriptive memoir (Strahan, 1890), and a later supplement (Strahan, 1898). Due to the economic and strategic importance of the Flintshire and Denbighshire Coalfields and of the Halkyn-Minera mineral province, a selected resurvey of the district was undertaken by the Survey during the first quarter of this century by C B Wedd, W B R King, G W Lamplugh and Dr H H Thomas. This resulted in the production of both Solid and Drift editions of new series one-inch Sheet 108 (Flint) and an accompanying memoir (Wedd and King, 1924). A separate report describing the history and extent of metalliferous (lead and zinc) mining in the district and adjacent areas was also produced (Smith, 1921).

The present resurvey at 1:10 000 scale was started in 1985. The early mapping phase, designed to produce thematic maps of the redevelopment areas of Deeside, was funded by the Department of the Environment on behalf of the Welsh Office. It was carried out by Drs S D G Campbell, J R Davies, B A Hains and D Wilson. The results of this phase were published in 1988 (Campbell and Hains, 1988). A thematic map set was also produced in 1991 for the Wrexham area (Hains 1991), which incorporated the results of revision mapping in the south of the district undertaken by Drs J R Davies, B A Hains and D Wilson in 1989. The resurvey of the remainder of the district was carried out by Drs R Addison and I T Williamson between 1989 and 1992. Subsequent to the resurvey, a desk revision of much of the Flintshire Coalfield area was undertaken by Drs J R Davies and D Wilson between 1994 and 1997 to take account of changes in stratigraphical nomenclature and correlation. The revised sheets are marked † on the following list.

The geological 1:10 000 scale National Grid maps included wholly or in part in 1:50 000 Series Flint Sheet 108 produced as part of the present resurvey are listed below, together with the initials of the geological surveyors and dates of the survey (those sheets marked * were resurveyed in part only). Copies of these maps are available for public reference in the library of the British Geological Survey in Keyworth and are also available from the London Information Office (see addresses at the end of this memoir). Copies may be purchased directly from BGS as black and white dyeline sheets.

SJ05NE* Gyffylliog RA 1990
SJO6SE* Llanrhaeadr RA 1990
SJ06NE* Denbigh ITW 1990
SJ07SE* Tremeirchion ITW 1991
SJ07NE* Dyserth Itw 1991
SJ15NW Ruthin RA 1990
SJ15NE Llanarmon-Yn-Ial RA 1989
SJ16NW Llandyrnog ITW 1990
SJ16NE Nannerch & Cilcain SDGC, DW 1986–87
SJ16NE Nannerch & Cilcain ITW 1990
SJ16SW Llangynhafal RA 1990
SJ16SE Llanferres RA 1985
SJ16SE Llanferres DW 1990
SJ17NW* Whitford ITW 1991
SJ17NE* Holywell SDGC, JRD 1987
SJ17SW Caerwys ITW 1991
SJ17SE† Brynford & Halkyn SDGC 1987
SJ25NW† Eryrys ITW 1990
SJ25NE† Hope Mountain JRD, DW 1992
SJ26NW† Northop SDGC 1986
SJ26NE† Connah’s Quay BAH 1986–87
SJ26SW† Mold JRD, DW 1985
SJ26SE† Buckley BAH, JRD 1985
SJ27NW† Bagillt (North) BAH 1987
SJ27NE* Neston BAH 1991
SJ27SW† Flint BAH, SDGC 1987
SJ27SE† Oakenholt BAH, ITW 1987, 1991
SJ35NW† Hope, Caergwrle & Llay BAH 1989
SJ35NE Rossett ITW 1989
SJ36NW† Hawarden BAH 1986
SJ36NE Blacon ITW 1990
SJ36SW† Kinnerton BAH, JRD 1985
SJ36SE Dodleston & Chester SW ITW 1989
SJ37NW* Willaston BAH 1991
SJ37NE* Hooton And Whitby BAH 1991
SJ37SW Deeside Indust. Park BAH 1987, 1991
SJ37SE Capenhurst BAH 1991

Geochemistry maps

Geophysical maps

Hydrogeological map

Minerals

Books and reports

Books

Memoirs

BGS Technical Reports and other reports

Other data sources

BGS borehole data set

At the time of going to press BGS holds borehole and site investigation records for the Flint district. These may be consulted and copies purchased at BGS Keyworth. The records are either hand written or typed and some of the older records are driller’s logs.

Details of boreholes cited in the memoir

Abbey Mills No 1 Borehole SJ17NE/1 [SJ 1949 7757] 363.93 m
Abbey Mills No 4 Borehole SJ17NE/4 [SJ 1949 7747] 243.8 m
Allington SJ35NE/45 [SJ 3961 5585] 20 m
Alyn Valley Borehole SJ15NE/8 [SJ 1889 5741] 312 m
April Rise Farm SJ27SW/248 [SJ 2267 7312] 25 m
Ball’s Hall, Rossett SJ35NE/31 [SJ 3603 5781] 22 m
Bellan SJ26NW/13 [SJ 2120 6505] 4.62 m
Blacon East Borehole SJ36NE/23 [SJ 3789 6686] 2265.88 m
Blacon West Borehole SJ36NE/24 [SJ 3662 6634] 1339.29 m
Bretton Farm SJ36SE/1 [SJ 3542 6392] 27.74 m
Bretton Road Well SJ36SE/11 [SJ 3517 6370] 17.37 m
Bretton, Chester and Flint Water Board SJ36SE/14A [SJ 3524 6308] 170.69 m
Bryn Alyn SJ26NW/12 [SJ 2115 6623] 5 m
Bryn-y-bâl shaft/borehole SJ26NE/5 [SJ 2648 6501] 121.92 m
Burton Green SJ35NW/45 [SJ 3441 5852] 20 m
Burton Meadows SJ35NE/26 [SJ 3532 5961] 18 m
Cae’r-Odyn (BGS) SJ16SE/2 [SJ 1911 6222] 4.4 m
Cae’r-Odyn (BGS) SJ16SE/4 [SJ 1919 6200] 4.3 m
Coppa House SJ26SE/32 [SJ 2766 6160] 23.8 m
Courtaulds Works Borehole 2 SJ27SW/272 [SJ 2399 7294] 10.08 m
Dee Estuary Borehole SJ27SE/46B [SJ 2650 7326] 21 m
Dee Estuary Borehole SJ27SE/64 [SJ 2777 7248] 20 m
Dee Estuary Borehole SJ27SE/79 [SJ 2902 7224] 18.29 m
Dee Estuary Borehole SJ27SE/90 [SJ 2974 7446] 12.19 m
Ferrybank Farm, Queensferry SJ36NW/25 [SJ 3280 6874] 76.2 m
Gorstella SE36SE/15A [SJ 3646 6218] 169.5 m
Gresford, A483 By-pass BH7/15, Moorfield Cottages SJ35NE/130 [SJ 3662 5993] 6.2 m
Gwernymynydd Borehole SJ26SW/10 [SJ 2111 6221] 206.04 m
Hawarden Castle Colliery No. 11 Borehole SJ36NW/38 [SJ 3352 6586] 124.05 m
Hawarden Castle Colliery No. 3 Borehole SJ36NW/33 [SJ 3339 6512] 163.86 m
Hawarden Castle Colliery SJ36NW/41 [SJ 3352 6586] 145.46 m
Hawarden Castle Colliery No. 6 Borehole SJ36NW/34 [SJ 3405 6561] 69.49 m
Hawarden,  Waterworks BH2 SJ36NW/26 [SJ 3202 6572] 141.43 m
Hendre SJ16NE/15 [SJ 1828 6767] 9.3 m
Hendre SJ16NE/6 [SJ 1822 6802] 48–77 m
Leeswood No.1 Test Borehole SJ26SE/16 [SJ 2636 6180] 333.1 m
Llanfynydd SJ25NE/10 [SJ 2785 5697] 13 m
Maes-y-Groes, (BGS) SJ16SE/3 [SJ 1887 6302] 6.7 m
Manor Industrial Estate, Phase 1, Borehole 1 SJ27SW/303 [SJ 2296 7456] 7.79 m
Manor Industrial Estate Borehole 5 SJ27SW/295 [SJ 2267 7481] 52.53 m
Mold SE26SW/3 [SJ 2418 6348] 123 m
Moorfield, Pulford SJ35NE/30 [SJ 3683 5976] 18 m
Nant (BGS) SJ16SE/5 [SJ 1911 6113] 4.8 m
Nercwys Hall SJ26SW/27 [SJ 2366 6019] 13.8 m
Oakenholt Paper Mill BH A SJ27SE/2 [SJ 2616 7161] 123.75 m
Oakenholt Paper Mill BH B SJ27SE/2 [SJ 2628 7150] 114.6 m
Oaklands Bridge Borehole SJ15NW/11 [SJ 1373 5556] 121.88 m
Pentre-cerrig-bach (BGS) SJ16SE/6 [SJ 1888 6048] 3.5 m
Pen-y-parc [SJ 279 598] no details available, extraction licence only
Plas Isaf Estate BH 2 SJ26SE/513 [SJ 2569 6253] 65.07 m
Plas-yr-Esgob Borehole No 1 SJ16SW/1 [SJ 1131 6190] 78.64 m
Plas-yr-Esgob SE16SW/3 [SJ 1125 6182] 92 m
Pont Glan-y-Wern (Dee and Clwyd Water Board) SJ06NE/10 [SJ 0906 6590] 79.17 m
Pontybodkin Telephone Exchange, BH1 SJ25NE/4 [SJ 2739 5987] 30 m
Pontybodkin Telephone Exchange, BH2 SJ25NE/5 [SJ 2742 5985] 31 m
Pontybodkin Telephone Exchange, BH3 SJ25NE/6 [SJ 2741 5984] 30 m
Rector’s Meadow, Hawarden BH1 SJ36NW/39 [SJ 3179 6614] 101.5 m
Rhesgoed (Clwyd County Council) SJ15NE/1–7 [SJ 156 589] 8–10 m
Ruthin SE15NW/2 [SJ 1195 5807] 122 m
Sandycroft Industrial Estate No.7 SJ36NW/265 [SJ 3290 6733] 18.59 m
Sandycroft SE36NW/ [SJ 3377 6742] no details
Sealand Exploration Co, No.1 SJ36NW/7 [SJ 3328 6813] 224.94 m
Sealand Exploration Co, No.3 SJ36NW/6 [SJ 3328 6813] 498.96 m
Shotton SE37SW/4 [SJ 3199 7258] 183.2 m
Shotton SE37SW/6 [SJ 3376 7166] 182.9 m
Shotton SE37SW/7 [SJ 3380 7112] 174.0 m
Singret Sand Pit SJ35NW/49 [SJ 3438 5586] 30.8 m
St Asaph Borehole (BGS) SJ07SW/15 [SJ 0366 7312] 150.57 m
Tai Bowen Borehole SJ26SE/29 [SJ 2680 6212] 22.5 m
Tan y Mynydd, Caergwrle SJ35NW/30 [SJ 3010 5817] 18 m
The Bridge, Colour Co Ltd, Shotton SJ36NW/1 [SJ 3190 6973] 67.67 m
Tyddyn-Onn SJ16NE/20 [SJ 1634 6995] 22.86 m
Tyddyn-y-Gwynt SJ26NW/11 [SJ 2137 6704] 14 m
Waen and Nerquis (Nerquis Colliery) SJ26SW/433 [SJ 2450 6240] 14.73 m
Waen, Llandyrnog SJ16NW/1 [SJ 1093 6564] 51.82 m
Wrexham A483 By-pass BH7/8 Moorfield Farm SJ35NE/70 [SJ 3670 5981] 3.8 m
Wrexham-Chester road SJ36SE/46 [SJ 3785 6139] 13.7 m
Wrexham-Chester road SJ36SE/23 [SJ 3778 6135] 17.4 m

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheet 108 Flint are held in the Lexicon database. This is available on Web Site http://www.bgs.ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS Petmin database

Thin sections and hand specimens of rocks from the district are held in the England and Wales Sliced Rocks and Museum Reserve collections at BGS Keyworth. Enquiries concerning all petrological material should be directed to The Manager, Petrological Collections, BGS Keyworth

BGS (Geological Survey) photographs

Copies of the photographs that appear in this memoir are deposited for reference in the British Geological Survey library, Keyworth, Nottingham NG12 5GG. Colour or black and white prints and transparencies can be supplied at a fixed tariff.

Geochemical data

Regional multi-element geochemical data are available for stream-sediment, stream-water and soil samples from the area. Enquiries should be directed to the Data Manager, G-BASE, BGS, Keyworth.

Fossils

Macrofossils and micropalaeontological residues for samples collected from the district are held at BGS Keyworth. Enquiries concerning all macrofossil material should be directed to the Curator, Biostratigraphy Collections, BGS Keyworth.

Minerals

Geological conservation

Geological conservation in Wales is administered by the Countryside Council for Wales, Plas Penrhos, Fford Penrhos, Bangor, Gwynedd LL57 2LQ (Telephone 01248–385500); and in England by English Nature, Northminster House, Peterborough PE1 1UA (Telephone 01733–340345).

Addresses for data sources

References

Most of the references listed below are held in the Libraries of the British Geological Survey at Edinburgh and Keyworth, ­Nottingham. Copies of the references can be purchased subject to the current copyright legislation. BGS Library catalogue can be searched online at: http://geolib.bgs.ac.uk

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Figures, plates and tables

(Front cover) View of Moel Fammau, the highest summit of the Clwydian Range, formed of Silurian turbidites and disturbed beds [SJ 200 580] (GS 1050). Photographer: I T Williamson

(Frontispiece) View looking westwards from Eryrys across the upper Alyn valley towards the Clwydian Range. Dinantian limestone crops out in the middle and foreground. Beyond the limestone escarpment, the fault-guided Alyn valley separates the Dinantian from the Silurian strata of the Clwydian Range, which forms the more rounded hills in the middle distance (GS 1051).

(Figure 1) Geology of the Flint district.

(Figure 2) Physical features of the district.

(Figure 3) Classification of the Silurian resedimented deposits in the district (after Davies et al., 1997). Types and inter-relationships of Silurian resedimented deposits of the district. Solid arrows connect deposits which may be part of an evolutionary continuum for individual flows. Broken arrows connect end members of a spectrum of deposits without necessarily implying an ­evolutionary linkage. The internal divisions for coarse-grained, Bouma and fine-grained turbidites are those of Lowe (1982), Bouma (1962) and Stow and Piper (1984). * applies to turbidites only ** combines features of both coarse-grained and Bouma turbidites *** many very thin-bedded varieties may represent ‘base-cut-out’ type E ­turbidites and properly lie within the field of fine-grained turbidites

(Figure 4) Lithostratigraphical, chronostratigraphical and biostratigraphical subdivisions of the Dinantian succession. *1 approximate position of P1c ammonoids on Hope Mountain *2 approximate position of probable P2b ammonoid at Bryn Mawr Quarry (see text for details)

(Figure 5) a Dinantian rocks of the district (inset) and of North Wales showing the position key boreholes and of suspected syndepositional faults. b to f Evolving Dinantian palaeogeography of North Wales. AF Aberdinlle Fault; AqF Aqueduct Fault; AVF Alyn Valley Fault; BF Berw Fault; BaF Bala Fault; BrF Bryn Eglwys Fault; C Corwen outlier; CF Conwy Valley Fault; DF Dinorwic Fault; GCF Glyn-Ceriog Fault; HF Hawarden Fault; LF Llanelidan Fault; LIF Llangollen Fault; MF Minera Fault; NNF Nercwys–Nant-figillt Fault Zone; PrF Prees Fault; VCF Vale of Clwyd Fault Boreholes: AV Alyn Valley; BE Blacon East; C Croxteth; G Gronant; MG Milton Green. Locality:1 disused quarry [SJ 1481 7144] 2 crags [SJ 1495 7180] 3 disused quarry [SJ 1547 7112] 4 crags [SJ 1510 7190] 5 disused quarry [SJ 1521 7201] 6 road cutting [SJ 1518 7163]; 7 stream exposure [SJ 1560 7269] 8 disused quarry [SJ 1549 7303]

(Figure 6) Schematic stratigraphical section illustrating the distribution of pre-Holkerian formations and facies in the district. Facies and thickness in the vicinity of the Vale of Clwyd are speculative. Sections indicated are: 1 River Clywedog 2 Galltegfa Dingle 3 Gelli-gynan Farm 4 Coed Maes-mynan 5 Alyn Valley Borehole and Pistyll Gwyn Quarry 6 Spring Quarry

(Figure 7) Composite log of the Alyn Valley Borehole and Pistyll Gwyn Quarry [SJ 1889 5741] showing the distribution of selected corals, brachiopods, foraminifera, algae and miospores (including data of Somerville and Strank, 1984b).

(Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8) Section in Foel Formation and basal Llanarmon Limestone, Coed Maes-mynan quarry [SJ 1212 7258]. Microfossils present in lower samples (1–6) are combined as Locality 2 in (Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see also (Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12).

(Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9) Geological map of the area west of Eryrys showing the position of localities listed in (Table 9) and/or described in the text. Grid references of localities 1 to 7 see (Table 9) 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11 Spring Quarry [SJ 1916 5968] 12 trackside crags [SJ 1927 5959] 13 crags [SJ 1918 5800] 14 disused quarry [SJ 1930 5957] 15 crags [SJ 1940 5964]

(Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10) Geological map of the Cilcain area showing the position of localities listed in (Table 4) or described in the text. Grid references for ­localities 1 to 9 see (Table 4) 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disused quarry [SJ 1800 6563]

(Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11) Geological map of the area around Ysceifiog and Pantgwyn showing the position of localities listed in (Table 5) or described in the text. Grid references for localities 1 to 8 see (Table 5) 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]

(Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12) Geological map of the Caerwys area showing the position of localities listed in (Table 6) or described in the text. Grid references for localities 1 to 10 see (Table 6) 11 disused quarry [SJ 1230 7354]

(Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13) Geological map of the area to the west of Ruthin showing the position of ­localities listed in (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782] to [SJ 1097 5780] 15 stream section [SJ 1034 5790] 16 disused quarry [SJ 1058 5837] 17 disused quarry [SJ 1067 5823] 18 disused quarry [SJ 1085 5804] 19 track section [SJ 1125 5720]

(Figure 14) Section in crags [SJ 1971 5875] to [SJ 1978 5876] north-west of Eryrys exposing the Leete Limestone–Loggerheads Limestone contact ((Table 9) and/or described in the text. Grid references of localities 1 to 7 see Table 9 8 crags [SJ 1896 5885] 9 crags [SJ 1884 5864] 10 crags [SJ 1894 5863] 11" data-name="images/P941719.jpg">(Figure 9), Locality 3).

(Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15) Geological maps of the Dinantian crops between splays of the Vale of Clwyd Fault showing the position of localities listed in (Table 8) or described in the text. Grid references for localities, see (Table 8).

(Figure 16) Sections in the Loggerheads Limestone showing suggested cycle correlations (for location of sections and key see previous page). (Substantially revises and updates cycle correlations previously attempted by Somerville, 1977, and Campbell and Hains 1988.)

(Figure 17) Sequence in the Gwernymynydd Borehole [SJ 2111 6221] showing the distribution of macrofauna (not corrected for tectonic dip). Note that at the time of logging (1958) pedogenic/karstic features were not identified and that the cycle boundaries indicated are therefore speculative.

(Figure 18) Sections in the Cefn Mawr Limestone showing suggested cycle ­correlations (for key to symbols see (Figure 16)) (Substantially revises and updates cycle correlations previously attempted by Somerville (1979) and Campbell and Hains (1988)). * horizon of P1c ammonoids on Hope Mountain; see text for details CB Coral Bed; MS Main Shale; TS Thick Shale

(Figure 19) Composite sections of the Minera Formation showing suggested cycle correlations (see also (Figure 18)). The correlations between Rainbow Quarry and Hendre Gorge, in particular, are highly speculative and a greater number of cycles may be cut out by the basal Cefn-y-fedw Sandstone ­disconformity than indicated (for key to symbols see (Figure 16)). Localities used to complile the Hope Mountain section: 1 Disused quarry south-east of Penrhiw farm [SJ 5409 5606] 2 Track section, Penrhiw farm [SJ 2874 5649] 3 Crags east of Penrhiw farm [SJ 2889 5636] 4 Disused quarries north-east of Penrhiw farm [SJ 2894 5650] 5 Disused quarry north-north east of Fynnon-y-garreg [SJ 2933 5644] 6 Crags east of Ty Ucha farm [SJ 2855 5662] 7 Disused quarry and crags north-east of Penrhiw farm [SJ 2897 5669] to [SJ 2893 5675] 8 Disused quarries north and north east of Ty Ucha farm [SJ 2866 5668] to [SJ 2843 5681] 9 Disused quarries and crags south of Coed-mawr-ucha [SJ 2930 5681] to [SJ 2947 5638] 10 Disused quarries south of Fron farm [SJ 2886 5677] to [SJ 2836 5694] 11 Disused quarry, Rhiwrob farm [SJ 2903 5697] 12 Coed-issa Quarry [SJ 283 565] 13 Quarries and crags north-north-west of Rhiwrob farm [SJ 2868 5710] to [SJ 2872 5716] 14 Crags south-east of Fron farm [SJ 2868 5687] 15 Disused quarry south-west of Ty Ucha farm [SJ 2826 5660] 16 Disused quarry, crags and farmyard, Fron farm [SJ 2832 5705 ]to [SJ 2839 5711] 17 Disused railway quarries north-west of Ffrith [SJ 2817 5561] to [SJ 2815 5569] 18 Disused railway quarry north-west of Ffrith [SJ 2810 5577] * alternatively, this sandstone could be assigned to the Minera Formation and the disconformity located at its top; see text

(Figure 20a) Millstone Grit Group of the district and adjoining areas. AF Abbey Mills Fault; BL Bala Linement; BF Bwlch-gwgn Fault; GEF Great Ewloe Fault; MF Minerva Fault; NF Neston Fault; N–NEZ Nercwys–Nant-figillt Fault Zone; SF Soughton Fault

(Figure 20b) Diagrammatic chronostratigraphical section for the late Dinantian, Namurian and early Westphalian succession of the Flint district and adjacent areas. Numbered ­localities refer to those detailed in (Table 11). Marine band determinations include data from Jones and Lloyd (1942), Shanklin (1956), Ramsbottom (1974) as well as this survey; arrows indicate maximum ranges of imprecise assemblages. * Marine band ­determinations include data from Jones and Lloyd (1942), Shanklin (1956), ­Ramsbottom (1974) as well as this resurvey

(Figure 21) Schematic section of the Namurian and early Westphalian succession of the Flint district and adjacent areas see also (Figure 20a), (Figure 20b). (GEF Great Ewloe Fault) * The disconformity may represent a compound feature created by very different ‘erosional’ processes in different areas (see text) NB The line of section intersects the Nercwys–Nant-figillt Fault Zone (N–NFZ) twice and the apparent change in attitude and throw of this structure is an artefact of the figure

(Figure 22a) Namurian marine band localities. a Coed-y-cra and Coed y Felin stream sections

(Figure 22b) Namurian marine band localities. b Warren Dingle

(Figure 23) Map of Westphalian crops of the district showing location of sections numbered and illustrated in (Figure 25)(Figure 26)(Figure 27)(Figure 28)(Figure 29)(Figure 30). GEF Great Ewloe Fault; HF Hawarden Fault; LF Llanelidom Fault; NaF Nesten Fault; N–FF Nercwys–Nant-figillt Fault Zone; NF Nant-figillt Fault; SF Soughton Fault

(Figure 24) Generalised vertical sections for the sub-areas of the Flintshire Coalfield described in the text. (See (Figure 23) for locations and p.89 for key.)

(Figure 25) Sections in the Leeswood area. (See (Figure 23) for location of sections and p.89 for key.)

(Figure 26) Sections in the Mold area. (See (Figure 23) for locations at sections and p.89 for key).

(Figure 27) Sections in the Buckley and Neston areas. (See (Figure 23) for location of sections and p.89 for key.)

(Figure 28) Sections in the Queensferry area. (See (Figure 23) for location of sections and p.89 for key.)

(Figure 29) Sections in the Bagillt area. (See (Figure 23) for location of sections and p.89 for key.)

(Figure 30) Section of Llay Main Colliery No. 1 Shaft in the Denbighshire Coalfield. (See (Figure 23) for locations of section and p.89 for key.)

(Figure 31) Regional setting of the Permo–Triassic rocks.

(Figure 32) Palaeocurrent ­measurements from the Kinnerton Sandstone and Chester Pebble Beds formations based on the inclination of dune foreset bedding, trough and planar cross-bedding.

(Figure 33) Section of the Permo–Triassic strata exposed at Burton Point [NGR] based on Macchi and Meadows (1987) and Macchi (1991).

(Figure 34) Location map of ‘Tertiary’ pocket deposits (based partly on data of Walsh and Brown, 1971).

(Figure 35) Principle structures of the Flint district. Names of mineral veins and cross-courses principally after Strahan (1890) and Smith (1921).Mwynbwll Fault * thought by miners to be a southward continuation of the Caleb Bell Cross-course

(Figure 36) Contoured white mica crystallinity data for the Silurian crop. RF Range Fault

(Figure 37) Bouguer gravity anomaly map of the district and location of profiles BB’ and CC’.Contours at 1 mGal intervals. Density for data reduction 2.60 Mg/m3. AqF Aqueduct Fault; AvF Alyn Valley Fault; AxF Axton Fault; BF Bryneglwys Fault; BwF Bwlch-gwyn Fault; C-y-B Cyrn-y-Brain, Denbigh Fault; GEF Great Ewloe Fault; HF Hawarden Fault; LF Llanelidan Fault; LM Llandegla Moor; MF Minera Fault; NF Neston Fault; N–NFZ Nercwys–Nant-figillt Fault Zone; VCF Vale of Clwyd Fault; WF Wrexham Fault; WaF Waverton Fault

(Figure 38) ­Aeromagnetic anomaly map (reduced to pole data) of the district and location of profile AA’. M indicates anomalies with probable man-made sources. Contours at intervals of 10 nT. AqF Aqueduct Fault; AvF Alyn Valley Fault; AxF Axton Fault; BF Bryneglwys Fault; BwF Bwlch-gwyn Fault; C-y-B Cyrn-y-Brain; Denbigh Fault; GEF Great Ewloe Fault; HF Hawarden Fault; LF Llanelidan Fault; LM Llandegla Moor; MF Minera Fault; NF Neston Fault; N–NFZ Nercwys–Nant-figillt Fault Zone; VCF Vale of Clwyd Fault; WF Wrexham Fault; WaF Waverton Fault

(Figure 39) Sketch map showing main geological and geophysical features. Numbered lineaments refered to in text. AqF Aqueduct Fault; AvF Alyn Valley Fault; AxF Axton Fault; BF Bryneglwys Fault; BwF Bwlch-gwyn Fault; CB Cyrn-y-Brain; Denbigh Fault; GEF Great Ewloe Fault; HF Hawarden Fault; LF Llanelidan Fault; LM Llandegla Moor; MF Minera Fault; NF Neston Fault; N–NFZ Nercwys–Nant-figillt Fault Zone; VCF Vale of Clwyd Fault; WF Wrexham Fault; WaF Waverton Fault

(Figure 40) Aeromagnetic profile AA′ across the southern part of the district (see (Figure 38)) and a model for magnetic basement rocks. susceptibility 0.01 SI units; background field 70 nT.

(Figure 41) Gravity profile BB’ and geological model across the Llandegla Moor area (see (Figure 37)). Densities used in modelling (in Mg/m3): Ordovician (O) 2.73; Silurian (S) 2.70; Dinantian (D) 2.65; Namurian (N) 2.50; low density Namurian sandstones (N(L)) 2.30; background field 0 km 22.5, 7 km 20.5 mGal

(Figure 42) Bouguer gravity anomaly map of the Dodleston area with locations of gravity observations, the BGS seismic survey (S) and profile CC’. Contours in 0.5 mGal intervals. TI Trevalyn inlier

(Figure 43) Gravity profile CC’ and geological model for the Dodleston area see (Figure 37), (Figure 42). Background field 26.5 mGal (0 km), 27 mGal (11 km). Densities used in modelling (in Mg/m3) Drift 2.00; Triassic (T1), Permo–Triassic (T2) 2.35; Westphalian and Namurian (W/N) 2.50; Dinantian (D) 2.65; at depth (not shown): Silurian 2.70 and Ordovician (south of Bala Lineament) 2.73

(Figure 44) Structure contour map of the base of the Permo–Triassic in the east of the district (interpreted from seismic and borehole data).

(Figure 45) Map of the Quaternary deposits of the district.

(Figure 46) Generalised ice-flow directions and other glacial features. Sections F, H and I of Thomas (1985) extend outside the area shown in the figure. SG Sarn Galed channel Section

(Figure 47) Subdrift bedrock topography in the east of the district illustrating the location and form of the palaeo-Dee valley.

(Figure 48) Schematic cross-sections through the Quaternary deposits of the River Dee and adjacent areas. (See (Figure 46) for location of sections.)

Plates

(Plate 1) Striped silty mudstone facies, Elwy Formation Lady Bagott’s Drive [SJ 1020 5950] (hammer 0.35 m) (GS 1052).

(Plate 2a) Sandstone facies in the Elwy Formation (hammer 0.35 m). a Thin-bedded sandstone facies (Bouma turbidites comprising sandstone-mudstone couplets), crags [SJ 1318 6374] south-east of Penycloddiau (GS1053A).

(Plate 2b) Sandstone facies in the Elwy Formation (hammer 0.35 m). b Thick-bedded sandstone facies, disused quarry [SJ 1467 6584], Moel Arthur (GS1053B).

(Plate 3a) Basement Beds and Foel Formation lithologies in the Alyn Valley Borehole [SJ 1889 5741] (figures are depths in metres below ground level). a Breccia of buff and red, weathered Silurian mudstone clasts, Basement Beds, 121.05 m depth (0.75 m above basal unconformity) (GS1054A)

(Plate 3b) Basement Beds and Foel Formation lithologies in the Alyn Valley Borehole [SJ 1889 5741] (figures are depths in metres below ground level). b ‘Teepe structure’ in cryptalgal laminite, Foel Formation, 105.65 m depth (GS1054B)

(Plate 3c) Basement Beds and Foel Formation lithologies in the Alyn Valley Borehole [SJ 1889 5741] (figures are depths in metres below ground level). c Brecciated cryptalgal laminate, Foel Formation, 84.20 m depth (GS1054C)

(Plate 3d) Basement Beds and Foel Formation lithologies in the Alyn Valley Borehole [SJ 1889 5741] (figures are depths in metres below ground level). d Mottled packstone-wackestone with colonies of the tabulate coral Syringopora, Foel Formation, 61.50 m depth (GS1054D)

(Plate 3e) Basement Beds and Foel Formation lithologies in the Alyn Valley Borehole [SJ 1889 5741] (figures are depths in metres below ground level). e Ostracode wackestone with small oncolites, note presence of both disarticulated and articulated ostracode valves, the latter with spar-filled internal cavities, 52.24 m depth (BGS micropalaeontology slide RHR 550, 3 63) (GS1054E)

(Plate 4a) Foel Formation lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are 3 63 unless stated otherwise). a Ooid peloid grainstone, note early-cemented intraclast centre left (JD 5006), Llwyn-y-frân farm [SJ 1894 5889] (GS1055A)

(Plate 4b) Foel Formation lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are 3 63 unless stated otherwise).b Ooid peloid packstone-wackestone, note vadose micrite-cement bridges between grains, early fringing spar cement and later, cavity-filling, blocky spar cement (JD 5213) (3 67), Coed Maes-mynan quarry [SJ 1212 7258] (GS1055B)

(Plate 4c) Foel Formation lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are 3 63 unless stated otherwise). c Peloid calcisphaere skeletal grainstone, JD 5210 (3 22), as b (GS1055C)

(Plate 4d) Foel Formation lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are 3 63 unless stated otherwise). d Calcretised skeletal peloid packstone, with spar filled rhizolith (root cast), crags [SJ 1698 6783] south of Penbedw Hall (GS1055D)

(Plate 4e) Foel Formation lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are 3 63 unless stated otherwise). e ‘Porcellaneous’ peloid wackestone with spar-filled fenestrae, note sub-horizontal desiccation fenestrae partially filled with internal sediment, displaying flat bottoms and irregular tops, and near-vertical tubular fenestrae formed by roots, or gas bubbles, or by burrowing, cut block (coin diameter 15 mm), as b (GS1055E)

(Plate 5a) Llanarmon Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are X25). a Poorly sorted, intraclast peloid skeletal grainstone, note large intraclast with micritic rind top right, and well developed micrite filled algal bores and micrite envelopes on skeletal grains (ON 725), basal bed of formation, landslip back scar [SJ 1026 5975] north of River Clywedog (GS1056A)

(Plate 5b) Llanarmon Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are X25). b Anthracoporella grainstone/framestone (TW 828), roadside exposure [SJ 1176 7406] near Rhos farm (GS1056B)

(Plate 5c) Llanarmon Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are X25). c Koninckopora peloid intraclast grainstone, note honeycomb-­structured Koninckopora plates and abundant wackestone intraclasts (JD 5011), disused quarry [SJ 1810 6471], Maes-mawr (GS1056C)

(Plate 5d) Llanarmon Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are X25). d Well sorted and well rounded skeletal peloid grainstone, note compound grains and extensive micritisation (ON 724), channel-fill sequence, cliff section [SJ 1029 5933], River Clywedog (GS1056D)

(Plate 5e) Llanarmon Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers; magnifications are X25). e Algal pisolith with fenestral wackestone intraclast core and thick, cryptalgal bindstone coating, note section through encrusting vermeetid gastropod on outer surface of intraclast (ON 723] as d (GS1056E)

(Plate 6a) Leete Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers). a Calcisphaere wackestone with calcite pseudomorphs of euhedral gypsum crystals (JD 5066) (X 25), disused quarry [SJ 1784 6580] north east of Cilcain (GS1057A)

(Plate 6b) Leete Limestone lithologies (numbers with letter prefixes are BGS micropalaeontology sample/slide numbers).  b Calcisphaere wackestone (JD 5065) (X 63), cliff [SJ 1861 6511] north of Nant Gain (GS1057B)

(Plate 7) Limestone escarpment [SJ 1995 6328] at Loggerheads showing the eponymous type section (GS1058).

(Plate 8) Palaeokarstic pit with overlying clay palaeosol, Loggerheads Limestone, Graig Quarry [SJ 205 565] (GS1059).

(Plate 9a) Cefn Mawr Limestone. a Thin-bedded, dark grey ­wackestones with black chert nodules, note ­articulated productid brachiopods in preserved life position, Pant Quarry [SJ 200 703] (GS1060A)

(Plate 9b) Cefn Mawr Limestone. b Trace fossil Zoophycus (lens cap diameter 60 mm), as a (GS1060B)

(Plate 9c) Cefn Mawr Limestone. c Ammonoid floatstone with abraded conchs (maximum width of block 26 cm), crags [SJ 2861 5595], Hope Mountain (GS1060C)

(Plate 10) Cross-bedded rudstone–grainstone units in the Cefn Mawr Limestone. a Pant quarry [SJ 200 703], arrows indicate bases of four separate lenticular bodies, (height of main quarry face about 60 m) (GS1061A). LoL Loggerheads Limestone; MS Main Shale b Graig quarry [SJ 205 565] (GS1061B).

(Plate 10) Cross-bedded rudstone–grainstone units in the Cefn Mawr Limestone. a Pant quarry [SJ 200 703], arrows indicate bases of four separate lenticular bodies, (height of main quarry face about 60 m) (GS1061A). LoL Loggerheads Limestone; MS Main Shale b Graig quarry [SJ 205 565] (GS1061B).

(Plate 11a) Synsedimentary ­disturbances in the Cefn Mawr Limestone. a Truncation surfaces (highlighted) bounding successive, internally bedded, synsedimentary slides, Waen Brodlas quarry [SJ 1875 7310] (GS1062A)

(Plate 11b) Synsedimentary ­disturbances in the Cefn Mawr Limestone. b Synsedimentary faulting, Pant quarry [SJ 200 703] (GS1062B)

(Plate 12a) Pentre Chert Formation. a Laminated and banded, glassy chert, disused quarry [SJ 1932 7335], Halkyn Mountain, note campactional ­deformation around early diagenetic nodules (GS1063A)

(Plate 12b) Pentre Chert Formation. b Pillow-like body of deformed bedded chert within the upper part of the Cefn Mawr Limestone, Bryn Mawr quarry [SJ 1877 7340] (GS1063B)

(Plate 12c) Pentre Chert Formation. c Highly disturbed and contorted bedded cherts, Pen-yr-henblas quarry [SJ 1900 7290] (GS1063C)

(Plate 13) Large-scale, dune cross-bedding in Kinnerton Sandstone, Burton Point [SJ 3023 7356] (note sheep centre right for scale) (GS1064).

(Plate 14) Junction (at head of hammer) between Kinnerton Sandstone and the overlying Chester Pebble Beds, Burton Point [SJ 3023 7356] (hammer 0.35 m) (GS1065).

(Plate 15) Northward downthrowing, mineralised normal fault (Pant y Gwlanod Vein) exposed in Graig Quarry [SJ 205 565] (height of quarry face about 50 m). The contact between pale and dark limestones (lower left) is a palaeokarstic surface equivalent in quarry sections farther north to the Loggerheads Limestone–Cefn Mawr Limestone contact, but here included within an expanded Cefn Mawr Limestone sequence (see Figure 16) and (Figure 18) (GS1066).

(Plate 16) Glacial erratic of ­Ordovician volcanic rock [SJ 1099 7029] on Moel-y-Parc (hammer 0.35 m) (GS1067).

(Plate 17) Kettlehole topography developed on glaciofluvial ice contact deposits, Wheeler valley, Nannerch (GS1068).

(Plate 18a) Glaciofluvial sheet deposits formerly exposed in Singret Sand Pit [SJ 344 558]. a Gravel-filled channels (L1692)

(Plate 18b) Glaciofluvial sheet deposits formerly exposed in Singret Sand Pit [SJ 344 558]. b Cross-bedded sand and gravel overlain by poorly bedded gravel (L1690)

(Plate 19) Moundy and pitted ground south of Waen Brodlas quarry [SJ 186 732], created during the 19th century exploration for metal ores and comprising numerous shafts, trail pits and associated spoil heaps. Such ground is characteristic of much of Halkyn Mountain (GS1069).

Tables

(Table 1) Summary of the geological sequence in the Flint district.

(Table 2) Current and former classifications of Silurian rocks of the district and adjacent areas.

(Table 3) Current and former stratigraphical classifications of Dinantian strata of the district and adjacent areas.* In the Vale of Clwyd the Basement Beds underlie the Arundian Llanarmon Limestone † in this account the Pentre Chert Formation is included in the Namurian † The precise position of the Dinantian–Namurian boundary is not known and it may or may not lie above the base of the Cefn-y-fedw Sandstone

(Table 4) Distribution of foraminifera and algae in the Llanarmon Limestone and Leete Limestone in the Cilcain area. Localities (see (Table 4) or described in the text. Grid references for localities 1 to 9 see Table 4 10 crags [SJ 1861 6511] 11 crags [SJ 1831 6459] 12 crags [SJ 1892 6482] 13 disus" data-name="images/P941720.jpg">(Figure 10) . 1 disused quarry [SJ 1729 6556] 4 disused quarry [SJ 1792 6458] 7 disused quarry [SJ 1784 6580] 2 disused quarry [SJ 1720 6596] 5 disused quarry [SJ 1799 6461] 8 stream exposures [SJ 1884 6509] to [SJ 1846 6511] 3 disused quarry [SJ 1766 6444] 6 disused quarry [SJ 1810 6471] 9 crags [SJ 1855 6510]

(Table 5) Distribution of foraminifera and algae in the Llanarmon Limestone and Loggerheads Limestone in the area of Ysceifiog and Pantgwyn. Localities (see (Table 5) or described in the text. Grid references for localities 1 to 8 see Table 5 9 road cutting [SJ 1521 7203] to [SJ 1520 7229]" data-name="images/P941721.jpg">(Figure 11)).

(Table 6) Distribution of foraminifera and algae in the Foel Formation and Llanarmon Limestone of the Caerwys area. Localities (see (Table 6)11 disused quarry [SJ 1230 7354]" data-name="images/P941722.jpg">(Figure 12)).Locality: 1 crags [SJ 1202 7262] 7 crags [SJ 1245 7342] 2 disused quarry [SJ 1212 7258] (lower part of section, see (Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8)) 8 crags [SJ 1176 7406] 3 as 2 (upper part of section, see (Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8)) 9 disused quarry [SJ 1149 7458] 4 disused quarry [SJ 1208 7278] 10 disused quarry [SJ 1125 7481] 5 disused quarry [SJ 1255 7268] 11 disused quarry [SJ 1230 7354] 6 crags [SJ 1239 7322]

(Table 7) Distribution of foraminifera and algae in the Llanarmon Limestone and Leete Limestone in the Vale of Clwyd, west of Ruthin. Localities (see (Table 7) or described in the text. Grid references for localities 1 to 12 see (Table 7) 13 river section [SJ 1023 5955] 14 stream sections [SJ 1049 5782]" data-name="images/P941723.jpg">(Figure 13)): 1 landslip scar [SJ 1026 5975] 2 river section [SJ 1029 5959] 3 river section [SJ 1042 5968] 4 river section [SJ 1079 5999] 5 river section [SJ 1074 5992] 6 river section [SJ 1064 5980] 7 disused quarry [SJ 1023 5870] 8 disused quarry [SJ 1040 5863] 9 river section [SJ 1049 5979] 10 Craig-y-ddywart quarry [SJ 1112 5932] 11 Craig-y-ddywart quarry [SJ 1112 5932] 12 disused quarry [SJ 1087 5942]

(Table 8) Distribution of foraminifera and algae in the Dinantian sequences preserved between splays of the Vale of Clwyd Fault. Localities (see (Table 8) or described in the text. Grid references for localities, see Table 8." data-name="images/P941725.jpg">(Figure 15)): 1 disused quarry [SJ 1171 6698] 2 disused quarry [SJ 1183 6696] 3 disused quarry [SJ 1385 6261] 4 disused quarry [SJ 1489 5948] 5 disused quarry [SJ 1172 6705] 6 disused quarry [SJ 1174 6703] 7 roadside exposure [1190 6694] 8 disused quarry [SJ 1390 6246] 9 crags [SJ 1425 6187] 10 house cutting [SJ 1481 5954] 11 roadside exposure [1505 5569] 12 disused quarry [SJ 1191 6676] 13 disused quarry [SJ 1263 6563] 14 crags [SJ 1514 5601] 15 crags [SJ 1609 5589] 16 stream exposure [SJ 1415 6185] 17 crags [SJ 1452 5619]

(Table 9) Distributions of foraminifera and algae in the Leete Limestone and Loggerheads Limestone in the area west of Eryrys. Localities (see (Table 6); sample 7 is Locality 3a and sample 8 is Locality 3b of (Table 6); see" data-name="images/P941718.jpg">(Figure 8)). 1 crags [SJ 1951 5809] 2 crags [SJ 1965 5905] 3 crags [SJ 1971 5875] to [SJ 1978 5876] 4 crags [SJ 1969 5889] 5 crags [SJ 1978 5809] 6 crags [SJ 1988 5772] 7 crags [SJ 1977 5769]

(Table 10) Namurian stages, biozones and subzones, with a list of the main marine bands recognised within the Pennine Basin. Mesothems are those of Ramsbottom (1977). * denotes biozone and/or specific marine band recorded in the Flint district and adjacent areas. B. Bilinguites; C. Cancelloceras; Cr. Cravenoceras; Ct. Cravenoceratoides; E. Eumorphoceras; F. Fayettevillea; H. Homoceras; Hd. Hudsonoceras; Ho. Hodsonites; Ht. Homoceratoides; I. Isohomoceras; N. Nuculoceras; R. Reticuloceras; T. Tumulites; V. Vallites; Ve. Verneuillites

(Table 11) Distribution of goniatite (ammonoid), brachiopods and other taxa obtained from selected Namurian marine band localities in the district (based on material in BGS collections).

(Table 12) Westphalian stages, marine bands, past and current lithostratigraphy of north-east Wales and the English Midlands. Abbreviations for selected coal seams: Ch Chwarelan; Lly Llwyneinion Half Yard; N Nant; R/UR Red or Upper Red; BB Black Bed; Dr Drowsell; Lst Lower Stinking; W Warras; USt Upper Stinking

(Table 13) Summary of current and former coal seam nomenclature used in the district. Based, with modifications on the former British Coal nomenclature scheme for the Flintshire and Denbighshire (north) Coalfields.

(Table 14) Classification and correlation of Permo- Triassic sequences in the Flint and adjacent districts.

(Table 15) Occurrence and lithology of probable Tertiary deposits in the district. * Walsh and Brown (1971), suggested that this locality refers to the Pant-du deposits described by Maw (1867), whereas Strahan (1890) stated that Maw’s Pant-du descriptions are probably those of the pocket deposits at Pwll Helyg. Abbreviations:- C Clay, S Sand, W White, Y Yellow, G Grey, BG Blue-grey, R Red, B Black (carbonaceous), L Laminated, M Mottled, V Variegated, P Pebbly The numbers refer to the localities in (Figure 34) Based on Walsh and Brown (1971) with supplementary data from Maw (1867) and Strahan (1890)

(Table 16) Summary of density values of the main rocks types based on results by Powell (1956), Wilson (1959), Smith (1986) and Cornwell (1987).

(Table 17) Active limestone quarries in 1998.

(Table 18) Mechanical properties of Carboniferous limestones.

(Table 19) Chemical analyses of Carboniferous limestones.

(Table 20) Licenced abstractions of groundwater.

(Table 21) Typical chemical analyses of groundwater from the district.* from Water Resources Board, 1973 All other analyses from National Well Record Archive at Wallingford

Tables

(Table 15) Occurence and lithology of probable Tertiary deposits in the district.

No. Name NGR Lithology
1 Wern Fawr [SJ 1217 7509] C
2 Plascerrig Main [SJ 1311 7497] S WC RC YC BGC GC LC
3 Ffrith-y-garreg wen (S) [SJ 1346 7489] WS WC
4 Ffrith-y-garreg wen (N) [SJ 136 753] WS WC
5 Plymouth Copse [SJ 1390 7520] S WC
6 Bwlch Farm [SJ 1811 7104] WS GS YS BS WC P
7 Rhes-y-cae Main [SJ 1946 7102] WS VS WC BC GC VC
8 Rhes-y-cae [SJ 1952 7073] YS C
9 Loggerheads Inn [SJ 1970 6294] WS WC YC RC
10 Colomendy [SJ 2028 6197] WS WC LC
11 Bryn Alyn [SJ 1993 5900] WS
12 Pwll Heli [SJ 1951 5867] WC BC LC
13 Nags Head [SJ 1717 7317] C
14 Pwll Melyn [SJ 185 716] WS WC BC
15 Berth ddu [SJ 2039 6954] WS YS C
16 Waen [SJ 202 648] WC
17 Pant Glas [SJ 206 635] WC
18 Vron Hall Mine [SJ 223 621] WC
19 Maeshafn [SJ 203 610] WS WC
20 Pant Ddu * [SJ 204 596] WC BR GL LC21
21 Llangynhafal [SJ 135 632] GC LC

(Table 16) Summary of density values of the main rocks types based on results by Powell (1956), Wilson (1959), Smith (1986) and Cornwell (1987).

Rock group Saturated density (Mg/m3)
Quaternary 2.00
Permo–Triassic Mercia Mudstone 2.24
Permo–Triassic Sherwood Sandstone Group (Cheshire Basin) 2.48
Permo–Triassic Sherwood Sandstone Group (Vale of Clwyd) 2.30
Carboniferous Westphalian 2.50
Carboniferous Namurian 2.45 (range 2.10–2.60)
Carboniferous Dinantian 2.65
Lower Palaeozoic Silurian
Lower Palaeozoic Ordovician

(Table 18) Mechanical properties of Carboniferous limestones.

Aggregate crushing value 17–23
Aggregate impact value 18–23
Aggregate abrasion value 9.2–10.9
10% Fines value 180–210 kN
Polished stone value 40–45
Water asbsorption (%) 0.4–0.9
Source: company data.

(Table 19) Chemical analyses of Carboniferous limestones.

Weight per cent 1 2 3 4
CaO 55.51 55.29 55.44 54.56
MgO 0.14 0.10 0.12 0.08
SiO2 0.12 0.21 0.13 0.00
Fe2O3 0.00 0.03 0.01 0.01
A12O3 0.00 0.00 0.00 0.00
Na2O 0.06 0.06 0.06 0.01
K2O 0.00 0.00 0.00 0.00
TiO2 0.00 0.00 0.00 0.00
P2O5 0.00 0.00 0.00 0.01
MnO 0.03 0.04 0.05 0.01
Loss 42.44 42.55 42.50 42.86

(Table 20) Licenced abstractions of groundwater.

a by aquifer Ml/a
Drift 525
Sherwood Sandstone Group 6944
Westphalian 38
Namurian 198
Carboniferous Limestone 527
Silurian 7
Total 8239
b by use
Public water supply 1321
Agriculture (including spray irrigation) 491
Industry (including non-evaporative cooling 4967
Transfer between sources (mainly river regulation) 1442
Sand and gravel washing 18
8239

(Table 21) Typical chemical analyses of groundwater from the district.

Location Halkyn Ruthin Mold Dodleston Burton Ruthin* Llandyrnog Pentre Mawr Plas-yr-Esgob Burton Burton
National Grid Reference [SJ 2029 7065] [SJ 1112 5920] [SJ 2418 6348] [SJ 3622 6082] [SJ 3099 7430] v1305 5720] [SJ 1092 6563] [SJ 1031 6693] [SJ 1107 6164] [SJ 3236 5859] [SJ 3184 5944]
Type of source Level tunnel spring borehole borehole spring borehole borehole well borehole shallow well catchpit
Aquifer Carboniferous Limestone Carboniferous Limestone Westphalian Kinnerton Sandstone Kinnerton Sandstone Kinnerton Sandstone Kinnerton Sandstone Kinnerton Sandstone Kinnerton Sandstone Drift Drift
Date of analysis 4/1875 18/12/79 24/4/34 24/2/74 9/10/81 15/10/79 7/11/80 17/11/80 25/1/84 25/1/84
pH 6.9 7.8 6.9 6.7 7.2 7.7 7.15 7.2 6.8
Electrical conductivity μmhos/cm 670 550 800 430 794 368 255 509
Total dissolved solids mg/1 297 372 2352 298 508 402 572 264 164 317
Total hardness (CaCO3) mg/1 825 245 101 213
Carbonate (CO3 -) mg/1 8.3 nil nil nil nil <1.0 <1.0
Bicarbonate (HCO3 -) mg/1 256 264 649 248 175 279 220 221 155 71 52
Sulphate (SO4 2-) mg/1 34 27 281 11 59.1 34.2 66.4 9.0 29.6 49.6
Chloride (C1-) mg/1 26 16 50 16 61 228 36 54 15.1 17.1 43.4
Nitrate (NO3-_N) mg/1 6.31 nil <0.5 9.49 4.86 13.99 2.78 2.16 15.49
Calcium (Ca2+) mg/1 94 97.0 70 109 149 70 90 56 36.5 66.7
Magnesium (Mg2+) mg/1 5 7.8 17 13.5 48 2.7 12.2 6.5 2.4 11.2
Sodium (Na+) mg/1 11 10.0 17 33.9 34 17.9 20.8 9.5 12 15.4
Potassium (K+) mg/1 2 4.4 6.7 3.4 5.1 1.73 48.0 2.2 6.3 10.1
Iron (total) mg/1 0.10 <0.1 0.112 0.02 0.71 0.06 0.093 0.068
Manganese (total) mg/1 0.01 0.0444 0.01 0.03 <0.02 0.086 <0.01
Silica (SiO2) mg/1 5.3 1.48 4.7 10.6 4.6 4.9 3.0 3.4