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Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. England and Wales.

By P. T. Warren, D. Price, M. J. C. Nutt and E. G. Smith.

Bibliographical reference: Warren, P. T., Price, D., Nutt, M. J. C. and Smith, E. G. 1984. Geology of the country around Rhyl and Denbigh. Memoir British Geological Survey, sheets 95 and 107, 217 pp.

• Contributors
• Stratigraphy: H. E. Wilson, D. A. C. Mills, T. R. W. Hawkins and W. H. C. Ramsbottom
• Palaeontology: P. T. Warren, A. W. A. Rushton, R. B. Rickards and W. H. C. Ramsbottom
• Geophysics: J. D. Cornwell

Natural Environment Research Council. London Her Majesty's Stationery Office 1984. © Crown Copyright 1984. First published 1984. ISBN 0 11 884225 0. Printed in the UK for HMSO. Dd717073 C20 9/84.

• Authors
• P. T. Warren, MA, PhD, MI Geol. The Royal Society, 6 Carlton House Terrace, London SW1Y 5AG
• D. Price, BSc M. J. C. Nutt, BSc, PhD E. G. Smith, BSc, CEng, FIMM British Geological Survey, Keyworth, Nottingham NG12 5GG
• Contributors
• H. E. Wilson, MSc, CEng, MIMM, MIGeol, J. D. Cornwell, MSc, PhD, D. A. C. Mills, BSc, MIGeol, and W. H. C. Ramsbottom, MA, PhD British Geological Survey, Keyworth
• T. R. W. Hawkins, BSc, PhD Middlesex Polytechnic, Enfield, Middlesex.
• A. W. A. Rushton, BA, PhD British Geological Survey, Exhibition Road, London SW7 2DE.
• R. B. Rickards, BSc, PhD, ScD, MIGeol Sedgwick Museum, Downing Street, Cambridge.

(Front cover)

(Rear cover)

Other publications of the Survey dealing with this and adjoining districts

Books

• British Regional Geology
• North Wales, 3rd Edition, 1961, reprinted 1979
• One-inch and 1:50 000 Sheet Memoirs
• Geology of the country around Liverpool, 1923
• Geology of the country around Flint, Hawarden and Caergwrle, 1924
• Geology of the country around Wrexham, 2 vols, 1927 and 1928
• Classical areas of British Geology. Explanation of 1:25 000 Geological Sheets
• Capel Curig and Betws-y-Coed, SH 75, 1978
• Dolgarrog, SH 76, 1981

Geological maps

• 1:625 000
• Geological Map of the United Kingdom, South Sheet, 3rd Edition, 1979
• Overprinted with sheet lines of one-inch and 1:50 000 Geological Series
• Quaternary map of the United Kingdom, South Sheet, 1977
• Aeromagnetic map of Great Britain, Sheet 2, 1965
• 1:250 000
• Liverpool Bay, 1978 Bouguer gravity anomaly
• Liverpool Bay, 1977 Aeromagnetic anomaly
• Liverpool Bay, 1978
• One-inch to one mile (1:63 360) and 1:50 000
• Llandudno (94) Solid, Drift, in press
• Bangor (106) Solid, Drift, in press
• Liverpool (96) Solid 1923, reprinted 1974; Drift 1923, reprinted 1975
• Flint (108) Solid 1924, reprinted 1974; Drift 1924, reprinted 1979
• Wrexham (121) Solid 1927, reprinted 1979; Drift 1927, reprinted 1968
• 1:25 000
• Conwy, SH 77–78, Solid and Drift, in press Dolgarrog
• SH 76, Solid and Drift, 1981 Capel Curig and Betws-y-Coed
• SH 75, Solid and Drift, 1976 Central Snowdonia, Special Sheet, Solid, 1972

Preface

This memoir describes the geology of the district covered by One-inch/1:50 000 New Series sheets 95 (Rhyl) and 107 (Denbigh) together with those small parts of sheets 94 (Llandudno) and 106 (Bangor) that are underlain by Silurian and Carboniferous rocks. The district was originally surveyed on the one-inch scale by W. T. Aveline, W. W. Smyth and A. C. Ramsay and the results published as Old Series sheets 79 NW and 79 SW in 1850. The geology was briefly described by Ramsay in his memoir on North Wales published in 1866, with a second edition in 1881. Old Series Sheet 79 NW (Rhyl) was surveyed on the six-inch scale by W. T. Aveline, C. E. De Rance, A. Strahan and R. H. Tiddeman and the one-inch map and memoir (by Strahan) published in 1885. The Carboniferous rocks of Old Series Sheet 79 SW (Denbigh) were revised by Tiddeman and Strahan in 1895, but no account was published.

The six-inch resurvey of the Rhyl area (New Series Sheet 95) was carried out by Messrs D. Price and D. A. C. Mills with some assistance from Dr P. T. Warren in 1963–65, under the supervision of Mr H. E. Wilson as District Geologist. Solid and drift editions on the one-inch scale were published on one sheet in 1970. The six-inch survey of the Denbigh area (New Series Sheet 107) with adjoining parts to the west was carried out by Dr P. T. Warren, Messrs. H. E. Wilson, E. G. Smith, D. Price and D. A. C. Mills and Drs T. R. W. Hawkins and M. J. C. Nutt between 1963 and 1969, Dr E. H. Francis succeeding Mr Wilson as District Geologist in 1967. Solid and drift editions on the 1:50 000 scale were published in 1973.

The greater part of the present memoir has been written by Dr Warren. Mr Price, Dr Nutt and Mr Smith are co-authors, and contributions to the stratigraphy and general geology have been made by Messrs Wilson and Mills and Drs Hawkins and W. H. C. Ramsbottom.

In addition to the study of large numbers of fossils collected from the Silurian rocks during the survey, extensive pre-existing collections, especially those of P. G. H. Boswell, have been re-examined. Drs Warren and R. B. Rickards have been responsible for the graptolites and Dr A. W. A. Rushton for the shelly faunas. Dr Ramsbottom has identified the Lower Carboniferous fossils except for the corals, which were named by Mr M. Mitchell.

Dr J. D. Cornwell has contributed Chapter 11 on geophysical investigations, including gravity and magnetic surveys. Mr D. J. Lowe and Mrs R. J. Tappin (nee Follows) have prepared most of the diagrams for publication. The memoir has been compiled and edited by Mr Smith.

Grateful acknowledgement is made to numerous organisations and individuals, including landowners, quarry operators, consulting engineers and public and local authorities, for generous help during the survey.

G. M. Brown, Director British Geological Survey Keyworth Nottingham NG12 5GG 18 June 1984.

Geology of the country around Rhyl and Denbigh—summary

The district covered by this memoir includes a major part of the Silurian outcrops of North Wales, an area of classic and, in the past, controversial geology and of beautiful if not dramatic scenery. The several thousand metres of Silurian rocks are described in detail, and particular attention is paid to the slump beds they contain—some of the best examples in the British stratigraphical sequence. An extensive collection of graptolites has led to the recognition of twenty-three faunal assemblages, which have been correlated with other British zonal sequences.

A comprehensive account is also given of the Carboniferous Limestone, which is spectacularly exposed on the Great and Little Orme, and occurs along the resort-studded coast and in the graben-like Vale of Clwyd. In the last area it is overlain by Upper Carboniferous and Permo-Triassic rocks, known chiefly from boreholes.

Much of the district is covered by glacial deposits, and there are many good examples of drumlins, sub-glacial channels and other phenomena associated with ice or its meltwaters.

Sections of the memoir deal with the tectonic history of the district, economic geology (including metalliferous mineral deposits and underground water) and geophysical investigations of the rocks at depth.

Geological sequence

(Geological succession) Geological sequence

The rock units represented on Sheets 95 and 107 are summarised below.

Superficial deposits (Drift)

Solid rocks

Six-inch maps

The six-inch National Grid maps included in One-inch map Sheet 95 and 1:50 000 map Sheet 107 are listed below with names of the surveyors and dates of survey. The surveyors were: T. R. W. Hawkins, D. A. C. Mills, M. J. C. Nutt,

D. Price, E. G. Smith, P. T. Warren and H. E. Wilson. The Silurian and Carboniferous outcrops forming parts of sheets 94 (Llandudno) and 106 (Bangor) and described in this memoir fall within six-inch maps: SH 76 SE and NE, SH 77 SE and NE, and SH 78 SE. Copies of the maps (except SH 85 NW, which is incomplete) are available for purchase in dye-line form from the British Geological Survey, Keyworth, Nottingham NG12 5GG.

Small parts of SH 87 NW and NE, 97 NW and NE and SJ 07 NW and NE are included in Sheet 107 below.

Notes

In this memoir the word 'district' means the area included in 1-in/1:50 000 geological sheets 95 (Rhyl) and 107 (Denbigh) together with those areas of the adjacent sheets 94 (Llandudno) and 106 (Bangor) that are underlain by Silurian and Carboniferous strata (Figure 1).

Figures in square brackets are National Grid references within 100-km squares SH and SJ.

Numbers preceded by the letter E refer to the Sliced Rock Collection of the British Geological Survey. The prefix NEX indicates an X-ray diffraction powder photograph by the Survey (Keyworth Office); L and MLD refer to the Survey's collection of photographs. Numbers with other prefixes refer to the Survey's Palaeontological Collection.

The authorship of fossil species is given in Appendix 1 and the Index of Fossils.

Chapter 1 Introduction

General and geographical

This memoir describes the geology of the district covered by the Rhyl (95) and Denbigh (107) sheets of the 1-inch/1:50 000 Geological Map of England and Wales (Figure 1). Also included are the adjoining parts of the Llandudno (94) and Bangor (106) sheets to the west that are underlain by Silurian or Carboniferous rocks. The district lies largely in the former county of Denbighshire, with part of Flintshire in the northeast and small parts of Caernarvonshire in the west. Except for the western edge, which falls within Gwynedd, it now forms part of the new county of Clwyd. The main centres of population are on the coast and include the resorts of Llandudno, Colwyn Bay, Rhyl and Prestatyn and the market town of Abergele. Away from the coast only Rhuddlan, Dyserth, St Asaph, Denbigh and Llanrwst are of appreciable size; elsewhere there are scattered villages and hamlets, chiefly in the valleys.

The district is delimited to the west by the outcrop of Ordovician sediments and volcanic rocks with intrusions, a small area of which is included in the south-western corner of Sheet 107. The greater part of the district is underlain by Silurian rocks, consisting of mudstones and siltstones with sandstones and containing a significant proportion of slumped, or disturbed, strata. Carboniferous rocks, mainly limestones at outcrop, occur along the coast and on both sides and below the graben-like Vale of Clwyd, which structure preserves areas of Permo-Triassic sandstones. The lower ground, especially in the valleys and near the coast, is extensively covered by Quaternary deposits, the chief of which is boulder clay.

Topographically the district can be divided into six areas; the eastern slopes of the Conway Valley, the Denbigh uplands, the Carboniferous Limestone country between the Great Orme and Denbigh, the coastal fringe, the Vale of Clwyd and the Clwydian Hills. The Denbigh uplands, comprising most of the Silurian outcrop and including the Denbigh Moors to the south, form more than half of the district. The Denbigh Moors consist of high dissected ground generally between 300 m (1000 ft) and 425 m (1400 ft) above OD and reaching 518 m (1698 ft) on Gorsedd Brân. To the north, where the general level of the uplands is 150 to 300 m (500 to 1000 ft) OD, the dissection is deeper, leaving numerous isolated hills up to 395 m (1300 ft) OD usually composed, wholly or partly, of disturbed beds, which are more resistant to weathering than the undisturbed strata.

The Carboniferous Limestone outcrop west of the Vale of Clwyd forms a line of rounded hills, much quarried in the north, which reaches a high point of 204 m (668 ft) at Cefn yr Ogof near Llanddulas. The coastal fringe, most extensive between Abergele and Prestatyn, where it merges southwards with the Vale of Clwyd, is broken by the limestone cliffs of the Great Orme, Little Orme and Penmaen Head. It consists of low-lying ground underlain by marine, glacial and windblown drift deposits.

The Vale of Clwyd, between the Denbigh uplands and the Clwydian Hills, is underlain by Upper Carboniferous and Permo-Triassic rocks, largely obscured by drift. It is land of gentle relief, mostly lying below 46 m (150 ft). The north-east of the district includes a small part of the Clwydian Hills which consists of Carboniferous Limestone in the north, rising to 264 m (867 ft) on Moel Hiraddug, and Silurian rocks farther south, where the high point is 304 m (998 ft) on Mynydd y Cwm.

Most of the district drains into the River Clwyd, which flows north-north-westwards along its wide vale to join the sea at Rhyl. The Clwyd's chief tributaries are the Elwy, into which flows the Aled below Llanfair Talhaiarn, the Ystrad, the Nant Mawr and the Clywedog from the west, and the Wheeler from the east. The coastal slopes west of Abergele drain direct to the sea via a number of small streams including the Nant y Groes and the Dulas. The western edge of the district is drained by minor tributaries of the Conway, and a small area of the Denbigh Moors in the south-west of the district is part of the Dee catchment.

The soils of the district and their relationships to parent material have been described in detail by Ball (1960). The most widespread soils, covering much of the Silurian outcrop are the brown earths, but there are podsolized and organic soils and peaty gleys on the Denbigh Moors, calcareous soils on the Carboniferous Limestone outcrops, and gleys and immature soils along the coast (ibid., fig. 10). Away from the coast, with its tourism and light industry, agriculture is the main occupation, with the emphasis on livestock rearing, and the small family farm as the norm. Although grass is the predominant crop, arable farming is widespread on the lower drift-covered ground and forms an important part of the agricultural economy of the Vale of Clwyd, one of the most fertile parts of Wales, and the coastal region. The majority of farms have an entitlement to mountain grazing for sheep, and it is to sheep and, more recently in the south, to forestry that the Denbigh Moors are devoted.

The only industry based on the minerals of the district is limestone quarrying in the north, but formerly mining for lead, zinc, copper and iron was important. EGS, DP.

Geological history

The upper Ordovician deposits in the south-west, the oldest known rocks in the district, indicate a basinal marine environment at this time. Tuffaceous horizons show that this area was near the margin of the Caradocian volcanic province of Snowdonia to the west. EGS.

Towards the close of the Ordovician period a glaciation centred on North Africa led to a world-wide lowering of sea level and consequent widespread development of shelf deposits. Subsequently, deeper and probably warmer waters returned, in which graptolites soon flourished and in which the highest Ordovician and lowest Silurian (Llandovery) mudstones were laid down. Uncertain environmental changes in late Llandovery times led to deposition of the Pale Slates, but at the close of the Llandovery the quiescent offshore conditions in which graptolite mudstones were deposited re-established themselves. All subsequent Silurian sedimentation points to a generally east–west basin of unknown extent to the east and bounded on the west by a Precambrian–Lower Ordovician land-mass. The sedimentation was affected by penecontemporaneous movements on the Conway Valley Fault.

From early in the Wenlock turbidity currents flowing from the south, west and north-west deposited sandstones and some finer grained sediments, all of which periodically suffered submarine sliding and slumping. After a brief period in which mainly fine-grained sediments were laid down, slumping again took place and on an increasingly extensive scale, culminating in wholly disturbed middle Wenlock deposits (Llanddoget Formation), which are believed to have slumped towards the north-north-east. They extend throughout the district, although thinning markedly eastwards. In middle to early upper Wenlock times quiescent conditions prevailed in which far-travelled turbidity currents, possibly from the south, deposited calcareous siltstones, silty mudstones and laminated (graptolite) muddy siltstones in a poorly oxygenated environment. In late upper Wenlock times a more or less synchronous world-wide event—possibly a glacial episode—created extensive shelf conditions resulting in the deposition of limestones and calcareous mudstones. In the North Wales basin, as in others in Europe and as far afield as Nevada (USA), bioturbated shelly mudstones were laid down, reflecting relatively calm, oxygenated and rather shallower waters. These conditions were short-lived and the basin returned before the close of Wenlock times to a deeper water regime.

In end-Wenlock and early Ludlow times the basin became unstable, and extensive slumping, almost certainly down-slope from west to east, took place, some of the slumped beds just reaching the Clwydian Hills. At first only in the north of the district, but later sporadically elsewhere, sandstones were deposited by easterly-flowing turbidity currents, possibly originating from slumps. As Ludlow time passed, the basin filled up and the proportion of slumped and coarser grained sediments diminished. The youngest strata comprise distal turbidites, and current-oriented graptolites in the east point to the probable influence of northerly-flowing currents from a Central Wales basin.

The presence of upper Ludlow pebbles in the Carboniferous Basement Beds in the north of the district probably indicates that deposition continued into late Ludlow times or even beyond, as it did in both the Welsh Borderland to the south and the Lake District to the north. PTW.

There followed an episode of emergence and mountain building—the Caledonian orogeny—during which the Silurian and older rocks of the district were folded, faulted and cleaved. Devonian/Old Red Sandstone strata are absent from the district, as they are from the whole of North Wales except a small part of Anglesey. This was a period of erosion, lasting well into Lower Carboniferous times, when the sea returned from the north.

The earliest Carboniferous rocks, the Basement Beds, apparently represent fluvial and littoral deposits. They were followed by the deposition of Asbian (D ) limestones, which, especially in the west, show shallow-water phenomena including reefs, though there is evidence of frequent fluctuations of sea level, and shorelines presumably lay south of the district during most of this time. After withdrawal in late Asbian times, the sea returned in the Brigantian (D₂) and limestones of a deeper water facies were laid down in the east of the district.

Sedimentation may have been continuous from the Lower Carboniferous into the Namurian and possibly through to the Westphalian in the north-east around Rhyl and Prestatyn, but most of the district apparently lay outside the Namurian basin of deposition. The succeeding Westphalian basin was more extensive, but Coal Measures of grey facies were laid down only in the north; elsewhere, and probably only in the east, red measures, indicative of marginal or continental conditions, accumulated.

Earth movements during the Carboniferous came to a head at the close of the period with the Hercynian orogeny. Though not as violent in this part of the world as the preceding Caledonian orogeny, it was responsible for the appreciable folding and faulting displayed by the Carboniferous rocks, and it was at this time that the graben structure of the Vale of Clwyd was initiated. Metalliferous mineralisation, of which there had been an earlier phase during the Carboniferous, apparently accompanied the tectonism and most of the mineral veins affecting both the Lower Palaeozoic and Carboniferous rocks are believed to date from the Hercynian.

Another period of erosion followed, and the next rocks laid down in the district are probably of late Permian or early Triassic age. They are continental deposits, aeolian and fluviatile red sandstones, which were probably confined to the Vale of Clwyd and not much more extensive than they are today.

The subsequent, pre-Quaternary, history of the district is speculative, though faults affecting the Permo-Triassic rocks record the influence of mid-Tertiary earth movements. For other evidence we must look outside the confines of the district. Lower Jurassic rocks to the east in north Salop and to the west at Mochras Farm Borehole near Harlech, the latter sequence almost 900 m thick (Woodland, 1971, p. 4), show no signs of marginal deposition, and the Lias may have covered most of North Wales, including the Rhyl and Denbigh district. It is possible that late Triassic deposits as well as younger Jurassic and Cretaceous deposits were similarly extensive.

The Tertiary era saw uplift and prolonged erosion, the only vestiges of which are the supposed erosional platforms present at various heights across North Wales (Smith and George, 1961, pp. 78–80). By the end of the era the district must have broadly assumed its present form, but with the rocks thickly covered by the products of prolonged deep weathering, partly originating under tropical conditions. This weathered material probably contributed significantly to the tills of the succeeding ice ages.

The district must have suffered the effects of at least two glaciations, but this can be recognised only along the coast. Here, near Llandudno and Conway, Devensian till derived from the Irish Sea is separated from underlying till of North Welsh type by marine or estuarine deposits apparently laid down in the Ipswichian interglacial. Apart from excursions up the Conway and Clwyd valleys, ice advancing from the Irish Sea affected only the coastal belt. Elsewhere the locally derived North Welsh till cannot be divided and there is no evidence that it is not all attributable to the Devensian glaciation.

Associated with the melt stages of the ice were deposits of glacial sand and gravel and overflow and sub-glacial channels. The retreating ice left lakes dammed by moraines or the ice itself, and in the prevailing periglacial conditions, head, pingos and landslips were formed. Rivers re-established themselves and, with amelioration of the climate, vegetation gave rise to deposits of peat on the moors as well as on the low coastal flats. As sea level rose the exposed coastal peats became submerged forests, and marine and estuarine alluvium spread inland. The most recent deposits include beach sands and gravels and blown sand. EGS.

References

BALL, D. F. 1960. The soils and land use of the district around Rhyl and Denbigh. Mem. Soil Surv. G.B. 139pp.

SMITH, B. and GEORGE, T. N. 1961. North Wales. 3rd edition. Br. Reg. Geol., Inst. Geol. Sci.

WOODLAND, A. W. (Editor). 1971. The Llanbedr (Mochras Farm) Borehole. Rep. Inst. Geol. Sci., No. 71/18. 115pp.

Chapter 2 Ordovician

Included here with the Ordovician rocks occurring in the south-western corner of Sheet 107 are those cropping out farther west, as far as the River Conway (Figure 2). Nothing has been published on this area since the work of the earliest surveyors, but a field meeting report by Boswell (1935) covers the ground immediately to the south.

Some 480 m of Caradoc and possibly Ashgill rocks crop out in the area, but they occur in a number of fault-bounded blocks, and nowhere is there a complete sequence. Black mudstones and grey siltstones make up the bulk of the succession, with two volcaniclastic formations, one of pink acid vitric tuff and the other of greyish tuffaceous siltstone in which there is an admixture of volcanic and epiclastic material (Figure 2). A small penecontemporaneous fine-grained acid intrusion is also present.

Details

Extending north of Picnic Point [SH 8055 5629] as far as the Rhŷd-y-creua Fault (Figure 2) a thick sequence of strongly cleaved and locally bedded grey siltstones form the oldest Ordovician rocks exposed. The brachiopods (DT 7827–55) Dalmanella cf. horderleyensis and Howellites cf. antiquior obtained from around Picnic Point indicate that these beds are late Soudleyan or early Longvillian (Caradoc) in age and compare with the Allt-ddu Mudstones and Gelli-grin Calcareous Ashes of the Bala area (Bassett and others, 1966) and with a part of the Glanrafon sediments of Snowdonia below the Crafnant–Snowdon Volcanics (Davies, 1936; Diggens and Romano, 1968). Evidence for older sandstones and tuffs as shown in the vertical and horizontal sections of Sheet 107 was taken from incomplete surveys to the south and west.

In the steep-sided wooded area of Coed Hafod to the north-east of the Rhŷd-y-creua Fault there occurs the only complete succession through the higher Ordovician rocks (Figure 2). At the base of this sequence there is a massive aphanitic, greenish grey, vitric tuff with shards that are barely distinguishable in thin section. A good exposure [SH 8037 5724] of this rock is seen by the A496 road to the north-east of Rhŷd-y-creua Farm and there are further sections to the south-west and south-east of Gallt-y-rhŷg [SH 8073 5646] and [SH 8114 5630]. Neither the base nor the top of the tuff, however, is anywhere exposed. The overlying black mudstones, 15 m thick, are poorly exposed in old mineral workings adjacent to the Rhŷd-y-creua Fault, but better exposures [SH 8120 5640] occur in a fault-bounded area south-east of Gallt-y-rhŷg. Nine metres of cleaved grey to dark grey siltstones separate these black mudstones from further black mudstones, 37 m thick, which immediately underlie the main tuffaceous sediments of the area. The siltstones are exposed [SH 8046 5737] to the north-east of Rhŷd-y-creua Farm and in the fault-bounded blocks about Gallt-y-rhŷg, and the overlying black mudstones are best examined in the old mineral workings [SH 8059 5737] north-east of Rhŷd-y-creua Farm and in the hummocky ground at Gallt-y-rhŷg.

The main tuffaceous sediments of the area, 91 m thick, form the major scarp and wooded dip-slope of Coed Hafod. On lithological and stratigraphical evidence these rocks are to be correlated with the Upper Tuff Bed of Davies (1936) in the ground west of the River Conway. The absence at outcrop of the major part of the Crafnant Volcanic Series between the Glanrafon Beds and the Upper Tuff Bed (ibid., fig. 2, p. 65) suggests that the Rhŷd-y-creua Fault has a throw in excess of 370 m.

The tuffaceous sediments of Coed Hafod consist mainly of massive, cleaved, grey to dark grey, locally mottled, megacryst-bearing siltstones. They normally contain scattered shards and angular to rounded lithic clasts, the latter predominantly of dark mudstone, but also of acid volcanic rocks. Generally the sediments are coarser with a higher percentage of clastic material in their lower part. Bedding is usually absent or very poorly developed, and current bedding is rarely seen. Silicification is a fairly constant feature, with many of the exposures weathering white.

A passage from cleaved, grey, tuffaceous siltstones, through cleaved, grey to dark grey, tuffaceous silty mudstones, into overlying cleaved black mudstones, takes place over approximately 3 m of sediment. Accompanying the change in lithology is a change of cleavage from fracture cleavage in the tuffaceous siltstones to slaty cleavage in the black mudstones.

These black slaty mudstones are exposed along the eastern edge of Coed Hafod and to the north-east of Cilcenus [SH 8105 5837]. They are some 92 m thick and are correlated with the thicker (183 m) Llanrhychwyn Slates of the area to the west (Davies, 1936). A graptolite fauna (DT 7958–84) was obtained from an horizon towards the top of the mudstones in a digging [SH 8129 5749] south-east of Coed Hafod. This fauna, which is in a poor state of preservation, contains possible orthograptids, probably of the Orthograptus truncatus group.

Completing the Ordovician succession of the district is a thick sequence (152 + m) of grey to dark grey, locally micaceous siltstones and silty mudstones with, in places, thin bands of pale sandstone or dark grey to black mudstone. This siltstone formation, correlated with the (?Ashgill) Grinllwm Slates of Davies (1936), is poorly exposed in the gorge [SH 8133 5760] east of Coed Hafod. No evidence was found for the presence of the Trefriw Mudstones (ibid.) at the base of the siltstones, although the dark, massive chlorite-rich silty mudstone with pale speckling exposed in the roadside quarry [SH 8132 5763] east of Coed Hafod probably represents the fine-grained, lateral equivalent of the chlorite-pumice-tuff associated with the Trefriw Mudstones (ibid.). From this quarry the Chitinozoa (SAL 406), Conochitina conulus and Siphonochitina cf. pellucida were obtained.

To the north-west of Picnic Point [SH 8045 5645], there is a small acid intrusion apparently composed of a pale green to greenish grey rhyolite, usually cleaved. Cavities formed by the weathering of amygdales are often present on exposed surfaces and quartz veining is common. Vertical contacts with the surrounding siltstones are exposed in a number of places, and the siltstones show slight baking. Rafts and xenoliths of slightly baked siltstones are to be found in the intrusion, the rafts occurring mainly towards the south-western end. In thin section the igneous rock is seen to be composed of well-defined, devitrified, non-welded shards in a groundmass consisting of quartz and albite with a little white mica and chlorite. The shards are composed of multiple-twinned albite with quartz. No preferred orientation has been noted, but small discrete rounded fragments composed of aligned shards have been seen. In these fragments the shards exhibit compaction and possible welding, giving a eutaxitic texture. The fragments may, therefore, have been derived from an older welded tuff. Megacrysts have not been seen, but rounded amygdales of quartz and albite or chlorite are present.

This vitroclastic rock appears to be an intrusion in the form of a vertical pipe. However, to the west of the River Conway there is a rhyolitic vitric tuff, the base of which intrudes the underlying sediments in a complex manner (Francis and Howells, 1973). Although this tuff is at a different stratigraphical level, it suggests that the present intrusion might be an outlier of the intruding base of a similar tuff horizon. MJCN.

References

BASSETT, D A., WHITTINGTON, H. B. and WILLIAMS, A. 1966. The stratigraphy of the Bala district, Merionethshire. Q. J. Geol. Soc. London, Vol.122, pp.219–271.

BOSWELL, P. G. H. 1935. Report of Easter field-meeting 1935. North-western Denbighshire. Proc. Geol. Assoc., Vol.46, pp.193–202.

DAVIES, D. A. B. 1936. The Ordovician rocks of the Trefriw district (North Wales). Q. J. Geol. Soc. London, Vol.92, pp.62–90.

DIGGENS, J. N. and ROMANO, M. 1968. The Caradoc rocks around Llyn Cowlyd, North Wales. Geol. J., Vol.6, pp. 31–48.

FRANCIS, E. H. and HOWELLS, M. F. 1973. Transgressive welded ash-flow tuffs among the Ordovician sediments of NE Snowdonia, N. Wales. J. Geol. Soc. London, Vol.129, pp. 621–641.

Chapter 3 Silurian: Llandovery

General account

Included here are the Llandovery rocks of the adjacent districts to the west (Sheets 94 and 106). The total outcrop on both sides of the River Conway covers about 7.5 sq km (Figure 1).

In the north Llandovery rocks have been found in the Llandudno Junction area and are believed to exist around Llandudno, Llanrhos and Tywyn, but all four areas are completely covered by superficial deposits. West of the River Conway Llandovery strata extend southwards from Conway through Henryd to Caerhun. Here they pass under the Vale of Conway alluvium and, continuing south, are largely buried beneath the Denbigh Grits Group which is faulted against the Ordovician (Figure 1). East of the river and to the south of Llanrwst, Llandovery strata reappear [SH 8110 5890] near Hendre House and continue south-eastwards across the southern boundary of the district.

Apart from the work of Elles, little attention has been paid to the Llandovery rocks. The earliest surveyors of the Geological Survey recognised the presence of the uppermost part of the Llandovery, which they equated with the Tarannon Shales. These—part of the Pale Slates of this memoir—were mapped west of the River Conway, but were believed to be faulted out over much of the southern part of the district. A brief mention of the Tarannon Shales was given by Ramsay (1866), who stated that from Conway to Llanbedr they rested on Ordovician rocks as a result of unconformable overlap.

The only detailed research into the Llandovery rocks of the district was conducted around Conway by Elles (1896, 1909), who gave an account of the lithologies and graptolite zones present. She described (1909, p. 171) a conformable sequence about 92 m thick, which she named the Gyffin Shales. Included at the base of the Llandovery was the Conway Castle Grit (op. cit., p. 172) even though she agreed that its correlation was with the (Ordovician) Hirnant Limestone (op. cit., pp. 183–184). Elsewhere in the district the only publications on the Llandovery are brief notes by Boswell (1935, pp. 156, 176; 1949, pp. 61–62, 303–309, 353).

Around Conway there are at least 230 m of Llandovery strata, within which all the accepted graptolite zones have been shown to occur. Because of faulting, there is nowhere a complete succession at outcrop, and the sequence has had to be compiled from scattered boreholes and exposures.

The Llandovery is composed almost entirely of mudstones, which are shown as two units on the map—undivided Llandovery below and Pale Slates above.

In the north, beyond Llanrwst, the undivided Llandovery, about 112 m thick, consists dominantly of dark grey to grey, bedded to unbedded mudstones, which exhibit a poor cleavage in parts. Thin paler bands are present and there are sporadic silty and siltstone bands. Bioturbation in the form of Chondrites (Simpson, 1957) is a fairly common feature throughout the succession, usually occurring in bands (see below). The graptolites also are usually found in bands.

South of Llanrwst the undivided Llandovery comprises a thick sequence, estimated at 520 m, of grey and dark grey silty mudstones and siltstones with sporadic paler bands. Small-scale current bedding is present in some of the siltstones. Cleavage is usually developed and the rocks are thus poor slates. No marker bands and few fossils have been found in the south.

The Pale Slates, 105 to 118 m thick, are lithologically identical throughout the district. They consist dominantly of pale greenish grey, occasionally mottled, mudstones, which weather pale buff. A fracture cleavage is commonly present. Towards the base and top interbedded dark grey to grey mudstones are present. With an increase of dark bands these beds pass down into the undivided Llandovery and up into the basal Wenlock Benarth Flags (p. 15). Hence the lowermost and uppermost parts of the Pale Slates are referred to as passage beds.

The base of the Llandovery has been defined only in boreholes (pp. 191–192), being taken some 60 m above the top of the Conway Castle Grit where dark grey to grey silty mudstones with thin paler siltier bands and a few Ordovician trilobites give way to grey mudstones with graptolitic and bioturbated bands. The trilobites include Mucronaspis mucronata (DJL 437–8) and M. cf. olini (MJN 89), the latter from about 1.5 m below the top of the silty mudstones. From the graptolitic bands a Llandovery Glyptograptus persculptus Zone fauna was obtained.

Graptolites, the dominant and only diagnostic fauna, are fairly common except in the Pale Slates, where they are apparently absent outside the passage beds. They are usually present in the dark grey to grey mudstones, commonly occurring in graptolitic bands, some of which are similar to the laminated muddy siltstones in the higher Silurian strata (p. 44). Within bioturbated strata graptolites are very rare or absent. The graptolite zonation of the Llandovery rocks around Conway, based on comparisons with other areas outside North Wales, is given in (Figure 4)." data-name="images/P1000807.jpg">(Figure 3). In support of this study several thousand graptolite-bearing specimens were collected from the Conway area.

The bioturbation common throughout the Llandovery sequence of the northern area has not been studied in detail, but much if not all of it is clearly attributable to the trace-fossil Chondrites. Chondrites is most obvious in borehole cores, where it is seen to consist of small dark elongate bodies, the orientation of which is commonly parallel to bedding (Plate 2). These bodies may exhibit dendritic branching and were once circular in cross-section. There are two types, larger darker forms and smaller lighter ones (Plate 2), but both have the same general morphology. This is a common feature of Chondrites (Simpson, 1957, p. 484). Simpson (ibid., pp. 493–494) deduced that beds with Chondrites in the position of growth are of marine origin and accumulated rapidly without being subject to wave action and reworking. From its recorded occurrences throughout the geological column and the facies in which it occurs it may also be concluded that Chondrites is in the main an indicator of shallow water, both near and far from shore, although records from the Mesozoic and Tertiary suggest that it can also occur in deep water (Ekdale, 1977, p. 166).

From north to south across the district there is an overall thickening of the Llandovery strata beneath the Pale Slates from around 112 to 520 m. It is accompanied by a slight coarsening of the sediments, increasing evidence of current activity and decreasing bioturbation. In contrast the Pale Slates exhibit a slight decrease in thickness southwards, from 118 to 105 m. The commencement and cessation of Pale Slates sedimentation were coeval throughout the district (within Monograptus turriculatus Zone to top of Monoclimacis crenulata Zone).

To the south and south-east, across the Corwen (120) district, little is known about the Llandovery strata except in the region of the Derwen Anticline. Here the Pale Slates are well developed and again of the same age, but the underlying Llandovery is either very thin or absent (R.B. Rickards and K.A. Alder, personal communication). In the western Berwyn Hills of the Bala (136) district recent work by BGS has shown that thin graptolitic basal Llandovery mudstones overlie Ashgill siltstones with conformity. These mudstones are overlain disconformably by Pale Slates of the M. griestoniensis Zone. Pale Slates deposition appears to have continued into the M. crenulata Zone (A.W.A. Rushton, personal communication).

Farther to the east and south, beyond the Derwen Anticline, the lower undivided part of the Llandovery reappears (Wedd and others, 1927, 1929; Bassett and others, 1966), though there appear to be a number of non-sequences. Arenaceous sediments are locally present here towards the Derwen Anticline and again in the Oswestry (137) district, where there is a Shelly fauna. The Pale Slates are present throughout this southern area, although in the Wrexham (121) district at least, they rest unconformably on the lower beds. They are of similar age and lithology to the Pale Slates of the Denbigh district, but they contain shells as well as graptolites. In addition in the Oswestry district interbedded thin bands of hard fine-grained siliceous sandstones are present in the Pale Slates.

To the south-west of the present district Llandovery rocks are next encountered on the Lleyn Peninsula in the vicinity of Llanystumdwy [SH 4750 3850]. Here much of the Middle and Upper Llandovery is apparently exposed, black to grey mudstones being overlain by 'the shales of the Tarannon' (Fearnsides, 1912, pp. 205–207), which are probably Pale Slates, although they contain thin bands of grit.

To the west of Conway Llandovery rocks occur in Anglesey where Greenly (1919) believed that all the graptolite zones up to and including that of Monograptus sedgwickii were present in the dark mudstones of Parys Mountain. Nutt and others (1980) have indicated, however, that only the upper M. gregarius Zone and the M. convolutus and M. sedgwickii zones are present, the last containing thin tuff horizons.

Lithologically there are close similarities between the Llan dovery succession of Conway and that of the Lake District. In general the beds below the Pale Slates at Conway correlate with the Skelgill Beds of the Lake District—28 to 40 m thick according to Hutt (1974)—and the Pale Slates are coeval with the Browgill Beds (30 to 50 m thick).

In Llandovery times the Welsh trough was a subsiding area receiving clastic sediments. Within it however there appear to have been a number of subsidiary basins separated by positive areas which probably emerged from the sea at times. These rises took place along well defined tectonic axes. The present district evidently formed part of one of the minor basins—the North Wales basin.

The North Wales basin, which included the Lake District, was elongated in a general east-north-easterly direction and was bounded to the south and south-east by a ridge in the area of the Derwen Anticline. In the west its limit is uncertain but Anglesey does not appear to have been a part of it. Up to and including part of the M. turriculatus Zone the area of maximum sedimentation appears to have been in the extreme south of the district. Subsequently maximum sedimentation appears to have occurred over a much wider area, including the whole of the district.

As indicated by the presence of Chondrites (see above) the sedimentation of the undivided Llandovery below the Pale Slates in the north of the district probably took place under quiet, shallow-water, almost lagoonal, conditions, continuing from the uppermost Ordovician. In the south the undivided Llandovery rocks appear to have been deposited under normal basinal conditions. There is at present no direct evidence of the source of these sediments, but it seems likely that they came from the south. This is suggested by the presence of derived Upper Cambrian to Arenig microfossils (SAL 4097–4103) in the highest Ashgill rocks of Conway.

The pale greenish grey mudstones of the Pale Slates were deposited not only in the minor basins of the trough, but also on the surrounding shelf and over the interbasinal areas. Sedimentation of the Pale Slates commenced at the same time throughout North Wales and the Lake District and must reflect a sudden widespread change in sedimentation. In Central Wales the pale mudstones appear to occur at a slightly later date (Smith and Long, 1969, p. 241), but this is because coincident turbidite deposition, probably initiating from the same event, swamped the earliest pale mudstones. When turbidite deposition ceased the pale mudstones appeared and persisted as they did elsewhere until the close of the Llandovery.

This change of sedimentation occurring during the M. turriculatus Zone extended far beyond Britain (Strachan, 1969, p. 253), implying a major event of continental dimensions which can only have had a tectonic, volcanic or glacial origin. Despite evidence of local tectonic activity in south-west Wales (Cave, 1980), there is nothing to suggest that a major tectonic event was responsible. The high chlorite-content of the Pale Slates could suggest mud derived from volcanic material or fine-grained tuff deposits. A volcanic origin is unlikely, however, because of the substantial and consistent thickness of the Pale Slates over wide areas, their persistence for a long period of time represented by several graptolite zones, and the absence both of recognisable volcanic material or the passage of Pale Slates into known volcanic deposits.

It may be deduced that the Upper Llandovery transgression of the Welsh Borderland was the result of a rapid rise in sea level, though Ziegler and others (1968, p. 777) would deny this, preferring a gradual rise throughout the Llandovery. McKerrow (1979, fig. 1) later indicated a rise in sea level commencing in the M. turriculatus Zone. A rapid rise is consistent with the end of a glacial event, and Silurian glacial formations are known in South Africa and South America (Harland and Herod, 1975, p. 195). An Ashgill glaciation is known from North Africa (Destombes, 1971) and it is not impossible that this extended into the Silurian (see also McKerrow, 1979, p. 141).

If the rapid rise of sea level resulting from ice melting is accepted for the Welsh trough in Upper Llandovery times, the Pale Slates can be explained as muds derived from land masses of relatively low relief and deposited under reducing conditions. MJCN.

Palaeontology

Except for the problematical fossil Chondrites (p. 7), the only identifiable macro-organisms found in the Llandovery rocks of the district are graptolites. These occur throughout the sequence except in the Pale Slates between the lower and upper passage beds. In bioturbated strata graptolites are rare and fragmented, but in the remaining grey to dark grey mudstones they are fairly prolific. In the passage beds of the Pale Slates graptolites are only to be found in the grey to dark grey mudstone bands.

Preservation of the graptolites is generally good, especially in borehole cores. Those obtained from the fore-shore exposures of Benarth and from weathered sections are less well preserved, though still identifiable. Deformation of the specimens has not proved to be a problem in identification. Many of those obtained from boreholes are excellent three-dimensional specimens preserved in pyrite. Other specimens have been preserved as flattened carbonised periderms.

The eleven graptolite zones used in this account (Figure 4)." data-name="images/P1000807.jpg">(Figure 3) and (Figure 4) are those generally accepted for the Llandovery of the British Isles (Cocks and others, 1971) and the faunas collected have been assigned to zones on the basis of work elsewhere. However, an attempt has been made to show the ranges of species within the district (Figure 4).

Within the district the following zonal assemblages have been found:

Glyptograptus persculptus ZONE — Climacograptus innotatus subsp., C. medius, C. miserabilis, C. normalis, C. rectangularis, Diplograptus modestus, G. persculptus

Orthograptus acuminatus ZONE — Akidograptus ascensus, C. medius, C. normalis, C. rectangularis, D. modestus, O. acuminatus, O. acuminatus praematurus, "O. truncatus" ( = "O." illustris Koren & Tzaj; A Kearsley det.).

Monograptus atavus ZONE — C. rectangularis, Dimorphograptus decussatus, Diplograptus modestus diminutus, M. atavus, M. revolutus, M. cf. strachani, Pseudoclimacograptus hughesii

Monograptus typhus ZONE — C. normalis, C. rectangularis, ?Cystograptus vesiculosus, D. modestus diminutus, Glyptograptus sinuatus, G. tamariscus (s.l.), M. atavus, M. cyphus, M. revolutus, M. strachani, Orthograptus cf. bellulus, P. hughesii, Rhaphidograptus toernquisti

Monograptus gregarius ZONE — Climacograptus normalis, C. rectangularis, C. cf. scalaris, Diplograptus magnus, Glyptograptus cf. enodis, G. sinuatus, G. tamariscus (s.l.), G. tamariscus distans, G. vas, M. argenteus, M. argutus, M. atavus, M. communis (s.l.), M. communis communis, M. communis rostratus, M. convolutus, M. cyphus, M. denticulatus, M. fragilis, M. gregarius, M. cf. jonesi, M. leptotheca, M. lobiferus, M. millipeda, M. pseudoplanus, M. revolutus (s.l.), M. triangulatus (s.l.), M. triangulatus fimbriatus, M. triangulatus triangulatus, Petalograptus ovatoelongatus, Pristiograptus concinnus, P. regularis, Pseudoclimacograptus hughesii, P. retroversus, P. undulatus, Rastrites hybridus, R. longispinus, R. aff. peregrinus, Rhaphidograptus toernquisti

Monograptus convolutus ZONE — Cephalograptus cometa, Climacograptus rectangularis, C. scalaris, Glyptograptus elegans, G. incertus, G. tamariscus (s.l.), G. tamariscus distans, G. tamariscus tamariscus, Monoclimacis crenularis, ?Monograptus angustus, M. argenteus, M. argutus, M. clingani, M. communis (s.l.), M. convolutus, M. decipiens, M. denticulatus, M. gregarius, M. cf. jonesi, M. knockensis, M. leptotheca, M. limatulus, M. lobiferus, M. millipeda, M. triangulatus (s.l.), M. triangulatus fimbriatus, O. cf. insectiformis, Petalograptus cf. minor, P. ovatoelongatus, Pristiograptus concinnus, P. regularis, Pseudoclimacograptus retroversus, P. undulatus, Pseudoplegmatograptus obesus, Rastrites aff. fugax, R. hybridus, R. longispinus, R. spina, ?Retiolites perlatus

Monograptus sedgwickii ZONE — Cephalograptus cometa, Climacograptus cf. simplex, G. incertus, G. tamariscus (s.l. ), ?M. angustus, M. argutus, M. capillaris, M. decipiens, M. knockensis, M. lobiferus, M. marri, M. pragensis ruzickai, M. sedgwickii, M. tenuis, Petalograptus altissimus, P. intermedius, P. kurcki, P. cf. minor, P. ovatoelongatus, Pristiograptus concinnus, P. jaculum, P. regularis, Pseudoclimacograptus retroversus, P. undulatus, Pseudoplegmatograptus obesus, Rastrites hybridus

Monograptus turriculatus ZONE — G. tamariscus (s.l. ), Monoclimacis galaensis?, Monograptus exiguus, M. gemmatus, M. halli, M. lobiferus, M. marri, M. nodifer, M. proteus, M. planus, M. pseudobecki, M. runcinatus, M. sedgwickii, M. cf. speciosus, M. turriculatus, Petalograptus altissimus, P. cf. minor, P. ovatoelongatus, P. wilsoni, Pristiograptus nudus, P. regularis, Pseudoclimacograptus undulatus, Pseudoplegmatograptus obesus, R. cf. distans, R. maximus

Monograptus crispus ZONE — Monoclimacis galaensis?, Monograptus crispus, M. dextrorsus, M. exiguus, M. gemmatus, M. marri, M. nodifer, M. proteus, M. runcinatus, M. turriculatus, Petalograptus altissimus, P. cf. ovatus, P. palmeus, Pristiograptus nudus, R. cf. distans

Monoclimacis griestoniensis ZONE — M. griestoniensis, Monograptus discus, M. priodon, M. spiralis, M. tullbergi, Petalograptus altissimus, Pristiograptus nudus

Monoclimacis crenulata ZONE — M. crenulata sensu Elles and Wood, Monograptus discus, M. nodifer, M. parapriodon, M. priodon, M. spiralis, M. tullbergi, Petalograptus altissimus, Pristiograptus nudus MJCN, RBR

The sequence of Llandovery graptolite faunas

The sequence begins with the G. persculptus Zone assemblage of biserial genera such as Climacograptus, Diplograptus, and Glyptograptus. Uniserial genera such as Dicellograptus, typical of the underlying Ordovician rocks, are absent, and the monograptids identified in other areas (Rickards and Hutt, 1970) have not yet been found in the district. Faunas containing abundant Climacograptus, Diplograptus and Glyptograptus, but lacking uniserial genera or genera with attenuated proximal ends (Akidograptus), probably indicate the G. persculptus Zone. The association of G. persculptus, a species not restricted to the G. persculptus Zone, with D. modestus, C. miserabilis, C. medius and C. normalis is a more specific indication of the zone.

The succeeding O. acuminatus Zone has a distinctive fauna, striking elements of which are Akidograptus ascensus and O. acuminatus. Biserial graptolites with attenuation of the proximal end region also occur at higher horizons (Dimorphograptus, Cephalograptus) but are there always associated with abundant monograptids. Otherwise the fauna of the O. acuminatus Zone is not dissimilar to that of the G. persculptus Zone.

In the succeeding M. atavus Zone the faunas take on distinctive new aspects with little trace of transitional elements from the Ordovician. In particular there are several monograptid species, the most common and widespread of which is M. atavus. Biserials such as C. normalis and C. miserabilis are still common, but form a lower percentage of the whole fauna, whilst a number of genera such as Dimorphograptus and Rhaphidograptus, with protracted proximal regions are found for the first time. In evolutionary terms it may well be that these latter genera represent subsequent abortive attempts by the biserial stock to produce more monograptids. The stratigraphical value of these relatively short-lived species, such as D. confertus, is high, but the nature of their environment and the mode of life is not known. The number of species in the M. atavus Zone is about 30, compared with 12 and 18 in the G. persculptus and O. acuminatus zones respectively, reflecting the tremendous diversification of the early Llandovery graptolites.

Above the three basal zones are a further eight, one of which (M. crispus) was identified by Elles (1909) but has not been confirmed in the district during the present survey. These are for the most part assemblage zones. They are arbitrarily defined, but they depend upon a framework of major natural events in the evolving graptolite faunas. The events and their stratigraphical significance are enumerated below.

1 The appearance of Glyptograptus tamariscus and other species such as G. cf. nickoleyevi in the M. cyphus Zone marks the beginning of a complex diversification of glyptograptids (Packham, 1962), which can be contrasted with the earlier G. persculptus plexus, essentially an 'Ordovician remnant'. The peak occurrence of Dimorphograptus species is probably also at this level.

2 The M. cyphus Zone is typified by the first appearance of biform monograptids such as the austerus and sudburiae groups (Hutt, 1974) as well as by the continued expansion of coronograptids like M. acinaces, M. cyphus and M. gregarius and of the G. tamariscus group.

3 Perhaps the most striking Llandovery graptolite event is the appearance at the base of the M. triangulatus Subzone (M. gregarius Zone) of the M. triangulatus group followed quickly by the genera Rastrites and Petalograptus. All these graptolites have a protracted proximal region, which in M. triangulatus and Rastrites takes the form of isolate metathecae and thread-like prothecae.

4 Typically, the cladia-bearing genera occur in the highest Llandovery and Wenlock rocks, but the Diplograptus magnus Subzone of the M. gregarius Zone sees their first appearance with Silurian Diversograptus ramosus or like species. Diplograptus magnus is abundant in this subzone, being separated, by a gap of at least one zone from earlier Diplograptus spp. The first Monograptus sensu stricto (Rickards, 1974) appears in the D. magnus Subzone, as do rare early representatives of the other important Silurian genera, Pristiograptus and Monoclimacis, although slender species of the latter genera may originate somewhat earlier.

5 The M. argenteus Subzone fauna, as well as being characterised by peculiar biform species such as M. argenteus and M. limatulus contains the first robust pristiograptids of Llandovery type (P. jaculum, P. concinnus) and the first robust monograptid (M. lobiferus). At the same level the first Silurian retiolitids appear after a known gap of at least eight zones, though they are rare. Species like Retiolites perlatus almost certainly evolved directly from biserial graptolites of the D. magnus type with a complete periderm.

6 In the M. convolutus Zone M. lobiferus is extremely abundant and is accompanied by other species of Monograptus s.s., whilst the Llandovery kind of Pristiograptus, including P. regularis and P. jaculum, is well established. M. convolutus itself is strongly coiled in a planispiral, and there is a general increase at this level in the number of spirally coiled graptolites, which persist until the Wenlock. Cephalograptus cometa, an extreme derivative of Petalograptus with a very elongate proximal region, is characteristic of the zone. It is represented in the succeeding M. sedgwickii Zone by the subspecies C. c. extrema, which probably has the longest thecal tubes, at 30 mm, of any known Silurian graptolite.

7 A further quite dramatic expansion of graptolites takes place in the M. sedgwickii Zone, where about 50 British species are recorded. M. sedgwickii is abundant and widespread, but it occurs first, though rarely, at the top of the M. convolutus Zone. This was the most robust species then developed in the Llandovery rocks, reaching a length of 1 m, and having strongly spinose thecae. These features are typical of the Monograptus s.s. stock throughout the rest of the Llandovery and the Wenlock.

8 Above the M. sedgwickii Zone the robust monograptids are represented by a succession of species including M. halli, M. marri, and M. priodon, the later forms persisting into the Wenlock. The lower half of the M. turriculatus Zone sees the acme and demise of bizarre rastritids such as Rastrites maximus, R. linnaei and R. distans, all of which have extremely long thecae and thread-like prothecal parts. The genera Rastrites and Glyptograptus have not been recorded above this zone, and Climacograptus extends only as high as the Monoclimacis griestoniensis Zone. As well as the firm establishment of robust monograptids from the Monograptus sedgwickii Zone upwards, an expansion of those monograptids with 'lobate' thecae takes place in the M. turriculatus Zone and persists until the basal Wenlock: such species as M. runcinatus, M. pseudobecki, M. nodifer and M. crispus occur at successively higher horizons.

9 The Monoclimacis griestoniensis and M. crenulata zones maintain the high proportion of monograptids of priodon type, but in addition are characterised by large numbers of specimens and several species of the genus Monoclimacis, the peak of which spans these highest Llandovery levels and the basal Wenlock rocks. Of particular stratigraphical value are M. griestoniensis, M. vomerina and M. crenulata sensu Elles and Wood.

10 Finally, the rare occurrence of the genus Cyrtograptus in the highest Llandovery rocks, both in this country and abroad, foreshadows the expansion of this cladia-bearing genus in the Wenlock alongside the persisting Monograptus and Monoclimacis stocks. RBR

Details of stratigraphy

Northern area, west of River Conway (Sheets 94 and 106)

In the drift-covered ground south of Conway, boreholes show an apparently conformable sequence from the Conway Castle Grit through Ordovician mudstones into the basal Llandovery sediments.

In five boreholes, Colcon 624, 625, 626, 634 and 634A (pp. 191–193), sited to the north of the Afon Gyffin, dark grey to grey bedded mudstones with graptolitic bands and bands of bioturbation have yielded the following composite fauna (DJL 54–64, 112–70, 197–288, 309–18, 319–404) indicative of the basal Llandovery G. persculptus Zone: Climacograptus innotatus, C. medius, C. miserabilis, C. normalis, C. rectangularis?, ?Diplograptus modestus, Glyptograptus persculptus.

Similar mudstones, but without bands of bioturbation, from Colcon 624A Borehole (p. 192) yielded (DJL 69–111): Akidograptus ascensus, ?C. medius, C. normalis, Glyptograptus avitus?, Orthograptus acuminatus, O. acuminatus praematurus, O. illustris, indicative of the O. acuminatus Zone. Above 8.50 m the borehole yielded (DJL 65–8) Dimorphograptus decussatus, indicating the M. atavus Zone. Elsewhere in the northern area this sequence is unknown. Bryn Dowsi Borehole [SH 7729 7602], sited just below the base of the Pale Slates, proved some 73 m of mudstones (pp. 188–189) dipping north-eastwards at 30 to 65°. All the graptolite zones from M. cyphus to M. sedgwickii inclusive are present. The M. cyphus Zone is represented by the following fauna (PT 2594–661) obtained between 118.75 and 120.10 m from the surface: Climacograptus cf. normalis, C. cf. rectangularis, ?Cystograptus vesiculosus, Dimorphograptus sp., Glyptograptus cf. sinuatus, G. tamariscus s.l., Monograptus cf. atavus, M. cyphus, M. revolutus s.l., Pseudoclimacograptus cf. hughesi, Rhaphidograptus toernquisti.

There is no reason to believe that the M. cyphus Zone does not continue beyond the bottom of the hole at 135.52 m. Below 120.10 m the only identifiable graptolites obtained were (PW 2696) Climacograptus normalis and (PW 2698) ?Rhaphidograptus toernquisti from 132.20 and 133.20 m respectively.

Fault-breccia separates the M. cyphus Zone from the overlying M. triangulatus Subzone. The latter is represented by a fauna (PW 1994–2584) obtained between 86.30 and 116.38 m: ?Climacograptus alternis, Glyptograptus sinuatus, G. tamariscus, ?G. (Pseudoglyptograptus) sp., Monograptus atavus, M. argutus?, M. communis communis, M. communis rostratus, ?M. cyphus, M. triangulatus fimbriatus, M. fragilis fragilis, M. gregarius, M. revolutus, ?M. aff revolutus, M. triangulatus triangulatus, Orthograptus cf. cyperoides, Petalograptus ovatoelongatus, ?Pristiograptus concinnus, Pseudoclimacograptus hughesii, P. retroversus, P. undulatus, Rastrites longispinus, Rhaphidograptus toernquisti.

From 77.00 to 86.30 m the Diplograptus magnus Subzone of the M. gregarius Zone is represented by (PW 1549–1993): Climacograptus sp.sp. Diplograptus magnus, D. sp.nov, Glyptograptus sinuatus, G. tamariscus, G. vas, Monograptus argutus, M. communis?, M. denticulatus?, M. triangulatus fimbriatus, M. fragilis, M. gregarius, M. pseudoplanus, M. aff. revolutus, M. triangulatus s.l., Petalograptus ovatoelongatus, Pristiograptus concinnus, Pseudoclimacograptus retroversus, P. undulates, Rastrites longispinus, R. aff. peregrinus, Rhaphidograptus toernquisti.

From 69.80 to 76.80 m the following fauna (PW 1460–548), believed to indicate the M. argenteus Subzone of the M. gregarius Zone, occurs between faults: Climacograptus scalaris, C. ex gr. rectangularis, Glyptograptus tamariscus distans, Monograptus communis?, M. convolutus?, M. gregarius, M. leptotheca, M. cf. lobiferus, Petalograptus aff. ovatoelongatus, Pristiograptus concinnus?, Rastrites sp.

From 32.50 to 69.50 m the following M. convolutus Zone fauna (PW 886–1459) was obtained:.Cephalograptus cometa, Climacograptus rectangularis?, C. scalaris, Glyptograptus elegans?, G. cf. incertus, G. tamariscus s.l., Monoclimacis crenulari s, ?Monograptus angustus, M. aff. argutus, M. clingani, M. aff. communis, M. convolutus, M. decipiens, M. denticulatus, M. gregarius?, ?M. gregarius mini sculus, M. cf. involutus, M. knockensis? (sensu Rickards, 1970), M. leptotheca, M. limatulus, M. lobiferus, M. aff. revolutus, M. triangulatus fimbriatus s. s., Orthograptus cf. insectiformis, Petalograptus cf. minor, P. aff. ovatoelongatus, Pristiograptus concinnus, P. regularis?, Pseudoclimacograptus retroversus, P. undulatus, Pseudoglyptograptus sp., ?Pseudoplegmatograptus obesus, Rastrites aff. fugax, R. cf. longispinus, R. spina, ?Retiolites perlatus.

Between 22.80 and 32.50 m the beds are barren, but between 22.80 and 5.79 m the following fauna (PW 628–864), indicative of the M. sedgwickii Zone, was obtained: Climacograptus sp., Glyptograptus incertus, Monograptus sedgwickii, M. pragensis ruzickai, M. tenui s, Petalograptus intermedius?, P. cf. kurcki Pristiograptus concinnus?, P. cf. regularis, ?Pseudoclimacograptus retroversus.

Above 5.79 m no core was recovered.

North-east of Gyffin Church [SH 7770 7698] in the north bank of the old Mill Race a small exposure of bedded grey mudstones yielded (RV 9009–22) Climacograptus ex gr. normalis, suggesting a pre- M. gregarius Zone horizon. This locality is apparently identical to the Pont Gyffin locality of Elles (1909, p. 185). In addition to C. ex gr. normalis (see below) she recorded Diplograptus modestus, C. rectangularis, G. persculptus and Pseudoclimacograptus hughesii. Material from her collection in the Sedgwick Museum, Cambridge, referred to as C. rectangularis and C. normalis, has been re-identified as C. cf. rectangularis and C. ex gr. normalis respectively. There is no confirmatory evidence for the other species recorded.

Dark grey to grey silty mudstones exposed in a small quarry [SH 7756 7646] south-west of Bryn Glorian Mawr have yielded (RV 9023–40) a ?chonetid and ?Climacograptus normalis. This quarry is one of the Bryn Glorian Mawr localities of Elles (1909, p. 185). She recorded a large fauna, now lost, similar in many respects to those found in the quarries near Bryn Dowsi (see below) for which she deduced a M. gregarius Zone horizon.

Elles also collected a graptolite fauna from a wood said to be to the north of Bryn Glorian Mawr (1909, p. 185), but probably to the north of Bryn Glorian Bach [SH 7737 7641], where dark grey mudstones are exposed on a scarp below the Pale Slates outcrop. Graptolites from this collection have been identified as Monograptus limatulus and Rastrites peregrinus, suggesting the M. convolutus Zone.

Dark grey to black mudstones, probably of M. convolutus Zone age, are exposed below the Pale Slates in the gorge [SH 7735 7662] of the Afon Gyffin, but no fossils have been collected here. Thinly bedded dark grey mudstones exposed on the hillside [SH 7726 7603] just above the old quarry at the base of the scarp north-west of Bryn Dowsi yielded (RV 9084–9125): Climacograptus sp., Glyptograptus tamariscus tamariscus, Monograptus lobiferus, Pristiograptus sp., Pseudoclimacograptus retroversus, Rastrites sp. andseveral fragments of a triangulate monograptid. The abundance of P. retroversus suggests the M. convolutus Zone and this is supported by Bryn Dowsi Borehole, which was drilled just to the east, beyond the top of the scarp.

In the old quarry immediately north-west of Bryn Dowsi cleaved grey mudstones, weathering brown and containing two thin hard bands, have yielded the following fauna (RV 9041–83 and Zp 92–4), which is referred to the basal part of the M. sedgwickii Zone: Climacograptus cf. simplex, ?Monograptus capillaris, M. decipiens, M. lobiferus, Petalograptus ovatoelongatus, Pristiograptus jaculum, Pseudoclimacograptus undulatus, Pseudoplegmatograptus obesus, Rastrites aff. hybridus and fragments of triangulate monograptids. The majority of the last are probably M. decipiens, but some may be M. communis rostratus. Elles (1909, p. 185) recorded C. rectangularis, G. tamariscus, M. fimbriatus, M. revolutus and Pseudoclimacograptus [C.] hughesii, but these are probably misidentifications of species in the above list.

Around and to the south-south-west of Plas Iolyn [SH 7719 7545], there are numerous small exposures of grey silty mudstones, some of which weather brown. They have failed to yield identifiable graptolites, but are referred to the Llandovery below the Pale Slates on the basis of their general lithology and the presence of Chondrites.

West of Henryd two small exposures [SH 7653 7465] and [SH 7663 7468] of cleaved grey silty mudstones have yielded no fossils to date, but the following fauna (ON 157–78) was collected from similar silty mudstones in a pipe trench [SH 7657 7465] between them: Climacograptus cf. rectangularis, Glyptograptus sp., ?Monograptus argenteus, M. denticulatus, M. cf. jonesi, M. cf. lobiferus, M. millipeda, M. triangulatus s.l., ?Pristiograptus regularis, Pseudoclimacograptus retroversus, P. undulatus, Rastrites cf. hybridus, R. cf. peregrinus. This fauna is probably of the M. argenteus Subzone, but may indicate the base of the M. convolutus Zone.

Cleaved dark grey to grey bedded mudstones occur in the gorge beneath the bridge at Pontwgan [SH 7665 7082], and are faulted against cleaved grey mudstones with Chondrites exposed downstream. A little farther downstream cleaved thinly bedded grey to dark grey siltstones appear to be overlain by the Pale Slates.

The junction between the undivided Llandovery and the lower passage beds of the Pale Slates is visible in only one place in the entire area—an old quarry [SH 7743 7570] by the footpath east of Gyffredin:

No fossils were found. The sections of Elles (1896, pp. 279–280) are no longer exposed. The junction has also been seen at 5.55 m in Colcon 625A Borehole (p. 192) south of Conway. Here the lower passage beds are underlain by grey to dark grey mudstones with paler bands and a 2-cm siltstone at 6.20 m. Bands of bioturbation are common. The following fauna (DJL 171–196) was collected between 6.0 and 9.7 m: Glyptograptus tamariscus s.l., Monograptus exiguus, M. cf. halli, M. cf. lobiferus, M. nodifer, M. planus, ?M. pseudobecki, M. cf. sedgwickii, M. turriculatus, ?Petalograptus minor, P. ovatoelongatus, P. wilsoni, Pristiograptus regularis, Rastrites maximus. This fauna is indicative of the Rastrites maximus Subzone in the Zone of Monograptus turriculatus. The R. maximus localities of Elles (1909, pp. 185–186) are no longer exposed.

The most complete sequence through the Pale Slates anywhere in the district is that exposed across the foreshore [SH 7860 7732] to [SH 7867 7718] at Benarth, to the east of Benarth Lodge:

Elles (1909, p. 185) reported that M. sedgwickii mudstones were exposed at low water below the above succession towards the mouth of the Afon Gyffin at exceptional spring tides. The whole section below high water mark is deeply weathered along both cleavage and bedding. Fossils are poorly preserved and difficult to find. Those collected from the lower passage beds elsewhere in the vicinity indicate the zones of Monograptus turriculatus, Monoclimacis griestoniensis and M. crenulata. Elles (1909, p. 186) reported the presence of the Monograptus crispus Zone from a locality apparently in the lower passage beds, and collected a Monoclimacis crenulata Zone fauna from beds now referred to the upper passage beds. Neither fauna was confirmed by recent collecting.

Colcon 632 Borehole (p. 193) on the west side of the Afon Gyffin south of Conway penetrated, beneath 9.00 m of drift, 4.80 m of the lower passage beds, consisting of pale greenish grey mudstones with sporadic dark grey laminae and minor bioturbation. A dark mudstone band 12.30 to 13.00 m below the surface yielded (DJL 289–308): Monograptus cf. marri, M. proteus, M. cf. runcinatus, Petalograptus wilsoni?, Pristiograptus cf. nudus, Rastrites cf. distans. This fauna is probably high in the Monograptus turriculatus Zone.

The following fauna (KR 3284–321) was collected from the lower passage beds exposed in a bank [SH 7843 7732] near the entrance to the Mussel Purification Station on Benarth Road, south of Conway Castle: Climacograptus sp Monoclimacis griestoniensis, Monograptus discus discus, M. discus s.l., M. priodon s.l., M. spiralis, ?M. tullbergi, Pristiograptus nudus, Pseudoplegmatograptus sp.This fauna is indicative of the Monoclimacis griestoniensis Zone.

Further small exposures [SH 7844 7732] of the lower passage beds in the vicinity yielded the following fauna (GF 7870–908 and KR 3245–83) indicative of the Monoclimacis crenulata Zone: ?Glyptograptus sp., ?Monoclimacis crenulata (sensu Elles and Wood), Monograptus nodifer s.l., M. parapriodon, M. priodon, M. spiralis, Petalograptus altissimus, Pristiograptus nudus, Retiolites (?Pseudoplegmatograptus) sp.

The beds yielding this fauna appear to be lower than those yielding the Monoclimacis griestoniensis Zone fauna, and a small fault may therefore exist between the two localities. However the M. crenulata Zone locality equates in part with the section described by Elles (1896, p. 208) as being opposite the forge and belonging, at least in part, to the Monograptus crispus Zone (Elles, 1909, p. 186), which is not otherwise recorded and must be thin. Likewise the overlying Monoclimacis griestoniensis Zone must also be thin at this locality. Alternatively, if no fault exists between the M. griestoniensis and the M. crenulata Zone localities, the M. crenulata Zone fauna collected (see above) ranges down into the Monograptus crispus Zone.

No other fossil localities are known within the Pale Slates of this area, even though the formation is fairly well exposed. North of Coed Bryn Glorian the lower passage beds are to be seen on the east side of the Afon Gyffin gorge [SH 7733 7653] and to the north-west [SH 7721 7657] across the Afon Gyffin. The Pale Slates are exposed around and to the south-west of Bryn Glorian Bach and to the north of Bryn Dowsi.

The full sequence of the Pale Slates appears to be present on the northern limb of a gentle syncline to the east of Plas Iolyn. The basal part has already been described (p. 15), the middle part may be seen in a scarp [SH 7739 7543], and the upper passage beds are exposed [SH 7736 7526] to the south-east of Plas Iolyn. On the southern limb of the syncline the middle part of the Pale Slates is seen to the northeast of the Rectory [SH 7748 7498]; the upper passage beds are apparently faulted out.

From east of the Groes Hotel [SH 7785 7400] to the north-west of Eiranws [SH 7750 7355], and from the east to the south-west of Gwernborter [SH 7677 7302] to [SH 7605 7250] the Pale Slates are seen in several places. The most southerly exposures of the Pale Slates west of the River Conway occur [SH 7700 7070] between Pontwgan and Caerhun. An X-ray diffraction analysis (NEX 1866) of a sample of Pale Slates from an exposure in the bank south-east of Pontwgan [SH 7689 7062] showed it to be predominantly chloritic with some muscovite and quartz. There was no trace of montmorillonite nor of any expanding lattice clay mineral.

Northern area, east of River Conway (Sheet 94)

Llandovery strata are believed from indirect evidence (p. 188) to occur beneath the drift-covered ground to the north of Bodafon Hall Farm [SH 8040 8175], at Tywyn [SH 7830 7890] and around Llanrhos [SH 7890 8000]. The only recorded occurrence of Llandovery mudstones below the Pale Slates is that provided by Colcon 614 Borehole (p. 191) to the south-west of Llandudno Junction station. Here 5.0 m of grey mudstones with paler grey laminations were seen below thick drift deposits.

Pale Slates have been proved beneath drift in two boreholes, Colcon 409 and 409C (pp. 189–190), west of Llandudno Junction station. In both boreholes pale greenish grey mudstones contain thin dark bands, with evidence of bioturbation in Colcon 409. This lithology is that of the passage beds, probably the upper, but no fossils were obtained from the cores.

From the evidence of all the boreholes in the Llandudno Junction area it is evident that the Pale Slates have a south-easterly dip and will therefore extend south-westwards across the Conway Estuary beneath the drift towards Benarth. To the north-east the Pale Slates are faulted against higher Silurian strata proved in the Llandudno Junction Borehole (pp. 61 and 195).

Southern area (Sheet 107)

A full conformable sequence is thought to be present around Moel Trefriw [SH 8201 5670], though the base has not been seen. The oldest Llandovery sediments seen during the present survey are exposed in a small roadside quarry at Pen-y-ffridd [SH 8186 5609] just beyond the southern boundary of Sheet 107. They consist of cleaved grey siltstones with thin (less than 1 cm) pale bands and thicker (up to 8 cm) hard darker bands. The section has yielded (DT 7985–8006): Dimorphograptus sp., Diplograptus modestus diminutus, Glyptograptus tamariscus s.l., Monograptus cf. revolutus and M. cf. strachani, a fauna indicative of the M. atavus to M. cyphus zones.

At a slightly higher horizon thinly bedded grey silty mudstones with thin laminated muddy siltstone bands (graptolite shale) are exposed in the roadside [SH 8179 5609] to the west-north-west of the Pen-y-ffridd quarry. They have yielded (RV 3160–80): Climacograptus cf. normalis, C. cf. rectangularis, Diplograptus modestus diminutus, Glyptograptus? and Orthograptus cf. bellulus, an assemblage probably indicating the M. cyphus Zone.

Some 3 m of interbedded grey and dark grey mudstones, smashed and contorted in part because they are adjacent to the Soflen Fault, are exposed in the northern bank of the Afon Gallt-y-rhŷg [SH 8144 5799] upstream from Oaklands. They have yielded (DT 7937–57): Climacograptus normalis, ?Diplograptus magnus, Glyptograptus cf. enodis and Petalograptus sp., an assemblage suggesting the M. gregarius Zone, though it could be younger.

Pale Slates are exposed at several places in the lower part of the scarp between Hendre House [SH 8136 5885] and Coed Soflen [SH 8190 5775]. X-ray diffraction analysis (NEX 1867) of a sample of Pale Slates from Coed Soflen [SH 8207 5762] showed it to be mineralogically identical with that obtained in the northern area (see above).

To the south-east the formation is seen at only one place [SH 8231 5700] between Coed Soflen and Bryn y fawnog. In the latter area [SH 8285 5645], however, it is well exposed though the top is faulted out. The basal 2 m or so are greyer and siltier than elsewhere and contain thin bands of siltstone and dark mudstone. This basal horizon, exposed in a small quarry at Nant Uchaf [SH 8231 5700] has yielded (DT 7922–36): Monograptus marri, Petalograptus cf. altissimus, and Pristiograptus? [fragment], indicating the M. turriculatus or possibly the M. crispus Zone.

The lower part of the Pale Slates above the basal section consists of pale greyish green, occasionally mottled, mudstones with bands, up to 3 cm thick, of grey or dark grey mudstone which become less common upwards. Throughout the main mass of the Pale Slates there is only the occasional dark mudstone band and a complete absence of fossils. The top of the Pale Slates is seen in Coed Soflen [SH 8197 5772]. With the incoming of dark grey mudstone and laminated silty mudstone bands, there is a passage over a few metres into the mudstones of the Denbigh Grits Group (p. 72). MJCN

References

BASSETT, D. A., WHITTINGTON, H. B. and WILLIAMS, A. 1966. The stratigraphy of the Bala district, Merionethshire. J. Geol. Soc. London, Vol.122, pp.219–271.

BOSWELL, P.G. H. 1935. The geology of North-western Denbighshire. Proc. Geol. Assoc., Vol.46, pp.152–186.

BOSWELL, P.G. H. 1949. The Middle Silurian Rocks of North Wales. (London: Edward Arnold.)

CAVE R. 1980. Sedimentary environments of the basinal Llandovery of mid-Wales. Pp.517–526 in The Caledonides of the British Isles—reviewed. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (Editors). Spec. Pub. Geol. Soc., London, No. 8.

COCKS, L. R. M., HOLLAND, C. H., RICKARDS, R. B. and STRACHAN, I. 1971. A correlation of Silurian rocks in the British Isles. J. Geol. Soc. London, Vol.127, pp.103–136.

DESTOMBES, J. 1971. L'Ordovician au Maroc: essai de synthese stratigraphique. Colloque ordovicien–silurien. Mem. Bur. Rech. Geol. Min., Vol.73, pp.237–263.

EKDALE, A. A. 1977. Abyssal trace fossils in worldwide Deep Sea Project cores. Pp.163–182. in Trace fossils 2. CRIMES, T. P. and HARPER. J. C. (Editors). (Liverpool: Seel House Press.)

ELLES, G. L. 1896. On the Llandovery and associated rocks of Conway (North Wales). Q. J. Geol. Soc. London, Vol.52, pp.273–288.

ELLES, G. L. 1909. The relation of the Ordovician and Silurian Rocks of Conway (North Wales). Q. J. Geol. Soc. London, Vol.65,pp.169–194.

FEARNSIDES, W. G. 1912. Report of an excursion to the Portmadoc and Criccieth district of south-east Carnarvonshire. Proc. Geol. Assoc., Vol.23, pp. 199–217.

GREENLY, E. 1919. The geology of Anglesey. 2 volumes. Mem. Geol. Surv. G. B.

HARLAND, W. B. and HEROD, K. N. 1975. Glaciations through time. Pp.189–216 in Ice Ages: Ancient and Modern. WRIGHT, A. E. and MOSELEY, F. (Editors). (Liverpool: Seel House Press.)

HUTT, J. E. 1974. The Llandovery graptolites of the English Lake District Palaeontogr. Soc. [Monogr.]: Part 1, pp.1–56.

MCKERROW, W. S. 1979. Ordovician and Silurian changes in sea level. J. Geol. Soc. London, Vol.136, pp.137–145.

NUTT, M. J. C., INESON, P. R. and MITCHELL, J. G. 1980. The age of mineralisation at Parys Mountain, Anglesey. Pp.619–627 in The Caledonides of the British Isles reviewed. HARRIS, A. L., HOLLAND, C. H. and LEAKE, B. E. (Editors). Spec. Pub. Geol. Soc. London, No. 8.

PACKHAM, G. H. 1962. Some diplograptids from the British Lower Silurian. Palaeontology, Vol.5, pp.498–526.

RAMSAY, A. C. 1866. The geology of North Wales. Mem. Geol. Surv. G.B.,V ol.III.

RICKARDS, R. B. 1974. A new monograptid genus and the origins of the main monograptid genera. In RICKARDS, R. B., JACKSON, D. E. and HUGHES, C. P. Graptolite studies in honour of O. M. B. BULMAN. Spec. Pap. Palaeont., Vol.13, pp.141–147.

RICKARDS, R. B. and HUTT, J. E. 1970. The earliest monograptid. Proc. Geol. Soc. London, No. 1663, pp.115–119.

SIMPSON, S. 1957. On the trace-fossil Chondrites. Q. J. Geol. Soc. London, Vol.112, pp.475–499.

SMITH, A. J. and LONG, G. H. 1969. The Upper Llandovery sediments of Wales and the Welsh Borderlands. Pp.239–253 in The Pre-Cambrian and Lower Palaeozoic Rocks of Wales. WOOD, A. (Editor). (Cardiff: University of Wales Press.)

STRACHAN, I. 1969. In discussion of SMITH, A. J. and LONG, G. H. q.v.

WEDD, C. B., SMITH, B. and WILLS, L. J. 1927. The geology of the country around Wrexham. Part 1. Lower Palaeozoic and Lower Carboniferous rocks. Mem. Geol. Surv. G. B.

WEDD, C. B., KING, W. B. R. and WRAY, D. A. 1929 The geology of the country around Oswestry. Mem. Geol. Surv. G. B.

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Chapter 4 Silurian: Wenlock–Ludlow

Introduction

Included in this chapter is an account of the Wenlock–Ludlow rocks that crop out in the adjacent districts to the west (Figure 1) and (Figure 5). The total outcrop lying within the four New Series Sheets (94, 95, 106 and 107) covers about 550 sq km, and extends from the high ground of the Denbigh Moors (up to almost 500 m above OD) to sea level along the coast. The outcrop continues eastwards to form the Ciwydian Range, only a small part of which occurs in the present district; and there is every reason to believe that Wenlock–Ludlow rocks are present at depth beneath the

Carboniferous and Permo-Triassic outcrops to the north and east. Drift masks a large proportion of the outcrop, but where it is absent the Wenlock–Ludlow rocks are generally well exposed.

The succession has an average thickness of about 3550 m, and comprises a complete Wenlock sequence and a partial Ludlow sequence up to approximately the Saetograptus leintwardinensis Zone (cf. (Figure 13)." data-name="images/P1000810.jpg">(Figure 6)). Thickness variations of individual units are detailed below, and although some thicknesses are closely comparable in the Conway Valley on the west and the Clwydian Range on the east, structural complexity and the complete absence of underground information makes the question of total thickness variation a matter of speculation.

The greater part of the sequence is composed of mudstone—the remainder consisting of sandstone, siltstone and an insignificant proportion of carbonate rock. The sandstones, which are referred to as such in this account, may be more precisely classified as greywackes (cf. Pettijohn, 1957). The finer grained sediments may be subdivided into: striped silty mudstones, ribbon-banded mudstones and mottled mudstones. The striped silty mudstones consist of irregular alternations of silty mudstone and siltstone or fine sandstone, the ribbon-banded mudstones of regular alternations of thin bands (averaging 20 mm) of silty mudstone and laminated muddy siltstone (cf. Boswell, 1949, p. 41) and the mottled mudstones of irregularly cleaved and mottled, calcareous silty mudstones (ibid., p. 37). All these rock types locally show contorted and/or fragmented bedding and the strata are then referred to as disturbed beds (cf. Jones, 1937). They are held to be the products of penecontemporaneous subaqueous slumping or sliding.

The succession is rich in graptolites, although they are almost entirely restricted to the laminated muddy siltstone units of ribbon-banded mudstone, which may thus be loosely termed graptolite shale'. Also present is a shelly' macro-fauna composed of corals, bryozoans, brachiopods, gastropods, bivalves, cephalopods, trilobites, ostracods and crinoids. One or more elements of this fauna occur in all the rock types described, but the shelly fauna is mainly present in disturbed beds and in calcareous siltstones that form part of striped silty mudstones and which, from their weathering characteristics, are referred to as 'gingerbread' horizons. Evidence suggests that only in the mottled mudstones can the shelly fauna be considered an autochthonous one. Microfloral (acritarch) assemblages have been obtained from a number of levels, and chitinozoans have also been recovered.

The lithostratigraphical and biostratigraphical classifications adopted for this Wenlock–Ludlow sequence are indicated in (Figure 13)." data-name="images/P1000810.jpg">(Figure 6), along with the distribution of the rock types in the succession and its correlation with the chronostratigraphical units, Wenlock and Ludlow Series, as defined in their type areas. The biostratigraphical classification is based entirely on graptolites, twelve faunal assemblages being recognised and defined by the zone names given (p. 52). In some measure, the graptolite faunas have also been used, as lithological marker bands, in the lithostratigraphical classification of the sediments.

The base of the Denbigh Grits Group is defined, in the Llanrwst area, as the base of the ribbon-banded mudstones of the Benarth Flags, although there is a lithological transition between the latter and the underlying Pale Slates. The boundary, however, appears to coincide, locally at least, with the base of the Cyrtograptus centrifugus Zone. In consequence elsewhere in the district where a lithological boundary could have been chosen at a slightly different level, the boundary has been decided in any one section on the basis of this zonal assemblage. The Denbigh Grits/Lower Nantglyn Flags Group boundary is a clearly defined lithological break, which is almost certainly that taken by early workers when defining these groups (see pp. 22 and 25). The Upper and Lower Nantglyn Flags groups are almost identical in lithology, and the subdivision of the Nantglyn Flags is based on the contained graptolite faunas, and hence coincides with the base of the lower Neodiversograptus nilssoni Zone. This horizon was chosen for the subdivision of the Nantglyn Flags in the belief that it correlates with the stratotype of the Wenlock–Ludlow boundary (Warren and others, 1966). Through much of the present district the boundary lies at or near to the top of a disturbed bed and can thus be mapped with some accuracy. The base of the Elwy Group is defined by the incoming of a sandier facies of mudstones, accompanied generally by the start of a very considerable thickness of disturbed beds. The latter have a well marked, though diachronous base, but the facies change in the undisturbed mudstones is gradual and does not provide a readily definable boundary. However, the facies change approximates to the base of the upper N. nilssoni Zone, as here defined, and as in the case of the base of the Denbigh Grits Group some boundary adjustments have been made in the light of the local faunas recovered.

The lithostratigraphical classification adopted is of necessity a simple one. Post-depositional tectonics (cleavage in the finer grained rocks, folding and particularly faulting), repetition of rock types and the rapid lithological changes associated with disturbed beds have resulted in a confused stratigraphy, especially in the Elwy Group. Mapping has depended essentially upon the tracing of topographic features, here reflecting the outcrops of sandstones, mottled mudstones (Plate 3), disturbed beds (Plate 4), and certain other hard bands. These have revealed the structural grain and stratal sequence in a local area—generally a fault block. Correlation of these local sequences is as Jones (1954, p. 255) remarked 'a notoriously hazardous proceeding'. Nevertheless on the basis of the graptolite and, to a lesser extent, shelly faunas, a consistent lithostratigraphical sequence has been established. However, a number of locally established formations, for example those of Jones (1937), are omitted from the published map, because it was found impossible to recognise them with precision even from one region (Figure 15) to another within the district.

The areal distribution and lithological features of the Wenlock–Ludlow rocks point to a basin of deposition elongated approximately east–west. This basin was effectively delimited to the west by the Conway Valley Fault, early hinge movements on which accommodated a sinking of the basin and the incursion of detritus from a land mass farther to the west which yielded Precambrian and Ordovician detritus. The sandstones of the Denbigh Grits Group are the products of near-shore slides and turbidity currents, whilst those of the Elwy Group resulted from more distal turbidity flows. Ribbon-banded mudstones are believed to have formed under quiet conditions of sedimentation well away from the basin margins, although on the evidence of the mottled mudstones, with which they are interbanded (p. 46), perhaps at no great depth. Striped silty mudstones represent an intermediate facies, probably deposited on a gentle slope. Disturbed beds are interpreted as the products of down-slope slumping, probably induced by earthquake shocks along the line of the Conway Valley Fault, and because of their origin on the basin slopes their main constituents are striped silty mudstones.

History of previous research

The earliest published work on Wenlock–Ludlow rocks in the district is that of Bowman (1838), who gave a brief account of the strata exposed in Fernant Dingle, near Abergele. He subsequently (1841) reported more generally on the 'Upper Silurian' rocks of Denbighshire, but Sedgwick (1844) was the first to provide a reasonably complete account of the strata. In 1845 a comprehensive survey on the scale of one inch to one mile was begun by the Geological Survey, the maps (79 NW Rhyl; 79 SW Denbigh) being published in 1850. In the first edition of an explanatory memoir Ramsay (1866) assigned all the Silurian rocks to the Wenlock, but in the second edition (1881) he allowed that they might include part of the Ludlow. McKenny Hughes produced papers (1879, 1885, 1894) on the strata bordering the Vale of Clwyd, and after a resurvey by the Geological Survey of the district covered by 1-in sheet (Old Series) 79 NW a memoir by Strahan (1885) was published.

A second major period of investigation dates from 1921 when Boswell began the intensive study of the north-west Denbighshire Silurian which he was to continue for nearly 40 years; 19 papers were published between 1926 and 1961. Parts of the present district were investigated by Edmunds (1923), Double (who published jointly with Boswell in 1934, 1938, 1940) and Simpson (1940). Boswell (1949) collated all this work in his book The Middle Silurian Rocks of North Wales, and notwithstanding the arguments that had been put forward by Jones (1937, 1940, 1943) on the sedimentary origin of disturbed beds, he maintained his view then and later (1953) that they were in general of tectonic origin.

More recently the only work on the strata in the district predating the present survey has been that of Cummins (1957, 1959a, b) on the petrography and provenance of the sediments.

Denbigh Grits Group

Terminology

The term Denbigh(shire) Grits was apparently first used by Murchison (1859), although the unit had been recognised much earlier and described under such terms as 'Denbighshire Sandstones' (e.g. Sedgwick, 1844). In the second edition of the Geological Survey Memoir on North Wales Ramsey (1881, p.280) refers to the Denbighshire flags, grits and shales of Sedgwick, and from the description—'The Lower Silurian [ = present-day Ordovician] rocks are bounded on the east by the TARANNON SHALE and DENBIGHSHIRE GRIT, the last of which is overlaid... by the WENLOCK SHALE'—it is evident that the beds coincide with the unit now termed the Denbigh Grits Group. Comparison of the Old Series 1-in sheets, 78 SE, 79 SW and 74 NW published in 1850–55 with the Denbigh (107) Sheet shows how closely the upper boundary of the Denbighshire Grits coincides with the present junction of the Denbigh Grits Group and the Lower Nantglyn Flags Group.

Boswell (1949, p. 35) abbreviated the Denbighshire Grit and Flag Series to Denbigh Grit Series, but had previously used the even briefer term Denbigh Grits (1926, pp. 560–561). Cummins (1957) used the same terminology, which is followed here with modification in conformity with present stratigraphical terminology for a lithostratigraphical unit that comprises a number of formations (see pp. 23–24).

Distribution

The Denbigh Grits Group crops out (Figure 5) immediately south of Conway, where it was described by Elles (1909) as the Benarth Flags and Grits; also west of the Conway Valley, around Ty'n-y-groes; along the eastern side of the Conway Valley between Glan Conway and Llanrwst; between Llanrwst and Nebo (Corwen 120 Sheet) and eastwards across the Denbigh Moors from Moel Seisiog through Llyn Alwen to the Brenig Valley. For convenience these parts of the outcrop are referred to as the Benarth, Ty'n-y-groes, Conway Valley, Llanrwst and Denbigh Moors areas, respectively.

In addition to this main outcrop, there is an inlier in the Clwydian Hills between Cwm and Rhuallt, and the Group underlies part of the extensive drift deposits at Llandudno Junction and probably also to the east of Llandudno.

Thickness

The thickness of the group can be established with any degree of certainty only in the Llanrwst area, where it averages 1100 m. Even here rapid thickness changes within the Group (see pp. 72–74) and scarcity of through successions probably mask a considerable variation in the total thickness. On the Denbigh Moors a minimum of 880 m is recorded whilst in the Clwydian Range the thickness exceeds 400 m. The Benarth and Ty'n-y-groes areas are heavily faulted and folded and there is a virtual absence of macrofaunas, and hence any thickness assessment must be very tentative. In the Conway Valley the thickness of the succession between Groes, near Henryd, and Eglwysbach is believed to be of the order of 1200 m. It would appear therefore that there is no significant thickness change in the Denbigh Grits Group northwards from the Llanrwst area. It is established from outside the district (Boswell, 1949; Cummins, 1957) that the Group thins south-eastwards towards Llangollen, and it may be surmised that this is also the direction of its thinning throughout the present district.

Lithology

The base of the Denbigh Grits Group, as previously indicated (p. 19), lies within an alternation of pale grey-green mudstones—typical of the Pale Slates of the Llandovery and dark blue-grey ribbon-banded mudstone with abundant graptolites. A prominent band of calcareous concretions (p. 63), some 6 to 25 m above the base of the group may be of more than local significance in correlation, for a similar band has been reported by Rickards (1967, 1969) in the basal Wenlock of the Howgill Fells and the Lake District. The bulk of the Denbigh Grits Group consists of an alternation of sandstone (p. 33), striped silty mudstone (with sparse and generally thin ribbon-banded mudstone seams) and disturbed beds. The sandstones vary from fine-grained through coarse-grained to pebbly sandstones and rocks that may be loosely termed conglomerates (see p. 35).

The proportions of the various rock types show marked variations both vertically and laterally within any one part of the succession, but it is possible to recognise several major features in the lithological succession. Thick sequences (up to 75 m) of massive sandstones are present only in the lower two-thirds of the succession. Although the sandstones vary markedly in coarseness from fine to pebbly, this part of the succession in general shows only one horizon of predominantly coarse-grained sandstone and conglomerate. Although individual bands of conglomerate, for example, cannot be correlated, it is believed that the broad stratigraphical level at which they occur is probably synchronous throughout the district. Notable examples of this horizon are the conglomerate at Oerfa Quarry [SH 8372 5881] (p. 74), the disturbed pebbly sandstone at Parc Farm [SH 8012 6470] and the coarse to pebbly sandstone at Croesau Farm [SH 8012 7340] (p. 65).

Except in the Clwydian Range, where it occurs a little below the top, there is a disturbed bed of characteristic lithology but variable thickness (9 to 344 m) at the top of the Group. This disturbed bed consists predominantly of dark grey, fine silty mudstone showing a very high degree of fracturing, and thus forming a pencil slate. Local inclusion of sand grains and lenses, balls and rafts of sandstone indicates that this horizon is indeed one of disturbed beds and not a result of tectonic deformation. The stratigraphical unit has been named the Llanddoget Formation by Warren (1971a, p. 452) from the village 2.5 km NNE of Llanrwst where it is well seen. The outcrop of the formation (Figure 5) extends from the Bryniau area, 1.6 km SSW of Glan Conway, through Bodnant Gardens and Estate, where it is particularly well developed, to Llanddoget and Llanrwst. South-east of Llanrwst in the Nant y Goron sections up to 60 m are exposed. The outcrop continues eastwards across the Denbigh Moors, with conspicuous exposures in the area around Oerfa [SH 839 592] and in the headwaters of the Afon Cledwen, south of Gwytherin. In the Clwydian Range the formation is represented by 9 m of characteristic disturbed bed some 67 m below the top of the Denbigh Grits Group.

The marked thickness variation of the Llanddoget Formation is complemented by an equally variable thickness of underlying striped silty mudstones with generally thin bands of ribbon-banded silty mudstone with graptolites. This lower unit, almost entirely devoid of significant sandstone bands, is termed the Berllan Formation, following Boswell (1943, p. 94; 1949, p. 35) from Berllan-fawr Farm, 0.8 km SSW of Eglwysbach. The term is here used to include the Berllan and Lletty Beds of Boswell (1949, p. 287 and fig. 14.7). The resurvey of the type areas has shown that the four divisions included in Boswell's two formational units are untenable. In particular, units (b) and (d) of his Berllan Beds are one and the same horizon in most places, whilst some areas mapped as Berllan Beds are clearly at the same horizon as the type Lletty Beds (at Lletty Farm 0.8 km WSW of Eglwysbach). The term Lletty Beds, although providing a more satisfactory and characteristic type locality for the presently defined Berllan Formation, is pre-empted by its use for Lower Ludlow strata in South Wales (Cantrill and Thomas in Strahan and others, 1907; Potter and Price, 1965, p. 392).

The lateral variation in the Denbigh Grits Group is shown in (Figure 7).

Palaeontology

Graptolites dominate the fauna of the Denbigh Grits Group although, as in all the Wenlock–Ludlow sequence, they are restricted to the ribbon-banded mudstones. They are plentiful at the base of the Group and reasonably numerous in the Berllan Formation, but are rare or absent from most of the remainder of the succession. The Denbigh Grits Group, in comparison with the rest of the Wenlock–Ludlow succession, is poorly fossiliferous in terms of both graptolites and other fossils (see Appendix 1). In addition to the graptolites recorded during the present survey, C. centrifugus has been recorded from the Benarth area by Rickards (in litt.); and there is no reason to doubt Elles's (1909, p. 186) records of C. murchisoni and Monograptus riccartonensis from the same area.

On the basis of the graptolite fauna, the Denbigh Grits Group corresponds to the following graptolite zones (in ascending order): Cyrtograptus centrifugus Zone, C. murchisoni and M. riccartonensis Zone, C. rigidus Zone and C. perneri Zone. The basal beds are characterised by faunas of the first two zones, with the C. centrifugus Zone restricted to the lowest 18 m or so. Separation of the overlying C. murchisoni–M. riccartonensis Zone into the two or more zones recognised elsewhere is not possible in the present district owing to the paucity of fossils at this horizon, nor is it certain where the upper boundary with the C. rigidus Zone lies in the succession. In the type locality it would appear to lie at the base of the Berllan Formation. The upper part of these beds is probably of C. perneri Zone age, but graptolites are few and the coeval Llanddoget Formation is unfossiliferous. The immediately overlying beds of the Lower Nantglyn Flags Group everywhere yield a C. perneri Zone graptolite fauna and, except in the Clwydian Range, the top of the Llanddoget Formation represents a synchronous facies change.

The shelly fauna of the Denbigh Grits Group (Appendix 1) it largely restricted to broken fragments of shells in sandstones and thin ' gingerbread' horizons in striped silty mudstones.

A preliminary micropalaeontological analysis by Dr T. R. Lister has shown the presence of miospores, acritarchs, chitinozoa and scolecodonts. The most prolific fauna was obtained from a locality [SH 8099 7265] near Bodnant Uchaf in the Llanddoget Formation. The total palynological content is listed in Appendix 1.

Facies development

Lateral facies variation in the Silurian of the district is probably at its greatest in the Denbigh Grits Group (Figure 7). In the absence of good faunal control, the details must remain conjectural, but the major changes are:

1. A thickening northwards from the Ty'n-y-groes area to the Benarth area in the basal, graptolite-bearing beds of the Group (Benarth Flags), probably in part related to (2).
2. A decrease in proportion and grain-size of the sandstones both north and south-east from Nebo [SH 836 561].
3. The absence of the Berllan Formation in the Llanrwst area, its appearance and rapid thickening northwards from Maenan [SH 795 648] to Eglwysbach [SH 804 705] and its re-appearance east of Oerfa [SH 837 588] in the Denbigh Moors area.
4. Complementary to (3), the marked thickness variation in the Llanddoget Formation.

The basal beds (Benarth Flags), south of Conway, consist of some 200 m of striped and ribbon-banded silty mudstone with thin sandstone and sandstone/mudstone bands. They include the C. centrifugus, C. murchisoni and a significant part of the M. riccartonensis zones. In contrast, south of Henryd the formation is approximately 110 m thick and is overlain by about 30 m of sandstone. No evidence is available in the Conway Valley area or at Llanrwst, where the Conway Valley Fault cuts out the lower part of the Denbigh Grits Group. However, in Coed Soflen [SH 819 578] the formation is only 80 m or so thick and represents little more than the C. centrifugus Zone.

Boswell (1949, pp. 314, 325) maintained that in the vicinity of Llanrwst the Denbigh Grits Group thins and contains no sandstone bands. The present survey indicates the presence of sandstone in Llanrwst and suggests that the basal two-thirds of the Group either lies below the alluvium of the Conway Valley or is cut out by the Conway Valley Fault. Nevertheless, from (Figure 7) (sections E–G) and Sheet 107 (Denbigh) it is evident that from the Nebo–Oerfa area to Llanrwst there is a marked decrease in the number of sandstones in the sequence, associated with the incoming of disturbed beds. Towards the Denbigh Moors a similar decrease in the sandstone proportion occurs and the sequence becomes one of striped silty mudstone. Direct evidence of facies changes to the north-east is missing, but the Clwydian Range outcrop shows a low proportion of sandstone. Accompanying the decrease in proportion of sandstone in the sequence, there is a decrease in the maximum grain-size of individual sandstones towards the north and east. The two variables are not, however, directly related. Thus to the north of Llanrwst, in the Conway Valley area, the trend in maximum grain-size is maintained, even though the proportion of sandstone in the area east of Dolgarrog is considerably greater than that at Llanrwst or in the Ty'n-y-groes and Benarth areas.

It is perhaps significant that the area of minimum sandstone proportion near Llanrwst coincides with the position of the Gwytherin Syncline, whilst the maximum proportions are found in the Nebo and Dolgarrog areas, coincident with anticlinal structures.

The complementary thickening and thinning of the Llanddoget and Berllan formations is perhaps the best-documented facies change in the Silurian of the district. Stratigraphical control is afforded by the consistent age of the basal Lower Nantglyn Flags and the generally abundant graptolite fauna of the Lower Berllan Formation. Between these levels a marked variation in proportion of the formations and total thickness of the sequence is evident.

The Berllan Formation can be traced from its type locality to Bodnant and Bryniau in the north and to the Maenan area [SH 805 665] in the south; maintaining in each direction a fairly constant thickness of 150 m. In the vicinity of Goppy [SH 803 663], however, they are quite suddenly replaced by disturbed beds of the type found in the Llanddoget Formation. Although local down-cutting of the latter is known to occur, it is believed that in general the Berllan Formation passes laterally into the Llanddoget Formation.

Because of facies change, the Llanddoget Formation, which north of Goppy averages 150 m, varies in thickness between its maximum of 344 m at Llanrwst and about 23 m in the Afon Brenig [SH 963 573], being about 150 m south of Gwytherin. Complementary to this thinning striped silty mudstones, with a fauna identical to that of the type Berllan Formation, reappear, and although not strictly in mapping continuity are referred to the Berllan Formation. They vary in thickness from about 120 m south of Gwytherin to a probable 210 m east of Llyn Alwen

In the Clwydian inlier biostratigraphical equivalents of the Berllan Formation are at least 350 m thick, and the Llanddoget Formation is 12 to 24 m. The latter is overlain by some 67 m of striped silty mudstone, believed to belong to the Denbigh Grits Group and to be the lateral equivalent of the upper part of the Llanddoget Formation elsewhere.

Lower Nantglyn Flags Group

Terminology

The term Nantglyn Flags may be said to have originated with McKenny Hughes' (1879) article on the Silurian of the Vale of Clwyd in which he refers (p. 694) to the Encrinite flags of Nantglyn'. A few years later (1894) he uses the term Nantglyn Flags and of them says (ibid., p. 154) '[they] form a fairly homogeneous series, splitting into flags, …or irregular blocks and slabs, or sometimes splitting into slates.… I take as a typical section that exposed along the hillsides and in the valleys near Nantglyn'. It is evident (ibid., p. 142) that he included all that part of the succession between the Denbigh Grits Group (Caer Drwyn Grit) and the present Elwy Group (Moelfammau Sandstone) in his Nantglyn Flags.

Boswell (1926, p. 562) divided the Nantglyn Flags into lower and upper parts, the latter including only the Nantglyn Flags of the type area and their correlatives (Lower N. nilssoni Zone strata), and regretted that the term could not be restricted to the strata forming the upper part. Later Boswell (1949) did so restrict the term, referring to the beds below as 'flags of Nantglyn type'.

Lithologically there is no justification for a division of the strata lying between the Denbigh Grits Group and the Elwy Group, and the term Nantglyn Flags is here retained, in its original sense, to include all these rocks. On biostratigraphic and chronostratigraphic grounds, however, it is convenient to subdivide the sequence at the base of the N. nilssoni Zone that is at the horizon correlated with the base of the Ludlow in its type area—into an Upper and a Lower group. Boswell's 1926 terminology is thus retained. The divisions have been raised to group status partly because formations, e.g. the Lower Mottled Mudstone, can be recognised in the Lower Nantglyn Flags Group, but largely in conformity with the terminology employed for the remainder of the Wenlock–Ludlow succession.

Distribution

The main outcrop of the Lower Nantglyn Flags Group, parallel to that of the Denbigh Grits Group, extends from Tywyn [SH 790 790] in the north, through the Eglwysbach area to the vicinity of Llanrwst, and thence through the Oerfa area to just south of Gwytherin and across the Denbigh Moors as far as Llech Daniel [SH 995 566]. It continues on the adjacent Corwen (120) Sheet through Clocaenog Forest and terminates against the southerly continuation of the Gwaynynog Fault, north of Derwen [SJ 070 507].

In the vicinity of the Sportsman's Arms [SH 952 591] the outcrop extends northwards into the area south of Llansannan, extending eastwards through Mod Grugoer to Bylchau [SH 976 630]. Only around Beidiog Uchaf [SH 9181 6350] in this area did any previous worker recognise that Wenlock strata were present (Boswell, 1949, p. 246). The area was in general previously assigned to the Ludlow (Neodiversograptus nilssoni Zone).

The lower Nantglyn Flags Group also crops out in the Clwydian Range in two small inliers, one around and to the south-east of Cwm [SJ 065 774] and the other between Rhuallt [SJ 075 751] and Tremeirchion [SJ 082 731].

Thickness

The Lower Nantglyn Flags Group ranges in thickness from an estimated maximum of 660 m in the Glan Conway area to a minimum of about 320 m to the south of Gwytherin. There is a progressive southerly decrease in thickness in the Conway Valley area between these two extreme values. Traced across the Denbigh Moors, the group thickens eastwards to 425 m in the vicinity of the Afon Brenig.

In the Llansannan area more than 400 m are recorded, excluding the lowest part of the group (see (Figure 8)), and the total thickness here probably exceeds 520 m. It is thus evident that there is a marked thickening in a northerly direction from the Sportsman's Arms area, comparable to the northward thickening observed in the Conway Valley.

In the Clwydian Range inliers the group has an estimated thickness of 600 m. The overall evidence therefore is of a succession thickening northwards, north-eastwards and eastwards, the area of minimum thickness probably coinciding with the zone of maximum thickness of the Denbigh Grits Group.

Lithology

The greater proportion of the Lower Nantglyn Flags Group consists of a regular alternation of 1.2 to 3.6 -cm bands of mudstone and laminated muddy siltstone with less common thin, and generally lenticular bands of calcareous siltstone. For this rock type Boswell (1926, p. 562; 1949, pp. 7, 41) proposed the term 'ribbon-banded', and this is employed in the present account. The rock when fresh is of a dark blue-grey colour, the mudstone being rather paler than the laminated muddy siltstone, but darker than the calcareous siltstones, which in weathered sections are yellow. The rock type has been described in detail by Cummins (1959a) and is further discussed below (p. 44).

The basal 18 m or less of the group locally lack the graptolitic laminated muddy siltstone bands and consist predominantly of massive dark blue-grey mudstones with siltstone bands. They thus form a passage sequence to the striped silty mudstones of the underlying Denbigh Grits Group.

Included in the ribbon-banded mudstone are ovoid calcareous nodules, composed of sideritic carbonate rock, which vary from a few centimetres to 1 m across. In general they increase in size upwards in the succession, reflecting an increasing carbonate content in the ribbon-banded mudstone.

The mottled mudstones (p. 46) on account of their hardness, distinctive mottling, goethite veinlets and shelly fauna form conspicuous marker bands in the ribbon-banded mudstone. They are dark blue-grey with darker and lighter blebs when fresh, but weather brown with dark red-brown veinlets.

The Lower Nantglyn Flags Group contains a number of disturbed beds, composed in part of ribbon-banded mudstone but more frequently of silty mudstone, partly striped, and locally of mottled mudstone. Of these the most important is the Brynsylldy Formation—type locality Brynsylldy Farm [SH 821 619] south-east of Llanrwst. At many localities in the district this unit demonstrably forms the uppermost part of the Wenlock succession, being overlain by graptolitic ribbon-banded mudstones of the Upper Nantglyn Flags Group. Its top is readily mappable and is believed to represent a single time horizon. Hence, where palaeontological control is absent, it is taken to indicate the boundary between the Lower and Upper Nantglyn Flags groups.

Palaeontology

Graptolites are the dominant element in the Lower Nantglyn Flags Group fauna and occur in abundance in the ribbon-banded mudstone throughout the succession. The grapolite fauna (see Appendix 1) allows the definition of the following zones (in ascending order): Cyrtograptus perneri, C. ellesae, C. lundgreni, Gothograptus nassa and Monograptus ludensis [ = vulgaris].

The shelly fauna of the group (Appendix 1), although consisting of many species, is largely restricted to the mottled mudstones. A number of species are also recorded from the Brynsylldy Formation and other disturbed beds and a limited fauna, notably of Scyphocrinites? pulcher, Cardiola interrupta and orthoceratids, is found associated with the graptolites in the ribbon-banded mudstone.

A preliminary micropalaeontological study of this group by Dr T. R. Lister has shown the presence of chitinozoa in ribbon-banded mudstones of the Cyrtograptus perneri, C. lundgreni and G. nassa zones.

Facies development

The Lower (and Upper) Nantglyn Flags Groups represent a period of relatively quiet sedimentation and in contrast to the Denbigh Grits Group show little lateral variation. Individual bands in the ribbon-banded mudstone sequences can be traced throughout large quarry sections and are believed to extend over much greater areas.

Mapping of the area in the Conway Valley between Bodnant [SH 801 724] and Pennant [SH 816 673], a distance of nearly 7 km, has revealed the consistent presence of 13 hard bands in the beds below the Lower Mottled Mudstone.

The facies change marked by the base of the Lower Mottled Mudstone is, in the absence of evidence to the contrary, taken as a synchronous event throughout the district. On this basis considerable lateral variation is believed to take place between this horizon and the top of the group (Figure 8). Most of this variation occurs within that part of the succession containing the mottled mudstones. Boswell (1949, p. 38 and fig. 14.1 for example) was of the opinion that the mottled mudstones were impersistent and changed horizon markedly, but this interpretation has not been confirmed by the present mapping, which indicates continuous outcrops of both the Lower and Upper Mottled Mudstones. Moreover, graptolites show that these formations lie at constant biostratigraphical horizons ((Figure 8), but see p. 56). However, it is true that locally the upper part of each formation passes laterally into ribbon-banded mudstone, and that such mudstone forms thin bands within the mottled mudstones and especially the upper formation. The Lower Mottled Mudstone ranges in thickness from 12 to 76 m and the Upper Mottled Mudstone from 12 to 64 m (but see below). Although some of this variation reflects the general thickness variation in the Lower Nantglyn Flags Group, this is of less significance than lateral facies changes associated with the mottled mudstones. Thus the thinnest and thickest sections in the Upper Mottled Mudstone both occur in 600-m sequences.

Marked lateral facies changes are also associated with the Brynsylldy Formation and to a lesser extent with other disturbed beds. Some 3 km north of its type locality, the Ffrithuchaf area [SH 831 651], the Brynsylldy Formation is about 6 m thick and is underlain by approximately 120 m of ribbon-banded mudstones that overlie the Upper Mottled Mudstone. This general situation, with minor thickness variations, pertains over much of the outcrop. In two areas however a very different situation holds. South of Ffrithuchaf the unit thickens rapidly towards the type locality and at the same time the base cuts down in the succession, so that at Brynsylldy itself the disturbed bed rests on the Upper Mottled Mudstone. At Pennant Farm [SH 825 609], 1.6 km to the south-east, the Upper Mottled Mudstone is completely cut out and the unit attains a thickness of 88 m. Farther east the position is reversed, and in the area south-west of Gwytherin the disturbed bed has thinned to 18 m.

Traced north from Ffrithuchaf the bed thins and in the area of Mwdwl Eithin [SH 829 683] dies out. About 1.5 km farther north a disturbed bed reappears, apparently by lateral facies changes, in the uppermost part of the Lower Nantglyn Flags Group. Northwards this disturbed bed thickens rapidly and its base progressively descends the succession until, in the area north-east of Eglwysbach, it rests on the lower part of the Upper Mottled Mudstone and has a thickness of about 180 m. Traced farther north the bed thins rapidly at first and then gradually, to die out south-east of Glan Conway.

East of Llyn Aled, in the vicinity of the Sportsman's Arms and in most of the Llansannan area the Brynsylldy Formation is absent and the junction of the Lower and Upper Nantglyn Flags groups occurs within a ribbon-banded mudstone sequence. In the same areas however, disturbed beds are common at lower levels down to the top of the Lower Mottled Mudstone. They all vary in thickness, the thickest reaching 40 m, and apparently die out eastwards; there are no disturbed beds in the Lower Nantglyn Flags south-east of Llyn Brân [SH 963 593], nor in the Clwydian Range.

Disturbed beds are developed in the lower part of the group only in the area between Pennant [SH 816 673] and Pantllinmawr [SH 814 640], north-north-east of Llanrwst. They vary in thickness up to 45 m.

In conclusion, the changes in overall thickness of the group, along with local facies variation, both vertically and laterally, combine to provide significantly differing sequences (Figure 8).

Upper Nantglyn Flags Group

Terminology

The Upper Nantglyn Flags are similar to the Lower Nantglyn Flags Group in that they consist predominantly of ribbon-banded mudstone, and the base of the Group, though locally related to a mappable horizon, is defined palaeontologically as the base of the Neodiversograptus nilssoni Zone (p. 20). The top of the Upper Nantglyn Flags Group, is marked by a significant facies change to sandstones, striped silty mudstones and disturbed beds. Except locally, ribbon-banded mudstones are no longer present in the succession and reappear in a modified form only in the upper Elwy Group (Saetograptus incipiens Zone).

Distribution

The main outcrop of the Upper Nantglyn Flags Group parallels that of the Lower group (p. 25). There are two small inliers in the Clwydian Range east and south-east of Rhuallt and an outier at Llandudno Junction (Figure 5).

Thickness

The Upper Nantglyn Flags Group shows a marked change of thickness from an established minimum of about 75 m in the Oerfa area to a possible maximum of about 600 m to the south of Llangerniew [SH 875 675]. The latter area, however, is heavily drift-covered and the presence of undetected strike faults might considerably reduce this maximum figure.

Northwards from the Oerfa area the thickness increases steadily to reach 220 m around Wenlli [SH 835 661], from where it decreases northwards to about 100 m in the Topanbach area [SH 821 726], east of Bodnant. The thickness increases again farther north to 220 m east of Glan Conway.

Eastwards from Oerfa across the Denbigh Moors the group thickens to about 180 m in the area south of Gwytherin, which figure is maintained eastwards to beyond the type area around the Nantglyn Quarries [SH 979 598].

Northwards from the southern margin of the outcrop, the group thickens appreciably, reaching an average thickness of about 450 m in the area south-east of Llangerniew. There is no direct evidence of the thickness changes north and east of Llansannan, but comparisons with the outcrop to the northwest and south-east suggest that the northerly thickening is not continued, and indeed may be reversed. In the Clwydian Range the group averages only 180 m. There is a complementary relationship between the thickness of the Nantglyn Flags as a whole and that of the Denbigh Grits Group.

Lithology

The group is composed almost entirely of ribbon-banded mudstones, which are blue-grey, thin-bedded to shaly in part, but more generally flaggy and in places massive. Being much more readily weathered than the overlying Elwy Group they form relatively low ground and in places, e. g. the headwaters of the Afon Aled, form spectacular gorge-sections. As in the Lower Nantglyn Flags Group, they contain ovoid concretions (Plate 5), up to 45 cm across and generally restricted to certain horizons, some 2 to 3 m apart. Locally in the uppermost part of the group the calcareous siltstone elements of the ribbon-banded mudstones are thicker and coarser, and here and there the group includes thin (less than 30 cm) sandstone bands, presaging the major facies change to the Elwy Group.

In addition to the ribbon-banded mudstones the group contains a number of disturbed beds, composed of mudstone, striped silty mudstone and ribbon-banded mudstone. These are, however, almost entirely restricted to the area between Llangerniew and Llansannan, where they are generally less than 45 m thick, but reach a maximum of 90 m.

Palaeontology

The ribbon-banded mudstones have a rich graptolite fauna (see Appendix 1, p. 182) which is indicative of the Lower N. nilssoni Zone. In contrast the shelly fossils of the Upper Nantglyn Flags Group (Appendix 1, pp. 184–187) are sparsely distributed.

No detailed micropalaeontological work has yet been undertaken on the Upper Nantglyn Flags Group.

Facies development

The only detectable facies changes are related to the occurrence of disturbed beds, those south-east of Llangerniew in particular. In the absence of close biostratigraphical control—the whole group being represented by the Lower N. nilssoni Zone—the details of this variation are hard to assess. Nevertheless there is direct evidence, notably in a quarry [SH 9078 6625] 1.2 km south-east of Hafod-y-gog (p. 90), of the westerly thickening, from 1.5 to 6 m, of a disturbed bed near the base of the Upper Nantglyn Flags Group, a thickening which is accounted for by the base of the disturbed bed transgressing lower levels in the succession. The major thickness of disturbed beds in the group is seen between Plas Mattw [SH 889 648] and Plas-onn [SH 894 653], around Foel Cathau [SH 895 647] and near Tŷ-uchaf [SH 901 658], and is believed to represent a single disturbed sheet, up to about 105 m thick.

The disturbed bed thins away to the south and east and, eventually to the west for it is absent west of the Llangerniew faults. Disturbance therefore appears to occur only in a limited area, though this may extend northwards beneath the overlying Elwy Group, and this area coincides with the thickest sequences of the Upper Nantglyn Flags Group.

Elwy Group

Terminology

Throughout the district the sequence of strata overlying the Nantglyn Flags consists of disturbed beds, striped silty mudstones and subordinate sandstones with rapid lateral facies changes. The term Elwy Group is here proposed to encompass all these rocks. As thus defined it is probably equivalent to Cummins's (1959a) term 'Lower Ludlow Grits', but for a number of reasons this term is considered inappropriate in North Wales. Firstly, the implication of a similarity of this Grits group to the Denbigh Grits Group is misleading, for sandstones form only a minor part of the Elwy Group succession as a whole. Secondly, Cummins's (ibid., p. 168) definition of his term based essentially on strata in Central Wales included the following: 'a series of greywackes and calcareous siltstones with interbedded mudstones', indicating a very distinct difference between his formation and the present group (see below). Thirdly, the two sequences are not in mapping continuity and it is therefore desirable to retain two lithostratigraphical terms.

The Elwy Group includes a very large number of formations recognised by earlier workers from Sedgwick (1845) onwards both within the present district and in the adjacent areas to the east—Clwydian Range (Woods and Crosfield, 1925)—and the south-east—Llangollen area (Wills and Smith, 1922). In particular it includes the formations defined by Jones (1937) in the Llangerniew area, and the Cynant Beds' of the present authors—a term used for that part of the sequence which contains a relatively high proportion of sandstone and is typically developed in the area around Cynant Isaf [SH 924 716], north of Llanfair Talhaiarn. The distribution of some of the formations recognised in the district is shown in (Figure 5) and a summary of many earlier terms for the Lower Ludlow strata of North Wales is given, with probable correlations, in (Figure 9).

The correlation of the Llangollen District differs from that given by Cocks and others (1971). In determining the age of the Dinas Brân Group, the evidence from Dayia navicula is inconclusive and the aspect of the whole fauna recorded by Wills and Smith (1922, p. 208) and by Lake (1895, p. 20) is distinctly Leintwardinian. Moreover, the fauna of the Vivod Group and the facies change at the top of the Group—to the 'thin-bedded flaggy shales with silty, often laminated beds' (Wills and Smith, 1922, p. 207)—together with the incoming of 'M. leintwardinensis'(as likely to be Saetograptus leintwardinensis incipiens as S. leintwardinensis leintwardinensis) all strongly suggest that the Vivod Group/Dinas Brân Group boundary lies at no higher horizon than the base of the S. leintwardinensis Zone (Figure 9). Indeed, it may well lie at an horizon not far removed from that of the base of the Upper Elwy Group (Bont-uchel Formation), that is close to the base of the S. incipiens Zone.

Distribution

The outcrop area of the Elwy Group exceeds that of any of the other groups recognised in the district (Figure 5). The main outcrop extends from Colwyn Bay [SH 850 790] through Llanfair Talhaiarn [SH 927 702] to Gyffylliog and the south-east corner of the district, where it continues south-eastwards to the Llanelidan area, north-west of Llangollen. On its northern and eastern sides the outcrop terminates against that of Carboniferous rocks.

South of the main outcrop there are a series of outliers forming the high ground at the northern edge of the Denbigh Moors and the group also crops out in the Clwydian Range, including the eastern margin of the district between Bodfari [SJ 093 701] and Rhuallt [SJ 075 751].

Thickness

No upper limit for the thickness of the group can be defined in the present district. The maximum thickness exposed is 1750 m, which probably compares with no more than 610 m for the equivalent strata in the Llangollen area (Wills and Smith, 1922 and (Figure 9)) and 450 m in the Clwydian Range (p. 102 and Boswell, 1949). This marked thinning of the group to the east and south-east is largely confirmed by thickness variation in the several parts of the Elwy Group succession defined on palaeontological grounds (see below).

Lithology

The predominant rocks in the Elwy Group succession are striped silty mudstones, but these contain laminae or thin bands of laminated muddy siltstone—the graptolite-bearing element of the ribbon-banded mudstones—from which the graptolite fauna has been obtained. In addition the mudstones contain a number of generally thin bands of 'gingerbread' rock (p. 19 and Boswell, 1949, p. 46) containing moulds of disarticulated and usually fragmentary shells.

Only locally do sandstones form a significant proportion of the succession. Sandstone units reach a maximum thickness of 55 m, but within these the individual beds are generally 0.3 to 1 m thick and do not exceed 2 m.

A substantial proportion of the Elwy Group consists of disturbed beds (Figure 10). These include all the various types recognised (p. 38), and range from pencil slate disturbed beds, composed of highly fractured silty mudstone, to sandy disturbed beds, which are locally composed entirely of sandstone. Mapped thicknesses of disturbed strata exceed 450 m, but estimates are taken from areas that are partly unexposed or drift-covered, and may thus include small thicknesses of undisturbed sediments.

In the Llangerniew area and elsewhere, completely exposed, 60 m thick sections of disturbed strata are seen, and individual disturbed bed units may exceed that figure.

Generally associated with disturbed strata in the Llansannan–Llanfair Talhaiarn area, are a number of occurrences (p. 85) of conglomerate bands containing subrounded pebbles up to 15 cm across of decalcified siltstone, decalcified fossils and pellets of black mudstone. All lie close to the base of the Elwy Group, but it is uncertain whether or not they represent a single horizon. A comparable bed was recorded by Wills and Smith (1922) from near the base of their Nant y Bache Group in the Llangollen area (Figure 9).

Although facies changes are numerous and complex (p. 32 et seq.), it is nevertheless possible to recognise a general threefold lithological division of the Elwy Group. The lowest division, averaging about 400 m in thickness, consists predominantly of disturbed beds, and sandstones are only significant locally. It includes the Llangerniew Disturbed Beds of Jones (1937). The middle division comprises an alternation of disturbed beds and striped mudstones with the latter predominant in the lower part and the former in the upper part. Sandstone bands are relatively numerous, particularly in the part of the succession containing Jones's (1937) Gribin Mudstones, Pentre Disturbed Beds, Bryn-hyfryd Mudstones and Henfryn Disturbed Beds, and the type area of the present authors' Cynant Beds'. The upper division of the Elwy Group comprises striped silty mudstones with relatively few disturbed beds which are generally restricted to the basal part. This division is here named the Bont-uchel Formation—type locality Bont-uchel [SJ 085 578], near Gyffyliog.

Palaeontology

The fauna of the Elwy Group is a rich one both in shelly fossils and graptolites. However, to a large extent these are mutually exclusive; the former occurring predominantly in disturbed beds but also in 'gingerbread' bands, and the latter being restricted to the striped silty mudstones of the normally bedded sequences. Thus in the lower part of the group graptolite faunas are generally poorly developed and shelly fossils are dominant, whereas in the upper part the converse is true.

The graptolite fauna (Appendix 1, p. 182) is interpreted as comprising three assemblages indicative of (in ascending order) the Upper Neodiversograptus nilssoni, Cucullograptus scanicus and Saetograptus leintwardinensis incipiens zones (p. 54). Within the limits of definition of these zones, they correspond to the lower, middle and upper lithological divisions of the Elwy Group (see above).

The Shelly fauna of the Elwy Group (Appendix 1, pp. 184–187) is abundant in numbers of species, but of restricted occurrence in the sequence, being confined in the main to 'nests' in disturbed beds and to seams of 'gingerbread' rock and laminated muddy siltstone (with graptolites) in the undisturbed sediments. Only the fossils from the last-named can be said to be in any sense indigenous, and many of these show evidence of current transport, winnowing and damage. There is, however, no evidence of wholesale derivation of faunas from significantly older strata.

A preliminary palynological investigation of the upper Elwy Group, carried out by Dr T. R. Lister, was primarily concerned with the south-east corner of the district and adjacent areas to the east (Flint (108) Sheet) and south (Corwen (120) Sheet). The fauna, entirely of chitinozoa, is listed in Appendix 1 (p.183), and its significance in correlation is discussed below.

Facies development

Lateral facies changes and thickness variations are numerous and significant in the Elwy Group, but for two main reasons they cannot be detailed accurately. Firstly, intense faulting makes it impossible to trace formation boundaries very far, to correlate sequences with certainty or to assess thickness variations. Secondly, the lower two zones of the Elwy Group have insufficiently distinct faunal assemblages for precise stratigraphical control. Although the upper C. scanicus Zone fauna is distinct from that of the lower Upper N. nilssoni Zone, there are transitional faunas which, in the absence of certain diagnostic species, cannot be assigned with certainty to one zone or the other. Many of the differences between the present maps (1:50 000 and six-inch) and those of Boswell (1949) stem from what are now believed to be incorrect interpretations of the relative horizons of Upper N. nilssoni and C. scanicus Zone faunas, and Jones (1954) was right to point out the limitations of graptolites in geological mapping. On the other hand, it is also true that Jones himself (1937, p. 14), in mapping his Llangerniew Series over a large part of the district, implied a correlation of strata now believed to range from the lower part of the Upper N. nilssoni Zone to the upper part of the C. scanicus Zone. Boswell's (1953, p. 150) critical analysis of Jones's formations is reproduced here (Figure 11) along with the present interpretation of these sections. It is evident that over a large part of the area the base of Jones' s Llangerniew 'Series' maintains a consistent horizon and coincides with the base of the Elwy Group. In a number of places, however (Figure 10), there is a serious divergence of opinion on the age of the disturbed beds, accounted for by the present interpretation of the top and bottom of the main disturbed bed sequences being diachronous boundaries.

No attempt has therefore been made to map formational or biostratigraphical boundaries within the Elwy Group. (Figure 10), which comprises generalised vertical sections for areas covering one or more six-inch National-Grid sheets, provides the basis of the facies interpretation given here.

The lower division of the group (Upper N. nilssoni Zone) consists almost entirely of disturbed beds and in the Llangerniew area averages nearly 500 m in thickness, locally reaching 550 m. Generally it maintains its thickness to the north and west, but it thins rapidly eastwards, averaging only 275 m in the area between Bylchau and Llannefydd. In this last area the proportion of disturbed strata falls locally to less than 30 per cent. To the north-west of Llangerniew, disturbed beds form only 50 per cent or so of the succession in the area between Egiwysbach and Colwyn Bay and the thickness of the Upper N. nilssoni Zone here may be not much more than 300 m. Traced south-eastwards from Llangerniew the beds thin to 180 m in the Nantglyn area, where disturbed beds may form no more than 30 per cent of the succession. In the Clwydian Range the thickness of the division exceeds 460 m and disturbed beds are absent. The lower division is devoid of sandstone in the Llangerniew area, but the occurrences elsewhere suggest an easterly as well as a substantial northerly increase in sandstone deposition during this period.

The middle division of the Elwy Group (C. scanicus Zone) shows the most pronounced facies changes of the Ludlow rocks, but it is difficult to detect any general pattern. The thickness varies between 275 m in the Nantglyn area and about 670 m in the Bettws-yn-Rhos area, but there does not appear to be any consistent pattern (Figure 10).

In most of the district this division is characterised by the presence of sandstones, which may form nearly 10 per cent of the succession. To the south-east of Nantglyn, however, sandstones are virtually absent although many of the disturbed beds here are of the 'sandy' type. Up to 75 per cent of the succession in the Nantglyn area consists of disturbed beds, whereas in the northern, sandstone-rich successions around Bettws-yn-Rhos the figure is less than 25 per cent.

In the upper division (S. incipiens Zone) of the Elwy Group the facies is similar throughout the district, with disturbed beds, locally thick, and sandstones at the base, and the remainder of the succession composed of striped silty mudstones with sparse disturbed beds (Bont-uchel Formation). In one respect, however, there is a marked change. In the north and north-east of the district the striped silty mudstones contain relatively few, and thin, bands of laminated muddy siltstone (with graptolites), but in the south-east these graptolitic bands are both thicker and more numerous. The facies thus approaches the ribbon-banded mudstone (Nantglyn Flag) facies (p. 44), though the graptolite-bearing muddy siltstones are siltier than those typical of the Nantglyn Flags and the proportion of siltstone in the rock as a whole is also greater. Nevertheless the similarity of facies misled Boswell (1949) and Jones (1937), and resulted in wrong correlations.

Petrography, sedimentation and provenance

Sandstones of the Denbigh Grits Group

The arenaceous and rudaceous rocks of the lower to middle Wenlock range from silty sandstones to conglomerates, but they are here all classed as sandstones. The classification of sandstones in general and of these rocks in particular is a vexed question which raises several problems (Folk, 1954; Pettijohn, 1954; Krynine, 1948). Many terminologies have been employed and all depend upon examination of the mineral and rock content, matrix proportion, grain size and texture under the microscope. In this study it has been possible only to examine a limited number of sandstones from the Denbigh Grits and Elwy Groups, and these are not thought to be representative of the full range. All the sandstones studied in this section may be classed as greywackes (of Pettijohn, 1957).

Macroscopic features

Sandstones exhibit marked variation in grain size and bed thickness. The latter varies from a few centimetres to a recorded maximum of 3 m. In general there is a relationship between bed thickness and grain size. Fine-grained sandstones seldom exceed 0.6 m thick, whereas beds of coarse sandstone most frequently fall in the range 1.5 to 3 m. Exceptions are not uncommon, but thin beds of very coarse sandstone are not found. In that the finer grained sandstones are the most abundant, individual sandstone beds or posts fall most commonly in the range of 0.3 to 1 m.

Sandstone posts may form a series of regular bedded units, separated by thin bands of silty mudstone or siltstone—as for example at Graig Goch Quarry, Nebo [SH 8361 5641] (p. 72)—or, more commonly, exhibit rapid lateral transition, with marked changes in bed thickness as a result of the joining of adjacent sandstones. The latter situation is particularly evident in sections along the eastern side of the Conway Valley, where it is commonly accompanied by marked changes of grain size in individual posts and by the passage of sandstone/silty mudstone sequences into sandy disturbed beds.

The sandstones are generally well jointed and in some cases cleaved. Quartz-veining is a characteristic feature, especially of those beds exhibiting rapid lateral changes.

The sandstones are generally grey to grey-green when fresh, the coarser grained examples being paler by virtue of their more conspicuous content of white quartz or pale grey acid igneous rocks. Small brown patches are frequently a reflection of decalcified shell, crinoid and bryozoan fragments; and in the case of more general brown coloration of deep weathering due to fracturing. Dark grey to black areas result from the inclusion of mudstone clasts, but these are generally confined to distinct levels, at or near the base or top of a sandstone post; their significance is discussed below (p. 35).

Microscopic characters<span data-type="footnote">This section and that on the micrometric analysis are based very largely upon a pilot, petrographic study of the sandstones conducted by Dr R. J. Bailey, University College of Bangor, North Wales.</span>

The sandstones typically consist of a poorly sorted mixture of angular sand-size grains of vein-quartz and rock fragments (which together form between 12 and 70 per cent of the rock), feldspars (2 to 8 per cent) and micas, set in a finer 'slate' matrix of chlorite and white mica flakes, the matrix in some cases exhibiting a marked schistosity. Thus described they may be said to be greywackes as defined by Helmbold (1952), and from the percentages of various elements present they fall within the class greywacke' of Pettijohn (1957).

Rock fragments are an important component, particularly in the coarser grained sandstones. They include chert, silty mudstone, metaquartzite, schistose pelite, granophyre, and acid and intermediate volcanic rocks. In no case is it possible, however, to relate the fragments to source areas, and the majority could have survived previous sedimentary cycles. Quantitative micrometric analysis was by point counter (1000 points per slide) and the grouping of components was into quartzose, volcanic, metamorphic and sedimentary rocks. The analysis of three slides from sample (E35808) gives a clear indication of the extent to which operator error and within-sample variation could influence the final range in the analyses.

Testing of the possibly significant variation, e.g. that in the schistose pelite proportion of the rock fragments, by a simple analysis of variance technique showed that variation was no greater than that obtainable from a common parent population. This may suggest origin of the greywackes in a common source terrain, but equally it may be said that the analysis failed to reveal statistically significant evidence of any pattern of variation in these sandstones.

Heavy minerals

Heavy minerals were obtained from eighteen 50-g rock samples<span data-type="footnote">This work was carried out by Dr D. A. Jenkins of the Department of Biochemistry and Soil Science, University College of North Wales, Bangor.</span>, which were gently crushed and, by combined sieving and sedimentation, 500 to 200 μm, 200 to 60 μm, 60 to 20 μm and less than 6 μm fractions obtained. The heavy minerals (S.G. greater than 2.95) were concentrated quantitatively by centrifugation in tetrabromoethane from the 200 to 60μm and 60 to 20μm fractions, preliminary tests having shown that the 500 to 200 μm fraction was negligible. The samples showed an impoverished assemblage with the following present (in general order of decreasing abundance): chlorite, zircon, tourmaline, apatite, rutile and brookite. Sphene, garnet and siderite occur rarely. Opaque minerals include leucoxene, ilmenite and goethite, but these were not studied in detail.

Chlorite

Chlorite dominates both size-fractions of all the samples. A range of varieties is present, from virtually colourless (some of which are hexagonal) to dark green and, occasionally, yellow-green. Some flakes show brown oxidation rims, and pinkish brown oxidised chlorite is common in many samples (e.g. (E35808) and (E35821)). Birefringence colours range from first order low greys to anomalous greens and browns: all the grains checked were biaxial negative, and the common pale grey-green variety (the 'Powys' chlorite of Smithson, 1953) has an S.G. ≏ 3.0 and β/γ = 1.62. This chlorite is thus in the Brunsvigite class of Hey (1954).

A distinctive fresh blue-green variety occurs exclusively in the 60 to 20μm fraction of sample (E35801), of which it is a common constituent. It also is biaxial negative with dark blue-green birefringence, S.G. ≏ 3.2 and ( β/γ = 1.66. This suggests that it belongs to the Daphnite class of Hey (1954).

Occasionally fine acicular length-slow inclusions showing high birefringence occur, often orientated in a triangular pattern (parallel to {010} and {110}): they are probably of rutile. Euhedral thin hexagonal prisms of apatite also occur in the chlorite, occasionally surrounded by a darkened halo. Such haloes also occur without identifiable inclusions. In sample (E35811) thin brown hexagons (a ferric oxide?) occur oriented in or on chlorite flakes.

Zircon

Zircon is sporadic to common in both size-fractions of all the samples. A number of common varieties can be distinguished: (a) rounded colourless, dusky and yellowish prismatic grains occasionally showing concentric growth lines and cemented fractures. None of the yellowish grains checked by the spectroscopic method was confirmed as monazite, (b) rounded pale pink to purple prismatic grains, (c) euhedral colourless varieties. The last can be distinguished as: (i) commonly {110)}, occasionally modified by {100}, terminated by {111}, (ii) occasionally {100} terminated by {111}, (iii) {110} and {100} terminated by {311} ± {111}. These euhedral varieties commonly have opaque inclusions and irregular gas cavities, and are thus reminiscent of zircons from the Ordovician rhyolitic rocks of Snowdonia.

Variety (a) composes 50 to 70 per cent of the total, (b) about 10 per cent of the total, but these zircons are more abundant in samples (E35809) and (E38814), and (c) about 30 to 40 per cent of the total.

Tourmaline

Tourmaline is a sporadic to common component of both size-fractions in all the samples. A number of general varieties were noted, including: (a) small indeterminate prisms, pleochroic from colourless straw to grey-brown or green; they are often dusky with small inclusions and often show colour banding, (b) larger clear prisms and fragments, pleochroic from pink to green-black, (c) large prisms and irregular grains, pleochroic from straw to brown, often containing inclusions, (d) rounded and angular fragments, pleochroic from colourless or pink to shades of blue.

Prismatic grains often show different coloured outgrowths at one end only. Varieties (a), (b) and (c) are of widespread occurrence, particularly in the case of (a) in the 60 to 20μm fraction.

Apatite

Apatite is a rare to common component of both size- fractions in all samples. It is most common as subrounded prisms, but euhedral prisms are occasionally present. Rare grains show pleochroic cores from brown (parallel to c) to colourless (E37659), or thin elongated inclusions parallel to c.

Rutile

Rutile was noted as a rare to sporadic constituent in all but four samples. It is present as rounded yellow, orange, or red-brown prismatic grains, rarely showing twin lamellae. As described elsewhere, it also occurs in association with chlorite and possibly brookite.

Brookite

Brookite is present as a common to rare component of the 60 to 20 μm fraction of all samples. It is also present as a sporadic to rare component of the 200 to 60 μm fractions, being most abundant in samples (E35802), (E35812), (E35813) and (E35820).

Grains have the characteristic tabular habit of the prism, flattened parallel to 010, elongated and deeply striated parallel to c. Forms identified include {010}, {120}, {221} and {001}, ± {111} (cf. Palache and others, 1944). The crystals range from colourless to yellow or pale bluish or greenish grey; occasionally they show a crude "hourglass" zonation around an opaque core from deep yellow to blue-grey. The grains display typical anomalous first order birefringence colours, usually red-blue: they have the normal optical orientation, giving biaxial interference figures showing crossed axial dispersion, with the o.a.p. (blue) parallel to (100) and (red) parallel to (001).

An interesting feature is the oriented association of the brookite with a pale yellow mineral, showing higher (3rd to 4th order) birefringence and crude lamellar twinning. The optical orientation of this mineral is consistent with it being rutile, with its twin-plane {011} transverse to the brookite prism (i.e. parallel to {001}). In some instances the brookite appears to be an oriented outgrowth on rounded grains of rutile, whereas in others the rutile appears to be replacing the brookite.

Sphene

Sphene is possibly present in several samples as rare to sporadic small irregular brownish granules, showing high birefringence. In one sample (E35804) a small characteristic lozenge-shaped grain was observed.

Garnet

Garnetis possibly present as rare brownish pink, isotropic (n> 1.74) fragments in samples (E35812) and (E35813).

Sedimentary structures

Internal structures

Graded bedding is a feature of the regularly bedded and finer grained sandstones. Uniform grading is uncommon and largely restricted to beds less than 30 cm thick. Thicker posts show 'delayed' grading (Walton, 1956), and the coarser sandstones and the thickest units of whatever grain size show poorly developed grading in general, with 'pen-symmetrical' (Ksiazkiewicz, 1954) and 'multiple' grading. The last includes individual bands showing 'interrupted', 'inverted' (Walton, 1956) and 'symmetrical' grading (Ksiazkiewicz, 1954). The gravelly and pebbly sandstones and conglomerates show the poorest grading, and lenses and bands of differing grain size occur without any clear indication of rhythmic repetition and with as much lateral as vertical variation. Nevertheless almost all the sandstone posts show clear-cut bases and indistinct gradational upper surfaces and to this extent may be considered graded overall.

Shell fragments, crinoid debris and many mudstone clasts are included in the sandstones in the basal position expected in a graded bed produced by deposition from an overladen current. In addition some mudstone clasts occur towards the top of individual posts,in sandstone with generally uniform grain size, and posts with such clasts commonly pass laterally into sandstone/mudstone melanges and thence into sandy disturbed beds. Similar sandstones have been reported by, for example, Marschalko (1970) and ascribed to turbidity-current action and, in part, to slumping. These beds will be considered more fully in the section on disturbed beds (p. 37 et seq.).

The uppermost part of many sandstone units is laminated, with small-scale repetitions of coarser and finer material. These horizons may also exhibit convolute lamination (Haaf, 1956) and minor washout structures. Lamination is occasionally seen at lower levels in the sandstone posts. Current bedding is almost entirely absent from the sandstones, although occasionally present in sandy siltstones of the Denbigh Grits Group.

Bedding-plane structures

A large number of sole-structures occur on the sandstone beds, although they are all generally poorly developed and their occurrence is very largely limited to beds less than 0.6 m thick. Load-casts and flame-structures, flute-casts, groove-casts and prod-casts, longitudinal ridge-casts (Craig and Walton, 1962) and interference-ripple-casts have all been recorded. In addition asymmetric ripples occur on the upper surfaces of some sandstones, notably in the Nebo area.

A number of the above may be termed directional current-structures (Bassett and Walton, 1960) and from them the directions of currents finally depositing the sandstones have been determined. In the field the pitch of the structure on the bedding plane was measured and this was converted to an azimuthal current direction by reorientation about the local or regional fold plunge, following Norman (1960). Because almost all the structures recorded were from right-way-up beds with relatively low dips the corrections involved are very small.

Directional current-structures plotted as a rose-diagram ((Figure 12)(b)) show that the sandstones were deposited by currents flowing from directions between south and northwest (through west). The range is thus considerable and the measurements few (30). Nevertheless 250° (west-northwest) is considered to represent the predominant current source. The results are very much in agreement with those obtained by Cummins (1957). The lack of precise stratigraphical control in the Denbigh Grits Group prevents any assessment of how far the range of current directions can be attributed to a changing current pattern with time.

Sedimentation and provenance

The nature of the sandstones—poorly sorted, often coarse-grained, pebbly or conglomeratic, and largely unstratified with an abundant clay matrix—suggests deposition by strong, rapid, short-lived currents of a high density. Such features are characteristic of turbidity currents (Kuenen and Migliorini, 1950), which would satisfactorily account for the graded bedding, directional current-structures and the sharp sandstone soles.

A number of features of all but the regularly bedded sandstone sequences—notably, the abundance of multiple grading and overall poorly developed grading, rapid lateral variations in thickness, the inclusion of numerous mudstone clast, and the association with slumped beds—indicate that the sandstones are deposits of turbidity currents close to their point of origin—the proximal turbidites of Walker (1967) or fluxoturbidites of Dzulynski and others (1959), Unrug (1963, 1964) and Dzulynski and Walton (1965). Walker (1967) has argued that the term fluxoturbidite should be abandoned in that it combines a bed, facies and a process description and in no case classifies concisely a division within any of the concepts. In terms of bed terminology it has been used for strata ranging from slide conglomerates (Stanley, 1961) to proximal turbidites. Although, as Walker (ibid., pp. 38–39) argues this is a continuous sequence, there does seem to be some justification for a term descriptive of the product of a process which is neither sliding or slumping on the one hand nor turbidity flow on the other, and fluxoturbidite is retained here to fulfil this need.

Four distinct facies are recognisable in the arenaceous sequences of the Denbigh Grits Group: viz, in order of increasing grain size, 1 the regularly bedded sandstone facies, 2 the irregular and thicker bedded coarse-grained sandstone facies, 3 the pebbly sandstone and mudstone facies, and 4 the conglomerate facies. The regularly bedded sandstone facies is interpreted as a distal turbidite facies as described by numerous authors (e.g. Walker, 1967).

The irregularly bedded, coarse-grained sandstone facies comprises beds typical of those described by Walker (1967) as proximal turbidites. The association of disturbed bedding is characteristic, and as Marschalko (1970) has shown this may be due in part to the impact of a turbidity current upon a mudstone substratum and in part to slumping. In the latter case the slumps are not necessarily from the same source as the turbidity flow. The amalgamation of sandstones is typical of this facies and its expression may reasonably be taken as an indication of the source direction of the rocks—the more distal turbidites showing inter-banded mudstone. Where recorded this feature indicates a southerly source.

The conglomerate facies is represented by deposits at only one locality, namely Oerfa Quarry [SH 8372 5881]. The emplacement of this horizon appears to have been by a mud-flow or slide, a view supported by the very close association of this bed with slump beds believed to have developed from mud-flows.

Intermediate in appearance, grain size and mode of origin between the conglomerate and proximal turbidite facies, lies the pebbly sandstone and mudstone facies. It is to the rocks of this facies that the term fluxoturbidite is applied. Following Walker (1967, p. 39) the series conglomerate–fluxoturbidite–proximal turbidite can be interpreted 'in terms of a general increase in intergranular movement and development of fluid turbulence during transportation... ' Depending upon the pore-water pressure in the sediments they appear to have variously responded by slumping, slurrying (see p. 34) or flowing. However, the transport mechanisms are very poorly understood (ibid., p. 38), and the terminology of these sediments is problematical (Warren in Smith, 1970 p. 34). It seems unlikely that any member of this series was deposited more than a few miles from the shoreline of a land mass situated, on the evidence of current directions, to the south-west of Llanrwst and consisting, on the meagre evidence of the petrography, of metamorphic and igneous terrain. Similar conclusions were reached by Cummins (1957) and have been summarised by Ziegler (1970).

Sandstones of the Elwy Group

The arenaceous rocks of the Elwy Group differ markedly from those of the Denbigh Grits Group in two important respects. They are invariably regularly bedded and they show relatively little variation in grain size. Petrographical studies<span data-type="footnote">Including those by Dr R. J. Bailey and Dr D.A. Jenkins, University College of North Wales.</span> show that they are greywackes and indicate a close similarity to the Denbigh Grits sandstones. The Elwy Group sandstones are in general well graded and show upward passage through siltstone into mudstone. The problem of nomenclature of such beds has been discussed by Warren (1963), who coined the terms greywacke-siltstone and greywacke-mudstone for the finer elements in the graded unit. In this account all three terms are covered by the term sandstone, which thus refers to the entire graded unit.

Macroscopic features

The sandstones are grey-green in colour when fresh, but they weather brown in most exposures. Except in a zone of lateral passage to disturbed beds, they exhibit regular bedding, with clear-cut bases and gradational tops. Beds vary from 2.5 cm to the maximum recorded of 4.6 m, and in general the thinner beds are the finer grained. Mud flakes are a common feature and locally give rise to mud-flake conglomerates. The fauna of the sandstones is sparse, but includes crinoid and shell debris in some cases. The sandstones are generally well jointed and locally cleaved, and they commonly contain quartz stringers.

Microscopic features

The sandstones appear to be petrographically identical to those of the Denbigh Grits Group, except in so far as the finer grain size renders an assessment of the rock-fragment content more difficult. Hence the account given on pp. 33–34 applies to these rocks also.

Sedimentary structures

Internal structures

Grading is almost entirely of the 'delayed' type, with most of the bed showing a more or less uniform grain size and the upper part grading rapidly into mudstone. Mudstone clasts, shell debris etc. occur in the expected position.

Lamination is a characteristic feature, and in terms of Bouma's (1962) divisions (intervals) of a complete turbidite, both the lower and upper parallel lamination divisions (B and D) are represented, the former being particularly evident in many sandstones and commonly forming the base of the bed. The division (C) of current ripple lamination is also generally apparent, and convolute lamination, festoon bedding and ripple-drift bedding all occur. Where sandstones have been noted resting on irregular surfaces of disturbed beds, current bedding has sometimes been recorded in sandstone infilling the hollows.

Bedding-plane structures

Asymmetric ripples have been noted on the upper surfaces of some sandstones. Some sandstones possess undulatory bases or less commonly show evidence of cut and fill, but in general the bases are parallel to the underlying bedding. Sole-structures are not common, but load-casts and flame-structures, flute-casts, longitudinal ridge-casts, groove-, bounce- and prod-casts, interference-ripplecasts and flowage-casts have been recorded.

The relationship noted by Warren (1963) between bed thickness and grain size on the one hand and sole-structures on the other apparently holds for the Elwy Group sandstones, with flute- and longitudinal ridge-casts restricted to the thicker, coarser beds and the bounce- and prod-casts confined to thinner, finer beds.

A number of sandstones show well marked flame-structures associated with pseudo-nodules (Macar and Antun, 1950), and in the unique case on the west side of Moel Fodiar a sandstone has formed into a series of boudins up to 1 m across and 0.6 m deep.

The origin of currents responsible for the deposition of these sandstones has been deduced from the directional current-structures ((Figure 12)(a)). The scatter, though considerable, is clearly distributed about a mean source direction of 280°. In addition to this predominant eastward flow there is some evidence of counter-currents (flowing from between 110° and 125°) and of currents from both north and south. In view of the close association of the sandstones with disturbed beds (p. 30), the formation of which must have displaced large masses of water in variable directions, it is perhaps surprising how predominant is the easterly flow deduced from the current structures. These results largely confirm those of Cummins (1959b), although it has not been possible to show a preponderance of northerly flowing currents in the eastern part of the present district as might be expected from Cummins' results (ibid., fig. 1).

Sedimentation and provenance

The Elwy Group sandstones belong to the regularly bedded facies (p. 33) and are interpreted as the distal products of turbidity currents. The frequency with which beds begin with the B division of Bouma (1962) and show well developed lamination suggests deposition in a relatively low flow regime (Walker, 1967) with the development of a thin traction carpet. The implications are that either the depositional area lay some distance from a landmass—and at a much greater distance than during the deposition of the Denbigh Grits—or that the initial impetus of the turbidity currents was small, implying relatively gentle slopes in the area. In the lower part of the

Elwy Group neither alternative seems likely, for disturbed (slumped) beds are abundant. It is possible therefore that the sandstones are derived from low-velocity turbidity currents generated within the basin of deposition by the slumping itself. In the middle part of the Group, where sandstones are universally present, the situation may have been one of relatively gentle slopes and turbid flows in a basin largely filled up by the extensive slumped beds.

Disturbed beds

Disturbed beds are here defined as strata in which the bedding is highly contorted and/or fragmented, sometimes to the extent that it is unrecognisable in individual exposures or at certain horizons. They vary in thickness from less than 1 m to more than 60 m, the thinner horizons usually having clearly developed folds and the thickest consisting of apparently unbedded, often highly fractured, silty mudstone. They are composed of any or all of the rock types of the Wenlock–Ludlow sequence, and are subject to rapid lateral variations in lithology. As a whole the disturbed beds are more resistant to weathering than the undisturbed strata, and they therefore form much of the higher ground in the district. Their characteristic morphological expression is irregular moundy ground with numerous small exposures (the golf- course topography), well illustrated by the summit of Tre-pys-llygod [SH 885 684], near Llangerniew. The origin of this topography lies partly in the rapid lithological changes and partly in the faulting and jointing of the disturbed beds. Probably as a result of the absence of bedding planes and hence of bedding-plane slip, the disturbed beds are preferentially faulted (Smith and others, 1965), and weathering, including ice action, has picked out and greatly enlarged the numerous fault lines as well as the joints. The disturbed beds are resistant not only to weathering but to mechanical removal, and the excavation of any substantial thickness almost invariably necessitates the use of explosives.

It is possible to recognise six distinct types of disturbed bed: 1 Bedded, 2 unbedded, 3 pencil slate, 4 pebbly/sandy mudstone, 5 raft-bearing, and 6 convolute.

1 Bedded disturbed bedsare relatively common and make up a high proportion of the upper Wenlock and Ludlow disturbed strata. They are characteristically composed of striped silty mudstone in which the original bedding is largely retained, but may also contain a relatively high proportion of sandstone. Slump folds are evident and vary in size from small puckers to folds some 2 m in wavelength and amplitude. The fold axial-planes lie in any position from horizontal to vertical. Jones (1940) maintained that there was a preferred orientation to the folds with a predominant east–west strike to the axes. The present study has not confirmed this view, but with an almost complete lack of three-dimensional sections any analysis of the fold orientations is severely limited.

In the most disturbed examples the original existence of a bedded sequence can be inferred only from the presence of discrete rolls, lenses and irregular masses of sandstone and silty mudstone, and these approach in character type (4) below.

2 Unbedded disturbed bedsare generally associated with bedded disturbed beds. In them foliation planes are always evident, often in the form of small scale isoclinal folds as in the Camaes area [SH 860 680] west of Llangerniew. Evidence that these planes represent bedding is lacking however, and in some instances the gently curved, nearly vertical planes might be folded cleavage, though no preferred orientation related to the structural history of the area can be identified. Most of these disturbed beds consist of silty mudstone, and it is possible that the general absence of bedding is essentially a reflection of the character of the original sediment.

3 Pencil slate disturbed beds consist predominantly of slightly silty mudstone of finer grain than any other in the Wenlock–Ludlow sequence. Intensely fractured, the rock breaks into fragments, approximately cuboidal and seldom more than 1 cm across. The fractures show no preferred orientation but two, and commonly three, sets of fractures at right angles to one another are generally present. Moreover, different parts of a single exposure commonly show differently oriented sets of fracture planes, although these maintain this same mutual agreement.

Boswell (1949, p. 113) clearly attributed these rocks to strong tectonic deformation of a uniformly fine-grained sediment. However, the lack of any preferred orientation to the fractures is inconsistent with this hypothesis, and during the present studies convincing evidence that these are slump beds has accumulated. At a number of localities, notably in Llanrwst Station Road [SH 7959 6209] (p. 72) and south of Tal-y-cafn [SH 7813 6963] (p. 65), the pencil slate disturbed beds contain irregular masses, lenses and balls of sandstone. The margins of these are not sharp and their shape is generally irregular on one side at least. They are interpreted as the remnants of sandstone bands. Further evidence that the pencil slate rocks are slumps is given by exposures, for example at Oerfa [SH 837 588], where a continuous lateral transition of these beds into type (1) disturbed beds is observed.

4 Pebbly/sandy mudstone disturbed bedsare closely related in appearance to the pencil slate variety, in that they exhibit similar intense fracturing. Everywhere, however, they show evidence of disturbance in that they consist of silty mudstone with isolated pebbles and scattered sand grains. This type of disturbed bed would appear to represent the ultimate stage in the disturbance of a sandstone/silty mudstone alternation.

5 Raft-bearing disturbed beds are a special variety of the foregoing types in which undistorted rafts and blocks of bedded strata occur. They are particularly common in the lower part of the Denbigh Grits Group and are well seen in the Benarth, Roewen and Llanrwst areas. Because they occur in this part of the stratigraphical column they are generally composed of sandstone, often pebbly or conglomeratic, and silty mudstone, and the rafts, which are always of local and penecontemporaneous rocks, are of similar strata. In one case at. Hafotty Bennett [SH 8366 6905] (p. 71), near Eglwysbach, a raft of striped silty mudstones and thin sandstone bands itself contains a thin disturbed bed of type (5).

6 Convolute disturbed beds are thin (less than 0.6 m) sheets of disturbed bed rock which show clear folds but lack precise lower boundaries. The best example is that cropping out on Mynydd Bodrochwyn [SH 9385 7225] (p. 87), near Llanfair Talhaiarn, and figured by Boswell (1949, pl. 13). This shows an horizon of striped silty mudstone with folds and overfolds which die out downwards and are terminated upwards by a thin, wavy band of silty sandstone. There is a close similarity between disturbed beds of this type—which are relatively rare—and horizons of convolute lamination recorded from the finer grained parts of sandstones, mainly in the Elwy Group. Only in the truncated upper margin do the structures differ, and suggest differing modes of origin (see below).

Fauna

Nests of Shelly fossils, including corals, bryozoa, brachiopods, mollusca, trilobites, ostracods and crinoids occur in the typical developments of bedded disturbed beds. The presence of composite moulds indicates that disturbance occurred at a time when some fossils still possessed their original shells. A large number of the shells are contorted, with individual measurements altered by a factor of up to 2. The faunal assemblages in the Elwy Group can be assigned to the Isorthis community of Calef and Hancock (1974, p. 787), that is the probable equivalent of the Stricklandia community of Llandovery (Cocks, 1967).

Similar, but much sparser faunas are found in the unbedded disturbed beds, but the other types lack a fauna almost completely. At Ty'n-y-ffordd Quarry [SH 8699 6525] (p. 79) graptolites in relief have been recovered from blocks of decalcified siltstone in the basal 1.5 m of a disturbed bed. The species, Monograptus uncinatus orbatus Wood, was not recorded from the normal-bedded strata immediately underlying the disturbed bed, despite their being rich in graptolites.

Basal contacts

With the exception of the convolute type, all disturbed beds have basal contacts which are sharp, or involve rapid transitions to undisturbed rocks. They may be divided into the following types: 1 uniform, 2 local downcutting, 3 regional downcutting, 4 lateral passage, 5 rip-up, and 6 conglomeratic.

1 Uniform In a high proportion of disturbed beds, and particularly in those less than 10 m thick, the basal contact may be said to be uniform, i.e. it parallels the bedding in the underlying strata, is sharp and clear-cut and gives no evidence of deformation of the underlying beds. This relationship, as well as being seen in exposures, can be inferred from mapping and palaeontological studies to hold on a regional scale.

2 Local downcutting In many exposures disturbed beds are seen to transgress but not deform the bedding of the underlying strata. This downcutting varies in extent from a few centimetres to several metres, and is also variable in form and direction. At Ffrithuchaf [SH 8308 6493] (p. 70) the Brynsylldy Formation shows channel cuttings, trending 112° and up to 2 m deep, on an otherwise uniform base. At Ty'n-y-ffordd Quarry [SH 8699 6525] (p. 79) the more characteristic form of downcutting is seen—a progressive transgression of the bedding, here totalling 1 m, across the exposure from west to east (Plate 1). A progressive transgression of some 3 m towards the north over a distance of some 30 m is evident in Creigiau Quarry, Llansannan [SH 9381 6537] (p. 90). In a quarry [SH 9077 6625] (p. 90) near Halod-y-gog, west of Llansannan the downcutting is very marked—6 m over less than 5 m—with some slight deformation of the underlying ribbon-banded silty mudstones (cf. type 5). No consistent picture of extent, characteristics or direction of downcutting is obtainable from these local occurrences.

3 Regional downcutting At three horizons, viz: the Llanddoget and Brynsylldy formations, the Llangerniew Disturbed Beds, stratigraphical control is sufficiently tight to show that major transgressions occur at their bases over large areas of their outcrops. Some exposures of the basal contacts of these units, e.g. that of the Llangerniew Disturbed Beds at Ty'n-y-ffordd Quarry [SH 8699 6525] (p. 79) indicate downcutting, but other exposures, for example those of the Llanddoget Formation near Goppy [SH 8027 6618], suggest that there is also lateral passage (see type (4) below).

4 Lateral passage There are two varieties of this type of basal contact. The first, and this is most commonly seen in sandstone/mudstone sequences, involves the interdigitation of disturbed beds with regularly bedded sequences. This relationship is particularly well exhibited by rocks of the Elwy Group on Cefn Du, north-east of Eglwysbach (pp. 62–63). On a larger scale such a relationship is implied by the mapping of the area around Goppy and Goelas Fawr, alluded to above (Figure 17).

The second variety of lateral passage from disturbed to normal-bedded strata involves the presence of 'incipient disturbed beds'. These generally consist of striped silty mudstones showing numerous 'fractures which are at right angles to the bedding and in anastomosing groups centred about the harder bands. These fractures, which decrease in number away from the disturbed bed contact, produce small-scale sedimentary boudinage or 'pull-apart' structure in the bedded sequence for some 3 to 5 m from the contact. 'Incipient disturbed beds' are particularly well developed in the Cefn Du area and immediately to the south-west of Colwyn Bay (see p. 62 and Jones, 1940, fig. 18).

5 Rip-up In contrast to the foregoing types of basal contact, in which the underlying strata show minimal disturbance, this type provides evidence that considerable erosion, plucking and buckling took place during the emplacement of the disturbed bed, which is frequently of the raft-bearing type. Deformation of the underlying beds ranges from the local disorientation of the bedding, seen for example at Cefn-y-groes fawr, to the incorporation of rafts, blocks and irregular masses of the substratum into the disturbed bed. In all cases material included in the disturbed bed can be related to the rocks upon which the disturbed bed rests. In the Nantglyn Flags, this type of disturbed bed commonly contains disorientated calcareous concretions identical to those in the normal-bedded flags (see p. 45).

6 ConglomeraticAlthough the underlying strata are undeformed, the basal contact in this group tends to be ill-defined. The blocks composing the conglomerates consist of silty mudstone, decalcified siltstone, black mudstone and collophane and are largely exotic. They vary in size from a few millimetres to 15 cm, which is the maximum thickness of a conglomerate band, and are generally well-rounded but have a low sphericity value. The decalcified siltstone fragments are mainly rectangular with only the corners rounded. This rock type contains fossils, notably coral-like organisms and in some instances graptolites (see p. 79).

Basal conglomerates of this type are known from two localities—Fron-goch [SH 909 699] (p. 82) and Dyffryn Aled [SH 9495 6710] (p. 85). These localities lie close to the base of the Elwy Group and at much the same horizon at Ffrith Bedwyn [SH 910 674] there is a similar conglomerate interbedded with sandstones which are themselves interbedded with disturbed beds. The conglomerates may reflect a period of particular instability, as do the conglomeratic sandstones of the Denbigh Grits Group (p. 36). Comparable conglomerates have been recorded by Bailey (1964) from south Central Wales and by Wills and Smith (1922) in the Llangollen area, both at approximately the same horizon and marking the base of a major disturbed bed sequence.

It should be added that elements of the conglomerates—in particular collophane pebbles—are present within and at the top (see below) of a number of disturbed beds throughout the stratigraphical sequence, decalcified siltstone blocks being recorded for example from Tyn-y-ffordd Quarry (see p. 79).

Upper contacts

In marked contrast to basal contacts, upper contacts appear remarkably uniform. The overlying strata, generally sandstone or striped silty mudstone, rest with marked unconformity' on the disturbed bed, folds in the latter being truncated, and irregularities in the upper surface filled by current-bedded sandstone. The latter irregularities are, however, the exception and the contacts are generally near-planar. At a number of localities, notably Mynydd Tryfan [SH 9703 6583], the uppermost 30 cm of the disturbed bed contain a relatively high proportion of shelly fossils and pellets, up to 12 mm across, of black collophane, the significance of which is discussed below.

At two localities—near Heskin [SH 9591 6668] (p. 95), Llansannan, and near Hafodty Farm [SH 8327 7790] (p. 62), Mochdre—sand volcanoes have been recorded from within the sandstone immediately overlying disturbed beds. Those from the latter locality are no longer visible, but were described by Gill (1957); they were up to 30 cm in diameter and some 7 to 10 cm high with a central 'vent' about 12 mm across. Those from the Heskin locality are poorly developed and can be seen only in cross-section.

Jones (1940) maintained that the sediment overlying a disturbed bed was always sandstone and that this fact was significant in terms of the origin of the disturbed beds. It is certainly true that in the large majority of examples of upper contacts seen there is overlying sandstone (Plate 6), but silty mudstones have been observed to overlie disturbed beds at a number of localities, and it is suspected, on physiographical grounds, that this situation may be quite common.

The importance of upper contacts in an interpretation of the origin of disturbed beds cannot be over-emphasised (see below) and hence those localities at which such contacts can be seen, or with the minimum of excavation could be seen, are included in the details of the regions (see index). Included are the localities listed by Jones (1940, pp. 375–376), most of which were confirmed during the present survey. No evidence has been found of distinct junctions within a mass of disturbed beds which might be construed as the contact between two separate disturbed sheets. It seems unlikely, however, that disturbed beds are not overlain by other disturbed beds in some areas, and the very considerable thickness of disturbed strata in, for example, the Llangerniew area, with local evidence of interbanded normal sediment is best interpreted in this way.

Origin

The genesis of the disturbed beds of north-west Denbighshire has been a vexed question (see e.g. Boswell, 1932, 1949, 1953; and Jones 1937, 1940, 1954). The present authors ascribe to the general view, held by Jones, that the disturbed beds are the products of penecontemporaneous slumping or sliding on the sea-floor. In detail, however, certain features of disturbed beds, noted particularly by Boswell and especially around Llangerniew (but see p. 43) are suggestive of additional processes in some instances.

The critical evidence in respect of the mode of formation of disturbed beds lies, as Jones (1937) pointed out, with the upper contact. The evidence adduced above clearly points to the deformation of disturbed beds prior to the deposition of the beds overlying the contact. The alternative hypothesis of relatively late-stage tectonic disturbance requires the presence of a plane, or planes, of decollement between the disturbed bed and overlying sediment for which no evidence has been found. Only in the case of the convolute disturbed beds (p. 38) is it likely that any sediment overlay the now-disturbed bed prior to its disturbance. The evidence is not conclusive, but this type of disturbed bed is believed to have formed in the manner described by Dzulynski and Walton (1965, p. 191) as a ' plastic glide' in which 'deformation of the beds took place within an envelope of soft plastic silts or clays'. They are rather later in formation than the majority of disturbed beds, although still penecontemporaneous with deposition.

In addition to the critical evidence, other features of disturbed beds imply a slump origin. The following features listed by Helwig (1970) occur in the present disturbed beds: detached fold blobs; disrupted bedding; isolated stratigraphic horizon of disturbances; variable and/or irregular fold style; curvilinear fold hinges; piled-up, recumbent, or nappe-like folds (Plate 7); secondary cleavage(s) nonparallel to fold axial surfaces; sedimentary breccia or pebbly mudstone; folded clasts. In addition, one disturbed bed contains a raft, itself showing a thin disturbed bed (see p. 71).

The concentration of pellets and shells at the top of some disturbed beds implies a period of winnowing of the slump whilst it formed the sea-floor, and the near-planar upper surfaces also suggest a period of erosion prior to further deposition in the area. These suggestions contradict the belief of Jones (1940, p. 356), that the overlying sandstone layer represents the settling of a cloud of material resulting from the subaqueous slumping. In support of his contention he noted the generally graded nature of this sandstone band. However, almost all the sandstones of the sequence are in some measure graded. It seems more probable that material sufficiently liquefied during slumping would behave as a suspension or turbidity current and this may in part explain the relative abundance of graded sandstone in the sequence containing the highest proportion of disturbed beds, viz. the Elwy Group.

The existence in some disturbed beds of nodules, the origin of which is known to be during early diagenesis of the enclosing sediments (see p. 45), indicates that disturbance post-dates some lithification and diagenesis. This is confirmed by the nature of slump folds and rolls and the deformation of the basement upon which the slump took place. In some instances it is possible to recognise the precise degree of lithification in that certain slump rolls preserve groove-casts on the sandstone bands at their base. In these cases the rocks were sufficiently cohesive to preserve the casts of fine grooves in the underlying mudstone, whilst behaving plastically to the extent of enrolling rather than fracturing.

The formation of the rolls may be similar to that of pseudo-nodules (Macar and Antun, 1950) which Kuenen (1958) showed experimentally to be the result of the sinking of sand into clay after the latter had received a shock. Macor and Antun considered their origin to lie in submarine slumping implying some forward movement of one layer over another. The possibility arises that vertical and differential loading and consequent collapse may have played as large a part as horizontal mass movements in causing the disturbance of some beds. Some parts of certain disturbed beds at least are believed to have originated in this way.

The rich shelly fauna of the disturbed strata is almost unique to them, being found in such profusion elsewhere only in the 'gingerbread' bands of normal sediments. This fauna belongs to a community normally existing in somewhat shallower water than the graptolitic facies of the normal sediments (Calef and Hancock, 1974, and p. 59), but mass movements may not necessarily have been horizontally extensive. The possibility that disturbance affected only certain lithofacies and that these with their shelly fauna were originally interbedded with normal sediments of graptolite facies is discounted in view of the general absence of an undisturbed shelly facies. Lateral passage of disturbed into undisturbed shelly facies has been observed only in the mottled mudstones and they, themselves, exhibit local disturbance.

The conglomerates and conglomeratic bases of disturbed beds afford the most direct evidence of horizontal displacement, in that they contain (decalcified) calcareous siltstone blocks, with in one instance corals and elsewhere graptolites unrecorded in the underlying normal sediments. Neither these blocks nor the collophane pellets can be related to contemporaneous normal sediments.

The absence of any indisputably shallow-water facies in the undisturbed Wenlock–Ludlow sequence of North Wales precludes any direct evidence as to the source of the slumped material now forming the disturbed beds. Nor is there evidence in the area of the removal of a part of the succession which could indicate the source of a disturbed bed. Basal and lateral contacts at various localities yield evidence of movement, but apparently from various directions. Jones (1940) suggested that the majority of slump-rolls were oriented east–west, and from this, taking into account over-folding and general considerations of palaeogeography, he concluded that slumping took place from north to south.

Whilst it is true, in the Colwyn Bay area in particular, that the Elwy Group disturbed beds show contacts and folds aligned east–west and that there is an increase in the proportion of disturbed beds present when traced from north to south, slumping from the north is not necessarily indicated. Helwig (1970) has shown how fold axes may be reoriented during slumping to give a very wide range of variation in the final deposit, and Dzulynski (personal communication) some years ago demonstrated experimentally that slump rolls are frequently formed and remain with axes parallel to the direction of slumping. The latter would appear to be the case with a roll occurring on Moel Fodiar which displays groove-casts that are subparallel to the length of the roll, unless it be accepted that slumping took place down a slope at right angles to that down which the groove-cast turbidite sandstone was deposited. Bailey (1967) has recently discussed such situations and interpreted them as indicating the lateral and axial slopes of a basin, or reflections of differing slopes over a period of time. Some indication of slumping direction is yielded by a study of the distribution of disturbed strata at particular horizons, particularly in the Llanddoget and Brynsylldy formations and the lower Elwy Group.

The Llanddoget Formation shows rapid thinning northwards and south-eastwards from Llanrwst, which is interpreted as reflecting local slumping from south-south-west to north-north-east. This direction is parallel to that indicated for the major proportion of the turbidity current flows responsible for the immediately underlying sandstones.

A similar situation occurs with the Brynsylldy Formation near Eglwysbach, the unit dying out not far to both the north and south (although reappearing still farther south). Again an east–west 'trough' is considered the most probable explanation. Farther south around Ffrithuchaf there appears to be another east–west trough affecting the Brynsylldy Formation. Here small-scale channels at the base are aligned 112–292° (p. 39) with ripping-off suggesting movement from west to east.

From the Mwdwl Eithin area [SH 829 683] as far south and east as the Gwytherin area, the Brynsylldy Formation maintains in general a constant thickness (see p. 27). However at the type locality and to the south it thickens considerably and cuts down in the succession, ultimately around Pennant removing the Upper Mottled Mudstone. This suggests a south-west–north-east channel.

Perhaps the most significant indication of the sources of disturbed beds lies in the areal distribution of the lower Elwy Group. The generally east-north-easterly plunge of the folded outcrop obscures the three-dimensional picture, but sufficient is seen of the lateral variation to show that changes from north to south (e.g. from the Llangerniew to Gwytherin regions) are less marked than those from west to east (e.g. from the Llangerniew to Llansannan regions) and hence to indicate that the slumped beds originated in the west and travelled in a general eastward direction. Jones' hypothesis of a northerly origin implies a consistent and extensive facies change from north to south, which is not found. Only in the Colwyn Bay area is there evidence of north–south movement and this is best interpreted as flow controlled by the proximity of the northern margin of a basin oriented east–west and with an axial slope towards the east. That this area is exceptional is borne out by the presence here alone of sandstones in the lower part of the Elwy Group.

The immediate cause of the slumping would, on experimental grounds, seem to be a sudden shock, for example an earthquake, resulting in the denser packing of a saturated, loosely packed, poorly drained sand layer, or to the rapid loading of a porous but ill-drained, saturated clay layer (Bailey, 1967). As Bailey (ibid., p. 1253) remarked, 'In either case the change in conditions causes a rise in the pore water pressure in the buried layer such that, in the extreme case, the overburden is supported by the pore-water rather than the grain to grain transmission of stress within the layer. Where this occurs, the effective normal stress on the buried layer can be reduced to zero and the shear resistance of the layer, which largely depends on this stress, will also tend to zero. The sediments above the layer, if lying on a slope, may thus begin to slide'. The ultimate nature of the slumped deposit (disturbed bed) will depend upon the degree of lithification of the sediments, the proportion of water to sediment in the initial and final stages of mass movement, the slope down which slumping occurred and, as Bailey (ibid.), notes, whether or not the slide plane remains below the sediment/water interface. If it does, or if layers become loaded by an initial superficial slump, the mechanism of 'progressive slumping' (Moore, 1961, p. 354) may occur, which may lead to the more distal disturbed beds showing marked disturbance and yet being essentially autochthonous.

Although there are many types of disturbed beds, all could have been produced by a single hypothetical slump. The progressive stages and associated critical factors are shown below.

The initial sediment collapse gives rise to disturbed beds of type (1) (p. 38), in which sand injection, collapse of sand layers into mud, etc. occurred, or, when restricted to only one horizon in the sequence, to the convolute disturbed bed (type 6). Dependent upon the presence and inclination of a depositional slope, the collapsed sediment will move as a sand or mud-flow and ultimately as a slump. Differences in cohesion will lead to different forms of movement, but at this stage some of the disturbed beds of types (3), (4) and (5) (p.38) will have formed. Such flows have enormous transporting power (Ksiazkiewicz, 1958) and are quite capable of carrying the conglomeratic material described (p. 39) without it disintegrating, for it can be held in a non-erosive medium of plastic flowing mud. Of interest here are the isolated sandstone masses in the type (3) (p. 38) disturbed beds, which may be interpreted as 'whirl-balls' (Dzulynski and others, 1957), formed by vortex structures in which the clean sand was concentrated during flow. Their preservation by instantaneous setting of the mud-flow (ibid.) would be consistent with the marked fracturing of this type of disturbed bed, explicable only in terms of residual forces in a mud-flow suddenly 'frozen' by rapid loss of pore-water pressure (Bailey, 1967).

Increasing slope or loading of the slump by further collapse leads to the erosional slump phase in those instances where the shear resistance at the slide plane is relatively high. Extensive scour channels (Crowell, 1957) and the incorporation of torn up sediment into the mud-flow (Sutton and Watson, 1955) have been described both in the present district and elsewhere. The presence of flow-rolls in disturbed beds overlying down-cut contacts strongly suggests a relatively low pore-water pressure, and hence relatively high shear resistance and plastic flow.

The contrast between rip-up and lateral passage contacts (p. 39) is interpreted as the difference between contacts essentially belonging to the original floor over which the slump passed, and by which it was affected, and those at the margins of slumps.

With decreased slope but high pore-water pressure the slump travels beyond the palaeoslope over sediments unaffected by collapse or by the overriding slump (Bailey, 1967), resulting in the uniform contacts displayed by so many disturbed beds. For these disturbed beds considerable horizontal movement is implied, and it is likely that shear resistance at the time was responsible for considerable deformation of both the sediment and its shelly fauna. Evidence of the latter has been given; evidence of the former lies in the conspicuous folding and 'cleavage' of disturbed beds like those of west of Llangerniew which Boswell considered to be the results of tectonic deformation. However, the characteristics of the deformation—variable and/or irregular fold style; curvilinear fold hinges; secondary cleavage(s) non-parallel to fold axial surfaces; absence of any preferred orientation in fold axial planes or cleavages—are all features now considered to be indicative of slump deformation (Helwig 1970). There appears to be no single feature that is uniquely associated with tectonic deformation. Furthermore, folded 'cleavage', far from implying a late stage deformation of the disturbed bed, may now be reinterpreted as a very early stage in the formation of cleavage—probably during the processes of lithification and slumping (cf. Elliston, 1963). A similar conclusion as to the age of cleavage formation and its relation to high pore-water pressure has been reached by Braddock (1970, p. 598), who concluded that 'The formation of cleavage and the intrusion of clastic dikes seem to be only different manifestations of the same process'.

At any stage an increase in water content would cause the slump to pass the liquid limit and become a suspension and, under slope conditions, a turbid flow (Dott, 1963). The close association of turbidites and slump beds in the Elwy Group suggests that this occurred, and most of the Elwy Group sandstones thus probably represent the distal products of slumps.

The final stage of settling of material thrown into suspension by the slumping can be expected to take the form of thin graded layers of fine sandstone, siltstone and mudstone deposited outside the area of deposition of the slump mass, and it is such a facies that forms the bulk of the striped silty mudstones (p. 49).

Ribbon-banded mudstones

This lithology is defined as a regular alternation of silty mudstone and laminated muddy siltstone bands, with locally developed thin calcareous siltstones (Plate 5). It thus comprises three rock types which are markedly different in structures, textures and fauna and thus probably in mode of formation, although from their close association they must have been deposited in a similar environment.

The ribbon-banded mudstones comprise the greater proportion of the Nantglyn Flags and have been described in detail by Cummins (1959a). Elements of them, in particular the laminated muddy siltstone, have been described by Warren (1963) and Rickards (1964) under the terms graptolite shale' and graptolitic mudstones', respectively from the Silurian of Southern Scotland and the Howgill Fells. Llewellyn (1960, 1965) has described and interpreted ribbon-banded mudstones from the Lake District Silurian.

The silty mudstone bands are tough, blue-grey rocks which weather brown. Fissility is poor, even in weathered examples. In thin section they are seen to be calcareous. The fine silt of these bands is mainly quartz, averaging 121.4 m across, but chlorite and mica flakes also occur. The dominant features of the silty mudstones, seen under high magnification (x500) are the illite flakes (averaging 20 by 4μm) generally aligned parallel to bedding. Fine opaque dust is optically indeterminable, but minute (2 to 5μm) rounded specks of blood-red hematite are common, along with sparse zircon crystals (20 by 50 μm). The silty mudstones are devoid of sedimentary structures and are quite unfossiliferous.

The laminated muddy siltstones are distinctly coarser grained beds, with quartz silt predominating. They show the same colour and weathering as the silty mudstones but differ greatly in appearance by virtue of the lamination. In hand specimen the lamination is parallel to bedding and a laminated band may itself include laminae of silty mudstone. In thin section it is evident that the lamination is discontinuous and results from an arrangement of lenses of relatively clean silt surrounded by silty mudstone and opaque, partly carbonaceous, matter. Surfaces parallel to bedding show a mottled appearance due to the silt patches, which are roughly equidimensional and have vague outlines. The silt fraction is predominantly angular grains of quartz, but plagioclase, chlorite and mica flakes and vein quartz fragments occur, along with accessory zircon, rutile, tourmaline and apatite. Graded bedding is evident in some of the silt lenses which are up to 1 mm across and 0.3 mm high. The finer fraction of this rock type is apparently identical to the silty mudstones described above. The opaque material consists largely of detrital magnetite and ?authigenic pyrite and pyrrhotite. Chemical analyses of comparable material by Warren (1963) showed up to 0.5 per cent carbon and 0.8 to 1.0 per cent sulphur, whilst Rickards (1964) gives figures of 1.55 per cent carbon decreasing stratigraphically upwards to 0.48 per cent for Wenlock–Ludlow laminated muddy siltstones, with a suggested mean value of 0.4 per cent. It is probable that Denbighshire examples contain a similar percentage of free carbon.

The laminated muddy siltstones are generally fossiliferous, and indeed, with the exception of the disturbed beds and mottled mudstone elements, they contain the entire Nantglyn Flags and upper Elwy Group faunas. The faunas are dominated by graptolites, invariably flattened and in many cases showing a preferred orientation. Straight nautiloids are a common associate, along with the bivalve Cardiola interrupta, commonly found with the two valves joined together. The crinoid Scyphocrinites? pulcher is found as complete specimens, sometimes attached to orthocones and, although flattened on the bedding, probably not far from its life position. Starfish are also recorded, notably a specimen of Lapworthura miltoni collected by Boswell, who also recorded an example of the trace-fossil Myrianites tenuis.

The third element in the ribbon-banded mudstones is the calcareous siltstone. The full description of these bands by Cummins (1959a, p. 164) is applicable only to the Nantglyn Flags of the area south-west of the present district and particularly around Llanfyllin and Newtown, Montgomeryshire. In the present district the bands seldom exceed 10 mm in thickness (although near Wern Uchaf [SH 980 602] bands up to 30 cm thick occur) and are generally lenticular and deeply weathered from an original pale grey colour to a yellow ochre. In consequence little can be made of the composition, although it is likely to have been essentially similar to that reported by Cummins (ibid.)—viz: quartz silt and calcite cement with some mica and minor pyrite. In the weathered bands of the present district the calcite is lost and the pyrite oxidised.

The calcareous siltstone bands are very evident in any section by virtue of their colour. They are also unique within the ribbon-banded mudstone in showing an obvious lenticular form, often with individual lenses load-casted into an underlying silty mudstone band. The bands are slightly graded at the base but lack an upper graded part and consequently have sharp upper boundaries. The lenses and bands may also show cross bedding. The cross bedding is unidirectional and of the ripple-drift or diagonal (Birkenmajer, 1959) type. No fauna has been obtained from the calcareous siltstones.

Although the calcareous siltstone bands are always thin, the other two elements form bands varying from a few millimetres to several centimetres thick, and characteristically form an alternation of equal-sized bands, typically 1 to 5 cm thick. The intercalation of the calcareous siltstones into this alternation takes various forms. They occur most commonly above a laminated muddy siltstone band, but may also occur within, or occasionally at the top of, a silty mudstone band.

Throughout all but a few sections in the ribbon-banded mudstones, the individual bands maintain a constant thickness, except in so far as the calcareous siltstones are lenticular and the presence of calcareous concretions affects the bands locally (see below). In one exposure on the northwest side of Gorsedd Brân [SH 9661 6054] (p. 96), near Bylchau, a series of thin calcareous siltstone bands, together 5 m thick, truncates an underlying alternation of laminated muddy siltstone and silty mudstone bands, with a maximum observed down-cut of 7.5 cm. The truncated surface dips at about 30° towards 150°, suggesting a cut-and-fill structure due to a north-easterly current (cf. Cummins, 1959a, table 1, area 14). No recorded directional current structures have been observed on any of the banding to confirm the current direction, though Cummins (1959a, p.172) observed five graptolite alignments in laminated muddy siltstone indicating a mean orientation of 83 to 263° and assumed an easterly flowing current. The only record of sole structures in the ribbon-banded mudstones is of shallow flute-casts observed on a siltstone band in the Afon Nant Geneth [SH 8162 7375], near Deunant, south-east of Glan Conway (p. 60). These indicated a current flow from the east-south-east (110°).

Characteristic of the ribbon-banded mudstone is the presence of calcareous concretions, which occur at discrete horizons up to 4 m but generally about 2 m apart. In general the concretions are oblate spheroids flattened in the plane parallel to the bedding, but some approach an almost perfectly spherical form, and smaller examples are commonly subtabular. In a quarry [SH 8177 6538] (p. 69) due east of Ffrithisaf some of the concretions are seen to be dumb-bell shaped or are multicentred forming irregular masses

The concretions are generally 10 to 20 cm in maximum dimension, but they vary from 2 cm up to 56 cm seen at Bryny-fran [SH 8083 7012], 0.5 km south-east of Eglwysbach.

At several localities (for example at Waen Isaf Las [SH 9052 6003] (see p. 92) it is possible to observe the relationship between ribbon-banding and concretions. The bands at the same horizon as the centre of the nodule pass through with little or no deflection, whilst the immediately overlying and underlying bands are seen to arch up and down respectively in passing through the concretion. Bands in the surrounding ribbon-banded mudstone above and below the concretion also bend around the concretion, and their thickness and the distance between individual bands is some 30 per cent less than within the concretion. This preservational contrast in the banding is also reflected in a comparison of the graptolites within and outside the concretion; the former are in full relief while the bulk of the latter are flattened in the plane of the bedding.

Analysis of unweathered concretions shows them to consist of the same minerals that are present in the ribbon-banded mudstone (quartz, micas, clay minerals and opaque grains including sulphide) with the addition of dolomite. In one concretion, from a temporary section [SH 8133 6922] southwest of Ty-du, Eglwysbach, septarian veinlets taper from centre to periphery in an irregularly radiating arrangement. The veins are lined with microcolumnar quartz succeeded by a zone of fibro-columnar dolomite which is dark grey because of a high content of dusty micro-inclusions. The fibres are arranged normal to the vein walls. The main component of the veins is white, coarsely crystalline dolomite. The extremities of the veinlets are vuggy with remnants of soft, friable brown powder. The same material forms the outer rim of a number of weathered concretions and in some instances the entire concretion is composed of it. X-ray powder photographs indicate no crystalline phase, except for quartz, but X-ray fluorescence analysis shows a predominant iron content, subordinate manganese, minor calcium, very minor barium and nickel and traces of strontium. The material is thus iron-manganese wad.

The evidence outlined clearly indicates a penecontemporaneous origin for the concretions, which preserve, on the evidence of the graptolites, the sediment in its uncompacted state. The significance of this in considering the origin of the ribbon-banded mudstone and particularly the laminated muddy siltstone element is discussed below (p. 46). The concretions clearly formed within, and not at the surface of the sediment as shown by the upward arching of banding in the upper halves of the concretions, and on the geochemical evidence their origin must be envisaged as due to the migration of dolomite ions along selected horizons (laminated muddy siltstone bands) within the sediment, with precipitation at selected foci. Rickards (1964, p. 444) has suggested that shell fragments, found in identical concretions in the Brathay Flags, provide such foci. In the present concretions no such evidence is available, and it may be that the distribution of concretions reflects very small differences in the pH or Eh values of carbon and sulphide-rich laminated muddy siltstones.

The formation of ribbon-banded silty mudstone is dependent upon the sedimentation in a single depositional environment of three rock types of potentially different origins. These may now be considered.

The silty mudstone bands, with their homogeneous, structureless character can be interpreted as the products of very rapid deposition, of slow uniform deposition, or of complete reworking by benthonic organisms during relatively slow deposition (Moore and Scruton, 1957). In contrast to the mottled mudstones (p. 46) the present silty mudstones lack any evidence of reworking or of a benthonic, or indeed any, fauna. Warren (1963) has interpreted similar mudstones as the extreme distal products of turbidity currents, a view accepted by Rickards (1964), following Marr (1925), for the similar barren green mudstones of the Upper Llandovery of the Lake District. Nevertheless, the slow uniform deposition of this sediment suggested by Cummins (1959a, p. 165) cannot be ruled out.

The calcareous siltstones show load-cast bases and diagonal cross-bedding, indicating rapid deposition from an essentially unidirectional current. This is consistent with a turbidity current origin, as argued by Cummins (1959a) for the better-developed bands in the Nantglyn Flags of Central Wales. The Denbigh siltstones are interpreted as the more distal products of turbidity currents similar to those responsible for the Central Wales siltstones, and this is consistent with the northerly flowing currents deduced by Cummins (1959a) for the latter. However, current directions in the present district appear to have been aligned more east–west and although too few for a consistent picture, the evidence militates against the present siltstones being the direct distal equivalents of those in the Central Wales area. Nevertheless, petrographic similarities—notably the relatively clean silt content and absence of upward grading to mudstone—strongly suggest a common or similar source area.

The origin of the laminated muddy siltstone bands has long been a matter of controversy. The significant distinguishing features are the relatively high carbon and sulphur contents and the lensoid structure. The latter has been interpreted by Jones (1954), Cummins (1959a) and Llewellyn (1965) as the compaction of faecal pellets or of mottled structures due to bottom-living animals. This hypothesis has been convincingly negated by Rickards (1965) and his evidence is confirmed by the present studies. In particular, the degree of compaction involved, as evidenced by the calcareous concretions would not be sufficient to yield lenses of the present form from equidimensional pellets (Cummins 1959a, p. 163). Rickards (1965) argues that the lenses are absent from the concretions and result from the compaction of a sediment showing irregularly positioned carbonaceous flakes in an ungraded mass of material. This too implies a degree of compaction greater than that seen in the present district, and primary lensoid structure is believed to have been present in the sediment. The consistent orientation of graptolites and the presence of laminated muddy siltstone within graded greywackes in the Wenlock of southern Scotland (Warren, 1963) both suggest turbidity currents as a possible source of the sediment. That these were weak and non-eroding and of low density is evident from the regularity of ribbon-banding, the occurrence of conjoined valves of Cardiola interrupta and near in situ preservation of some crinoids. Similar evidence is adduced by Rickards (1964). Warren (1963, p. 245) suggested that the lithology 'resulted from the entrapping of pelagic organisms by drifting clouds of fine turbidite sediment' and argued that "Such clouds if consisting of 'a turbulent aggregate of finer fractions with interspaces containing even finer material' (Wood and Smith, 1959) would, on settling, probably lead to the characteristic lamination of these beds". This hypothesis of sudden entrapment of graptolites by a turbidity current also goes some way to explain the monotypic and single growth stage assemblages of graptolites so frequently found in graptolite shales (cf. Hills and Thomas, 1954). It is significant that the progressive development of less variable graptolite faunas through the Silurian is paralleled by an increase in the evidence for turbidity current activity in the formation of graptolite shale as witnessed by the increasing grain size (Rickards, 1964).

The origin of the carbonaceous matter is open to question. Much is almost certainly land derived, the remainder perhaps of marine origin. The significant feature, however, is its preservation, which has generally been taken to imply reducing conditions in the area of sedimentation. Comparison has frequently been made with black shales forming in the Black Sea (Archanguelesky, 1927) and the Norwegian fjords (Strom, 1939) in barred basins where bottom waters are stagnant. In these cases the carbonaceous matter is attributed to the annual fall of phytoplankton and the banded sediment interpreted as varved. That the present ribbon-banding is not the product of varve deposits is suggested by the very variable thickness of laminated muddy siltstone bands. Furthermore an approximate indication of the duration of deposition of a silty mudstone and laminated muddy siltstone pair can be found as follows: the thickness of the Lower and Upper Nantglyn Flags Group is approximately 700 m, estimated to represent some 20 000 rhythms. The Silurian System is now believed to have lasted approximately 50 Ma, indicating an average figure of some 1 Ma for each graptolite zone, and hence a minimum of 4 Ma for the deposition of the Nantglyn Flags. Thus each rhythm of the ribbon-banded mudstone was formed over a period of about 200 years. This figure, however approximate, is clearly not of the same order as that of annual varves. Calvert (1964) and Hillsemann and Emery (1961) have recorded sediments with organic layers from the Gulf of California and the Santa Barbara Basin respectively, which are being laid down in bottom waters containing small amounts of oxygen. As Hallam (1967, p. 487) points out 'this is evidently sufficient to support a sparse fauna of soft-bodied dwellers on the sediment surface but insufficient to support burrowers which would destroy the lamination'. Just such a situation may have obtained with the present laminated muddy siltstones, for there is a sparse fauna of crinoids and bivalves which may possibly represent a restricted epifaunal benthos. The regular banding clearly indicates deposition of the present strata below wave-base, but not necessarily at much greater depth. Some confirmation of the relatively shallow water is provided by the closely associated mottled mudstones deduced to have been deposited at or near wave-base (p. 49).

The evidence thus points to the deposition of the ribbon-banded mudstones by weak turbidity currents (silty mudstones) far-travelled turbidity currents (calcareous siltstones) and turbid suspensions (laminated muddy siltstones), in a relatively shallow-water environment, largely lacking oxygen. It remains to consider the relation of the several turbidity flows. The random position of the calcareous siltstone bands, and their presence in mottled mudstones (pp. 47–48) suggests a different source from the other elements, possibly, as argued, a southerly source. This contrasts with a probable derivation of the other elements from a land mass, relatively close, and presumably to the west. The situation was thus one of axial and lateral sediment supply.

Mottled mudstones

These beds are among the most distinctive in the district. They may be defined as massively bedded, irregularly fractured silty mudstones with mottling, associated goethite veining and a shelly fauna. They thus contrast strongly with the ribbon-banded silty mudstones generally adjacent to them and, because of their massive nature, are more resistant to erosion and form readily mappable features (Summ. Prog. Geol. Surv. for 1965)(Plate 3).

Boswell and Double (1940, p. 158), in first distinguishing these beds, referred to them as 'irregular mottled mudstones' and Boswell (1949, p. 38) noted that 'they bear some superficial resemblance to the fireclays of the Carboniferous'. He thus aptly described their characteristic weathering features, which are an irregular fracturing and a pale brown coloration with veinlets of iron resembling rootlets.

The fracturing of these beds is characteristic and takes the form of two sets of irregular breaks, one of which is subparallel to the bedding, and the other approximately normal to the bedding and parallel to the cleavage in adjacent strata. The resulting effect is that the bed is composed of phacoidal masses, averaging some 2 to 5 cm thick and 15 to 23 cm across (Plate 8). In some sections the fracturing is on a coarser scale and weathering may be loosely termed sphaeroidal; whereas in others, where the cleavage is more regular and intense, the rock splits more or less along the cleavage.

In unweathered sections the fracturing is largely absent, although some evidence of cleavage is apparent. The characteristic mottled pale and darker brown colour and the iron veinlets are also largely lost, and the rock has a dark grey-green colour with paler and darker blotches. Such sections also reveal that the mottled mudstones are in part bedded and besides containing concretions and interbedded mudstones or siltstones identical to those which together make up the ribbon-banded mudstones may consist of at least three separate lithological elements. The elements are calcareous siltstone, dark grey to black mudstone and the irregular mottled mudstone lithology itself.

The calcareous siltstone bands are essentially similar to those in the ribbon-banded mudstone (p. 44), although they seldom exceed 6 mm in thickness and are usually only 1 to 2 mm thick. They commonly show parallel lamination and a number of bands are lenticular and show diagonal cross-bedding. They are pale grey in colour, weathering yellow.

In the overall units mapped as mottled mudstone, the siltstone bands show a variable frequency, ranging from between 1 to 5 cm to over 9 m apart. In general they are commonest at the base and top of a unit, and in the latter position in particular are accompanied by laminated muddy siltstone bands identical to those that form one of the three elements comprising the ribbon-banded mudstones. Interbedded ribbon-banded and mottled mudstone sequences to 0.6 m are also recorded and form transitional beds between the two lithological groups.

Generally associated with the thin siltstone bands are thicker bands, commonly about 1 cm thick, of dark grey to black mudstone. In thin section these are seen to contain some silt and a relatively high proportion of carbonaceous matter and opaque iron minerals, probably including sulphides. This rock type thus has some similarities to the laminated muddy siltstones already described, but is finer grained and lacks their characteristic texture and pseudo-lamination. Locally mottled mudstone units contain a high proportion of dark mudstone with relatively few siltstone bands, but in general it represents a small proportion of any one unit.

Accompanying the calcareous siltstone and dark mudstone bands in many sections are small calcareous concretions of the type described under the ribbon-banded mudstones (p. 45). These are commonly 5 to 7.5 cm across and 1 to 2.5 cm high and are in the form of oblate spheroids flattened in the plane of the bedding. Some concretions are bigger and exceptionally, as at a quarry [SH 8092 6397] (p. 75) west-south-west of Pen-y-garth, Llanddoget, reach a maximum width of 45 cm. The concretions are pale brown or whitish when weathered and are frequently decalcified. In one instance they are entirely replaced by pale blue-grey clay. They are believed to have originated in a similar way to the concretions occurring in the ribbon-banded mudstones (p. 45).

The main component, on average some 90 per cent, of the mottled mudstone units is the irregular mottled mudstone itself. On polished surfaces (Plate 2) this can be seen to consist largely of patches of pale and darker grey-green siltstone/mudstone. The patches are generally irregular in form, but some are circular or tubular, and they frequently penetrate the regularly bedded stripes already described. In addition irregular areas of much darker mudstone occur, along with 'clasts' of black mudstone. The 'clasts' have distinct margins and vary from 1 to 35 mm in length but seldom exceed 2 mm in thickness. Many are subparallel to the bedding, but others are at right angles or form curving stringers through the mottled matrix.

In thin section the palest patches are seen to consist predominantly of silt-grade quartz, with calcite as a cement or as anhedral grains, or rarely as rounded mosaics (0.1 mm across). Feldspar, opaque iron ores, zircon and leucoxene are also recorded. The slightly darker material, which may be considered the matrix of this rock type, consists of relatively homogeneous clay material comprising illite and/or degraded muscovite, hydrobiotite, porphyroblastic pale green chlorite or chlorite/illite intergrowths (cf. leverrierite), charged with calcite and quartz silt. Opaque grains, including framboidal aggregates of ferric oxide and the heavy minerals noted above also occur. The rock type is thus a silty mudstone, similar in composition but rather 'coarser grained than the silty mudstone of the ribbon-banded mudstones (p. 44). The darker mudstone is finer grained than the above and richer in carbonaceous material and iron ore, and the 'clasts' of black mudstone are still finer and richer in opaque material. In general 'clasts', silt lenses and layered minerals tend tcr be parallel to bedding, although the last are in many cases oriented along an irregular shear direction, more or less at right angles to bedding. In a few instances circular patches of calcite siltstone reveal mineral orientation parallel to the margin and a similar orientation is also present in the immediately adjacent silty mudstone of the 'matrix'.

The characteristic iron veinlets of the mottled mudstone are seen only in weathered material. They seldom exceed 1 cm in length and 2 to 3 mm in width. X-ray powder photographs indicate that they are composed of goethite. They generally appear in exposures to have no preferred orientation, to have a flattened tubular form in many cases and to bifurcate. At one or two localities there is a concentration of veinlets along discontinuities parallel to bedding, and evidence from hand specimens and thin sections indicates that the ferric oxide structures are in places aligned along shear directions at angles to the bedding. When fresh, the mottled mudstone shows little evidence of the veinlets and the goethite is evidently a weathering product. Pyritic streaks are associated with the darkest mudstone 'clasts' and these may have provided the iron necessary for the development of goethite, or even be the unweathered equivalent of some of the veinlets. However, the general association of goethite with planes of fissility and other interspaces in the rock, including fossil moulds, strongly suggests a secondary mobilisation of iron.

The structures and textures in the irregular mottled mudstone are clearly the result of bioturbation, there being evidence of burrows, disruption of banding, and churning of the sediment (Plate 2). Three elements of the irregular mottled bands, the calcareous siltstone, silty mudstone and dark mudstone are recognisable as bands in bedded strata either within the mottled mudstone units or in the adjacent ribbon-banded mudstones, and it is from such bands that the mottled divisions are believed to originate. Confirmation of this is given by ghost bands of dark mudstone. The only element whose origin can be in doubt is the black mudstone. The elastic appearance of this lithology suggests that in part at least it does not entirely originate from bioturbation, although some part may represent the infilling of burrows, seen in section as ovoid 'clasts'. The occurrence of the black mudstone 'clasts' subparallel to bedding and apparent absence of this element elsewhere in non-reworked sediment, suggest the possibility of the introduction of these 'clasts' from outside the immediate area of sedimentation of the mottled mudstones, or at least penecontemporaneous erosion and redeposition. Some evidence of penecontemporaneous erosion is given by two exposures of the Lower Mottled Mudstone; one is a road section [SH 8095 6247] (p. 75) due west of Pentre-tafarn-y-fedw Chapel; the other a small quarry [SH 8115 7465] (p. 60) east of Ffordd-lâs Chapel, Glan Conway. At the former a bed up to 7.5 cm thick of silty mudstone with dark silty mudstone pebbles and collophane pellets occurs, and at the latter there is a 0.53-m band of mudstone with collophane pellets, solitary corals and bryozoan fragments. It is believed that these beds have a similar origin to the conglomerates associated with disturbed beds (p. 39).

The calcareous siltstone bands in the mottled mudstones, as in the ribbon-banded mudstones, are believed to have originated from far-travelled turbidity currents (p. 45). The few graptolites recorded from mottled mudstone occur in this rock type and commonly are preferentially oriented, indicating a unidirectional depositional current. By way of contrast, the darker mudstone was probably derived from turbidity currents, relatively rich in carbonaceous matter and thus possibly originating more proximally.

The shelly fauna, predominantly of brachiopods, mollusca and trilobites (p. 57), is interpreted as an indigenous benthos. It differs markedly from the faunas of the contemporaneous shallow-water shelf (e.g. in the Welsh Borderland) and cannot have been derived thence. The state of preservation suggests a relatively quiet area of sedimentation and, along with the preservation of fine banding and a chaotic fabric, indicates that the sedimentation took place below wave-base in an area swept in the main only by gentle currents. The 'clast' material may indicate periodic wave or vigorous current action, the latter perhaps induced by slumping. McCabe and Waugh (1973) have interpreted the sedimentation of an identical rock type to the mottled mudstone, making up the Arcow Formation (Wenlock) of the Austwick area of Yorkshire, entirely in terms of turbidity current deposition. They also postulated a derived origin for the fauna. Warren (in McCabe and Waugh, 1973, p. 464) refuted this view, believing the fauna of the mottled mudstones to be indigenous, and suggested that some at least of the bioturbated sediment was derived from other than turbidity currents. Subsequently Hancock and others (1974) have interpreted the fauna as a Visbyella community, the deepest indigenous brachiopod community and one that lived at a depth possibly as great as 1000 to 1500 m. Certainly mottled mudstones are interbedded, sometimes interleaved, with ribbon-banded mudstones believed to have been laid down by turbidity currents in quiescent, anoxic conditions. But equally clearly, mottled mudstone deposition took place in oxygenated conditions with an adequate food supply to support bottom-dwellers. The widespread contemporaneous formation of these latter mudstones is probably to be explained, at least in part, by a global lowering of sea level of some magnitude. However, other factors probably also played a role in creating an ecological regime which may not be closely depth-dependent. That the depth was over 200 m is hardly in doubt, but to the present authors the setting of any upper limit must remain a matter of conjecture.

Some of the trilobites and bivalves may have effected part of the bioturbation but it is inferred that soft-bodied organisms, for example, Chondrites, were responsible for most of the burrowing. The question arises as to whether the iron-ore veinlets are in part trace fossils. Boswell (1949, p. 38) interpreted the goethite as 'penetrating plant remains' and earlier had suggested that they were remains probably of algae (Boswell and Double, 1940, p. 158). More recently Holland and others (1963, p. 128) have described identical veinlets in the Wenlock Limestone of the Ludlow district as 'reddened tubular bryozoa'. The present studies do not confirm either of these views, for only exceptionally does iron ore replace fossils, and tubular veinlets are uncommon. In general the present authors interpret the veinlets as due to mobilisation of iron during the compaction of the sediment and diagenesis. The initial bioturbation and water loss on lithification in an irregular mottled bed would lead to cavitation either directly or as a result of differential compaction strains, and the iron ore is believed to have filled such cavities. Its original chemical composition is unknown.

Striped silty mudstones

This rock-type comprises irregular alternations of silty mudstone and siltstone or fine sandstone. Two further elements are generally involved—thin bands of laminated muddy siltstone and ' gingerbread rock' (p. 19).

Striped silty mudstones make up a substantial proportion of the Denbigh Grits Group—in particular the Berllan Formation—and of the Elwy Group—occurring mainly in the middle and upper divisions. Variation in detailed lithology is marked (see below), but shows no consistent change with time.

The striped silty mudstones show great variation in the thickness of individual bands, from single laminae to thicknesses exceeding 1 m, although in general bands are less than 2.5 cm thick. The form of alternation thus contrasts markedly with that in the ribbon-banded silty mudstone (p. 44) and this is epitomised in the terms 'irregular' and 'regular' respectively, applied to the alternation.

Sequences of striped silty mudstone also vary in maximum grain size from an alternation of sandstone and thin silty mudstone bands, through an alternation of thin sandy siltstones and silty mudstone (striped beds) to silty mudstones with sporadic bands of siltstone.

The rocks vary in colour from dark grey-green to pale grey, depending upon the proportion of clay minerals present.

Brown weathering is characteristic of the more calcareous siltstone bands and shelly layers, e.g., ' gingerbread' bands. Lamination is invariably present and is generally subparallel to bedding, the exceptions being the laminae associated with load-cast siltstone bases and those forming ripple-drift bedding in some siltstones. Small concretions, apparently ferruginous, are seen in a few sections. Graded bedding in siltstone laminae and bands is evident, especially in thin section.

Mineralogically the striped silty mudstone may be treated as a whole, for the main elements show only varying proportions of the same minerals. Quartz is predominant, occurring in silt-grade grains 0.01 to 0.06 mm across. Other silt-size components are feldspar, including oligoclase, microdine and perthite, chert fragments, and heavy minerals including zircon, tourmaline (golden brown or dark green to colourless), glauconite, apatite, anatase, leucoxene and magnetite. The clay minerals represented are chlorite (green flakes), chlorite/illite intergrowths, illite, hydrobiotite and, in some cases, kaolinite.

Faunally, these rocks are relatively impoverished. Graptolites are numerous in the laminated muddy siltstone bands, which however represent probably less than 1 per cent. of any sequence of these beds. The 'gingerbread' bands, which likewise form a small percentage of the striped silty mudstones, have an allochthonous fauna of disarticulated and fragmentary shells, crinoid columnals etc., along with some graptolites. They show strong evidence of penecontemporaneous derivation. Some of the thicker bands of siltstone also contain a shelly fauna which is probably derived.

In origin, the striped silty mudstones are believed to represent the lateral equivalents of the sandstones and disturbed beds already described (pp. 33–44). They are the distal products of turbidity currents and slump movements, and without exception the elements of the rock type represent derived material from one or more of the sources deduced for the other rock types. The siltstones are seen mainly as the equivalents of the sandstones of the Denbigh Grits and Elwy groups. The muddy siltstone laminae and silty mudstone elements are identical to those of the ribbon-banded mudstones in mineralogy and texture, and presumably have the same mode of deposition (see p. 46). The origin of the ' gingerbread' horizons is more obscure. The faunas suggest a source rich in shells and hence a shallow shelf region; possibly these bands are directly related to slumps and are the distal equivalents of some disturbed beds.

In terms of water depth and distance from land, the environment of deposition of the striped silty mudstone is believed to be similar to that of the distal sandstones (pp. 35–37) and the ribbon-banded mudstones. Turbidity current action must have been somewhat less intense than in the case of the sandstones, but much greater than in the case of the ribbon-banded mudstones. Decrease in such action in upper Elwy Group times, in particular in the south-eastern part of the district, led to the mixed facies of striped and ribbon-banded mudstones (p. 33) PTW et al.

Palaeontology

Introduction

The macrofossils of the Wenlock–Ludlow sequence fall into two groups, which may be termed graptolitic and shelly. Graptolites occur chiefly in the laminated muddy siltstones, either in their nearly continuous sequences in the Nantglyn Flags or where they occur as thin bands in the striped silty mudstones. They are recorded rarely from disturbed beds and silty sandstones and less rarely from ' gingerbread' horizons. In the typical graptolitic facies of the Nantglyn Flags the graptolites are invariably accompanied by thin-shelled bivalves, notably Cardiola interrupta and orthocone nautiloids. The shelly faunas (Plate 9) occur predominantly in disturbed beds and mottled mudstones, but are also found in sandstones, siltstones and ' gingerbread' horizons.

As well as in occurrence, the two groups differ in their value in stratigraphy, sedimentology and palaeoecology. In conjunction with the detailed lithostratigraphical mapping, the graptolites have proved invaluable in erecting a stratigraphical succession. Moreover, through them it has proved possible both to assign isolated sequences within the district to their correct stratigraphical levels, and to effect a detailed correlation of the complete succession with those elsewhere in Britain and abroad. By contrast, the shelly fauna has proved remarkably uniform throughout the sequence and has provided little assistance in correlation. Unlike the graptolites, however, the shelly fauna has been significant in sedimentological interpretations, for example of the origin of mottled mudstones and disturbed beds, and is of particular interest in palaeoecological terms.

Warren (1971) has provided an account of the North Wales graptolite sequence and its world-wide correlation, and the present account is therefore restricted, in the main, to highlighting the more significant or specific points, particularly in the Welsh and British contexts. In the absence of any previous account of the shelly faunas, these are given a more comprehensive coverage.

Graptolites

Preservation and distribution Although largely restricted to one rock type, graptolites are found throughout most of the Wenlock—Ludlow sequence; the exceptions being the disturbed strata composing the Llanddoget Formation and the sandstone/striped bed sequence comprising the lower part of the Denbigh Grits Group.

In general the graptolites are preserved as flattened films on bedding planes, although there is no evidence of distortion due to this flattening. Distortion by tectonic deformation, however can be considerable. Up to 30 per cent crustal shortening at right angles to the cleavage is implied by thecal counts of up to 20 per 10 mm for species for which the maximum is normally 13 per 10 mm. Rickards (1970, p. 27) and Hutt (1974, p. 16) have discussed the implications of this kind of deformation in the identification of graptolite species. Suffice it to say here that tectonic deformation varies markedly in intensity between adjacent sections in the North Wales sequence. Moreover, on many bedding planes the graptolites are randomly oriented with respect to the cleavage trace, and, despite the high degree of deformation that has occurred in many cases, no serious difficulties have been experienced as regards identification.

In the less cleaved and jointed rocks, for example the upper Wenlock (Cyrtograptus lundgreni Zone) strata around Llanrwst, extensive bedding planes with giant rhabdosomes of Monograptus flemingii have been seen (Bulman, 1965), and in the Nantglyn Flags in general preservation is good. In some instances the graptolites are preserved in relief, usually in pyrite or limonite, but in the more calcareous bands, and notably in the concretions, as carbonised periderms. It has not proved possible to isolate this last material.

Assemblages and zones

Twelve graptolite assemblages have been recognised (Warren, 1971), and these define the following zones (given in ascending order with approximate thicknesses): Cyrtograptus centrifugus (15–20 m), C. murchisoni—Monograptus riccartonensis (610 m), C. rigidus (320 m), C. perneri (175 m), C. ellesae (20 m), C. lundgreni (230 m), Gothograptus nassa (5–10 m), M. ludensis (150 m), Lower Neodiversograptus nilssoni (215 m), Upper N. nilssoni (410 m), Cucullograptus scanicus (490 m) and Saetograptus leintwardinensis incipiens (850 m + ).

The full assemblages present in each zone are given below. For strata above the base of the C. rigidus Zone the ranges of the majority of species, and hence the basis for the zonal boundaries, are shown in (Figure 13). No such chart is given for the basal two zones because (a) the greater proportion of the C. centrifugus Zone fauna is restricted to that zone, and (b) records are too sparse and correlation too poor in strata assigned to the C. murchisoni—M. riccartonensis Zone.

It is stressed that the zonal sequence established is similar to that generally accepted for these rocks elsewhere in Britain, but that precise correlation of identically named zones is not necessarily implied (see p. 54 et seq.).

Cyrtograptus centrifugus ZONE — ?Barrandeograptus pulchellus, C. centrifugus, C. cf. insectus, Monoclimacis griestoniensis nicoli, M. linnarssoni, M. shottoni, M. vomerina basilica, M. vomerina vomerina, Monograptus cf. danbyi, M. cf. firmus firmus, M. cf. flexuosus, M. kodymi, M. ?minimus cautleyensis, M. priodon, M. simulatus, Pristiograptus sp., Retiolites geinitzianus angustidens, R. geinitzianus geinitzianus.

Cyrtograptus murchisoni–Monograptus riccartonensis ZONE — Monoclimacis cf. flumendosae flumendosae, M. vomerina basilica, M. vomerina vomerina, Monograptus flexilis cf. belophorus, ?M. riccartonensis, M. cf. riccartonensis, Pristiograptus dubius cf. dubius, P. dubius latus, P. cf. praedubius. In addition at Conway, Elles (1909, p. 186) recorded: C. murchisoni, Monograptus priodon, M. riccartonensis, R. geinitzianus (s.l. ).

Cyrtograptus rigidus ZONE — C. rigidus, Monoclimacis flumendosae flumendosae, M. flumendosae aff. kingi, Monograptus flemingi zflemingii, M. flemingii primus, M. ex. gr. flexilis, M. sp.of priodon/flemingii lineage, M. retroflexus, Pristiograptus dubius, P. pseudodubius. Upper part only: C. cf. linnarssoni.

Cyrtograptus perneri ZONE — Cyrtograptus linnarssoni, C. perneri, Monoclimacis flumendosae flumendosae, M. flumendosae kingi, Monograptus flemingii flemingii, M. flemingii primus, P. dubius, P. pseudodubius, retiolitid. Uppermost part only: C. ellesae, M. flemingii aff. elegans, P. lodenicensis, P. aff. lodenicensis, P. aff. meneghinii.

Cyrtograptus ellesae ZONE — Cyrtograptus ellesae, C. lundgreni, C. perneri, Monoclimacis flumendosae flumendosae, M. flumendosae kingi, Monograptus flemingii cf. elegans, M. flemingii flemingii, M. flemingii primus, M. flemingii var. 6, Pristiograptus dubius, P. lodenicensis, P. aff. lodenicensis, P. aff. meneghinii, P. pseudodubius.

Cyrtograptus lundgreni ZONE — C. hamatus, C. lundgreni, C. aff. pseudolundgreni, Monoclimacis flumendosae kingi, Monograptus flemingii compactus, M. flemingii elegans, M. flemingii flemingii, M. flemingii var. δ, ?Plectograptus lejskoviensis, Pristiograptus dubius, P. lodenicensis, P. aff. lodenicensis, P. pseudodubius, Pristiograptus sp. nov? aff. curtus. At the base only: C. ellesae, C. perneri, Monoclimacis flumendosae flumendosae, Monograptus flemingii primus. In the upper 61 m: Monograptus aff. deubeli, M. aff. flemingii elegans, Pristiograptus aff. jaegeri, retiolitids.

Gothograptus nassa ZONE — Typically: G. nassa, Pristiograptus pseudodubius. Also recorded: G. intermedius, Monograptus aff. deubeli, M. flemingii aff. elegans, Plectograptus? aff. dubius, Pristiograptus? aff. dubius, Pristiograptus dubius, P. jaegeri, P. aff. jaegeri, Pristiograptus sp. nov? aff. curtus.

Monograptus ludensis ZONE — Typically: M. ludensis, Pristiograptus dubius, P. jaegeri, P. aff.jaegeri, P. sp. nov? aff. curtus. Also recorded: Acanthograptus sp., Pristiograptus auctus. At base only: G. nassa, Monograptus aff. flemingii elegans, P. pseudodubius, retiolitids indet. In top 50 m: Monograptus aff. uncinatus orbatus, M. colonus colonus.

LOWER Neodiversograptus nilssoni ZONE — Monoclimacis(?) cf. haupti, M. colonus colonus, M. colonus compactus, M. cf. roemeri, M. uncinatus orbatus, N. nilssoni s.l., Pristiograptus bohemicus bohemicus, P. curtus, P. dubius, Saetograptus varians cf. pumilus, S. varians varians, Spinograptus spinosus. Upper part only: Pristiograptus comis, P. aff. curtus.

UPPER Neodiversograptus nilssoni ZONE — Cucullograptus(?)crinitus, C. scanicus, Monoclimacis(?) haupti, M. micropoma [rare], M. (?) ultima (sensu Wood, 1900), Monograptus colonus colonus, M. colonus compactus, M. aff. uncinatus, N. nilssoni s.l., Pristiograptus bohemicus bohemicus, P. aff. curtus, P. dubius, P. vicinus, P. aff. welchae, Saetograptus chimaera chimaera, S. chimaera salweyi, S. chimaera cf. semispinosus, S. varians pumilus, S. varians varians. In basal 137 m: Lobograptus progenitor, Monograptus uncinatus orbatus, Pristiograptus curtus.

Cucullograptus scanicus ZONE — Basal part (150 m): M. colonus s.l., N. nilssoni s.l., S. varians varians. Remainder: C. (?) crinitus, C. scanicus, Monoclimacis (?) haupti, M. micropoma, M. (?) ultima, P. bohemicus bohemicus, P. bohemicus tenuis, P. tumescens, P. vicinus, P. aff. welchae, S. chimaera chimaera, S. chimaera salweyi, S. chimaera semispinosus, S. clunensis, S. leintwardinensis incipiens, S. varians pumilus.

Saetograptus leintwardinensis incipiens ZONE — Monoclimacis (?)haupti, Pristiograptus bohemicus bohemicus, P. tumescens, P. vicinus, S. clunensis, S. leintwardinensis incipiens, S. leintwardinensis leintwardinensis?, S. varians pumilus. Basal 122 m only: C. scanicus, M. micropoma, S. chimaera salweyi.

The faunal lists include a number of forms not previously recorded in Britain or not recorded from the same horizon. In addition to Monograptus cf. firmus firmus (Warren, 1971, p. 454), these are: M. kodymi, a characteristically Czechoslovakian form from the basal Wenlock; P. dubius latus, recorded from the M. riccartonensis Zone of Poland and Czechoslovakia; P. cf. praedubius, recorded previously in Britain and Poland only from the Cyrtograptus centrifugus Zone (Rickards, 1967; Teller, 1969); C. perneri (see Warren, 1971, p. 456); P. lodenicensis, a form first recognised in Czechoslovakia (Pfibyl, 1944) and subsequently in Poland (Teller, 1969) and apparently restricted to the C. lundgreni Zone in both countries, C. aff. pseudolundgreni; G. intermedius (see Warren, 1971, p. 454); and Plectograptus? aff. dubius.

The main faunal changes are as follows:

1. a rich, varied basal Wenlock fauna (C. centrifugus Zone) followed by a relatively depleted fauna in the C. murchisoni—M. riccartonensis Zone, probably only in part a reflection of lack of preservation
2. a progressive development of cyrtograptids, monoclimacids, monograptids and pristiograptids through most of the Wenlock, terminated by a depletion of the fauna to essentially Pristiograptus pseudodubius and Gothograptus nassa in the G. nassa Zone
3. a restricted, mainly pristiograptid, fauna in the M. ludensis Zone followed by the incoming of a rich, varied fauna in the Lower N. nilssoni Zone
4. the incoming of S. chimaera group forms, along with Cucullograptus scanicus (s.l.) and Monoclimacis micropoma, and the disappearance of M. uncinatus orbatus and Spinograptus spinosus at the base of the Upper N. nilssoni Zone
5. an increasing abundance of pristiograptids through the C. scanicus Zone, leading to monotypic assemblages in the upper part of the zone and in succeeding strata
6. a rapid increase in abundance of forms close to Saetograptus leintwardinensis and accompanying loss of most of the typical lower Ludlow forms, at the base of the S. incipiens Zone.

Correlation

On the basis of the detailed faunas, and particularly on the faunal changes outlined above, Warren (1971, pp. 456 et seq.) provided a correlation of the North Wales Wenlock—Ludlow sequence with those elsewhere in Britain and Continental Europe. His summary diagram (ibid., fig. 2) is reproduced here (Figure 14) with some modification taking into account more recent work (see below).

Early in the present study of North Wales graptolites, it was evident that the sequence of faunas could be more closely correlated with that in eastern Europe, and in particular Poland, than with those elsewhere in Britain. The work of Tomczyk (1956, 1962), Urbanek (1966) and Teller (1969) in Poland allows a very precise correlation to be made, even though some zonal definitions and species complements differ. Through this correlation, the similarity of the North Wales faunas to those in Czechoslovakia and Germany, and the work of Boueek (1933, 1960), Horny (1962), Jaeger (1959, 1964) and others, it has proved possible confidently to correlate a large number of European sequences with each other. The work of Regnell and Hede (1960), Ulste (1964), Obut (1960) and Obut and others (1965) suggests that precise correlation will in time be possible with the Scanian (Swedish), Latvian and other peri-Baltic sequences and with much of the USSR.

By contrast, correlation of the North Wales sequence with those elsewhere in Britain, whether in the basinal or shelf facies, is much less precise. This is due to three main factors. Firstly, a number of British Wenlock–Ludlow successions lack a full sequence of faunas because of preservational failure—e.g. south Scotland (Warren, 1964) and the Lake District (Rickards, 1969)—or non-sequences—e.g. Howgill Fells (Rickards, 1967; Warren in Rickards, 1967). Secondly, only a limited number of up-to-date and comprehensive studies of Wenlock–Ludlow successions have been published, particularly for Wales. Thirdly there are real differences, even within Wales, in the species present and their ranges (see below).

Despite these limitations a reasonable correlation can be made with the Welsh Borderland sequence and hence with the standard stages of the Wenlock and Ludlow Series as defined in the Much Wenlock/Wenlock Edge and Ludlow areas of Shropshire (Bassett and others, 1975; Holland and others, 1963). This is summarised in (Figure 14), but some points deserve elaboration. The zonal sequence in the Sheinwoodian is comparable to that in North Wales, but effectively the stage can only be correlated as a whole. This is also true of corre lation with the Wenlock sequence up to the base of the C. lundgreni Zone in Central Wales (Elles, 1900). Much more accurate correlation is possible with the Homerian Stage, for the C. lundgreni, G. nassa and Monograptus ludensis zone assemblages are comparable with those of North Wales. The North Wales sequence from the top of the C. lundgreni Zone to the base of the N. nilssoni Zone has been correlated by Holland and others (1969) with the sequence in the Ludlow area.

Precise correlation of the Ludlow stages awaits a detailed modern account of the graptolite faunas of the type area; the correlation presented is based on the records of Wood (1900); Holland and others (1963); Shergold and Shirley (1968); Palmer (1971) and Lawson (1973). The absence or rarity of M. colonus and N. nilssoni in the C. scanicus Zone of North Wales suggests that the present N. nilssoni and C. scanicus zones are approximately coeval with Wood's; moreover Shergold and Shirley (1968, fig. 1) show a distinct fauna, including M. uncinatus, N. nilssoni and S. spinosus and lacking saetograptids of the S. chimaera group, at the base of the Middle Elton Beds of the Much Wenlock area, and this horizon is therefore correlated with the upper part of the Lower N. nilssoni Zone of North Wales. There are inherent dangers in correlation by monotypic assemblages (Warren, 1968), but the numerous monotypic assemblages of P. vicinus (? = P. tumescens in part) in the upper part of the present C. scanicus Zone are consistent with a correlation with Wood's tumescens Zone and hence the Upper Eltonian. Lawson's (1973) record of S. clunensis from the uppermost Elton Beds at Aymestry suggests that this horizon falls just within the incipiens Zone of the present sequence.

Environmental and faunal implications

The most significant palaeoenvironmental point to emerge from the faunal studies and correlations is the recognition of a probable coeval development of limestone or calcareous, shelly bioturbated mudstone over a wide region extending from Sweden (Scania) through Britain—Welsh Borderland (Wenlock Limestone); Long Mountain (Glyn Member of the Trewern Brook Mudstone Formation of Palmer, 1970); Newtown area of central Wales; North Wales (Lower and Upper Mottled Mudstones); north-west Yorkshire (Arcow Formation of McCabe, 1972); Lake District (Middle Coldwell Beds)—to Nevada (U.S.A.).

British correlations suggest that the facies change marked by the base of the Elwy Group in North Wales (base of Upper N. nilssoni Zone) may be more or less synchronous throughout the basin (cf. Holland and others, 1963, table II). Correlation with the basin facies of central Wales (Earp, 1938, 1940) is only tentative and there is some evidence that the range of M. uncinatus orbatus in the two areas may be different, but the general pattern of graptolite development supports the sedimentological conclusion. Likewise, the facies change associated with the base of the incipiens Zone in North Wales and the Howgill Fells may be coeval.

The similarity throughout Europe of the basal Wenlock faunas and again of the C. lundgreni—Lower N. nilssoni Zone faunas stands in marked contrast to the dissimilarity of the middle Wenlock (C. rigidus—C. ellesae zones) faunas and of the post-N. nilssoni Zone faunas. Specific examples have been given by Warren (1971), who has concluded that there is evidence not only for species migration at variable rates but for ecologically or geographically controlled graptolite communities. Even at times when the faunas were comparatively uniform throughout Europe and, in the case of the C. lundgreni—M. ludensis Zone interval as far afield as North America (Nevada) and North Africa, there are significant differences between any two graptolite successions. Thus M. testis has yet to be recorded in North Wales, although it is present in the Long Mountain and throughout Continental Europe; P. meneghinii has not been recorded from North Wales but it occurs in the Howgill Fells, where P. lodenicensis is unrecorded; and G. nassa and M. flemingii occur together in the Welsh Borderland and Sweden but not in North Wales (although this might be attributable to the presence there of the Lower Mottled Mudstone).

It is fair to point out that there are inevitably some monographic problems in comparing faunas; thus the con-specificity of M. ludensis, M. vulgaris (pars) and Pristiograptus gotlandicus (Perner) recognised during the present studies (Warren and others, 1966) and the solution of the 'vulgaris problem' (Holland and others, 1969) led to a major simpli fication of M. ludensis Zone faunas in Europe. Nevertheless as Warren (1968, 1971) concluded there appear to be real differences between coeval graptolite faunas in the Wenlock and Ludlow, and it is now becoming evident that environment played a bigger part in this than was previously thought. The environmental controls are as yet unknown, though depth is one popular contender (e.g. Berry and Boucot, 1972). It is perhaps significant, as Berry and Boucot (1973) have also pointed out, that the near-extinction of the graptolites at the Ordovician—Silurian boundary coincided with the maximum of the North African glaciation (Destombes, 1971) which produced a lowering of sea-level and consequent widespread extension of shelf seas, but which must also have had a marked effect on many aspects of both marine and terrestrial climate, for example food resources (cf. Berry, 1974). The almost complete extinction of characteristic Wenlock species and lineages just below the G. nassa Zone, which coincided with a widespread development of a calcareous lithofacies, may similarly reflect a glacial episode, with changes of sea-water depth, temperature or food supplies, or a combination of these, affecting the graptolites. PTW

Shelly faunas

Introduction

The shelly fossils from the Wenlock—Ludlow rocks are distributed unequally through the four stratigraphical groups. Full lists are given in Appendix 1. Faunal variations of stratigraphical significance are comparatively slight, a more important factor being the lithofacies in which the fossils are found. Many of the shells are allochthonous in the sense that after the animals died their shells were moved by sedimentary processes which also influenced the lithofacies, whereby contemporaneous faunal associations representing very different biotopes have been brought into juxtaposition. Comparisons were therefore made between faunas of different ages (as determined by the graptolite zones) in each facies, and between faunas from beds of similar ages but differing facies. In a particular lithofacies the proportion of localities in one graptolite zone yielding a given species was compared with the proportions in other zones; and in a particular zone the proportion of localities in one facies yielding a given species was compared with the proportions in other facies. Chi-squared contingency tests (with Yates' correction) were computed in each case and compared with the value for 95 per cent significance. Apart from some of the commoner brachiopods and a few other fossils the numbers of records were too low for the significance of most distributions to be established statistically.

Coarse greywackes

The coarse greywackes of the Denbigh Grits Group yielded crinoid columnals and rare shells at a few localities, and locally, as near Plas Madoc, Llanrwst (p. 74; Salter in Ramsay, 1866, p. 277 et seq.), they contain jumbled masses of diverse but poorly preserved fossils. The fauna consists mainly of brachiopods, especially rhynchonelloids and orthoids, with fewer molluscs and trilobites, and is quite typical of the Homoeospira and Isorthis communities in the Wenlock faunas of the Welsh Borderlands (Calef and Hancock, 1974). The fossils were derived, together with the clastic material, from a shallow sea floor. Typical Wenlock forms not occurring at higher horizons in Denbighshire include Stegerhynchus decemplicatus, S. diodonta, Rhynchotreta cuneata and Meristina sp.

Ribbon-banded mudstones

The restricted shelly fauna typical of the ribbon-banded mudstones is similar in character throughout the succession, consisting mainly of orthocone nautiloids and bivalves (Cardiola, Butovicella and pterineids), and rarer crinoids and entomozoid ostracods (Entomozoe and Bolbozoe spp.). These are found sporadically in association with graptolites in ones and twos or, rarely, in large numbers. The fossils are generally poorly preserved, flattened and fractured by compaction, and sometimes tectonically distorted. Some fossils, however, were evidently neither transported far nor disturbed by scavengers after death and before fossilisation, for Cardiola interrupta, small pterineids and ostracods are to be found with both valves side by side where they came to rest, with the shell open but still conjoined; more rarely there occur complete crowns of Scyphocrinites? pulcher and small, possibly juvenile, crinoids attached to orthocones. Brachiopods, thick-shelled molluscs and vagrant benthic forms are typically absent except in some flags in the Bont-uchel Formation at the top of the Elwy Group, where Dayia navicula, Lanceomyonia sp., Protochonetes minimus, Shagamella minor and occasionally other brachiopods are added to the fauna. There are also local aggregations of Aegiria grayi, beyrichiid ostracods and other fossils, but these generally occur in the layers of calcareous siltstone (see below).

The ecological significance of the faunal association typical of the ribbon-banded mudstones is debatable: there is no evidence for the activity of benthic organisms, such as burrowing, and the restricted nature of the shelly fauna indicates an unsuitable environment for the development of an extensive benthos such as is found in the mottled mudstones; the water may have been too deep and the oxygenation or food supply inadequate. Furthermore, though the shelly animals present may possibly represent a restricted epifaunal benthos, their modes of life are open to doubt and can be interpreted as pelagic. The orthocones may be nekroplankton, their empty shells having drifted far after the death of the animals (cf. MO, 1969, p. 113); whether the small crinoids fastened on to the shells as they drifted or after they sank to the bottom is unknown. The larger crinoids are tentatively assigned to Scyphocrinites (Ramsbottom, 1958), a genus characterised by a chambered bulbous root ('Camarocrinus') which is thought to be a float for these supposedly pelagic crinoids; 'Camarocrinus'-like fragments were found associated with Scyphocrinites? pulcher at one locality (p. 97) in the present district. According to Kra" (1969) Butovicella migrans was epiplanktic. The widespread Cardiola interrupta was stated by McAlester (in Berry and Boucot, 1967, p. 1519) to be epifaunal in habit, whereas Kříž considered some Cardiola species to have been epiplanktic. The pterineids are represented by small, weakly ornamented specimens recalling Pterinea hians (McCoy, 1852, pl. 1.1, fig. 6), but they are too poorly preserved to be determined; they were presumably byssate and can be interpreted as epiplanktic in habit.

Some orthocones, for example 'Orthoceras'subannulare and 'Orthoceras' filosum, were not found above the Nantglyn Flags, nor was Scyphocrinites? pulcher recorded with certainty from higher horizons. Bolbozoe cf. bohemica, on the other hand, does not appear below the base of the Elwy Group. In all these cases the numbers of records are too small for the absence of any species to be regarded as significant (at P = 0.05).

Mottled mudstones

The mottled mudstones yield a diverse shelly fauna of some 90 forms, dominantly benthic brachiopods, molluscs and trilobites. The fossils are indigenous and, to judge from a number of articulated brachiopods (Glassia, Mezounia) and some trilobite moults (Ananaspis), accumulated in calm water. The fauna is distinctive, made up largely of species unknown from other horizons in Denbighshire. Corals and bryozoa are present. The commonest of the brachiopods are Bracteoleptaena cf. bracteola, Clorinda' dormitzeri, Giraldiella sp. nos., Glassia sp., Mezounia sp., Strophochonetes sp. and Visbyella cf. trewerna. Aegiria grayi occurs but is rarer in the mottled mudstones than in the adjacent flags. Of the Bivalvia, Cardiola interrupta is much the most widespread form, but several others are represented by one or two specimens, for example small nuculoids and the thin-shelled edentulous genera Lunulacardium, Maminka and Stolidotus. Gastropods are rare, but a number of orthocone species is present, judging from the variety of surface ornaments displayed by the fragments collected. The commonest trilobites are phacopids (Ananaspis) and dalmanitids (Dalmanites, Struveria), but Calymene sp., Miraspis mira and other forms occur rarely. Amphitoxotid ostracods occur gregariously in some places, and Sleia cf. kochi was found very rarely; these ostracods appear to have favoured the environment of the adjacent ribbon-banded horizons rather than that of the mottled mudstones themselves. Echinoderm remains were found in two starfish beds associated with the mottled mudstones, at Pantllin-mawr (p. 75) and Brynsylldy (p. 75), where the faunas are especially varied.

A feature of the faunas of the mottled mudstones (and to some extent of the ribbon-banded mudstones adjacent to them, which yield a more varied fauna than elsewhere in the succession) is the presence of several species unknown in typical Wenlock rocks in the Welsh Borderland but described from strata of Wenlock age in Bohemia and from the top of the Cyrtograptus Shale at Smedstorp in Scania, Sweden (Hede, 1915)—e.g. Bracteoleptaena bracteola, Maminka? cf. suecica, Stolidotus siluricus, Cryptocaris cf. scanica, Miraspis mira and Mezounia sp.(= Leptaena comitans Barrande of Hede). Analysis of the fauna of the mottled mudstones and associated ribbon-banded mudstones shows that 14 out of 18 of the forms identical to or comparable with species recorded at Smedstorp by Hede are confined to the mottled mudstones, and only one species (Parakionoceras ori ginale) is confined to the ribbon-banded beds. In contrast, a low proportion of forms comparable with those from the Welsh Borderland is confined to the mottled mudstones.

The brachiopods from the mottled mudstones indicate the Visbyella community of Calef and Hancock (1974, p. 791), who stressed the lack of variety of the brachiopod assemblage and stated that it was adapted to deep-water conditions. Hancock and others (1974, p. 155) observed a Visbyella community in mottled mudstones near Llanrwst and concluded that Wenlock 'Brachiopod communities occur [in the C. lundgreni Zone] down to the maximum depths reached in Wales'; but they did not remark that the fauna of the mottled mudstones contrasts with the absence of autochthonous brachiopods through much the greater part of the thickness of the Lower and Upper Nantglyn Flags groups. If it is agreed that water depth controlled the distribution of brachiopod communities then the Nantglyn Flags basin in Denbighshire seems to have been too deep to be colonised except during temporary shallowing at the time of deposition of the mottled mudstones. Comparable changes of depth in late Wenlock to early Ludlow times were inferred in South Wales by Calef and Hancock (1974, p. 799, text-fig. 5). However, Berry and Boucot (1967) emphasised the influence of water temperature and it is possible that a temporary climatic change affected the spread of the fauna of the mottled mudstones as well as the evolution of the graptolites (p. 56).

Calcareous siltstones

Some of the decalcified siltstones or 'gingerbread' rocks contain a profusion of fossils, but these are generally fragmentary and indeterminable. The chief exceptions are (1) various small fossils, including Aegiria grayi, Craniops implicatus, beyrichiid ostracods and the young of larger animals, (2) those recognised by fragments with a distinctive surface, for example Cardiola and 'Orthoceras'argus, and (3) durable fossils like crinoid columnals. These beds were deposited from turbidity currents (p. 45) and their fossils were thus probably derived from distant sources. The original community relationships cannot be determined because so many of the fossils are indeterminable.

A number of thin calcareous siltstone layers, especially in the Nantglyn Flags, are characterised by an abundance of beyrichiid ostracods associated with Aegiria grayi, and on occasion with aligned graptolites also. Other shells are very rare, but Dicoelosia biloba and Visbyella cf. trewerna have been found. These assemblages were probably derived from a Visbyella community living in deep water. Holland (1959, p. 474) has remarked that the assemblage of Aegiria grayi and ostracods was tolerant of conditions adverse to other benthos.

Disturbed beds

The distribution of shelly fossils in the disturbed beds is uneven, but in many places, notably in the Elwy Group, they are abundant both in bands or 'nests' and scattered through the rock. They generally occur as limonitic moulds, sometimes showing fine details. The stouter shells are often found undamaged, but the thinner or larger shells were generally broken before fossilisation. Conjoined valves are rare, but composite moulds occur locally. Compaction caused brittle fracture of some shells, and others show plastic deformation attributable to slumping or tectonism. It is inferred that the shelly material was in general dissolved away after slumping.

The disturbed beds yield a greater variety of shelly fossils than other facies; besides the abundance of brachiopods, a majority of the corals, bryozoa, bivalves and gastropods are from these beds. They are mostly typical of the Elton and Bringewood Beds of the Welsh Borderland but, despite some similarities in the vertical distribution of species, no detailed comparison can be made; nor can they be compared in detail with the contemporaneous faunas of the Welsh basinal facies, in spite of a general resemblance to the Cyrtoceras Mudstone of Builth, the Lower Baily Hill Beds of Knighton and the Wilsonia wilsoni Grits of Kerry.

The faunas of the disturbed beds of the Elwy Group vary slightly according to stratigraphical horizon (determined by means of graptolites) as follows:

Upper N. nilssoni ZONE — The commonest forms are Atrypa reticularis, Isorthis clivosa and I. cf. clivosa, together with Aegiria grayi, Hyattidina sp., Leptaena depressa, Microsphaeridiorhynchus nucula, Protochonetes ceratoides and Pholidostrophia sp.Apparently more typical of this zone than the C. scanicus Zone are Amphistrophia funiculata, Dicoelosia sp.[rare], Skenidioides lewisii [rare] and Sphaerirhynchia wilsoni, but they are not common enough for their distribution to be counted significant (at P = 0.05). Certain other rare forms have not been found higher than the N. nilssoni Zone, namely Dicoelosia biloba, Glassia sp., Visbyella cf. trewerna, Praectenodonta ludensis, Maminka cf. tenax, Parakionoceras originale and Leonaspis cf. coronata.

C. scanicus ZONE — The commonest fossils are Atrypa reticularis and Isorthis spp. with fewer Leptaena depressa, Leptostrophiafilosa, Sphaerirhynchia aff. wilsoni, Howellella spp., Microsphaeridiorhynchus nucula, Protochonetes ceratoides, P. ludloviensis, Pholidostrophia sp. and Shagamella minor. Isorthis aff. orbicularis (Walmsley, 1965, pl. 64, fig. 10) accompanies the I. clivosa group in significant numbers. A. reticularis, Howellella spp., L. filosa, P. ludloviensis, S. minor and Cardiola interrupta (in disturbed beds) are significantly commoner in this zone than below. Dalejina hybrida and Homoeospira baylei are rare, and Dayia navicula appears in small but significant numbers. The Ostracoda include species of Frostiella, Londinia, Sleia and cruminate Bollia' cf. unexa.

S. leintwardinensis incipiens ZONE — The shelly faunas are far less diverse than those at lower horizons. Dayia navicula is much commoner than in the C. scanicus Zone and is apt to occur gregariously, commonly in association with Shagamella. Also present are 'M.' nucula, Lanceomyonia sp. and I. aff. orbicularis. Notably less common than in the C. scanicus Zone are A. reticularis, Howellella spp., L. depressa, Protochonetes minimus and beyrichiid ostracods. Other species common enough in lower zones to be expected in the disturbed beds at this horizon, but which were not found at all, comprise A. grayi, C. implicatus, L. filosa, Pholidostrophia sp., P. ceratoides, P. ludloviensis, S. aff. wilsoni and Whitfieldella sp.Bivalves, gastropods and trilobites are all rare.

The shelly faunas of the disturbed beds evidently inhabited a shallow muddy sea, and the bulk of the faunas can be assigned to the Isorthis community (Calef and Hancock, 1974, p. 788). Slumping brought their remains into water apparently too deep to support an extensive shelly benthos, and they are thus juxtaposed with the restricted fauna of the ribbon-banded mudstones. However, in the Bont-uchel Formation a restricted Dayia navicula association (Calef and Hancock, 1974, p. 796) appears at times to have colonised the sea floor, suggesting that it was able to inhabit comparatively deep water. AWAR

Details of stratigraphy

In the following account the district has been divided into 16 regions (Figure 15) as follows: (I) Colwyn Bay, (II) Benarth—Ty' n-y-groes, (III) Eglwysbach, (IV) Llanrwst, (V) Bettws-yn-Rhos, (VI) Llangerniew, (VII) Gwytherin, (VIII) Llanfair Talhaiarn, (IX) Llansannan, (X) Denbigh Moors, (XI) Llannefydd, (XII) Bylchau, (XIII) Nantglyn, (XIV) Denbigh, (XV) Gyffylliog, and (XVI) Clwydian Range. In each region the rocks are considered in ascending stratigraphical order.

I Colwyn Bay region

This region lies to the north-east of the Pentrefelin Fault and includes the small area of Silurian rocks east of Llandudno. The eastern boundary with the Bettws-yn-Rhos region (V) is arbitrary.

A complete succession from the Lower Nantglyn Flags (?Cyrtograptus perneri Zone) to the Elwy Group (Cucullograptus scanicus Zone and possibly younger rocks) is present. In addition the presence of the Denbigh Grits Group beneath thick drift deposits to the south of Llandudno Junction Railway Station is confirmed by boreholes. This Group may also underlie three areas in the vicinity of Tywyn, Llanrhos and Llandudno.

Extensive drift obscures the outcrops from Llandudno to Tywyn and Llandudno Junction, between Llandudno Junction and Mochdre in the Afon Ganol, between Glan Conway and Bronyferiaeth and between Llanelian-yn-Rhos and Colwyn Bay. Elsewhere, except on the heights behind Mochdre, at Ffrithymynydd and Cefn Du, where there are extensive exposures, sections are generally restricted to isolated crags and quarries and short lengths of streams.

The area is important in the interpretation of disturbed beds, as was recognised by Jones (1940), and because it shows a distinct sandstone-rich facies of the Elwy Group. Surface mapping and borehole data have led to a complete reinterpretation of the solid geology of the Glan Conway and Tywyn–Llandudno Junction areas (compare (Figure 16) with Elles, 1909, pl. 8 and Boswell, 1949, p. 309, fig. 14.1, etc.). PTW, MJCN

Denbigh Grits Group and Lower Nantglyn Flags Group below Lower Mottled Mudstone

North-east of Craig-y-don, Bodafon Hall Farm Borehole (p. 188) proved the presence of Ordovician rocks beneath 25.33 m of drift (Rep. Inst. Geol. Sci for 1971, 1972, p. 122). From this and structural evidence the Denbigh Grits Group is inferred to occur beneath the low-lying drift-covered area north-west of Penrhyn-side and southwest of Little Ormes Head. The Group is also believed to succeed the Llandovery in the Llanrhos area (Figure 16).

At and to the south of Llandudno Junction Railway Station numerous boreholes for a road scheme have shown rocks of Benarth Flags facies (p. 24) dipping at 30° to 40° to the south-east and overlying Llandovery Pale Slates. One borehole, Colcon 612B (p. 191), yielded a mixed graptolite and shelly fauna (DJL 31–42) from ribbon-banded mudstones 28 to 38 m from the surface. The fauna comprises: horny brachiopod indet., Lunulacardium?; 'Orthoceras' cf. subundulatum, 'O.' cf. undulocinctum and graptolites including Monograptus firmus cf. sedberghensis. It is believed that this fauna is from the basal Cyrtograptus centrifugus Zone, for the rocks between 40 m and the bottom of the hole at 45.8 m appear to be passage beds into the Pale Slates, which are recorded in an adjacent borehole (Colcon 409C p. 190). A further borehole, Colcon 611, yielded (DJL 412–5 and 417–9) 'O.' argus and a number of distal fragments of pristiograptids, finds consistent with a low Wenlock horizon. A major fault must therefore separate the Denbigh Grits Group to the south of the railway station from the higher Wenlock and Ludlow strata to the north (see below and p. 61). MJCN

Recent temporary sections in ribbon-banded flags at a new warehouse site [SH 8054 7770] south-west of Hendre-Wen, and in a temporary excavation [SH 7969 7804] north-east of Llandudno Junction Station, together with Tywyn Borehole (see below and p. 199), have shown that the Lower Nantglyn Flags Group crops out extensively in the Llandudno Junction area (Figure 16). The beds are believed to dip consistently to the east or south-east at around 35°.

The 6 m of cored Lower Nantglyn Flags from Tywyn Borehole yielded a fauna (PW 585–627) of: Cyrtograptus aff. pseudolundgreni, Monoclimacis flumendosae cf. kingz, M. flumendosae subsp. ? nov., Monograptus flemingii s.l., M. cf. flemingii elegans and ?Plectograptus lejskoviensis, indicating the lower part of the C. lundgreni Zone. The dip in the borehole is 43–45°, whilst the Conway Castle Grit exposed immediately north-west of the bounding fault at Tywyn dips at 77°. On this evidence it is probable that part of the Tywyn foreshore is underlain by Denbigh Grits.

The temporary excavation in Llandudno Junction yielded (MJN 75–84) Monoclimacis flumendosae, Monograptus flemingii flemingii and Pristiograptus cf. pseudodubius, and on this and mapping evidence is probably also in the C. lundgreni Zone. MJCN, PTW

Between the Afon Ganol and Bryn-rhys faults the Lower Nantglyn Flags are intensely cleaved and weathered. They comprise ribbon-banded mudstones with bands of possibly mottled or disturbed mudstones. Determinable fossils are sparse, but the road cutting [SH 803 768] north of Glan Conway has yielded indeterminate graptolites (Zp 116–121) possibly referable to M. flemingii and P. pseudodubius, which, together with the lithology, indicate Lower Nantglyn Flags. In part of the cutting [SH 8030 7680] the mudstones include horizons with black shale chips and phosphatic pellets associated with small nodules and sandy inclusions. Sporadic larger mudstone nodules also occur. The following shelly fauna has been recorded from another part of the cutting [SH 8030 7676]: bryozoa, Aegiria sp., cf. Glassina laeviuscula, Howellella elegans, Visbyella cf. trewerna, Whitfieldella?, Hemsiella sp. andcrinoid columnals.

Farther east, but still to the north of the Bryn-rhys Fault, two localities [SH 8050 7631 (Zp 37–44); SH 8083 7631 (RK6108 6121)] in the vicinity of Bryn-rhys have yielded Gothograptus nassa, accompanied at the former locality by Pristiograptus aff. jaegeri and at the latter by Giraldiella sp.(RK 6122). These faunas confirm the presence of the Lower Nantglyn Flags and, by comparison with other regions, suggest that the local succession may include the Lower Mottled Mudstone.

The main outcrop of the Lower Nantglyn Flags in this region lies between the Bryn-rhys Fault and the Pentrefelin Fault at Deunant. The measures are well seen, though poorly fossiliferous, in Nant-yGareg-ddu, and are sporadically exposed in and to the south of Glan Conway and in Nant Geneth. At a number of localities [SH 8008 7540] (Zp 508–514); [SH 8042 7569] (RK 5011–5027); [SH 8041 7535] (Zp 463–495); [SH 8082 7557] (RK 607–688)] the ribbon-banded mudstones have yielded characteristic C. lundgreni Zone faunas. The graptolite fauna of the easternmost locality is: C. hamatus and M. flemingii cf. flemingii. Atrypids, dalmanellids, a lunulacardiid and cf. Scyphocrinites? pulcher were also found. The westernmost locality yielded C. cf. lundgreni, and the total fauna from these several exposures included M. flemingii cf. compactus and M. flemingii cf.elegans.DACM, PTW

Lower Nantglyn Flags Group

Lower Mottled Mudstone and higher measures

The Lower Mottled Mudstone is seen in quarries near Ty-du, [SH 8082 7596 and SH 8092 7548] and in small sections west of Penrhiwisaf. In general it consists mainly of mottled mudstone, but with a higher than average proportion of calcareous siltstone stripes. A small quarry [SH 8115 7465] near Penrhiwisaf shows 0.53 m of grey-brown mudstone with pellets of black collophane and sporadic solitary corals and bryozoan fragments, interpreted as a disturbed bed, overlying 1.20 m of grey-brown, irregularly mottled and well bedded mudstone. The following fauna (RU 3232–3244) was recorded: Bractoleptaena bracteola?, Glassia sp., Howellella sp., Mezouni a sp., Visbyella cf. trewerna and Hemsiella sp.. In Nant Geneth the Lower Mottled Mudstone is almost entirely represented by disturbed beds.

The measures between the Lower and Upper Mottled Mudstones generally form a topographical slack and are poorly exposed; continuous sections are, however, seen in the stream west of Ty-du and in Nant-y-Gareg-ddu and Nant Geneth. At an exposure [SH 8162 7375] in Nant Geneth, where 2.40 m of ribbon-banded flags are seen, one sandy mudstone band has shallow flute-casts, indicating a depositional current from the east-south-east (110°). This record, unique in the Group, does not conform readily to the pattern of current directions deduced for the preceding or succeeding groups (pp. 35 and 36). An old quarry [SH 8129 7456] in interbedded mottled mudstone and ribbon-banded silty mudstone has yielded Gothograptus nassa (RV 1060–1067), and the bank [SH 8099 756] east-north-east of Ty-du yielded (RK 4908) ' 'Orthoceras' arenosum.

The Upper Mottled Mudstone is very poorly exposed. An old quarry [SH 8152 7461] in Nant-y-Gareg-ddu, west-south-west of Penrhiwuchaf shows over 10 m of mottled mudstone with numerous siltstone stripes and one 60-cm band of ribbon-banded mudstone. PTW

A borehole, Colcon 401 (p. 189), west-north-west of the A55/A496 road junction proved 6 m of Upper Mottled Mudstones' beneath 9.2 m of drift and yielded (DJL 405–411) a ?Tentaculid, Monograptus ludensis and Pristiograptus jaegeri, indicating the M. ludensis Zone.

The succeeding measures are seen only in sporadic exposures west of Glan Conway and in the stream sections cited above. Near Deunant, a gorge [SH 8183 7364] in Nant Geneth south-west of the farm, exposes the Brynsylldy Formation, here represented by 3 m of disturbed silty mudstone of which the uppermost part consists of collophane pebbles, decalcified concretions (to 9.5 cm) and small blocks of silty mudstone in a disturbed matrix. To the north, this formation is again exposed west of Penrhiwuchaf, where it is 4.5 m thick.

The low north–south spur to the east of Tywyn and the parallel feature east of the small stream through Llandudno Junction are interpreted as outcrops of the Lower Mottled Mudstone, and the low feature to the north-east of the Hotpoint Factory [SH 803 778] is taken to be the outcrop of the Upper Mottled Mudstone beneath the boulder clay.

Upper Nantglyn Flags Group

The Group crops out under part of Llandudno Junction (Figure 16), in the Hendre Wen area, and between the Cilglasin area and Grugfryn. It varies in thickness from around 150 m in the Glan Conway and Llandudno Junction areas to perhaps 100 m west of Bronyferiaeth, where the upper boundary is obscure, lying in a transitional sequence of ribbon-banded and striped mudstones largely obscured by drift. MJCN, DACM, PTW

Characteristic faunas occur at a number of localities, notably: a section [SH 8196 7342] in Nant Geneth which yielded (RV 1182–1193) Monograptus uncinatus orbatus and Neodiversograptus nilssoni; a road section [SH 8153 7639] east of Bryn-rhys Chapel yielding the same fauna (Zp 292–311); a further road section [SH 8149 7643] from which Monoclimacis aff. haupti, M. uncinatus orbatus, ?Neodiversograptus nilssoni and Spinograptus spinosus have been obtained (Zp 312–318); and an old quarry [SH 8212 7358] northeast of Ty'n-y-Coed (Zp 1161–1172) with M. colonus cf. colonus and Saetograptus cf. varians varians. Llandudno Junction Borehole (p. 195) yielded (BLE 5453–5483) ?Pristiograptus dubius and an indeterminate specimen (BLE 5480) which may be M. uncinatus orbatus. MJCN, PTW

Elwy Group

Most of the Colwyn Bay region is underlain by the Elwy Group, which is here more than 1200 m thick. In the Mochdre–Ffrith-ymynydd area the lower 300 m are believed to belong to the Upper N. nilssoni Zone, and 450 m are so ascribed in the Cefn Du area.

Characteristic graptolite faunas have been obtained: at Bronyferiaeth [SH 8267 7369] (Zp 1362–1383)—Monoclimacis micropoma, S. varians pumilus, S. cf. varians varians, Pristiograptus cf. comis; at Goetre Farm [SH 8212 7603] south-east of Cefn-y-garlleg (Zp 323–351a)—Cucullograptus cf. scanicus, N. nilssoni [abundant], Saetograptus chimaera chimaera, S. chimaera salweyi, S. aff. leintwardinensis incipiens, S. varians cf. pumilus; and at Ty-gwyn [SH 8206 7794] (Zp 130–162)—M. colonus colonus, ?N. nilssoni, P. dubius cf. ludlowensi s, P. curtus, P. aff. curtus and S. chimaera cf. chimaera.

Shelly faunas have also been recorded. The stream section [SH 8267 7336] to [SH 8297 7295] south-south-east of Bronyferiaeth yielded (RV 1332–1345) Aegiria grayi, Dicoelosia cf. biloba, plectodontid, Skenidioides lewisii, Visbyella sp., calymenid and dalmanitid, Leonaspis cf. coronata, Hemsiella sp. andcrinoid columnals; in addition the graptolites, M. cf. colonus, S. varians varians and S. cf. varians were collected. An exposure [SH 8140 7697] at Cilglasin Farm yielded (RK 4959–4978) Favosites sp., bryozoa, A. cf. reticularis, Isorthis sp., Microsphaeridiorhynchus cf. nucula, Protochonetes sp., Salopina sp., Sphaerirhynchia aff. wilsoni, Whitfieldella sp., beyrichiid and crinoid columnals. The stream section [SH 829 777] at Graianelyn yielded (RK 6763–6767) Cardiola interrupta and Bolbozoe cf. bohemica.

Graptolite faunas indicative of the C. scanicus Zone are few, but have been recorded from a crag [SH 8238 7685] north-east of Cefny-garlleg (RK 6959–6976)—C. cf. crinitus, C. cf. scanicus, P. cf. vicinus—and from the old quarry [SH 820 788] at Mochdre. At the latter locality three collections (Zp 169–184, RK 5768–5776 and RK 5802–5814) from the 110-m sequence of silty mudstones with numerous siltstones and sandstones and four disturbed beds have together yielded C. scanicus, P. aff. curtus, P. cf. dubius ludlowensis, P. aff. vicinus, S. chimaera chimaera and S. chimaera cf. salweyi. In addition the following shelly fauna (RK 5778–5800) was recorded: bryozoa, Aegiria grayi, Atrypa sp., Craniops implicatus, Homoeospira baylei, Howellella sp., Protochonetes cf. ceratoides, Shagamella minor, strophomenid, Whitfieldella?, 'Orthoceras' cf. argus, 'Bollia' sp., Entomozoe cf. tuberosa, and crinoid columnals.

A thin disturbed bed in an exposure [SH 8433 7622] east of Bryny-maen yielded (RK 6798–6846) Howellella elegans, Isorthis cf. clivosa [common], I. aff. orbicularis [common], Marklandella sp., Sphaerirhynchia aff. wilsoni [common], Loxonema sp., Dalmanites sp. andcrinoid columnals; and from the quarry [SH 8394 7465] north-east of Ffrith-wen the commoner forms listed above were recorded together with Leptaena sp., Leptostrophia sp., Protochonetes ceratoides?, Cardiola interrupta and molluscan fragments (RV 1194–1242).

Mainly on lithological grounds, some 400 m of strata in the Cefn Du-Mynydd Llanelian area are referred to the C. scanicus Zone. By the same token, and by comparison with the Bettws-yn-Rhos region (pp. 78–79) the striped silty mudstones with thin disturbed beds typical of the Mynydd Llanelian-Coed Bryndansi to Old Colwyn area are attributed to the S. leintwardinensis incipiens Zone. There is, however, no palaeontological evidence. Typical sections in silty mudstones with disturbed beds up to 2.4 m thick are seen along the Bettws-yn-Rhos–Glan Conway road east of Lletty Adar. Farther north striped silty mudstones are seen in the stream section [SH 8581 7617] east of Tyn-y-coed, around Bryn-y-person [SH 8618 7617] and near St Hilary's Church [SH 8635 7642], Llanelian-yn-Rhos, where a 45-cm sandstone band occurs. The highest beds in the succession are probably these cropping out close to the base of the Carboniferous Basement Beds, a typical locality being an old quarry [SH 8763 7743] south-east of Peulwys, where 3.6 m of variably cleaved, poorly bedded striped beds and mudstones with rare shells are visible. The fauna (RK 9353–9376) comprises Atrypa reticularis, Dayia navicula [common], Microsphaeridiorhynchus nucula, Lanceomyonia?, Salopina sp.[rare], Shagamella sp. and Bolbozoe?—a fauna characteristic of the S. incipiens Zone elsewhere in the district (see e.g. p. 79). PTW, DP

The most significant exposures in the Colwyn Bay area are those of the lower Elwy Group seen in the high ground south-east of Mochdre and in the Cefn-y-garlleg- Pant and Cefn Du areas, and described in some detail by Jones (1940). The areas are unique in north-western Denbighshire in showing a sandstone facies of the Upper nilssoni Zone. The sandstones, up to 30 m thick, are regularly bedded or massive and graded in part as in the crag [SH 8326 7351] south of Tyddynuchaf, or in the form of disturbed masses irregularly interbedded with disturbed beds as in the sections [SH 8239 7591] and [SH 8240 7579] immediately south of Pant. Sandstones are well exposed in the quarry [SH 8262 7602] west of Geufron, where 6.4 m of interbedded graded sandstones, with flute and load-casts, and silty mudstones are seen, and at the eastern end of Ffrithymynydd in a scarp [SH 8300 7533] west-south-west of Bryn-y-maen Farm, where 10 m of comparable beds include a 4.5-m massive fine-grained sandstone. Sole markings are also seen in the old quarry [SH 8312 7941] south-west of Bryn-dinarth (flute-casts), the scarp [SH 8461 7658] west-south-west of Cilgwyn-mawr (? bounce-casts), the scarp [SH 8434 7626] south-west of Cilgwyn-mawr (flute-loadcasts), the disused quarry [SH 8398 7533] south-east of Bryn-ymaen Farm (bounce and prod-casts), the old quarry [SH 8337 7486] north-west of Tyn-llwyn (?flute-casts, ?longitudinal ridge-casts, ?groove and bounce-casts); the old quarry [SH 8382 7414] south-south-east of Ffrith-wen (groove, bounce and load-casts), the small waterfall [SH 8346 7389] east of Tyddynuchaf (asymmetrical ripple mark), and the scarp [SH 8284 7354] south-west of Bronyferiaeth (furrow-casts).

In the same areas—Mochdre, Ffrithymynydd and Cefn Du there are notable exposures of disturbed beds, including several showing lateral and upper surface contacts. A scarp [SH 8346 7344] south-east of Tyddynuchaf shows:

and a further scarp [SH 8335 7359] some 200 m to the north-west exposes:

The old quarry [SH 8327 7790] west-south-west of Hafodty Farm, Mochdre, described by Jones (1940, p. 355, fig. 18) shows:

Gill (1957) reported sand volcanoes from the upper sandstone/silty mudstone sequence but these are no longer visible (p. 39).

Additional localities of interest are: an exposure [SH 8458 7685] west of Cilgwyn-mawr, where 2.1 m of 'incipient disturbed' sandstone are seen, and a crag [SH 8228 7802] north-east of Ty-gwyn, which shows an excellent upper surface contact with 7.5 to 15 cm of fine-grained sandstone at the base of a 2.4-m sequence of striped silty mudstones, resting on the highly uneven surface of a 2.4-m pencil slate disturbed bed. Other top contacts are or were formerly visible in the yard [SH 8223 7562] at Tan-y-bryniau and in the road-bank [SH 8349 7537] beside Llwydcoed School, south-west of Bryn-y-maen Farm. The section seen at the former locality is:

Slump folds are seen in a crag [SH 8360 7503] south-south-east of Caethiwed ((Plate 7); Jones, 1940, pl. 23). Sandstone rolls with load-casts occur in a small quarry [SH 8211 7562] south-west of Pant.

One of the finest surviving pavements of disturbed beds (i.e. on a plane more or less parallel to the bedding) occurs at Cefn-y-garlleg farm-yard (Jones, 1940, p. 348, fig. 11). Jones' interpretation of the exposures was confirmed during the present survey although less significance is attached to the dip and strike within the disturbed beds. However, Jones' (1940, fig. 8, pl. 24) correlation of the disturbed beds either side of the Pant Fault in the Cefn-y-garlleg area is not accepted, and comparison of Jones's plates 24 to 26 with 1-in Sheet 95 (Rhyl) and 1:50,000 Sheet 107 (Denbigh) reveals rather different interpretations of the importance of faulting and lateral passage in the Colwyn Bay region, with a greater emphasis given in the present survey to the latter in the Cefn Du area and to the former in the Mochdre area. PTW

II Benarth–Ty'n-Y-Groes region

This region includes all the Wenlock rocks west of the River Conway. The succession ranges from the basal Wenlock (Cyrtograptus centrifugus Zone) to an horizon low in the C. rigidus Zone, but structural complications make a detailed stratigraphical appraisal impossible.

Numerous small quarry, crag and roadside exposures occur on the high ground south of Gyffin and in the area between Eirianws and Bryn-cŵn and south-westwards to Roewen. There are also extensive coastal and estuary bank sections at Benarth, east of Gorse Hill and in the Tal-y-cafn area. The classic section on the Benarth foreshore (Elles, 1909) is the type locality for the Benarth Flags (and Grits) and provides the only exposed Llandovery-Wenlock succession in the district outside the Llanrwst region (p. 72).

Denbigh Grits Group

All the rocks of the region belong to this Group. They are considered below under two areas—the Benarth area to the north and, separated from it by the drift-covered anticlinal area around Henryd, the Ty'n-y-groes area to the south.

Benarth area

The foreshore below Benarth Hall provides one of the few continuous, measurable sections in the district:

The eight fossil localities A: [SH 79001 76996]; B: [SH 78995 77012]; C: [SH 78946 77072]; D: [SH 78780 77146]; E: SH 78710 77163; F: [SH 78701 77166]; G: [SH 78698 77170]; H: [SH 78697 77172] yielded:

• A (RV 9299–9314): Monoclimacis cf. flumendosae flumendosae, Monograptus flexilis cf. belophorus, Monograptus sp., Pristiograptus cf. praedubius.
• B (RV 9315–9337): Dalmanellid, small plectambonitacean, Nuculites?, ?Monoclimacis vomerina basilica, M. vomerina s.l., Pristiograptus cf. dubius, P. cf. praedubius.
• C (RV 9338–9343): P. dubius cf. dubius, P. dubius latus.
• D (RV 9344–9372 and GF 7828–7833): Orthoceras sp., Cyrtograptus cf. insectus, Monoclimacis cf. linnarssoni, M. vomerina s.l., M. vomerina basilica, M. vomerina vomerina, Monograptus cf. flexuosus, M. kodymi, M. priodon, Pristiograptus?, Retiolites geinitzianus angustidens, R. geinitzianus geinitzianus.
• E (GF 7834–7837): Monoclimacis vomerina aff. vomerina, Monograptus cf. priodon.
• F (GF 7838–7841): Monoclimacis vomerina basilica, M. vomerina aff. basilica.
• G (GF 7842–7855): Monoclimacis vomerina s.l., M. vomerina aff. basilica, Monograptus priodon.
• H (GF 7856–7867): Monoclimacis vomerina s.l., M. vomerina basilica, M. vomerina aff. basilica, M. vomerina cf. basilica, M. vomerina ?vomerina.

They are assigned thus: A: high in the M. riccartonensis Zone; B and C: the M. riccartonensis Zone, and D to H: the C. centrifugus Zone.

The term Benarth Flags, taken from Elles's (1909) Benarth Flags and Grits, which is synonymous with the Denbigh Grits Group, is used here for the flaggy, predominantly silty mudstone with some ribbon-banding forming the basal 200 m of the foreshore section. Thus defined, the formation at its type locality extends from the base of the Wenlock to an M. riccartonensis Zone horizon.

The basal Benarth Flags have also been seen near Gyffin in a road-side section [SH 7805 7720] at Castle View Estate where the section is:

Farther south in an old quarry [SH 7738 7512], south-east of Plas Iolyn, the following are exposed:

At both the above localities a C. centrifugus Zone fauna was obtained. As well as several of the species recorded from the Benarth foreshore, the Gyffin Section yielded (DT 8013–8058) Monoclimacis griestonensis nicoli, M. linnarssoni and Monograptus simulatus during the present survey and in addition C. centrifugus and Monoclimacis shottoni? (fide Dr R. B. Rickards). The fauna (RV 8728–8756) from the Plas Iolyn section is comparable to that from Benarth, but slender monograptids are absent and the section probably lies higher in the zone.

In the absence of sufficient exposure, thickness variations in the Benarth Flags cannot be precisely determined. However, the disturbed bed cropping out at Benarth Point is believed to be the same one that occurs: near the Rectory at Gyffin; at Bryn-seiri, 4 km south of Gyffin; at Cae Cregin, 0.8 km south of Gyffin; and on the high ground immediately east of Plas Iolyn. This being so, the Benarth Flags have a fairly constant thickness, although the upper part is cut out by the disturbed bed in the Bryn-seiri area. The disturbed bed is well exposed at Bryn-seiri [SH 7800 7627], where it consists of highly cleaved silty mudstone, locally pencil slate, with sporadic sandstone balls to 22 cm and includes a 12-m sandstone band. The section at Cae Cregin [SH 7752 7613], south-south-west of Gyffin, shows disturbed sandy siltstone with sandstone lenses and brachiopods, overlying massive sandstone with a band of flaggy siltstone. The scarp [SH 7760 7515] west of Gorse Hill Farm shows up to 15.2 m of pencil slate with a few irregular sandstone masses and small pebbles. MJCN, PTW

Overlying the disturbed beds in the foreshore section at Benarth Point (p. 63) there are, in ascending order, approximately 18 m of fine-grained and partly disturbed sandstone, about 360 m of striped silty mudstone with thin sandstone bands, some of which show sole structures, and about 15 m of disturbed mudstone with sandstone lenses. The structural complexity of the section, including faulting, folding and locally inverted sequences, is such that these are very approximate figures. In the absence of palaeontological evidence the beds are assigned to the base of the C. rigidus Zone (cf. Elles, 1909, p. 187). DP, PTW

Inland, the succession includes thin (0–10 m) sandstones and, 0.8 km NE of Gorse Hill, up to 67 m of pencil slate disturbed bed. A section [SH 7810 7550] north-north-east of Gorse Hill Farm, for example, shows:

East of Baclaw there is, in the river bank [SH 7829 7476] to [SH 7840 7500], a sequence of highly folded, locally inverted beds, the stratigraphical position of which is believed to be immediately above the Benarth Point disturbed bed. They consist of pale grey striped silty mudstones with laminated siltstone beds and multiple-graded sandstones up to 30 cm thick, which are convoluted and laminated in part. Flute-casts were recorded from the foreshore [SH 7839 7495], east of Baclaw. Sole structures, respectively flute-casts and longitudinal ridge-casts are also seen at two localities [SH 7900 7688], [SH 7904 7655] on the Benarth foreshore.

Ty'n-y-groes area

Basal Wenlock strata crop out, more or less continuously, between Eirianws and the Afon Roe. A temporary exposure in the roadside [SH 7772 7362] north-north-east of Eirianws showed grey mudstone with thin graptolitic laminated muddy siltstone bands and yielded a C. centrifugus Zone fauna (RV 8696–8727), comprising Cyrtograptus sp.(of C. murchisoni group), ?Monoclimacis linnarssoni, M. vomerina basilica, M. vomerina cf. vomerina, Monograptus?, Pristiograptus sp. and Retiolites geinitzianus cf. geinitzianus. The same horizon is poorly exposed in the stream northwest of the farm. Overlying striped, and in part ribbon-banded, silty mudstones, are also assigned to the Benarth Flags. They are succeeded by a unit of sandstones and disturbed beds which crops out sporadically between Eirianws and the Roewen area and is correlated with the disturbed horizon at Benarth Point. Striped silty mudstones with sandstones up to 3 m thick are exposed in the yard and old quarry at Eirianws.

In an old quarry [SH 7802 7337] east of Eirianws the section is:

Further sections are seen at a number of localities, notably in the old quarries [SH 7746 7337], [SH 7746, 7335] 170 to 180 m west of Eirianws. The northern quarry shows:

Overlying sandstones, 6 m thick, are seen in the southern quarry; they have sole structures (bounce, groove and prod-casts) indicating currents flowing from south-west to north-east. The unit appears to thicken, with an increasing proportion of disturbed beds, southwards. A quarry [SH 7652 7202], south-south-west of Glyn Uchaf shows cleaved, disturbed sandstones and siltstones with thin conglomerate lenses composed of mudstone and siltstone fragments. A roadside quarry [SH 7690 7307] south-south-west of Llangelynin yielded (RV 6903–6924) ?Monoclimacis and ?Monograptus riccartonensis, and probably lies in the M. riccartonensis Zone.

In the Tan-y-bryn area highly cleaved striped silty mudstones with thin bands of laminated siltstone and fine-grained sandstone and sporadic sandstone beds up to 45 cm thick are seen, for example, in the scarp [SH 7754 7289] due west of the house. They directly overlie the sandstone/disturbed bed unit described above. A scarp [SH 7856 7349] east of Tremorfa exposes 9 m of striped silty mudstone comprising thin alternations of laminated siltstone and dark grey silty mudstone. The position of these and similar strata around Tremorfa is uncertain, but they are probably the correlatives of striped silty mudstones in the Tan-y-bryn area. Immediately east of Tan-y-bryn itself two old quarries [SH 7780 7289]; [SH 7782 7279] expose sandstones up to 1.67 m thick, some of which show sole structures (groove-casts and ?flute- casts). The southern quarry shows:

Immediately east of these quarries the sandstones are disturbed in part and are associated with disturbed silty mudstone. This is also the case on Bryn-cŵn, where 10-m scarps in pencil slate disturbed bed occur, and farther east where a quarry [SH 7898 7297] north of Llwydfaen, shows:

A major pencil slate disturbed bed unit, approximately 61 m thick, occurring to the west of Bryn-cŵn is probably to be correlated with a similar bed spectacularly exposed at Ty' n-y-groes. In the road cutting [SH 7743 7177] through the village highly cleaved, pencil slate, silty mudstones with lenses of siltstone and sandy siltstone showing disturbed bedding are seen. Small rounded pebbles of dark mudstone and shells are scattered throughout. Farther west, and apparently underlying the road section, a scarp [SH 7725 7171] exposes massive disturbed sandstones with thin lenticular disturbed siltstones and pebbly conglomerate. MJCN, PTW

East of Ty'n-y-groes, near Tal-y-cafn uchaf, a disturbed bed unit composed predominantly of pencil slate and about 45 m thick is exposed in a scarp [SH 7825 7130] south-west of Tal-y-cafn railway station and in the west bank of the Conway [SH 7833 7102] south-south-west of the station. It probably continues east of the Conway Valley as the disturbed bed/sandstone unit immediately east of Tan-yr-allt (Figure 17) in the Eglwysbach region. The implication is that almost the entire sequence of the Ty'rt-y-groes and Benarth region underlies the lowest beds of the Denbigh Grits Group of the Eglwysbach region; and on this basis the thickness of the Denbigh Grits Group in the northern part of the district is estimated to be 1050 to 1200 m.

III Eglwysbach region

A complete succession from the Denbigh Grits Group (rigidus Zone?) to the basal Elwy Group (Upper Neodiversographus nilssoni Zone) is present in this region. Exposure is generally good, particularly on the relatively high ground adjacent to the Conway at Bodnant, Llyn Syberi and Plas Maenen, and in the area between Moel Gyffylog and Gwern-bwys. The region provides some of the best exposures of the Denbigh Grits Group and includes the type area of the Berllan Formation.

Denbigh Grits Group

Measures below the Berllan Formation

Conglomeratic sandstones and disturbed beds are seen in the Bodnant area, notably in the old quarries [SH 8012 7340] at Croesau, where the section is:

Farther south around Llyn Syberi a succession consisting predominantly of striped silty mudstones contains four units of sandstones and disturbed beds (Figure 17). The lowest unit crops out near the River Conway, and is seen in the old quarry [SH 7821 7001] south-west of Tan-yr-allt, and in the road cutting [SH 7813 6963] north of Dyto. At the former exposure sandstone, one bed of which shows longitudinal ridge-casts, is overlain by a disturbed bed, the base of which cuts down to both north and south (Plate 10). The road cutting shows 6 m of pencil slate disturbed bed, which there consists of highly cleaved, dark grey silty mudstone with lenses and balled masses of sandstone up to 30 cm across.

The second unit forms the conspicuous sandstone scarps north of Dyto. One exposure [SH 7824 6922] north of Dyto, shows:

A little to the south-east [SH 7827 6916] 12 m of fine to coarse-grained sandstones, in beds to 2.5 m, are exposed. One bed shows flute-casts. Siltstone laminae, sandy mudstone bands and silty disturbed beds separate the sandstones. The overlying third unit, when traced northwards, shows a rapidly increasing proportion of disturbed beds. Around Heather House, west of Llyn Syberi, there are at least three disturbed beds, one in the second unit and two in the third. Each is up to about 8 m thick, but only the lowest persists northwards to be seen in crags [SH 7849 7023] near Tan-yr-allt, where it is over 12 m thick, in Coed Cerrig-y-wyallt and in crags east of Brickyard House, south-south-west of Tal-y-cafn Station.

Underlying the highest of the sandstone/disturbed bed units is a sequence of striped silty mudstones. A section in the stream [SH 7872 6916] west of Hafotty shows 9 m of such beds with sporadic fine-grained laminated sandstone bands, up to 12 cm thick, showing groove and flute-casts and interference ripple-casts. The disturbed unit is thickest between Hafotty and Ffridd-lom, where it consists predominantly of highly cleaved, in part pencil slate, disturbed beds. Locally the highly fractured silty mudstone making up these beds contains pebbles to 2.5 cm of vein-quartz, silty mudstone and black mudstone. Fine to coarse-grained sandstones occur, and sections show interbedding with disturbed strata, for example in a crag [SH 7919 6878] near Ffridd-lom. Lateral passage of sandstone into disturbed bed is also seen in numerous sections, and many individual sandstone beds thin rapidly northwards. Traced northwards, this unit thins rapidly, and at the northern end of Llyn Syberi is reduced to a single 6-m disturbed bed. This disappears south-west of Esgairheulog in Coed Newydd, and the unit is replaced by striped silty mudstones of Berllan Formation type.

South of the Dyto Fault (Figure 17) the succession of predominantly striped beds, seen for example in the stream section at Cefnycoedisaf, contains three sandstone/disturbed units. Correlation across the Dyto Fault is, however, uncertain. The lowest unit, possibly correlating with the second unit north of the fault, consists only of sandstones from Dyto southwards almost to Bodhyfryd. However, in the wood east of Bodhyfryd the sandstones are almost entirely replaced by disturbed beds.

The middle unit south of the Dyto Fault forms the prominent scarps of Cader Ifan Goch, east of Plas Maenan, where there are sections in thick sandy disturbed beds and sandstones. To the north of Plâs Maenan the unit consists of varying proportions of sandstones and disturbed beds; to the south it thins, becomes more regularly bedded and includes a large proportion of striped beds. The stream section immediately east of Maenan Abbey shows about 120 m of striped beds with bands of fine-grained sandstone and disturbed beds up to 3 m and 5 m thick respectively.

The upper unit south of the Dyto Fault consists predominantly of sandstones with disturbed beds around Clwt-Bedw. Here a crag [SH 7960 6767] shows 6 m of massive, fine-grained sandstones interbanded with disturbed and regularly bedded silty mudstone. To the south, around Maes-y-groes, disturbed beds predominate. A scarp [SH 7960 6723] at Maes-y-groes-isaf for example shows 4.5 m of highly cleaved, dark grey silty mudstone with sand grains throughout, overlying 3 m of massive, fine-grained sandstones with irregular cleaved silty mudstone partings, the whole passing laterally into sandy disturbed beds northwards, and being represented by sandstone alone to the south. Rapid lateral passage can also be seen around Ty'n-y-celyn and in the Pen-y-graig area a little to the south-south-east.

Farther south the unit is composed of massive sandstones in beds up to 2.4 m thick, and in the Goelas area there are associated disturbed beds and evidence of the joining of sandstone beds. A scarp [SH 7996 6591] north-north-east of Pont-y-gatn, Maenan, shows: 3.6 m of fine-grained sandstones in 10-cm to 1-m beds alternating with striped silty mudstones. Abundant sole structures, including longitudinal ridge-casts, elongate load-casts and interference ripple-casts, occur, and the sandstone beds all show southward thickening. To the south a scarp [SH 7998 6562] shows 6 m of massive sandstones with much disturbance, passing laterally northwards into more regularly bedded sandstones in beds from 30 cm to 1 m thick, alternating with silty mudstones. Prod-casts occur on one bed.

Sole structures, many of which are indicative of a generally westerly current source (p. 35), are also found at a number of other localities in the region: a scarp [SH 7824 6932] north of Dyto (linear load-casts); the old quarry [SH 7885 7445] 1 km west of Bryniau (interference ripple-casts, load-flowage casts); the road bank [SH 7982 7436] south of Bryniau (torose load-casts and flute-casts); and the road cutting [SH 7815 6969] north of Dyto (linear load-casts).

Berllan Formation

The outcrop of this formation extends from the Croesau area to between Goppy and Goelas, where it is replaced by beds of the Llanddoget Formation. It includes the type locality, Berllan-fawr [SH 8019 6985], south of Eglwysbach.

An old quarry [SH 8043 7331] near Croesau shows 7.6 m of well cleaved, dark grey, mostly laminated, silty mudstones and siltstones with thin laminated muddy siltstone ('graptolite shale') bands and sporadic bands up to 2.5 cm thick of fine-grained sandstone. Abundant joints trending 190° give a blocky weathering.

East and south-east of Tal-y-cafn numerous exposures show similar strata. One quarry [SH 7916 7157], south-south-east of Penrhyd, exhibits 6 m of striped silty mudstone with a single band of small (2.5 cm) grey, 'rotten' nodules. Streams at Esgairheulog and around Lletty provide sections up to 15 m in silty mudstones and siltstones. In the stream west of Berllan-fawr a section [SH 7967 6973] at Ffridd-lŵyd consists of 3 m of ribbon-banded silty mudstone.

The most complete sections in the formation are seen in the tributaries of Nant y Rhiw-felen, south-west of Pentre'r-Felin. One [SH 8026 6918], west-south-west of Pentre'r-Felin Smithy, shows 6 m of striped silty mudstones, and in the nearby road [SH 8027 6909] the same rock type contains a 35-cm nodule.

A 3.5-m scarp [SH 8013 6722] east of Maes-y-groes-isaf is in dark grey, thinly bedded silty mudstone with ribs up to 10 cm thick of fine-grained, poorly graded and partly current-bedded sandstone in the upper part. East of Ty'n-y-celyn a quarry [SH 8005 6663] shows 1.5 m of dark grey silty mudstone with 1 to 3.5-cm ferruginous nodules and a few thin sandstone ribs. Evidence of the lateral passage of the Berllan Formation into a disturbed bed sequence is provided by the small scarp [SH 8034 6675] in Twhwnt-i'r-fawnog farmyard, west of Cefn-llech, Maenan, which shows 1.5 to 7 m of pencil slate disturbed bed on 1 to 6.3 m of striped silty mudstone. Southwards the base of the disturbed bed is at progressively lower levels. Comparable evidence is seen at Goppy [SH 8027 6618].

A small scarp [SH 8029 6680] immediately north-west of Twhwnt-i'r-fawnog shows 3 m of partly contorted striped silty mudstone containing rounded sandstone balls (to 0.75 m) near the base.

Graptolite faunas from the Berllan Formation are indicative of the Cyrtograptus rigidus Zone. A temporary section in the foundations of a pylon [SH 8028 6891] west of Croes-onen, Pentre'r Felin, yielded (DT 6340–6364): C. rigidus, Monoclimacis flumendosae flumendosae, Monograptus flemingii cf. primus, M. retroflexus, Pristiograptus sp.A comparable fauna (Zp 702–730, Zp 8048), with the addition of M. flemingii cf.flemingii and P. pseudodubius, was found in the stream bank [SH 8021 6905], immediately to the north. Farther north, collections (Zp 683–695, RV 3266–3286) from the stream bank [SH 7968 6974] to [SH 7987 6975] at Ffridd-lŵyd, south-west of Eglwysbach, yielded C. rigidus, M. flumendosae, Monoclimacis sp.aff. vonzerina, M. flemingii and P. pseudodubius. In addition a nearby stream section [SH 7951 7014] yielded (Zp 603–617) Monograptus cf. priodon.

Llanddoget Formation

Except in the Hiraethlyn valley and between Cefn-llech and Nant, where drift cover is extensive, the Llanddoget Formation gives rise to topographically prominent areas with irregular, hummocky form and numerous small exposures of pencil slate disturbed bed (p. 38) and some exposures of beds or irregular masses of sandstone.

In the Bodnant area, the basal 1.8 m of the formation with 11 m of well bedded underlying measures is exposed at Bwlch [SH 8052 7312]. In the same area two old quarries [SH 8020 7279] and [SH 8023 7282] in Terrace Wood, west of Bodnant Uchaf, show sandstones interbedded with pencil slates. The section in the eastern quarry is:

The sequence is folded into a large anticline and syncline, and a pair of smaller (2-m wavelength) folds are also seen. Flute-casts are seen in the comparable section in the western quarry.

Sections to 15 m of pencil slate disturbed bed are seen for example in the grounds of Bodnant House, where the Afon Hiraethlyn has cut a gorge through the lower part of the Llanddoget Formation.

Farther south in Coed Ty-gwyn there are scarp exposures in disturbed bed, and in one area [SH 794 711] quarries in beds of fine to medium-grained sandstones in a melange of sandstone and cleaved silty mudstone.

East of Eglwysbach the uppermost beds of the formation are seen in Nant-y-Rhaglaw (see below).

South of Pentre'r-Felin extensive exposures in quarries, crags and Nant y Rhiw-felin show sections in pencil slate disturbed bed. Masses of sandstone occur locally at the base of the formation.

At Rhiwlas [SH 8067 6851], south-south-west of Croes-onen, regularly bedded striped silty mudstones of the Llanddoget Formation have yielded a graptolite fauna (RV 1917–1932) assigned to the C. rigidus Zone: Monoclimacis sp., Monograptus flemingii cf. flemingii, M. flemingii cf. primus, M. retroflexus, Pristiograptus sp.

In the Cefn-llech area there are numerous exposures of pencil slate with some sandstone beds up to 1 m thick. East of the Pentre-tafarny-fedw Fault sections to 9 m in pencil slate disturbed beds are seen in the stream south-west of Pennant.

A section [SH 8004 6555] south-south-west of Goelas-fawr shows about 12 m of disturbed beds, with sandstone bands up to 1.2 m thick in the partly undisturbed basal 6 m, and pencil slate in the uppermost 3 m. The section is interpreted as the lower part of the Llanddoget Formation resting upon the upper sandstone unit of the measures below the Berllan Formation. An old quarry [SH 8043 6562] west of Bryn-glâs reveals 3 m of highly fractured silty mudstone with lenses and beds of coarse sandstone up to 1.2 m thick, which contain quartz pebbles as much as 2.5 cm in diameter. In the valley to the south exposures show medium to coarse-grained sandstone with pebbles of a similar size [SH 8044 6530] and conglomeratic sandstone with larger (3.7 cm) pebbles [SH 8033 6523].

Fossils have been recorded from this formation only at Rhiwlas (see above).

Lower Nantglyn Flags Group

Measures below the Lower Mottled Mudstone

These measures are well exposed in the eastern tributaries of the Afon Hiraethlyn, notably Nant Maes Adda, Nant-y-Rhaglaw and those east of Pentre'r Felin. The last-mentioned provide more or less continuous sections through the greater part of the succession. Throughout the region these rocks consist largely of ribbon-banded mudstones. Feature mapping east of Eglwysbach has shown a remarkable consistency in the occurrence of hard bands over distances of more than 3 km, but the reason for their relative hardness is not entirely clear. In the stream [SH 8212 6661] to [SH 8193 6655] west-northwest of Pennantuchaf three waterfalls over hard bands reveal thin-bedded ribbon-banded silty mudstone, 1.5 m of massive dark grey silty mudstone and 1.8 m of thin-bedded ribbon-banded mudstone respectively. The first and last apparently differ little from the intervening 'soft' bands.

The basal beds of the Lower Nantglyn Flags Group differ somewhat from the succeeding measures. A road section [SH 8100 7283] at Bodnant Uchaf shows 3 m of striped silty mudstone only a metre or so above the Llanddoget Formation, and in Nant Maes Adda [SH 8086 7231] strata at the same horizon consist of massive dark grey silty mudstone with a few sandy stripes. Farther south in Nanty-Rhaglaw [SH 8084 7046] the basal contact can be seen, and the lowest beds of the Nantglyn Flags here consist of tough, dark grey silty mudstone with some laminated muddy siltstone ('graptolite shale') beds. Upstream [SH 8087 7046], and higher in the succession, the beds are blocky, silty mudstones with calcareous siltstone stripes, and these pass up gradually into ribbon-banded mudstones with the incoming of laminated muddy siltstone bands. The basal facies is here approximately 30 m thick.

A number of other sections show variations from the standard ribbon-banded mudstone succession. In the stream [SH 8157 6890] south-west of Fron-goch massive ribbon- banded mudstones forming a series of waterfalls and rapids contain sparse bands, 15 to 30 cm thick, of mottled mudstone. In the stream [SH 8141 6948] south-south-east of Llwyn-du a waterfall exposes 1.2 m of ribbon-banded mudstone overlying about 2.5 m of massive silty mudstone, which is possibly a disturbed bed.

Between Pennant and Ffrithisaf disturbed beds form mappable horizons. One such bed, seen in the stream banks north-east of Pennant [SH 8165 6731], [SH 8173 6729] is about 6 m thick and can be traced southwards to the Ffrithisaf area. In the Afon Hiraethylyn below Pennantuchaf a waterfall exposes over 15 m of disturbed beds, and a minimum of 12 m is probably present in the feature 300 m east of Ffrithisaf. An old quarry [SH 8177 6538] in this feature shows:

In the road section [SH 8210 6587] at Ffrith-arw, east of Ffrithlas, 1.8 m of thinly bedded, ribbon-banded silty mudstones can be seen overlying about 2.4 m of disturbed bed. The former has yielded a graptolite fauna (Zp 3307–3310, RV 1966–1980) of Monograptus flemingii flemingii, M. flemingii (narrow variety), and Pristiograptus pseudodubius during the present survey. It is the type locality for M. flemingii elegans (Boswell and Double, 1940, fig.22) and is assigned to the Cyrtograptus lundgreni Zone. The upper contact of the disturbed bed is obscured by vegetation at this locality but is well exposed in the stream [SH 8215 6601 to 8215 6613] north of Frith-arw:

Fossils, predominantly graptolites, are present in almost every section in these measures and the faunas are those ascribed to the C. perneri, C. ellesae and C. lundgreni zones. The more important localities in the C. perneri Zone are as follows: old quarry [SH 8094 7263] at Bodnod Bach Farm (RV 1080–1124), which yielded C. cf. perneri, M. flemingii flemingii, M. flemingii cf. primus and P. pseudodubius; a section [SH 8095 7224] in Nant Maes Adda (Zp 1254–1276) and exposures [SH 8110 6502] in Nant farmyard (DT 5881–5888, Zp 3450), from which comparable faunas were recorded plus P. aff. dubius and Monoclimacis flumendosae flumendosae respectively. A roadside exposure [SH 8046 7179] north of Brymbo yielded (Zp 563–566, Zp 8047) C. cf. perneri, M. flemingii flemingii (small variety), P. lodenicensis, P. cf. pseudodubius and Spinograptus?, a fauna ascribed to the C. ellesae Zone. A comparable assemblage (Zp 1572–1607, Zp 8063) but, also including Cyrtograptus sp.(of ellesae/rigidus group), Monoclimacis flumendosae?, Monograptus flemingii flemingii, M. flemingii cf. compactus and P. cf. dubius was found at the old quarry [SH 8089 6785] west of Fron-lepa and ascribed to the basal C. ellesae Zone. C. lundgreni Zone faunas have been recovered from the following localities: a bank [SH 8176 7311] at Erw-goch, north-east of Ty'n-y-Coed, where M. flemingii s.l., M. flemingii elegans, P. lodenicensis (including new varieties) and P. cf. pseudodubius (Zp 1207–1234) were found; a nearby stream section [SH 8188 7280] which yielded C. cf. hamatus, C. lundgreni, M. flemingii flemingii, M. sp.of priodon/flemingii group, P. cf. dubius and P. pseudodubius (ZP 1127–1153); a roadside exposure [SH 8079 7188] north-north-east of Brymbo (Zp 599–602), a roadside section [SH 8119 7018] east-south-east of Eglwysbach (Zp 1548–1557), and a nearby stream section [SH 8141 6948] (Zp 1559–1560); a bank [SH 8148 6822] north-west of Fron-lepa (Zp 1608–1629); and an old quarry [SH 8121 6726] west of Pennant, which yielded C. lundgreni, C. cf. ellesae, M. flemingii flemingii, M. flemingii var. (5 and P. cf. pseudodubius (Zp 3360–3375) ascribed to the base of the zone. In addition to M. flemingii cf. flemingii and P. pseudodubius, the old quarry [SH 8220 6596] east-north-east of Ffrith-las yielded Cardiola interrupta, Orthoceras sp. and Scyphocrinites? pulcher (RV 1981–1994, Zp 3299–3306).

Lower and Upper Mottled Mudstones and intervening measures

These strata form conspicuously featured and partly wooded ground with abundant small exposures of the Mottled Mudstones (Plate 3). More or less continuous sections through the complete succession are seen in the streams at Esgair-Ebrill, Tŷ-mawr, Bryngwian and Pennantcanol.

Both the Mottled Mudstones are composed of a number of separate lithological elements, including, in addition to mottled mudstone sensu stricto (pp. 46–47), calcareous siltstone, dark grey mudstone and silty mudstone and ribbon-banded mudstone. The measures between the Lower and Upper Mottled Mudstones are composed of ribbon-banded mudstones similar to those in the rest of the Lower Nantglyn Flags Group, though they locally contain a disturbed bed. A composite section showing the lithological variation in the complete succession, as exposed at Tŷ-mawr, was given by Holland and others (1969, fig.4).

A scarp section [SH 8095 7112] east of Brymbo shows 3.6 m of well cleaved mottled mudstone with stripes 15 to 30 cm apart. Solitary corals occur. In the old quarry [SH 8089 7066] south-east of Brymbo 9 m of mottled mudstone occur, the lower 6 m of which are entirely devoid of bedding planes and contain very few stripes. In the Cefn-gwyn area an old quarry [SH 8147 7026] west- northwest of the farm shows a 3.6-m section in the Lower Mottled Mudstone, and the base of the Upper formation is seen in the road section [SH 8159 7024] nearby.

A more or less continuous sequence in the stream [SH 8185 6889 to SH 8209 6895] between Esgair-Ebrill and Fron-goch shows the following succession (thicknesses in part estimated):

A similar succession but lacking the disturbed bed is seen at Tŷ-mawr and at Pennantcanol [SH 8237 6667], where the lower part of the Upper Mottled Mudstone (estimated at 45 m in total) comprises:

To the south, a section [SH 8237 6510] near the top of the Lower Mottled Mudstone north-west of Bryn-Madoc shows:

Gothograptus nassa has been recorded from this locality.

The Mottled Mudstones and intervening measures are sporadically fossiliferous throughout the region, with the ribbon-banded mudstones in particular yielding graptolites at most exposures. The Lower Mottled Mudstone has yielded graptolites at two localities the old quarry [SH 8161 7148] east-north-east of Brymbo Chapel (PW 256–257: ?Monograptus flemingii) and the bank [SH 8181 6830] south-east of Croes-onen (Zp 1445–1447: Pristiograptus sp.). The overlying ribbon-banded mudstones in a roadside exposure [SH 8159 7027] east-south-east of Eglwysbach yielded (Zp 1472–1479) Cyrtograptus?, Gothograptus nassa and P. pseudodubius, and the nearby quarry [SH 8160 7023] in the overlying Upper Mottled Mudstone (RU 3360–3388) yielded 'Clorinda'dormitzeri? Glassia sp., 'Ctenodonta' aff. securiformis, Stolidotus siluricus?, Bellerophon buccinatus?, Gyronema?, Hyolithes cf. fabaceus, 'Orthoceras' cf. undulocinctum, O. sp .A, phacopid and echinoderm fragments. Gothograptus nassa and Pristiograptus spp. (RV 2046–2056) were recorded from a section [SH 8102 7063] at Nant-Uchaf. At Cefn-madoc Quarry [SH 8257 6536], north-east of Caerfaban, tip material (DT 6134–6152, Zp 3530–3531) yielded Cardiola interrupta, Orthoceras sp., eurypterid(?) fragments, ostracods, G. nassa, Plectograptus? sp. Pristiograptus dubius and P. sp. nov?, and from an old quarry [SH 8233 6501] a little farther south have come (Zp 3573–3583) G. nassa and fragments of other retiolitids, P. aff. pseudodubius, P. jaegeri and P. aff. ludensis. The adjacent quarry [SH 8237 6504] yielded a shelly fauna including Glassia sp., Cardiola interrupta, Orthoceras cf. subannulare, 'O.' cf. ekmgatocinctum, Miraspis and phacopid fragments. Further shelly faunas from the mottled mudstones have been recorded from the old quarry [SH 8212 7267] north-east of Topanbach, where the Lower Mottled Mudstone yielded (RU 3245–3302) 'Clarinda' dormitzen, Glassia sp.[common], Mezounia sp.[common], Cardiola interrupta, 'Orthoceras' cf. 'elongatocincturre, Struveria howgillensis, and Turrilepas sp.: the quarry [SH 8232 6501] near the gate to Waen Oer yielded (RU 3303–3324) Glassia sp., Cardiola interrupta, 'Ctenodonta' aff. securiformis, Stolidotus siluricus, Dawsonoceras annulatum, 'Orthoceras' sp.A, phacopid and echinoderm fragments from the Lower Mottled Mudstone; and to the east an excavation [SH 8273 6503] in the Upper Mottled Mudstone at Cefn Madoc yielded (RU 3341–3359) a limited shelly fauna including Stolidotus siluricus. The graptolite faunas obtained are thus those characteristic of the uppermost C. lundgreni Zone, the G. nassa Zone and the basal M. ludensis Zone (see p. 54 and (Figure 13)).

Measures above the Upper Mottled Mudstone, including the Brynsylldy Formation These are well exposed and reach their maximum thickness of 205 m in the Fox Hall–Henrhyd area, and there are more or less continuous stream sections around Tŷ-mawr and Bryngwian, with extensive quarry and crag exposures farther south.

The succession, with the exception of the Brynsylldy Formation is entirely of ribbon-banded silty mudstones. These are rich in graptolites, everywhere characteristic of the M. ludensis Zone. From a stream section [SH 8229 6796] east of Fron-lepa Monograptus ludensis and Pristiograptus cf. jaegeri were obtained (Zp 1647–1650), and farther south a road section [SH 8305 6502] at Ffrithuchaf yielded M. ludensis, M. aff. ludensis, P. cf. jaegeri, and P. aff. jaegeri, together with Metaconularia cf. bilineata (RV 1995–2021). A roadside exposure [SH 8165 6993] at Pwll-y-clai yielded M. ludensis, P. dubius and P. jaegeri (Zp 1410–1429). A number of other notably graptolitic localities yielding the same fauna with the addition of P. aff. curtus occur: [SH 8228 6704] (Zp 3321–3338); [SH 8269 6638] (Zp 3507–3517); [SH 8144 7052] (Zp 1533–1542); and [SH 8166 7010] (Zp -1430–1433). The last is a roadside exposure at Cefn-gwyn from which Elles and Wood (1908, p. 344) recorded G. nassa.

There are numerous exposures of the Brynsylldy Formation around Henrhyd, the finest of which are those in Nant-y-Rhaglaw [SH 8127 7048] to [SH 8160 7045]. Waterfalls and gorge sections show about 140 m of disturbed beds, consisting of cleaved silty mudstone and mottled mudstone, resting on the basal 3.6 m of the Upper Mottled Mudstone, and overlain by ribbon-banded flags of the Upper Nantglyn Flags Group. At one place [SH 8154 7043] west of Penrhyn 1.5 m of sandy-striped and ribbon-banded silty mudstone are seen below the disturbed bed.

The Brynsylldy Formation is exposed at numerous localities around Cefn-gwyn and in the streams and fields north-east of Esgair-Ebrill. It is absent in the continuous stream section north- east of Tŷ-mawr, but reappears in the stream [SH 8245 6795] east of Bryngwian, where it is 4.5 m thick. From here it continues as a feature-forming bed (Plate 4) to the southern margin of the region, with a constant thickness of approximately 6 m.

A scarp section [SH 8245 6731] east of Pennant shows the basal 2 m of a conformable disturbed bed. There is a shallow channel cut into the underlying ribbon-banded mudstones, and more extensive channeling is seen in the vicinity of Ffrithuchaf. The old quarry [SH 8309 6492] north-north-east of Maes Madog shows over 2 m of disturbed bed largely occurring in channels (p. 39) in 3 m of thinly bedded siltstone. The fauna (Zp 3468–3473) comprises Dalmanites myops and M. ludensis. In contrast the base of the Brynsylldy Formation is seen to be strictly conformable in many sections east of Pennantuchaf and Gwern-bwys. One of these, a quarry [SH 8291 6623] east-south-east of Pennantuchaf, shows the following sequence:

The mudstones yielded (Zp 3493–3499) a monotypic assemblage of M. ludensis.

Upper Nantglyn Flags Group

The outcrop of these rocks is extensively drift-covered and significant sections are restricted to Nant-y-Rhaglaw, south of Gyffylog, and isolated quarries and small streams, notably in the Nant-ywrach–Foel-fawr area.

In the north the strata are silty mudstones with calcareous siltstone stripes and local ribbon-banding. A quarry [SH 8338 7213] at Croesengan Ucha for example, shows 3.6 m of thinly bedded (2.5 to 5 cm) silty mudstone with stripes of current-bedded, partly decalcified, calcareous siltstone. A few concretions are developed and the section shows spheroidal weathering.

The fauna obtained (RV 1291–1301) comprises: Aegiria grayi, rhynchonelloids, Skenidioides lewisii, Hemsiella sp., crinoid columnals, ostracods, ?Monograptus uncinatus orbatus and Pristiograptus cf. dubius. A further fauna has been recorded from the quarry [SH 8264 7209] south-west of Croesengan Ucha and comprises (RV 1247–1275, Zp 1324–1326) A. grayi, Dicoelosia sp., Howellella sp., Visbyella?, Whitfieldella sp.[common], orthocone, Hemsiella sp., crinoid columnals, Monograptus uncinatus orbatus and Pristiograptus cf. vicinus.

An old quarry [SH 8175 7049] east-south-east of Gyffylog exposes 7.5 m of silty mudstone with laminated muddy siltstone (graptolite shale) bands, 5 to 25 cm apart, and numerous discontinuous stripes of calcareous siltstone.

A small quarry [SH 8331 6690] west-south-west of Foel Cledeiriau shows 1.2 m of disturbed bed, the only one recorded in this region.

Quarries in the area between Wenlli, Nant-y-wrach and Foel- fawr (Plate 5) show flaggy ribbon-banded silty mudstone with sporadic concretions 5 to 15 cm across. The old quarry [SH 8381 6504] south-west of Wenlli yielded a fauna (Zp 3669–3671) of M. uncinatus orbatus, Neodiversograptus nilssoni and Spinograptus spinosus, and the first two species are also recorded (DT 6217–6223) from a bank [SH 8404 6467] north-east of Farm Yard. Fossils, predominantly graptolites, occur in most sections and further characteristic lower N. nilssoni Zone faunas were obtained from the following localities: the quarry [SH 8432 6568] at Wenlli (DT 6161–6184, DT 6202–6216): Monograptus colonus colonus, M. colonus compactus, ?M. uncinatus orbatus, Saetograptus cf. varians varians; the old quarry [SH 8302 6645] west-south-west of Foel Cledeiriau (DT 6187–6198): M. uncinatus orbatus, N. nilssoni, Pristiograptus sp. nov?; the old quarry [SH 8298 6729] west of Foel Cledeiriau (Zp 3682–3707); M. colonus colonus, M. colonus compactus, M. aff. colonus, M. cf. roemeri, S. varians pumilus and P. cf. curtus; and the stream section [SH 8178 7046 ] in Nant-y-Rhaglaw (Zp 1509–1521): M. uncinatus orbatus, S. cf. varians, P. cf. comis, Pristiograptus sp.

Elwy Group

Drift obscures much of the outcrop of these rocks and extensive exposures occur only on Moel Gyffylog and in the Hafotty Bennett–Fferm area. In the north much of the succession seen consists of striped silty mudstones, locally with thin disturbed beds and sandstones. An old gravel pit [SH 8387 7255] north of Chweffordd shows 10.6 m of thinly bedded striped silty mudstone with sporadic siltstone bands up to 7.5 cm thick.

A scarp [SH 8341 7158] west of Pen-y-fron exposes the lowest disturbed beds and sandstones of the local succession in the group; the section being:

Comparable sequences with upper contacts of disturbed beds are seen in the vicinity of Rhandir [SH 8273 7145]; [SH 8261 7097], and an alternating sequence of sandstones and silty mudstones in the old quarry [SH 8311 7114] south of Rhandir includes sandstone with linear load-casts and ?flute-casts all aligned east–west.

The sequence on Moel Gyffylog is interpreted as lying close to the base of the group. The proportion of disturbed beds here is higher, with individual beds up to 4.5 m thick. A well exposed undulatory upper contact of a disturbed bed is seen in Nant-y-Rhaglaw [SH 8217 7056] due east of Gyffylog, where the stream bank shows:

Higher in the succession sections up to 15 m thick in disturbed beds are seen in the gorge [SH 8291 6993] north-north-west of Penmynydd, and extensive exposures, almost entirely in disturbed beds, occur in Coed Pant-glas.

In contrast to the Colwyn Bay region to the north, mappable sandstones are absent, with the exception of a thin bed on Mynydd Esgair-Ebrill. This represents the southernmost extension of sandstone development in the lower Elwy Group.

In the Mynydd Esgair-Ebrill area disturbed beds make up much the greater proportion of the succession, and there are numerous exposures in small crags and quarries around Hafotty Bennett and Fferm. One quarry [SH 8350 6904] south of Hafotty Bennett shows:

A second quarry [SH 8366 6905] south-east of Hafotty Bennett shows about 6 m of near-vertical striped silty mudstone with fine-grained laminated sandstones up to 17 cm and one 30-cm disturbed bed consisting of silty mudstone with collophane pebbles and sandstone rolls. The whole sequence may be inverted and is interpreted as a large raft within the local disturbed bed succession.

Fossils, mainly graptolites, occur sparsely. A scarp exposure [SH 8275 7146] west of Rhandir yielded (RV 1276–1290, Zp 1327–1330): Saetograptus varians cf. pumilus, S. varians varians, Pristiograptus sp.; and the quarry south of Hafotty Bennett (see above) yielded (Zp 1725–1784): crinoid columnals, Monoclimacis? aff. haupti, Monograptus cf. colonus, M. colonus compactus, S. cf. varians pumilus, S. varians varians, P. bohemicus, P. cf. comis, P. vicinus. A further quarry [SH 8343 6859] in the area, south of Hafotty Bennett, yielded a Shelly fauna (Zp 1709–1724) including various mollusca: Dalejina hybrida, Glassia sp., Salopina sp., Nuculites anti quus, Myalina?, Cymbularia? and Plectonotus trilobatus. These and other faunas are all interpreted as indicating the Upper N. nilssoni Zone (p. 54). PTW

IV Llanrwst region

Complete successions of the Denbigh Grits and Lower and Upper Nantglyn Flags groups are present in the region, together with the lower part of the Elwy Group (Upper N. nilssoni Zone).

Denbigh Grits Group

Measures below the Llanddoget Formation

These measures crop out between Maenan and the Nebo–Oerfa area, and, except around Llanrwst, are well exposed.

The basal beds consist of thinly interbedded dark grey silty mudstones, pale grey-green, partly mottled, mudstones identical to those in the Llandovery (Pale Slates) below, calcareous siltstones, and ribbon-banded mudstones. They are exposed in Coed Soflen, north-west of Nebo, and a forestry track cutting [SH 8197 5772] south-west of Tyddyn Llywarch shows about 9 m of predominantly dark grey silty mudstone with paler grey-green bands and one band of ribbon-banded silty mudstone. The graptolite fauna (PW 206–255) from the last is: ?Barrandeograptus pulchellus, Monoclimacis shottoni, M. vomerina basilica, M. eomerina cf. vomerina, Monograptus cf. danbyi, M. cf. firmus firmus, ?M. minimus cautleyensis, M. priodon, M. simulatus, Pristiograptus? and Retiolites geinitzianus geinitzianus, and is indicative of the Cyrtograptus centrifuges Zone.

In the Tyddyn Llywarch–Hafotty Fawr area these basal beds, about 60 m thick, are succeeded by striped silty mudstones with sandstone units, and north of Tyddyn Llywarch itself they include a unit of disturbed beds. Numerous exposures occur. A road section [SH 8220 5771] south-south-west of Tyddyn Llywarch shows 21 m of dark grey silty mudstones, in part rusty and/or spheroidally weathered, with a few thin, pale grey-weathering laminated muddy siltstones (graptolite shales). A crag [SH 8207 5817] north-west of Tyddyn Llywarch, shows 3 m of disturbed strata consisting of highly and irregularly cleaved, unbedded, fine-grained sandstone and silty mudstone, which passes northwards into undisturbed sandstone and mudstone. At Ty'n-twll [SH 8201 5842] south-east of Penrallt, at least 21 m of interbedded fine and coarse-grained sandstone are seen.

Traced north-westwards, the succession thins, with a rapid decrease in the thickness and proportion of sandstones. The lowest sandstone unit is well seen in the old quarry [SH 8151 5873] between Penrallt and Hendre House, which shows:

Immediately to the north the sandstones die out, and in the area between Hendre House and Bryn Derwen numerous exposures show only striped silty mudstones with thin sandstone bands. Only one sandstone, with associated disturbed beds in the neighbourhood of Bryn Derwen, is known to be present in the Melin-y-coed area. It is well seen in the old quarry [SH 8107 5983] north-north-east of Bryn Derwen, which shows 12 m of alternating silty mudstone, laminated siltstone and fine to coarse-grained sandstone in beds up to 2 m thick. Each bed locally shows rapid lateral passage to sandstone/ mudstone melange, the top 4.5 m being predominantly melange of coarse sandstone in a mudstone matrix. The middle 3 m are mainly well bedded, coarse and fine-grained, graded sandstones with interbedded laminated siltstones and dark grey silty mudstones, showing poor longitudinal ridge and flute-casts. The basal 4.5 m are dominantly coarse-grained sandstones and melange.

A number of exposures of striped silty mudstone, sandstone and disturbed beds occur in Llanrwst. One, close to The Square [SH 7988 6162], due east of St Grwst's Church, shows:

Another small exposure [SH 7959 6209] in Station Road, Llanrwst, shows crudely cleaved silty mudstone with a few small slump-folded sandstone lenses and fragments, indicating disturbed strata (see p. 38).

Between Llanrwst and Maenan the succession seen comprises striped silty mudstones with two groups of sandstone and disturbed beds (Figure 17) and (Figure 18). Disturbed beds forming the upper part of the lower group are seen in road and railway cuttings at Plas Madoc Lodge [SH 7932 6292], south-west of Plas Madoc, where they consist of sandy siltstone enclosing isolated sandstone and siltstone masses. These beds die out north of Tan-y-fron. At Tan-lan a quarry [SH 7915 6352] in the underlying sandstone element shows:

Disturbed beds reappear in the upper part of this group at Maenan.

The upper group is composed mostly of disturbed beds, although sandstones occur west of Belmont and south-west of Plas Madoc. Sections to the south of Plas Madoc show disturbed sandstones, siltstones and mudstones passing laterally into massive sandstones with thin siltstone bands or disturbed sandy siltstones containing rafts of sandstone.

East of the Maes-swyn Fault, in the Nebo–Uerfa area, the succession shows a much greater proportion of sandstone—in regularly bedded groups without disturbed beds—than does that in the Tyddyn Llywarch area described above. In the absence, due to faulting, of the base of the succession, correlation of the sequence is uncertain. However, ribbon-banded silty mudstones in the old quarry [SH 8264 5856] east of Garth-y-pigau may, on both lithological and faunal grounds, correlate with similar rocks seen east of the Maes-gwyn Fault in the stream banks [SH 8312 5833], [SH 8436 5838] respectively west-north-west of Hwylfa-ddu and north-east of Clytiau-têg. The fauna from the first locality (RV 3149–3159, DT 7889–7904) is ?'Orthoceras' sp., Cyrtograptus sp.(of C. rigidus group), Monoclimacis flumendosae, Monograptus flemingii and Pristiograptus?, and that from the latter (DT 7859–7877, DT 8009–8011): C. cf. rigidus, cf. Mcl. flumendosae flumendosae and M. flemingii cf. primus. Both faunas are ascribed to the C. rigidus Zone.

Sandstones in the Nebo–Oerfa area are predominantly fine to medium-grained and thickly bedded. They are well exposed in numerous scarps north-west of Hafodlas and around Creigiau'r-lwrch and in the quarry [SH 8361 5641] at Graig Goch, which shows 8 m of thickly bedded sandstones with thin (2.5 to 30 cm) siltstone bands. Longitudinal ridge-casts and groove-casts are present on the bases of some sandstones. Directional current-structures are also recorded from a nearby stream section [SH 8346 5655] to [SH 8358 5652] (asymmetrical ripple-mark and load-casts); a scarp [SH 8370 5688] east of Creigiau'r-lwrch (asymmetrical ripple-mark), a scarp [SH 8165 5841] east of Oaklands (longitudinal ridge-casts); and a scarp [SH 8222 5928] south-west of Ffron-wen (flute-casts, prod-casts and groove-casts).

Except for ribbon-banded mudstones in the upper part of these measures, referred to above, which yield a C. rigidus Zone graptolite fauna, the measures below the Llandogget Formation are largely barren. However, a stream exposure [SH 8328 5693] due west of Creigiau'r-lwrch yielded Monograptus cf. riccartonensis and may therefore lie in the M. riccartonensis Zone. The Coed Soflen locality (see above) yielded a C. centrifugus Zone fauna. Shell and crinoid fragments are seen in some sandstones, and shelly faunas have been recorded from the Plas Madoc area west of Llanddoget. A shelly collection (Z19311–9362) mentioned by Salter (in Ramsay, 1866, pp. 277–278) as from Plas Madoc [c. SH 797 633] comprises favositid, bryozoa, Eoplectodonta?, Leptaena depressa, Meristina?, Microsphaeridiorhynchus nucula, Rhynchotreta cuneata, Stegerhynchus borealis, S. decemplicatus, S. diodonta, 'Ctenodonta' sp., Cypricardinia sp., Modiolopsis sp., Nuculites cf. antiquus, N. cf. coarctatus, Cymbularia?, Gyronema?, Hormotoma?, Loxonema sp., Platyceras haliotis, Plectonotus trilobatus, orthocone, Acaste sp., Calymene sp., Dalmanites sp., Encrinurus sp., proetid fragments and crinoid columnals. This is the best shelly collection from the Denbigh Grits Group of the whole district. A second collection (Z1 9363–9372) from '1 mile [400 m] west of Plas Madoc' [SH 794 632] is similar but much less rich; it includes, however, a solitary coral, Atrypa reticularis and cf. 'Phragmoceras' nautileum.

Llanddoget Formation

This formation crops out in a continuous strip from the northern margin of the region at Bryn Morfydd, through Llanddoget and Melin-y-coed to the Oerfa area. Exposures are sporadic, however, due to drift cover. It is composed almost entirely of pencil slate disturbed beds (p. 38), although at a number of places, and notably around Parc, Fron-wen and Oerfa, the lower part contains beds of coarse sandstone and conglomerate, and in the last two localities there is interdigitation of disturbed material with the underlying sandstones and striped beds.

In Parc farmyard a partly disturbed sandstone with pebbles of quartz and mudstones is underlain by disturbed and highly fractured siltstone with sandstone and mudstone clasts. Some 180 m to the south massive, in part coarse, quartz-veined sandstone with small pebbles is exposed. Farther south an old quarry [SH 8001 6408] east-north-east of Belmont Farm shows:

Around the type locality, Llanddoget, numerous small sections in pencil slate disturbed bed occur, and a 7.6-m section in highly fractured and contorted, fine blue siltstone with small (< 5 cm) concretions is seen east of Llanrwst in a quarry [SH 8031 6223] south-south-east of Gorffwysfa.

The best sections in the formation, however, are seen in Nant y Goron, south-east of Melin-y-coed. Here, the gorge shows sections in up to 60 m of pencil slate disturbed beds, with no evidence of sandstones. In contrast, south-west of Fron-wen interbedded sandstones and disturbed beds occur. A scarp [SH 8206 5940] west-south-west of Fron-wen shows 6 m of highly fractured silty mudstone with pebbles up to 3.8 cm in diameter of vein-quartz, quartzite, sandstone and siltstone; sand grains occur sporadically throughout.

To the south and east, e.g. in the scarp [SH 8239 5909] due south of Fron-wen, underlying sandstones are coarse-grained and contain disturbed beds composed of silty mudstone with sand grains and sandstone with pebbles to 2 cm. In this transitional and, on the evidence of mapping, laterally varying sequence the base of the Llanddoget Formation is arbitrarily defined.

A similar situation pertains at Oerfa where the old quarry [SH 8372 5881] west-south-west of Bryn-y-gwynt shows:

The lowest bed contains thin bands of medium-grained sandstone and lenses of disturbed and contorted mudstones with intraformational breccia-conglomerates and slumped sheets, best developed at the south-eastern end of the quarry. Fragments of contorted mudstone occur throughout the 'tapioca rock' and sandstone. Contacts between the sandstones and 'tapioca rock' are sharp, and there are occasional flame-structures developed as a result of sandstone being squeezed into the overlying 'tapioca rock'. North and west of the quarry numerous exposures in pencil slate disturbed beds occur.

Fossils have not been recorded from this formation in the Llanrwst region.

Lower Nantglyn Flags Group

Measures below the Lower Mottled Mudstone

These measures are not extensively exposed except in stream sections and gorges. They consist of ribbon-banded silty mudstones, varying from thin-bedded or flaggy to massive, with concretions and local thin disturbed beds.

East of Llanddoget, a 60-m disturbed bed occurs at the top of these measures. As seen at Bryn-saith-eglwys quarry [SH 8069 6323], south-south-east of Llanddoget, it consists essentially of bedded disturbed silty mudstone, but of mottled mudstone with disoriented concretions in part. Some 410 m to the south the bed terminates. East of the Pentre-tafarn-y-fedw Fault, the correlative of this disturbed bed is ribbon-banded mudstones, with two disturbed beds, seen in the stream below Nant-y-glyn-uchaf [SH 8145 6348], south of Pantllin-bach.

Farther north in the vicinity of Caerfaban a disturbed bed at a much lower horizon is a continuation of that seen between Pennant and Ffrithisaf (p. 69). It is well seen in the quarry [SH 8169 6467] at Caerfaban, which shows 3 m of bedded disturbed bed, within unbedded disturbed bed, overlying 2.5 m of ribbon-banded siltstones and silty mudstones with a slightly irregular contact. The latter yielded a graptolite fauna (DT 5995–6054, p 3387–3425, Zp 6483–6487) of: Cyrtograptus?, Monograptus flemingii flemingii, Pristiograptus lodenicensis, P. sp. nov. aff. curtus and P. aff. ludensis, indicative of the C. lundgreni Zone. A shelly fauna (DT 6060–6069) from a nearby exposure [SH 8174 6466] in the disturbed strata comprises: 'Clorinda' dormitzeri, Visbyella cf. trewerna, Lunulacardium cf. acuminatum, L. cf. excellens, 'Orthoceras' spp. , Hemsiella sp., Lepidocoleus sp. andcrinoid fragments. The outcrop does not extend south of Pantllin-mawr.

A disturbed bed, probably about 12 m thick, occurs at Bwlch-ygwynt and is exposed in the farmyard.

All the undisturbed measures noted are richly fossiliferous, with graptolites predominating, but also with orthocone nautiloids and thin-shelled bivalves. The graptolite faunas are those ascribed to the C. perneri, C. ellesae and C. lundgreni zones.

The more important localities in the C. perneri Zone include a stream-bank [SH 8163 6443] east-north-east of Pen-y-garth (Zp 4204–4217), which yielded Cyrtograptus sp.(close to C. perneri), Monoclimacis flumendosae cf. flumendosae, Monograptus flemingii cf. flumendosae, Monograptus flemingii cf. flemingii, M. flemingii primus and Pristiograptus sp.; Cyrtograptus perneri itself was found in a nearby roadside exposure [SH 8121 6377] (Zp 4163–4203). Comparable faunas (DT 5913–5923) with, in addition, P. pseudodubius were found in the stream section [SH 8115 6475] south of Nant Farm, and the stream bank [SH 8189 6055] east of Capel Bethel, Melin-y-coed (Zp 5602–5610). A roadside section [SH 8090 6431] north-west of Pen-y-garth yielded a probable C. ellesae Zone fauna (Zp 4218–4225): Cyrtograptus sp.(of rigidus/ellesae group) and Pristiograptus? aff.pseudodubius. C. lundgreni Zone faunas were found at many localities including the track exposure [SH 8126 6255] north-west of Cae'r-ceiliog (Zp 3882–3887) and the stream banks [SH 8136 6220; 8106 6158] south-west of Cae'r-ceiliog (Zp 3903–3932; Zp 3939–3943 and Zp 8085–8087). The more northerly of the last two localities yielded a fauna of C. hamatus, C. lundgreni, M. flemingii flemingii, M. flemingii cf. flemingii var δ and P. pseudodubius. Two further important localities are an old quarry [SH 8207 6081] at Bwlch-y-gwynt (Zp 5649–5652, Zp 6488–6498) and an old [SH 8375 5974] east-south-east of Bryniog Uchaf. The latter yielded (Zp 5701–5713, PW 193–194) M. flemingii cf. flemingii, M. flemingii cf. elegans, P. cf. pseudodubius and Pristiograptus aff. jaegeri.

Lower and Upper Mottled Mudstones and measures between

A complete succession crops out between the Caerfaban area in the north and the Oerfa area in the east, with notable exposures around Plasuchaf, Cae'r-ceiliog and Brynsylldy, east of Llanrwst, and the headwaters of Nant Bwlch-y-gwynt. In general the measures are well exposed although drift cover is extensive locally.

The Lower Mottled Mudstone and a few metres or so of the overlying ribbon-banded mudstones also crop out west of the Pentre-tafarn-y-fedw Fault, from Pen-y-garth to just south of Garth-y-foel. The former locality is of interest in showing a disturbed bed within the Lower Mottled Mudstone.

As in the Eglwysbach region, the Mottled Mudstones consist of a number of feature-forming elements, in this region three or four in the Lower formation and three in the Upper. Completely exposed sequences and notably those near Brynsylldy, confirm that three hard bands are composed of mottled mudstone. The relatively soft horizons consist of mottled mudstone with thin calcareous siltstones, dark mudstones and ribbon-banded mudstones, the last identical to those below and above the Lower and Upper Mottled Mudstones. The following are more notable sections in these strata.

At the roadside [SH 8095 6247] due west of Pentre-tafarn-y-fedw Chapel a 2-m section in thin irregularly bedded mottled mudstones of the Lower Mottled Mudstone shows a thin (< 7 cm) lenticular band of disturbed silty mudstone with small silty mudstone and collophane pebbles. This locality is on the same horizon as a possible thin disturbed bed at Garth-y-foel, 457 m to the south. The Upper Mottled Mudstone is well exposed in the roadside [SH 8188 6323] at Plasuchaf, which shows about 6 m of horizontally fracture-cleaved mottled mudstone, weathering light brown with dark lenticular mudstone inclusions (< 2–5 cm) probably resulting from bioturbation, goethite veinlets and a shelly fauna (DT 7724–7731, RU 3146–3231). This, the richest shelly assemblage collected from mottled mudstone during the present survey, comprises: Bracteoleptaena bracteola, chonetid, Glassia sp., Howellella cf. elegans, Protathyris sp., Visbyella cf. trewerna, Cardiola interrupta, 'Ctenodonta' sp., Maminka? cf. suecica, Stolidotus siluricus, Bellerophon cf. buccinatus, Hormotoma?, Platyceras?, Kionoceras?, 'Orthoceras' cf. dimidiatum, 'O.' cf. mocktreense, 'O.' cf. primaevum, 'O.' sp. A., Hyolithes cf. forbesii, Acaste?, Ananaspis aff. communis, Dalmanites myops, Miraspis mira, Hemsiella sp., Sleia cf. kochii, Aptychopsis sp., crinoid columnals and echinoderm plates. A single thin bed of ribbon-banded mudstone also occurs.

The most completely exposed section in the present measures lies between Coed-y-brain, east-north-east of Poethfoel, and Brynsylldy in the road and stream banks. In the road cut [SH 8167 6176] northeast of Poethfoel, about 6 m of massive, irregularly fractured, pale brown-weathering mottled mudstone with goethite veinlets form the basal member of the Lower Mottled Mudstone (Plate 8). A shelly fauna (RU 3389–3409), including a solitary coral, 'Clorinda' sp., Eoplectodonta sp., Dawsonoceras annulatum, Kionoceras sp., 'Orthoceras' cf. mocktreense and Corptocaris [ = opercula of Orthotheca], has been recorded.

A scarp [SH 8204 6182 ] at Brynsylldy in the Upper Mottled Mudstone shows interbedded ribbon-banded and mottled mudstone and has provided the most extensive fauna, including both shells and graptolites, from the Upper Mottled Mudstone of the entire district. The late Sir Arthur Dixon discovered this locality and presented material from it to the Sedgwick Museum, Cambridge, to Kingswood School, Bath and to the Institute of Geological Sciences. The shelly fossils lie scattered on a few bedding planes: some species are very abundant but most are represented by very few specimens. The fauna (RU 3090–3145; ZI 1817–1827, ZI 8549–8560, Zs 433–435) comprises: bryozoa, Aegiria grayi [common], 'Camarotoechia' sp., Cyrtia sp., Dicoelosia biloba, Glassia sp., Howellella elegans, Leptaena aff. amelia, Strophochonetes sp.[common], Visbyella cf. trewerna, Cardiola interrupta, 'Ctenodonta' sp., Ptychopteria sp., Dawsonoceras annulatum, 'Orthoceras' cf. subannulare?, Parakionoceras?, Calymene cf. blumenbachii, Dalmanites sp., Leonaspis crenata cf. brutoni, Warburgella cf. stokesii, Hemsiella sp.[common], Anatifopsis sp., Echinocystites pomum, Furcaster?, Lapworthura miltoni, ophiuroid, Palaeodiscus ferox?, Dendrocrinites sp., Dimerocrinites sp., Placocystites [plates], Plumulites sp., dendroid graptolites, Monograptus auctus (? = M. ludensis), M ludensis, P. dubius, P. jaegeri, problematicum 1 [large orbicular impressions] and problematicum 2 [chevron-shaped plates].

Sychbant, north-east of Bryniog Uchaf, are numerous, and although small they indicate an almost complete succession in the present measures. Isolated exposures north and east of Oerfa show mottled mudstone, for example in the road bank [SH 8426 5932] north-east of Bryn-y-gwynt.

Rich graptolite and shelly faunas occur at a large number of localities and can be ascribed to the uppermost part of the C. lundgreni Zone, the G. nassa Zone and the M. ludensis Zone. A shelly fauna has been collected from the Lower Mottled Mudstone at two localities. The quarry [SH 8092 6397] north-east of Llanddoget Church yielded (DT 5924–5954) bryozoa, 'Clorinda' dormitzeri:, Slava fibrosa?, Dawsonoceras annulatum, 'Orthoceras' aff. coralliforme, 'O'. aff. recticinctum, Dalmanites caudatus [common], Hemsiella sp., and crinoid columnals. The stream section [SH 8186 6408] east-north-east of Pantllin-mawr yielded small shells and several echinoderms (including the first record of a solutan carpoid from Silurian rocks) from flags within the Lower Mottled Mudstone. The fauna (DT 6112–6115, RU 2997–3089) comprises: Bracteoleptaena bracteola? [common], Glassia sp.[common], Hyattidina sp., lingulid, Mezounia sp., Protathyris?, Strophochonetes sp., Platyceras?, Polygrammoceras?, Orthotheca sp., Odontopleura sp., Raphiophorus parvulus, Amphitoxoti s sp., Hemsiella sp., Furcasta leptosoma, Lapworthura miltoni, ophiuroid, solutan carpoid (nov. ?) and Gothograptus nassa.

Comparable faunas (Zp 4406–4415) were found in the track section [SH 8259 6123] south of Llwyn-Goronwy and the old quarry [SH 8229 6098] north-west of Bryniog Uchaf (PW 157–171); and a bank [SH 8280 6080] in the latter area also yielded (Zp 5720–5735; PW 172–192) cf. Scyphocrinites? pulcher,, Plectograptus? and Pristiograptus aff. pseudodubius. Zp 5720 from this locality is one of the paratypes of P. jaegeri. Further mixed graptolite and shelly faunas were recorded from a quarry [SH 8204 6165] south of Brynsylldy, in the flags between the two Mottled Mudstone formations, which yielded (Zp 4012–4020) Cardiola interrupta, Clorinda?, Praectenodonta ludensis, Entomozoe?, Hemsiella sp., Sleia cf. kochii and Monograptus ludensis; and a bank [SH 8206 6135] nearby yielded (Zp 4142–4146) Howellella elegans, Mezounia sp., the ostracods noted above and P. cf. jaegeri. From the Upper Mottled Mudstone in the quarry [SH 8236 6447] east of Caerfaban came Cheirurus sp. and Dalmanites cf. myops (DT 5955–5963).

A stream section [SH 8177 6176] west-south-west of Brynsylldy yielded a G. nassa Zone fauna (Zp 4090–4127): cf. G. intermedius, G. nassa, Pristiograptus aff. jaegeri and P. pseudodubius; and a further stream section [SH 8195 6172] south-south-west of Brynsylldy yielded a M. ludensis Zone fauna (Zp 4041–4070) comprising M. ludensis, P. dubius, P. jaegeri and P. aff. jaegeri.

Measures above the Upper Mottled Mudstone, including the Brynsylldy Formation

The sequence below the Brynsylldy Formation everywhere consists of ribbon-banded mudstones. They are seen in road and stream sections east of Plasuchaf, where they have yielded graptolites characteristic of the M. ludensis Zone at a number of places, the most notable being the stream bank [SH 8216 6305] east-south-east of the farm, which has yielded (Zp 3852–3879) M. ludensis, Pristiograptus sp. nov? aff. curtus and P. cf. dubius, and the nearby bank [SH 8221 6287], which has yielded (Zp 3783–3801) the first two forms. Owing to the downcutting of the base of the Brynsylldy Formation (see below) the ribbon-banded mudstones vary in thickness and are absent between Brynsylldy and the Afon Sychbant.

The Brynsylldy Formation, which is well exposed between the type locality and the Afon Sychbant, varies in thickness from an estimated 15 m around Madoc to over 100 m in the area between Llwyn-Goronwy and Pennant Farm. At Brynsylldy, where it is well exposed in the yard and in the stream and road sections to the south, it is approximately 36 m thick. It here rests on the Upper Mottled Mudstone, the contact being seen in a road bank [SH 8210 6165] south of the farm.

In the Pennant Farm area the base of the formation lies at a number of different horizons. In the farmyard it rests on ribbon-banded mudstones below the Upper Mottled Mudstone, whereas to the north it rests on rocks within that formation. On Cefn-y-Fran exposures suggest a cross-cutting relationship from an horizon near the top of the Upper Mottled Mudstone to one in the measures below it. Overall the relationships in this area suggest a north-easterly trending channel.

The Brynsylldy Formation in this region has generally proved unfossiliferous, but a M. ludensis Zone graptolite fauna comprising M. ludensis and P. dubius has been recorded from a track section [SH 8230 6147] south-west of Llwyn-Goronwy.

Upper Nantglyn Flags Group

The outcrop of this group is extensive but very largely drift-covered. Exposures are seen in quarries and streams around Farm Yard, and Swchyrhafod, and almost continuous sections through the succession are visible in the streams at Henblas, north of Llwyn Richard and at Llwyn-Goronwy, where additional sections occur along the road to the north.

The measures vary in thickness from 90 to about 120 m and consist of ribbon-banded mudstones with calcareous concretions. They are richly fossiliferous and have yielded graptolites indicative of the Lower N. nilssoni Zone at a large number of localities, the most notable of which are the stream section [SH 8375 6356] west-southwest of Swchyrhafod Farm, where the fauna recorded (DT 6255–6268) was Monograptus colonus colonus, Saetograptus varians varians and Pristiograptus bohemicus; and the stream [SH 8401 6311] south of the same farm, which yielded (DT 6324–6337) M. uncinatus orbatus, S. varians varians and cf. Neodiversograptus nilssoni. A bedded disturbed bed is seen in the stream [SH 8396 6319] between these localities. Other notable graptolite localities occur in the Henblas area. The stream exposure [SH 8273 6153] south of the farm yielded (Zp 4331–4361) M. uncinatus orbatus, N. nilssoni, cf. Spinograptus spinosus, a fauna which is characteristic of the upper part of the Lower N. nilssoni Zone. From the quarry [SH 8260 6185] south of the farm a fauna (Zp 4248–4260) of M. colonus cf. colonus, M. ludensis, N. nilssoni and Pristiograptus sp.was collected. This last locality is one of those showing tectonic ripples (p. 144).

Elwy Group

Measures belonging to this group are well exposed between Rhos-yffarm and Pant-glas, particularly in the vicinity of Nant-y-fedwen. They consist largely of disturbed beds, with numerous small exposures visible in the areas north of Pant-glas and south-east of Llwyn Richard. A roadside quarry [SH 8347 6151] south-west of Nant-yfedwen and another [SH 8424 6205] at Maelogen-bach show excellent examples of slump folds with horizontal and vertical fold axial planes respectively.

South of Nant-y-fedwen the disturbed beds are large lenticular bodies. The irregular and peculiar topography in that area is believed to be due in part to this situation and in part to crag-and-tail formation by glacial action (see p. 151). The striped mudstones here and elsewhere in the region have yielded graptolite faunas characteristic of the Upper N. nilssoni Zone. Two of the more important localities are the stream section [SH 8369 6156] south of Nant-y-fedwen, from which Cucullograptus scanicus, Saetograptus chimaera salweyi and S. chimaera cf. salweyi (broad variant) have been obtained (Zp 4503–4516), and the old quarry [SH 8499 6174] east of the farm, which yielded (Zp 4527–4559) M. colonus cf. colonus, M. cf. roemeri, P. comis, P. cf. frequens, P. cf. vicinus, S. chimaera cf. chimaera, S. chimaera salweyi and S. varians cf. pumilus.

A single sandstone, about 10 m thick, is recorded in an old quarry [SH 8415 6198], now filled, due east of Nant-y-fedwen. MJCN, PTW

V Bettws-Yn-Rhos region

This region includes the high ground of Mynydd Branar, Mynydd Glyn-lws, Moelfre Uchaf and Mynydd-dir, which form the watershed between the rivers Elwy and Dulas. Exposures on this ground are plentiful though confined to crags, quarries, road-cuts and short sections of stream. In the northern half of the region, however, drift cover is almost complete and exposures are largely restricted to an area between Sirior Goch and Nant Fawr, east of Betws-yn-Rhos, and to the vicinity of the Abergele Sanatorium.

Fossils, shelly and graptolitic, have been recorded from numerous localities and indicate the presence of the Cucullograptus scanicus and Saetograptus incipiens zones and possibly the basal part of the S. leintwardinensis Zone. They also facilitate division of the Elwy Group in this region, as in some others — e.g. the Llangerniew region (p. 79), into separate formations; in this instance, the Gribin Mudstones, Cynant Beds' and Bont-uchel Formation. PTW

Gribin Mudstones

The Gribin Mudstones of Jones (1937) crop out along the southern margin of the region between Mynydd Branar and the Garthewin area. They consist almost entirely of striped silty mudstones, as at the type locality (p. 81), and are well exposed, particularly in the stream north-east of Rhosmarch-ganol.

Near Dafarn-bara-ceirch two mappable disturbed beds occur, and in the quarry [SH 8926 7120] at the farm a disturbed bed up to 0.6 m thick is seen which thins out rapidly towards the east. Graptolites from this and several other localities are characteristic of the basal C. scanicus Zone.

The more notable collections came from temporary exposures for the foundations of two pylons [SH 8712 7107]; [SH 8830 7105] respectively east of Glan-y-gors and north of Pen-y-fyddin. The former yielded (RV 1746–1771) Cucullograptus scanicus and S. varians, the latter (RK 1806–1830) these forms and Monoclimacis micropoma and Pristiograptus cf. comis. The bank [SH 8913 7105] south-west of Dafarn-bara-ceirch has yielded (Zp 1993–2002) C. scanicus, Neodiversograptus nilssoni and Pristiograptus dubius? and the quarry [SH 8963 7128] east of the farm (Zp 1981–1992) ?C. scanicus, S. chimaera cf. chimaera and S. chimaera cf. salweyi. The old quarry at the farm itself yielded (Zp 1963–1980) M. colonus colonus, P. cf. vicinus, S. chimaera cf. salweyi and S. varians.

North of Garthewin the formation includes a sandstone bed, 1.5 m of which are seen in two crags [SH 9095 7124]: [SH 9180 7088]. HEW

'Cynant Beds'

The 'Cynant Beds' are a sequence of sandstones and disturbed beds interbedded with and showing rapid lateral passage into striped silty mudstones. The formation yields a graptolite fauna indicative of the upper C. scanicus Zone. The type area, around the Cynant farms, includes numerous exposures in crags and old quarries and forms the locally highest ground. The highly irregular topography of the Cynant area is interpreted as being due to lateral facies changes and a complex faulting pattern.

A temporary exposure [SH 9175 7203] east-north-east of Cynant Uchaf showed a disturbed bed high in the local sequence and yielded a rich, although fragmentary, shelly fauna (RK 1908–1952): Aegiria grayi, Atrypa reticularis, Dalejina sp., Dayia navicula, Howellella elegans [common], Hyattidina?, Isorthis clivosa [rare], Leptostrophia filosa [common], Microsphaeridiorhynchus nucula, Pholidostrophia sp., Protochonetes cf. minimus, P. cf. ludloviensis, Shagamella minor, Sphaerirhynchia?, Whitfieldella?, Cardiola interrupta, Kionoceras?, Sleia sp. andcrinoid columnals. An associated 'gingerbread' horizon has substantially the same assemblage but with fewer determinable species. A quarry [SH 9179 7194] nearby yielded (RK 1083–1089) a comparable fauna, and Skenidioides lewisii and Kionoceras angulatum were also recorded. These faunas suggest an horizon very high in the C. scanicus Zone. PTW

In the Dawn area thick sandstones and disturbed beds occur. For example, the crags at Ffrith Wen [SH 8635 7228], west-north- west of Deheufryn, show:

A similar sequence in the crag [SH 8694 7251] east-south-east of Barnhill shows a 1.8 to 2.4-m disturbed bed with a basal 2.5-cm sandstone band, downcutting northwards into striped silty mudstone.

A roadside quarry [SH 8688 7237] in this area yielded a graptolite fauna (RK 3090–3113) of C. scanicus, Pristiograptus bohemicus bohemicus, P. aff. tumescens, S. chimaera, S. chimaera cf. salweyi and S. chimaera semispinosus. A shelly fauna (RK 3069–3089) from a 1-m disturbed bed comprises: A. reticularis, Isorthis sp., Leptaena depressa and Cyclonema?, and a 'gingerbread' horizon 2.7 m up in the 3.3 m of striped mudstone present yielded (RK 3116–3133) A. reticularis, Isorthis?, cf. Protochonetes ceratoides, Cardiola interrupta, cf. Mila innotata, Ophioceras? cf. articulatum, 'Orthoceras' argus, 'O.'ibex and 'O.' cf. rectincinctum.

Traced eastwards, both sandstones and disturbed beds thin rapidly, and in the area north of Rhosmarch-ganol the 'Cynant Beds' consist very largely of striped silty mudstones.

On Mynydd Glyn-lws thick disturbed beds and sandstones up to 30 m thick reappear. An old quarry [SH 8874 7305] south-east of Bron-y-pistyll shows up to 7.6 m of graded sandstones and sandy siltstones with sole structures and current bedding. Eastwards sandstones again die out, and they are found only locally in the Bethgeth area, where, for example, the old quarry [SH 9119 7188] west of Cynant Uchaf shows 4.8 m of massive sandstone, in two beds with 0.3 m of intervening disturbed siltstone.

Between Mynydd Glyn-lws and the Bethgeth area the number and thickness of disturbed bed units diminishes; and in this and the Cynant area there are numerous examples of rapid lateral passage. A crag [SH 9093 7154] south-east of Bethgeth shows the westward passage of irregularly cleaved, striped silty mudstones into a disturbed bed.

The thin disturbed beds of the Cynant Ganol and Cynant Isaf areas are interbedded with ripple-marked sandstones (Plate 11). The scarp [SH 9230 7241] north-north-east of Cynant Ganol shows:

This is one of several exposures of the upper surfaces of disturbed beds.

A scarp [SH 9227 7152] west of Cynant Isaf shows:

Comparable sequences are seen in the cliff [SH 9178 7312] east-north-east of Cynant Uchaf and a small quarry [SH 9229 7161] north-west of Cynant Isaf.

In the stream south-west of Nantyfedw, near Bettws-yn-Rhos, over 15 m of striped silty mudstones occur, providing some evidence that there is a northerly element in the thinning of sandstones and disturbed beds. HEW

Bont-uchel Formation

The highest beds of the Elwy Group of this region consist of striped silty mudstones with, locally, thin disturbed beds, correlated on the basis of their faunas (see below) with the Bont-uchel Formation (p. 101).The outcrop, which is almost entirely drift-covered, extends from around Dolwen through Bettws-yn-Rhos to the Sirior Goch–Pen-y-bryn area, and thence into the Fynnonau–Nant Fawr and Ysgeirallt areas.

The Bont-uchel Formation of the Dolwen area, whose correlatives are part of the sequence described from the Llanelian-yn-Rhos area to the west (p. 62), consist of striped silty mudstones. They have yielded (Zp 912–919) a shelly fauna of Dayia navicula, Hyattidina sp., Lanceomyonia sp. and Shagamella sp.from the stream section [SH 8826 7482] west of Glan-yr-afon Farm; D. navicula and Shagamella minor, together with Isorthis sp., Schizocrania sp. anda pterineid (RK 3288–3309), are recorded from a disturbed bed visible in the yard [SH 8896 7326] and stream [SH 8897 7321] at Gwyndy-uchaf near Bron-y-pistyll, and D. navicula is recorded from the old quarry [SH 8971 7411] due west of Plas-yn-Betws. PTW, HEW

Farther east, around Bettws-yn-Rhos itself, a quarry [SH 9153 7480] north-east of Peniarth-fawr exposes thickly bedded mudstone with crushed shells typical of the less fossiliferous undisturbed strata of the S. incipiens Zone. The fauna (RK 644–651) collected comprises: Lanceomyonia? and Cardiola interrupta. A disturbed mudstone exposed in a roadside quarry [SH 9073 7296] due south of the Wesleyan Chapel in Bettws, has yielded (RK 1047 1082) a shelly assemblage comparable to that from near Cynant Uchaf (see above) with the following additional forms: favositid, bryozoa, Leptaena depressa [common], Strophonella euglypha, Loxonema? and Frostiella?.

Exposures in the Sirior Goch–Pen-y-bryn area show a relatively high proportion of disturbed bed units, individually up to about 6 m thick as can be seen, for example, in the old quarry [SH 9275 7440] at Pen-y-bryn. A small quarry [SH 9330 7383] at Pen-yr-efail shows cleaved silty mudstone with sandy ribs, and has yielded (Zp 2217–2218, RK 597–616, RK 3223–3263) Bolbozoe cf. bohemica, S. leintwardinensis incipiens, S. leintwardinensis cf. leintwardinensis and S. cf. varians pumilus, and perhaps exposes the highest beds in the whole district.

Further faunas are recorded from the Nant Fawr and Ysgeirallt areas. A disturbed bed exposed in the scar [SH 9310 7567] west of Nant-uchaf yielded (RK 4735–4759) Atrypa reticularis, Dayia navicula [common], Lanceomyonia?, Microsphaeridiorhynchus nucula?, Salopina sp., Shagamella minor, pterineid and crinoid fragments; nearby exposures [SH 9328 7577] (RK 1499–1504), [SH 9287 7523] (Zp 8053–8058)] yielded S. leintwardinensis incipiens and S. leintwardinensis cf. leintwardinensis; a track-side exposure [SH 9272 7507] south-east of Ffynonnau exposes a shell band in thickly bedded mudstone with a fauna (RK 1507–1584) of 'Syringopora' bifurcata, A. reticularis, D. navicula [common], Howellella cf. elegans, M. nucula [common], Isorthis cf. clivosa, I. aff. orbicularis, Lanceomyonia? Leptaena sp., Shagamella minor, Cardiola interrupta, orthocone, Bolbozoe cf. bohemica and crinoid columnals; and exposures in the right bank of Nant-y-Bryniau [SH 9482 7504] near Ysgeirallt have yielded (Zp 928–949, RK 1454–1493) C. scanicus, S. cf. leiwardinensis incipiens, and S. cf. varians pumilus. All these faunas indicate the S. incipiens Zone. PTW

VI Llangerniew region

With the exception of mainly peripheral areas of Upper Nantglyn Flags, this region is underlain by Elwy Group rocks belonging to the Upper Neodiversograptus nilssoni and Cucullograptus scanicus zones. Numerous crag and quarry sections give exposures on the high ground, but there is extensive drift cover along the Elwy and Melai valleys and between Llangerniew and Wenlli.

The region includes the classic locality at Ty'n-y-ffordd [SH 8699 6525] (see below) and the type localities of Jones's (1937) Llangerniew Disturbed Beds, Gribin Mudstones, Pentre Disturbed Beds, Brynhyfryd (March Aled) Mudstones and Henfryn Disturbed Beds (Figure 19).

Upper Nantglyn Flags Group

Rocks of this group crop out north of Pandy Twdwr, in an inlier in the Elwy Valley around Ysgubor-newydd, and in the Afon Cledwen around Pont Sylltu. The outcrops are extensively or entirely drift-covered, and exposures are restricted to streams and quarries in the Pandy Twdwr area.

As in other regions the rocks are ribbon-banded mudstones with sporadic concretions, yielding a Lower N. nilssoni Zone fauna. An old quarry [SH 8514 6479] at Llwyn Llydan shows 3 to 4 m of such beds with a fauna (Zp 3729–3730) of Saetograptus varians pumilus and ?N. nilssoni. The uppermost 15 m of the group comprise siltier ribbon-banded mudstones that locally include silty sandstone bands; for example, a stream bank [SH 8577 6508] west-south-west of Ty-gwyn shows a 30-cm grey silty sandstone in a 3-m section of striped and ribbon-banded silty mudstone. The most important section in the uppermost beds of the Upper Nantglyn Flags Group is, however, the classic locality of the roadside quarry [SH 8699 6525] at Ty'n-y-ffordd described by Jones (1937) and Boswell (1949, p. 259). This also exposes the basal strata of the Elwy Group, the section being:

The fauna from the Elwy Group (RK 3816–3858, RK 3967–3983) comprises 'Syringopora' bifurcata, Atrypa reticularis, Hyattidina sp.[common], Isorthis clivosa, I. cf. clivosa, Leptostrophiafilosa, Microsphaeridiorhynchus cf. nucula, Pholidostrophia sp., cf. Maminka tenax, 'O.'undulocinctum, Monograptus uncinatus orbatus, N. nilssoni, and Pristiograptus dubius. The graptolites are preserved in relief. The Upper Nantglyn Flags yielded (Zp 3718–3728, Zp 8083–8084, RK 873–896) chiefly Saetograptus chimaera salweyi, associated with Cardiola interrupta, N. nilssoni and P. cf.frequens, a fauna characteristic of the basal Upper N. nilssoni Zone. The graptolites from the Elwy Group rocks indicate the Lower N. nilssoni Zone and are evidently a derived fauna.

Elwy Group

The outcrop in this region may conveniently be described under three areas, viz: west of the West Llangerniew Fault, between the Llangerniew faults and east of the East Llangerniew Fault.

West of the West Llangerniew Fault

This area includes some of the type localities of Jones's (1937) formations and these form a satisfactory local subdivision of the Elwy Group.

Llangerniew Disturbed Beds

The Llangerniew Disturbed Beds are extensively exposed in their type area in crags, fault scarps, old quarries and stream sections in the Ty'n-y-ffordd—Tyddynuchaf area, where the base of the formation is seen (see above); between Llangerniew and Rhos-ymawn; and most spectacularly west of Llangerniew in the banks of Nant Camaes and Nant Bodgwn fod, along the Afon Collen and in the area south of Ynys-Rhys, Gribin and Fron-wen, where the uppermost beds of the formation are seen. TRWH, PTW

In all these areas the formation is composed almost entirely of disturbed beds, forming a sequence approximately 500 m thick. Many of the exposures show numerous small plunging, often recumbent, slump folds, which it is not always possible to distinguish from tectonic folds (see p. 43). Some of the finest sections displaying folding are as follows. An exposure [SH 8681 6828] near Fron-wen shows numerous small and large-scale irregular slump folds although tectonic folding is also present. A 4 to 5-m exposure [SH 8612 6803] near Camaes shows intense small-scale slump folding, with a prominent large synclinal fold at the northern end. The small-scale folds are tight and plunge in several directions, but mainly to the south-west, while the larger syncline is more open and may be of tectonic origin. Much of the 9-m gorge section [SH 8543 6795] near Tyddyn-bach is composed of pencil slates, but there are scattered small irregular folds, both tight and open and plunging in various directions. Near the base of the section the beds are less disturbed, and contain sandy laminae.

In the same vicinity there is an appreciable proportion of undisturbed strata. Several poorly laminated but undisturbed mudstone bands, each about 3 m thick, occur interbanded with disturbed mudstones along a fault scarp [SH 8632 6798] east of Camaes. Most exposures are of silty and sandy laminated mudstones, but ribbon-banded beds occur, for example at a quarry [SH 8618 6806] near Camaes. These undisturbed rocks usually display a dip far in excess of the regional dip.

Relationships of disturbed rocks to the undisturbed layers within the Llangerniew Beds are seen at two localities. One, a small hillside exposure [SH 8550 6782] south-east of Tyddyn-bach, shows disturbed beds sagging around a small patch of undisturbed laminated mudstone which appears to be a mass caught up in the slumping. The other is a nearby quarry [SH 8554 6784] where about 2.5 m of undisturbed mudstone overlie 1.8 m of disturbed beds, filling in the slight undulations at the top of the latter. TRW H

In the north of the region the Llangerniew Disturbed Beds form the core of an anticline which extends eastwards from the Pen-y-Bryn Cloliau area. Here the proportion of undisturbed strata increases and includes a number of sandstone bands, particularly between Trofarth and Ty Isaf Trofarth. A quarry [SH 8540 7114] west-north-west of Tai shows 3 m of massive fine-grained sandstone with a band of mud-flake conglomerate.

The relationship of the undisturbed silty mudstones and sandstones to the disturbed strata in this area is shown, for example, in a crag [SH 8446 7089] south-south-west of Pen-y-bryn Cloliau, which shows:

A temporary exposure [SH 8413 7026] west of Llyn Ty'n-y-llyn showed a similar disturbed bed with a shelly top overlain by a 5-cm graded sandstone. HEW, PTW

Shelly fossils have been recorded from a number of localities in the area; the more important are: an exposure [SH 8687 6839] southeast of Fron-wen, which yielded (RK 1220–1264) solitary corals, bryozoa, Atrypa reticularis [common], Howellella sp., Eospirifer radiatus, Isorthis sp., Leptaena depressa [common], Leptostrophia sp., Pholidostrophia sp., Protochonetes sp., Palaeopteria?, Plectonotus trilobatus, Hormotoma sp., Dalmanites sp., proetid fragment and crinoid columnals; a temporary excavation [SH 8484 7067] west-north-west of Trofarth, which yielded (RK 1981–2018) a rich, but atypical, fauna remarkable for the diversity of mollusca and including the earliest undoubted trigoniacean bivalves recorded (Newell and Boyd, 1975, p. 97, fig 33). The forms present are Favosites sp., 'Syringopora' bifurcata, bryozoa, A. reticularis, Craniops implicatus, Dicoelosia sp., Howellella sp., Isorthis clivosa, L. depressa, Microsphaeridiorhynchus nucula, Protochonetes ludloviensis, Salopina lunata?, Sphaerirhynchia aff. wilsoni, Actinodonta cf. cuneata, 'Ctenodonta'angli ca, Nuculites cf. pseudodeltoideus, Toechomya sp., Bucanopsis expansus, Gyronema octavia glabrum, Hormotoma cf. corallii [common], Plectonotus trilobatus, Temnodiscus murchisoni, 'Orthoceras' sp., beyrichiid, 'bolliid', Londinia sp., Turrilepas?, crinoid columnals and a monograptid; a temporary excavation [SH 8529 7084] due west of Bryndwdan, which yielded (RK 1648–1701) a rich and well preserved fauna typical of the Llangerniew Disturbed Beds, namely: Cornuliles serpularius, Aegiria grayi, Atrypa reticularis [common], Craniops implicatus, Howellella sp., Leptostrophia filosa [common], M. nucula, Pholidostrophia sp., Protochonetes ludloviensis, P. cf. ceratoides, Sph. aff. wilsoni, Stegerhynchus lewisii, Strophonella?, 'Ctenodonta', Palaeopteria?, Bembexia sp., Platyceras cf. cornutum, Acaste downingiae, Frostiella sp. andcrinoid columnals; a further excavation [SH 8595 7109] north-east of Bryndwdan, which yielded (RK 1709–1741) fairly complete specimens of Antihomocrinus?; a quarry [SH 8571 7176] south-east of Hafod-lom, which yielded (RK 3140–3173) Keilorites sp., A. grayi, A. reticularis, Howellella sp., I. clivosa, I. cf. orbicularis, L. depressa, M. nucula, cf. P. ceratoides, Nuculites sp., 'Orthoceras' cf. recticinctum and Acaste sp.; a comparable fauna (RK 3789–3799), but with Amphistrophia funiculata, Gypidula sp., Shagamella sp. and Toechomya sp., was found at the quarry [SH 8594 6640] south-west of Caer-llo, which also yielded (RK 3781–3788) Monograptus cf. colonus, Saetograptus cf. varians pumilus and S. cf. varians varians.

The foregoing shelly faunas give no precise indication of the age of the Llangerniew Disturbed Beds. However, from the few graptolites recorded (see above and below), but mainly on the basis of the ages of the overlying and underlying formations, they are considered to be of Upper N. nilssoni Zone age. PTW

The Gribin Mudstones, some 640 m of which are present north of the type locality, Gribin, crop out from the West Llangerniew Fault, east of Fron-wen, westwards through Gribin, to Ynys-Rhys. Thence the outcrop extends northwards to the area around Gell and then north-eastwards in a series of synclinal fault-bounded outliers to Trofarth. Further outcrops occur along the northern margin of the region west of Ty Isaf Trofarth.

The beds are everywhere striped silty mudstones with sporadic thin sandstone bands and very thin laminated muddy siltstone (graptolite shale) bands. Exposures are numerous in small crags, old quarries and streams. The type locality—the quarry [SH 8624 6839] at Gribin—shows 3.6 m of grey, well bedded silty mudstone with rare stripes, which has yielded a graptolite fauna (RK 749–780, Zp 1864–1875) of C. scanicus, ?Monograptus colonus, P. cf. vicinus, S. chimaera cf. semispinosus and ?S. varians varians. Graptolites have been recorded from numerous other places, notably the old quarry [SH 8501 6963] west-south-west of Ty-isaf, Gell, which yielded (Zp 1901–1910) C. scanicus, M. colonus, S. chimaera cf. chimaera and S. chimaera cf. salweyi; the exposure [SH 8549 6991] in Lletty Watkin yard (Zp 1934–1942), C. scanicus; the scarp [SH 8483 7007] due east of Dol-goch where C. scanicus and S. chimaera chimaera were collected (RV 1321–1331); and the temporary excavation [SH 8457 7056] north of Dol-goch, which yielded (RK 1953–1980) Atrypa?, C. scanicus, ?N. nilssoni, P. cf. comis, S. varians pumilus and S. varians varians. Fragmentary plant remains (RK 1702–1708) were recorded from a further temporary excavation [SH 8571 7100] north-north-east of Bryndwdan.

The strata at Gribin lie 15 m above the base of the formation and the fauna (see above) suggests an horizon close to the base of the C. scanicus Zone. Locally at least, the junction of the Gribin Mudstones and the Llangerniew Disturbed Beds may be considered to coincide with the boundary of the N. nilssoni and C. scanicus zones. PTW, TRWH

The Pentre Disturbed Beds, Brynhyfryd Mudstones and Henfryn Disturbed Beds (Jones, 1937) are believed to be the lateral equivalents of the 'Cynant Beds' of the region to the north (p. 77), and like these beds they overlie Gribin Mudstones of lower C. scanicus Zone age.

A roadside exposure [SH 8640 6962] in the Henfryn Disturbed Beds north-north-east of Henfryn yielded the following shelly fauna (RK 1278–1298, RK 4166–4213): bryozoa, Atrypa reticularis, Microsphaeridiorhynchus cf. nucula, Dayia navicula? [rare], Howellella cf. elegans, Hyattidina sp., Isorthis sp., Leptaena depressa?, Protochonetes cf. ceratoides, Shagamella sp., Ctenodonta' aff. anglica, Rterinea' aff. rectangularis, Loxonema sp., 'Orthoceras' cf. recticinctum, beyrichiids and crinoid columnals. This, and in particular the presence of D. navicula? suggests an horizon for the top of the Henfryn Disturbed Beds closely similar to that of the boundary between the Bont-uchel Formation and the Cynant Beds' (p. 78). PTW

The Penti-e Disturbed Beds crop out in the Cefn-coch and Nant areas, and in an outlier north-west of Plas-yn-y-Trofarth. They consist of about 60 m of disturbed silty mudstones and siltstones with rare thin sandstones; slump folds are rarely seen. Much of the ground underlain by the Pentre Disturbed Beds is drift-covered but good sections occur along the lane south-east of the chapel [SH 8673 6900] near Cefn-coch, in the gorge section of the Afon Gell eastwards from Melin Trofarth [SH 8623 6926], and in the southern bank of the Afon Gell [SH 8568 6919], [SH 8565 6920] near Plas-yn-y-Trofarth. A shelly fauna comprising Atrypa reticularis [common], Hyattidina sp., Isorthis sp., Leptaena depressa, Protathyris?, Protochonetes?, Sph. aff. wilsoni [common], Rollia' sp., Leperditia sp., and crinoid columnals has been recorded from an exposure [SH 8623 6883] in the west bank of the stream west-south-west of Cefn-coch.

The Brynhyfryd (March Aled<span data-type="footnote">The mudstones at the type locality are here interpreted as belonging to the Gribin Mudstones (see p. 82), hence the use of the alternative term 'March Aled Beds'.</span>) Mudstones consist of 15 to 45 m of striped silty mudstones overlying the Pentre Disturbed Beds. They crop out in the Melin Trofarth and March Aled areas between the Tŷ-mawr and West Llangerniew faults, and are well exposed east of the Tŷ-mawr Fault along the Afon Gell as far as Melin Trofarth. North of Melin Trofarth the basal beds can be seen for about 30 m in a small tributary stream. Some 3 m of striped silty mudstone can be seen in the old quarry [SH 8641 6991] at March Aled.

The Henfryn Disturbed Beds crop out only in the area between Henfryn and the Ty-mawr Fault to the west. About 41 m of disturbed beds similar to the Pentre Disturbed Beds occur, the best exposures being along the lane immediately north-east of the farm and in the stream [SH 8613 6963] to the west-north-west.

The bank [SH 8599 6967] west-south-west of March Aled shows an excellent example of a small recumbent slump-fold, and yields a shelly fauna (RK 3919–3946) comparable to that from the temporary excavation west of Bryndwdan (see above) although less rich, but including Butovicella migrans and Dalmanites myops. TRWH

Between the Llangerniew Faults

Although the type localities of the Pentre Disturbed Beds and the Brynhyfryd Mudstones (see above) occur in this area, extensive drift cover in the Elwy valley and the structure allow for more than one interpretation of the Elwy Group sequence in terms of its correlation with the succession west of the Llangerniew faults.

Disturbed beds, exposed in numerous small crags west of Ty'n-ydal are assigned to the Llangerniew Beds because they dip northwards under a striped and ribbon-banded silty mudstone sequence, 4.5 m of which are seen in the quarry [SH 8769 6645] at Tan-y-ffos, north-north-east of Hendre-ddu. These beds have yielded a fauna (RK 781–792) of C. scanicus, P. bohemicus bohemicus and S. chimaera semispinosus?, unequivocally indicative of the C. scanicus Zone, and are hence interpreted as Gribin Mudstones. TRWH, PTW

To the north only two small disturbed-bed exposures [SH 8777 6806] occur before the disturbed-bed sequence above Pentre Isaf is reached. These disturbed beds appear to dip south under the Gribin Mudstones and, unless a fault intervenes in the Pentre Isaf area, they and the disturbed beds between Pentre Isaf and Ty Isaf Trofarth to the north-west are probably part of the Llangerniew Disturbed Beds. A shelly fauna (RK 3883–3918) comparable to that from the exposure [SH 8687 6839] near Fron-wen (see above), but including Aegiria grayi, Dinobolus?, Euomphalopterus and Gyronema spp., has been recorded from the quarry [SH 8705 6992] due north of Pen-y-ffridd. A graptolite fauna (Zp 1827–1842) from the old quarry [SH 8695 7050] north-east of March Aled, high in the sequence, comprises Monograptus sp., Pristiograptus cf. comis and P. cf. tumescens, and is of uncertain age. There is no unequivocal evidence that any part of this sequence of disturbed strata north of Pentre Isaf, totalling some 240 m and showing 40 m of disturbed beds in one cliff section [SH 8718 6913] north-west of Pentre Isaf crossroads, is the equivalent of the Pentre Disturbed Beds of the Cefn-coch area (cf. Jones, 1937, pl. 14). Likewise the striped silty mudstones cropping out around Ty-newydd and including the locality of Bryn Hyfryd [SH 8687 7013] belong to the Gribin Mudstones and not to the March Aled Beds.

Farther north around Wenallt there are numerous exposures of disturbed beds interpreted as belonging to an up-folded sequence of the Llangerniew Disturbed Beds. One scarp at Ty'n-y-terfyn [SH 8693 7090] shows the gradual lateral passage over 30 cm of striped silty mudstone into disturbed beds. As in the areas to the west and east (see above and below), the upper part of the Llangerniew Disturbed Beds contains several sandstones, including a unit up to 45 m thick. Several sections showing up to 2.5 m occur in the vicinity of Wenallt, and a crag [SH 8635 7172] east-south-east of Ty Isaf Trofarth shows 2 m of coarse sandstone with quartz pebbles. TRWH

East of the East Llangerniew Fault

Numerous exposures of disturbed beds on Mod Pentre-wern, Tre-pys-llygod and to the north of the Elwy valley are attributable in the main (see below) to the Llangerniew Disturbed Beds because the sequence to which they belong directly overlies the Upper Nantglyn Flags in the Elwy valley west of Llanfair Talhaiarn. The base of the sequence is exposed only near Fron-goch, east of Ysgubor-newydd, where the lowest disturbed beds include a conglomerate band (p. 39). This is seen in the scarp [SH 9119 6995] east of Ysgubor-newydd which shows:

Within the predominantly disturbed sequence south of the Elwy valley there are a number of undisturbed striped silty mudstone bands, up to 6 m thick. A quarry [SH 9000 6849] at Tan-y-graig shows 3 m of disturbed beds with a uniform base resting on one such band, and a fault scarp [SH 8989 6845] west of the farm shows a disturbed bed cutting down some 2.4 m into another band. Further bands of striped beds occur in the Nant Mawr area. A 4.5-m exposure [SH 8881 6956] at Bryn-yr-awr, due west of Nant Mawr, shows a disturbed bed of grey silty mudstone with layers containing numerous small shelly fossils—brachiopods (small and coarse-ribbed) and crinoids—and sporadic small ironstone pebbles. It also includes siltstone and fine-grained sandstone, brown-weathering, with shelly fossils, in balled-up and twisted masses. These suggest slumping from the south.

Shelly faunas have been recorded from a number of localities. The bank [SH 9056 6945] west-south-west of Henllys yielded (RK 7747–7757) A. reticularis, C. implicatus, Isorthis sp., M. nucula, Pholidostrophia sp., Protochonetes cf. ceratoides, S. wilsoni, beyrichiid and crinoid columnals, and in addition the graptolites S. varians cf. pumilus and S. varians cf. varians. A steep scarp [SH 8787 6908] south-south-west of Glan-yr-afon yielded (RK 1265–1277) Isorthis sp., Leptostrophia sp., Sphaerirhynchia sp., 'Ctenodonta' anglica, 'C.' cf. lingualis, Nuculites cf. pseudodeltoideus, Loxonema sp. and Acaste sp.TRWH, PTW

North of the Elwy valley, numerous crags expose disturbed beds in the area between Cefndrydwy and Garthewin in a sequence that also includes both striped silty mudstone and sandstone bands. The old quarry [SH 8865 7011] north-west of Bryngwylan shows 4.8 m of sandstone, the uppermost, 0.6-m bed having large flute-casts indicating a current from due west. At the northern end of this exposure the flute-cast sandstone overlies a disturbed bed, up to 2.4 m thick, which cuts across the other beds of sandstone. TRWH

Graptolite faunas have been recorded from two localities in undisturbed mudstones: a scarp [SH 8759 6982] west of Bryngwylan, which yielded (Zp 2417–2418) Cucullograptus? cf. crinitus, and a stream section [SH 8892 7007] north of Bryngwylan from which Monoclimacis aff. haupti, M? aff. ultimus and Pristiograptus sp.were recovered (Zp 2419–2427). Both faunas are consistent with an Upper N. nilssoni Zone age for this disturbed-bed sequence. PTW

The Gribin Mudstones are represented along the northern margin of this area by a sequence of striped silty mudstones with at least one disturbed bed up to 15 m thick. As already indicated (p. 77), several temporary exposures in these beds in the Bettws-yn-Rhos region, have yielded C. scanicus Zone graptolites. Although the faunas suggest that the base of the Gribin Mudstones here and at the type locality falls at approximately the same stratigraphical horizon, the upper part of the present sequence which, at Deheufryn Gorse, includes a disturbed bed up to 60 m thick probably correlates with the lower part of the 'Cynant Beds' in the area west of the Llangerniew faults.

A similar problem of correlation applies to the Tre-pys-llygod sequence. An undisturbed striped silty mudstone band near the top of the sequence is exposed near the summit of the hill, in a scarp [SH 8890 6857] east-north-east of Tyn-y-caeau, and has yielded a C. scanicus Zone graptolite fauna (Zp 2429–2465) of C. scanicus, M. micropoma, ?Pristiograptus cf. vicinus, S. cf. leintwardinensis incipiens and S. varians pumilus. This, together with the thick sequence of disturbed beds present as compared with that of the Llangerniew Disturbed Beds of the type area, suggests that the uppermost part of the Trepys-llygod sequence correlates with the type Gribin Mudstones only 6 m of which are represented in the same lithology—and the basal Pentre Disturbed Beds. Taken farther east into the Llanfair Talhaiarn Region and south into the Gwytherin Region correlation of Jones's formations becomes even less certain and they can no longer be sensibly used. TRWH, PTW

VII Gwytherin region

A complete stratigraphical succession from the C. rigidus Zone strata of the Denbigh Grits Group to the Elwy Group (Upper N. nilssoni Zone) is represented in this region. Drift cover is extensive, but there are good stream sections in the Afon Cledwen and its tributaries, and crags and quarries provide numerous exposures on the high ground above Taipellaf and Merddyn, and around the summits of Moel Seisiog and Graig-hir on the Denbigh Moors.

The region is unique in possessing an Upper Nantglyn Flags sequence locally exceeding 460 m and including disturbed-bed units more than 100 m thick. PTW

Denbigh Grits Group

Measures below the Berllan Formation

The outcrop of these measures extends across the southern margin of the region from the Moel Seisiog area to the vicinity of Llyn Alwen. East of the Taipellaf Fault the outcrop may extend as far north as the southern end of Creigiau Llwydion.

The sequence consists of a variable alternation of sandstones and striped silty mudstones. The proportion of the former is greatest in the Moel Seisiog–Graig-hir area, where they comprise some 40 per cent of the succession and consist of fine-grained to pebbly sandstones. Eastwards the sandstones thin and are represented by only a few thin bands in the Llyn Alwen area. Exposures are, in the main, restricted to small scarps and old quarries. On Moel Seisiog the sandstone units seldom exceed 10 m in thickness and are commonly 4.5 to 6 m. They are mainly fine to medium-grained at the base, passing upwards into coarser, often pebbly and shelly, sandstone, and are succeeded by disturbed silty mudstones and sandstones overlain by silty mudstone. The interbedded striped silty mudstones contain rare graptolites. A characteristic section is seen in an old quarry [SH 8604 5693] south of the summit of Moel Seisiog, where the sequence is:

A further old quarry [SH 8573 5703] nearby shows the lateral transition of sandstone into disturbed silty mudstone, the overall sequence being:

This disturbed bed and its possible correlative to the north-west are the only mappable disturbed beds in these measures.

Massive and bedded sandstones in the Graig-hir area vary from fine-grained to pebbly with mudstone clasts, and are locally fossiliferous. An exposure at the summit [SH 8691 5725] shows 2.4 m of grey, green or brownish, fine to coarse-grained, bedded sandstones, shelly in part and locally with vein-quartz. Marklandella sp. andcrinoid columnals (DT 7420–7426) were recovered.

A shallow-water benthonic fauna, similar to that from Plas Madoc (p. 74), was recorded by Ramsay (1866, p. 212) from Moel Seisiog (Z19295–9310). DACM

Berllan Formation

In the neighbourhood of the Afon Cledwen, near Ddol-frwynog, there is a sequence of some 120 m of striped silty mudstones with rare thin sandstone bands below the presumed Llanddoget Formation. A stream section [SH 8755 5785] east-north-east of Moel Seisiog yielded (DT 7239–7251) Cyrtograptus cf. rigidus, Monoclimacis flumendosae cf. flumendosae, Monograptus flemingii cf. primus, Monograptus sp.[of flexilis type], Pristiograptus cf. pseudodubius [early form] and Pristiograptus sp.—a fauna indicative of the C. rigidus Zone, suggesting that these are strata of the Berllan Formation (p. 67). PTW

Llanddoget Formation

These distinctive, predominantly disturbed, beds crop out from the western margin of the region to the north–south fault along Nant Caledfryn, and are particularly well exposed in the Afon Cledwen area.

Near Tu-hwnt-ir-afon numerous sections up to 15 m thick occur in pencil slate disturbed bed, and in a crag [SH 8685 5844] south-south-west of Ty-draw the sequence is:

In the Cefnen area the base of the formation is largely composed of disturbed sandstone, which thickens rapidly eastwards from Afon Cledwen to the Pant-y-fotty Fault. Numerous exposures up to 2 m thick show fine to pel. bly sandstones, predominantly massive and apparently disturbed. Quartz veins and stringers are common. DACM

At the south end of Creigiau Llwydion medium to coarse-grained, locally pebbly sandstones alternate with silty mudstones. This succession is tentatively correlated with the basal part of the Llanddoget Formation of Cefnen and may thus reflect an easterly thinning of the disturbed strata and their replacement by a regularly bedded succession.

Lower Nantglyn Flags Group

The outcrop of this group is extensively drift covered, but there are numerous small exposures of the Mottled Mudstones and Brynsylldy Formation north of Taipellaf and between Merddyn and Pant-y-fotty.

The measures below the Lower Mottled Mudstone, consisting of ribbon-banded silty mudstones with sporadic calcareous concretions, are well seen only in the stream south-west of Pant-y-fotty, where 54 m of the basal part of the succession are exposed. The sequence includes a 15-cm disturbed bed, and the lowest beds are silty mudstones with sandstone laminae, lying close above the Llanddoget Formation. Graptolites from this area and from exposures in Nant y Fawnog, west of Taipellaf, are indicative of the C. perneri to C. lundgreni zones.

The more important localities are: a crag [SH 8790 5880] southwest of Pant-y-fotty, which yielded (DT 7252–7274) Cyrtograptus sp.[of perneri/linnarssoni type], Mcl. flumendosae flumendosae, M. flemingii cf. flemingii, M. flemingii cf. primus and P. pseudodubius [early form], suggesting the C. perneri Zone; the stream bank [SH 8792 5883] to the north-east, from which a probable C. ellesae Zone fauna (Zp 5969–5980) of Cyrtograptus sp.(? ellesae), Monoclimacis? (possibly flumendosae), M. flemingii and P. cf. pseudodubius was recovered; the stream bank [SH 8795 5889] farther north-east, which yielded a C. lundgreni Zone fauna (Zp 5938–5959) of cf. Scyphocrinites? pulcher, C. hamatus, C. cf. lundgreni, Mcl. flumendosae kingi, M. flemingii cf. compactus, M. flemingii cf. flemingii and P. pseudodubius; and a stream-section [SH 8656 5934] west of Taipellaf, from where M. flemingii and M. flemingii var. δ Elles [25 cm + long] were recorded (Zp 5862–5865).

The Lower and Upper Mottled Mudstones are exposed in the Moel Gydia–Taipellaf area, approximately 10 m of the former being seen [SH 8650 5952] west-north-west of Taipellaf. This sequence also crops out east of the Afon Cledwen around Merddyn, where it is intensely cleaved and brown-weathering. A section [SH 8843 5926] east-south-east of Pant-y-fotty shows 3.6 m of mottled mudstone belonging to the Lower formation. A shelly and graptolite fauna, assigned to the C. lundgreni Zone has been recovered (DT 7294–7331) comprising: bryozoa, Glassia sp., Mezounia?, Kionoceras sp., 'Orthoceras' cf. filosum, M. flemingii cf. flemingii, M. cf. flemingii [narrow variety], P. pseudodubius and echinoderm fragments. A C. lundgreni Zone graptolite fauna (Zp 5911–5924) has also been recovered from the Lower Mottled Mudstone exposed in a bank [SH 8792 5907] east of Tu-hwnt-ir-afon. The Upper formation is seen as massive, striped, irregularly mottled mudstone in the stream [SH 8851 5942] east-north-east of Pant-y-fotty. Ribbon-banded mudstones exposed in the quarry [SH 8823 5933] near the same farm yielded a fauna (DT 7283–7293) of Gothograptus nassa, Pristiograptus cf. dubius and Pristiograptus sp.[close to pseudodubius]indicating the G. nassa Zone. P. dubius and M. ludensis (Zp 6048–6066), indicating the M. ludensis Zone, were recovered from the bank [SH 8865 5961] to the north-east. In the area immediately east of Merddyn, the stream section [SH 8773 5986] east-north-east of Merddyn yielded (DT 7394–7401) P. jaegeri and P. aff. jaegeri, suggesting the lower M. ludensis Zone, and in the same stream [SH 8783 5979] farther east 'Orthoceras'?, P. dubius and M. ludensis (DT 7366–7385), suggesting the upper M. ludensis Zone, were recovered. An intervening locality [SH 8779 5983] shows a tectonic ripple horizon (p. 144) in the ribbon-banded mudstones, which yielded (DT 7386–7393) ostracods and M. ludensis.

The uppermost unit of the Group, the Brynsylldy Formation, comprises disturbed silty mudstones which thicken westwards from 20 to 30 m and are exposed in numerous small crags all along the outcrop. Notable exposures occur in the stream bank [SH 8644 5990] north-west of Taipellaf, where 8 m of ribbon-banded silty mudstone are overlain by more than 12 m of disturbed silty mudstone, and at Pen Bryn y Clochydd [SH 8808 5982], where 9 m of disturbed bed are exposed. The most important locality, however, is an old quarry [SH 8578 5996] west-north-west of Taipellaf, which shows:

The fauna (RK 3363–3435, HEW 2–47, Zp 5800) comprises those species from the ribbon-banded flags, namely: cf. Maminka tenax, Dawsonoceras annulatum, 'O.' cf. primaevum, cf. Parakionoceras originale, Scyphocrinites? pulcher?, Monograptus colonus cf. colonus, M. ludensis [dominating the graptolite fauna], Pristiograptus cf. curtus and P. cf. dubius; those from the striped beds at the top of the section, probably part of the disturbed bed: Aegiria grayi, 'Clorinda' sp., Glassia sp., Protathyris sp., Strophochonetes sp. Hemsiella maccoyiana, cf. S. pulcher and problematicum [cf. Drbiculoidea' beckettiana];and those from the obviously contorted part of the disturbed bed: 'Syringopora' bifurcata, A. grayi, Atrypa reticularis, cf. Resserella canalis, Skenidioides? and Hemsiella sp.The graptolite fauna is undoubtedly of uppermost M. ludensis Zone age (p. 54) and, as in other regions, the top of the Brynsylldy Formation apparently approximates to the Wenlock–Ludlow boundary. DACM, PTW

Upper Nantglyn Flags Group

The outcrop of the Upper Nantglyn Flags is extensively drift covered, and exposures are generally poor and scattered. With the exception of one major and one minor disturbed-bed unit, the rocks are entirely ribbon-banded silty mudstones, typically flaggy with sporadic concretions. They are about 180 m thick in the southern part of the region, thickening to at least 350 m and possibly to as much as 600 m in the north-east.

Notable exposures in the ribbon-banded mudstones occur: in the Afon Derfyn valley; on the western slopes of Afon Cledwen; in Gwytherin, where the old quarry [SH 8743 6153] shows 3.3 m of beds yielding Neodiversograptus nilssoni (Zp 4988); at Tyddynuchaf, where an old quarry [SH 8805 6352] yielded a mixed Shelly and graptolite fauna (Zp 5041–5042, RK 3462–3495) comprising: Butovicella migrans, Cardiola interrupta, 'Orthoceras'dimidiatum, Entomozoe sp., Monograptus colonus colonus, M. colonus compactus, N. nilssoni, P. aff. comis; and in the Ty' r-felin-isaf area. Other localities that have yielded graptolites and associated orthocone nautiloids and thin-shelled bivalves characteristic of the Lower N. nilssoni Zone are: an old quarry [SH 8891 6637] south-south-west of Hafod-lom, from where M. cf. colonus, N. nilssoni, cf. P. comis and Saetograptus cf. varians (Zp 8100–8103, Zp 4635–4641) were recovered; the gorge [SH 8575 6405] of Afon Derfyn, south-south-west of the Methodist chapel at Pandy Twdwr—'Orthoceras' cf. subannulare [of Blake] (RK 949), M. colonus cf. colonus, P. bohemicus bohemicus, S. cf. varians pumilus, S. varians varians (RK 949–980); the small bank exposure [SH 8580 6051] on the eastern bank of the Afon Derfyn—Cardiola interrupta, Kionoceras cf. angulatum, 'Orthoceras' cf. bullatum, 'O.'cf. filosum, D. ' mocktreense, 'O.' cf. primaevum, 'O.' aff. recticinctum, 'O.' spp. indet., Parakionoceras originale, Polygrammoceras cf. bullatum, Bolbozoe cf. anomala, M. colonus colonus, M. colonus compactus, Pristiograptus cf. dubius, P. cf. curtus and S. varians varians (RK 849–872); and the disused quarry [SH 8573 6022] at the head of Afon Derfyn, which yielded N. nilssoni in a limited graptolite fauna (RK 793–815, Zp 5890–5893).

The major disturbed unit crops out in three areas: around Plas Mattw and Cae-coed, where it is 70 to 80 m thick and contains sandstone bands up to 1 m in diameter; south-east of Foel Cathau, where it is about 55 m thick and has sandstone lenses and balls, well seen for example, in the roadside [SH 8985 6455] east-south- east of the farm, where there is also an excellent example of a slump fold, and around T9-uchaf where the thickness exceeds 100 m. An exposure [SH 8891 6534] in the northern bank of Afon Cledwen, south-south-east of Wern, shows the upper contact of the disturbed unit with grey, thin-bedded silty mudstone. The base of the unit is exposed [SH 8844 6409] due west of Cae-coed, where striped silty mudstones pass up into disturbed beds of the same lithology. In the stream bank [SH 8939 6447] due south of Foel Cathau the disturbed unit is underlain by striped silty mudstones with thin sandstone bands. There is thus a strong suggestion of an association between disturbed beds and facies change from ribbon- banded to striped silty mudstone in the undisturbed measures. Shelly faunas have been recovered from the disturbed unit at a number of localities, for example the quarry [SH 8860 6447] north-west of Tyn-y-caeau which yielded (RK 4133–4165) favositid, cf. Amphistrophiafuniculata, Craniops implicatus, Hyattidina sp., Isorthis clivosa, Leptaena depressa, Leptostrophia filosa?, Microsphaeridiorhynchus nucula, P. cf. ceratoides, Shagamella sp., Sphaerirhynchia sp., Whitfieldella?, Dawsonoceras sp., 'Lituites' ibex?, Acaste downingiae and crinoid columnals.

A disturbed bed up to 15 m thick occurs some 100 m above the major unit in the Hafod-lom area.

Elwy Group

These rocks, up to 400 m thick, consist almost entirely of disturbed beds, and are well exposed to the west of Gwytherin. As in other areas they locally show numerous 'slump contortions' including folds and balled-up structures, while at other localities, including most of those east of Gwytherin, they appear to be almost without bedding. A notable section is provided by a 4-m gorge [SH 8527 6271] south-south-east of Bodwrach. Near the northern end of the section the rocks are a jumble of irregularly plunging slump folds, but there is gradual upward passage through open-folded beds into almost undisturbed mudstones with only very slight undulations. There is a break in the section at this point but disturbed strata are visible above and it is considered that all the rocks are part of a disturbed unit.

A number of generally thin, undisturbed silty mudstones occur sporadically throughout the disturbed bed sequence. The contact between a disturbed bed and underlying undisturbed strata is visible in a small quarry [SH 8584 6146] south-west of Bwlch-y-gwynt. In the middle of the quarry 2.4 m of disturbed silty mudstone rests with little apparent discordance on 1.8 m of undisturbed mudstone, but at the southern end the disturbed bed cuts down sharply into the undisturbed rocks, truncating their bedding.

Striped silty mudstones, probably at one horizon and up to 20 m thick, form a discontinuous outcrop in the Maelogen—Graig-bach—Bryn Goleu area, near the core of the Gwytherin Syncline and hence high in the sequence. A rich and well preserved shelly fauna (RK 3654–3696, RK 3984–4026) has been recovered from a thin disturbed horizon in striped mudstones exposed in the old quarry [SH 8615 6226] east-north-east of Graigbach. The fauna comprises: Favosites sp., solitary coral, bryozoa, Atrypa reticularis, Dalejina hybrida, Howellella sp., Hyattidina sp., Isorthis clivosa, I. cf. clivosa, Leptaena depressa, Leptostrophia filosa, Protochonetes ceratoides, Sphaerirhynchia wilsoni, Strophonella?, Cardiola interrupta, 'Ctenodonta' sp., Cyrtodonta?, Nuculites sp., Pterineid, Gyronema sp., Loxonema sp., 'Orthoceras' cf. argus, 'O. ' cf. ibex, 'Orthoceras' spp. indet., Calymene aff. bayeri, Dalmanites caudatus, crinoid columnals, Monograptus?, and Saetograptus chimaera cf. chimaera. Further graptolite faunas have been recorded from the old quarries [SH 8655 6219] (Zp 4577–4582) and [SH 8625 6226] (Zp 4583–4596)] east-north-east of Graig-bach, and [SH 8718 6212] (Zp 4994–5004) northwest of Gwytherin church. Together these localities have yielded: Cardiola interrupta, 'Orthoceras'primaevum, Cucullograptus scanicus, Monograptus colonus cf. colonus, Pristiograptus cf. tumescens, P. cf. vicinus and Saetograptus chimaera chimaera, which suggest the lowest part of the C. scanicus Zone. Thus part at least of the disturbed bed sequence may correlate with the Gribin Mudstones of the Llangerniew Region (p. 81), although this would imply some southerly thinning of the Llangerniew Disturbed Beds. That such thickness variations do occur is suggested by the thinning, west-south-west of Gwytherin, of the Upper Nantglyn Flags Group from about 230 m to 100 m, believed to be due, at least in part, to westerly downcutting and consequent thickening of the Llangerniew Disturbed Beds.TRWH, PTW

VIII Llanfair Talhaiarn region

With the exception of small outcrops of Upper Nantglyn Flags in the Elwy, Melai and Aled valleys and along the southern margin of the region, the strata belong entirely to the Elwy Group (Upper Neodiversograptus nilssoni and Cucullograptus scanicus zones).

There is a drift cover on the low ground, but extensive exposures of disturbed strata occur on the high ground. The area around Cefntreflech and Bryn-nantllech provides some of the finest 'golf-course' topography in the district and some notable fault-scarp exposures (Smith and others, 1965).

Upper Nantglyn Flags Group

A number of poor exposures in the Melai Valley, notably in the stream at Tyn-y-cyll, and on Ffrith Bedwyn, show striped and ribbon-banded mudstones immediately underlying disturbed beds of the Elwy Group. In the Aled valley, north-east of Llansannan, an old quarry [SH 9486 6701] at Dyffryn Aled, east-north-east of Foel, shows 3.6 m of ribbon-banded flags with thin calcareous siltstone bands and calcareous concretions flattened parallel to the bedding at two horizons 1.2 m and 1.8 m from the base. Graptolites from this locality (RK 8640–8650) and from others, for example a bank [SH 9094 6692] east-north-east of Hafod-y-gog (Zp 4722–4746), are those typical of the Lower N. nilssoni Zone.

On the south side of the Aled valley, the succession around Pencleden, east of Llansannan, includes a disturbed bed approximately 10 m thick. PTW, TRWH, EGS

Elwy Group

Although no distinct formations can be recognised and the fossils recovered (see below) are insufficient to give close palaeontological control, the succession in this group can be divided into three parts.

The lower part comprises predominant disturbed beds, composed of silty mudstone, siltstone and subordinate sandstone, with bands of striped silty mudstone and sandstone. The disturbed beds are well seen in numerous exposures on, for example, Moel Unben and Y Foel. TRWH, EGS

A 6-m scarp section [SH 9023 6796] north-east of Melai shows disturbed rocks containing numerous irregular bands and balls of fine-grained sandstone as well as jumbled masses of silty mudstone and sandstone. It is obvious here that contortion of the cleavage around sandstone balls is not due to later folding. A 9-m scarp [SH 9062 6840], just east of Plasbychan, shows disturbed rocks consisting of mudstone and blocky siltstone with a few balls and lenticular bands of sandstone, small dark rounded concretions and thin quartz veins. The whole mass is irregularly contorted. Disturbed beds belonging to this and probably the middle part of the succession occur on the high ground north of Moel Unben as far as Llanfair Talhaiarn, except in a synclinal area around Ty-celyn [SH 9177 6907]. Some of the best sections in this area are: 5.4 m of well cleaved disturbed silty mudstone in the old roadside quarry [SH 9204 7000] south by west of Llanfair Talhaiarn; 12 m of cleaved silty mudstone in the old roadside quarry [SH 9171 6987] north-east of Coedcochion; 3 m of disturbed striped silty mudstone in a scarp [SH 9141 6941] south of Coedcochion; and 3.6 m of silty mudstone in a small quarry [SH 9098 6941] south-south-east of Henllys. A section worthy of a more detailed description is that in a quarry [SH 9245 6985] south-west of Llanfair Talhaiarn. Here 6 m of striped silty mudstone with sporadic thin sandstone bands are exposed, the upper half being more disturbed than the lower. Small folds can be seen in the centre of the quarry. The sandstone bands vary in thickness from about 1 to 7.5 cm; most are grey, but two are brown and have irregular load-casted bottoms and undulating tops. Also some of the sandy striped mudstones show small anticlinal rises which often have a fracture cutting through them.

Sandstones are also present as discrete, mappable units, notably on Ffridd Bedwyn. In the quarry [SH 9096 6737] due south of the chapel the exposed section is: disturbed silty mudstone 0.9 m, on yellow-brown striped, fine-grained sandstone with some rolled pebbles at the base 2.1 m, on disturbed mudstone with sandy laminae and enclosed blocks of sandstone 0.3 to 1.2 m (this band fills a local disharmonic synclinal depression in the sandstone below), on fine-grained sandstone with pebble beds and some load-casts 1.8 m, on disturbed mudstone. The upper two disturbed bands truncate the bedding of the underlying sandstones. There is therefore, considerable evidence here of contemporaneous disturbance. TRWH

In the Y Foel area two localities are of particular interest. The first, an old quarry [SH 9495 6710] near Dyffryn Aled Mine, south-southwest of Pen-y-coed, shows a 7.5 to 15-cm pebble bed at the base of a disturbed bed and probably at the base of the Elwy Group. It is composed predominantly of purple-brown, decalcified, subangular, 'gingerbread' blocks up to 10 cm across; smaller (up to 2.5 cm) black mudstone and ?collophane pebbles also occur. The pebble bed apparently dies out westwards. The second notable locality is an old quarry [SH 9358 6657] west-north-west of Plas-yn-cornel, which shows a 17.5-cm disturbed bed with a few pebbles of black mudstone and 'gingerbread' rock containing shelly fossils, (RK 6808–6833) comparable to those recovered elsewhere in the region (see below). Graptolites from the same quarry comprise (Zp 4804–4830, RK 8590–8607) C. scanicus, Monograptus cf. colonus, Pristiograptus vicinus, Saetograptus chimaera cf. chimaera, and S. chimaera cf. salweyi. The bed thins and dies out westwards and to the east it ends abruptly against a fine-grained sandstone which is apparently its lateral equivalent. PTW

The topography of Moel Dowel suggests that there is here a greater proportion of striped beds in the lower Elwy Group strata than in the Y Foel sequence. Exposures of pencil slate disturbed bed occur on the southern slopes of Mod Dowel, and an old quarry [SH 9317 6681] west-north-west of Plas-yn-cornel shows the basal 0.6 m of a disturbed bed consisting of cleaved silty mudstone resting on 1.2 m of massive grey siltstone. The contact is irregular in part. EGS, PTW

Other faunas from this lower part of the Elwy Group succession include that from a quarry [SH 9398 6734] west-south-west of Mostyn-uchaf (RS 6755–6800), comprising 'Syringopora' bifurcata, bryozoa, Amphistrophia funiculata, Homoeospira cf. baylei, Hyattidina sp., Isorthis clivosa [common], Leptaena depressa, Protochonetes cf. ceratoides, Ptychopteria pleuroptera, cf. Nuculites ovatus, Ecculiomphalus sp., Gyronema sp., Liospira?, Loxonema sp., Acaste sp., Dalmanites sp., homalonitid and crinoid columnals, a typical assemblage from the N. nilssoni Zone disturbed beds. Many of the fossils, which are flattened, occur as composite moulds, indicating that the disturbed bed sediments were rather plastic. A similar shelly assemblage (RK 7688–7704) was collected from a quarry [SH 9092 6743] east-south-east of Melai, and an exposure in a field [SH 9281 6669] north-west of Plas-uchaf yielded (RK 8356–8369) bryozoa, Atrypa reticularis, Craniops implicatus Homoeospira?, Howellella sp., Leptostrophia filosa, Microsphaeridiorhynchus nucula, P. ceratoides, S. minor, Whitfieldella sp., Goniophora? 'Orthoceras'recticinctum, beyrichiid and crinoid columnals. A bank [SH 9475 6720] east-north-east of Foel yielded (Zp 4865–4879, RK 8674–8678) Aegiria grayi, Visbyella trewerna?, Hemsiella sp., cf. M. colonus colonus, P. vicinus, P. cf. comis, P. cf. dubius ludlowensis and ?S. varians. All these faunas suggest an Upper N. nilssoni Zone age for this part of the succession.

The middle part of the Elwy Group succession probably crops out between Bryn-nantllech and Mynydd Bodrochwyn and underlies all the ground east of the Llansannan–Abergele road. The sequence consists chiefly of disturbed beds of two kinds: namely, sandy disturbed beds typified by those in the area around Cefntreflech, and highly cleaved, pencil slate disturbed beds as seen for example around Bryn-Kenrick. The sequence also contains a significant proportion of undisturbed sandstone bands. It thus varies in several respects from either the lower or upper part of the succession in this region. Nevertheless it is possible that it may correlate with at least part of the sequence described above.

Graptolite and shelly faunas have been recovered from a number of localities, the most notable being: an old quarry [SH 9389 7042] east-north-east of Llanfair Talhaiarn, which yielded (Zp 2468–2490) Monoclimacis? cf. ultima, Monograptus sp., N. aff. nilssoni, P. cf. ludlowensis, P. cf. vicinus, ?S. leintwardinensis incipiens; the stream section [SH 9417 6956] due north of Foel, from which were collected (Zp 2946–2957, RK 7554–7560) C. scanicus, P. bohemicus, Pristograptus? and ?S. varians; and a quarry [SH 9403 6838] west of Bryn-nantllech, which yielded (Zp 2887–2898, RS 6810–6816) bryozoa fragments, Craniops implicatus, Howellella elegans, Hyattidina?, Isorthis sp., Tentaculites sp., trilobite fragment, beyrichiid, crinoid columnals, Pristiograptus vicinus [short form] and P. aff. tumescens from a 'gingerbread' band. These faunas suggest an Upper N. nilssoni Zone age, and hence the same age as the lower part of the group (see above). In view of the thickness of this zone in the district generally (p. 32), however, this affords little help in establishing local correlations. EGS, PTW, TRWH

A number of sections in this sequence deserve individual mention. The old quarry [SH 9403 6838] near Bryn-nantllech referred to above shows the undisturbed measures assumed to separate individual disturbed beds in the Bryn-nantllech area (Smith and others, 1965). About 4.5 m of disturbed, highly contorted sandstone and silty mudstone rest on graded bands of grey silty mudstone and light grey, false-bedded, fine-grained sandstone with flute-casts. As indicated, a 'gingerbread' horizon in the section has yielded numerous small, fragmentary, shelly fossils. EGS

Farther north, a cliff section [SH 9378 6953] on the west side of Moel Iago comprises:

Another example of the upper surface of a disturbed bed is seen in a small scarp [SH 9588 6910] in the Aled valley south-east of Ty'n-ycoed, where 15 cm of fine-grained laminated sandstone are seen to overlie 1.2 m of sandy disturbed bed. A nearby scarp [SH 9565 6886] exposes 2 m of alternating fine-grained sandstones, up to 20 cm thick, with silty bands and cleaved silty mudstone; the sandstones show flute-casts. PTW

At approximately the same horizon as that of the Moel Iago section, the following sequence has been established on the western face of Mynydd Bodran:

Sandstones at probably the same horizon crop out north of the Elwy valley on Mynydd Bodrochwyn and are exposed in old quarries in the vicinity of Caenewydd [SH 9371 7194].

A scarp [SH 9430 7183] to the east just over 1 km east-north-east of Bronheulog, shows:

The upper division of the Elwy Group of this region consists predominantly of striped silty mudstones. The outcrop is in three areas: the north side of Mynydd Bodrochwyn, the east side of Mynydd Bodran around Tycanol and between Llanfair Talhaiarn and Moel Emwnt. In the last-mentioned area the sequence includes sandstones up to 3 m thick and two disturbed beds, one up to 18 m thick.

Graptolite faunas have been recorded from all three areas, notably, in the first two areas, from: an old quarry [SH 9313 7242] near Capel Tabor, Wern-bach, which yielded (Zp 2172–2182, RK 532–546) C. scanicus, ?M. colonus, S. varians cf. pumilus, S. varians cf. varians; a scarp [SH 9386 7228] due south of Bodrochwyn-fawr, from where C. scanicus, S. chimaera cf. chimaera and S. chimaera cf. semispinosus (RK 636–642, Zp 2213–2216) were recovered; a bank [SH 9422 7240] west-south-west of Cae-march, which yielded (Zp 2357–2359) C. ? crinitus; and a small disused quarry [SH 9455 7047] near Bodchwil, from a 'gingerbread' horizon in which a shelly fauna (RK 990–1011) of bryozoa, chonetid, Howellella sp., Hyattidina?, Sphaerirhynchia, beyrichiid, 'Balla'and crinoid and machaeridian fragments was obtained as well as a graptolite fauna (RK 981–989) of C. scanicus and S. cf. leintwardinensis incipiens. In the third area, both shells and graptolites were obtained from an old quarry [SH 9285 6974] south-south-east of Llanfair Talhaiarn, where 6 m of disturbed silty mudstone resting on 2.5 m of striped beds with sandstone bands are exposed. The fauna (RK 7531–7553, Zp 2589–2608) comprises solitary coral, A. funiculata, A. reticularis, L. depressa, M. nucula, Protochonetes?, Bellerophontid, Hormotoma sp., crinoid columnals, C. scanicus, P. cf. vicinus and P. aff. tumescens. Further graptolite faunas were recovered: from a roadside exposure [SH 9377 6840] west-north-west of Mostyn Isaf, which yielded (Zp 2901–2909) C. scanicus, P. bohemicus bohemicus, P. bohemicus cf. tenuis, P. cf. vicinus and S. chimaera semispinosus; and from the quarry [SH 9257 6763] south of Pen-y-ffrith (Zp 2787–2821), where in addition to the first two and the last, C.? crinitus, M. colonus and S. chimaera chimaera were collected.

All these faunas indicate a C. scanicus Zone age for this upper part of the group, but it is nevertheless possible that the lower part of the sequence in the Llanfair Talhaiarn area correlates with part of the middle division described above. In general terms, however, it may be suggested that this upper division correlates with the Gribin Mudstones, and in part with higher strata, of the Llangerniew region (p. 81), thus implying a correlation of both the lower and middle divisions with the Llangerniew Disturbed Beds of that region.

A number of sections of particular interest occur in this part of the Elwy Group succession. The scarp [SH 9386 7224] on Mynyddd Bodrochwyn, referred to above, shows:

This section was described by Boswell (1949, p. 209, fig. 9.3, pl. XIIIa). In the Aled valley, an exposure [SH 9505 7100] west of Fron Fawr shows 7.6 m of highly cleaved silty mudstone with fine-grained sandstone bands to 60 cm resting on about 9 m of fine-grained sandstones with flute-casts in beds up to 1.5 m thick, one of which contains abundant fragmentary shelly fossils 7 to 18 cm above its base. Two exposures [SH 9309 6812, SH 9332 6828] on the eastern slopes of Moel Emwnt show spheroidal or nodular-weathering siltstones. PTW, HEW

IX Llansannan region

This region includes all the enclosed, mainly arable land south of Llansannan as far as the northern edge of the Denbigh Moors, drained by the Afon Aled and its tributaries. It also includes the relatively high ground of the Beidiog area, Moel Grugoer, Foel Lwyd and Rhos Pen-llwyd.

The succession in the region extends from the Lower Nantglyn Flags Group (Cyrtograptus lundgreni Zone) to the lower part of the Elwy Group (Upper Neodiversograptus nilssoni Zone). In general, exposure is poor with extensive drift cover on both high and low ground, especially in the Foel Lwyd area in the south-west corner of the region. Exposure is largely restricted to short sections of streams, particularly in the Afon Aled, to quarries, and to small natural exposures in the drift-free areas.

The region is of interest in being underlain largely by Upper Wenlock strata, forming almost an inlier well to the north of the main Denbigh Moors outcrop. Although Boswell (1949, pp. 246–247) recorded a Wenlock graptolite fauna from Beidiog Uchaf (see below), he did not map Wenlock rocks in the region, and the extent of the present inlier has not hitherto been recognised. The succession in the Upper Wenlock is of added interest in showing certain features, notably the existence of two leaves to the Upper Mottled Mudstone, unique to the district. PTW, EGS, TRWH

Lower Nantglyn Flags Group

Measures below the Lower Mottled Mudstone

The outcrop of these beds is restricted to five fault-controlled inliers, two of which are in the Clwt-grugoer area near Bylchau, and the others around Tany-fron, Melin Gadeg and Beidiog Uchaf. In each of the areas the sequences consist of ribbon-banded silty mudstones yielding sparse shelly faunas and graptolites indicative of the C. lundgreni Zone.

Notable localities are: the quarry and stream bank [SH 9181 6350] at Beidiog Uchaf, which yielded (Zp 5096–5113, RK 7709–7744) C. cf. hamatus, Monoclimacis cf. flumendosae kingi, Monograptus flemingii cf. compactus, M. flemingii elegans, M. flemingii flemingii, M. flemingii var. b, Pristiograptus aff. lodenicensis, P. pseudodubius; the road section [SH 9255 6407] south-east of Pentrebeidiog, from where Giraldiella?, Howellella sp., Mezounia sp., Skenidioides sp., Entomozoe sp., Hemsiella?, Scyphocrinites? pulcher, M. flemingii cf. flemingii and Pristiograptus? have been recovered (RV 1755–1768); the exposure [SH 9380 6330] opposite Capel Glan-rhyd, where Cyrtograptus?, M. flemingii and P. cf. pseudodubius were recorded (RS 7019–7026); the bank [SH 9461 6295] near the ford at Fforest, which yielded (RK 8418–8421) M. flemingii cf. flemingii and P. lodenicensis; the disused quarry [SH 9596 6378] north-north-east of Wenallt-isaf, which yielded (RK 382–407) the same fauna and in addition M. aff. ludensis and P. dubius; and an old quarry [SH 9638 6298] from which were recovered (RK 8679–8699) M. flemingii flemingii, M. flemingii aff. elegans, and P. cf. pseudodubius.

In addition, at Clwt-grugoer and Tan-y-fron, an impersistent disturbed bed, up to 36 m thick, occurs. Also in the Clwt-grugoer area, an old quarry [SH 9572 6280] near Fron shows 3.6 m of spheroidally weathered siltstone with pyritic sandy bands and interbedded ribbon-banded mudstone. One 5-cm band consisting of 3.7 cm of ribbon-banded mudstone overlying 1.3 cm of white-weathering mudstone contains small calcareous concretions, up to 6.5 cm across, flattened parallel to the bedding.

In the Tan-y-Fron area up to 7.6 m of characteristic ribbon-banded silty mudstones with concretions are seen in the old quarries [SH 9593 6402], south-west of Bryn Bigad.

In the vicinity of Melin Gadeg the River Aled from 120 m south of the farm flows through a gorge and provides a spectacular section in ribbon-banded mudstones. (Boswell, 1949, pl. VIb).

The inlier around Beidiog Uchaf and Gilfach-lwyd is almost entirely obscured by drift.

Lower and Upper Mottled Mudstones and measures between

These measures are found in fault-bounded outcrops between Plas Madoc in the north-west and the Hendre-Aled to Bryn area in the Aled valley, in the Pont-y-Nant to Pen-cae'r-cwm area, and in the Moel Grugoer and Plas Panton areas. In general the measures, and particularly the mottled mudstones and disturbed beds, are resistant to erosion and are feature-forming, and hence are reasonably well exposed. The sequence is variable in both lithological content and thickness. It is 155 m thick in the Gilfachlwyd—Hendre-Aled and Cae Gronw areas, but less than 60 m in part of the Beidiog area. The mottled mudstones themselves show a similar degree of thickness variation (Lower 15 to 85 m; Upper 15 to 54 m), although this may in part reflect the inclusion or exclusion of transitional facies (see below) within tire mapped units. PTW, EGS

A number of individual exposures and sequences deserve particular mention: near Llyn Beidiog, an old quarry [SH 9160 6377] north-north-west of Beidiog Uchaf shows 2.1 m of (Upper) mottled mudstone with numerous undulating and partly current-bedded calcareous siltstone stripes, 6 to 12 mm thick and 1.3 to 5 cm apart. Small lenticular galls of pale blue-grey clay (5–7.5 x 1.3 cm) occur along one bedding plane and probably represent concretions. TRWH

At Gilfach-lwyd [SH 9214 6325] most of the Lower Mottled Mudstone, here estimated to be 18 m thick, is exposed in the approach road to the farm and in the farmyard, and a specimen ((E33898)) from here forms part of the material from which the rock type has been described (p. 46 et seq.). EGS

In the stream north-west of Ochr-y-cefn an exposure [SH 9161 6258] shows 4.5 m of mottled mudstone. The underlying ribbon-banded mudstones have yielded (RK 8569–8571) crinoid debris and Pristiograptus aff. jaegeri in the stream bank [SH 9163 6266] south-south-east of Blaen-y-weirglodd, indicating that the mottled mudstone belongs to the Upper Mottled Mudstone.

The Hendre-Aled area is unique, both in the region and the district, in showing a sequence that includes two leaves to the Upper Mottled Mudstone. An old quarry [SH 9270 6275] north-northwest of the farm lies in strata immediately below the lowest mottled mudstone of the area. It shows about 6 m of ribbon-banded silty mudstone with graptolites, viz: Pristiograptus jaegeri and P. aff. curtus, which clearly indicate the M. ludensis Zone and hence an horizon above the Lower Mottled Mudstone. Mapping shows that the overlying sequence contains two distinct mottled mudstone units, part of the lower of which is seen in an old quarry [SH 9284 6268] north-northwest of Hendre-Aled, where 2 m of massive mottled mudstone with goethite veinlets, shells and characteristic mottling occur. The base of the upper leaf is seen in the track section [SH 9261 6240] west of the farm, where about 3.5 m of mottled mudstone overlie at least 1.8 m of ribbon-banded silty mudstone with relatively few laminated muddy siltstone bands. This leaf is again seen in the west bank of the Afon Aled, south-west of the farm, where it immediately underlies disturbed beds.

East of the Afon Aled, across the Hendre Aled Fault, the sequence in equivalent measures is quite different. It comprises the Lower and Upper Mottled Mudstones, poorly exposed and rather atypically developed with some disturbance, and intervening measures consisting almost entirely of disturbed beds, composed in part of ribbon-banded mudstone. These are exposed in the track at Chwibren-isaf [SH 9339 6253] and in the yard at Chwibren-uchaf [SH 9359 6259].

East of the Afon Hyrdd in the vicinity of Pont-y-Nant the Lower Mottled Mudstone is some 85 m thick. A section at the base of the formation, in an old quarry [SH 9427 6442] east of the bridge, shows 3.6 m of mottled mudstone with numerous calcareous siltstone stripes, some showing disturbance on a minute scale, with interbedded ribbon-banded mudstone at the base.

At Fedw Uchaf, a quarry [SH 9451 6441] in the uppermost part of the formation shows 1.5 m of mottled mudstone, which passes downwards in the basal 0.6 m to ribbon-banded silty mudstone. The latter, 2.4 m thick, contains abundant Gothograptus nassa and Pristiograptus pseudodubius at one horizon, together with Plectograptus?, P. aff. pseudodubius [straight form] and an indeterminate retiolitid with affinities to G. nassa, a fauna indicative of the G. nassa Zone. Such interbedded ribbon-banded mudstones may explain the unusual thickness of the formation in this area.

Farther south around Cae Gronw numerous small exposures in both Mottled Mudstones and in the intervening ribbon-banded mudstones are seen. A quarry [SH 9500 6397] at Brynadar, north-north-west of Eithinfferwd, exposes 7.6 m of mottled mudstone with calcareous siltstone stripes and sporadic small concretions, and yields a fauna (Zp 5191–5195) of Pristiograptus jaegeri and Pristiograptus sp.EGS, PTW

East of the Pont-y-Ddol Fault, along Nant y Fleiddiast, the measures are best exposed on the north-west face of Moel Grugoer above Fron, where they form a series of well marked features. The sequence and estimated thicknesses are:

A track-widening [SH 9587 6271] at Fron revealed 4.5 m of characteristic mottled mudstone, and farther north near Eithenfferwd the stream bank [SH 9568 6348] east-north-east of the farm shows 9 m of intensely cleaved mottled mudstone with bedding stripes, but with complete mixing of siltstone and silty mudstone in parts.

The base of the Lower Mottled Mudstone is seen in the old quarry [SH 9638 6298] near Clwt-grugoer farm (see above), where 3 m of pale brown mottled mudstone with shell fragments overlie 6.4 m of flaggy, ribbon-banded silty mudstone with numerous calcareous siltstone stripes and one horizon of poorly developed concretions 5.1 m from the base.

The disturbed bed in the Moel Grugoer sequence appears to die out south-eastwards. It is, however, present to the east at Cil-lidiart [SH 9700 6269], north of Plas Panton, where a quarry at the farm shows:

In the same area, a scarp [SH 9704 6255] north-north-east of Ylas Panton, shows a sequence ascribed to the Upper Mottled Mudstone:

Graptolites are recorded from the measures between the base of the Lower Mottled Mudstone and the top of the Upper Mottled Mudstone are those characteristic of the C. lundgreni, G. nassa and M. ludensis zones. The most notable localities are: a roadside exposure [SH 9119 6426] south of Moel Wilym, which yielded (RK 7643–7655) Glassia sp., Mezounia?, Visbyella cf. trewerna, Hemsiella sp.[common], crinoid columnals, Gothograptus nassa and P. pseudodubius—a fauna assigned to the G. nassa Zone; the disused quarry [SH 9266 6339] east-north-east of Gilfach-lwyd, from which the same graptolites were recovered together with P. jaegeri and Pristiograptus sp.(RS 6710–6721); the old quarry [SH 9270 6275] north-north-west of Hendre-Aled, where P. cf. curtus and P. jaegeri were recorded (RS 6957–6984), indicating the M. ludensis Zone; and the old quarry [SH 9499 6354] at Cae Gronw, which also yielded an M. ludensis Zone fauna (RK 8391–8417), here comprising M. cf. ludensis, P. cf. dubius, P. jaegeri, P. aff. jaegeri, P. aff. curtus and P. aff. pseudodubius.

Measures above the Upper Mottled Mudstone, including the Brynsylldy Formation

These measures are reasonably well exposed by virtue of the feature-forming elements they contain. The sequence consists of ribbon-banded mudstones with one, or rarely two, disturbed beds, and averages 106 m in thickness.

In the Hafod-y-gog–Plas Madoc area 137 m of ribbon-banded mudstones occur and are overlain by a disturbed bed assigned to the Upper Nantglyn Flags Group (see below). PTW

To the south at Beidiog Isaf [SH 9207 6437], west of Pentrebeidiog, 3 m of disturbed strata are seen, and what is believed to be the same bed crops out in the stream at Nant Melae Fawr. This bed, varying in thickness up to 21 m is considered to represent the Brynsylldy Formation. PTW, TRWH

In the Cleiriach and Bryn areas in the Aled valley disturbed beds are seen in sections up to 3 m high on the west bank of the river, as well as in numerous small exposures. They are interpreted as belonging to a disturbed unit 40 or more metres thick. This is overlain by some 45 m of strata seen for example in the stream section [SH 9282 6187] south-east of Cleiriach, where they consist of massive silty mudstones with thin sandy bands and some ribbon-banding.

Graptolites (Zp 5183–5188) from these strata have been obtained from a quarry [SH 9486 6133] south-west of Cefn-Fforest and are assigned to Monograptus sp.[of ludensis type]. On this basis the strata are referred to the Lower Nantglyn Flags Group (M. ludensis Zone). Hence the disturbed unit, although at the same horizon as strata elsewhere included in the Brynsylldy Formation, is not believed to represent the same period of slumping. This disturbed unit has a downcutting base and rests on the Upper Mottled Mudstone around Hendre-Aled and Chwibren-uchaf. The outcrop configuration suggests a north-east–south-west channel with local north-easterly downcutting.

The same disturbed formation crops out around Cae Gronw and is exposed in small crags and a road section north-east of the farm. The finest exposure, however, occurs in the gorge of the Afon Hyrdd immediately south of Pen-cae'r-cwm, where it is 30 m thick and is overlain and underlain, without any evidence of a transgressive base, by ribbon-banded mudstones. PTW, EGS

Gorge sections [SH 9597 6156] in the headwaters of the Nant y Fleiddiast, show the same disturbed bed to be only 6 m thick. To the south-west, however, it appears to be in two leaves, each about 4.5 m thick. The overlying beds are seen in the old quarry [SH 9566 6092] east of Tan-y-rhiwiau and consist of ribbon-banded flags with lenticular calcareous siltstone stripes and a graptolite fauna (RK 235–253) indicative of the M. ludensis Zone.

In the Plas Panton–Moel Erydd area there is a similar sequence of ribbon-banded flags with the disturbed bed varying from 3 to 7.5 m in thickness. An old quarry [SH 9712 6211] south-south-east of Plas Panton shows:

A similar disturbed bed section, also with a uniform base, is visible in an old quarry [SH 9750 6258] south-south-west of St Thomas's Church, Bylchau.

Graptolites recorded from a large number of sections in ribbon-banded mudstone are those characteristic of the M. ludensis Zone.

Notable localities are the roadside quarry [SH 9203 6231] at the southern end of Coed Ffridd-newydd, from which was obtained a shelly and graptolite fauna (RK 8436–8511) comprising: coral (solitary), 'Syringopora' bifurcata, Glassia sp., Leangella?, Strophochonetes sp., Visbyella cf. trewerna, Butovicella?, Poleumita cf. globosa, 'Orthoceras' sp., Ananaspis cf. stokesii, Calymene sp., Dalmanites caudatus [common], Hemsiella sp., Lepidocoleus sp., crinoid columnals, M. ludensis, P. cf. dubius, P. jaegeri and P. sp. nov. aff. jaegeri; the gorge section [SH 9594 6158] of Afon-uchaf, east-south-east of Cefn-Fforest, which yielded (RK 352–370) M. ludensis, P. jaegeri, P. sp. nov. aff. jaegeri; and an old quarry [SH 9713 6211] south-south-east of Plas Panton, from which were recovered (RV 1786–1806) the same forms together with indeterminate material which may include forms close to M. uncinatus orbatus and if so indicate an horizon at the very top of the M. ludensis Zone. PTW

Upper Nantglyn Flags Group

The thick sequence of the Upper Nantglyn Flags of the Gwytherin region (p. 84) is apparently maintained in the area north of Nant Melae Fawr, although strike faults may exist in the Cornwal area, obscured by drift. Elsewhere the sequence varies from about 182 m in the southern part of the region to some 250 m in the Afon-uchaf area. The sequence consists primarily of ribbon-banded mudstones with sporadic concretions and at least one impersistent disturbed bed. The latter is locally shelly, for example in a bank [SH 8999 6676] west-north-west of Hafod-y-gog, where it has yielded (Zp 4719–4721) a solitary coral, Aegiria grayi and Hemsiella sp.The ribbon-banded mudstones are rich in graptolites characteristic of the Lower N. nilssoni Zone. The quarry [SH 9073 6601] west-south-west of Cefny-groes-fawr yielded (RK 7668–7687, Zp 4715–4718) Monograptus colonus, M. cf. colonus compactus, Monograptus sp. nov.[aff. orbatus], Pristiograptus cf. curtus, Saetograptus varians varians; from the stream section [SH 8982 6211] south-south-east of Brynhafod M. colonus cf. colonus, N. nilssoni, P. bohemicus bohemicus, S. varians cf. pumilus and S. cf. varians varians were recovered (Zp 5054–5072); the old quarry [SH 9096 6190] near Ty-nant yielded (RK 8546–8558) M. uncinatus orbatus, M. sp. nov. [aff. orbatus]and P. cf. dubius; a bank [SH 9183 6543] east-north-east of Plas Madog yielded (Zp 4753–4771) excellent material of M. uncinatus orbatus, N. nilssoni and Spinograptus spinosus. Other comparable faunas were recorded from the quarry [SH 9046 6329](Zp 5053) at Cornwal Bach; an old quarry [SH 9080 6037] (RK 7496–7516, Zp 6092–6093) west of Rhwyngyddwyffordd; the gorge section [SH 9162 6000] to [SH 9170 6020] downstream of the Aled Isaf Reservoir Dam (RK 7336–7404); a track exposure [SH 9027 6764] south-east of Melai (Zp 2497–2501); the stream section [SH 9848 6191] to [SH 9821 6171] north-west of Bryn-llwyd (RV 1809–1819, RV 1857–1881). A number of sections are of particular interest.

The northern bank of Nant Melai [SH 9068 6682], east-northeast of Hafod-y-gog, shows 3.6 m of striped and ribbon-banded mudstone overlying 2.4 m of disturbed silty mudstone which forms the upper part of a 45-m disturbed unit. The latter also crops out around Cefn-y-groes-fawr, where it is up to 64 m thick, but it thins rapidly and disappears to the south. Although the outcrop is complicated by faulting it is evident that the disturbed bed is largely underlain by strata belonging to the Lower N. nilssoni Zone and hence to the Upper Nantglyn Flags Group, as in the old quarry [SH 9073 6601] south-west of the farm (see above). A quarry [SH 9077 6625] south-east of Hafod-y-gog shows, at its eastern end, 1.8 m of disturbed sandy striped mudstones overlying 7.6 m of undisturbed ribbon-banded silty mudstone with sporadic yellow-brown irregular calcareous and sandy laminae and bands. The base of the disturbed bed cuts down some 9 m towards the west and the underlying beds are distorted. These ribbon-banded mudstones, in contrast, yield a fauna (Zp 4704–4714, RK 7656–7667) of Monograptus ludensis, M. sp. nov.[aff. orbatus]and P. cf. dubius, indicating the uppermost M. ludensis Zone and hence showing that the strata belong to the Lower Nantglyn Flags. TRWH, PTW

In the Bryn-cnap area several thin disturbed beds occur, but it is uncertain whether any one correlates with the major unit described above.

Likewise, there is no certain correlation with the disturbed bed of the Cornwal–Brynhafod area which is well exposed in the quarry [SH 9046 6329] at Cornwal Bach, where the section shows:

A disused quarry [SH 9009 6366] north-west of Cornwal-ganol shows up to 3 m of typical ribbon-banded mudstone, forming flaggy beds, with sporadic concretions 7.5 to 12.5 cm across, which show well the effects of compression on the adjoining mudstone (see p. 45). In a nearby section [SH 9022 6365] the concretions are up to 38 x 10 cm, flattened parallel to the bedding, and show limonite weathering and manganese wad (see p. 45). TRWH, PTW, EGS

Perhaps the finest exposures in the Upper Nantglyn Flags of the whole district are those in the Afon Aled gorge north of Rhaeadr y Bedd. The cliff sections, up to 45 m high, show ribbon-banded mudstones, varying from thin-bedded through flaggy to massive, with considerable variation in the development of calcareous siltstones and concretions and in the thickness of individual bands.

Exposures to the east are poor. One thin disturbed bed up to 10 m thick occurs in the Afon-uchaf area and its conformable base is seen in an old quarry [SH 9633 6145] north-north-west of the farm. PTW

Elwy Group

This Group comprises disturbed beds, composed predominantly of silty mudstone, with rare thin striped silty mudstone bands.

Faunas from Creigiau Quarry (see below), from a tributary of the Afon Aled (see below), and from a roadside section [SH 9235 6570] north-east of Clwt-y-ddafod-ddu (RK 8311–8318), comprise Pristiograptus cf. dubius, and Saetograptus varians variansand are, by comparison with those from other regions, suggestive of an Upper N. nilssoni Zone age for the group in this region.

The outcrop west of Llansannan is largely drift-covered, although sections in disturbed beds overlying ribbon-banded silty mudstones of the Upper Nantglyn Flags Group are seen in Nant y Plwm, between Llansannan and Bron-y-llan.

The Pencraig-fawr outcrop is exposed in numerous small crags north of the farm and spectacularly in Creigiau Quarry [SH 9381 6537], south-east of Llansannan.

The section in the quarry is:

It has been referred to by Boswell (1949, p. 245) and Warren and others (1970, p. 53, fig. 1). The fauna (A) from the disturbed bed (RS 6817–6857) comprises: 'Syringopora' bifurcata, bryozoa, Aegiria grayi, Amphistrophia funiculata, Atrypa reticularis, Dicoelosia biloba, Gypidula sp., Hyattidina sp., Isorthis clivosa, Leptostrophia?, Pholidostrophia sp., Protochonetes minimus?, Shagamella sp., Sphaerirhynchia aff. wilsoni, Loxonema sp., Gomphoceras sp., 'Orthoceras' cf. recticinctum, 'O.' cf. tenuiannulatum, tentaculitid, proetid (fragment), beyrichiid and crinoid columnals. The 'gingerbread' horizon (B) yielded a poor fauna (RS 6858–6866); the thin disturbed bed (F) a shelly fauna (RS 6900–6918) comparable to the foregoing; and other fossiliferous horizons (C, D, E, G, H) yielded (RS 6867–6899, 6919–6931) some of the above shelly fauna and in addition Cardiola interrupta and Loxonema cf. pseudofasciatum, together with Monograptus cf. colonus colonus, Pristiograptus bohemicus bohemicus, P. comis, P. curtus and P. dubius. To the right of the quarry entrance a shelly fauna (RS 6932–6948, RK 6168–6170) comparable to that listed but with Craniops implicatus, Leptaena depressa, Protochonetes ceratoides, Plectonotus trilobatus, Calymene sp. and Monoclimacis? ultima was recovered. An isolated outcrop of disturbed beds also occurs in the east bank of the Aled, 0.4 km to the south. EGS

Numerous small crag exposures occur north of Rhwngyddwyffordd, and farther west on Moel Goch a thin undisturbed striped silty mudstone band is exposed. Such undisturbed strata represent a significant proportion of the lower part of the succession in this southern area. In the east bank tributary of the Aled at Nant Merddyn the sequence is as follows:

The three upper divisions are all interpreted as belonging to the Elwy Group although the only faunal evidence (RK 7436–7447) is from part [SH 9268 6100] to [SH 9260 6111] of the stream section, and, comprising N. nilssoni and S. cf. varians, is inconclusive. A similar situation pertains on Moel Bengam, where crags [SH 9285 5965] show 12 m of disturbed strata, with apparently undisturbed striped silty mudstones higher in the sequence. In contrast, between Moel Bengam and Rhos Pen-llwyd poor crag exposures are apparently of disturbed beds, and one fault-scarp section [SH 9303 6019] shows 3 m of disturbed, cleaved silty mudstone overlying 1.8 m of silty ribbon-banded flags with numerous lenticular and load-casted calcareous siltstone/fine sandstone bands up to 5 cm thick. These beds are probably correlatives of those between the disturbed strata in the tributary of the Afon Aled. PTW

X Denbigh Moors region

This region comprises the moorland and afforested ground (Clocaenog Forest) on the northern flank of Mynydd Hiraethog. It includes the high ground of Moel y Bryniau, Gwylfa Hiraethog, Cerrig Caws, Cefn Brenig, Llech Daniel and Tir Mostyn, and li,es at the headwaters of the Aled, Alwen, Brenig, Clywedog and Concwest, forming the watershed between the Clwyd and Dee drainage systems.

The stratigraphical succession extends from the base of the Cyrtograptus rigidus Zone or an even lower horizon to the lower part of the Upper Neodiversograptus nilssoni Zone (Elwy Group). Drift cover is extensive and exposures are generally restricted to streams and to small quarries and crags in feature-forming horizons such as sandstones in the Denbigh Grits, the Llanddoget Formation and the Mottled Mudstones.

Denbigh Grits Group

The main outcrop extends from Cefn Mawr to the Taipellaf- Tany-graig area, and there are isolated outcrops along the southern margin of the region at Cerrig Caws and near Pont Ty'n-y-ddôl. The group is assumed to consist predominantly of striped silty mudstones, but there are few exposures not in sandstone or disturbed beds.

On Cefn Mawr sandstones are exposed in a number of small crags. One [SH 9063 5714] shows 3 m of massive sandstone, another [SH 9069 5723] 1.8 m of medium-grained sandstone resting on highly cleaved silty mudstone, possibly disturbed. The sequence in this area is believed to correlate with sandstones in the lower part of the C. rigidus Zone in the adjoining Gwytherin Region (p. 83).

At Taipellaf crags show up to 3 m of highly cleaved (pencil slate) silty mudstone, which are thought to form part of the 130-m thick Llanddoget Formation. Similar beds are seen south of Cerrig Caws along the track to Pont y Brenig, where the Llanddoget Formation is less than 70 m thick.

The small outcrop shown on the map near Pont Ty'n-y-ddôl is entirely drift covered, but on the evidence of exposures in the Hafodyronnen area in the Corwen district consists of a disturbed bed unit about 22 m thick, again believed to be the Llanddoget Formation.

No faunal evidence of the age of the Denbigh Grits Group sequence has been obtained from this region.

Lower Nantglyn Flags Group: beds below the Lower Mottled Mudstone

These beds crop out around Moel y Gaseg-wen and Llyn Aled, and thence through Cefn Tan-y-graig, the Sportsman's Arms and Cefn Brenig to the vicinity of Hafoty Siôn Llwyd. Much of the outcrop is drift covered and there are few exposures, but in sporadic stream and quarry sections the sequence is seen to consist entirely of ribbon-banded mudstones containing graptolites characteristic of the Cyrtograptus lundgreni Zone. Notable localities are: a quarry [SH 9116 5807] west-north-west of Llyn Aled Reservoir Dam, from where a solitary coral, Monograptus flemingii flemingii, M. flemingii var.

Monograptus sp.[flemingii lorbatus transient] and Pristiograptus cf. pseudodubius have been recovered (Zp 6094–6110); a scarp [SH 9081 5758] north-east of Ty'n-llyn, which yielded (Zp 6129–6130) Cyrtograptus cf. hamatus; a quarry [SH 9704 5673] north-west of Pont Ty'n-y-ddôl, which yielded (RV 2172–2184) cf. Monoclimacis flumendosae, Monograptus flemingii flemingii and cf. Pristiograptus lodenicensis; and the stream bank [SH 9758 5613] west-southwest of Hafoty Siôn Llwyd, from where 'Orthoceras' sp. and C. cf. lundgreni (Zp 6693–6695) were recovered. Comparable faunas came from a gully section [SH 9225 5683] west of Pont-y-clogwyn, a scarp [SH 9843 5693] north-west of Hafoty Siôn Llwyd, and an old quarry [SH 9468 5870] south-west of the Sportsman's Arms (see below). A total of some 12 m of these ribbon-banded mudstones are visible in the Afon Brenig [SH 9688 5677] north-west of Pont Ty'n-y-ddôl, and a nearly continuous section in about 30 m of beds is exposed [SH 9768 5622] in the tributary stream, Afon Fechan, north-east of the bridge.

Lower Nantglyn Flags Group: The Lower Mottled Mudstone and overlying beds

The outcrop of these beds extends from the Moel y Bryniau area in the north-west to around Pont y Clogwyn, and from near Hafod Dafydd south-eastwards to Llech Daniel. In the Hafoty Siôn Llwyd area in particular the beds form a substantial feature and are largely drift-free. Nevertheless, exposures are restricted to sporadic quarries, short sections of stream, and poor sections on the moors and in the forest.

The sequence is basically the same as in other regions, namely 18 to 42 m of Lower Mottled Mudstone and 50 to 150 m of Upper Mottled Mudstone separated by 15 to 39 m of ribbon-banded mudstone and overlain by up to 94 m of the same rock type. In some areas one or more disturbed beds also occur.

Graptolites have been recorded from both ribbon-banded and mottled mudstones and are those characteristic of the uppermost part of the C. lundgreni Zone, the Gothograptus nassa Zone and the Monograptus ludensis Zone. Faunas belonging to the uppermost C. lundgreni Zone were recovered from localities in the Lower Mottled Mudstone or immediately overlying beds. These include a crag [SH 9407 5925] south of Hafod Dafydd, which yielded (RK 9701–9704) Cyrtograptus cf. hamatus and M. flemingii, and a crag [SH 9918 5633] east of Hen Ddinbych, from which was recovered (Zp 6390–6391, RV 2299–2306) Giraldiella sp., Orthoceras sp.A and Pristiograptus cf. pseudodubius. A probable G. nassa Zone fauna (Zp 6143–6150) of G. nassa and P. cf. dubius was recorded from a bank [SH 9235 5731] west-north-west of Ty-isaf, and the same species, with in addition 'Orthoceras' aff. recticinctum, M. cf. ludensis, P. jaegeri and P. aff. jaegeri, was obtained from the stream section [SH 9059 5885] in Nant Bach and are assigned to the basal M. ludensis Zone. Other M. ludensis Zone faunas were recorded from stream and bank exposures [SH 9188 5872] in Nant-y-Foel-frêch, which yielded (RK 9718–9732) ostracods, M. cf. ludensis, P. cf. dubius, P. jaegeri and P. aff. jaegeri; the stream section [SH 9297 5740] north of Pont-y-Clogwyn, from where M. ludensis and P. jaegeri were recovered (RK 9552–9577); and a quarry [SH 9622 5958] at the northern end of Llyn-Bran, which yielded (RK 9530–9535) cf. Scyphocrinites? pulcher,, P. dubius and P. aff. curtus. PTW, HEW

In the north-west of the region a section in Nant Bach (see above) shows 5.4 m of ribbon-banded silty mudstone with some decalcified 'gingerbread' bands overlying 1.8 m of Lower Mottled Mudstone. Downstream [SH 9076 5886] the section in the Upper Mottled Mudstone is:

On Moel y Bryniau crags up to 7.6 m high expose irregularly cleaved, shelly siltstone and silty mudstone—a disturbed unit believed to form part of the Brynsylldy Formation. This latter is exposed in Nant y Foel-frêch [SH 9196 5873], immediately north of Cesyg Aled, in a 9-m high gorge and is here estimated to be 12 m thick. A 1.8-m disturbed unit also occurs some 30 m below the base of the Brynsylldy Formation.

Between Llyn Aled and Pont-y-Clogwyn the lower part of the beds is well exposed. The old roadside quarry [SH 9264 5655] south-west of the bridge shows 6 m of Lower Mottled Mudstone consisting of pale grey, mottled mudstone with thin pale yellow calcareous siltstone stripes. The stream at and to the north of Pont-y-Clogwyn shows the following seaqence (thickness estimated):

This is one of only two places in the entire district where the Upper Mottled Mudstone rests on a disturbed bed. However, in the Hafod Dafydd–Bryn Trillyn area of the present region a disturbed bed occurs between the mottled mudstone formations at a slightly lower level. It is well exposed in crags [NH 9427 5947] north-west of Gwylfa Hiraethog, where it is estimated to be about 15 m thick. The bed dies out southwards in the Bryn Trillyn area and thins to 6 m in the Hafod Dafydd area to the north. The mottled mudstones of this area are well seen in an old quarry [SH 9468 5870] west-south-west of the Sportsman's Arms, where the section at the base of the Lower Mottled Mudstone is:

The locality yields a C. lundgreni Zone fauna (Zp 6111–6119) of M. flemingii cf. flemingii, Monograptus sp. [aff flemingii elegans, aff. orbatus], P. pseudodubius and P. sp. nov. aff. jaegeri. HEW

There are sections to 1.8 m in the Upper Mottled Mudstone in the old quarry [SH 9476 5902] at Gwylfa Hiraethog and in a roadside quarry [SH 9513 5881] south-south-west of the Sportsman's Arms. A quarry [SH 9571 5927] east-north-east of the Sportsman's Arms shows a 2.4-m section in the Lower Mottled Mudstone. The highest beds in the Lower Nantglyn Flags are seen in the old quarry at the north end of Llyn Brân (see above), where 6 m of ribbon-banded silty mudstones with one prominent horizon of concretions occur, and in the crags east of the lake [SH 9645 5930], where up to 3 m of a disturbed bed, estimated to be 7.6 m thick, are seen. This bed dies out at the southern end of the lake, but is believed to thicken northwards and be the same bed as that found in the adjacent Llansannan Region, east and south-east of Cefn-Fforest (p. 89). This disturbed bed is thought to lie approximately 30 m below the base of the Upper Nantglyn Flags Group.

A scarp [SH 9868 5675] behind Hafoty Siôn Llwyd shows 3.6 m of Lower Mottled Mudstone consisting of brown-weathering, fractured, mottled mudstone with abundant goethite veinlets and a few siltstone stripes, and further sections up to 6 m thick are seen at Hen Ddinbych. The higher beds in this vicinity are poorly exposed.

Upper Nantglyn Flags Group

Strata of this group crop out around the Aled Isaf Reservoir and between Llyn Brân and Diffwys on the eastern boundary of the region. The sequence, approximately 180 m thick in both areas, consists essentially of ribbon-banded mudstones with calcareous concretions and a rich graptolite fauna characteristic of the Lower N. nilssoni Zone. PTW, HEW

Notable localities are quarries [SH 9040 6000] to [SH 9066 6002] at Waen Isaf Las, from where Monograptus uncinatus orbatus and N. nilssoni (RK 7561–7566) were collected; an old quarry [SH 9685 5939] north west of Pant-y-maen, which yielded (RV 2691–2701) M. cf. colonus, M. uncinatus orbatus, Saetograptus cf. varians and P. cf. dubius; and stream exposures [SH 9862 5906] south-south-west of Hafod-Caradoc, from which a M. ludensis/M. colonus transient, M. cf. uncinatus orbatus, Monograptus sp. and N. nilssoni were recorded (RK 9616–9628). Other localities from which comparable faunas, with in addition S. varians cf. pumilus and M. colonus compactus, were collected are the exposure [SJ 0025 5745] in Aber Waen Llydan (RV 2185–2225), the section [SJ 0175 5726] in Ceunant Diffwys (RV 2791–2810) and the stream section [SJ 0235 5699] south of Diffwys (RV 2860–2883). PTW

In the Aled Isaf area the poorly exposed sequence contains two impersistent disturbed beds, each up to 6 m thick. Sections in these are seen on the east side of Llys Dymper [SH 9013 5946], on Cefn Du [NH 9309 5871], and in a stream on the south side of Cesyg Aled. At the last locality 3 m of disturbed bed are seen above 0.9 m of ribbon-banded silty mudstone in one place [SH 9236 5831], and a little farther downstream [SH 9218 5829] a 1.8-m disturbed bed lies within ribbon-banded mudstones at a lower stratigraphical level. HEW

The ribbon-banded mudstones of this outcrop are well seen in sections up to 12 m in the stream south-west of Rhwyngyddyffordd. One locality [SH 9052 6003], at Waen Isaf Las, is of interest in showing calcareous siltstone bands, with 6-mm deep load-casts in 7.6 m of ribbon-banded silty mudstone. Concretions, approximately 23 cm x 23 cm x 7.5 cm, occur in bands at about 1 m intervals and illustrate well the effect of differential compaction compared with the adjacent flags (p. 45). PTW

In the eastern outcrop the measures are well exposed in quarries south-east of Gorsedd Brân, the best being the Aber Quarries [NH 977 593], in which up to 9 m of ribbon-banded mudstones with bands of concretions are seen. The stream at and to the south of Hafod-Caradoc reveals a continuous section of about 60 m of the same rock type. South-east of Tir Mostyn in the headwaters (Ceunant Diffwys) of the Afon Clywedog between Rhyd Galed and Diffwys a continuous section reveals about 45 m of the same lithology overlying 30 m of beds with relatively few laminated muddy siltstone bands and hence composed essentially of homogeneous silty mudstone. In the Afon Terfyn [SH 0188 5698], to the south, a section exposes over 30 m of ribbon-banded silty mudstone. HEW

Elwy Group

Rocks of this group crop out immediately west of the Groes Fault between Brynbach and Diffwys, and in a small area south of Diffwys. Up to 90 m of strata are probably present, consisting of striped silty mudstones with two impersistent disturbed beds. The disturbed bed at the base of the succession is seen in crags [SJ 0245 5715], [SJ 0250 5721] south and south-south-east of Diffwys, where up to 3.6 m of an estimated 9 m of disturbed silty mudstone occur.

In the stream (Nant Selator) flowing north-westwards through Brynbach, steep banks [SJ 017 583] show a local succession interpreted as:

Graptolites have been recorded only from a quarry [SJ 0084 5793] south of Hafotty-las (Zp 6202–6205, RK 7915–7937); they were identified as Monoclimacis? cf. ultimus, Monograptus?, Pristiograptus?, and Saetograptus spp., possibly including S. leintwardinensis incipiens, thus confirming the Elwy Group age of these strata (Upper N. nilssoni Zone or younger). The basal boundary of the group is largely conjectural in this region and is based on the facies change from dominantly ribbon-banded to striped silty mudstones. HEW,PTW

XI Llannefydd region

This region includes the high ground of Moelfre Isaf, Mynydd y Gaer and Fron Fawr and the Elwy Valley between Pont y Gwyddel and the Carboniferous outcrop in the east. Silurian strata at outcrop belong entirely to the Elwy Group and are largely, if not entirely, of Cucullograptus scanicus Zone age. Exposure is good on the higher ground, but elsewhere there is extensive drift cover.

The region is heavily faulted (Smith and others, 1965, fig. 4) and local successions are not readily correlated. Nevertheless a sandstone horizon, well exposed in several areas, can be widely traced.

The most complete succession in the region can be established in the Llannefydd area, between the summit of Mynydd y Gaer, northwest of the village, and Pen-y-bryn Farm south-west of the village.

The sequence is as follows:

Elements of these six divisions are seen in a number of crag and quarry sections. The upper sandstone unit of (1) is well exposed in an old quarry [SH 9748 7177] east of the summit of Mynydd y Gaer, where 10.5 m of sandstone with silty mudstone bands are seen.

Some siltstones in these beds show sole structures, namely prod-casts, bounce-casts and interference ripple-casts. Other sole structures have been recorded: from a roadside section [SH 9710 7335] south-west of Nant-Meifod, where flute-casts occur in a sandstone in a 3-m sequence of striped beds; in a quarry [SH 9801 7178] northwest of Pentre-isaf, in the northern part of which flute-casts occur on a 1.2-m sandstone in a 5-m interbedded sequence of fine-grained sandstones and thin silty mudstones; an exposure [SH 9777 7035] at Pen-y-bryn, where sporadic graded, fine-grained sandstones up to 22 cm thick show flute-casts; and in an old quarry [SH 9515 7378] north-north-west of Tynewydd, where linear-load-casts are seen. In every case the sole structures indicate currents flowing from the west.

The disturbed beds forming division (2) are well exposed in numerous fault scarps, crags and small quarries on the southern face of Mynydd y Gaer. The old quarry [SH 9738 7138] near Bronhwylfa, west-south-west of Pentre-isaf, has yielded a rich fauna (RS 7239–7326) of fragmentary but otherwise well preserved shelly fossils and graptolites comprising: Favositid, bryozoa, Aegiria grayi, Atrypa reticularis, Craniops implicatus, Howellella elegans, Isorthis clivosa, I. aff. clivosa, I. cf. orbicularis, Leptaena depressa, Leptostrophi afilosa, Microsphaeridiorhynchus nucula, Protochonetes ceratoides [common], Protochonetes minimus, Sphaerirhynchia aff. wilsoni, Cardiola interrupta, Actinoceratid?, Phragmoceras [fragment], beyrichiid, phacopid, crinoid columnals, Cucullograptus scanicus, Monograptus sp., Pristiograptus vicinus and Saetograptus chimaera chimaera. This fauna is indicative of the C. scanicus Zone, and the same age is assigned to the fauna (Zp 2375–2409, RS 7044–7045, 7052–7092) from the disused quarry [SH 9686 7180] on the western side of Mynydd y Gaer, which comprises: 'Orthoceras'fimbriatum? [fragment], P. aff. tumescens, P. cf. comis, P. vicinus [abundant] and Saetograptus sp.

The striped beds of division (3) occur south-east of the Dolwen Reservoir and north-north-westwards to the Plas-uchaf Reservoir. In the steep, wooded bluff [SH 9711 7086] north-east of Blaen-yweirglodd up to 5 m of laminated grey siltstone with silty mudstone and thin sandstone bands are seen. A quarry [SH 9709 7110] on the Llannefydd—Pont y Gwyddel road, east of the bridge over Plas-uchaf Reservoir, shows 2.4 m of grey silty mudstone with thin (0–6 mm) yellow sandy bands at 1.2 to 2.5-cm intervals. The striped beds apparently die out eastwards, and the numerous small crags in the fields west of Llannefydd reveal a continuous succession in disturbed beds, mainly highly cleaved silty mudstones, forming division (4) and the upper part of division (2).

The striped beds of division (5) are well exposed in an old quarry [SH 9783 7045] north-north-east of Pen-y-bryn, where 9 m of silty mudstone and siltstone with laminated muddy siltstone and thin, current-bedded, sandy siltstone bands up to 3.7 cm thick occur. The disturbed beds forming division (6) are seen in a small outcrop [SH 9787 7037] east-north-east of Pen-y-bryn.

West of the Llannefydd Fault, in the area between Fron Fawr and the River Elwy, a series of fault-bounded sequences of interbedded striped silty mudstones and disturbed beds with sporadic sandstones are believed to be equivalent to parts of the foregoing succession. Palaeontological evidence, though not unequivocal, certainly suggests a C. scanicus Zone age for the sequences: a bank [SH 9641 7117] east-south-east of Fron-fawr farm yielded (Zp 2305–2308) Monoclimacis? cf. ultima, Pristiograptus aff. welchae and Saetograptus clunensis; a fault scarp [SH 9591 7186] north-east of the farm yielded (Zp 2277–2299) ?C. scanicus, P. bohemicus bohemicus and Saetograptus sp.; a crag [SH 9565 7119] south-south-east of the farm yielded (Zp 2316–2319) C. scanicus and ?P. bohemicus; and from a fault scarp [SH 9562 7096] (see below) P. bohemicus bohemicus, S. chimaera cf. chimaera and S. cf. clunensis were recovered (RK 515–526).

A notable sandstone band forms the wooded bluff [SH 9580 7215] south of Plasisaf, the maximum thickness seen being 4.5 m. It crops out from near Pont y Gwyddel as far east as Coed Melin Caerau [SH 9650 7210], south of Ty'n-y-bedw, and is believed to lie at approximately the same stratigraphical level as the sandstones forming the summit of Mynydd y Gaer. It is overlain by some 240 m of striped beds alternating with sandstones, silty mudstones and disturbed beds, extending as far south as Pen-y-coed.

Fron Fawr is composed of a similar alternation of rock types which may correlate with divisions (3) and (4) of the Llannefydd area. The Fron Fawr disturbed beds are, however, markedly sandier. A prominent scarp [SH 9562 7096] on the west face of the hill shows a characteristic section:

Moelfre Isaf and the adjacent high ground to the east are predominantly formed of sandstone, although there are disturbed beds with abundant sandstone bands and 'nodules' interbedded with and overlain by striped beds, to the north-east of Tynewydd. Owing to structural complexity, no generalised succession can be established. The striped beds have yielded graptolites, notably from an old quarry [SH 9598 7281] east of Tynewydd, where C. scanicus, P. bohemicus bohemicus, P. aff. tumescens and P. vicinus were found (RK 474–514, Zp 2320–2336), and in the quarry [SH 9511 7280] north of Caemarch, where C. scanicus, ?Pristiograptus sp. and S. chimaera chimaera were recorded (Zp 2360–2374). These records indicate a C. scanicus Zone age for the strata, and it is therefore believed that the sandstones in this area lie at approximately the same stratigraphical level as those on Mynydd y Gaer. The Moelfre Isaf sandstone unit is estimated to be up to 60 m thick, although the maximum visible thickness is 10 m seen in an old quarry [SH 9552 7344] north of Tynewydd.

Comparison with the adjacent Denbigh Region (p. 98) suggests that the mainly drift-covered area in the south-east of the Llannefydd Region may be underlain by Elwy Group rocks of S. incipiens Zone age. Faunas recovered from exposures at Myfoniog are consistent with an horizon of this or upper C. scanicus Zone age. A disturbed bed in the northern bank of the stream [SH 9908 7177] south-south-west of the farm yielded a shelly fauna (RS 7123–7218) very similar to that from the old quarry [SH 9738 7138] on the southern side of Mynydd y Gaer (see above), but lacking L. depressa and P. ceratoi des and including Pholidostrophia sp., Protochonetes ludloviensis, Modiolopsis gradatus, Bernbexia lloydii, Loxonema sp., 'Orthoceras' cf. mocktreense and 'O.' aff. recticinctum. The abundant and variable Isorthis specimens from this locality probably represent only one species with a wide range of morphology. A nearby exposure [SH 9901 7174] in the southern bank of the stream shows 4.5 m of striped beds and yielded a graptolite fauna (RS 7093–7122) of Mcl. ?cf. ultima, M. cf. colonus, P. dubius, P. cf. frequens, S. cf. leintwardinensis primus and S. varians varians, a somewhat anomalous assemblage (see (Figure 13)). A nearby section [SH 9918 7175] yielded (Zp 3007–3012) a favositid, Isorthis aff. orbicularis, a nuculoid and crinoid columnals. A river cliff [SH 9859 7259] to [SH 9870 7257] on the northern side of the Elwy at Pont y Ddôl yielded an assemblage of fragmentary shelly fossils (RS 7219–7238) similar to that recorded from Myfoniog, but poorer and including Dayia navicula, which is rare in the C. scanicus Zone and more characteristic of the S. incipiens Zone.

XII Bylchau region

This region includes the high ground of Mynydd-y-Gyrt, Moel Fodiar, Mynydd Tryfan, Moel Tywysog and Foel Greon, together with the lower ground of the Nant y Terfyn and Afon Deunant in the Aled catchment. A complete succession from the Upper Nantglyn Flags Group to the top of the Cucullograptus scanicus Zone in the Elwy Group is present. It comprises ribbon-banded mudstones overlain by striped silty mudstones and disturbed beds with subordinate sandstones. The rocks are well exposed on the higher ground in numerous crags and small quarries; elsewhere drift cover is extensive, and exposure is generally restricted to short sections of streams.

The region provides the best exposure in the district of the upper contact of a disturbed bed (p. 96 and (Plate 6)) and the most complete sections in the sandstones of the middle Elwy Group.

Upper Nantglyn Flags Group

These beds crop out in two separate areas—around Bryn-cnap and Afon Deunant, and between Wenallt and Tycerrig. They consist almost entirely of ribbon-banded mudstones with concretions, and up to 120 m are estimated to be present. Graptolites are common and are those typical of the Lower Neodiversograptus nilssoni Zone. Notable localities are: stream exposures [SH 9577 6625] north-west of Penglogor, which yielded (RK 4474–4491) Monograptus colonus compactus, Pristiograptus cf. comis and Saetograptus cf. varians varians; an old quarry [SH 9548 6521] north-north-east of Hwlffordd, from where Cardiola interrupta, M. colonus cf. colonus, N. nilssoni and P. cf. curtus were recovered (RK 8634–8639); and the disused quarry [SH 9637 6370] at Wenallt-isaf, which yielded (RK 408–425) 'Orthoceras' sp., M. uncinatus orbatus, N. nilssoni and P. cf. vicinus. The characteristic rock type is well seen in the old quarry [SH 9660 6355] at Wenallt-uchaf where 6.7 m of ribbon-banded mudstones with lenticular calcareous siltstone bands (up to 2.5 cm) and four concretion horizons are exposed. A similar section, with concretions up to 20 cm diameter at two horizons 2.1 m apart is seen in an old quarry [SH 9837 6359] north-east of Bylchau.

In the Afon Deunant area, an old quarry [SH 9522 6653] west-south-west of Heskin shows 3.6 m of ribbon-banded flags with sporadic concretions up to 10 cm diameter and a 2.5-cm band of current-bedded sandstone. The section lies almost immediately below the base of the Elwy Group. Other sections in flags are seen in the streams at and to the north-west of Penglogor. A scarp at Pont Deunant [SH 9611 6528] shows 6 m of a probable 9-m disturbed bed, consisting of ribbon-banded mudstone with small recumbent folds, overlying 1.8 m of undisturbed flags.

Elwy Group

Except in the areas outlined above the region is underlain by Elwy Group rocks, which on faunal evidence (see below) belong to the Upper N. nilssoni and C. scanicus zones. This limited range, along with the structural complexity of the region, renders correlation difficult and three separate successions are recognised—in the Foel Greon–Bryn-mawr, Plasisaf (Bryn Rhyd-yr-Arian)- Mynydd Tryfan and Moel Fodiar areas.

The succession in the Foel Greon area is:

The disturbed bed in unit 1 is seen at the roadside at Bylchau and in the old quarry [SH 9718 6318] west of the village. The overlying strata, seen in the quarry [SH 9774 6312] at Bylchau Rectory and an old quarry [SH 9722 6330] to the north-west, are ribbon-banded and striped mudstones with thin sandstone bands and horizons of small concretions. At the former locality these beds have yielded (RK 296–331) Monograptus uncinatus orbatus, P. curtus, P. aff. dubius, S. cf. chimaera chimaera and S. cf. varians, and are here assigned to the Upper N. nilssoni Zone.

The disturbed beds of unit 2 are exposed in numerous crags on Foel Greon, where they consist of silty mudstone, highly and irregularly cleaved and jointed, commonly with spheroidal weathering and containing a variable proportion of sandstone, present as stringers, balls and irregular inclusions. In the area around Bryn-du, east of Tan-y-fron, sandstone forms a high proportion of the disturbed beds, and interbedded, undisturbed sandstones up to 0.6 m thick occur.

Units 1 and 2 thicken to about 200 m when traced westwards to the Tan-y-fron area, and are there composed entirely of disturbed beds, which have probably cut down into the Upper Nantglyn Flags.

Beds of unit 3 are seen at Brynrhedyn; in the old quarry [SH 9801 6339] north-east of St Thomas's Church, where they include laminated, sandy siltstone bands up to 0.3 m thick and a 'gingerbread' horizon, and in the quarry [SH 9810 6496] at Brynmawr, where bands of fine to medium-grained sandstone up to 15 cm thick are recorded.

In the quarry [SH 9755 6403] at Brynrhedyn about 4.5 m of silty mudstone, graptolitic shales and spheroidally weathering siltstones are exposed. ?S. varians has been recorded (Zp 5360–5364).

Numerous crags in the Brynmawr–Tryfan area reveal sections in units 4 to 8. Unit 4 appears to correlate with the sequence of sandstones, silty mudstones and disturbed beds at Bryn-bod. Traced eastwards across Moel Tywysog, units 5 to 8 show a notable increase in the proportion of striped silty mudstone.

The succession in the ground between the Heskin–Coedorros area and the summit of Mynydd Tryfan is as follows:

The basal disturbed beds are seen in numerous crags west of Heskin, and the top of unit 1 is exposed in a scarp [SH 9591 6668] east of the farm, where the overlying sandstone is seen resting on truncated folds in the disturbed bed (Plate 6), the section being:

Disturbed beds of unit 3 are exposed in sections up to 10 m thick in the eastern bank of Nant y Terfyn and around Bwrdd Arthur. They vary from highly cleaved silty mudstone to sandstone. Units 3 and 4 are exposed in an old quarry [SH 9641 6666] north-north-east of Coedorros, where the section is:

Flute-casts and upper-surface ripple-marks are recorded from other sandstones in the region. An old quarry [SH 9730 6694] north of Arllwyd shows 4.5 m of predominantly spheroidally weathered siltstone, massive at the base, but in thin (7 to 15 cm) graded beds, which show asymmetrical flute-casts at the top. The quarry has yielded (RK 4446–4450) C. scanicus and P. comis. Flute-casts are also seen: in a scarp [SH 9649 6809] north-east of Plasisaf; an exposure [SH 9713 6751] south of Minffordd, where the sandstones also show load-casts, current-bedding and convolute lamination; the stream bank [SH 9610 6691] east of Tynyfedw; and an exposure [SH 9640 6879] east-south-east of Beniar. Upper-surface ripple-marks are seen on laminated, cross-bedded sandstones overlying a sandy disturbed bed and exposed in a scarp [SH 9691 6768] south-south-west of Minffordd. In every case the sedimentary structures indicate a generally westerly origin for the sandstones (p.36).

Sections up to 10.6 m thick in sandy disturbed beds of unit 5 are seen on the ridge between Bwrdd Arthur and Mynydd Tryfan, where there are also numerous small crags exposing the overlying units.

The sandstone units, together with structural interpretation of the area immediately east of the Aled Valley, enable a correlation to be made between the foregoing succession and the sequences on Mynydd-y-Gyrt and Pengwern. The middle sandstone unit crops out near the summit of Pengwern and is well exposed at three localities north-west of Pengwern Farm [SH 9640 6879], [SH 9665 6783] and [SH 9678 6768]. Up to 3 m of part-laminated and current-bedded fine-grained sandstones are seen resting on the undulatory upper surface of pencil slate disturbed beds correlated with unit 3. Pencil slate is also seen in a 12-m exposure [SH 9690 6749] west-north-west of Pengwern Farm. The disturbed beds, however, are predominantly sandy and there is some suggestion in the area of individual disturbed beds up to 30 m thick which are graded from sandstone to pencil slate. Sandstones up to 2 m thick lower in the succession, seen for example in a scarp [SH 9630 6770] east-north-east of Plasisaf, where they exhibit folds and are evidently disturbed, may correlate with the lower sandstone unit of the Mynydd Tryfan sequence.

The middle sandstone unit crops out again in the Beniar area, where it is about 30 m thick. The underlying disturbed beds are again of pencil slate and sandstone, one section [SH 9626 6836] at Cae'r-groes consisting of 10 m of contorted sandstone. Some 3 m of graded, fine to medium-grained sandstones with silty mudstone bands at Fron-haul [SH 9614 6918] may represent the lower sandstone unit.

Sandstones are also exposed on Mynydd-y-Gyrt at, for example, an old quarry [SH 9646 6970] south-south-east of Tan-y-gyrt and in small quarries [SH 9648 6969] south-south-west of Pen-y-bryn. The latter locality yielded a comparable, but less varied shelly fauna (RK 4401–4444) to that collected from the quarry on the eastern side of Moel Fodiar (see below), but including Protochonetes ceratoides, Ptychopteria sowerbii and 'Orthoceras' dimidiatum, and a graptolite fauna (RK 4384–4400, Zp 2968–3001) comprising P. bohemicus bohemicus, P. cf. tumescens, P. vicinus, S. chimaera salweyi and S. cf. varians, indicative of the C. scanicus Zone. It is believed that the sandstones on Mynydd y Gyrt correlate with the upper sandstone unit of Mynydd Tryfan.

Correlation of the two major successions in the region is based solely on the supposed equivalence of the main sandstone of the Foel Greon sequence with the upper sandstone unit of Mynydd Tryfan. If correct, it implies a rapid thinning of the Upper N. nilssoni Zone strata from 457 to 260 m between the Tany-y-fron and Heskin areas. This could reasonably be accounted for by a south-westerly trending trough of disturbed beds in the Tan-y-fron area (see above). Correlation of the Mynydd Tryfan succession to the west is not possible even in general terms.

Even within the present region correlation east of the Llanefydd Fault (Figure 24), which separates Pengwern from Moel Fodiar and Mynydd Tryfan from Moel Tywysog, is tentative. The succession around Moel Fodiar is:

The highest beds of the upper group of striped silty mudstones are exposed in an old quarry [SH 9832 6795] on the eastern side of Moel Fodiar, where the section is:

The disturbed bed yielded a rich and well preserved shelly fauna (RK 4214–4246, RU 3410–3484, Zp 3042–3049), one of the best in the whole district and the sole locality for several forms. This fauna comprises: favositid and bryozoa, Amphistrophia funiculata, Atrypa reticularis, Craniops implicatus, Dayia navicula, Homoeospira baylei, Howellella cf. elegans, Hyattidina sp., Isorthis cf. clivosa, I. aff. orbicularis, Leptaena depressa, Leptostrophia filosa, Microsphaeridiorhynchus nucula, Morinorhynchus sp., Pholidostrophiid, Protathyri s sp., Protochonetes ludloviensis, P. minimus, Salopina sp., Shagamella minor, Skenidioides?, Sphaerirhynchia aff. wilsoni, stropheodontid, Cardiola interrupta, 'Ctenodonta' sp., Cypricardinia subplanulata, Nuculites sp., Ptychopteria (Actinopteria) sp., Liospira sp., Loxonema sp., Plectonotus trilobatus, 'Orthoceras' cf. tenuiannulatum, Ceratotheca adunca, Calymene sp., Dalmanites myops, Hemiarges cf. bucklandi, amphitoxotids, 'Botha' cf. unillexa [cruminate], primitiids, Lepidocoleus sp. and crinoid columnals.

The quarry also yielded a graptolite fauna (RK 4247–4264) of M. aff. uncinatus, M. cf. uncinatus orbatus, S. cf. leintwardinensis incipiens, S. varians pumilus and S. varians cf. varians.

On the western side of Moel Fodiar, the old quarry [SH 9759 6821] south-west of Foel shows an alternation of graded, fine-grained sandstones, siltstones and silty mudstones. A comparable shelly fauna (RK 4278–4291) to that given above, and in which Gypidula sp. and Whitfieldella sp.are present and L. filosa is very common, was collected. The graptolite fauna recorded (RK 4265–4277, RK 4292–4295, Zp 3064–3092) comprises Monoclimacis cf. ultima [of Wood], Monograptus colonus cf. colonus, M. cf. uncinatus, P. cf. praeultimus, P. aff. tumescens, P. vicinus and S. cf. varians.

On structural considerations the lower disturbed unit may be equivalent to that associated with the upper sandstone unit on Mynydd-y-Gyrt, and the upper disturbed unit would appear to correlate with that at the summit of Moel Tywysog, and here lie above the upper sandstone unit of Mynydd Tryfan. This conclusion is supported by the foregoing palaeontological evidence.

The remainder of the region—mainly between Cefn Berain and the northern slope of Moel Tywysog—is poorly exposed and is probably largely underlain by striped silty mudstones with only thin disturbed beds. The rocks are probably similar to the upper Elwy Group strata (Bont-uchel Formation) of the Bettws-yn-Rhos (p.78), Denbigh (p.99) and Gyffylliog regions (pp.100–101), which are assigned to the S. leintwardinensis incipiens Zone. However, in the absence of direct palaeontological evidence and on general structural grounds the present strata are assigned to the upper C. scanicus Zone.

The only significant exposures in these rocks are at Waen Tywysog [SH 9945 6617], south of Bronfelen, where a disturbed bed up to 2 m thick overlies 3 m of striped silty mudstones, and in an old quarry [SH 9941 6749] at Fron, east-south-east of Foel, where the succession is:

At the latter locality a shelly fauna (RK 4349–4361) comparable to that from Moel Fodiar, but also including Aegiria grayi and Leperdita sp., was collected, together with a graptolite fauna (Zp 3129–3144) comprising Monograptus?, S. chimaera salweyi and Spinograptus?.

XIII Nantglyn region

This region is centred on the villages of Nantglyn and Waen and includes the headwaters of the Afon Ystrad, the high moorland area around Gorsedd Brân and the relatively high ground at Moel Fechan, Foel Gasyth and Moel Prion. It is highly faulted and the succession, which ranges from near the base of the Upper Nantglyn Flags Group (Lower Neodiversograptus nilssoni Zone) to the upper Elwy Group (Saetograptus incipiens Zone), is probably incomplete. With the exception of the Gorsedd Brân area, almost the whole southern half of the region is drift covered, as are the Rhydgaled area and the northern margin of the region. The high ground, mainly composed of disturbed beds of the Elwy Group, has numerous small crags and a number of quarry sections. PTW

The region is of interest in including the type locality of the Nantglyn Flags—the Nantglyn Quarries 3.3 km south-west of Nantglyn (see below).

Upper Nantglyn Flags Group

These beds crop out over much of the southern half of the region, and some 152 m of strata are present. Almost everywhere they consist of ribbon-banded mudstones, splitting into flags, with sporadic concretion horizons and a rich fauna of graptolites and orthocone nautiloids indicative of the Lower N. nilssoni Zone.

At the type locality [SH 9790 5980] south-west of Wern Uchaf, up to 10 m of flaggy ribbon-banded mudstones with rare concretions are seen. A fauna [RK 9578–9615] of Cardiola interrupta, 'Orthoceras' sp., Monograptus colonus colonus, N. nilssoni, Pristiograptus bohemicus, Pristiograptus? sp. and Saetograptus cf. varians varians was recorded. Between here and Nantglyn village numerous sections up to 9 m thick are seen in streams and quarries. Nearly all show the characteristic rock type, but sections in the Bryn-llwyd area are in part composed of striped silty mudstones. A number of quarry sections in the south of the region are of particular interest. HEW, PTW

A roadside quarry [SH 9661 6054] on the north-west side of Gorsedd Brân, in addition to showing 11.5 m of typical Nantglyn Flags, is almost unique in showing thin calcareous siltstones, 5 m thick, which truncate the underlying bands of laminated muddy siltstone and silty mudstone, with a maximum observed down-cut of 7.5 cm. The down-cut surface dips at about 30° towards 150°, suggesting a cut-and-fill structure caused by a north-eastward flowing current. The cut is compensated for by a variable thickness of silty mudstone overlying the calcareous siltstones. The section also exhibits a tectonic-ripple horizon (p.144). The abundant fauna recorded (RK 146–234) comprises 'Orthoceras' sp., M. colonus colonus, M. colonus compactus, N. nilssoni, S. varians cf. pumilus and S. varians varians.

In the ribbon-banded mudstones exposed in an old quarry [SH 9863 6102] west of Gwaen-y-mywion the few, poor specimens of graptolites are preserved in pyrite.

Farther north at Waen, an old quarry [SH 9963 6257] shows 10 m of typical ribbon-banded mudstone with an impersistent horizon of concretions, forming the highest beds in the group. The strata have yielded (RK 8783–8796), M. uncinatus orbatus and P. dubius. East of Nantglyn, in the river bank [SJ 0075 6183] north-north-east of Felin Hên, beds at approximately the same stratigraphical level consist of interbedded striped and ribbon-banded silty mudstones.

Other notable fossiliferous localities are: the stream bank [SH 9857 6277] north-east of Aberdeunant-isaf, from where M. cf. colonus colonus, N. nilssoni and S. cf. varians were recorded; the bank [SJ 0027 6248] north-north-west of Nantglyn, which yielded a similar graptolite fauna (Zp 5396–5407) together with Dawsonoceras annulatum, Scyphocrinites? pulcher and fragments of the latter's supposed bulbous Camarocrinus'root (RV 1519–1521, 1523–1524); the stream section [SJ 0087 6053] north-north-west of Tynyffynon, from which were recorded (RV 1709–1725) M. cf. uncinatus orbatus, P. bohemicus and cf. S. chimaera salweyi; the quarry [SH 9957 5896] south of Tynewydd, which yielded (Zp 6206–6212) N. nilssoni and S. varians cf. pumilus; and an old quarry [SH 9814 6015] west of Wern Uchaf, where M. cf. colonus compactus, N. nilssoni, S. varians cf. pumilus and S. varians varians were collected (RK 267–292, Zp 6175–6176).

Elwy Group

Numerous small crags on the western slope and summit of Gorsedd Brân are of disturbed silty mudstone, and an estimated 143 m are present. On the eastern side of the hill the disturbed beds are underlain by striped silty mudstones, and a section [SH 9799 6015] west of Wern Uchaf shows 1.5 m of such beds with bands of siltstone and fine-grained, cross-bedded sandstone. These undisturbed strata total some 75 m and apparently represent that amount of northwesterly downcutting by the base of disturbed beds.

A similar relationship between Elwy Group disturbed beds and underlying strata is evident in the Nantglyn—Cader area. In an old quarry [SJ 0079 6187] south-east of Nantglyn, striped silty mudstones with S. cf. chimaera salweyi, indicative of the Elwy Group, rest on mudstones of the Upper Nantglyn Flags Group. At Ty-Angharad, north-west of Tynffynon, the latter are immediately overlain by disturbed beds. South of Ty-Angharad, however, the disturbed beds apparently die out; west of Hafod Ty Ddu the Elwy Group comprises striped silty mudstones with only a single thin disturbed bed. These strata are seen in scattered poor exposures, for example in a quarry [SJ 0117 5916] east-south-east of Gareg-lywd, from where P. cf. dubius and Pristiograptus sp.[close to P. vicinus]have been recorded (Zp 6192–6197).

North and west of Nantglyn and Waen the basal Elwy Group strata consist of 15 to 128 m of striped silty mudstones with, notably at Coed yr Erwi [SJ 002 627], thin sandstone bands. Higher in the succession are disturbed beds, forming the main mass of Moel Fechan and the high ground east of Pentre. They consist predominantly of silty mudstones with sandstone balls, lenses and bands and are up to about 90 m thick. Separated from these beds by a faulted outcrop of more than 60 m of striped silty mudstones, exposed in the stream at Rhydgaled, is a second unit of disturbed beds. As exposed in the area between Rhydgaled and Groes, for example in 10-m crags [SJ 003 645] west-south-west of Groes chapel, these consist of sandy and silty mudstones with a near-vertical raft composed of 5.4 m of fine-grained, partly laminated, poorly graded sandstones alternating with silty mudstones. A sandstone band within overlying striped silty mudstones in this area is a possible correlative of the upper sandstone unit of Mynydd Tryfan in the Bylchau region (p. 95).

The outcrops east of the Groes Fault are interpreted as belonging largely to the middle part of the Elwy Group. They are known to underlie beds of the upper part (S. incipiens Zone) in this and the Denbigh Region (p.98), but correlation between the faulted outcrops extending from the Tan-y-gyrt area to Moel Prion and southwards to Bryn Goleu is speculative.

On Y Gyrt, west of the Afon Ystrad and south-west of Tan-y-gyrt, and in the high ground east of the valley and between Segrwyd and Cornych [SJ 013 622] up to 106 m of disturbed cleaved silty mudstones with balls and sporadic thin bands of sandstone are exposed in numerous crags.

They are underlain by up to 75 m of striped silty mudstones, exposed, for example, in the stream [SJ 0097 6349] to [SJ 0078 6355] north-west of Tan-y-gyrt, where they yielded (RV 1558–1581) P. vicinus. In the stream [SJ 0128 6283] west-north-west of Bryn Hyfryd the section is:

East of a north—south fault through Bryn Hyfryd, in the area between Segrwyd and Bron-haul [SJ 0245 6189] on the south side of Foel Gasyth, the following tentative succession has been deduced:

The units are exposed in numerous small crags, road sections and quarries, five of which deserve special mention. Beds of unit 10 are exposed in a road section [SJ 0277 6272] north-north-west of Foel and in the streams to the east. The road section, which shows 1 m of disturbed strata with a more or less conformable base on 1.2 m of massive fine-grained sandstone, yielded (Zp 5494–5517, RV 1620–1632) 'Orthoceras' sp., Monoclimacis?, P. aff. vicinus, S. chimaera salweyi, S. chimaera cf. chimaera and S. cf. clunensis. This fauna suggests that unit 10 belongs to the basal S. incipiens Zone, and thus by implication the rest of the succession is of Cucullograptus scanicus Zone and possibly older ages. The majority of other faunas recorded, here and in the Y Gyrt—Moel Prion area in general, are near monotypic assemblages of P. vicinus with P. cf. tumescens. The most notable locality is the old quarry [SJ 0240 6191] at Bron-haul, referred by Boswell (1949, p. 54) to the P. tumescens Zone. The quarry exposes about 4.5 m of striped silty mudstone with numerous white to pale yellow sandy bands of graptolitic shale, belonging to unit 2 above. The beds yielded (PW 17–54, Zp 5458–5493) P. cf. corn's [short form], P. vicinus, Pristiograptus sp.aff. tumescens and Saetograptus?. Other localities are the stream section [SJ 0106 6305] east of Parc-y-maes (Zp 5366–5376), the stream section [SJ 0097 6200] to [SJ 0103 6207] east of Nantglyn (RV.1552–1557), and an old quarry [SJ 0405 6160] west-south-west of Pant-pastynog (RV 1699–1709, Zp 5535–5551). All these assemblages are from beds believed to be no younger than the C. scanicus Zone.

An old quarry [SJ 0186 6232] south of Bryn Hyfryd shows the following section of unit 7:

The underlying disturbed beds (unit 5) are seen in an old quarry [SJ 0187 6210] to the south, where a 9-m section consists predominantly of spheroidal-weathering siltstone, but contains fine-grained sandstone in rolls, lenses and bands. The rolls trend 130–310° and plunge about 18° to the north-west. Disturbed beds of unit 3 are also sandstone-rich, one crag section of pencil slate [SJ 0222 6239] east-south-east of Bryn Hyfryd showing a raft 1.5 m thick and at least 3 m across of fine-grained sandstone, which is laminated at the top, convoluted in the middle part and massive in the basal 1 m.

In the Ty'n-twll area, south of Foel Gasyth and the Dyffryn Maelor Fault, a gorge [SJ 0168 6150] 21 m deep, shows up to 15 m of disturbed silly mudstones, sandy at the top, with a raft, 2.4 by 4.5 m, of striped beds with sandstone bands. Sandstone also occurs in an old quarry [SJ 0198 6128] west-north-west of Cefn-maen-uchaf, where the section is:

The sandstone unit here is possibly the correlative of the upper sandstone (unit 7) on Foel Gasyth, and in an adjacent quarry [SJ 0202 6123] the disturbed bed has yielded a shelly fauna (Zp 5425–5439, 5445–5456) comprising solitary corals, 'Syringopora' bifurcata, bryzoan, Atrypa reticularis, Howellella sp., Isorthis clivosa, Leptaena depressa and Loxonema sp.

In the area between Foel Gasyth and Pant-pastynog, and notably on Moel Prion, numerous crags expose predominantly sandstone-rich disturbed beds with interbedded impersistent units of striped silty mudstones. In a fault scarp [SJ 0459 6178] west-south-west of St James' Church, Pant-pastynog, 3 m of striped silty mudstones with fine-grained sandstones up to 30 cm thick overlie 7.6 m of disturbed beds with sandstone rolls.

XIV Denbigh region

This region lies to the south and west of Henllan and Denbigh, between Cefn Berain and Bont-newydd in the north-west and Pont Ystrad in the south-east. The stratigraphical succession, although exceeding 820 m in thickness, consists entirely of rocks of the upper part of the Elwy Group, ranging in age from upper Cucullograptus scanicus Zone to Saetograptus incipiens Zone. The beds of the latter zone alone are more than 600 m thick.

The entire region is heavily drift covered. The only notable exposures—in the form of small crags and roadside quarries—are in the Cefn Berain area, around Llechryd and Hafod Wood, and in the area south-east of Groes. Elsewhere exposure is restricted to isolated quarries and short stream sections.

Elwy Group

These strata, with the exception of sporadic and generally thin disturbed beds, consist of striped silty mudstones with numerous thin fine-grained sandstone bands and local developments of ribbon-banded silty mudstone. They have yielded both graptolite and shclly faunas, notable localities being: the stream section [SJ 0131 6860] east-north-east of Hafod, which yielded (Zp 3177–3186) Pristiograptus cf. tumescens minor and Saetograptus varians pumilus, a road bank [SJ 0133 6507] east-north-east of Groes Bach (Zp 5298–5313); the stream section [SJ 0353 6334] to [SJ 0385 6362] between College and Segrwyd Uchaf (RV 1633–1658); and the river bank [SJ 0351 6505] south of Gwaynynog (Zp 5342–5359). From the last three localities monotypic assemblages of a short variety of S. leintwardinensis incipiens were recorded. A section in the Afon Ystrad [SJ 0190 6396] to [SJ 0240 6408] upstream of Pandy Farm yielded (RV 1600–1619) Dayia sp., Cardiola interrupta, Leptodesma sp., orthocones and crinoid fragments, Bolbozoe cf. bohemica and Pristiograptus bohemicus [small form].

East of the Groes Fault the faunas, where unequivocal, indicate the S. incipiens Zone. In the area between Twywsog and Rhydgaled, west of the Groes Fault, faunal evidence is inconclusive, and on lithological grounds the beds are assigned to the upper C. scanicus Zone.

Significant sections in the striped silty mudstones are seen mainly in stream banks; several have yielded shelly faunas, for example a disturbed bed in the old quarry [SJ 0036 6832] at Hafod Farm yielded (RV 2461–2478) Atrypa reticularis, Dayia navicula [common], Howellella elegans, Isorthis aff. orbicularis, Shagamella minor [common] and Cardiola?. From overlying striped mudstones, fragments of most of these forms were recorded together with Lanceomyonia? (RV 2457–2460). In Hafod Dingle, north-east of the farm, an exposure [SJ 0065 6846] shows 10 m of thinly bedded silty mudstone with laminated fine-grained sandstone bands averaging 25 to 50 mm but occasionally reaching 230 mm. Similar strata are exposed downstream towards Coed, along with interbedded disturbed silty mudstones, seen for example upstream of the section [SJ 0150 6873] at Bont y Coed, north-west of Henllan, which yielded (RV 2412–2432, Zp 3170, 3172) Dayia?, Lanceomyonia?, Protochonetes minimus, Pterinea sp.[common], Cardiola interrupta and Odontopleura sp.Comparable strata are also seen in the area west and north of Llechryd, where one disturbed bed locally reaches a thickness of 76 m.

Farther south, around Fron-haul, striped silty mudstones are seen in the stream banks, as in the 1.8-m section [SJ 0141 6767] west-south-west of the farm, where laminated muddy siltstone bands, up to 12 mm thick, and fine-grained laminated and in part current-bedded sandstone bands up to 2.5 cm thick occur. There is one 'gingerbread' horizon with Atrypa sp.In the scarp [SJ 0165 6784] behind the farm similar beds with a basal sandstone band showing flute-casts rest on a shelly disturbed bed. Sporadic thin sandstone bands exposed throughout the region show flute-casts and linear load-casts, for example in the river cliff [SJ 0336 6506] north-west of Dr Johnson's Cottage, and in the stream bank [SJ 0395 6491] north-west of Segrwyd.

South-westwards, the supposed C. scanicus Zone strata around Tywysog are well seen in the stream banks and quarry at the farm and consist predominantly of silty mudstone, but contain sandy bands up to 2.5 cm, laminated muddy siltstone bands up to 1.2 cm and sporadic dark grey to black, laminated, very silty mudstone bands up to 30 cm. The siltstone bands, where common, are responsible for thicknesses of ribbon-banded mudstone.In the Eriviat Hall area to the south, The Dingle exposes similar beds with sporadic, spheroidally weathered siltstone. An exposure at the Hall [SJ 0132 6607] shows:

In the Groes area, sections in undisturbed strata in the stream banks around Nant-yr-hengoed are composed almost entirely of ribbon-banded mudstones, in places micaceous and everywhere rather siltier than the Nantglyn Flags. They contain a rich graptolite fauna indicative of the S. incipiens Zone. A section [SJ 0069 6439] yielded (RV 1453–1500) 'Orthoceras' sp., S. leintwardinensis incipiens and cf. S. leintwardinensis [form close to S. varians pumilus]. The road cutting [SJ 002 6467] at Groes shows:

The fauna (RK 1134–1177), which comprises P. cf. frequens, 'Monograptid' [slender curved fragments], S. leintwardinensis incipiens [short form] and S. varians cf. varians, is an almost monotypic assemblage of the zonal fossil S. incipiens.

In the Groes area, an old quarry [SJ 0132 6394] east-north-east of Bryn Robin shows:

Striped silty mudstones are exposed in numerous small sections in the Afon Ystrad below Segrwyd Mill [SJ 0172 6387], and sections up to 9 m thick occur in the north bank at and to the east of Dr Johnson's Cottage. One section [SJ 0336 6506] at the cottage shows 7.6 m of mudstone, predominantly very silty, which is spheroidally weathered and has bands, up to 5 cm thick, of laminated and convoluted fine-grained sandstone grading to siltstone. Linear load-casts, with an east—west trend, occur on one of the sandstone bands, and sporadic concretions (up to 5 cm in diameter) are present at the eastern end of the section. Similar beds, in which the sandstone bands reach 15 cm thick, are exposed in a 9-m section [SJ 0395 6491] at Dolyhyfryd. PTW

XV Gyffylliog region

This region includes most of the drainage area of the Afon Clywedog and its tributary the Nant Mawr, along with the high ground west and north of Gyffylliog, namely Bryn Ocyn, Foel Uchaf, Foel Ganol and Ffridd Fawr.

The succession, though incomplete due to faulting, comprises about 1200 m of strata ranging from the Upper Nantglyn Flags Group to the upper part of the Elwy Group (Saetograptus incipiens Zone). Drift cover is extensive in the north and in the main valleys. However, there are exposures in small quarries and short sections of stream, notably the Nant Mawr, and there are numerous small crags on the higher ground. A more or less continuous exposure of the higher strata present in the region is found in the Clywedog between Bont-uchel and the base of the Carboniferous rocks near Rhewl.

Upper Nantglyn Flags Group

Beds of this group crop out along the southern margin of the region in the entirely drift-covered Nant Ladur and in the partly drift-covered Pen-y-lan—Tai Ucha area. Several quarries in this latter area reveal ribbon-banded silty mudstones which have yielded graptolite faunas indicative of the Lower Neodiversograptus nilssoni Zone. Two of the more important localities are an old quarry [SJ 0344 5647] east of Tai Ucha, which yielded (RK 8064–8092, Zp 6396–6400) crinoid stems, Monograptus colonus colonus, M. cf. ludensis, M. sp. nov.[aff. orbatus], Saetograptus varians pumilus, S. cf. varians varians, cf. Spinograptus spinosus, and a roadside exposure [SJ 0285 5638] east of Tai Ucha, from which ?Lobograptus progenitor, M. cf. colonus colonus, S. varians pumilus and S. varians varians (RK 8020–8063) were recorded.

Elwy Group

Because of faulting there is nowhere a continuous section between this group and the Upper Nantglyn Flags Group. For convenience of description the outcrop is divided into three areas, two of which contain rocks assigned to the C. scanicus Zone and the third, rocks assigned to the S. incipiens Zone. HEW, EGS, PTW

The area west of the Gwaynynog Fault

The area west of the Gwaynynog Fault includes the ground around Pentre Saron, Moel Yta, and Bryn Ocyn, and is underlain largely by a sequence of striped silty mudstones with interbedded disturbed beds ranging in thickness from a metre or so to over 30 m. In the ground east of Pentre Saron the following local succession has been established:

Disturbed beds form the summits of Foel Uchaf and Bryn Ocyn, and approximately 60 m are present on the latter. Crags [SJ 0374 5859] north-north-west of Cernyfed show about 30 m of disturbed, cleaved silty mudstone with sporadic small concretions.

In the Afon Concwest, disturbed beds up to 6 m thick occur in a striped silty mudstone sequence. In one section[SJ 0410 5924] north-east of Tai-isaf a slipped mass of silty mudstone includes a 15-cm band or lens of calcareous conglomerate, with angular and rounded pebbles of silty mudstone set in a dark grey calcareous matrix. In the north of the area, in the Afon Clywedog and Afon Corris, disturbed beds are largely absent. HEW

At a number of horizons, throughout the succession described above, graptolites indicative of the Cucullograptus scanicus Zone have been recorded. The bank [SJ 0288 5777] and nearby forestry road section [SJ 0285 5775] south-west of Hafotty Bach have yielded (Zp 6271–6300, RK 7964–7987), C.? cf. crinitus, C. scanicus, cf. Monograptus colonus, Pristiograptus bohemicus [small form], P. comis, P. cf. vicinus, Saetograptus chimaera chimaera, S. chimaera salweyi and S. chimaera cf. semispinosus.

Similar faunas have been recorded from: the quarry [SJ 0249 5793] south-south-east of Brynbach (Zp 6222–6234), where cf. Scyphocrinites? pulcher was also collected; a track-side exposure [SJ 0444 5849] on the south-western side of Foel Uchaf (RK 7988–8002, Zp 6355–6360); the quarry [SJ 0248 5873] east of Brynbach (DT 5828–5859, Zp 6257–6270) where 'orthoceratids' and Pristiograptus cf. tumescens also occur; a track exposure [SJ 0454 5944] south-south-west of Porth (PW 55–63, Zp 6319–6353), which yielded an almost monotypic assemblage of P. vicinus and close or conspecific forms, an orthoceratid' and S. varians cf. varians; an old quarry [SJ 0360 5945] south-south-west of Meifod (RK 7758–7779), and the track exposure [SJ 0200 6000] north of Brynbach. PTW

There is a fault-bounded outcrop extending from Porth to Coed y Porth, east of Rhiwlas, which consists of approximately 120 m of disturbed beds, overlain and underlain by striped silty mudstones. The disturbed beds are exposed in numerous small crags around Porth. This isolated succession is interpreted as underlying the main succession of the area and, in the absence of palaeontological evidence, can be only tentatively assigned to the C. scanicus Zone (see below). HEW, PTW

The Gyffylliog area

The Gyffylliog area is bounded by the Gwaynynog, Clocaenog and Parc Postyn faults (Figure 24). In the north over 180 m of disturbed beds, overlying striped silty mudstones, crop out on Ffridd Fawr and again on the high ground immediately east of Nant-y-ffridd. The disturbed-bed sequence, which dips generally westwards but forms a shallow syncline on Ffridd Fawr itself, consists of cleaved silty mudstone with, so far as can be seen from exposures and loose blocks lying about on the mountain, quite a considerable proportion of sandstone. The sandstone occurs in contorted bands and as balls. In a small disused quarry [SJ 0537 6100] south-west of Llewesog Hall 4.5 m of striped silty mudstone rest on disturbed mudstone with shells. Thin interbedded units of undisturbed sediments also occur elsewhere. The whole disturbed bed sequence is believed to be that of the Porth area (see above) and on this correlation and the relatively high sandstone content is assigned to the C. scanicus Zone. EG S, HEW

South of the north-west trending fault through Carreg-y-gath a sequence of disturbed beds is exposed in numerous crags and scarps between Foel Ganol and Ysgeibion. This sequence overlies, and in part passes laterally into some 450 m of striped silty mudstones with sporadic thin disturbed beds. These mudstones are exposed in quarries, banks and the bed of the Afon Clywedog between Gyffylliog and The Mill, west of Bont-uchel. For example a quarry [SJ 0736 5774] east-south-east of Pen-rhiw-bach shows 12 m of striped silty mudstone with sandstone bands, some of which show slight disturbance. The outcrop of this sequence extends northwards to the vicinity of Ysgubor-bach and southwards to the southern margin of the region around Cae'r-hafod, with notable exposures in the stream immediately north of Ysguborisaf, where there is a nearly continuous section in about 90 m of striped, and in part ribbon-banded, silty mudstone with sporadic fine-grained sandstone bands.

From a number of small exposures typical of the outcrop, graptolite faunas have been recorded that suggest a C. scanicus Zone age: the river bed [SJ 0693 5760] west of Bont-uchel yielded P. cf. bohemicus and P. comis (DT 5773–5780); a track exposure [SJ 0587 5833] north of Gyffylliog yielded P. aff. comis, cf. S. chimaera, S. cf. clunensis and S. varians cf. pumilus (Zp 6310–6318); and from an old quarry [SJ 0642 5794] south-west of Pen-rhiw-bach P. comis and P. cf. vicinus were recorded. The age indicated by these faunas thus supports the evidence for a similar age for the major disturbed units of Porth and Ffridd Fawr (see above), with which the sequence is correlated.

A disturbed bed seen in a roadside exposure [SJ 0622 5612] near Bryn Llwyd yielded a shelly fauna (RK 7801–7847) of Atrypa reticularis [common], Howellella sp., Isorthis aff. clivosa and I. aff. orbicularis [both common], Leptaena sp. and Sphaerirhynchia aff. wilsoni; and the river section [SJ 0670 5756] east-south-east of Gyffylliog yielded (DT 5784–5785) Bolbozoe cf. bohemica. HEW, PTW

The Llanrhaiadr–Bont-uchel area

The Llanrhaiadr–Bont-uchel area lies along the northern and eastern margins of the region, and is underlain by a sequence of rocks at least 670 m thick. In the north, the oldest beds exposed are probably the striped silty mudstones revealed in the Nant Goch stream section [SJ 0821 6086] to [SJ 0867 6129] north-west of Tŷ-mawr.

In the old quarry [SJ 0837 6095] near the road bridge over the stream there are 9 m of nodular-weathering, striped silty mudstone with (DT 7110–7156) 'Orthoceras' sp., P. bohemicus, S. leintwardinensis incipiens, cf. S. varians pumilus and cf. S. varians varians—a fauna dominated by S. incipiens.

The best sections in the vicinity of Llanrhaiadr, however, are those in Nant Mawr and its tributary valleys, notably Nant Prŷf, east of Parc Postyn. East of Llewesog Hall, Nant Mawr roughly follows the axis of a westerly-plunging syncline. Repetition of the succession occurs because of the easterly downthrowing faults which cross the axis. The amounts of throw of these faults in the Silurian rocks are, however, unknown, and between 300 and 1250 m of strata may be present. Generally speaking, the lower part of the sequence consists of striped silty mudstones and the upper of ribbon-banded mudstones or of striped silty mudstones with ribbon-banding in parts. Ferruginous concretions are not uncommon in the striped silty mudstones, another feature of which is nodular weathering, sometimes with exfoliation, which may indicate incipient concretion formation. The striped silty mudstones also contain sandy bands, some of which show lamination or small-scale false bedding. Red laminae or bedding planes in striped silty mudstones have been noted at several localities [SJ 0626 6155]; [SJ 0719 6149]; [SJ 0737 6188]. These exposures are neither close to the sub-Basement Beds unconformity nor near major faults, and it is possible that the reddening is primary.

Shelly fossils and graptolites have been collected from several localities along and near Nant Mawr and from the tributary valleys. Both shelly and graptolite assemblages are fairly restricted, in some instances monotypic, and indicate an S. incipiens Zone age. The best exposure for shells is the 4.3 m of ribbon-banded silty mudstone with sandy bands (showing east-west oriented flute-casts) seen in a lane section [SJ 0661 6222] near Pen-y-cae, west-south-west of Pen-y-bryn. The fauna (Zp 5573–5586, DT 7493–7527) comprises Dayia navicula, Lanceomyonia?, Microsphaeridiorhynchus nucula, Shagamella sp., Cardiola interrupta, 'Orthoceras'undulocinctum and Saetograptus leintwardinensis incipiens. A similar shelly fauna (DT 7527–7529) was also recorded from a 'gingerbread' horizon in a section [SJ 0724 6178] south-east of Pen-y-cae Farm, where S. cf. leintwardinensis incipiens was also recorded (DT 7530–7531). The more notable graptolite localities, however, are: the quarry [SJ 0464 6187] near St James' Church, Pont-pastynog (RV 1671–1686); a scarp [SJ 0544 6227] north-west of Croesgota Chapel (DT 7194–7221)—Cardiola interrupta, P. bohemicus, P. aff. dubius ludlowensis and S. leintwardinensis incipiens); an old quarry [SJ 0560 6181] west-north-west of Llewesog Hall (DT 7656–7662)—P. tumescens, P. cf. vicinus and S. cf. leintwardinensis incipiens; an exposure [SJ 0638 6189] in Coed-y-Plas Wood, east-north-east of the Hall (DT 7436–7464, ZP 5560–5566)Cardiola interrupta, orthocone, P. vicinus and S. leintwardinensis incipiens; the lane section [SJ 0710 6200] east-south-east of Pen-y-Cae Farm (DT 7465–7492)—Cardiola interrupta, pterineid, orthocone, S. cf. clunensis and S. cf. leintwardinensis incipiens; the stream section [SJ 0713 6146] west of Pen y Waen, which yielded (DT 7559–7610) a monotypic assemblage of S. clunensis and a shelly fauna (DT 7611–7618) comprising Dayia navicula, Pterinea sp., orthocones [indet.]. The stream section [SJ 0747 6188] in Coed Nant Mawr, north-north-east of Pen y Waen, yielded a mixed fauna (DT 7535–7545, Zp 5587–5591) of Leptodesma sp., Ptychopteria (s.l.) sp., Pterinea sp. and S. leintwardinensis incipiens. Finally, mention should be made of the oldest fauna in the Nant Mawr area, which comes from a section [SJ 0814 6226] east-south-east of Pen-y-bryn; it comprises an orthoceratid' and Monoclimacis micropoma (Zp 5592–5594) and may be of uppermost C. scanicus Zone age.

A nearby bank section [SJ 0719 6149] yielded a shelly fauna (Zp 5595–5597) comprising Dayia navicula, Lanceomyonia?, Protochonetes minimus, Cardiola interrupta, Ptychopteria sp. and 'Orthoceras' dimidiatum. EGS

Southwards, these upper Elwy Group strata are exposed in small crags and quarries only in the Maesannod area and to the south of Bont-uchel. However, in the Afon Clywedog between Bont-uchel and the eastern margin of the region, near Pen-tor, an almost continuous, though faulted, sequence of beds is seen, and this is the type locality of the Bont-uchel Formation. They consist predominantly of striped silty mudstones with disturbed beds at intervals. The latter are generally thin (up to 10 m), but one reaches 38 m. It is exposed in a scarp [SJ 0915 5914] at Tany-y-bryn, where 4.5 m of silty mudstone and sandstone with numerous small slump folds are seen. HEW

At the eastern end of the Clywedog section and continuing beyond the district boundary the beds are of a mixed ribbon-banded and striped silty mudstone facies, sporadically rich in oriented graptolites indicative of the S. incipiens Zone.

Monotypic assemblages of S. leintwardinensis incipiens, with in places Cardiola interrupta and S. varians cf. pumilus, have been recorded from the stream [SJ 0997 5943] east of Pant Glas Isaf (DT 5506–5526), and the stream sections [SJ 0980 5922], [SJ 0944 5903], [SJ 0930 5885] farther west (DT 5555–5577, DT 5623–5650 and DT 5668–5679 respectively).

A roadside exposure [SJ 0966 5917] south-east of Pant Glas Isaf yielded a mixed fauna (DT 5596–5622) comprising Athyrid [ribbed], Dayia navicula, Lanceomyonia?, Microsphaeridiorhynchus nucula, Shagamella sp., Cardiola interrupta, 'Orthoceras'dimidiatum, P. cf. vicinus and S. leintwardinensis incipiens. Similar graptolite faunas have also been recorded farther west, for example in a bank [SJ 0871 5879] west-south-west of Pen-tor (Zp 6432–6450), and in the vicinity of Bont-uchel, where the road cutting [SJ 0809 5782] west of the bridge (DT 5752–5763) and a small cliff [SJ 0888 5849] north-east of the village also yielded (DT 5699–5731) Dayia navicula, Shagamella? and crinoid columnals [pentagonal]. Notable lithological sections in the neighbourhood of Bont-uchel are the old quarry [SJ 0835 5778] west of the chapel, where 24 m of striped silty mudstone with thin hematite-stained, partly decalcified, shelly bands occur, and the river, where about 90 m of striped silty mudstones with five disturbed beds, 1.8 to 6 m thick, are exposed. HEW, PTW

XVI Clwydian Range region

Separated from the main outcrop of Silurian rocks by the Carboniferous and Triassic rocks of the Vale of Clwyd is the inlier of the Clwydian Range, part of which, between Cwm and Bodfari, falls within the present district and forms this region.

A complete, or almost complete, 1675-m succession, ranging from the Denbigh Grits Group (Cyrtograptus rigidus Zone) to the lower Elwy Group (Upper Neodiversograptus nilssoni Zone), crops out in the region. In general it is reasonably well exposed, but there is considerable drift cover along the western margin of the region, at and to the south-east of Cwm at the junction with the overlying Carboniferous, and along the eastern margin, east of Rhuallt.

Wenlock rocks were first recognised in the Clwydian Range during the present survey (Summ. Prog. Geol. Surv. for 1964); and the region is of further interest because the Elwy Group succession, over 450 m thick, is virtually devoid of disturbed beds.

Denbigh Grits Group

Rocks of this group crop out between Rhuallt and Cwm, forming the high ground of Mynydd y Cwm. They comprise approximately 420 m of highly cleaved blue-grey and dark grey mudstones with subordinate siltstones, sandstones and one or more disturbed horizons. Exposures are generally poor and sporadic because of intensive afforestation.

Along the Forestry Commission track, from its start [SJ 0719 7578] north of Rhuallt to a place [SJ 0695 7680] east-north-east of Tycelyn there are a number of poor sections showing generally highly cleaved silty mudstones with subordinate striped mudstones and sandstones. The sandstones are grey and fine to coarse-grained, and in places contain decalcified shell and crinoid fragments, sometimes concentrated in the top few centimetres of individual bands, suggesting winnowing. Strongly cleaved mudstones are seen on the hillside north-east of Brynllithrig Hall, where 21 m of beds are exposed in an old quarry [SJ 0739 7624].

Sandstones form a significant proportion of the succession in the Ty-Cêlyn area and on the summit of Mynydd y Cwm. An old quarry [SJ 0659 7662] immediately south of Ty-Cêlyn shows 3 m of dark grey massive sandstone with abundant crinoid and shelly fragments, underlain by 3 m of grey sandstone with silty mudstone lenses. A second quarry [SJ 0648 7683] north of Ty-Cêlyn reveals:

On the southern ridge of Mynydd y Cwm a section [SJ 0738 7675], apparently in the same sandstone unit, shows:

The base of the unit lies approximately 90 m below the top of the Denbigh Grits Group.

Above the sandstone there occurs a 12 to 24-m unit of highly and irregularly cleaved, disturbed mudstone with sporadic sandstone inclusions, some of which define slump structures. The unit forms a prominent scarp on the north slope of Mynydd y Cwm and can be traced westwards to the Vale of Clwyd Fault at Tan-y-bryn. It is well seen in a crag [SJ 0700 7692] south-east of Cwm church and in the Tan y Bryn area [SJ 0642 7695, SJ 0648 7695]. The intensely cleaved nature of this disturbed bed, which frequently gives it a pencil slate appearance, along with its position in the succession indicate that it is almost certainly part of the Llanddoget Formation of other regions (p. 68 et seq.).

What is probably the same disturbed bed crops out with underlying sandstone at Rhuallt. A section in the western part of an old quarry [SJ 0760 7508] south-south-east of Rhuallt Hall on the east side of the village is as follows:

A few yards to the east a second quarry [SJ 0764 7509] and exposures on the prominent ridge running 45 m eastwards show:

A small faulted inlier of the Denbigh Grits Group occurs northwest of Cwm, where dark grey, massive, fine-grained sandstone is seen [SJ 0614 7770] beside the Rhuallt–Dyserth road.

Only two graptolite faunas have been recorded from the group in this region. A disused quarry [SJ 0760 7508] south-south-east of Rhuallt Hall yielded (RS 9597–9627) Monoclimacis cf. flumendosae, Monograptus flemingii cf. compactus and Pristiograptus cf. pseudodubius indicating either a middle or upper Wenlock fauna. A quarry SJ 0719 7578] north of Brynllythrig Hall yielded Cyrtograptus cf. rigidus, Monoclimacis?, M. cf. flemingii and P. aff. dubius (PD 708–717, RK 4760), indicating a C. rigidus Zone age for the lower part of the succession. DP

Lower Nantglyn Flags Group

These beds crop out around Cwm, south of Rhuallt, and east of the Marian Ffrith and Gop faults (Figure 24), but they are poorly exposed, and parts of the succession are probably faulted out.

Throughout the Cwm area the beds are ribbon-banded, flaggy, silty mudstones with sporadic, thin, calcareous siltstone bands. A 5.1-m section is seen in an old quarry [SJ 0714 7722] south of Bwlch, and 12 m are exposed in another old quarry [SJ 0669 7742] west-north-west of Bwlch, where P. cf. dubius and P. pseudodubius (Zp 402–407) were collected, possibly indicating the presence of the Gothograptus nassa Zone. A fauna (RS 9571–9587, Zp 408–412) from a roadside quarry [SJ 0715 7730] south-west of Marian Cwm comprising M. flemingii flemingii, ?P. pseudodubius and P. cf. pseudodubius suggests the Cyrtograptus lundgreni Zone. Biscuity' bands at the same locality also yielded Glassia?, a smooth orthocone and a crinoid columnal.

South of the Rhuallt Fault exposures are few, but a small section of silty mudstone [SJ 0765 7495] south-south-east of Rhuallt Hall has yielded probable C. lundgreni Zone graptolites (RS 9628–9639, Zp 1013–1018), namely M. flemingii flemingii, M. flemingii cf. compactus and P. pseudodubius. Numerous fragments of pale blocky mudstone, lithologically similar to the Mottled Mudstones of other regions occur on the slope below this locality, but they are thought to have fallen from above it. Farther south and stratigraphically below the above locality, in a copse [SJ 0784 7428] west-north-west of St Beuno's College, highly cleaved, apparently unbedded, buff and grey mudstones are seen in place and are interpreted as belonging to the Lower Mottled Mudstone. On this basis, the previously mentioned slope, along with a roadside exposure [SJ 0773 7442], north-west of the college, of pale brown siltstones and silty mudstones belong to the Upper Mottled Mudstone.

East of the Marian Ffrith Fault a fauna (Zp 414–422) of C. lundgreni Zone age is recorded from mudstones formerly exposed in a bank [SJ 0768 7691] south-east of Bwlch. It comprises M. flemingii cf. compactus and P. cf. lodenicensis. The apparent absence of the Mottled Mudstones in this area is almost certainly due to exposure failure, though they may be faulted out.

Upper Nantglyn Flags Group

Outcrops of this group are found east of Rhuallt and Aelwyduchaf, but exposures are poor. Where seen, the rocks consist of ribbon-banded silty mudstone, locally with calcareous siltstone stripes and ferruginous concretions. The estimated thickness of the group is 180 m.

The beds are best seen near Pennant Mine [SJ 086 754], east of Rhuallt, where there are numerous exposures in the short valley which extends from the main buildings to the A55 road. An exposure [SJ 0854 7544] north of the mine yielded (RS 9795–9812, Zp 423–455) crinoid ossicles, Monograptus colonus colonus, M. colonus cf. compactus, N. nilssoni, Saetograptus leintwardinensis incipiens [short form close to S. varians], S. varians varians, S. varians pumilus and Spino graptus spinosus—with the exception of the S. incipiens form, a characteristic Lower N. nilssoni Zone assemblage. Comparable but less varied faunas occur in a quarry [SJ 0891 7502] south-south-east of Ty'r-ali (RS 9706–9729) and a quarry [SJ 0827 7470] east-south-east of Rhuallt (Zp 950–975). The A55 road cutting [SJ 0855 7571] south-west of Ty'r-ali yielded (RS 9640–9667) poorly preserved shells, ostracods, Monograptus uncinatus orbatus and N. nilssoni, and from an exposure a little to the south-west [SJ 0852 7568] came Aegiria grayi and Hemsiella sp.(RS 9701, 9702–9704).

A small quarry [SJ 0815 7434] north-east of St Beuno's College, in the faulted inlier (see above), shows 3.4 m of silty mudstone, in part ribbon-banded but mainly striped, with numerous sandy bands. The fauna (RK 9433–9442, Zp 996–1008) of ?Cardiola interrupta and M. uncinatus orbatus strongly suggests the Upper Nantglyn Flags Group, and the inlier is classified as such on the maps although the rocks are lithologically atypical. DP, PTW

Elwy Group

Rocks of this group crop out between Moel Maenefa and the River Wheeler at Bodfari. They are seen in numerous, mainly small, sections and comprise mudstones and silty mudstones, commonly striped and, especially towards the base of the succession, ribbon-banded.

The absence of a pronounced lithological change between this and the underlying Upper Nantglyn Flags Group, and the limited faunal evidence available make accurate demarcation of the group impossible. About 450 m of strata are estimated to be present. DP

More than 100 m of rather thinly bedded, striped silty mudstones crop out on Moel Maenefa, but exposures are poor. At Maen Efa Farm [SJ 0819 7463] 3.3 m of silty mudstone with wavy, sandy ('gingerbread') bands and a few small ferruginous concretions are seen. They yield a fauna (RK 9403–9417, Zp 976–995 of M. cf. colonus, ?N. nilssoni, Pristiograptus vicinus, Saetograptus varians pumilus and S. varians varians. Other fossiliferous localities are: a roadside quarry [SJ 0827 7470] north-east of Maen Efa, from which were obtained (RK 9380–9402, Zp 950–957), M. colonus, M. colonus compactus, M. uncinatus cf. orbatus [narrow form], N. nilssoni, N. cf. nilssoni [late form, close to Lobograptus progenitor]and S. varians cf. pumilus; a bank [SJ 0836 7454] east-south-east of Rhuallt, which yielded (Zp 1019–1022) P. vicinus; and a roadside exposure [SJ 0826 7446] north of Benarth, from where cf. Scyphocrinites? pulcher, cf. Cucullograptus scanicus, N. nilssoni, N. cf. nilssoni [close to L. progenitor]and P. vicinus were collected. These faunas all suggest a position for the Maen Efa sequence low in the Upper N. nilssoni Zone.

In the Cefn Du area beds probably also low in the group comprise fossiliferous ribbon-banded and striped silty mudstones. The old quarry [SJ 0945 7313] west-south-west of Glyn Uchaf showing 5 m of ribbon-banded mudstone yielded (RK 9492–9501, Zp 3235–3242) crinoid ossicles, ?N. nilssoni, S. chimaera cf. chimaera and S. cf. varians varians; a bank [SJ 0912 7314] south-west of Pantglas yielded (Zp 3251–3253) S. chimaera salweyi. These faunas indicate an Upper N. nilssoni Zone age.

Near Sodom, a scarp [SJ 0970 7188] shows about 4 m of well bedded, ribbon-banded silty mudstone, and 10.6 m of similar beds are exposed in an old quarry [SJ 0992 7173] in Sodom Covert, just beyond the eastern margin of the district. The former locality yielded (RK 9475–9491) M. cf. colonus, P. bohemicus, P. cf. dubius, Pristiograptus? and S. cf. varians varians; from the latter locality came (RK 9443–9474, Zp 6499–6504) Scyphocrinites? pulcher, Monograptus? [cf. colonus], M.? [cf. uncinatus group], S. chimaera chimaera, S. chimaera salweyi and Saetograptus sp. [cf. varians]. Although Boswell (1931, p. 304) referred to these rocks as 'typical Nantglyn Flags', these faunas indicate an Upper N. nilssoni Zone age, and they are here assigned to the Elwy Group. Stratigraphically higher beds to the west of Sodom include flaggy striped silty mudstones. DP, PTW

The slopes of Moel-y-Gaer consist of striped and thinly bedded ribbon-banded mudstones. A section [SJ 0951 7087] south-southwest of Adwy-wynt, near the summit of Moel-y-Gaer, shows 9 m of such beds which have yielded (RK 9508–9529, Zp 3199–3204) S. chimaera cf. chimaera, S. chimaera salweyi [abundant] and S. cf. varians an Upper N. nilssoni Zone fauna. In the same area the rather more silty striped beds include small lenses of disturbed mudstone, and on the south-western side of the hill similar beds contain sandstone partings.

Sandstone units have been mapped south of Graig Tremeirchion, and an old quarry [SJ 0866 7186] east of Henblas shows up to 1.8 m of grey to dark grey, fine-grained sandstone with mudstone partings.

Sandstone is also present south-east of Tycerrig and is here associated with a disturbed bed, the section in an old quarry [SJ 0893 7369] being:

The stratigraphical position of these sandstones is uncertain, but they and all the disturbed beds probably lie in the upper part of the succession present. DP

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