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Geology of the Hay-on-Wye district — a brief explanation of the geological map Sheet 197 Hay-on-Wye

P R Wilby

Bibliographic reference: Wilby, P R. 2004. Geology of the Hay-on-Wye district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 197 Hay-on-Wye (England and Wales). Keyworth, Nottingham: British Geological Survey

Keyworth, Nottingham: British Geological Survey, 2004.

© NERC 2004 All rights reserved

Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) Front cover. Silurian strata, cropping out in the bed of the River Wye. Looking north-west from Erwood Bridge [SO 0896 4374]. (Photograph: C F Adkin; P535177).

(Rear cover)

(Geological succession) Geological succession in the Hay-on-Wye district.

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 197 Hay-on-Wye. National Grid references are given in square brackets and all lie within the 100 km square SO. Symbols in round brackets and lithostratigraphical names given in bold text are the same as those used on the geological map.

Acknowledgements

This Sheet Explanation was compiled by P R Wilby and edited by D Wilson (scientific) and A A Jackson (series editor); figures were drawn by R J Demaine. Geophysical maps were produced by C P Royles. Details of the Built–Llandrindod Inlier were provided by D G Woodhall; details of fish localities and sections in the Old Red Sandstone sequence were given by D Hawley, University of Wales, Swansea. Data on current water abstraction in the district were kindly provided by the Environment Agency, Wales.

The geological map was compiled by P R Wilby, mainly from air photo interpretations using ImageStation software, with a limited amount of field checking. Digital photogrammetry was performed by P Turner, and cartography was by M B Ledgard.

We gratefully acknowledge the co-operation of landowners in allowing access to their lands, and thank Tarmac (Western) for permitting access to their operations at Gore, Dolyhir and Strinds.

Maps and diagrams in this book use topography based on Ordnance Survey mapping.

© Crown copyright. All rights reserved. Licence Number: 100017897/2004.

Geology of the Hay-on-Wye district (suumary from the rear cover)

An Explanation of sheet 197 1:50 000 series map (England and Wales)

This Sheet Explanation contains a brief description of the geology of the Hay-on-Wye district, which covers the picturesque ground between the southern edge of Radnor Forest and the north-eastern margin of the Black Mountains, including the towns of Kington and Hay-on-Wye. As such, the district provides a transect across the south-east margin of the Lower Palaeozoic Welsh Basin. The bedrock geology comprises sedimentary, volcanic and intrusive rocks of late Precambrian, mid Ordovician, Silurian and early Devonian age, which formed between approximately 700 and 390 million years ago. A summary of the lithological characteristics and environments of deposition for each formation is presented.

The district was subject to a long history of fault movement that culminated in an episode of regional folding during the latest Silurian and early Devonian. A brief history of these movements and an outline description of the resulting structures is presented.

The distribution and composition of superficial Quaternary sediments in the district is described. These were deposited mainly during the last (late Devensian) major ice advance, between approximately 20 000 and 14 000 years ago, and form a patchy cover of unconsolidated material, particularly over the low-lying ground. Since the ice retreated, head has accumulated under periglacial conditions, and peat has developed in upland areas. Fluvial sediments continue to accumulate today in the rivers Wye, Arrow, Edw, Dore and their tributaries.

As well as summarising the traditional aspects of the geology, the new map (Bedrock and Superficial) and this accompanying Sheet Explanation provide valuable information on the applied geological aspects and heritage of the district. These include mineral and water resources, potential geological hazards, engineering ground conditions and geological conservation, all of which are important considerations for planning and development. Lists of information sources and relevant references are included.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 197 Hay-on-Wye, published as a Bedrock and Superficial Deposits edition in 2004.

The district lies in the counties of Powys and Herefordshire, and includes a small part of the Brecon Beacons National Park. In the south of the district the picturesque River Wye passes through a deeply incised valley near Erwood before abruptly changing direction and opening out into a broad floodplain near Hay-on-Wye (Plate 1). To the south-east are the Black Mountains, dissected near their eastern margin by the Golden Valley. To the north-west are the steep, deeply incised Rhulen Hills which reach a maximum height of 542 m on the summit of Gwaunceste Hill [SO 1582 5555]. The district is sparsely populated, the most important centres being the market towns of Hay-on-Wye and Kington. The local economy is primarily based on agriculture and tourism; commercial quarrying operations are present near Old Radnor.

The exposed bedrock consists mostly of sedimentary and volcanic rocks deposited at various times between about 550 and 390 million years ago (Ma) in the late Precambrian, mid Ordovician, Silurian and early Devonian periods (see inside cover). Intrusive rocks are confined to the Precambrian Stanner–Hanter and Ordovician Builth–Llandrindod inliers. A granophyre in the Stanner–Hanter Inlier has been radiometrically dated as 702 ± 8 Ma (Patchett et al., 1980), representing one of the oldest fragments of intrusive basement in England and Wales. The bedrock is mantled by extensive tracts of superficial Quaternary sediments (Drift). These include Pleistocene glacial sediments, deposited during the last (late Devensian) major ice advance, and Head, deposited under periglacial conditions following the retreat of the ice. Holocene alluvial deposits continue to accumulate today in many of the valleys.

The oldest rocks in the district are preserved in the Stanner–Hanter and Old Radnor inliers, and represent isolated fragments of Neoproterozoic (late Precambrian) basement caught up within the Church Stretton Fault Zone. The Stanner–Hanter intrusive complex may represent the deeper parts of a volcanic island arc system that formed on the margin of Avalonia, a component of the ancient continent of Gondwana (Pharaoh and Carney, 2000). The Longmyndian (late Neoproterozoic) alluvial sediments of the Old Radnor Inlier were deposited much later than the Stanner–Hanter complex, in a marginal basin formed by rifting and/or sinistral strike-slip dismemberment of the arc (Gibbons and Horák, 1996). A pronounced unconformity separates these rocks from the overlying Lower Palaeozoic sequence.

During the Lower Palaeozoic the district was situated on the south-east margin of the Welsh Basin, which was an area of enhanced subsidence characterised by turbiditic sequences. The Midland Platform, a relatively stable area dominated by shallow shelf sediments, lay to the south-east. The Welsh Basin and the Midland Platform were separated by the Welsh Borderland Fault System (Woodcock and Gibbons, 1988), a complex belt of long-lived, north-east-oriented, basin-bounding faults. The most easterly of these, the Church Stretton Fault Zone, passes through the middle of the district, forming a prominent lineament on satellite images (Plate 1). The subparallel Pontesford Lineament, locally represented by the Cwm-Mawr and Howey faults, intersects the north-west margin of the district. A third major structure, the Swansea Valley Disturbance, defines the northern side of the Wye valley in the vicinity of Hay-on-Wye.

Widespread rifting in the early Ordovician Llanvirn period initiated the development of a number of volcanic centres in Wales, including that preserved in the Builth–Llandrindod Inlier. The inlier lies along the Pontesford Lineament and contains the products of acid, intermediate- and basic volcanism, suggesting emplacement and eruption in an ensialic, marginal basin setting (Bevins et al., 1984; Kokelaar et al., 1984). In the adjacent Builth Wells district (Sheet 196), a marked unconformity separates these Ordovician rocks from overlying Silurian (Llandovery) strata. The unconformity is not apparent in the Hay-on-Wye district, where contacts with adjacent Wenlock and Ludlow strata are faulted, but it is probably present at depth. The Wenlock and Ludlow successions are dominated by distal shelf facies, mainly consisting of mudstone, and shallower, more proximal shelf siltstones and sandstones. Bioturbation and shelly fossils, including trilobites, brachiopods, bivalves, orthoconic nautiloids and crinoids, are locally abundant, particularly within the siltstones and sandstones. Sedimentation was greatly affected by a series of eustatic changes in sea level, as well as by local, intrabasinal subsidence. Delicately laminated, graptolitic mudstone units, representing alternations of organic and turbiditic fall-out (i.e. hemipelagites), correspond to periods of basin deepening and record deposition under stagnant, oxygen depleted (anaerobic) bottom conditions. Progradations of the shelf are recorded by large scale coarsening-upwards sequences, accompanied by an increase in the importance of bioturbation and shelly fossils, indicating basin shallowing and improved circulation. Slumped and destratified strata probably correspond to regressions and periods of enhanced fault activity, in which mass wasting of the sediment pile took place.

The late Silurian (Přídolí) and early Devonian saw a transition from marine to continental sedimentation. Deposition of the lower Old Red Sandstone sequence took place on a broad, semi-arid, coastal alluvial floodplain that was subjected to frequent floods, prolonged soil-forming episodes and intermittent marine incursions. The thick sequence was accommodated by renewed subsidence and was fed by highlands developing to the north in response to the closure of the Iapetus Ocean and the terminal collision of the palaeocontinental plates of Avalonia and Laurentia. This event, the Acadian Orogeny, reached its acme in the mid-Devonian and resulted in the uplift and erosion of parts of the pre-existing rock sequence, and reactivation of the major basin-bounding faults. Although continental fluvial sedimentation resumed in the late Devonian, no strata younger than the early Devonian St Maughans Formation are preserved within the district. However, a substantial proportion of the modern drainage system is inherited from the Cainozoic (Tertiary) and probably originated on a former Cretaceous cover (Rice, 1957a). Remnants of planation surfaces, formed during periods of fluvial stability in the Cainozoic, are preserved as a series of high platforms, isolated during intervening phases of river down-cutting (Rice, 1957b).

During the Pleistocene, global climate change brought about a succession of ice ages that affected much of the British Isles and considerably modified the landscape. The glacial deposits preserved in the district correspond to the Late Devensian glaciation, which reached its acme around 20 000 BP (Campbell and Bowen, 1989, p.18). Deglaciation was completed by about 14 000 BP and the pre-Devensian drainage system was largely re-established during the Holocene.

History of research

The area has a long history of geological research, starting with early investigations by Murchison (1839). It was systematically surveyed at one inch to one mile and published as parts of [Old Series] sheets 42 and 56 between 1845 and 1857. The age and the nature of the Precambrian inliers was considered by Callaway (1900), Garwood and Goodyear (1918), Holgate and Hallowes (1941) and Woodcock and Pauley (1989). Important contributions towards understanding the stratigraphy and palaeontology of the Builth–Llandrindod Inlier were made by Elles (1940), Jones and Pugh (1941) and Hughes (1969, 1971); the structure of the inlier has been described by Woodcock (1987). Aspects of the Wenlock and Ludlow strata have been dealt with by Straw (1937), Holland and Lawson (1963), Bailey (1964) and Woodcock and Tyler (1993). Details of the Ludlow–Prídolí boundary in the district are given by Banks (1856), Stamp (1923) and Holland and Williams (1985). Sections in the Old Red Sandstone are described by Clarke (1954), Barclay (2004) and Hawley (2004); details of a fish-bearing locality are summarised by Dineley (1999). The structure of the district has been discussed by Kirk (1947, 1952), Weaver (1974, 1975), Woodcock (1984, 1988) and Woodcock and Gibbons (1988). The district has been included in numerous regional studies of drainage and landscape evolution, notably by Clarke (1936) and Rice (1957a, b). The Quaternary glaciation and deposits have been documented in greatest detail by Dwerryhouse and Miller (1930), Pocock (1940) and Luckman (1970).

Chapter 2 Geological description

The geological succession present at outcrop in the district is shown inside the front cover. Most of the district is underlain by shallow marine Silurian strata and terrestrial sedimentary rocks of the Lower Old Red Sandstone succession. Precambrian igneous and alluvial sedimentary rocks occur as inliers along the northern margin of the district, and Ordovician mudstones and volcaniclastic rocks occur in the north-west. Glacial and postglacial deposits form a discontinuous mantle over the bedrock and occupy the majority of the valleys and low-lying ground.

Precambrian

The oldest rocks in the district crop out in two fault-bounded inliers within the Church Stretton Fault Zone. The Stanner– Hanter Inlier comprises wholly intrusive igneous rocks that have yielded a Rb-Sr age of 702 ± 8 Ma (Patchett et al., 1980). The Old Radnor Inlier consists of steeply dipping sedimentary rocks that, based on structural considerations and on lithological comparisons with the type Long Mynd (Callaway, 1900; Garwood and Goodyear, 1918; Woodcock, 2000), are presumed to be of Longmyndian age. Together, the inliers form the most western exposures of Precambrian basement in the Welsh Borderland.

Stanner–Hanter Inlier

The Stanner–Hanter Inlier is dominated by basic igneous rocks but contains important acid components, both extensively faulted and altered. They are interpreted as remnants of a feeder to a subduction-related, calc-alkaline, volcanic island arc (Jones, 2000) and were probably emplaced in four stages (Holgate and Hallowes, 1941). The oldest component, a fine-grained Dolerite (DPC), is well exposed in crags [SO 2525 5754] above Middle Hanter. It forms the majority of the inlier and was injected as a series of sills. These have been intruded by coarse-grained, quartz-free Gabbro (EG) which, locally e.g. [SO 2537 5710], forms a series of coalesced, near-vertical sheets, each with chilled margins. A larger and more homogeneous gabbro body with a shallow dipping lower contact and relict primary layering occurs on the southern slopes of Hanter Hill. Its top, near the summit of the hill, is notably more pegmatitic and leucocratic, and the body as a whole may represent a remnant of a small magma chamber. Granophyric granite (gGG) veins were subsequently injected and interacted (both mechanically and chemically) with the gabbros to produce hybrid melts. Hydrothermal activity linked with this phase of intrusion may also have been responsible for the widespread epidotisation and amphibolitisation exhibited by the inlier (Holgate, 1977; Holgate and Hallowes, 1941). At a late stage, fine-grained doleritic dykes were intruded. These cut the earlier gabbros in a small quarry [SO 2492 5690], but nowhere are cross-cutting relationships with the acid rocks exposed.

Old Radnor Inlier

Precambrian rocks are well exposed in working quarries within the Old Radnor Inlier, including those at Gore [SO 2565 5922], Dolyhir [SO 2430 5842] and Strinds [SO 2420 5790] (Plate 2). The Yat Wood Formation (YtW) consists of pale green siltstone and ripple cross-laminated sandstone, thinly laminated grey mudstone, and subordinate pale green-grey fine-grained massive sandstone.

The Strinds Formation (Str) forms the majority of the inlier and is believed to lie stratigraphically above the Yat Wood Formation (Woodcock and Pauley, 1989), although the boundary is everywhere faulted. It comprises pale green-grey and purple-grey, fine- to medium-grained, micaceous sandstone and pebbly sandstone. Thin matrix-supported conglomerates containing well-rounded clasts of vein-quartz, rhyolite and mica schist occur locally, but their sedimentological context is difficult to interpret because of pervasive tectonic brecciation. Both formations are interpreted as alluvial plain deposits, derived directly or indirectly from Avalonian island arc material (Woodcock and Pauley, 1989), with inputs from sources containing rocks comparable to those comprising the Stanner–Hanter Inlier (Holgate and Hallowes, 1941).

Ordovician

Ordovician rocks are confined to the Builth–Llandrindod Inlier in the north-west of the district. Here, they are faulted against the surrounding Wenlock strata but, to the west, in the Builth Wells and Rhayader districts (Sheet 196 and Sheet 179, respectively), they are overlain by Llandovery strata in a largely unconformable relationship (Davies et al., 1997). The succession (Figure 1) consists of a thick sequence of mudstones and volcanic rocks with high-level basic intrusions of the Builth Volcanic Group. The mudstones and tuffs have yielded important, locally rich, shelly and graptolitic faunas (Elles, 1940; Hughes, 1969, 1971; Sheldon, 1987), including the type specimens of the international zonal fossil Didymograptus murchisoni. These faunas indicate that the volcanism is entirely of late Llanvirn age. Marked lateral facies and thickness variations across major fractures, in particular the Cwm-mawr Fault, suggest that volcanism and sedimentation were contemporaneous with the faulting.

The oldest unit, the Camnant Mudstones Formation (Cte), ranges from the D. artus into the D. murchisoni Biozone (Elles, 1940). It consists of a monotonous sequence of dark grey, fossiliferous mudstone with intercalated thin beds (typically less than 0.1 m thick) of micaceous, fine-grained sandstone e.g. [SO 0883 5756]. The formation was deposited in an oxygen-starved, low energy, marine environment, below storm-wave base. Its base is nowhere exposed within the inlier, but up to 915 m of strata are present in the district, several hundred metres of which are exposed in Camnant Brook [SO 0888 5771] to [SO 0959 5560]. Mappable tuffs (Z) within the mudstone represent precursors to the main phase of volcanism. The Bettws Mill Tuff Member (Btw) is up to 50 m thick and consists of a stacked sequence of sharp-based, planar-, cross-bedded or normally graded, basic lapilli-tuffs, each 0.2 to 1 m thick. Subordinate rhyolitic tuffs occur locally and intercalations of mudstone are common at the top of the member (Plate 3). The Gelli Tuff Member (Glh) is of a similar thickness. It consists predominately of massive, basic lapilli-tuffs which, south of the Cwm-mawr Fault, contain moderately thick mudstone interbeds. Locally, rhyolitic clasts are abundant (Figure 2). The member wedges out on the south side of Gilwern Hill [SO 0928 5814] and to the east of Hirllwyn Bank [SO 1080 5540].

Lenticular, sill-like intrusions of medium- to coarse-grained Dolerite (Do), ranging up to 90 m in thickness and over 1 km in length, occur at two main levels within the Camnant Mudstones Formation. They represent high level intrusions into only partially lithified sediments and are probably genetically linked with the overlying Builth Volcanic Group. Typically, contacts with the country rocks are sharp and regular, and baking extends outwards for up to 5 m (Jones and Pugh, 1948). The largest intrusions form prominent, craggy, elongate ridges such as Castle Bank [SO 0880 5620].

The Builth Volcanic Group succeeds the Camnant Mudstones Formation and crops out in a series of faulted folds with north-east striking axis. It formed during a period of widespread early Ordovician (Tremadoc– Llanvirn) igneous activity in Wales and the Borderlands that included centres at Shelve, Llanwrtyd Wells, Cadair Idris and Fishguard. Both the chemistry of the erupted materials and plate tectonic reconstructions suggest that the centres developed in an ensialic, marginal basin setting (Bevins et al., 1984; Kokelaar et al., 1984). The Builth Volcanic Group is dominated by subaqueous pyroclastic and poorly sorted mass flow deposits, but includes mudstones and subordinate effusive lavas. The precise locations of the vents are unknown and probably lay outside of the district. The group is divided into six formations.

The basal division, the Llandrindod Tuff Formation (LdT), sharply overlies the Camnant Mudstones Formation. It is up to 60 m thick on the western side of the Cwm-mawr Fault, but on the eastern side it is attenuated and not everywhere present. It consists of an acid ash-flow tuff containing relict glass shards, pumice fragments and feldspars. The tuff is typically massive or poorly sorted, although on the north side of Howey Brook [SO 0925 5903] a secondary (alteration) bedding-parallel fabric is developed. A basal pyroclastic breccia crops out immediately to the north and west of the district e.g. [SO 0832 5940]; [SO 0875 5965].

The overlying Gilwern Volcanic Formation (Gwn) displays marked lateral and vertical variations in thickness, facies, and in the relative proportion of mudstones to volcaniclastic rocks. East of the Cwm-mawr Fault it consists predominately of massive and normally graded, basic lapilli-tuffs and fine-grained tuffs. At the base on Perthi Common [SO 0962 5485] is a prominent lapilli-tuff, up to 80 m thick, which thins northwards towards the fault. North of the Dolfawr Fault the base of the formation is defined by 25 to 30 m of reworked lapilli-tuffs and fine-grained tuffs, some displaying cross-bedding e.g. [SO 0922 5904], but the upper part of the formation is dominated by mudstone (md). The mudstone is dark grey and locally contains interbedded, normally graded tuffs, 0.1 to 4 m thick e.g. [SO 0909 5920] to [SO 0922 5904]. Subsidiary lenticular sheets and pods of acid tuff breccia e.g. [SO 0918 5464], massive dacite (RD) and highly amygdaloidal basalt lavas (B) occur at various levels in the formation.

The Carneddau Volcanic Formation (Cdu) is present only in the south of the inlier, forming a large part of Perthi Common. It consists of up to 35 m of basic lapilli and fine-grained tuffs, distinguishable from those of the underlying Gilwern Volcanic Formation by their greater abundance of feldspar crystals. West of Llansantffraed-in-Elwel [SO 0954 5472], the tuffs form structureless, normally graded, planar laminated or cross-laminated beds up to 0.5 m thick with sharp upper and lower contacts. Farther west, in the Builth Wells district, the unit dies out northwards against dacite lava domes, which formed substantial contemporaneous sea floor highs (Schofield et al., 2004).

The Llanelwedd Volcanic Formation (Ldd) is also restricted to the south of the inlier. It comprises feldsparphyric and amygdaloidal basaltic lavas, 40 to 60 m of which crop out to the south and the west of Llansantffraed-in-Elwel [SO 0984 5468].

The Cwm-amliw Tuff Formation (CaT) occurs only in a narrow tract of land north of Gilwern Hill [SO 0926 5922]. It consists of about 30 m of blue-grey, fine-grained, acid ash-flow tuffs, the eruption of which brought the Llanvirn phase of volcanic activity in the Builth Wells area to a close.

The succeeding Llanfawr Mud-stones Formation (LrM) consists of monotonous, dark grey, locally finely laminated, fossiliferous mudstone. To the west, the formation is up to 700 m thick and passes up into the Glyptograptus teretiusculus (upper Llanvirn) and the Nemagraptus gracilis (Caradoc) biozones, but in the present district only about 55 m of the unit are preserved against the Howey Fault [SO 0916 5923]. It is interpreted as having accumulated in a persistently low-energy, dysaerobic, offshore environment of uncertain depth, but certainly below storm-wave base (Davies et al., 1997), conditions comparable to those in which the Camnant Mudstones Formation were deposited.

Silurian

Silurian strata crop out over most of the district. Rocks of Wenlock age are known to flank the Precambrian and Ordovician inliers in the north (e.g. see Woodcock, 1988), and are likely to crop out elsewhere, but are not generally distinguished on the map from rocks of Ludlow age. These are succeeded by Přídolí strata of continental aspect (Old Red Sandstone), which occupy several fault-bounded outliers within the Church Stretton Fault Zone, and the majority of the region to the south of it. The position of the Silurian–Devonian boundary within the Old Red Sandstone sequence is uncertain because of a lack of biostratigraphical control, but is believed, based on miospore evidence elsewhere (Richardson and McGregor, 1986), to lie within the upper part of Raglan Mudstone Formation.

Wenlock and Ludlow

The outcrop of the Dolyhir Limestone Formation (Dlh) is confined to the Old Radnor Inlier, where an estimated 25 to 30 m of strata (Garwood and Goodyear, 1918) are well exposed in the upper faces of the Strinds (Plate 2) and Dolyhir Quarries. The formation is largely of early Wenlock age (Shein-woodian), although its lower part may range down into the latest Llandovery (Telychian; Kirk, 1951). It lies with marked unconformity on the Precambrian rocks and comprises massive, pale grey, coarsely crystalline limestone with abundant calcareous algae, corals and bryozoa, and scarce conodonts, trilobites, gastropods and brachipods. Locally, a thin (less than 2 m thick) crinoidal rudite is developed at its base which contains clasts derived from the underlying Precambrian strata, and corals (Favosites sp.) preserved in life position. It was deposited in a shallow marine setting, possibly on a local bathymetric high associated with the Church Stretton Fault Zone, or on the more extensive Midland Platform. The top of the formation is presently nowhere exposed; however, a possible conformable contact with Wenlock mudstones was recorded within the inlier by Garwood and Goodyear (1918), and Woodcock (1988) has suggested that the western margin is also conformable.

The complex stratigraphy of the Wenlock and Ludlow strata has not been mapped in detail within the district; accordingly, they are depicted on the map as undifferentiated Wenlock and Ludlow strata (WLu). Although the detailed succession is uncertain, it appears to be broadly comparable with sequences in adjacent districts to the north and west (Straw, 1937; Jones, 1947; Holland, 1959; Holland and Lawson, 1963; Tyler, 1987; Tyler and Woodcock, 1987; Zalasiewicz and Williams, 1999; Schofield et al., 2004). Strata comparable with the Builth Mudstones Formation of the adjacent Builth Wells district form the base of the sequence along the western margin of the sheet and against the Builth–Llandrindod Inlier. They consist of very thinly laminated, graptolitic, hemipelagic mudstone that was deposited under predominately anoxic conditions in an offshore setting. They are undated in this district, but probably span the Wenlock Series and may range up into the lower Ludlow. The overlying succession comprises over 1000 m of mudstone, siltstone and sandstone, deposited in mid-ramp to proximal shelf settings, and compares with the Irfon, Cwm Craig-ddu and Aberedw formations of the Builth Wells district (Schofield et al., 2004). Discrete units of slumped and destratified sediment record periods of mass wasting, initiated by fault activity and/or regressions. In the north of the district, the succession includes shelly, cross-stratified, sandy siltstones interbedded with finely laminated, hemipelagic mudstones and siltstones, comparable with the Bailey Hill Formation of Tyler and Woodcock (1987).

Overall the Wenlock and Ludlow strata show a large-scale coarsening upwards motif (Woodcock and Tyler, 1993), accompanied by an increase in bioturbation and shelly fauna, and a concurrent decrease in hemipelagites. This records the northward progradation of facies belts off the Midland Platform into the Welsh Basin, and improved circulation associated with the shallowing. It culminates in the upper part of the Ludlow with a storm-generated sequence consisting of calcareous, muddy siltstone and fine-grained sandstone, which is variably shelly and largely homogenised by bioturbation. These rocks form the well exposed, craggy ground along the Twmpath west of [SO 0882 4357] and on the north and west slopes of Llandeilo Hill [SO 0960 4660]. Evidence from elsewhere indicates that a short-lived deepening event affected the region prior to further shoaling and the deposition of the Ludlovian Cae'r mynach Formation (Car) that succeeds the undifferentiated strata. This division is only locally differentiated, and consists of interbedded fine- to medium-grained sandstones, and thin, burrowed mudstones and siltstones that are well exposed in quarries north of Crickadarn [SO 0875 4285] and near St Teilo's Pool [SO 0916 4567]. The sandstones are typically hummocky cross-bedded, generally less than 0.3 m thick, and are interpreted as storm-reworked sublittoral sheet deposits.

Přídolí

The Wenlock and Ludlow strata are conformably overlain by basal Přídolí strata of the Old Red Sandstone (Figure 3), corresponding to the onset of predominantly terrestrial sedimentation across the region. The Temeside Mudstone Formation (Tem) forms the basal division of the sequence across much of the district, although it has only been differentiated locally. It consists of a dull, olive-green siltstone with abundant calcrete glaebules. Where fresh, it is hard and splintery, but where weathered it softens considerably and readily breaks up into small fragments. It appears largely massive at outcrop, although bedding is locally discernible due to the presence of calcrete-rich horizons or thin, micaceous, ripple-laminated sandstones as exposed near Newchurch [SO 2212 4995]. The formation is up to 30 to 50 m thick (Barclay and Wilby, 2003). It is interpreted as a sand-starved lagoonal or coastal tidal flat deposit that experienced frequent and prolonged periods of emergence.

Locally, a sequence comparable to the Downton Castle Sandstone Formation defines the base of the Old Red Sandstone, but it is not differentiated on the map. It was formerly well exposed in several quarries and a road-cutting on the southern side of Bradnor Hill (see Banks, 1856; Stamp, 1923; Holland and Williams, 1985), but is now only easily accessible in Newton Lane [SO 2901 5715]. The base is marked locally by beds assigned to the Ludlow Bone Bed Member which, on Bradnor Hill, comprises 0.42 m of light olive-grey, thinly bedded, micaceous siltstones and fine-grained sandstones. Four separate bone beds are present, each one a few centimetres thick, containing abundant carbonaceous fragments, a scarce shelly fauna, and fish material including thelodont scales, and spines of the acanthodian Onchus sp. The bone beds are succeeded by 0.84 m of lithologically similar strata comparable to the Platyschisma Shale Member of the type area and likewise characterised by abundant specimens of the gastropod Turbocheilus [Platyschisma] helicites. The succeeding strata are compared with the Sandstone Member of the type area. They are at least 16 m thick and consist of dusky yellow sandstone, which is fine to coarse grained, highly micaceous, well laminated and, in places, cross-bedded. The sandstone yields a restricted fauna that includes scattered fish material (Dineley, 1999), eurypterids and the inarticulate brachiopod Lingula minima. Similar facies are well exposed in a small working quarry near Hengoed [SO 2176 5326] and were previously worked at Lodge Farm [SO 2632 5338]. The Sandstone Member was formerly seen underlying the Temeside Mudstone Formation in an abandoned quarry in Park Wood [SO 2792 5636] by Stamp (1923).

Overall, the Downton Castle Sandstone Formation is a regressive sequence and preserves an important faunal change at its base. The fully marine biota of the underlying Ludlow strata is progressively replaced by a new biota characterised at Kington by the bivalve Modiolopsis complanatus, and by L. minima, T. helicites, eurypterids, plants, and abundant fish remains. No appreciable time break is indicated, the marked change in biota probably resulting instead from a sudden regression and subsequent transgression (see Miller, 1995). The bone beds are interpreted as shallow subtidal or low intertidal deposits in which repeated reworking during a period of reduced sediment supply led to the accumulation of vertebrate-rich lags (Smith and Ainsworth, 1989). Proximity to land is indicated by the abundant plant fragments. The strata assigned to the Platyschisma Shale Member are interpreted as intertidal deposits, whilst the overlying Sandstone Member probably represents a series of storm-reworked sand shoals, behind which the Temeside Mudstone Formation accumulated (Siveter, 2000).

The Raglan Mudstone Formation (Rg) succeeds the Temeside Mudstone and Downton Castle Sandstone formations and records the onset of red-bed conditions. Although not examined in detail within the district, its character has been documented in adjacent areas (Brandon, 1989) and it is well exposed at a number of river localities, most notably The Scar [SO 3540 4440] (Barclay, 2004; (Plate 4)). It comprises up to 900 m of red, micaceous mudstone and siltstone, much of which exhibits evidence of pedogenic calcretisation. This is generally displayed as vague, green-purple, calcareous mottling, or as small dispersed nodules (glaebules). Where the effects of calcretisation have been minimal, ripple cross-lamination, desiccation cracks or simple burrows may be preserved (Hawley, 2004). Locally, larger calcrete nodules are present, and at certain levels they have coalesced to produce rubbly or massive limestones. These are most notable in the upper part of the formation, the Bishop's Frome (Psammosteus) Limestone Member (BFL) being the most prominent and best developed example. This unit forms an easily recognised regional marker that is present throughout much of south Wales and the Welsh Borderlands (King, 1934). In Merbach Brook [SO 3082 4496] it consists of four, closely spaced limestones, the main one being 1.5 m thick (Clarke, 1954).

Sandstones, typically less than 2 m thick, but locally up to 5 m, occur sporadically in the Raglan Mudstone Formation and form prominent low scarps. They range from red-brown and purple to green or grey-brown, and are fine to medium grained, cross-bedded and commonly micaceous. Palaeocurrent directions are variable but are predominately towards the south-east. Rarely, they contain thin, red or green basal intraformational conglomerates, some of which yield fish debris. Clarke (1954) recovered a diverse fish fauna, including the acanthodian Onchus sp., the osteostracan Didymaspis sp., and an undetermined heterostracan, from an abandoned sandstone quarry near Merbach [SO 2985 4520]. Acanthodian material also occurs south and south-east of Hay-on-Wye [SO 2300 4127] and [SO 2490 4027].

The distinctive Townsend Tuff Bed (TwT) occurs approximately 97 m below the top of the Raglan Mudstone Formation in Merbach Brook [SO 3067 4516], and 40 m below it in Cusop Dingle [SO 2486 4026]. It is the thickest and most widespread of a series of volcanic air-fall tuffs and is traceable across much of south Wales and the Welsh Borderland, where it has been taken as a convenient position for the Silurian–Devonian boundary (Allen and Williams, 1981). In Merbach Brook, it forms a 3.61 m high waterfall consisting of two green, well jointed porcellanites separated by a purple and cream crystal dust tuff (Parker et al., 1983). The upper surface of the lowest porcellanite is crowded with faecal pellets.

The Raglan Mudstone Formation was deposited on a semi-arid, coastal alluvial floodplain that was subjected to frequent floods, prolonged soil-forming episodes and intermittent marine incursions. The sandstones represent the fills and crevasse-splays of through-flowing, shallow streams that experienced pronounced seasonally controlled variations in their discharge. The Bishop's Frome Limestone Member, and its regional correlatives, correspond to a major depositional hiatus when mature carbonate soil profiles, representing a period in the general order of 10 000 years (Allen, 1986), formed on an interfluvial pediment (Allen, 1985).

Devonian

The St Maughans Formation (SMg) occupies the steep slopes immediately to the south of Hay-on-Wye and on either side of the Golden Valley. It consists of interbedded sandstone, siltstone and mudstone, commonly arranged in fining-upwards cycles. Typically, each cycle overlies an erosional surface cut into the top of the previous one, and is defined by a scatter of intraclasts or by a basal intraformational conglomerate. The conglomerates are particularly characteristic of the St Maughans Formation: they are up to 1 m thick, purple, red-brown or green, and consist mainly of calcrete clasts up to 5 cm diameter, with subordinate siltstone and sandstone clasts in a calcareous sandstone cement. Planar or low-angle cross-bedding is common. The overlying sandstones are generally well sorted, fine to medium grained, typically 3 to 4 m thick, and predominantly green-grey or red-brown in colour. Many show an upward progression from trough cross-bedding to planar bedding and ripple cross-lamination. They are interpreted as the channel-fill deposits of perennially charged meandering rivers. The sandstones fine upwards through a facies indicative of abandonment into floodplain mudstones, many of which are calcretised. Fish and plant fragments are common in both the conglomerates and the sandstones. Clarke (1954) recorded the acanthodians Ischnacanthus sp. and Onchus sp., and the heterostracans Anglaspis macculloughi, Corvaspis kingi, Tesseraspis sp. and Traquairaspis symondsi from an old quarry at the base of the formation near Arthur's Stone [SO 3145 4297]. Another quarry, stratigraphically higher up, above the source of the River Dare [SO 3080 4390], yielded fragments of the plants Prototaxites and Pachytheca, and the fish Pteraspis and Poraspis. Skolithos-like burrows were recorded by Clarke (1954) in a sandstone near Crafta Webb [SO 3182 4406].

Structure

Three major north-east-trending zones of complex faulting and localised folding extend across the district. These are, sequentially from the north-west, the Pontesford Lineament, which traverses the Builth–Llandrindod Inlier; the Church Stretton Fault Zone, which contains the Old Radnor and Stanner–Hanter inliers; and, the Swansea (or Tawe) Valley Disturbance ((Plate 1) compare with (Figure 4) and (Figure 5)). Both the Pontesford Lineament and the Church Stretton Fault Zone have, at different times, defined the south-eastern margin of the Welsh Basin and strongly influenced sedimentation in the region during the Early Palaeozoic (Woodcock and Gibbons, 1988). They both consist of anastomosing arrays of steep faults with variable vertical offsets and significant inferred strike-slip displacements (Woodcock, 1984, 1987, 1988). The Cwm-mawr Fault, a component structure of the Pontesford Lineament, dominates the Builth–Llandrindod Inlier, and has an overall down-throw of approximately 500 m to the east. The Builth Volcanic Group of this area is repeated in three open synclines with north-east-trending axial planes. Within the tract between the Pontesford Lineament and Church Stretton Fault Zone, the dip of strata is variable. However, south of this area the structure is relatively simple, the strata dipping gently towards the south, except in the vicinity of the Swansea Valley Disturbance, where dips locally exceed 40o (Weaver, 1975). The joint pattern in this region has been documented by Weaver (1974).

All three fault zones are sited above ancient basement fractures with long and complicated histories, including periods of extension, compression and strike-slip. Evidence of early tectonism is recorded by the pronounced angularity of the sub-Wenlock unconformity in the Old Radnor Inlier (Woodcock, 1988), and by rapid thickness and facies variations in the Builth Volcanic Group across major fault strands. The Builth volcanic centre itself is thought to have been initiated by activity on the Pontesford Lineament during a period of regional, crustal extension in the mid Ordovician (Kokelaar, 1988). Subsequent movements in the late Ordovician or early Silurian (Schofield et al., 2004), possibly with a component of strike-slip, resulted in folding of the volcanic sequence. Possibly the greatest movement occurred during the late Silurian to mid Devonian Acadian Orogeny, in response to the closure of the Iapetus Ocean and the terminal collision of the palaeocontinental plates of Avalonia and Laurentia. Sinistral transpression associated with this event probably led to the isolation of the Precambrian inliers within the Church Stretton Fault Zone along bounding faults that may converge at depth in a flower structure (Coster et al., 1997).

Minor reactivation of many of the faults probably occurred during the Variscan Orogeny (late Carboniferous to early Permian). The suggestion by Weaver (1975) that significant movement took place on the Swansea Valley Disturbance in the late Neogene has been discounted by George (1980). Small magnitude, upper crustal earthquakes still effect the district (Ritchie and Wright, 1991).

Regional geophysics

The gravity anomaly pattern shows a general north-westwards increase in values, consistent with an increase in thickness of relatively high-density, Lower Palaeozoic mudstone (typically 2.80 to 2.85 Mg/m3) towards the centre of the Welsh Basin (Figure 4). This trend is overprinted by a north-east-orientated complex pattern of localised anomalies, corresponding to the major fault lineaments in the district. The lateral extent of the gravity high on the northern edge of the district far exceeds the area of Precambrian rocks exposed there, suggesting that other high-density rocks may occur in similar structural settings beneath the cover of Lower Palaeozoic strata. Coster et al. (1997) noted a discordance between the location of the high-density rocks in the Stanner–Hanter Inlier and the position of the gravity maximum in the associated anomaly. This, they suggested, implies that the inlier is isolated from the surrounding basement at a relatively high structural level.

The aeromagnetic anomaly data exhibit a similar north-east to south-west pattern (Figure 5). The central tract, between the Church Stretton Fault Zone and the Swansea Valley Disturbance, is occupied by a marked magnetic low, implying the presence of a sub-basin containing a greater thickness of non-magnetic sedimentary rocks. The Stanner–Hanter Inlier coincides with a well defined positive magnetic anomaly. There is close agreement between the gravity and magnetic anomalies in the south-eastern part of the district, the pattern suggesting the presence of a concealed lineament to the south of the Swansea Valley Disturbance. The complex pattern of high and low anomalies probably reflects the juxtaposition of concealed basement rocks, igneous intrusions, and local pull-apart sedimentary basins within the plexus of faults.

Quaternary

The majority of the Quaternary deposits in the region were formed during the late Devensian glaciation, the limit of which lay just beyond the eastern margin of the district (Lewis, 1966; Campbell and Bowen, 1989, p.86–87). Ice derived from the Cambrian Mountains ('Welsh Ice') is believed to have converged on the topographic depression formed by the Irfon and Ithon valleys around Builth Wells and, at its acme, may have extended over the entire district. It overrode the Rhulen Hills and escaped southwards down the Wye valley to be joined near Boughrood [SO 1280 3930] by a tongue diverted northwards from the Usk valley (Dwerryhouse and Miller, 1930). This united mass of ice continued eastwards, probably as a piedmont lobe, part of it overriding the col at the head of the Golden Valley. Ice also flowed eastwards from the Builth Wells region over the Builth–Llandrindod Inlier into Summergil Brook [SO 1800 5814] and on into the Arrow valley. During deglaciation, glaciers appear to have been largely confined to the major valleys, in particular the Wye.

Glaciation significantly modified the landscape by over-deepening valleys and by depositing glacial materials. Early postglacial drainage patterns have been superimposed onto a preglacial buried topography resulting in the diversion of several rivers from their earlier courses and in the cutting of numerous bedrock 'bypass' gorges. Terminal moraines in the Wye valley have diverted the river from its preglacial course in two places: at Letton [SO 3350 4650] it turns abruptly to the south away from its former course to the north of Oaker's Hill [SO 3462 4604], and at Hay it now follows a narrow bedrock channel [SO 2260 4230] to the south of its original course [SO 2200 4360]. Choking of Cwmila Brook by a moraine at Dol-y-cannau [SO 2030 4923] has prompted the bulk of the valley to drain westwards into the River Wye via a deep bedrock channel [SO 1182 4344], rather than eastwards into the Arrow valley as it seems to have formerly done. Similarly, sections of the River Arrow have been modified by glacial deposits, parts of its course now being entrenched in a gorge that cuts across preglacial bedrock interfluves (Luckman, 1970) and drift-filled buried valleys e.g. [SO 3204 5708]. Glacial meltwater channels are common in the district. A series of dry, subparallel channels close to Elsdon [SO 3200 5460] may record successive stages of ice-marginal drainage formed during the retreat of the Arrow valley glacier. The age of a deep, abandoned bedrock channel at Red Hill Wood [SO 2590 5320] is uncertain, but it may have developed whilst dead ice still occupied the main valley.

In addition to glacial deposits, the district includes important postglacial (Holocene) alluvial, head and landslip deposits. None of the Quaternary deposits have been examined in detail as part of this study, but their characteristics are well known from published sources, and supplementary data are available from site investigations and borehole records.

Undifferentiated Glacial Deposits

Undifferentiated Glacial Deposits occupy much of the low ground and most of the tributary valleys in the district. Small spreads also occur on the higher peaks (Dwerryhouse and Miller, 1930) and in depressions on the interfluves. Their distribution, and the composition of erratics within them, has provided evidence for the direction of ice movement across the district (Dwerryhouse and Miller, 1930). Typically, the deposits are characterised by sporadically marshy, irregular or smooth, concave slopes and undulating, gently sloping ground. Near Erwood [SO 1075 4260], and elsewhere (Luckman, 1970), they also form cross-valley moraines. The deposits consist mainly of Till (glacial diamicton), but locally contain Head and Glaciofluvial Sand and Gravel. The Till is lithologically variable. In the north and west it is blue-grey and dominated by Ordovician and Silurian erratics, but in the south and east it is red-pink and contains, in addition, Old Red Sandstone erratics. It ranges from a gravelly sandy silty clay to a sandy clayey gravel. The clasts are subangular to well rounded and range up to boulder size. The thickness of till deposits is highly variable; at least 15 m have been observed in the north-west e.g. [SO 1125 5900]; [SO 1255 5890] and 12 m overlie bedrock in boreholes near Llanbedr [SO 1245 4611] and Aberedw [SO 0877 4881].

Hummocky Glacial Deposits

Hummocky Glacial Deposits are characterised by irregular, moundy ground with abundant kettleholes. They occur mostly in the south and the east of the district in the Wye valley between Hay [SO 2240 4300] and Clyro [SO 2130 4360], along the Arrow valley and its upper slopes to the east of Milton [SO 2408 5084], and in the ground between Blackmere [SO 3620 4118] and Calver Hill [SO 3718 4836]. The deposits of the Arrow valley and Blackmere–Calver Hill areas are contiguous with more extensive spreads in adjacent districts and form part of the substantial Kington–Orleton and Staunton moraines (Luckman, 1970). The Hummocky Glacial Deposits comprise a combination of melt-out and ice-marginal deposits, formed by still-stands or oscillations in the ice front during retreat. Typically, they are heterolithic and include bedded sand and gravel, clay-bound gravel, till and subordinate lenses of clay, silt and peat. At Hay, Dol-y-cannau [SO 2030 4923], Hergest Court [SO 2830 5550] and near Llanbadarn-y-garreg [SO 1180 4940], the deposits form well defined cross-valley halt moraines. To the south of Llowes, on the northern side of the Wye valley [SO 1760 4010], they form a kame terrace. Locally, the deposits may be extremely thick: 15 m were formerly exposed in the Staunton moraine near Bredwardine [SO 3330 4540] (Grindley, 1923), over 25 m have been drilled at a number of places in the Kington–Orleton moraine [SO 3312 5899]; [SO 2760 5440], and at least 39 m were recorded on the north side of the Wye valley west of Rhydspence [SO 2290 4740] (Luckman, 1970).

Glaciofluvial Deposits

Glaciofluvial Deposits represent outwash from melting glaciers that, in places, reworked earlier deposits. They are confined to the river valleys and typically form flat, or very gently sloping, smooth, dry ground, consisting predominately of interbedded sand and gravel. Glaciofluvial Sheet Deposits comprise poorly sorted, cobble pebble gravels and sands, locally slightly clayey or silty, and in places containing boulders up to 2 m in diameter (Pocock, 1940). In the Wye valley, upstream of Llyswen [SO 1340 3800], they form a series of steep-fronted terraces totalling up to 15 m high, each terrace being several metres high. Glaciofluvial Ice Contact Deposits occur in tributaries to the River Wye in the west of the district. They are generally more heterolithic, commonly containing tills and glaciolacustrine silts and clays, giving rise to irregular, locally waterlogged, ground.

Undifferentiated River Terrace Deposits

Undifferentiated River Terrace Deposits lie on either side of the River Wye downstream of Llyswen [SO 1340 3800]. They form flat-topped features, up to a few metres above the present floodplain, and consist principally of sand and gravel, of which 10 m metres were proved in a borehole [SO 1872 4095]. They are the preserved remnants of a broad, early postglacial alluvial tract, formed of reworked glacial deposits by the re-established River Wye. Williams (1968) recognised two terraces (not distinguished on the geological map), each corresponding to separate episodes of down-cutting in response to postglacial regrading and isostatic readjustment.

Lacustrine Deposits

Lacustrine Deposits, consisting of interlaminated sand, silt, clay and peat have accumulated in lakes e.g. [SO 1190 4653], kettleholes e.g. [SO 3175 5890]; [SO 2390 4510] and abandoned meander loops of the River Wye e.g. [SO 1860 4015] throughout the Holocene. The most extensive developments of lacustrine deposits, however, formed in the latest Devensian during ice retreat, but are presently nowhere exposed. Over 2 m of variably calcareous, shelly, silty and organic-rich clay concealed beneath Rhosgoch Common [SO 1950 4830] (Bartley, 1960) were deposited in a shallow, late glacial lake impounded by the cross-valley moraine at Doly-y-cannau [SO 2030 4923]. Similar glacial lakes may have developed in the vicinity of Hundred House [SO 1130 5440] (Dwerryhouse and Miller, 1930) and Waterloo [SO 3410 4760] (Aldis, 1904; Grindley, 1905), but have not been proved in the present study. A grey, stiff, pebble-free clay yielding abundant Pleistocene marine fossils, including foraminifera and fragmentary bryozoa, was formerly exposed in the bed of River Wye near Bredwardine [SO 3360 4545] (Grindley, 1923). Its precise age and relationship to the juxtaposed Hummocky Glacial Deposits are uncertain. However, the presence of marine fossils implies that Irish Sea ice situated in the Cheshire–Shropshire lowlands to the north (or deposits associated with this ice sheet) drained southwards, at least temporarily, into the district.

Alluvial Fan Deposits

Alluvial Fan Deposits have developed where tributaries intersect main valleys and emerge from the confines of their channels. They occur in all of the major valleys of the district and typically form small, cone-shaped features, dissected by a braided array of feeder channels. They consist predominately of silty sandy clay and variably clayey sand and gravel. The majority of fans were probably most active immediately after the retreat of the ice, but deposition continues to the present day.

Alluvium

Alluvium occurs as discontinuous tracts within all of the major river valleys and their most important tributaries within the district. The widest alluvial spread occurs in the Wye valley, and is over 1.5 km wide downstream of Whitney-on-Wye [SO 2690 4740]. Typically, the Alluvium forms flat or very gently sloping, intermittently saturated, ground. It is laterally variable and consists of clay, silt, sand and gravel with varying proportions of fine-grained matrix. A basal gravel lag is common and thin lenses of Peat and Lacustrine Deposits occur locally.

Peat

Peat and organic-rich clay are widespread in small waterlogged depressions, particularly kettleholes within the Hummocky Glacial Deposits, and in enclosed hollows on upland areas underlain by Wenlock and Ludlow strata. The largest expanse occurs on Rhosgoch Common where over 2 m of Peat have been proved (Bartley, 1960).

Head

Head occurs as a thin (less than 1 m) blanket over much of the district, but these slope deposits have been mapped only where they attain a significant thickness or have a marked topographic expression. They formed under periglacial conditions through the downslope movement of material by frost creep or saturated flow, and are most commonly preserved in the bottom of narrow valleys. The deposits are variable, depending on their up-slope source, but generally consist of clayey silt or sand with variably abundant, locally derived, angular clasts and/or rounded glacial erratics.

Landslips

Landslips are present, both within bedrock e.g. [SO 3465 4655] and superficial deposits e.g. [SO 0860 4030]. Most are relatively small and have developed on glacially over-steepened valley sides. Two landslips in bedrock [SO 2950 4155]; [SO 3395 4210] occur at, or near, the level of the Bishop's Frome Limestone Member, and may be related to sapping along a spring line.

Chapter 3 Applied geology

Earth science factors have a significant influence on land-use, planning and development. Their early consideration can help to ensure that developments are compatible with ground conditions, and that, where appropriate, mitigation measures are taken. Exploitation of mineral resources frequently conflicts with agricultural land-use, pre-existing developments and the environment. Geological hazards may present a potential public health risk or require costly remediation. Engineering ground conditions and designated conservation sites strongly influence the location and design of any new development.

Mineral resources

Parts of the Hay-on-Wye district are included in three reports on mineral resources and their economic potential in relation to planning issues (Department of the Environment, 1992; Highley et al., 1997; Bloodworth et al., 1999). Commercial operations are currently limited to four quarries in the north of the district. Hard-rock aggregate is being produced from the Dolyhir Limestone Formation and the underlying Precambrian rocks in the Old Radnor Inlier at Gore [SO 2565 5922], Dolyhir [SO 2430 5842] and Strinds [SO 2420 5790] (Plate 2). A 'Tilestones' facies near the base of the Old Red Sandstone sequence is being worked for building stone at Little Gwernilla Quarry [SO 2176 5326].

The district has a long record of small-scale workings, particularly for the local provision of building materials. Thick sandstones within the Old Red Sandstone were formerly quarried on Cusop Hill [SO 2580 4060] and Merbach Ridge [SO 3034 4472] for building stone, and the Bishop's Frome Limestone was worked for lime along much of its crop. The Sandstone Member of the Downton Castle Sandstone Formation has been extensively quarried on a small scale for roofing tiles, and locally, north of the River Wye, mudstones within the Raglan Mudstone Formation have been dug for brick-making. In the north of the district there are numerous small quarries exploiting sandstones and 'flaggy' facies within the Wenlock and Ludlow Series strata, and also dolerites and tuffs in the Builth–Llandrindod Inlier. Peat was formerly cut for fuel.

Small scale mining for lead took place at Hanter Hill in the late 18th century, and galena was probably the target of a trial level driven into the north side of Gilwern Hill [SO 096 589] (Hall, 1971). Copper mineralisation was recorded by Murchison (1839) at Stanner and Old Radnor Hill, but no remains of any former workings are visible.

Sand and gravel has been exploited only on a small scale. The value of any such deposit as a workable resource is constrained by environmental designations, distance from major markets, the sand to gravel ratio, the proportion of fines and oversize clasts, the quantity of deleterious rock types, the position of the water table, the abundance of interbedded unwanted material, and the thickness of the deposit and of its overburden. Hummocky Glacial Deposits cover a sizeable area and are locally over 30 m thick, but display considerable variation in thickness and grading. In addition, the proportion of deleterious mudstone clasts may be too great. River Terrace Deposits are likely to include high quality resources, but their commercial value may be limited by the quantity of material that can be extracted above the water table. The Glaciofluvial Deposits represent an important potential resource, although they may contain a significant proportion of oversize clasts.

Water resources

Water is supplied principally from sources lying outside of the district. Groundwater is currently abstracted for public supply only in the vicinity of Stanner, but private supply boreholes and springs are widely utilised elsewhere in the district. These tap aquifers that are generally of limited or local potential, and that lie at shallow depths within superficial deposits or the weathered bedrock zone. The Old Red Sandstone sequence has limited permeability and a generally low primary porosity: the dominant groundwater flow mechanism and much of the storage is within joint- and fault-related fracture systems (Jones et al., 2000). Glaciofluvial and River Terrace deposits may have limited aquifer potential, particularly where there is hydraulic continuity with rivers. The variable nature of the Hummocky Glacial Deposits renders them only suitable for local supply and, as with other drift-hosted aquifers, they are vulnerable to surface contamination.

Potential geological hazards

River flood plains within the district are prone to regular flooding. An indication of those areas which are susceptible is given by the extent of Alluvium on the map, and are shown on Environment Agency maps. In addition, areas shown as Peat are likely to experience regular water-logging, and Head and Alluvial Fan deposits may be inundated during exceptional weather events.

Slope instability, typically manifested as landslip, occurs on a small scale within the district. About a dozen landslips were identified by the air-photo interpretation, mostly in drift deposits on the sides of steeply incised valleys. Several, however, occur in the bedrock at, or just below, the level of the Bishop's Frome Limestone Member and are probably related to a spring line at this level. The current activity of the landslips in the district is unknown, but continued toe erosion by water courses has the potential to trigger further movement.

Radon is a naturally occurring ionising gas produced by the radioactive decay of uranium, which is present in small quantities in all natural rocks and soils. Site-specific reports assessing the potential risk from radon, and the level of remedial measures required for new developments, can be obtained from the BGS Enquiries Office, Keyworth.

Methane is an explosive, asphyxiant gas capable of migrating through permeable strata and accumulating in voids and poorly ventilated spaces. It is likely to be generated by the decomposition of organic material in landfill sites and organic-rich deposits such as peat. Risks associated with it can be mitigated through the correct design of landfills and developments in susceptible areas.

Engineering ground conditions

Engineering ground conditions vary depending on the physical and chemical properties of the underlying materials, the topography, and the behaviour of ground- and surface water. Most of the bedrock units in the district have high bearing capacities, except in weathered zones. Dissolution of calcretes within the Temeside Mudstone and Raglan Mudstone formations may lead to a considerable loss in rock strength and potentially, where calcretes are thick, to the development of voids. Pyritic mudstones in the Builth Mudstones Formation are generally unsuitable for fill, as weathering of the pyrite may corrode concrete, and the mudstones are susceptible to heave.

The engineering properties of the superficial deposits are more variable. They generally overlie a highly irregular rockhead surface and are therefore likely to exhibit considerable lateral variations in thickness. Peat and Lacustrine Deposits have low bearing capacities and can give rise to high settlements. In contrast, Glaciofluvial and River Terrace deposits generally present good foundation conditions, but may include running sands. Hummocky Glacial Deposits and the alluvial deposits are highly variable and are potentially susceptible to differential settlement. The same is true of former waste-burial sites [Weythel Walton [SO 242 577]; Lillapool [SO 340 432]] which, in addition, may generate methane and include contaminated land that may require remediation. Landslips are areas of known instability: their development requires careful investigation and appropriate foundation design.

Geological conservation

The geological heritage of the district forms a resource for tourism, education and scientific research, and is also an issue in planning and development. Geological localities considered to be of national or international importance are protected as Sites of Special Scientific Interest (SSSIs). These are statuary designated conservation sites that are protected under the Wildlife and Countryside Act, 1981. Existing SSSIs, and sites being considered for notification, are described in the Geological Conservation Review Series, published by the Nature Conservancy Council. Within the district there are six such sites, including examples designated for their palaeontological, stratigraphical and/or structural significance. Further information on the extent and designation of these, and of Regionally Important Geological Sites (RIGS), can be obtained from the Countryside Council for Wales, Plas Penrhos, Penrhos Road, Bangor, Gwynedd, LL57 2LQ.

Information sources

Further geological information held by the British Geological Survey relevant to the district is listed below. It includes memoirs, technical reports, digital data, documentary and material collections. Searches of indexes to some of the collections can be made on the Geoscience Data Index (GDI) system available in the BGS libraries and on the web site (see back cover for addresses). BGS Catalogue of maps, books, data and services is available on request. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre (NGRC), BGS, Keyworth. Geological advice for the district should be sought from the Regional Geologist, Integrated Geological Surveys (South), BGS, Keyworth. BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS web site or at Maclean Building, Crowmarsh, Gifford, Wallingford, Oxfordshire, OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

Maps

Published and unpublished geological maps, listed below, can be purchased through the regional BGS Sales Desks and the London Information Office in the Natural History Museum, or consulted at the BGS libraries. Original 1:63 360 scale maps are out of print but can be provided as facsimiles, or be consulted at the BGS library, Keyworth. Unpublished 1:25 000 scale maps are available on request. Many BGS products and data are available in digital form under licensing agreement, details of which are available from the Intellectual Property Rights manager at BGS, Keyworth. Current availability can be checked on the BGS web site. Groundwater vulnerability maps are published by the Environment Agency from data commissioned from the BGS and The Soil Survey and Land Research Centre (National Soil Resources Institute), and are available from BGS Sales Desks and The Stationery Office (020 7873 0011).

Geological maps

The district was originally surveyed at the scale of one inch to one mile by W T Aveline, H T de la Beche, H W Bristow, T E James, W E Logan, A C Ramsey, J Rees and D H Williams. The results were published in 1845–1857 as parts of [Old Series] sheets 42 and 56. The Builth–Llandrindod Inlier was surveyed at the 1:10 000 scale by D G Woodhall in 1988–1993. A small area along the western boundary was surveyed at the 1:10 000 scale by P R Wilby in 1999–2000 as part of the survey of the 1:50 000 Geological Sheet 196 Builth Wells.

Digital geological map data

In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights manager at BGS Keyworth. The main datasets are:

The current availability of these can be checked on the BGS web site at:

http://www.bgs.ac.uk/products/ digitalmaps/digmapgb.html

Books, reports and other publications

Books, reports and other publications relevant to the district are listed in the References. Most of the these are available for consultation at the BGS library, Keyworth.

Documentary collections

Records of boreholes and site investigations pertaining to the district are held at NGRC, Keyworth, and are available for consultation. Copies of most records are available for purchase through the Sales Desk and may be viewed through the GDI on the BGS web site.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on the accompanying map sheet are held in the Lexicon database, available on the BGS web site.

Material collections

Material collections from the district, including fossils, petrological hand specimens and thin sections, are available for inspection at the BGS, Keyworth. Index data for petrological specimens is listed in the BRITROCKS database, and for fossils in the Palaeosaurus database, both searchable through the GDI on the BGS web site.

References

Most of the references listed below are held in the Library of the British Geological Survey, Keyworth, Nottingham. Copies of the references may be purchased from the Library, subject to current copyright legislation. Bibliographical services and catalogue search facilities are available on line at geolib.bgs.ac.uk

Allen, J R L. 1985. Marine to freshwater: the sedimentology of the interrupted environmental transition (Ludlow–Siegenian) in the Anglo-Welsh region. Philosophical Transactions of the Royal Society of London B, Vol. 309, 85–104.

Allen, J R L. 1986. Pedogenic calcretes in the Old Red Sandstone facies (late Silurian–early Carboniferous) of the Anglo-Welsh area, southern Britain. 58–86 in Paleosols: their recognition and interpretation, Wright, V P (editor). (Oxford: Blackwell Scientific Publications).

Allen, J R L, and Williams, B P J. 1981. Sedimentology and stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh borders. Journal of the Geological Society of London, Vol. 136, 361–366.

Aldis, T S. 1904. Drift in the Wye valley. Transactions of the Woolhope Naturalists' Field Club, 325–329.

Banks, R W. 1856. On the Tilestones, or Downton Sandstones, in the neighbourhood of Kington, and their contents. Quarterly Journal of the Geological Society of London, Vol. 12, 93–101.

Bailey, R J. 1964. A Ludlovian facies boundary in south central Wales. Geological Journal, Vol. 4, 1–20.

Barclay, W J. 2004. The Scar. in The Old Red Sandstone of Great Britain. Barclay, W J, Browne, M A E, McMillan, A A, Pickett, E A, Stone, P, and Wilby, P R (editors). Geological Conservation Review Series, No. 31 (Peterborough: Joint Nature Conservation Committee.)

Barclay, W J, and Wilby, P R. 2003. Geology of the Talgarth district. Sheet explanation of the British Geological Survey, Sheet 214 (England and Wales).

Bartley, D D. 1960. Rhosgoch Common, Radnorshire: stratigraphy and pollen analysis. New Phytologist, Vol. 59, 238–262.

Bevins, R E, Kokelaar, B P, and Dunkley, P N. 1984. Petrology and geochemistry of Lower to Middle Ordovician igneous rocks in Wales: a volcanic arc to marginal basin transition. Proceedings of the Geologists' Association, Vol. 95, 337–347.

Bloodworth, A J, Cameron, D G, Harrison, D J, Highley, D E, Holloway, S, and Warrington, G. 1999. Mineral resource information for development plans phase one Herefordshire and Worcestershire: resources and constraints. British Geological Survey Technical Report, WF/99/4.

Brandon, A. 1989. Geology of the country between Hereford and Leominster. Memoir of the British Geological Survey, Sheet 198 (England and Wales).

Callaway, C. 1900. On Longmyndian inliers at Old Radnor and Huntley (Gloucestershire). Quarterly Journal of the Geological Society of London, Vol. 61, 511–520.

Campbell, S, and Bowen, D Q. 1989. Quaternary of Wales. Geological Conservation Review Series, A4.1. (Peterborough: Nature Conservancy Council.)

Clarke, B B. 1936. The post-Cretaceous geomorphology of the Black Mountains. Proceedings of the Birmingham Natural History and Philosophical Society, Vol. 16, 157–172.

Clarke, B B. 1954. The Old Red Sandstone of the Merbach Ridge, Herefordshire, with an account of the Middlewood Sandstone, a new fossiliferous horizon 500 feet below the Psammosteus Limestone. Transactions of the Woolhope Naturalists' Field Club, Vol. 34, 195–219.

Coster, D, Milsom, J, and Livermore, M. 1997. Gravity field variations around the Old Radnor Inlier: a preliminary study. The Radnorshire Society Transactions, Vol. 67, 14–19.

Davies, J R, Fletcher, C J N, Waters, R A, Wilson D, Woodhall, D G, and Zalasiewicz, J A. 1997. Geology of the country around Llanilar and Rhayader. Memoir of the British Geological Survey, Sheets 178 and 179 (England and Wales).

Department of the Environment. 1992. An appraisal of the land based sand and gravel resources of South Wales. Engineering Geology Unit, Department of Earth Sciences, University of Liverpool.

Dineley, D L. 1999. Bradnor Hill Quarry. 104–106 in Fossil fishes of Great Britain. Dineley, D L, and Metacalf, S J (editors). Geological Conservation Review Series, No. 16. (Peterborough: Joint Nature Conservation Committee.)

Dwerryhouse, A R, and Miller, A A. 1930. The glaciation of the Clun Forest, Radnor Forest, and some adjoining districts. Quarterly Journal of the Geological Society of London. Vol. 86, 96–126.

Elles, G L. 1940. The stratigraphy and faunal succession in the Ordovician rocks of the Builth–Llandrindod Inlier, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 95, 383–445.

Garwood, E J, and Goodyear, E. 1918. On the geology of the Old Radnor district, with special reference to an algal development in the Woolhope Limestone. Quarterly Journal of the Geological Society of London, Vol. 74, 1–30.

George, T N. 1980. Landform and structure in the terrain of the Tawe and Neath disturbances in South Wales. Proceedings of the Geologists' Association, Vol. 91, 155–168.

Gibbons, W, and Hórak, J M. 1996. The evolution of the Neoproterozoic Avalonian subduction system: evidence from the British Isles. 269–280 in Avalonian and related peri-Gondwana terranes of the circum-Atlantic. Nance, R D, and Thompson, M D (editors). Geological Society of America Special Paper, No. 304.

Grindley, H E. 1905. The glaciation of the Wye valley. Transactions of the Woolhope Naturalists' Field Club, 163–164.

Grindley, H E. 1923. A foraminiferous clay at Bredwardine, Herefordshire. Geological Magazine, Vol. 60, 88–90.

Hall, G W. 1971. Metal mines of southern Wales. (Westbury-on-Severn: G W Hall).

Hawley, D J. 2004. Cusop Dingle. in The Old Red Sandstone of Great Britain. Barclay, W J, Browne, M A E, McMillan, A A, Pickett, E A, Stone, P, and Wilby, P R (editors). Geological Conservation Review Series, No. 31 (Peterborough: Joint Nature Conservation Committee.)

Highley, D E, Cameron, D G, Linley, K A. 1997. Mineral resource information for development plans phase one south Wales: resources and constraints. British Geological Survey Technical Report, WF/97/10.

Holgate, N. 1977. Tourmaline from amphibolized gabbro at Hanter Hill, Radnorshire. Mineralogical Magazine, Vol. 41, 124–127.

Holgate, N, and Hallowes, K A K. 1941. The igneous rocks of the Stanner–Hanter district, Radnorshire. Geological Magazine, Vol. 78, 241–267.

Holland, C H. 1959. The Ludlovian and Downtonian rocks of the Knighton district, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 114, 449–482.

Holland, C H, and Lawson, J D. 1963. Facies patterns in the Ludlovian of Wales and the Welsh Borderland. Liverpool and Manchester Geological Journal, Vol. 3, 269–288.

Holland, C H, and Williams, E M. 1985. The Ludlow–Downton transition at Kington, Herefordshire. Geological Journal, Vol. 20, 31–41.

Hughes, C P. 1969. The Ordovician trilobite faunas of the Builth–Llandrindod inlier, central Wales. I. Bulletin of the British Museum (Natural History) (Geology), Vol. 18, 39–103.

Hughes, C P. 1971. The Ordovician trilobite faunas of the Builth–Llandrindod inlier, central Wales. II. Bulletin of the British Museum (Natural History) (Geology), Vol. 20, 115–182.

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication 68.

Jones, K A. 2000. Hanter Hill. 118–122 in Precambrian rocks of England and Wales. Carney, J N, Hórak, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, and Bevins, R E (editors). Geological Conservation Review Series, No. 20 (Peterborough: Joint Nature Conservation Committee.)

Jones, O T. 1947. The geology of the Silurian rocks west and south of the Carneddau range, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 103, 1–36.

Jones, O T, and Pugh, W J. 1941. The Ordovician rocks of the Builth district. A preliminary account. Geological Magazine, Vol. 78, 185–191.

Jones, O T, and Pugh, W J. 1948. The form and distribution of dolerite masses in the Builth–Llandrindod Inlier, Radnorshire. Quarterly Journal of the Geological Society of London, Vol. 104, 71–98.

King, W W. 1934. The Downtonian and Dittonian strata of Great Britain and north-western Europe. Quarterly Journal of the Geological Society of London, Vol. 90, 526–570.

Kirk, N H. 1947. The geology of the anticlinal disturbance of Breconshire and Radnorshire: Port Faen to Presteigne. Unpublished PhD thesis, University of Cambridge.

Kirk, N H. 1951. The Upper Llandovery and Lower Wenlock rocks of the area between Dolyhir and Presteigne, Radnorshire. Proceedings of the Geological Society of London, Vol. 147, 56–58.

Kirk, N H. 1952. The tectonic structure of the anticlinal disturbance of Breconshire and Radnorshire: Port Faen to Presteigne. Proceedings of the Geological Society of London, Vol. 148, 87–91.

Kokelaar, P. 1988. Tectonic controls of Ordovician arc and marginal basin volcanism in Wales. Journal of the Geological Society of London, Vol. 145, 759–775.

Kokelaar, P, Howells, M F, Bevins, R E, Roach, R A, and Dunkley, P N. 1984. The Ordovician marginal basin of Wales. 245–269 in Marginal basin geology, volcanic and associated sedimentary and tectonic processes in modern and ancient marginal basins. Kokelaar, B P, and Howells, M F (editors). Special Publication of the Geological Society of London, No. 16.

Lewis, C A. 1966. The Breconshire end-moraine. Nature, Vol. 212, 1559–1561.

Luckman, B B. 1970. The Hereford Basin. 175–196 in The glaciations of Wales and adjoining regions. Lewis, C A (editor). (London: Longman.)

Miller, C G. 1995. Ostracode and conodont distribution across the Ludlow–Prídolí boundary of Wales and the Welsh Borderland. Palaeontology, Vol. 38, 341–384.

Murchison, R I. 1839. The Silurian System. (London: John Murray.)

Parker, A, Allen, J R L, and Williams, B P J. 1983. Clay mineral assemblages of the Townsend Tuff Bed (Lower Old Red Sandstone), South Wales and the Welsh Borders. Journal of the Geological Society of London, Vol. 140, 769–779.

Patchett, P J, Gale, N H, Goodwin R, and Humm, M J. 1980. Rb-Sr whole-rock isochron ages of the late Precambrian to Cambrian igneous rocks from southern Britain. Journal of the Geological Society of London, Vol. 137, 649–656.

Pharaoh, T C, and Carney, J N. 2000. Introduction to the Precambrian rocks of England and Wales. 3–17 in Precambrian rocks of England and Wales. Carney, J N, Hórak, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, and Bevins, R E (editors). Geological Conservation Review Series, No. 20 (Peterborough: Joint Nature Conservation Committee.)

Pocock, T I. 1940. Glacial drift and river terraces of the Herefordshire Wye. Zeitschrift fur Gletscherkunde, Vol. 27, 98–117.

Rice, R J. 1957a. The drainage pattern and upland surfaces of south-central Wales. A re-examination in the light of existing theories. Scottish Geographical Magazine, Vol. 73, 111–122.

Rice, R J. 1957b. The erosional history of the upper Wye basin, central Wales. Geographical Journal, Vol. 123, 356–370.

Richardson, J B, and McGregor, D C. 1986. Silurian and Devonian spore zones of the Old Red Sandstone continent and adjacent regions. Bulletin of the Geological Survey of Canada, Vol. 364, 1–79.

Ritchie , M E A, and Wright, F. 1991. The 1988 Hay-on-Wye earthquake sequence. British Geological Survey Technical Report, WL/91/31.

Schofield, D I, Davies, J R, Waters, R A, Wilby, P R, Williams, M, and Wilson, D. 2004. Geology of the Builth Wells district. Sheet explanation of the British Geological Survey, Sheet 196 (England and Wales).

Sheldon, P R. 1987. Parallel gradualistic evolution of Ordovician trilobites. Nature, Vol. 330, 561–563.

Siveter, David J. 2000. Ludford Lane and Ludford Corner. 432–437 in British Silurian Stratigraphy. Aldridge, R J, Siveter, David, J, Siveter, Derek J, Lane, P D, Palmer, D, and Woodcock, N H (editors). Geological Conservation Review Series, No. 19 (Peterborough: Joint Nature Conservation Committee.)

Smith, R D A, and Ainsworth, R B. 1989. Hummocky cross-stratification in the Downton of the Welsh Borderland. Journal of the Geological Society of London, Vol. 146, 897–900.

Stamp, L D. 1923. The base of the Devonian, with special reference to the Welsh Borderland. Geological Magazine, Vol. 60, 385–410.

Straw, S H. 1937. Higher Ludlovian rocks of the Builth district. Quarterly Journal of the Geological Society of London, Vol. 93, 406–456.

Tyler, J E. 1987. Clastic marine facies in the Ludlow of the central Welsh region. Unpublished PhD thesis, University of Cambridge.

Tyler, J E, and Woodcock, N H. 1987. The Bailey Hill Formation: Ludlow Series turbidites in the Welsh Borderland reinterpreted as distal storm deposits. Geological Journal, Vol. 22, 73–86.

Weaver, J D. 1974. Jointing along the Swansea Valley Distrurbance between Clydach and Hay-on-Wye, South Wales. Geological Magazine, Vol. 111, 329–336.

Weaver, J D. 1975. The structure of the Swansea Valley Distrurbance between Clydach and Hay-on-Wye, South Wales. Geological Journal, Vol. 10, 75–86.

Williams, G J. 1968. Contributions to the Pleistocene geomorphology of the Middle and Lower Usk valley. Unpublished PhD thesis, University of Wales.

Woodcock, N H. 1984. The Pontesford Lineament, Welsh Borderland. Journal of the Geological Society of London, Vol. 141, 1001–1014.

Woodcock, N H. 1987. Kinematics of strike-slip faulting, Builth Inlier, Mid-Wales. Journal of Structural Geology, Vol. 9, 353–363.

Woodcock, N H. 1988. Strike-slip faulting along the Church Stretton Lineament, Old Radnor Inlier, Wales. Journal of the Geological Society of London, Vol. 145, 925–933.

Woodcock, N H. 2000. Dolyhir and Strinds Quarries. 114–118 in Precambrian rocks of England and Wales. Carney, J N, Hórak, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, and Bevins, R E (editors). Geological Conservation Review Series, No. 20 (Peterborough: Joint Nature Conservation Committee.)

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Woodcock, N H, and Pauley, J C. 1989. The Longmyndian rocks of the Old Radnor Inlier, Welsh Borderland. Geological Journal, Vol. 24, 113–120.

Woodcock, N H, and Tyler, J E. 1993. The Ludlow and Prídolí of the Radnor Forest to Knighton area. 209–228 in Geological excursions in Powys, central Wales. Woodcock, N H, and Bassett, M G (editors). (Cardiff: University of Wales Press.)

Zalasiewicz, J and Williams, M. 1999. Graptolite biozonation of the Wenlock Series (Silurian) of the Builth Wells district, central Wales. Geological Magazine, Vol. 136, 263–283.

Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland.The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.

The area described in this sheet explanation is indicated by a solid block.

(Index map)

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Ordovician succession of the Builth–Llandrindod Inlier.

(Figure 2) Interpretative log of the lower part of the Gelli Tuff Member, east bank of Camnant Brook [SO 0962 5577].

(Figure 3) Old Red Sandstone (Siluro-Devonian) succession in the district.

(Figure 4) Bouguer gravity anomaly map of the district (framed) and the surrounding area. The anomalies are shown as colour-shaded relief, illuminated from the north, and have been calculated using a variable Bouguer reduction density. Contours are at 1 milligal (=10-5m/s2) intervals.

(Figure 5) Total field aeromagnetic anomaly map of the district (framed) and the surrounding area. The anomalies, relative to IGRF95, are shown as colour-shaded relief, illuminated from the north, and are based on data from the Hi-Res 1 survey (1998). Although the data were subject to manual de-culturing, some anomalies of non-geological origin may still be present. Contours are at 10 nanotesla intervals.

Plates

(Plate 1) Satellite image of the Hay-on-Wye district. The Church Stretton Fault Zone (CSFZ) and Swansea Valley Disturbance (SVD) form clear lineaments, the former including the topographically prominent Stanner–Hanter (SH) and Old Radnor inliers (OR). Quarries in the Old Radnor Inlier appear blue. The arcuate south-eastern margin of the Builth–Llandrindod Inlier (BI) is well defined. Wenlock and Ludlow strata form the dissected high ground north-west of the CSFZ; rocks of the Old Red Sandstone sequence underlie most of the more subdued ground to the south-east. (Winter false colour Landsat 5 TM composite image of bands 4, 5 and 7). Abbreviations: BM Black Mountains; E Erwood; GV Golden Valley; HW Hay-on-Wye; K Kington; RH Rhulen Hills

(Plate 2) Strinds Quarry [SO 2420 5790], north face, Old Radnor Inlier. Massive limestones of the Silurian Dolyhir Limestone Formation (upper face) lie with marked unconformity on steeply dipping sandstones and conglomerates of the Precambrian Strinds Formation (lower face). Both units are worked for hard-rock aggregate (P535 175).

(Plate 3) Bettws Mill Tuff Member, disused quarry [SO 1191 5669]. Turbiditic basic lapilli-tuffs and red-purple mudstones are interbedded with subordinate, thin, pale weathering acid tuffs. Lens cap for scale (GS1298).

(Plate 4) Raglan Mudstone Formation. The Scar [SO 3540 4440]. Sharp-based fluvial sandstone fining upwards into a siltstone and mudstone-dominated sequence. Hammer for scale (GS1297).

(Front cover) Front cover. Silurian strata, cropping out in the bed of the River Wye. Looking north-west from Erwood Bridge [SO 0896 4374]. (Photograph: C F Adkin; P535177).

(Rear cover)

(Geological succession) Geological succession in the Hay-on-Wye district.

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey

Figures

Figure 1 Ordovician succession of the Builth–Llandrindod Inlier

Lithostratigraphy Lithology Sections Depositional environment
LLANFAWR MUDSTONES FORMATION (LrM) (up to 55 m in the district) dark grey mudstone, fossiliferous Gilwern Hill [SO 0916 5923] Low energy, dysaerobic, offshore setting.
BULITH VOLCANIC GROUP CWM-AMLIW TUFF FORMATION (CaT) (30 m) fine-grained acid ash-flow tuff Gilwern Hill [SO 0926 5922] Subaqueously deposited, non-welded, pyroclastic flow (ignimbrite), the product of a large, explosive eruption from un unknown centre.
LLANELWEDD VOLCANIC FORMATION (Ldd) (40-60 m in the district) feldspar-phyric, locally amygdaloidal, basalt lavas Perthi Common [SO 0984 5468] A series of effusive, basaltic eruptions, possibly from the same centres as the Carneddau Formation, supplied subaerial, basalt lava flows from the south.
CARNEDDAU VOLCANIC FORMATION (Cdu) (up to 35 m) massive and graded, crystal-rich, basic lapilli-tuffs and fine- grained tuffs Perthi Common [SO 0954 5472] A prolonged phase of explosive basaltic volcanism which supplied crystal-rich, pyroclastic debrites (massive tuffs) and turbidites (graded tuffs).
GILWERN VOLCANIC FORMATION (Gwn) (185-235 m) massive or graded, basic lapilli-tuffs and reworked tuffs; inter- bedded mudstones (md) and graded tuffs in upper part; subsidiary dacite intrusions/lavas (RD) and basalt lavas (B) Perthi Common [SO 0962 5485]; Howey Brook [SO 0922 5904]; (md) [SO 0909 5920] to [SO 0922 5904]; [SO 1076 5729]; (RD) [SO 0940 5945]; [SO 0918 5646]; (B) [SO 1072 5694] to [SO 1093 5734] Amalgamated, pyroclastic debris flows (massive tuffs) and turbidites (graded tuffs) emplaced during an initial series of large, explosive eruptions at a centre to the south or west of the inlier. Interbedded mudstone and tuff sequences record the subsequent decline in volcanic activity. Dacite bodies occur as high level intrusions and extrusive lava-domes.
LLANDRINDO TUFF FORMATION (LdT) (up to 60 m) acid ash-flow tuff Gilwern Hill [SO 0908 5848], [SO 0990 5920]; Careg-wiber Bank [SO 0904 5942]; Howey Brook [SO 0925 5903] Large volume, largely non-welded, subaqueously deposited pyroclastic flow (ignimbrite) generated by one or more violent silicic eruptions from a vent to the north of the district.
CAMNANT MUDSTONES FORMATION (Cte) (900 m+) dark grey, fossiliferous mudstone with sub- ordinate micaceous sandstones and basic lapilli-tuffs (Z), including GELLI TUFF (Glh) and BETTWS MILL TUFF (Btw) MEMBERS, both up to 50m thick Camnant Brook [SO 0888 5771] to [SO 0959 5560]; (Btw) [SO 1191 5669]; [SO 1299 5834]; (Glh) [SO 0981 5815]; [SO 0962 5577]; [SO 1056 5539]; [SO 1122 5793] Low energy, dysaerobic, offshore setting with this sandstones introduced by storm-generated density currents. Associated lapillituffs were emplaced as pyroclastic debris flows initiated by explosive volcanic eruptions from nearby vents, and by remobilisation of pre-existing unconsolidated pyroclastic piles.

Geological succession in the Hay-on-Wye district

See (Geological succession) for accurate layout.

Age Period Lithostratigraphy/deposit Thickness (m)
0.01 QUATERNARY HOLOCENE Alluvium, Lacustrine Deposits, Landslip, Peat, Alluvial Fan Deposits, River Terrace Deposits, Head
0.026 DEVENSIAN Head, Lacustrine Deposits, Till, Hummocky Glacial Deposits, Glaciolacustrine deposits, Glaciofluvial Deposits
Age Period Lithostratigraphy/deposit Thickness (m)
Unconformity
417 DEVONIAN LOWER DEVONIAN St. MAUGHANS FORMATION (SMg) red mudstone and siltstone; purple, red-brown and green sandstone (sal; some calcretes (lc) and intraformational conglomerates
SILURIAN PRIDOLI SERIES RAGLAN MUDSTONE FORMATION (Rg) red mudstone and siltstone; some sandstones (sal and calcretes, including Bishop's Frome Limestone Member (BEL) at top. Includes the Townsend Tuff Bed (TwT) 650-900
TEMESIDE MUDSTONE FORMATION (Tem) olive-green siltstone with calcrete glaebules, locally overlying yellow micaceous sandstone of the Downton Castle Sandstone Formation (not shown separately 30-50
419
WENLOCK AND LUDLOW STRATA CAE'R MYNACH FORMATION (Car) grey-brown sandstone and siltstone, some horizons bioturbated and shelly; subordinate mudstone. 80
UNDIFFERENTIATED (WLu) grey mudstone, siltstone and sandstone, locally shelly and/or bioturbated. Includes the Bailey Hill Formation in the north of the district and black laminated mudstone of the Builth Mudstones Formation at its base (not shown separately) >1000
DOLYHIR LIMESTONE FORMATION grey limestone, shelly 25-30
429
Unconformity
ORDOVICIAN LLANVIRN SERIES LLANFAWR MUDSTONE FORMATION (LrM) dark grey mudstone, fossiliferous 55
BULITH VOLCANIC GROUP basic tuff; grey and red-purple mudstone (md); subordinate acid tuff and breccia; some basalt lavas 255-355
(B), dolerite sills (Do) and dacite lavas/intrusions (RD)
CAMNANT MUDSTONES FORMATION (Cte) dark grey mudstone, fossiliferous, subordinate sandstone and basic tuff (Z), including Bettws Mill (Btw) and Gelli 915
Unconformity
PRECAMBRIAN EDIACARAN STRINDS FORMATION (Str) pale green-grey or purple grey sandstone
550 YAT WOOD FORMATION (YtW) pale green siltstone and sandstone; grey mudstone
Unconformity
702 CRYOGENIAN Stanner-Hanter Inlier dolerite (Dpc) and gabbro (EG); subordinate granophyric granite (gGG)