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Geology of the country between Hereford and Ross-on-Wye — a brief explanation of the geological map Sheet 215 Ross-on-Wye

W J Barclay and N J P Smith

Bibliographic reference: Barclay, W J, and Smith, N J P. 2002. Geology of the country between Hereford and Ross-on-Wye — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 215 Ross-on-Wye (England and Wales). Keyworth, Nottingham: British Geological Survey

Keyworth, Nottingham: British Geological Survey, 2002.

© NERC copyright 2002

(Front cover) Front cover Ross-on-Wye viewed from the west. The old part of the town is built below and on top of an old river cliff of Brownstones Formation (Old Red Sandstone). (Photograph R E Collins; A13272).

(Rear cover)

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

Notes

The word 'district' refers to the area of geological map, 1:50 000 Series Sheet 215 Ross-on-Wye. National grid references lie within 100 km square SO. Symbols in brackets after lithostratigraphical names are the same as those used on the geological map. Numbers given in plate captions refer to the BGS photograph collection.

Acknowledgements

This Sheet Explanation was compiled by W J Barclay and edited by S G Molyneux. It gives a brief overview of the geology of the area of provisional geological Sheet 215 Ross-on-Wye, using data available at the time of compilation of the map. The map was compiled by W J Barclay and D Wilson, following a brief field visit to the Old Red Sandstone outcrop (mainly in the area of faulted ground in the west). It incorporates available BGS 1:10 000 mapping and the results of examination of air photographs and satellite imagery at broad reconnaissance level by E A O'Connor. N J P Smith compiled the subsurface part of the horizontal cross-section from seismic data. No systematic 1:10 000 survey of the whole sheet has been carried out by BGS. Apart from small peripheral areas, BGS 1:10 000 surveying is restricted to the area south of Hereford, by A Brandon and B Hains in 1977–78, and the area of the town of Ross-on-Wye, by D B Smith in 1978–79. We are grateful to H C Squirrell and E V Tucker for making available their manuscript maps of the Woolhope Inlier.

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty's Stationery Office. © Ordnance Survey licence number GD272191/2002

Geology of the country between Hereford and Ross-on-Wye (summary from rear cover)

This Sheet Explanation is the first general account of the 1:50 000 Series Sheet 215 Ross-on-Wye geological map. The picturesque Wye valley, with its spectacular, wide, deeply incised meander loops, crosses the area from north-west to south-east. Both the Wye valley and the Fownhope–Woolhope area to the east are designated Areas of Outstanding Natural Beauty.

The district includes most of the Woolhope Inlier, an upfold of Silurian rocks that continues to attract geological attention, and contains several Sites of Special Scientific Interest. It also contains, in Perton Quarry, the only working quarry in the district. Fownhope Borehole, drilled in 1991, proved that a previously unknown Lower Palaeozoic rift basin, for which there is no surface evidence, underlies the inlier. Most of the district is underlain by Old Red Sandstone rocks of late Silurian to Devonian age, which form the red soils and fertile agricultural land. Two small areas of Carboniferous strata at the northern extremity of the Forest of Dean are present in the south-east; the highest strata in the district are the coal-bearing beds of the Upper Coal Measures.

Quaternary deposits are most widespread in the north-west, where they were laid down as moraine at the terminus of the Late Devensian Wye glacier about 13 500 years ago. Large, flat alluvial areas mark the sites of former glacial lakes that were dammed by ice and moraine. Sands and gravels are most extensive in the north, in the suburbs of Hereford. They were formed as outwash from the Wye glacier and as river terrace deposits.

The sandstones were formerly worked for building stone, and the limestones (including calcretes of the Old Red Sandstone) were quarried for stone and lime-burning. Today, the only working quarry in the district is Perton, where rock aggregate is being produced from the Aymestry Limestone Formation and adjacent Upper and Lower Ludlow Shales. Sand and gravel was worked until about 1990 at Belmont Pit, Hereford. Brickclay was formerly dug from the Raglan Mudstone Formation at Grafton. Coal was extracted from Great Howle opencast site from 1972 to 1977. Seismic surveys and drilling indicate that the hydrocarbon potential of the district is low.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Provisional Series Sheet 215 Ross-on-Wye, published as a solid and drift geology edition in 2000.

The area described lies in the county of Herefordshire in the Welsh Borderland, the picturesque Wye valley crossing it from north to south. The small market town of Ross-on-Wye lies in the south-east; the southern suburbs of the city of Hereford lie in the north. Most of the district is underlain by Old Red Sandstone rocks of late Silurian to Devonian age (Figure 1), which form the fertile, agricultural lower ground as well as the extensive area of hill farming in the south-central parts. Older Silurian strata outcrop in the Woolhope Anticline in the east, and small areas of Carboniferous strata are present south of Ross-on-Wye.

The oldest rocks at the surface belong to the Haugh Wood Formation of late Llandovery (Silurian) age, which crop out in the core and highest part of the Woolhope Anticline (or Inlier) in the east. These are succeeded by the Woolhope Limestone, Coalbrookdale and Much Wenlock Limestone formations of Wenlock age, and the Lower Ludlow Shales, Aymestry Limestone Formation and Upper Ludlow Shales of Ludlow age. All these formations were deposited as shallow marine sediments on a storm-affected shelf, on the stable Midlands Microcraton (or Platform) on the south-east flank of the Lower Palaeozoic Welsh Basin. A thick (1300 m) sequence of Llandovery strata below the exposed rocks in the core of the Woolhope Anticline demonstrates that rifting of the Midlands Microcraton took place in the early Llandovery, with sandstone deposition in basins separated by the Woolhope Fault. Later inversion of this fault, probably during the end-Carboniferous Variscan Orogeny, produced the Woolhope Anticline in its hanging wall.

The Upper Ludlow Shales are overlain by the Rushall Formation of Přídolí age. This consists mainly of shallow-marine, near-shore sandstones, and represents the final shoaling and infilling of the Welsh Basin. Collision between the continents of Avalonia/Baltica and Laurentia from mid-Silurian to mid-Devonian times produced the Caledonian mountain chain. Its uplift and erosion resulted in continental deposition of the alluvial red beds of the Old Red Sandstone in the Přídolí and throughout the succeeding Devonian Period. The Lower Old Red Sandstone rocks represent a coarsening-upwards succession, deposited as the alluvial facies belts shifted southwards. The mudstone-dominated Raglan Mudstone Formation was deposited in a marine-influenced coastal plain, subject to repeated subaerial exposure and carbonate soil formation. The carbonate soils (calcretes) formed in an arid, seasonally wet climate and are today seen as limestone nodules within the mudstones and, less commonly, as massive, rubbly beds of limestone. One such bed, the Bishop's Frome (or Psammosteus) Limestone, is a thick, regional calcrete marking the top of the formation.

The early Devonian St Maughans Formation comprises sandstones and mudstones deposited in meandering streams and their floodplains. It is succeeded by more proximal, high-energy, low-sinuosity stream deposits of the Brownstones Formation. This formation is truncated by a regional unconformity, which is overlain by fluvial strata of the late Devonian Upper Old Red Sandstone. These are present only in small areas south of Ross-on-Wye, and comprise the Quartz Conglomerate Formation and the Tintern Sandstone Formation. The Quartz Conglomerate was deposited in south-flowing, coarse gravel-bed braided streams, and the Tintern Sandstone in the channels of more sinuous streams. Early Carboniferous marine transgression of the alluvial plain resulted in a transition into the marine limestones and fissile mudstones of the Lower Limestone Shale Group, the basal unit of the Carboniferous Limestone. Only small areas of the Lower Limestone Shale Group are present in the south-east of the district, in the cores of the Howle Hill and Wigpool synclines. Dolostones of the overlying Lower Dolomite Formation, present only in the Wigpool Syncline, represent deposition on a tropical, marine carbonate ramp. They are absent in the Howle Hill Syncline, where a small area of coal-bearing Upper Coal Measures (Trenchard Formation) unconformably overlies the Lower Limestone Shale Group. The Trenchard Formation, of Westphalian D age, is the youngest rock stratum in the district. It is a small part of the thick succession of fluviodeltaic deposits that accumulated in the Carboniferous Variscan foreland basin to the north of the advancing Variscan mountain chain, from which the sandstones were derived. There is no record of the period from the late Carboniferous until the

Quaternary, when the superficial ('Drift') deposits of the district were laid down. The late Devensian Wye glacier reached the north-west margin of the district, where its ablation and retreat left a wide spread of hummocky morainic till and gravels. Extensive alluvial spreads mark the sites of lakes where meltwater was impounded by the retreating ice and its moraine. The dissected remnants of river terraces in the district predate and postdate the late Devensian stadial. The deeply incised, spectacular meanders of the River Wye point to a time when the river valley was much wider than today. Some gravels beyond the limit of the late Devensian glacier may be glaciofluvial outwash deposits laid down during an earlier (?Anglian) glaciation. Alluvium, head deposits and landslips have formed since the late Devensian until the present day.

History of research

The district is included in Sheet 43 of the Geological Survey published at 1:63 360 scale in 1845 and 1855. This followed publication of Murchison's (1839) study of the Silurian rocks of the Welsh Borderland, including the Woolhope Inlier. Phillips (1848) also described the Woolhope Inlier. Symonds (1872) gave some details of the Old Red Sandstone rocks around Eywas Harold, but the Silurian rocks of the Woolhope Inlier continued to attract most geological attention (Strickland, 1853; Brodie, 1871; Gardiner, 1927; Pocock, 1930). Gardiner's account was the most comprehensive until the work by Squirrell and Tucker (1960), who mapped the biostratigraphical/lithostratigraphical units of the Ludlow Series recognised in the type area at Ludlow (Earp and Hains, 1971, fig. 33). Squirrell and Tucker's paper remains the definitive account. Since then, the Woolhope Inlier has continued to attract attention in sedimentological studies of the Welsh Borderland (e.g. Hurst, 1975; Watkins, 1979; Cherns, 1980). It is included in a Geologists' Association guide (Squirrell and Tucker, 1967; revised 1982) and in a recent volume on Silurian stratigraphy (Aldridge et al., 2000). The northernmost part of the inlier, as well as smaller inliers to the north are described in the BGS Hereford memoir (Brandon, 1989).

The Old Red Sandstone first received detailed sedimentological attention by Allen (1971). His analysis of the pebbles in the Brownstones at Ross-on-Wye (Allen, 1974) was followed by two papers (Allen, 1983a, b) on the detailed facies architecture of the alluvial sequences of the Brownstones in the excellent exposures in road cuttings on the M50, on the Ross-on-Wye bypass (A449T) and near the Royal Hotel in Ross. The most recent published work synthesises the results of seismic reflection surveys and the subsequent drilling of the Fownhope Borehole in the Woolhope Inlier (Butler et al., 1997). In addition to revealing an exceptionally thick Llandovery sequence, the borehole proved the underlying Ordovician, Cambrian and Precambrian strata. Detailed, 1:10 000 geological surveys were carried out by BGS in the urban areas of Hereford in 1977–78 (Brandon and Hains, 1979) and Ross-on-Wye in 1978–79 (Smith, 1980). Apart from small areas of overlap from Sheet 233 Monmouth to the south, mapped by W C C Rose in 1933 (Welch and Trotter, 1961), from Sheet 198 Hereford to the north by A Brandon in 1977–78 (Brandon, 1989) and from Sheet 216 Tewkesbury to the east by B S P Moorlock on 1981–82 (Worssam et al., 1989), the remainder of the district remains to be surveyed in detail. The Quaternary terrace deposits of part of the district are depicted on the 1:25 000 scale maps of the Soil Survey (Hodgson and Palmer, 1971; Whitfield, 1971) and were described by Hey (1991). Richardson (1935) described the water supply of the district in detail, and the regional geology is included in Earp and Hains (1971).

Chapter 2 Geological description

Silurian

Marine Silurian strata outcrop in the Woolhope Inlier (Figure 2). The youngest Silurian strata, the Raglan Mudstone Formation, of Přídolí to early Devonian (Lochkovian) age and continental Old Red Sandstone facies, outcrops extensively around the inlier and in a large tract in the north-west of the district. The inlier is a faulted, asymmetrical pericline, which lies in the hanging wall of a major reverse north-west-trending fault (the Woolhope Fault; see p.22). The exposed rocks comprise a shallow-water shelf succession of mudstones, siltstones, sandstones and limestones ranging in age from late Llandovery to Přídolí. The geology and structure of the inlier is reflected in the topography, with the two main limestones, the Much Wenlock Limestone Formation and the Aymestry Limestone Formation, forming prominent ridges, the argillaceous rocks forming the intervening valleys, and the resistant sandstones of the Haugh Wood Formation forming the central topographic dome. The following description is a summary of the detailed account of the inlier by Squirrell and Tucker (1960).

Llandovery

The oldest strata exposed in the core of the Woolhope Inlier are the topmost 90 m of the Haugh Wood Formation (HW), subdivided into the sandstone-dominated Lower Haugh Wood Beds and the mudstone-dominated Upper Haugh Wood Beds. The Lower Haugh Wood Beds are flaggy, grey-blue, calcareous sandstones, decalcified by weathering to yellow and brown, with subordinate olive mudstones. The fauna includes Cyrtia exporrecta (Wahlenberg), Leangella cf. segmentum (Lindström), Clorinda cf. undata (J. de C. Sowerby), Rhabdocyclus binus (Lonsdale) and Costistricklandia lirata (J. de C. Sowerby). The lowest 6 m of the Upper Haugh Wood Beds are purple and olive argillaceous siltstones with thin, fossiliferous argillaceous limestones and blue-grey, calcareous siltstones. Coarser siltstones with common impure and crinoidal limestones and calcareous siltstones characterise the upper 3 m. Among these is the Petalocrinus Limestone, a distinctive pale grey to yellow, 0.02 m-thick bed of crinoid remains, many in growth position. It rests on a bed of large tabulate corals, the combined beds being 0.07 to 0.15 m thick (Pocock, 1930; Squirrell and Tucker, 1960, 1982). Eospirifer radiatus (J. de C. Sowerby), Leangella segmentum and Costistricklandia lirata are the commonest brachiopods, with the corals Goniophyllum pyramidale (Hisinger) and Ptychophyllum patellatum (Schlotheim) almost confined to these beds.

Wenlock

The Woolhope Limestone Formation (WoL), here in its type area, comprises 36 m of olive-grey to dark greenish grey, nodular, argillaceous limestones and pale olive-grey, rubbly, calcareous siltstones. Thicker, massive limestones occur in the middle of the formation and there are four bentonitic horizons. The formation rests with sharp lithological change on the Upper Haugh Wood Beds, but has a gradational top. A limited fauna dominated by the brachiopods Atrypa and Plectodonta (Squirrell and Tucker, 1960; Derek Siveter, 2000) is attributed to the Eoplectodonta duvalii community (Hurst, 1975). Corals are less common than in the underlying beds, but include massive and solitary forms. The basal beds have yielded one specimen of Ptychophyllum patellatum. The Wenlock trilobite Bumastus barriensis Murchison is also found. One of the four bentonite layers, and the one which persists throughout the inlier, was formerly exposed in Scutterdine Quarry [SO 577 368]. It is 0.6 m thick and divided by a 10 cm layer of cuboidally jointed, ferruginous, argillaceous limestone.

The Coalbrookdale Formation (Cbrd; for merly the Wenlock Shale) comprises 300 to 365 m of pale olive, calcareous, argillaceous siltstones. Thin, fossiliferous, argillaceous limestones occur in the middle of the formation, and calcareous nodules are present in the upper part. Small brachiopods including Dicoelosia biloba (Linnaeus) and Dalejina hybrida (J. de C. Sowerby) dominate the fauna. Atrypa spp., Antirhynconella linguifera (J. de C. Sowerby) and Parastrophinella rotunda (J. de C. Sowerby) are common in the calcareous beds and small turbinate corals are common in the mudstones. Fragments of the trilobites Otarion, Proetus and Phacops and large specimens of Dalmanites myops (König) occur at some horizons. Molluscs are quite common, but poorly preserved.

The Much Wenlock Limestone Formation (WeL; formerly the Wenlock Limestone) consists of 45 to 51 m of irregularly bedded, greyish olive, impure limestones and cleaner, blue-grey limestones. These are interbedded with buff-olive, calcareous, argillaceous siltstones. The base and top of the formation are limestone nodule beds, transitional with the underlying and overlying formations. Massive, lenticular bodies of fine-grained, micritic limestone ('ballstones') were patch reefs, one of which at Fownhope Park [SO 5760 3576] is over 3 m high and 6 m in diameter. Derek Siveter (2000) lists a varied fauna of at least ten corals (including branching corals such as Heliolites parvistella var. caespitosa Salter), two algae, three stromatoporoids, five bryozoans, two crinoids (including Crotalocrinus sp.), about twenty brachiopods, one bivalve, three gastropods, one cephalopod, three trilobites (of which Calymene is the commonest) and beyrichiacean ostracods. The brachiopods, which include large strophomenids, Atrypa reticularis (Linnaeus), Eospirifer, Leptaena, Meristina and Sphaerirhynchia, are assigned to the shallow water Sphaerirhynchia wilsoni community (Hurst, 1975). The patch reefs contain corals and stromatoporoids, some of which are in growth position.

Ludlow

The Ludlow Series is represented by up to 400 m of strata in the north of the inlier at Perton, but marked southerly attenuation results in only 60 m of beds in the south, around Upton Court, where there are local disconformities and the Aymestry Limestone Formation is absent. Several lithostratigraphical schemes have been applied to the succession, including local names (Squirrell and Tucker, 1960, 1982) and names used for the Ludlow type series (Squirrell and Tucker, 1967). In this account, Squirrell and Tucker's (1960) names are used, together with the traditional tripartite division of the Ludlow Series used in the adjacent districts of Hereford (Brandon, 1989) and Tewkesbury (Worssam et al., 1989). (Figure 3) shows their correlation. The series is rich in fossils, mainly species of small brachiopods. The formations erected by Squirrell and Tucker (1960) are based primarily on their brachiopod assemblages, and partly on their lithology.

The Lower Ludlow Shales (LLu) are subdivided into the Wootton Beds and overlying Lower Sleaves Oak Beds. The Wootton Beds are further subdivided into lower and upper units. The lower unit comprises 15 to 90 m of poorly bedded, pale grey and olive-grey, calcareous, argillaceous siltstones with a few thin, impure limestones. The upper unit, 22 to 97 m thick, is similar, but argillaceous limestones are more abundant and the bedding is more distinct. The units are distinguished faunally by the abundance of graptolites, including Pristograptus tumescens (Wood), and the appearance of Protochonetes cf. minima (J. de C. Sowerby) in the upper unit, in which Aegiria grayi (Davidson) is less common. The Lower Sleaves Oak Beds are 84 m thick in the north, thinning to 45 m at Sleaves Oak and about 15 m at Whittocks End. They are greenish grey, flaggy siltstones and fossiliferous limestones, with limestone nodules increasing in abundance in a gradational succession into the overlying Aymestry Limestone Formation. A rich and varied fauna includes an abundance of strophomenid brachiopods, as well as the gastropod Poleumita globosa (Schlotheim) and the trilobite Dalmanites myops.

The Aymestry Limestone Formation (AL) is 30 m in the north, thins to 2 m at Sleaves Oak and is absent at Upton Court. Named the Upper Sleaves Oak Beds by Squirrell and Tucker (1960), the formation consists mainly of nodular, olive-blue, argillaceous limestones with greenish grey, calcareous mudstone interbeds. Limestone development is best in the north, the formation becoming more argillaceous as it thins southwards. Limestone nodules are abundant in the upper 15 m of the formation, concomitant with a decrease in the

abundance of fossils and in the number of species. Many species are common throughout the Sleaves Oak Beds, but solitary corals, Gypidula sp. and Lingula lewisii (J. de C. Sowerby), together with Kirkidium knighti (J. Sowerby), are more abundant in the upper unit. Perton Quarry [SO 5970 3982] (Plate 1), straddling the border of the Ross-on-Wye and Hereford districts, provides the best section of the formation (Squirrell and Tucker, 1982; David Siveter, 2000). It contains several bentonites, which acted as slip-planes for landsliding in the quarry in 1979 (Brandon, 1989).

The Upper Ludlow Shales (ULu) were subdivided by Squirrell and Tucker into the Bodenham Beds and the overlying Perton Beds, each being further subdivided into upper and lower units. The Lower Bodenham Beds are 6 to 40 m thick and consist of thinly bedded and flaggy, pale olive-grey, calcareous, argillaceous siltstones with thin limestones. The limestones include intraformational conglomerates with calcareous pebbles, interpreted as reworked hardgrounds (Cherns, 1980), as well as coquina-type beds of articulated brachiopod shells and beds of broken shell debris. The base of the Lower Bodenham Beds is placed at the base of the lowest limestone conglomerate. There is a marked faunal change at this level, with many species disappearing, for example Poleumita globosa, Strophonella euglypha (Hisinger), Dalmanites myops and solitary corals, and forms such as Salopina lunata (J. de C. Sowerby) and Serpuloides longissimus (J. de C. Sowerby) appearing. The Upper Bodenham Beds are 0.15 to 5 m thick and consist of thinly bedded, dusky yellow, calcareous, argillaceous siltstones with lenticular beds of dark green-grey, shelly and argillaceous limestone. They are coarser and better bedded than the Lower Bodenham Beds. Their fauna is distinctive, with most of the species of the underlying unit dying out and few species surviving into the overlying Perton Beds. Dayia navicula (J. de C. Sowerby), Sphaerirhynchia wilsoni (J. Sowerby) and Hyattadina canalis (J. de C. Sowerby) decline or disappear and Shaleria ornatella (Davidson) appears in abundance. The Lower Perton Beds are 27 m thick at Perton in the north, thinning to 2.7 m at Upton Court. They are poorly bedded, pale olive-grey to dark yellow-brown argillaceous siltstones with calcareous beds in the lower half and softer mudstones in the upper. Many limestone beds are highly fossiliferous, the fauna of the beds being characterised by an abundance of individuals but a reduction in the number of species in comparison to the beds

below. Protochonetes ludloviensis (Muir-Wood) and Microsphaeridiorhynchus nuculus (J. de C. Sowerby) predominate. Thin layers of comminuted shell debris, conodonts and fish denticles (Squirrell, 1958), together with hardgrounds, point to reworking and depositional breaks. The Upper Perton Beds range in thickness from 2.4 to 21 m, but do not show the consistent pattern of southerly thinning of the underlying units. They are richly fossiliferous, well-bedded, calcareous, argillaceous olive-grey siltstones with common flaggy and massive calcareous siltstones and argillaceous limestones. Their well-bedded nature contrasts with the underlying poorly bedded unit. A 0.3 m-thick calcareous siltstone near the base persists throughout much of the inlier. Biotite-rich condensed beds with fish fragments, conodonts and broken and whole shell fragments underlie this siltstone. The faunal assemblage of the Upper Perton Beds is broadly similar to that of the Lower Perton Beds.

Přídolí

The Přídolí Series comprises the Rushall Formation and much of the Raglan Mudstone Formation. Both were included in the Devonian when the base of that period was placed at the base of the Ludlow Bone Bed, at the base of the Rushall Formation. The Silurian–Devonian boundary is now placed at a higher level. Its exact position is unknown, but is thought to lie in the upper part of the Raglan Mudstone Formation, below the Bishop's Frome Limestone. The formations are included in the continental Old Red Sandstone facies, the Raglan Mudstone Formation being the lowermost red bed unit. The Rushall Formation (Rus) is the local name given by Squirrell and Tucker (1960) to the Downton Castle Sandstone Formation of the Ludlow area. Allen and Dineley (1976) used the name Clifford's Mesne Sandstone for the formation in the M50 section, as named in the Gorsley and May Hill areas. Here, the formation comprises intercalated sandstones, mudstones and siltstones, and may include the Temeside Shale Formation of the West Midlands. The sandstones are hard, brown to yellow-brown, medium to coarse grained and cross-bedded. The mudstones and siltstones are pale greyish brown, medium brown and pale grey. Fossils are common, with abundant eurypterid remains. The carbonised alga Pachytheca, plant fragments, horny brachiopods, ostracods and fish fragments are also common. The Ludlow Bone Bed is represented locally by pockets containing abundant phosphatised fish fragments; the trilobite Acastella spinosa (Salter) has been recorded at one locality [SO 5762 3901]. Fish fragments are also present in the overlying 2.5 m of sandstone.

Siluro-Devonian

The Raglan Mudstone Formation (Rg) comprises a thick succession of red-brown mudstones and siltstones, with subordinate sandstones. The formation was proved to be about 385 m thick in the M50 to the east of the district (Allen and Dineley, 1976), but thickens westwards to about 700 m. The succession was deposited in coastal alluvial floodplains, sporadically influenced by marine influxes to produce wetland conditions. The climate was tropical, with seasonal rainfall, resulting in pedogenic alteration of the floodplain sedi ments. Carbonate soils (calcretes) developed to varying degrees of maturity. Small limestone nodules ('race') and calcified root traces (rhizoliths) are abundant, and more mature calcretes, now seen as massive, rubbly, pale green, red-brown and purple-mottled limestones, occur throughout. The best developed calcretes lie at the top of the formation, where the mature, regionally developed Psammosteus Limestone is given the local name Bishop's Frome Limestone Member (BFL) (Brandon, 1989).

In the west of the district, the formation's outcrop is repeated by a series of north-west-trending strike faults and it forms extensive dip slopes. Close to the east of the district on the M50, fourteen calcrete profiles have been recorded in the highest 85 m of the formation, the topmost being the thickest (about 10 m) and containing up to about 5 m of rubbly limestone at its top (Allen and Dineley, 1976). Sandstones are mainly red-brown, green and purple, and are commonly micaceous. They have sharp bases, with conglomerates of quartz pebbles and intraformational clasts. The sandstones represent wet-season, channelised, shallow ephemeral stream and unconfined sheet flood deposition. Bentonitic mudstones in the upper part of the formation exposed on the south bank of the Wye near Munderton [SO 4862 3876] are tentatively correlated with the Townsend Tuff Bed, a regionally developed volcanic ashfall tuff (Brandon and Hains, 1979; Allen and Williams, 1981; Brandon, 1989;).

Devonian

The Devonian lithologies of the district are shown in (Figure 4).

Lower Devonian

The St Maughans Formation (SMg) consists of about 630 m of red-brown mudstones and siltstones, purplish brown sandstones and subordinate intraformational conglomerates. Calcrete nodules and beds occur at some horizons, but calcrete is most common as reworked clasts in the conglomerates. A characteristic feature of the formation is the cyclic architecture of its component facies in fining-upwards alluvial cycles, representing deposition in the channels and floodplains of high-sinuosity rivers. The cycles range from metres to tens of metres in thickness. Mudstones and siltstones make up at least 60 per cent of the succession. The sandstones are purplish brown, red-brown and grey-green, fine- to coarse-grained, calcareous, finely micaceous, parallel-laminated and cross-bedded. The intraformational conglomerates are generally of pebble-grade and contain rounded to subangular clasts of mudstone, calcrete, and less commonly, sandstone in a coarse sandstone matrix. Fish fragments occur in the conglomerates and the sandstones. Tesseraspis, Pteraspis, Traquairaspis cf. symondsi (Lankester), Cephalaspis cf. lyelli Agassiz and acanthodian scales were collected by Allen and Dineley (1976). Cephalaspis langi Stensiö, C. whitei Stensiö and Securiaspis kitchini Stensiö have been recorded from the Kentchurch–Pontrilas area (Stensiö, 1932). The eurypterid Pagea symondsii (Salter) has been recorded from Rowlestone (Symonds, 1872).

The Brownstones Formation (Brs) forms the highest part of the Lower Old Red Sandstone magnafacies of South Wales and the Welsh Borderland. It reaches its maximum thickness in this area, with about 1200 m present. The formation consists of pebbly, cross-bedded, red-brown sandstones, which formed in an alluvial braided-river system. There are excellent, well-documented sections on the M50 and A449T, near the Royal Hotel in Ross-on-Wye (Plate 2); and on the A40(T) (Plate 3); Allen, 1971, 1974, 1983a, b; Allen and Dineley, 1976; Smith, 1980). The formation is unconformably overlain by the Upper Old Red Sandstone Quartz Conglomerate Formation.

Overall, the formation shows an upward transition from sandstones with subordinate mudstones to a succession largely of sandstones. In the lower 300 to 400 m of the formation, very fine- and fine-grained sandstones with some intraformational conglomerates are interbedded with equally thick red mudstones. In the higher beds, sandstones with intraformational conglomerates are 10 to 20 times more abundant than mudstones. Here, the sandstones are mainly fine grained, but locally medium grained, and the intraformational conglomerates include cobbles as well as pebbles. Also, these higher conglomerates contain scattered exotic clasts, including vein quartz, quartzite, acid lavas, sandstones, greywackes, tuffs and a wide range of other lithologies, all thought to have been derived from Ordovician, Silurian, early Lower Old Red Sandstone and perhaps Precambrian outcrops in north Wales and Anglesey (Allen, 1974; Allen and Dineley, 1976); Allen and Crowley, 1983. Medium- and very coarse-grained sandstones become abundant in the higher beds, with numerous exotic pebbles and cobbles as well as intraformational mudstone clasts in the conglomerates. The deposits were carried by southerly flowing, high-energy, braided streams, the composition of detrital garnets suggesting dispersal systems different from those of the underlying St Maughans Formation (Haughton and Farrow, 1989).

Upper Devonian

The Quartz Conglomerate Formation (QC) is 4 to 15 m thick and rests unconformably on the Brownstones. The formation outcrops around Chase Wood and Penyard Park (Smith, 1980), and around the rims of the Howle Hill and Wigpool synclines to the south (Kellaway and Welch, 1955; Welch and Trotter, 1961), forming prominent crags locally. It comprises red-brown, medium- to coarse-grained, pebbly sandstones, with beds of quartz pebble conglomerate particularly common in the lower part. Bedding is mainly parallel, in sets up to 2.5 m thick, but 2 m-deep channels are also present. Cross-bedding in these and pebble imbrication indicate a generally southward transport direction. Pebbles are mainly rounded to well rounded and less than 3 cm, although some cobbles are also present. The clasts consist mainly of vein quartz (80%), with quartzite, jasper and acid lavas making up the remainder. The sandstone matrix is generally medium to coarse grained and well sorted. Grains are angular to rounded, with some coarse grains being well rounded. There are also some detrital feldspar and mica grains. Ferric oxide grain coatings are ubiquitous and some grains have authigenic quartz overgrowths, which cement the rock locally. Elsewhere, selective dissolution of calcite cement has produced honeycomb weathering (Smith, 1980). The formation is interpreted as the deposits of fast-flowing, braided, gravel-bed streams (Allen, 1965, 1971). Old shafts and trenches on the southern slopes of Penyard Park Hill may have been trials for gold, small uneconomic quantities of which were found in the formation in the Forest of Dean (Hart, 1944).

The Tintern Sandstone Formation (TSG) caps Chase Wood and Penyard Park Hill, and also outcrops around the Howle Hill and Wigpool synclines to the south (Welch and Trotter, 1961; Smith, 1980). It is 45 to 75 m thick and consists of variegated, fine- to medium-grained, lithic, weakly cemented sandstones and subordinate beds of red, purple and green mudstone and siltstone, with a few intraformational conglomerates and nodular, sandy calcretes. The sandstones are mainly pale yellow-brown and pale greenish grey and composed of subangular to rounded grains. Some dark green sandstones are packed with mica flakes (Kellaway and Welch, 1955; Lovell, 1978). They are arranged in fining-upwards units up to several metres thick, with a basal conglomerate resting on a scoured or channelled surface and grading up progressively through coarse- to very fine-grained sandstone, siltstone and mudstone. The basal conglomerates commonly contain scattered, well-rounded quartz pebbles and fish fragments. Iron oxide coatings on grains suggest that the sandstones were formerly red. In addition to the nodular calcretes in the finer lithologies, some of the sandstones are also calcretised, with a nodular fabric.

A 1.5 m-thick conglomerate, 10 to 15 m above the base of the formation at Chase Hill and Penyard Park Hill, is similar to those in the underlying Quartz Conglomerate Formation.

The formation is interpreted as the deposits of southerly flowing, high sinuosity, sand-bed streams, with the sandstones being channel-fills and the finer lithologies floodplain deposits (Allen, 1965, 1971). The regional eastward thickening of the combined Quartz Conglomerate and Tintern Sandstone formations, along with a north-eastward increase in conglomerates, suggests a source to the north-east, probably of metamorphic and older sedimentary rocks (Wallis, 1927).

Carboniferous

Carboniferous strata (Figure 5) are present only in two small areas in the south, in the cores of the Howle Hill and Wigpool synclines. In the former, Dinantian (Carboniferous Limestone) rocks are overlain unconformably by the Westphalian Trenchard Formation. Only Dinantian strata are present in the small area of the Wigpool Syncline within the district, comprising the Lower Limestone Shale Group and overlying Lower Dolomite Formation. The Lower Dolomite is absent in the Howle Hill Syncline, the Trenchard Formation resting unconformably on the Lower Limestone Shale Group.

The Lower Limestone Shale Group (LSh) outcrops round the Howle Hill and Wigpool synclines. It consists of up to about 55 m of thinly bedded limestones and mudstones (Sibly and Reynolds, 1937; Trotter, 1942; Welch and Trotter, 1961). A basal 8 m-thick unit comprising cross-bedded, gritty oolite is overlain by interbedded grey crinoidal limestone and oolite. Above, a further 8 m of interbedded mudstones and limestones, including sandy, crinoidal and ostracod-rich limestones, are overlain by 9 m of 'Modiola-phase', thinly bedded, grey and yellow calcareous mudstones, many rich in ostracods, and fine-grained micritic and argillaceous limestones, some of which are algal- and annelid-rich. A fine-grained, fossiliferous, 1.6 m-thick, crinoidal limestone above is overlain by 1.2 m of fissile mudstone with thin fissile limestones. These are overlain by about 29 m of mudstones and thin crinoidal, fossiliferous dolomitic limestones, which are abundant in the topmost 12 m.

The Lower Dolomite Formation (LD) is present only in a small area in the Wigpool Syncline, where there are about 25 m of massive, unfossiliferous dolostone (Welch and Trotter, 1961). Numerous disused quarries and pits in the area worked compact, fine-grained, grey and buff dolostones (Sibly and Reynolds, 1937).

A small area of Upper Coal Measures is present in the south of the area, in the core of the Howle Hill Syncline. The beds are assigned to the Trenchard Formation and rest unconformably on the Carboniferous Limestone. About 25 m of strata consist mainly of grey mudstones and siltstones. They include the Trenchard Coal, which is 0.6 m thick and was worked at Great Howle opencast site from 1972 to 1977.

Concealed rocks

The Fownhope Borehole [SO 5929 3561], sited on the Woolhope Anticline, commenced in the Woolhope Limestone Formation and proved 2000 m of strata (uncorrected for dip), thereby proving about 1900 m of beds not exposed at the surface ((Figure 6); Butler et al., 1997). Precambrian rocks from 1933 m to the terminal depth are described as altered lavas, tectonised intrusive igneous rocks (possibly tonalite-diorite) and pale green sandstones. Cambrian strata, 141 m thick, are correlated with the White-leaved Oak Shale Formation (Woa) of the Malverns and consist mainly of dark grey to black mudstones. Thin sandstones and siltstones lie near the base of the formation, and a 10 m-thick, pale grey, green-speckled, very hard, fine- to medium-grained sandstone lies in the middle. Thin igneous intrusions are present towards the top of the formation. Grey Ordovician siltstones, 686 m thick, with a few thin sandstones, are correlated with the Tremadoc Series Bronsil Shale Formation (BSh) of the Malverns. To the west of the inlier, seismic evidence indicates a wedge of strata up to 1200 m thick above the Tremadoc rocks. A Caradoc age is inferred, but as none of these beds are present in the borehole, their lithologies are unknown.

About 1075 m (uncorrected for dip) of Llandovery strata were proved in the borehole. They rest unconformably on the Bronsil Shale Formation and are assigned to the May Hill Sandstone Group. The basal 370 m comprise orange sandstones with minor red-brown siltstones, overlain by red-brown siltstones and sandstones. These previously unproved beds are named here the Fownhope Formation (Fow). Above them, at a distinct colour change to grey-green beds, lie 380 m of grey-green sandstones and siltstones correlated with the Huntley Hill Formation (HH) of the May Hill inlier (Worssam et al., 1989). The overlying beds are correlated with the Haugh Wood Formation, the topmost part of which is exposed in the core of the inlier.

Quaternary

Quaternary deposits are most widespread in the north-west of the district, where kettled moraine marks the limit of the Late Devensian Wye glacier. Gravel terraces that predate and postdate this glaciation are present in the Wye valley, but much of the district is drift-free. (Figure 7) shows the various classifications of the deposits, suggested correlations and oxygen isotope stages.

Gravels beyond the Late Devensian limit are presumed to be the outwash of an earlier (? Anglian) glaciation and are termed Older Glaciofluvial Deposits. A long low ridge of sand and gravel between Kivernoll and Much Dewchurch extends to the col at the head of the Gamber valley near Wormelow Tump [SO 493 303] (Richardson, 1935; Luckman, 1970). A pit [SO 489 307] formerly worked 4.5 m of horizontally bedded fine-grained gravels with involutions caused by frost disturbance. There is also a small gravel capping on Blakemore Hill [SO 479 348] tentatively correlated with the Portway Sand and Gravel (Brandon and Hains, 1979; Brandon, 1989).

The river terrace deposits of the district are differentiated between those within the late Devensian ice limit and those outside it, some of the latter being interpreted as older (Brandon and Hains, 1979; Brandon, 1989; Hey, 1991). Four terraces are recognised between the late Devensian ice limit at Hereford and the Wye-Lugg confluence at Mordiford.

Terrace deposits occur as remnants in the Wye valley around and to the north of Ross. Whitfield (1971) recognised two terraces south of Holme Lacy, but because those in the vicinity of Ross were not recognised by Smith (1980) as belonging to a recognisable flight of terraces, they were mapped by him as River Terrace Deposits of the River Wye, undifferentiated, of probable fluvioglacial origin. They occur as sloping, patchy sheets up to 2 m thick. Cross-bedding and pebble imbrication indicate southerly transport, and cryoturbation points to permafrost conditions subsequent to deposition. Hey (1991) recognised the remnants of four terraces south of Hereford on altimetric grounds. He noted that the remnants of the two terraces recognised by Whitfield (1971) remain parallel to each other at 10 and 30 m above the floodplain, with elevations of 65 and 49 m above OD near King's Caple around [SO 552 290], 48 and 40 m above OD west of Ross, and 39 and 33 m above OD at Walford near the southern margin of the district. In addition, there are three deposits that do not conform to the profiles of these two terraces. One forming the top of a hill at Homme Farm [SO 577 224] lies at 53 m above OD. The second (not shown on the 1:50 000 map) is a small gravel patch [SO 558 288] of less than 100 m² west of King's Caple church, its top defined by a break in slope at 77 m above OD. The third extends for 1.5 km to the west and north-west of Walford Court and is the largest and best preserved terrace remnant in the Wye valley south of Mordiford. It lies at 43 m above OD in the west and 39 m above in the east, 15 to 11 m above the modern floodplain and below a nearby remnant of the '30 m terrace' at 59 m. It was mapped as the first terrace of the Wye (Welch and Trotter, 1961), and is shown as such on the 1:50 000 map, but may be older. Hey (1991) attributed its preservation to its position in and near a dry valley, 20 m deep and 1 km wide, that probably marks a former course of the Wye. Between Ross and Walford, an earlier, wide meander loop of the Wye flowed eastwards from Ross towards Weston under Penyard, then southwards and westwards to join the present course at Walford.

Fourth Terrace Deposits of River Lugg and Proto-Wye occur mainly between Dinedor and Holme Lacy (Brandon and Hains, 1979; Brandon, 1989). The base of the terrace lies about 20 to 30 m above the present floodplain. The terrace comprises about 3 to 4 m of reddish brown, poorly sorted gravel. The base of the Third Terrace Deposits of River Lugg and Proto-Wye lies at 57 to 64 m above OD, 15 to 18 m above the present floodplain (Brandon and Hains, 1979; Brandon, 1989). The largest of two spreads north of the Wye caps a spur in the Hampton Park area of Hereford. The smaller patch caps a hill to the east [SO 542 393]. The base of the Second Terrace Deposits of River Lugg and Proto-Wye lies 53 to 54 m above OD, 4.5 to 6 m above the present floodplain. There are numerous fragments of the terrace on both sides of the Wye in Hereford, with the largest extending more than 3 km from near Blackmarstone [SO 501 388] south-eastwards to north of Rough Hill [SO 529 375]. Like the other Lugg and Proto-Wye terraces, the deposit has a 'Welsh' clast suite of greywacke, siltstone, conglomerate, volcanic rocks and vein quartz, with some local Old Red Sandstone limestone and sandstone pebbles.

After deposition of the Second Terrace Deposits of the Lugg and Proto-Wye, the Late Devensian Wye glacier advanced down the Wye valley as far as the area south-west of Hereford, where, on melting, it deposited a wide area of recessional morainic debris (e.g. Brandon, 1989; Luckman, 1970). The deposits include sandy till, sand, gravel and clay; constructional hummocky, kettled topography is present locally. The hummocky morainic areas have been depicted on the adjoining Hereford sheet to the north and in the part of the Ross-on-Wye district north of grid line 35 and east of grid line 45, but in the absence of detailed mapping elsewhere, all the deposits are shown on the map as Glacial deposits undifferentiated. The erratics of the deposits are 'Welsh' Lower Palaeozoic greywackes, vein quartz and volcanic rocks, as well as Old Red Sandstone calcrete, siltstone and sandstone of more local origin (Brandon and Hains, 1979). Southwards from a belt of kettled ground in the north-west of the district, a broad till plain extends from the Madley area south-eastwards to Clehonger and southwards to Thruxton. The till is apparently thickest in the Allensmore area around Birch Hill [SO 451 370], where red-brown, pebbly, gravelly silt probably exceeds 3 m. Wide and flat alluvial areas within and at the margin of the glacial deposits probably mark the positions of former glacial lakes (see below). South-east of Clehonger, the till plain merges into ridged and hummocky ground of the end moraine that extends from Belmont south-westwards to Allensmore and beyond. The deposits of this belt are varied, but more gravelly than the till to the north, consisting of granule- to boulder-size clasts in a matrix of clayey, silty sand. In the west, till at the summit of Brampton Hill [SO 400 356] shows that the Wye glacier topped the hill to reach the area of Blackmoor Farm in the Grey valley to the west.

Outwash gravels (Glaciofluvial sand and gravel) from the Wye glacier occur on both sides of the Wye around the Belmont area of Hereford, where at least 7.5 m of gravel underlie a thin layer of silt (Plate 4). Thicknesses are up to 10.5 m in the west, decreasing to 7 m in the east. The altitude of the spread falls from 78 m above OD in the west to 72 m at Belmont and then markedly to 57 m before disappearing below modern alluvium. The broad alluvial area [SO 742 353] south of Allensmore is underlain by a thin spread of interbedded fine gravel and red-brown silty sand. Recorded thicknesses range from 0.7 to 1.9 m. Farther south, boreholes at Tram Inn [SO 464 337] proved gravels, up to 5 m thick, below thin, patchy alluvial cover. Up to 4 m of gravel underlie clay and silt in the wide alluvial area south of Madley. The First and Second River Terraces of the Wye are post-Devensian in age. The Second Terrace Deposits of River Wye occur only as narrow strips on both sides of the river above the Breinton gorge, representing the remnants of deposits of postglacial aggradation before downcutting of the gorge. The front of the terrace lies 8 m above the present floodplain in the west, falling to 6 m above the floodplain to the east, at an elevation of 58 m above OD. The back of the terrace lies 3 to 4.5 m higher. First Terrace Deposits of River Wye cover a wide area south of the river between Blackmarston and Sink Green, as well as smaller patches on both sides of the river. The front edge of the terrace lies about 1.5 m above the floodplain, with a gentle rise to 4.5 m above the floodplain at its back edge where it is widest, south-east of Lower Bullingham. The deposits generally comprise a variable thickness of silt, silty clay and sandy clay (up to 3 m thick) on gravel (up to 5 m thick).

Two areas of Lacustrine Alluvium are shown on the map. Between Perry Hill and Clehonger Court [SO 470 382] over 1 m of brown silty clay with a few pebbles is recorded. The second site, on the west bank of Cage Brook [SO 449 382], has its base at 62 m above OD; the lacustrine alluvium consists of dark brown silt, and is correlated with the Second Terrace of the Wye (Brandon and Hains, 1979). The large alluvial flats between Madley and Kingston, and at Tram Inn and Howton are underlain by lacustrine alluvium above glaciofluvial gravels. A variable thickness of alluvial silts and clays, up to 3.2 m thick, is present above gravels south of Madley.

Boreholes near Tram Inn proved up to 10 m of laminated silt and clay underlying glaciofluvial sand and gravel (see above). These deposits are glaciolacustrine, having formed in lakes ponded by the outwash gravels.

The largest of the Alluvial Fan Deposits mapped in the district [SO 535 355] is a complex of fans feeding from three tributary valleys into the broad north-east-trending valley south of Dinedor. In addition to the alluvial flats in the end-morainic area of the Wye glacier (see above), there are extensive areas of Flandrian Alluvium of the Wye and, in the north, of the Lugg and Frome. The area of their confluence is underlain by a floodplain up to 3 km wide. The Wye alluvium is narrowest in the Breinton gorge, where the floodplain lies about 4.5 m above the river bed and the alluvium consists of buff, homogeneous, clayey silt, locally with gravel beneath. Downstream of Breinton, the floodplain lies 4 to 5 m above normal river level and consists of pale brown or buff silt with gravel layers in the basal metre. The incised meanders of the Wye to the south of Hereford, as well as the abandoned meander loop east of Ross, mark the former extent of a very broad valley over 6 km wide (Miller, 1935). Around Ross, the Wye alluvium generally comprises up to 3.5 m of red-brown, silty sand and sandy silt, overlying a more heterogeneous sequence of brown and grey sands, silts and clays, locally rich in plant remains and thin gravel lenses. Modern deposits of the river are mainly sand, with up to 3 m of gravel present in some point bars of the river.

Two minor Peat occurrences are known. Up to 1.3 m of peat were augered [SO 393 397] in hummocky, kettled moraine near Tyberton. The other occurrence [SO 432 372] lies in the former glacial lake site near Madley Satellite Earth Station. Head is ubiquitous on the valley slopes of the district as a variable deposit of clay, silt and sand, locally with pebbles or angular blocks of local origin. It is the product of solifluction or gelifluction under periglacial conditions following the Late Devensian glaciation and recent colluvial downwash. The deposit on the Old Red Sandstone outcrop is reddish brown and up to 3 m thick, with local thicker accumulations up to 5 m below steeper slopes. Many dry valleys in the outcrop of the Brownstones Formation are floored with appreciable thicknesses of very recent red-brown, sandy colluvium. Head (Gravel) (not differentiated on the map) is mapped in two occurrences, the larger of which is along the base of Ridge Hill. It consists of up to 2 m of unbedded, unsorted rubble of rounded to subrounded limestone (calcrete) nodules and a few purplish brown sandstone fragments in a matrix of coarse quartz and limestone sand, quartz silt and red-brown clay. Extensive Head deposits flank the Woolhope Dome. The deposits near Mordiford [SO 572 373]; [SO 568 365] and Fownhope [SO 573 351] consist of interbedded clay and red, micaceous silts and sands; thicknesses of between 5.5 and 13 m are recorded in the Fownhope–Woolhope area (Luckman, 1970). Apart from two small bedding plane slips south of Hereford [SO 5345 3755] (not shown on map); [SO 507 350] and three small slips [SO 4815 3873]; [SO 4586 3931]; [SO 4545 3940] in the Raglan Mudstone Formation in the Breinton gorge (Brandon and Hains, 1979; not shown on map), the Landslip deposits of the district have not been systematically mapped. The ones shown are recognisable from air photographs as areas of disturbed and hummocky ground. The landslip at The Wonder [SO 635 365] is a failure in the Upper Ludlow Shales caused by bedding plane slip along bentonite layers (e.g. Richardson, 1905).

The predominantly rural nature of the district precludes extensive areas of Made Ground and Worked Ground, the only large mapped areas being in and around the gravel quarries of Hereford.

Structure

The structure of the district can be divided broadly into three main areas by the north-east-trending Neath Disturbance and the north-west-trending Woolhope Fault (Figure 8). The area south and west of these faults is structurally simple, with gentle southerly dips, but with minor folding, as for example in the Howle Hill and Wigpool synclines and intervening Hope Mansel Anticline in the extreme south. Dips are steeper and north-west-directed in the south-west, with gentler southerly dips around Ross-on-Wye, the overall structure being generally synclinal in the northern extension of the main Forest of Dean syncline. North-west of the Neath Disturbance, a series of north-west-trending faults repeats the Bishop's Frome Limestone and enclosing strata at outcrop. In the east, the Woolhope Anticline (Squirrell and Tucker, 1960) lies in the hanging wall of the Woolhope Fault. These structures and the adjacent sector of the Neath Disturbance are well imaged by seismic data, which reveal considerable syntectonic activity, with changes in depositional thickness of the Silurian formations controlled by faulting (Butler et al., 1997). The main thickening, for which there is no evidence at the surface, is restricted to beds beneath the Woolhope Limestone Formation, in the footwall of the Woolhope Fault. This fault trends north-west and extends beyond the south-east margin of the district. It terminates in the north-west against the south-east downthrowing Shucknall Fault, the local name of the Neath Disturbance (Brandon, 1989). The base of the Silurian is well-imaged in the footwall of the Woolhope Fault, unconformably truncating rocks of probable Ordovician age, which dip more steeply southwards than the basal Silurian.

These rocks have not been drilled, but may correlate with the Caradoc strata of Shropshire (Smith, 1987). They are thinner west of the Llandinabo Fault (see below). Early Cambrian strata may thicken east of a fault that crosses the Woolhope Anticline. Faults to the east of this fault appear to be antithetic, dipping west, and some of them show reverse displacement at the base-Silurian unconformity.

The Woolhope Fault was reactivated during the Variscan Orogeny, as a thrust with about 900 m of uplift to the east, forming the Woolhope Anticline. The south-east continuation of the Woolhope Fault probably lies between steep, west-dipping rocks associated with the Woolhope and May Hill anticlines and more gently dipping exposures farther west. It crosses the M50 motorway between exposures of the Brownstones Formation labelled H and J by Allen and Dineley (1976, fig. 2). To the east, the Newhouse Fault is the south-east extension of the Woolhope Fault. It underlies the May Hill Anticline, which is also interpreted as an inversion structure formed by Variscan thrusting.

The Neath Disturbance dips south-east and has a small downthrow in that direction in strata above the Woolhope Limestone Formation, with substantial thickening of the pre-Woolhope Limestone Llandovery strata. A second fault to the south shows similar growth in this interval and was reactivated as a thrust in the younger strata near the surface. Both faults appear to have been antithetic growth faults. In the ground mapped to the south-west, a disturbed belt up to 1.3 km wide along the Neath Disturbance (the Rough Hill Fault of Brandon and Hains, 1979) includes faulted blocks of St Maughans Formation forming the high ground of Dinedor Hill, Ridge Hill and Twyford Common. To the north of the fault, the large-scale structure in the Clehonger–Hereford area is broadly synclinal, with a north–south axis passing close to St Michael's Abbey [SO 4838 3813]. West of there, strata dip west-south-westwards in strike-faulted ground between Wormbridge and the Golden Valley.

The Pontypool Road Fault is an east-downthrowing fault, which controlled early Llandovery thickening, and the pre-Silurian basement has a different seismic appearance on the high to the west. The Llandinabo Fault trends north-west and although it may have been gently inverted, its latest displacement was normal, down to the east. It is shown at the surface as a fault conjectured to displace the outcrop of the base of the Brownstones Formation north-west of Much Birch. Tremadoc strata are thicker on its eastern side, and the sub-Llandovery strata, affected by pre-Silurian reverse faulting, dip eastward in its footwall.

The disjointed north–south line of late Carboniferous basins, which includes the Bristol, Forest of Dean and Newent basins, were possibly formed and preserved on footwall blocks bounded by Variscan reverse faults that splay off the Malvern Lineament. Superimposed on the main Forest of Dean synclinal structure north of Ross-on-Wye are a minor north-west-trending syncline near Hom Farm [SO 577 223] and a north-north-west-trending anticline that crosses the A449(T) 1 km north of Ross. The Ross Fault is one of several near the axis of the latter structure and appears to persist southwards to near Coughton (Smith, 1980).

Chapter 3 Applied geology

Mineral resources

Rock aggregate is being produced from the Aymestry Limestone Formation and adjacent beds of the Upper and Lower Ludlow Shales at Perton Quarry [SO 595 399] on the northern margin of the area (Plate 1). The other limestones of the Woolhope Inlier were formerly quarried extensively, for example the Woolhope Limestone Formation at Scutterdine Quarry. Elsewhere, the pedogenic limestones of the Old Red Sandstone, particularly the Bishop's Frome Limestone, were formerly dug for lime burning. The Carboniferous dolomites of the Forest of Dean were also quarried. Sand and gravel was worked until about 1990 at Belmont Pit [SO 488 386], Hereford, from terraced glaciofluvial deposits about 9.5 m thick ((Plate 4); Brandon and Hains, 1979). Other terrace deposits south of Hereford are largely sterilised by development, but some resources remain. Brickclay was formerly dug from the Raglan Mudstone Formation at Grafton [SO 502 363]. Sandstones throughout the area were formerly dug in small pits for local building and aggregate use.

Coal was dug from the Trenchard Seam in the south of the area at Great Howle opencast site [SO 610 204] from 1972 to 1977. The hydrocarbon potential of the district appears to be low, following seismic surveys and the drilling of the Fownhope Borehole in the Woolhope Inlier. This is one of three wells drilled in the Lower Palaeozoic succession in the region (the others being Collington and Usk), none of which discovered oil and gas. However, not all possible targets have been drilled.

The water supply of the district formerly came from the numerous wells and springs in the Old Red Sandstone (Richardson, 1935; Cradock-Hartopp in Brandon and Hains, 1979), but is now largely obtained from the River Wye. The wells and springs yield variable but generally small amounts of water from sandstones and massive calcretes. The Brownstones Formation is the largest potential aquifer in the district, but groundwater is compartmentalised by mudstone beds and the potential is limited by low to moderate porosity and variable permeability; the water is generally of good quality, but very hard (Mosley in Smith, 1980). The gravels of the district are less important as aquifers. Those at Belmont are in hydraulic continuity with the River Wye only at Hunderton Gardens [SO 498 391] and transmissivities are low (100 m²/day). The larger spreads of terrace gravels are generally in hydraulic continuity with the River Wye and may have limited aquifer potential, but they are vulnerable to surface contamination.

Geology and planning

Geological assessment of the urban area of Hereford and its vicinity was carried by the Institute of Geological Sciences (now BGS) for the Department of the Environment (Brandon and Hains, 1979). A similar exercise was conducted for Ross-on-Wye (Smith, 1980). The district was included in an assessment of the mineral resources of Herefordshire and Worcestershire by BGS for the Department of the Environment, Transport and the Regions (Bloodworth et al., 1999). This relates current or potential economic mineral interest to planning issues that may constrain extraction.

The Wye valley south of Hereford and most of the Woolhope Inlier are designated an Area of Outstanding Natural Beauty (AONB). Also, there are several geological Sites of Special Scientific Interest (SSSI) in the Woolhope Inlier. Both AONB and SSSI categories preclude mineral working without rigorous scrutiny of any proposal. The SSSIs are described in Aldridge et al. (2000), in which their designation as Geological Conservation Review (GCR) sites emphasises the importance of conserving the geological heritage that they represent. The sites are Scutterdine Quarry [SO 577 368] (Woolhope Limestone Formation), Little Hill [SO 603 387] to [SO 613 381] (Much Wenlock Limestone Formation) and Perton Road and Quarry [SO 5951 3990] to [SO 5970 4040] (Ludlow Series). The work on establishing a network of Regionally Important Geological Sites (RIGS) in Herefordshire is in progress.

The Old Red Sandstone rocks and the sand and gravel deposits of the district generally present good foundation conditions below the surface weathered zone. However, soft, weakened ground may occur in mudstone outcrops below springs. Similarly, superficial Head deposits and Landslips on slopes in the Old Red Sandstone need careful investigation and appropriate foundation design. The extensive alluvial

areas, comprising interbedded compressible clays, silts and sands as well as peat lenses, provide poor foundation conditions, and are subject to flooding. Alluvial sands may be thixotropic, and running sands may also be encountered in the glacial deposits. The bentonitic clay horizons in the Woolhope Inlier provide a mechanism for bedding plane slip, examples of which are The Wonder landslip and the 1974 failure in Perton Quarry. Currently, there are no waste burial sites in the district, but development of former sites requires consideration of unstable foundation conditions and the possibility of methane generation from the decomposition of domestic waste. There is also potential for aquifer pollution from former or proposed disposal sites in sand and gravel.

Information sources

The Geological Data Index (GDI) is now available on the Internet at http//www.bgs. ac.uk. This provides a searchable database of index level information on data and other products available from BGS. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Information on BGS products is listed in the current Catalogue of geological maps and books.

Books

• British Regional Geology
• Welsh Borderland, third edition, 1971
• Memoirs
• Geology of the country between Hereford and Leominster (Sheet 198), 1989
• Geology of the country around Worcester (Sheet 199), 1997
• Geology of the country around Tewkesbury (Sheet 216), 1989
• Geology of the South Wales Coalfield, Part II, the country around Abergavenny (Sheet 232), 1989
• Geology of the country around Monmouth and Chepstow (Sheets 233 and 250), 1961
• Sheet Explanation
• Geology of the Talgarth district (Sheet 214)

Maps

• Geological maps
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:1 625 000
• United Kingdom (South Sheet Solid Geology, 1979; Quaternary geology 1977)
• 1:250 000
• 51N 04W, Bristol Channel, Solid Geology, 1988
• 52N 04W, Mid Wales and Marches, Solid Geology, 1990
• 1:50 000
• Sheet 198 Hereford, 1989
• Sheet 199 Worcester, 1993
• Sheet 216 Tewkesbury, 1988
• Sheet 232 Abergavenny, 1990
• Sheet 233 Monmouth, 1974
• Sheet 234 Gloucester, 1972
• 1:10 000
• Maps at 1:10 000 scale are available for the area immediately south of Hereford and for the town of Ross-on-Wye itself. The maps are listed below, together with the surveyor's initials and the dates of survey. The surveyors were: A Brandon, B A Hains and D B Smith.
• The 1:10 000 maps are not published but are available for public reference in the libraries of the British Geological Survey at Keyworth and Edinburgh and BGS London Information Office in the Natural History Museum, South Kensington, London. Uncoloured dyeline sheets or photographic copies are available for purchase from the BGS Sales Desk.

• Geophysical maps
• 1:500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996 Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:250 000
• 51N 04W Bristol Channel, Bouguer gravity anomaly, 1986
• 52N 04W Mid Wales and Marches, Bouguer gravity anomaly, 1986
• 51N 04W Bristol Channel, Aeromagnetic anomaly, 1980
• 52N 04W Mid Wales and Marches, Aeromagnetic anomaly, 1980
• Hydrogeological maps
• 1:250 000
• South Wales
• Groundwater vulnerability maps (Prepared for the Environment Agency by BGS and the Soil Survey and Land Research Centre).
• 1:100 000
• Sheet 28 (Powys), 1996

Sheet 29 (Worcestershire). 1995

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 Series Sheet 215 Ross-on-Wye are held in the Lexicon database. This is available on BGS web site. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

BGS holds a number of photographs from the district; these are held in the National Archive of Geological Photographs. The photographs may be viewed at BGS Libraries in Keyworth and Edinburgh. Part of the collection has been digitised and can be accessed on our web site. Copies of the photographs are available at a fixed tariff.

Other relevant collections

Requests for access to the palaeontological, petrological and materials collections should be made to the Chief Curator, Keyworth.

References

Most of the references listed below are held in the libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright legislation.

Aldridge, R J, Siveter, David J, Siveter, Derek, J, Lane, P D, Palmer, D, and Woodcock, N H. 2000. British Silurian Stratigraphy. Geological Conservation Review Series No. 19. Joint Nature Conservation Committee.

Allen, J R L. 1965. Upper Old Red Sandstone (Farlovian) palaeogeography in South Wales and the Welsh Borderland. Journal of Sedimentary Petrology, Vol. 35, 167–195.

Allen, J R L. 1971. The sedimentation of the Old Red Sandstone in the Forest of Dean. 9–19 in Geological excursions in South Wales and the Forest of Dean. Bassett, D A, and Bassett, M G (editors). (Cardiff: Geologists' Association South Wales Group.)

Allen, J R L. 1974. Source rocks of the Lower Old Red Sandstone: exotic pebbles from the Brownstones, Ross-on-Wye, Hereford and Worcester. Proceedings of the Geologists' Association, Vol. 85, 493–510.

Allen, J R L. 1983a. Studies in fluviatile sedimentation: bars, bar-complexes, and sandstone sheets (low-sinuosity braided streams) in the Brownstones (L Devonian), Welsh Borders. Sedimentary Geology, Vol. 33, 237–293.

Allen, J R L. 1983b. Gravel overpassing on humpback bars supplied with mixed sediment: examples from the Lower Old Red Sandstone, southern Britain. Sedimentology, Vol. 30, 285–294.

Allen, J R L, and Crowley, S F. 1983. Lower Old Red Sandstone fluvial dispersal systems in the British Isles. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 74, 61–68.

Allen, J R L, and Dineley, D L. 1976. The succession of the Lower Old Red Sandstone (Siluro-Devonian) along the Ross–Tewkesbury Spur Motorway (M50), Hereford and Worcester. Geological Journal, Vol. 11, 1–14.

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, London, Vol. 138, 15–29.

Bassett, M G, Cocks, L R M, Holland, C H, Rickards, R B, and Warren, P T. 1975. The type Wenlock Series. Report of the Institute of Geological Sciences, No. 75/13.

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. 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).

Brandon, A, and Hains, B A. 1979. Geological notes and local details for 1:10 000 sheets: S O 43N E, S O 44S E, S O 53N W, S O 54S W (Hereford City). (Keyworth: Institute of Geological Sciences.)

Butler, A J, Woodcock, N H, and Stewart, D M. 1997. The Woolhope and Usk Basins: Silurian rift basins revealed by subsurface mapping of the southern Welsh Borderland. Journal of the Geological Society, London, Vol. 154, 209–223.

Brodie, P B. 1871. On the 'Passage-beds' in the neighbourhood of Woolhope, Herefordshire and on the discovery of a new species of Eurypterus and some new land plants in them. Quarterly Journal of the Geological Society of London, Vol. 27, 256–263.

Cherns, L. 1980. Hardgrounds in the Lower Leintwardine Beds (Silurian) of the Welsh Borderland. Geological Magazine, Vol. 117, 311–326.

Earp, J R, and Hains, B. 1971. British Regional Geology: the Welsh Borderland. Third Edition. (London: H MS O.)

Gardiner, C I. 1927. The Silurian inlier of Woolhope (Herefordshire). Quarterly Journal of the Geological Society of London, Vol. 83, 501–550.

Gradstein, F M, and Ogg, J. 1996. A Phanerozoic time scale. Episodes, Vol. 19, 3–5.

Hart, C E. 1944. Gold in Dean Forest. Transactions of the Bristol and Gloucester Archaeological Society, Vol. 65, 98–104.

Haughton, P D W, and Farrow, C M. 1989. Compositional variation in Lower Old Red Sandstone detrital garnets from the Midland Valley of Scotland and the Anglo-Welsh Basin. Geological Magazine, Vol. 126, 373–396.

Hey, R. 1991. Pleistocene gravels in the lower Wye valley. Geological Journal, Vol. 26, 123–136.

Hodgson, J M, and Palmer, R C. 1971. Soils in Herefordshire I. Sheet S O53 (Hereford South). Soil Survey Record, No. 2.

Hurst, J M. 1975. Wenlock carbonate level bottom communities from Wales and the Welsh Borderland. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 17, 227–255.

Kellaway, G A, and Welch, F B A. 1955. Upper Old Red Sandstone and Lower Carboniferous of Bristol and Mendip, compared with that of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No. 9, 1–21.

Lawson, J D, and White, D E. 1989. The Ludlow Series in the Ludlow area. 73–90 in A global standard for the Silurian System. Holland, C H, and Bassett, M G (editors). National Museum of Wales, Geological Series, No. 9.

Lovell, R. 1978. The sedimentology and stratigraphy of the Upper Old Red Sandstone and Lower Limestone Shales of the South Wales Coalfield. PhD thesis, University of Bristol (unpublished).

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

Maddy, D. 1999. English Midlands. 28–44 in A revised correlation of Quaternary deposits in the British Isles. Bowen, D Q (editor). Geological Society Special Publication, No. 23.

Miller, A A. 1935. The entrenched meanders of the Herefordshire Wye. Geographical Journal, Vol. 85, 160–178.

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

Pocock, R W. 1930. The Petalocrinus Limestone horizon at Woolhope, Herefordshire. Quarterly Journal of the Geological Society of London, Vol. 86, 50–63.

Phillips, J. 1848. The Malvern Hills compared with the Palaeozoic districts of Abberley, Woolhope, May Hill, Tortworth and Usk. Memoir of the Geological Survey, Vol. 2, Part 1.

Richardson, L. 1935. Wells and springs of Herefordshire. Memoir of the Geological Survey of England.

Sibly, T F, and Reynolds, S H. 1937. The Carboniferous Limestone of the Mitcheldean area, Gloucestershire. Quarterly Journal of the Geological Society of London, Vol. 93, 23–51.

Siveter, David J. 2000. The Ludlow Series. 325–425 in British Silurian Stratigraphy. Aldridge, R J, Siveter, David J, Siveter, Derek J, Lane, P D, Palmer, D, and Woodcock, N H. Geological Conservation Review Series No. 19. Joint Nature Conservation Committee.

Siveter, D J. 2000. The Wenlock Series. 181–323 in British Silurian Stratigraphy. Aldridge, R J, Siveter, David J, Siveter, Derek J, Lane, P D, Palmer, D, and Woodcock, N H. Geological Conservation Review Series No. 19. Joint Nature Conservation Committee.

Smith, D B. 1980. Ross-on-Wye Special 1:10 000 Sheet. (Keyworth: British Geological Survey.)

Smith, N J P. 1987. The deep geology of central England: the prospectivity of the Palaeozoic rocks. 217–224 in Petroleum geology of northwest Europe. Brooks, J, and Glennie, K (editors). (London: Graham & Trotman.)

Squirrell, H C. 1958. New occurrences of fish remains in the Silurian of the Welsh Borderland. Geological Magazine, Vol. 95, 26–332.

Squirrell, H C, and Tucker, E V. 1960. The geology of the Woolhope inlier (Herefordshire). Quarterly Journal of the Geological Society of London, Vol. 116, 139–185.

Squirrell, H C, and Tucker, E V. 1967. Woolhope and Gorsley. 8–21 in The Silurian inliers of the south-eastern Welsh Borderland. Curtis, M L K, Lawson, J D, Squirrell, H C, Tucker, E V, and Walmsley, V G. (London: Geologists' Association Guide, No. 5.)

Squirrell, H C, and Tucker, E V. 1982. Woolhope and Gorsley. 9–17 in The Silurian inliers of the south-eastern Welsh Borderland. Second Edition. Lawson, J D, Curtis, M L K, Squirrell, H C, Tucker, E V, and Walmsley, V G. (London: Geologists' Association Guide, No.5.)

Stensiö, E A. 1932. The Cephalaspids of Great Britain. (London: British Museum: Natural History.)

Strickland, H E. 1853. On the distribution and organic contents of the 'Ludlow Bone Bed' in the districts of Woolhope and May Hill. Quarterly Journal of the Geological Society of London, Vol. 9, 8–11.

Symonds, W S. 1872. The record of the rocks. (London: John Murray.)

Trotter, F M. 1942. Geology of the Forest of Dean coal and iron-ore field. Memoir of the Geological Survey of Great Britain.

Wallis, F S. 1927. Old Red Sandstone of the Bristol district. Quarterly Journal of the Geological Society of London, Vol. 83, 760–787.

Watkins, R. 1979. Benthic community organisation in the Ludlow Series of the Welsh Borderland. Bulletin of the British Museum (Natural History): Geology Series, Vol. 3, 175–280.

Welch, F B A, and Trotter, F M. 1961. Geology of the country around Monmouth and Chepstow. Memoir of the Geological Survey of Great Britain, Sheets 233 and 250 (England and Wales).

Whitfield, W A D. 1971. Soils in Herefordshire II. Sheet S O52 (Ross-on-Wye West). Soil Survey Record, No. 3. Harpenden.

Worssam, B, Ellison, R A E, and Moorlock, B S P. 1989. Geology of the country around Tewkesbury. Memoir of the British Geological Survey, Sheet 216 (England and Wales).

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.

(Index map)

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

British geological maps can be obtained from sales desks in the Survey's principal offices, through theBGS London Information Office at the NaturalHistory 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) Simplified geological map of the district.

(Figure 2) Silurian lithostratigraphy of the district.

(Figure 3) Classification and nomenclature of the exposed Silurian succession.

(Figure 4) Devonian rocks of the district.

(Figure 5) Carboniferous rocks of the district.

(Figure 6) Graphic log of Fownhope Borehole [SO 5929 3561]. (KB Kelly bushing)

(Figure 7) Correlation of the Quaternary alluvial, terrace and glaciofluvial deposits.

(Figure 8) Geological structure of the district: minor faults east of the Woolhope Fault are not shown.

Plates

(Plate 1) Perton Quarry in the Aymestry Limestone Formation [SO 5970 3982] (GS 1190).

(Plate 2) Brownstones Formation near Royal Hotel, Ross-on-Wye [SO 5969 2408] (GS 1191).

(Plate 3) Brownstones Formation, Glewstone cutting on A40(T) near Pencraig, Ross-on-Wye [SO 5665 2218]. Parallel sets up to 1.2 m thick of fining-upwards, trough cross-bedded and planar bedded red-brown sandstones (A 13273).

(Plate 4) Belmont gravel pit [SO 4920 3877] in 1979. Glaciofluvial outwash gravels viewed from south-west; 2.5 m of coarse cobble gravels overlie finer gravels. Imbrication of the larger clasts indicates current flowing from left to right (to east-south-east) (A13296).

(Front cover) Front cover Ross-on-Wye viewed from the west. The old part of the town is built below and on top of an old river cliff of Brownstones Formation (Old Red Sandstone). (Photograph R E Collins; A13272).

(Rear cover)

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

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

Figures

(Figure 2) Silurian lithostratigraphy of the district

(Figure 3) Classification and nomenclature of the exposed Silurian succession

(Figure 5) Carboniferous rocks of the district