Content and licensingview original scan buy a printed copy

The geology of the Craven Arms area (Explanation of 1:25 000 Geological Sheet SO48)

By B. A. Hains, B.Sc., Ph.D.

Bibliographic reference: Hains, B. A. 1969. [2^nd impression 1980]. The geology of the Craven Arms area  (Explanation of 1:25 000 Geological Sheet SO48).

Institute of Geological Sciences. Geological Survey of Great Britain

London: Her Majesty's Stationery Office 1969. © Crown Copyright 1969. First published 1969. Second impression 1980. ISBN 0 11 880062 0

Natural Environment Research Council. The Institute of Geological Sciences was formed by the incorporation of the Geological Survey of Great Britain and the Museum of Practical Geology with Overseas Geological Surveys and is a constituent body of the Natural Environment Research Council

(Front cover) Cover picture: Callow Hill, near Craven Arms

Preface

The 1:25 000 sheet of the Craven Arms area is one of a number of such sheets which are being produced by the Institute of Geological Sciences for selected areas of Great Britain. It shows all the topographical detail available on the larger six-inches to one mile maps, at which scale the area was surveyed, and it carries contours at 25 ft intervals. In complex areas details of the geology can be delineated in greater detail than is possible on the one-inch to one mile maps.

Craven Arms lies south of the classic area of Church Stretton (Sheet SO49) with which it shares many geological formations. This account of the geology is designed to be read in conjunction with the map, to which the reader is referred. Extensive use has been made of the co-ordinates of the National Grid in defining the position of localities.

K. C. Dunham, Director. Institute of Geological Sciences, Exhibition Road, South Kensington, London, S.W.7. 9th December, 1968

Addendum to 1980 reprint

Since the publication of this booklet in 1969, some changes in stratigraphical nomenclature have been suggested or agreed. The most significant of these are summarised below.

Bassett and others (1975) have proposed a new lithostratigraphical classification of the type Wenlock Series and related their suggested terminology to a number of new stages and chronozones and to the existing graptolite biozones. Their lithostratigraphical terms relate to those used in the present account as follows:

Lawson (1977) has discussed the proposed new classification and argues against the replacement of the terms Wenlock Shale and Wenlock Limestone until international agreement is reached on the modernisation of traditional stratigraphical names.

International agreement has now been reached on the position of the Silurian/Devonian boundary (see p. 26). This has been defined in Czechoslovakia at the base of the Monograptus uniformis Zone, and this horizon is also taken as defining the base of the Gedinnian Stage. In the Welsh Borders this boundary occurs within the lower part of the Lower Old Red Sandstone but its exact 'position cannot yet be specified (House and others, 1977, pp. 5–6 and 42–43).

Additional references

BASSETT, M. G., COCKS, L. R. M., HOLLAND, C. H., RICKARDS, R. B. AND WARREN, P. T. 1975. The type Wenlock Series. Rep. Inst. Geol. Sci.,No. 75/13, 19pp.

HOUSE, M. R., RICHARDSON, J. B., CHALONER, W. G., ALLEN, J. R. L., HOLLAND, C. H. AND WESTOLL, T. S. 1977. A correlation of Devonian rocks in the British Isles. Spec. Rep. Geol. Soc. Lond., No. 7, 110 pp.

LAWSON, J. D. 1977. The name 'Wenlock Limestone'. Geol. J., 12, 189–90.

B.A.H.

Chapter 1 Introduction

The area shown on 1:25 000 Geological Sheet SO48 lies in the southern part of the county of Shropshire, in the pleasant hill-country of the Welsh Borders. The largest settlement is at Craven Arms, on the banks of the River Onny, which serves as an agricultural centre for the surrounding country and is noted for its annual sheep sales. It lies on the main N.–S. road (A.49) and railway line through the Welsh Borders, and is also well situated for access to much of Central Wales.

In the N.W. corner of the area the south-eastern slopes of the Long Mynd rise to over 1400 ft O.D. South-east of this high ground, a belt of irregular topography between Hopesay Hill [SO 403 839] in the S.W. and Marshbrook in the N. reflects the great variety of geological formations and the intricate structure within the Church Stretton Fault Complex. The highest points within this belt, both exceeding 1000 ft O.D., are Hopesay and Wart hills, formed of Longmyndian and Uriconian rocks respectively. The steep-sided valley extending N.E. from Horderley [SO 409 871] to Cwm Head is guided by the main fracture (F1) of the Church Stretton Fault Complex.

South-east of the Fault Complex the topography is dominated by a series of scarps and dales which are formed by hard and soft beds within a south-easterly dipping series of Ordovician, Silurian and Old Red Sandstone sediments. North of Lower Dinchope [SO 451 843] the two main scarps are formed by the Wenlock Limestone and Tickwood Beds, and the Aymestry Group, with the complementary valleys of Ape Dale, Hope Dale and Corve Dale floored by the softer Wenlock Shales, Lower Ludlow Shales and Ledbury Group. The Aymestry Group scarp rises to 1112 ft O.D. at Callow Hill [SO 4607 8504]. South of Lower Dinchope the scarps are interrupted and displaced by the Dinchope, Bache and Stokesay faults.

Most of the area is drained by the River Onny and its tributaries. South of Craven Arms this river flows through a steep-sided gap in the Aymestry Group scarp, along the line of the Stokesay Fault. The south-eastern corner of the area is drained by the River Corve. Both of these rivers flow southwards to join the River Teme near Ludlow.

Geological history

The sequence of geological formations in the area is shown in the vertical sections on either side of the map. The solid rocks range in age from Pre-Cambrian (probably more than 700 million years old) to Lower Old Red Sandstone (about 400 million years old). Over considerable parts of the area they are obscured by superficial deposits of various types which were laid down during Pleistocene and Recent times (approximately during the last two million years). In some cases formations are separated by unconformities representing periods of time in which earth movements and erosion took place.

The oldest rocks are the Uriconian volcanics of Wart Hill. They are succeeded by Longmyndian sediments which crop out in the extreme N.W. of the area and within the Church Stretton Fault Complex. It is thought that the Longmyndian was deposited in a narrow trough, possibly fault-bounded, and both the shales and sandstones of the Stretton Series and the sandstones of the Wentnor Series show evidence of shallow-water deposition. Strong folding and erosion followed the deposition of the Longmyndian sediments, and there is a gap in the succession up to the Caradoc Series of the Ordovician. Cambrian rocks are present N. of the area, at Comley, and S., at Pedwardine, so it is possible that they were also laid down within the present area but were removed by erosion before the deposition of the Caradoc Series.

During Ordovician and Silurian times, sedimentation in the Welsh Borders was dominated by two major palaeogeographical features, the relatively stable Midland shelf area to the east and the Welsh Geosyncline to the west. There is a contrast in facies between the geosyncline, where subsidence was fairly continuous and rapid with the deposition of shales and greywackes, and the shelf, where subsidence was slower and a thinner sequence of sandstones, mudstones and limestones was laid down. Only a small part of the Ordovician sequence, the Caradoc Series, is represented in the Craven Arms area where it comprises shelf facies sediments—sandstones with an abundant shelly fauna. West of the Long Mynd, in the Shelve district, a full pre-Caradoc sequence is present. Folding and denudation took place at the end of the Ordovician Period, and the highest Ordovician (Ashgill Series) and lowest Silurian rocks are absent. In Upper Llandovery times there was an eastward transgression of the sea from the Welsh Geosyncline, and near-shore grits and conglomerates were laid down around the S.E. margin of the Long Mynd. These beds are succeeded and overlapped by the shallow water limestones and siltstones of the Pentamerus Beds and the mudstones of the Hughley Shales. During Wenlock and Ludlow times the broad boundary zone between the shelf and the geosyncline appears to have lain in the general region of the Church Stretton Fault Complex. The sediments laid down in the Craven Arms area were siltstones, silty mudstones and limestones, largely of the shelf facies. At the close of the Silurian Period earth-movements caused the gradual elevation of the geosyncfinal sediments in Wales into a folded mountain chain, and this caused the conversion of the shelf sea of Ludlow times into a subsiding delta plain. The consequent transition from a marine to a continental environment is seen in the sediments of the Downton Series of the Lower Old Red Sandstone.

There is a gap in the local sedimentary record between the Downton Series (about 400 million years old) and the superficial deposits of Pleistocene times (up to about two million years old). During the latter period the Craven Arms area was invaded by ice from the Shropshire Plain to the N. and the Welsh hills to the west. The glacial deposits, which are largely boulder clay, provide evidence for only one period of glaciation.

Chapter 2 Pre-Cambrian

Uriconian

Uriconian rocks occur in fault-bounded inliers within the Church Stretton Fault Complex at Wart Hill [SO 4006 8476] and one-third of a mile to the north-east. In both inliers the exposure is poor and the relationship between the various rock types uncertain. The faulted boundaries of the inliers are also largely conjectural.

The southern inlier includes Wart Hill and extends southwestwards, largely beneath drift, for about half a mile beyond the margin of the map. Coarse-grained tuffs and tuffaceous greywackes are seen in several exposures on the southern side of the hill and andesite is exposed [SO 4000 8462] 170 yd S.22°W. of the summit. These beds are believed to be faulted against rhyolites and lithic tuffs which crop out to the north. The rhyolites and tuffs appear to underlie porphyritic andesites and tuffs with a northerly dip of 45° to 80° which form the steep northern slopes of the hill. In this part of the inlier the highest beds are homogeneous rhyolites which crop out [SO 4018 8502] 300 yd W.12°S. of Palace. Banded and brecciated rhyolites occur in a small faulted block [SO 4030 8485] E.N.E. of the summit of the hill.

In the second inlier, altered basalt with amygdales filled with chlorite, quartz and epidote occurs in scattered outcrops [SO 4056 8524] in the wood 400 yd E.7°S. of Upper Carwood, and in the stream 150 yd to the north.

Longmyndian

Longmyndian rocks are present in two separate areas. The N.W. corner of the map includes a small part of the S.E. corner of the Long Mynd, where beds of the Stretton Series (Burway, Synalds and Lightspout groups) occur. These rocks are in continuity with those of the main part of the Long Mynd to the north (Wright 1968). Longmyndian rocks also occur within the Church Stretton Fault Complex, between the W. margin of the map and Cwm Head [SO 426 890]. Both the Stretton Series (Stretton Shale Group) and Wentnor Series are present in this outcrop.

Stretton Series

Stretton Shale Group

From the W. margin of the map to Cwm Head there is a narrow outcrop of the Stretton Shale Group between faults F1 and F2 of the Church Stretton Fault Complex. It is interrupted by a faulted wedge of Wenlock Shales immediately W. of the River Onny (Figure 3). About a quarter of a mile wide at its southern end, the outcrop narrows gradually towards the N.N.E. as the two faults converge. Locally there is a narrow zone of Wentnor Series rocks between it and F1. A small area of the Stretton Shale Group occurs to the E. of F2 near Brokenstones [SO 421 882].

The Stretton Shale Group comprises greenish grey shaly mudstones and siltstones, commonly contorted and crushed, usually with a N.N.E. strike and steep dips.

Near the southern end of the outcrop contorted olive-green shale is seen at Upper Carwood [SO 402 853] and green and grey shales are exposed at intervals in Dunslow Hollow and Day Batch [SO 4008 8590]–[SO 4076 8650]. Similar beds are exposed in the roadside between Horderley and Brokenstones. The Stretton Shale Group appears to be faulted against Wentnor Series grits in the stream [SO 4170 8804] about 950 yd S.W. of Cwm Head church and also in the plantation [SO 4199 8817] about 300 yd to the east-north-east. There is some doubt as to whether the boundary between the two series is really faulted or whether it is essentially an unconformity (Greig and others 1968, pp. 78, 80).

Burway Group

Rocks of the Burway Group crop out along the S.E. margin of the Long Mynd, overlain unconformably to the S.E. by the Upper Llandovery Series. In the Church Stretton area (Wright 1968) the Burway Group has a full thickness of about 2000 ft. Due to the steady southward overstep of the Upper Llandovery Series, its exposed thickness in the present area decreases from about 1000 ft in the N. to almost nothing in the west. It comprises flaggy, greenish grey sandstones, siltstones and mudstones, with a group of massive, micaceous greenish grey sandstones (the Cardingmill Grit) at the top.

The main part of the Burway Group, below the Cardingmill Grit, is extensively exposed on the valley sides [SO 408 889] about half a mile S.W. of Priors Holt. The beds dip at 60° to 80° to the S.E. and local cross-bedding suggests that they are overturned. Massive and shaly green siltstones are seen in the old quarry [SO 4134 8934] at Priors Holt.

The Cardingmill Grit is well exposed, though much faulted, on the sides of Mount Gutter [SO 4026 8826]–[SO 4032 8856] where it comprises about 190 ft of massive, green, quartz-veined sandstone with interbedded shale and siltstone. Northwards, in Woolers Batch [SO 4066 8900], it appears to consist of three beds of similar sandstone, 20, 30 and 20 ft thick, with a total of about 125 ft of interbedded purple shales.

Synalds Group

The outcrop of the Synalds Group lies immediately N.W. of the Cardingmill Grit, on the S.E. slopes of the Long Mynd. Its base is taken at the top of the Cardingmill Grit, and its upper limit at the colour change from dominantly purple rocks to the greenish grey rocks of the Lightspout Group. About 2800 ft thick, it consists mainly of purple shaly mudstones and siltstones with subordinate beds of purplish grey sandstone and local thin tuffs. The regional dip of the beds is steep to the west-north-west.

The Synalds Group is well exposed along the sides of Mount Gutter [SO 403 883]–[SO 401 888] and to a lesser extent on the sides of the next valley to the N., Woolers Batch [SO 406 890] [SO 405 895]. A dolerite intrusion in Mount Gutter [SO 4016 8844] is cut by barytes veins up to 4 in thick. A 6 ft 6 in bed of greenish grey and purple tuff is exposed in the crags [SO 4003 8863] on the S. side of Mount Gutter. This is the lower of two tuff beds which can be traced in the southern part of the Long Mynd, W. of the present area. Two tuff beds, about 4 ft and 7 ft 6 in thick, seen in Wooler's Batch [SO 4066 8950], [SO 4056 8954] may be equivalent to those traced farther south. The occurrence of tuffs in the Longmyndian suggests that the volcanic activity of Uriconian times was not yet completely extinct (Wright 1968, pp. 22–3).

Lightspout Group

Rocks of the Lightspout Group occur in the extreme N.W. corner of the area. They are almost entirely obscured by superficial deposits, mainly head, but green shales and sandstones are exposed in the stream [SO 4026 8962] 1200 yd W.N.W. of Old Churchmoor.

Wentnor Series

Rocks of presumed Wentnor Series age crop out within the Church Stretton Fault Complex between Hopesay Hill and Cwm Head [SO 425 888]. They lie mainly to the E. of fault F2 and are succeeded eastwards by Ordovician sediments, which in places overlie them unconformably and elsewhere are faulted against them. Three narrow fault-bounded outcrops occur between F1 and the Stretton Shale Group to the east.

The rocks are purple, or locally green, grits and micaceous sandstones with subsidiary mudstones, siltstones and conglomerates. They have been included in the Wentnor Series because of their general lithological similarity, but it has not been possible to make a correlation with either the Bayston–Oakswood or Bridges groups of the Long Mynd outcrop (Greig and others 1968, p. 49).

Wentnor Series rocks form the high ground of Hopesay Hill. There are no sections on the hill, but there is much debris of purple sandstone in the soil. Purple sandstone is seen in the stream [SO 4045 8313] 180 yd N.W. of Oldfield with Hoar Edge Grit exposed 30 yd downstream. They also form the ridge between Upper Carwood and the River Onny, though an unconformable outlier of Hoar Edge Grit, fault-bounded to the E., caps the ridge near its southern end. The Ordovician–Pre-Cambrian junction is exposed in an old quarry [SO 4118 8614] W.S.W. of Glenburrell, where the Hoar Edge Grit rests on soft weathered purple grit (pp. 24–5). Northwards the Wentnor Series outcrop extends to the faulted Brokenstones area, where purple grit is exposed on the hilltop [SO 4198 8782].

Purple grit in the stream [SO 4042 8642] S. of Ridgway Wood indicates the presence of a narrow strip of Wentnor Series beds between F r and the Stretton Shale Group. Two similar strips occur along F1 between Horderley and Cwm Head.

The Wentnor and Stretton series are seen in close proximity (p. 5) in the wood [SO 4199 8817] 680 yd S.30°W. of Cwm Head church and S. of Pillocksgreen [SO 422 887], and also about 950 yd S.W. of the church where Wentnor Series grits are cut by narrow andesite intrusions [SO 4170 8804]. Two further fault bounded strips of Wentnor Series beds trend N.N.E. from the cross-fault at Brokenstones to Cwm Head.

Chapter 3 Ordovician: Caradoc Series

In the Craven Arms area Ordovician rocks occur within and to the E. of the Eastern Uriconian Axis (p. 53), their outcrop extending N.E. from the southern end of Hopesay Hill to Marshbrook and Acton Scott. The higher part of the Ordovician sequence is cut out S.W. of Wistanstow by overstep of the basal Silurian. In their most westerly outcrops the rocks generally dip steeply to the E. or S.E. and are locally overturned to the west. Eastwards the dips decrease, usually abruptly, to i o to 20 degrees.

Only part of the Ordovician, the Caradoc Series, occurs in the Craven Arms area. In the Shelve district, W. of the Long Mynd, the Arenig, Llanvirn and Llandeilo series underlie the Caradoc while in other parts of the country the Ashgill Series lies above it. It comprises shallow-water sandstones and siltstones with a shelly fauna, chiefly brachiopods and trilobites, locally so abundant as to form thin limestones. These sediments were probably laid down in a shelf area which was relatively stable and was not submerged until Caradoc times, in contrast to the more rapidly subsiding basin which extended into Wales from W. of the Long Mynd, and in which deposition took place throughout the earlier part of the Ordovician Period.

Subdivisions of the shaly rocks of the basin facies are based on graptolite faunas, the zones recognized in the Caradoc Series of Wales being those of Nemagraptus gracilis, Diplograptus multidens, Dicranograptus clingani and Pleurograptus linearis in upward succession. The shelf facies of the Craven Arms area has a predominantly shelly fauna, but a few graptolites have been found which enable a partial correlation to be made with the succession in the basin area to the west. Dean (1958, pp. 226–30) discussed the correlation of the shelly and graptolite faunas and concluded that all the beds above the Chatwall Flags (Soudleyan) could probably be assigned to the D. clingani Zone and that there was no acceptable evidence of the presence of the P. linearis Zone in south Shropshire.

The palaeontological classification of the Caradoc Series is based on the work of Bancroft who developed a system dependent on his recognition of a succession of distinctive brachiopod–trilobite faunas. Dean (1958) amplified this work and made a new correlation between the shelly and graptolitic faunas. The lithological classification used in this account follows that of Greig and others (1968). Although some of the lithological units used are diachronous, in so far as they are units they constitute a classification which is useful to the layman and the geologist in the field. Table shows the relation of the palaeontological classification to Dean's lithological units and those used in this account. It is based largely on the work of Dean (1958, figs. 3 and 4;1964, p. 273) and refers to the succession in the Onny valley.

Hoar Edge Grit

The Hoar Edge Grit comprises coarse-grained sandstones and pebbly sandstones, often calcareous, locally with thin limestone beds. The basal beds are usually white and buff quartz-conglomerates. It crops out in four separate areas within the Church Stretton Fault Complex and, except at Brokenstones where the crop is fault-bounded, it appears to rest unconformably on the Wentnor Series.

The southernmost outcrop extends from the W. margin of the map to Oldfield where it ends against a N.–S. fault. At Oldfield, buff and grey pebbly grit crops out in the stream [SO 4046 8312] within a few yards of purple sandstone of the Wentnor Series. A downfaulted outlier of the formation lies between 150 and 600 yd N.N.E. of Upper Carwood.

The largest area of Hoar Edge Grit extends N.N.E. for a mile and a half from a place [SO 406 854] 500 yd E.N.E. of Upper Carwood. The formation is fully exposed in an old quarry [SO 4118 8614] 180 yd W.S.W. of Glenburrell, where it is about 80 ft thick and consists of brown calcareous grits with interbedded thin lenses of grey crystalline limestone and occasional pebbly beds, with an easterly dip of about 70 degrees. The relationship of the Hoar Edge Grit with the underlying Wentnor Series (at the W. end of the quarry) is uncertain. Whittard (1953, p. 243 and fig. 1) and Dean (1964, p. 274) both considered that the Ordovician–Pre-Cambrian junction is faulted, and that this accounts for the thinning of the Hoar Edge Grit from about 550 ft further south. Greig and others (1968, p. 19), however, considered that it rests unconformably on the Wentnor Series and that its thinning may be an original depositional character. The failure of the Hoar Edge Grit outcrop N.W. of Woolston is shown on the map as being due to faulting, but the evidence is not conclusive. North of Brokenstones much of the lower Caradoc succession is cut out by faulting but near Church Stretton, on Hazier Hill and nearby, rocks of Harnagian age rest unconformably on the Pre-Cambrian (Strachan and others 1948). Thus the outcrop failure at Woolston may be due to Harnagian overlap.

The Hoar Edge Grit at Brokenstones appears to be almost completely fault-bounded. Fossiliferous coarse-grained sandstones in Brokenstones Plantation [SO 4189 8794] have yielded the bryozoan Batostoma murchisoni Spjeldnaes, the brachiopods Heterorthis Patera (Davidson) and Horderleyella plicata Bancroft and the trilobite Flexicalymene cf. acantha Bancroft.

Harnage Shales

The outcrop of the Harnage Shales extends northwards from the W. margin of the map to the Onny valley at Glenburrell [SO 413 862] and thence to the faulted area of the Brokenstones. They succeed the Hoar Edge Grit except between Oldfield Wood [SO 406 833] and Wood House [SO 408 855] where they are faulted against Pre-Cambrian rocks to the west. Because of their relatively soft character they are poorly exposed.

West of Sibdon Carwood the Harnage Shales appear to be about 500 ft thick, but some of the beds may be repeated by faulting. They are buff, flaggy, micaceous, silty mudstones, locally shaly, and commonly with orange-stained cleavage and bedding planes. Beds of siltstone and fine-grained sandstone occur towards the top of the formation and there is a gradual upward passage into the Chatwall Flags. An old gravel pit [SO 4066 8324] 700 yd W.8°N. of Sibdon Carwood church and another pit 300 yd to the N. both show obscure exposures of steeply dipping or vertical mudstones and dark grey basalt. The field relations of the basalt are difficult to make out, but Whittard (1952, p. 162) and Dean (1964, p. 275) both considered it to be a lava flow. It is a carbonated and chloritized porphyritic basalt, and is the only definite evidence of vulcanicity in the Ordovician rocks of the Caradoc district.

In the Onny valley the Harnage Shales are about 400 to 450 ft thick. The basal 4 ft of the formation, yellowish buff, micaceous shale dipping E. at 65°, are seen at the E. end of the old quarry [SO 4118 8614] in the Hoar Edge Grit (p. 24). Green shale and flags form two small outcrops [SO 4130 8624] 35 and 70 yd W.N.W. of Glenburrell farmhouse, and higher beds are seen behind the farmhouse. Farther N., green shale with occasional thin beds of shelly grit is exposed [SO 4208 8755] 500 yd N.W. of Woolston where the fauna includes the trilobite Primaspis harnagensis (Bancroft) and the ostracoc Tallinnella scripta (Harper).

Chatwall Flags

The southern limit of the Chatwall Flags is obscured by drift, but they appear to be overstepped by the Wenlock Shales near Long Meadowend. Northwards their outcrop extends to the cross-fault S.W. of Whittingslow. In the S., between Long Meadowend and Spring Coppice [SO 410 849], there is partial repetition of the formation by fault F3. They succeed the Harnage Shales and usually have a steep easterly dip.

The Chatwall Flags comprise green and buff micaceous flags with some beds of shale and of more massive sandstone. The higher beds are locally dark reddish brown, many of the bedding planes carrying abundant impressions of ossicles of the crinoid Balacrinus [Glyptocrinus] basalis (McCoy), from which the popular name 'Glyptocrinus Flags' is derived. Their thickness varies between 120 and 250 ft, being least in the Onny valley.

Twenty-one feet of fine-grained purple sandstone, high in the formation, are exposed in a quarry [SO 4100 8350] about 500 yd N.W. of Sibdon Carwood church, and there are further sections some 400 yd to the south. North of the River Onny the Chatwall Flags occupy the steep W. slope of Briar Edge, while N.E. of Brokenstones their unusually broad outcrop may be due to partial repetition of the beds by a strike fault. At Blakemoor [SO 4259 8810] interbedded green micaceous flags and shales are exposed with a fauna including crinoid debris, the brachiopod Onniella cf. soudleyensis (Bancroft) and the trilobite Broeggerolithus cf. soudleyensis (Bancroft).

Chatwall Sandstone

The Chatwall Sandstone extends N. from Sibdon Carwood, where it is overstepped by the Wenlock Shales, to the cross-fault S.W. of Whittingslow. West of Cheney Longville it is displaced sinistrally for about 500 yd by a cross-fault and from here to the N. side of the Onny valley there is partial repetition of the formation by the fault F3. Along most of its crop the Chatwall Sandstone forms a distinct topographic feature, and at Briar Edge it caps a prominent escarpment.

The formation consists predominantly of massive dark green and brown sandstone, commonly current-bedded and occasionally flaggy. Purple banding is characteristic of the middle beds, and a uniform purple colour is developed locally. A number of shelly limestones up to 3 ft in thickness are present in the uppermost beds. Its thickness varies from 300 to 500 ft, being greatest in or near the Onny valley.

Drift deposits obscure the Chatwall Sandstone as far N. as Longlane Quarry [SO 413 842]. This quarry is now largely obscured, but in 1959 about 52 ft of greenish and purple banded sandstone with thin shelly limestones in the upper part were visible at its N. end. The fauna includes the brachiopods Dalmanella horderleyensis (Whittington), Howellites antiquior (McCoy), Kjaerina jonesi Bancroft and Sowerbyella soudleyensis Jones, and the trilobites Brongniartella minor subcarinata Dean, Flexicalymene cf. planimarginata (Reed) and Kloucekia apiculata (McCoy). North of the cross-fault at Spring Coppice, the basal beds, transitional to the Chatwall Flags, are seen in a quarry [SO 4101 8524] half a mile E. of Upper Carwood, where they contain the brachiopod Macrocoelia expansa (J. de C. Sowerby) and the trilobite Broeggerolithus cf. soudleyensis. They dip at 68° to the E.S.E. in contrast to the dip of 17° in higher beds Zoo yd to the E.S.E. on the E. side of F3.

The Chatwall Sandstone is well displayed in the Onny valley. About 350 ft of steeply dipping dark green and purple sandstone are seen in the old railway cutting [SO 414 860] 300 yd E. of Smeathen. The fault F3 appears to cut this section about 360 yd E. of Smeathen where the dip changes abruptly from vertical to 28° to the east-south-east. A further go ft of beds are exposed to the E.S.E., with shelly limestones in the top 20 ft.

At Woolston, 30 ft of vertical sandstone with shelly limestone beds, especially in the lower part, are seen in an old quarry [SO 4224 8734]. The fauna includes the brachiopods Dalmanella cf. horderleyensis, Howellites antiquior and Sowerbyella soudleyensis, and the trilobites Broeggerolithus, nicholsoni (Reed) and Kloucekia apiculata. Northwards the Chatwall Sandstone forms a distinct feature to a cross-fault S. of Blake-moor, and the outcrop ends at another cross-fault about 300 yd S.W. of Whittingslow.

alternata Limestone

The crop of the alternata limestone adjoins that of the underlying Chatwall Sandstone. Because of lack of exposure it has not been possible to distinguish this formation S. of the cross-fault at Cheney Longville and here it has been included within the Cheney Longville Flags. Northwards it can be traced to the cross-fault S.W. of Whittingslow.

The formation comprises a succession of lenticular shelly limestones up to 2 ft thick characterized by abundant Heterorthis alternata (J. de C. Sowerby) and interbedded with green micaceous flags and shales. Usually about 70 ft thick, it appears to thicken to about 100 ft near Woolston. It is well exposed in the roadside [SO 416 852] N.W. of Cheney Longville where as well as Heterorthis alternata the large fauna includes the brachiopods Bancroftina robusta (Bancroft), Kjaerina bipartita (Salter), Sowerbyella sericea (J. de C. Sowerby), Strophomena grandis (J. de C. Sowerby) and Trematis punctata (J. de C. Sowerby), the supposed scaphopod Tentaculites sp.and the trilobite Kioucekia apiculata. Green flags and grey bedded limestone crop out in the old railway cutting [SO 418 857] S. of New House, and in the River Onny to the north. At Woolston, vertical interbedded green flags and grey limestone are exposed in the roadside [SO 4227 8735] 230 yd from the road junction, where they contain the brachiopods Heterorthis alternata and Sowerbyella sericea, and the trilobite Broeggerolithus sp.

Although Bancroft's (1933) choice of the alternata Limestone as the lowest member of the Upper Longvillian Stage may have been founded on his recognition in places of a non-sequence at its base, the faunal lists published by Dean (1958, pp. 220–2) show that, in addition, the fauna of the alternata Limestone has more in common with that of the beds above than with that of the Lower Longvillian. In the Onny valley there does not appear to be a non-sequence at the base of the formation.

Cheney Longville Flags

The crop of the Cheney Longville Flags extends from S. of Cheney Longville, where the formation is overstepped by the Wenlock Shales, to the northern limit of the area near Marshbrook. Near Whittingslow there is partial repetition of the beds by the fault F3, and around Marshbrook the crop is further complicated by other faults. Over most of its outcrop the formation gives rise to a broad east-south-easterly dipping slope part of which is drift-covered.

Cheney Longville Flags, together with the alternata Limestone, make up the Upper Longvillian and Marshbrookian stages of Bancroft. The rocks are predominantly yellowish weathering, greenish grey flags with interbedded shales and rubbly siltstones. Thin beds of fine-grained sandstone occur and thin shelly limestones, usually decalcified, are quite common. Brachiopods and trilobites are common throughout the formation and amongst other fossils the most conspicuous and characteristic is Tentaculites. The Cheney Longville Flags are about 800 ft thick near Cheney Longville, thinning northwards to 400 or 500 ft over most of the outcrop.

The formation is well displayed in the W. part of Cheney Longville village, particularly in the vicinity of the Motte and Bailey [SO 4186 8496] where it has yielded a fauna including the brachiopods Dalmanella wattsi (Bancroft) and Sowerbyella cf. sericea, the gastropod Cyrtolites nodosus (Salter), Tentaculites sp.,and the trilobites Brongniartella bisculata (McCoy) and Chasmops extensa (Boeck). Further N., at Woolston, green flags and sandstones are exposed in the roadside [SO 4233 8730] between go and 2 I o yd N.W. of the road j unction. These beds have a steep W.N.W. dip which contrasts with the more gentle and variable easterly dip in the yard immediately W. of the road junction. The fault F3 is believed to pass between the two sections. Between Woolston and the Marsh Brook valley the dip-slope is largely drift-covered. The quarry and track section in Marsh Wood [SO 4445 8904], type section for the Marshbrookian Stage (Bancroft 1945, pp. 195–6; Dean 1958, pp. 209–10), is now mostly overgrown. About 90 ft of flaggy siltstones are intermittently exposed, the highest beds being transitional to the Acton Scott Group.

Cheney Longville Flags are intermittently exposed around Whittingslow, to the N.W. of F3. Green and brown sandstones and flags, with some shales and shelly limestones, crop out locally in the roadside and stream for about half a mile N.E. of a place [SO 4297 8924] 150 yd N.W. of Crosspipes. Another roadside section [SO 4410 8978] 200 yd W.S.W. of Marshbrook Station shows 35 ft of beds in the middle of the formation which contain the brachiopods Dalmanella unguis (J. de C. Sowerby), Kjerulfina sp.and Strophomena grandis, Tentaculites sp.and the trilobite Broeggerolithus transiens (Bancroft).

Acton Scott Group

The main crop of the Acton Scott Group extends from Cheney Longville, where it is overstepped by the Silurian, to the N. margin of the area at Acton Scott. A further small area lies to the N. of a W.S.W. trending fault at Marshbrook, and there is an isolated faulted outlier [SO 437 898] S. of New House. The Acton Scott Group, as defined by Greig and others (1968, p. 115), appears to be co-extensive with Bancroft's Actonian Stage (1945, pp. 183, 186), at least in the Onny valley.

In the Onny valley the formation is about 200 to 300 ft thick and consists of grey mudstones with thin shelly limestone beds. At Acton Scott, where it is about 200 ft thick, the beds are mainly olive-green silty mudstones and siltstones with an 80-ft lens of hard sandy limestone or calcareous sandstone, the Acton Scott Limestone, developed in the middle of the succession.

The most southerly exposure of the Acton Scott Group is in the River Onny [SO 4238 8528] 300 yd S. of Upper Barn, where blocky mudstone with two thin clay beds contains a fauna of brachiopods and gastropods (pp. 21–2). Owing to an extensive drift cover there are no good sections between the River Onny and the Marsh Brook. The Acton Scott Limestone caps the high ground between the village and the Marsh Brook, and is well displayed in a quarry [SO 4495 8955] 700 yd W. of Acton Scott Hall where 15 ft of fine-grained very calcareous sandstone have yielded crinoid ossicles, many brachiopods such as Cryptothyris paracyclica (Bancroft), Hedstroemina robusta (Bancroft), Onniella cf. grandis Bancroft, Reuschella cf. bilobata (J. de C. Sowerby) and Strophomena grandis and the trilobite Platylichas sp.Similar beds are exposed in a stream section [SO 4585 8962] 300 yd E.N.E. of the Hall. To the E. the Acton Scott Limestone dies out and the feature formed by the Acton Scott Group diminishes.

North of the W.S.W. trending fault at Marshbrook, the best section is at the overflow [SO 4460 8992] from Chuney Pool where 20 ft of olive-green micaceous siltstone are folded into a broad anticline with an E.N.E. axis. They contain a varied fauna including the scyphozoan Metaconularia sp.,the brachiopods Leptaena sp., Onniella cf. aspasia Bancroft and Reuschella cf. bilobata, Tentaculites sp.,the trilobite Onnicalymene cf. salteri (Bancroft) and the graptolite Climacograptus sp.

Onny Shales

The lenticular outcrop of the Onny Shales is limited to the E. by the unconformable basal Silurian which oversteps the formation in the S. at Cheney Longville and in the N. at Hatton, just N. of the present area. The Onny Shales are extensively covered by drift, and exposures are almost confined to the Onny valley and the area around Henley [SO 451 883].

Lithologically the Onny Shales resemble the underlying mudstones of the Acton Scott Group, from which they are differentiated mainly on palaeontological grounds. They contain numerous cryptolithid trilobites which are almost unknown in the Acton Scott Group. The rocks are mainly grey and buff micaceous siltstones and mudstones but in the River Onny sections they appear to be more greenish and better stratified than the underlying beds. They are about 400 ft thick near Henley but it is not known if any horizons present there are higher than those in the River Onny where only about 150 ft of beds crop out W. of the Silurian. The Onny Shales are equivalent to the Onnian Stage of Bancroft (1933) and the Onnia Beds of Dean ( r958, p. 213).

The best known section in the formation is the famous cliff section in the River Onny [SO 4262 8530] 250 yd W. of the Manor House, Wistanstow. Thirty feet of green micaceous shale are exposed, overlain unconformably by the basal Silurian (p. 31). The extensive fauna includes many brachiopods and trilobites (p. 21). Near Henley there are small exposures 50 yd N. of Marsh Mill [SO 4483 8810] and in the disused railway cutting [SO 4528 8852] N.E. of the farm. Buff flaggy siltstone crops out at Hatton Pool [SO 4622 8942].

Onny River Section

The well known Onny River Section comprises a number of exposures in and near the River Onny between Glenburrell and Cross Way, Wistanstow. Within this short distance there are exposures, many of which are very fossiliferous, in all the formations of the Caradoc Series and also in the lowest Silurian (Figure 1). For this reason it has been thought desirable to describe the section as a separate item although some of the individual exposures have been mentioned earlier in the text.

The Onny Valley is visited by many geologists, and it is imperative to obtain permission from the various landowners before examining the exposures. In particular, two localities (Nos. 15 and 20) are adjacent to houses and the collection of specimens is not normally permitted. Access to the south bank of the river is by footbridges near Glenburrell [SO 4124 8616], New House [SO 4182 8571] and Cross Way [SO 4276 8510].

The first three localities are reached along the north bank of the river from the footbridge near Cross Way. A few yards upstream from the footbridge (Locality r) soft grey shales of the Wenlock Shales, the highest beds of the Onny River Section, are exposed in the river bed.

The cliff section (Locality 2) displays the unconformity between the basal Silurian (Hughley Shales) and the Onny Shales, topmost formation of the Caradoc Series. About 35 ft of Hughley Shales are exposed, comprising green shales, commonly purple and purplish green in the upper beds, with many beds of hard micaceous calcareous flags up to 8 inches thick. The underlying Onny Shales are overstepped and successive Silurian beds overlap towards the N.W. apart from an apparently persistent basal bed of grey shelly limestone, half an inch thick. The Onny Shales, of which about 30 ft are visible, consist of green micaceous shale weathering to orange-brown above water level. A one-inch bed of grey crystalline limestone occurs within a few inches of the unconformity. The extensive fauna of the Onny Shales includes the brachiopods Onniella broeggeri Bancroft and 'Strophomena' holli Davidson, the trilobites Onnia superba (Bancroft), Onnicalymene onniensis- (Shirley), Remopleurella burmeisteri (Bancroft) and Triarthrus cf. linnarssoni Thorslund, the ostracod Primitia sp.and the graptolite Climacograptus sp.

Lower beds in the Onny Shales are exposed a few yards upstream (Locality 3) where a ledge of calcareous mudstone across the river contains Onniella broeggeri and the trilobites Illaenus cf. fallax Holm, Onnia gracilis (Bancroft) and Onnicalymene onniensis. Beds near the base of the Shales are seen in the south bank of the river about 45 yd east of Batch Gutter (Locality 4) where rubbly mudstones contain Onniella cf. broeggeri and abundant specimens of the trilobite Onnia cobboldi (Bancroft). Bancroft (1949) has recorded several other small exposures in the basal Onny Shales and the Acton Scott Beds between localities 3 and 5.

The Acton Scott Beds can be seen in two sections in the south bank of the river. At Locality 5, 70 yd west of Batch Gutter, grey blocky mudstone with two thin beds of clay yields the brachiopods Hedstroemina cf. robusta, Onniella sp.and Sericoidea sp.,and the gastropods Sinuites bilobatus (J. de C. Sowerby), S. pseudocompressus Reed and Tropidodiscus acutus (J. de C. Sowerby). A few yards farther upstream (Locality 6) brown weathering grey shaly mudstones contain an extensive fauna including the brachiopod Onniella cf. aspasia, the gastropods Archacinella cf. oblongata (Portlock), Sinuites bilobatus and Tropidodiscus acutus and the trilobites Ampyxella edgelli Reed and Loncltodomas pennatus (La Touche). Acton Scott Beds can also be seen in the roadside just west of Batch Gutter (Locality 7) where about 5 ft of olive-brown blocky weathering mudstone are exposed. As with the other sections in the Acton Scott Beds this section contains an extensive fauna with the brachiopods Nicolella actoniae (J. de C. Sowerby), Onniella sp.and Sowerbyella sp.,the gastropod Tropidodiscus acutus and the ostracod Primitia sp.

Green flags and decalcified yellow flaggy sandstones in the south bank of the river (Locality 8) yield a fauna including the brachiopods Kjerulfina polycyma Bancroft, Onniella sp., Strophomena grandis and Reuschella sp., Tentaculites sp.,and the trilobites Broeggerolithus sp.and Brongniartella bisulcata typical of the upper part of the Cheney Longville Flags (Marshbrookian). Similar beds are also exposed a few yards upstream (Locality 9).

The Lower Cheney Longville Flags (Upper Longvillian) are seen in an old river cliff and adjacent farm track (Locality o) on the south side of the old railway. About 55 ft of beds are exposed with an east-south-easterly dip of 20°. They comprise green flags with massive sandstone beds, up to 1 ft thick, in the upper half of the section, and with some thin beds of shelly crystalline limestone. There is a large brachiopod fauna including Bancroftina sp., Dalmanella sp.,Dolerorthis cf. duftonensis Reed and Kjaerina typa Bancroft and also Tentaculites sp. Grey micaceous flags and limestone, also of Upper Longvillian age, crop out in the river bank to the northwest (Locality ). There are no sections in the lowest part of the Flags.

The best section in the alternata Limestone is in the old railway cutting (Locality 12) south of New House where green flags and grey bedded limestones with abundant specimens of the brachiopod Heterorthis alternata are exposed intermittently over a distance of some 70 yd. Small exposures of similar beds are seen in the south bank of the river 25 yd east of the footbridge near New House (Locality 14) and in the north bank 35 yd farther east (Locality 13). In addition to H. alternata the latter locality has yielded the brachiopods Bancroftina robusta, Dalmanella sp., Kjaerina bipartita and Strophomena grandis and the trilobites Brongniartella bisulcata and Kloucekia apiculata.

There are several good sections in the Chatwall Sandstone in the Onny Valley. The topmost beds of the Sandstone are exposed in the quarry immediately west of New House (Locality 15). Although partly obscured this quarry displays about 18 ft of massive brown and green sandstone with an east-south-easterly dip of 17°. A bed of rotted sandstone full of specimens of the brachiopod Sowerbyella soudleyensis occurs near the top of the section. Other fossils include the brachiopods Bancroftina typa (Whittington), Dalmanella cf. lepta (Bancroft) and the trilobite Brongniartella sp.On the south side of the old railway a series of quarries and cliffs (Locality i6) shows a total of about go ft of massive purple and greenish brown sandstone. There are occasional shell beds in the lower part of the sequence and many beds of shelly limestone in the top 20 ft. A large roadside quarry (Locality 17) in the Chatwall Sandstone about 300 yd S.E. of Glenburrell displays about 50 ft of beds with a gentle south-easterly dip. The lowest 20 ft consists of dark green massive sandstone with many thin purple bands and a few beds of shelly limestone. The higher beds are less massive with more abundant beds of shelly limestone. The fauna includes the brachiopods Dalmanella horderleyensis, Howellites antiquior and Sowerbyella soudleyensis and the trilobites Flexicalymene cf. planimarginata and Kloucekia apiculata. The old railway cutting (Locality 18) south of Glenburrell, though now rather overgrown, still shows the contrast between the steeply dipping Chatwall Sandstone west of the fault F3 and the more gently dipping beds to the east. To the west of the fault the dip is 80° to 90°, to the E.S.E. in the lower beds and to the W.N.W. (overturned) in the rest of the section. There is an abrupt change to a dip of 28° to the E.S.E. at the south-east end of the section where F3 cuts the beds. About 350 ft of dark green and purple sandstone are intermittently exposed with a 3-inch shelly limestone bed near the top and a thin limestone bed near the base.

The Chatwall Flags and Harnage Shales are poorly exposed in the Onny Valley. The Flags are represented by brown flaggy sandstones which crop out in the river south of Glenburrell (Locality 19) and which contain the brachiopods Macrocoelia expansa, Reuschella horderleyensis Bancroft and Sowerbyella soudleyensis, the trilobites Broeggerolithus cf. broeggeri Bancroft, Brongniartella minor subcarinata and Proetidella sp.and the ostracod Tallinnella scripta. Beds in the upper part of the Harnage Shales form a low cliff immediately behind Glenburrell farmhouse (Locality 20). They comprise brown weathered shales with occasional beds of sandstone of an average thickness of 2 inches but exceptionally up to seven inches thick. This section was taken by Dean (1958, p. 203) to be the type locality of Bancroft's 'Glenburrell Beds' (1933). Slightly lower beds, green shales with flaggy sandstone beds up to four inches thick, crop out in the river (Locality 21) about 50 yd east of the footbridge near Glenburrell.

The full thickness of the Hoar Edge Grit is exposed in the quarry (Locality 22) about 75 yd west of the Glenburrell footbridge. The quarry is in two parts, the rocks of the eastern part appearing to succeed those in the western part. In the eastern part of the quarry about 57 ft of beds are exposed, the topmost 4 ft comprising yellowish buff micaceous shales being included in the Harnage Shales. The remaining 53 ft (Hoar Edge Grit) consist of flaggy and massive calcareous grits with occasional thin limestones and several shelly and pebbly beds. Two thin beds of pale grey and brownish grey clay occur about 2 ft and 2 ft 6 in respectively below the top of the Grit. About 30 ft of Hoar Edge Grit are exposed in the western part of the quarry, mainly brown calcareous grit with rounded quartz pebbles at one horizon near the base. The topmost 18 ft are rather flaggy, but the lower beds, up to 4 ft thick, are massive. Lenses and beds of crystalline limestone, commonly weathered back, are interbedded with the grits. At the west end of the quarry, which has recently been cleaned, the Hoar Edge Grit is in contact with purple weathered grits of the Western Longmyndian. Beneath the lowest hard grit of the Hoar Edge Grit, which has an easterly dip of 70°, there is about 1 ft 6 in of broken buff grit with a contained 2 in bed of pale buff clay. The broken grit rests on an irregular surface of weathered purple Longmyndian grit. Whittard (1953, pp. 243, 245) considered that the broken grit was Longmyndian and that the contact was faulted. On the other hand, Greig (in Greig and others 1968, p. 119) considered the broken grit to be Hoar Edge Grit resting unconformably on the Pre-Cambrian. He also states that the absence of disturbance in the weathered Pre-Cambrian grit and the course of the Hoar Edge Grit northwards across the River Onny both suggest that the contact is not faulted (see also p. 12).

Chapter 4 Silurian

Silurian rocks are present in two areas, known as the Western Outcrop and the Main Outcrop, which are separated by the Church Stretton Fault Complex and the Ordovician terrain on its S.E. side. The rocks of the two areas differ considerably in facies at some horizons and separate classifications have been used in their description. These classifications are shown in detail on the side margin of the map. The upper limit of the Silurian System has been taken at the top of the Ludlow Series, and E. of the Church Stretton Fault Complex this horizon is equated with the base of the Ludlow Bone Bed.

On the W. side of the Fault Complex Silurian rocks (Western Outcrop) with a general south-easterly dip occupy the area between the slopes of the Long Mynd and the Church Stretton Fault (F1). Small areas of Silurian occur within the Complex near Horderley, Cwm Head and New House.

Silurian rocks of the Main Outcrop occupy a broad area between the S.W. and N.E. corners of the map. The Wenlock and Ludlow series are fully developed along the whole length of the outcrop, but S. of Cheney Longville the Llandovery Series is absent, cut out by the overlap of the Wenlock Shales. There is a general south-easterly dip of 8° to 10° with local swings to an easterly direction in the southern part of the area.

In Silurian, as in Ordovician, times the Craven Arms area was situated marginally between a relatively stable shelf area to the E. and an intermittently subsiding basin to the west. The boundary between the two areas lay in the general region of the Church Stretton Fault Complex, but it must be considered as a diffuse zone rather than a sharp boundary exactly along the line of the Complex. The oldest Silurian rocks, the Upper Llandovery Series, do not show a contrast in facies between the shelf and the basin. They were laid down as the earliest deposits of a transgressive sea and rest with strong unconformity on Ordovician and older rocks. On the margin of the Long Mynd the basal deposits exhibit a near-shore facies with sandstones and conglomerates; elsewhere they are of finer grain, comprising siltstones and mudstones. By the close of Upper Llandovery times the sediments were of uniformly fine grain over the whole area.

The earliest obvious difference in facies between the shelf and basin sediments is in the upper part of the Wenlock Series. There is a marked contrast between the shallow-water reef limestones near Much Wenlock, to miles E.N.E. of Church Stretton, and the graptolitic shales of the same age in the Long Mountain area, about 15 miles to the northwest. Whittard (1952, pp. 170–1) and Dineley ( 1960, pp. 101–2) considered that the facies change took place across the Church Stretton Fault Complex with an intermediate facies at Brokenstones within the Complex. However, later work (Dean 1964, p. 285; Greig and others 1968, pp. 143–4) indicates that the westward change of facies, S. of the Long Mynd, is more gradual. Wenlock sediments of the Rowton [SO 410 805] and Edgton [SO 388 858] areas, E. and W. of the Fault Complex respectively, are essentially similar and consist of siltstones with a mixed graptolite and trilobite fauna and with nodular limestones in the higher beds. This sequence can be considered as intermediate between the limestones of Much Wenlock and the graptolitic shales of the Long Mountain.

The distribution of sedimentary facies in the Ludlow Series of the Welsh Borders was discussed by Holland and Lawson (1963) who concluded that the margin between the shelf and basin facies was not a straight line such as the Church Stretton Fault Complex but was curved and convex to the east. In addition the position of the boundary fluctuated considerably in Ludlow times. In the Upper Ludlow rocks there is no distinction between shelf and basin facies although the thickness of the sediments (calcareous shelly siltstone) increases towards the west. This uniformity in the highest Silurian rocks heralds the great palaeogeographical changes which were to follow in the Old Red Sandstone Period.

Upper Llandovery Series

The stratigraphy of the Upper Llandovery Series of south Shropshire has been studied in detail by Whittard (1925, 1928, 1932), and the subdivisions used in the present account closely follow those adopted by him. The subdivision of the Pentamerus Beds of the Western Outcrop into grits and conglomerates and shales and limestones corresponds with the At enaceous and Mudstone phases of Whittard (1932).

Western outcrop

Pentamerus Beds

The basal division of the Pentamerus Beds, the grits and conglomerates, occurs in a discontinuous outcrop along the S.E. margin of the Long Mynd. These beds, up to 180 ft thick, comprise rather massive grey and purple sandstones, pebbly sandstones and conglomerates which rest with strong unconformity on a very irregular surface of Longmyndian rocks. Their south-easterly dip of up to 30° is in part an original depositional dip. The only fossils are occasional lingulid brachiopods. The best exposure of the grits and conglomerates is in the stream [SO 4104 8863] about 100 yd W. of Churchmoor Hall, where about 140 ft of fine-grained conglomerates and pebbly grits with a south-easterly dip of 23° are seen. A small outlier of Pentamerus Beds occurs to the E. of the Church Stretton Fault (F1) at New House [SO 436 899]. The grits and conglomerates are widely exposed near the farm and apparently rest unconformably on the Caradoc Series. At Hamperley [SO 4217 8912] a Geological Survey borehole proved 158 ft of sandstone and conglomerate resting unconformably on Pre-Cambrian rocks (Figure 2). This evidence shows that the marginal conglomeratic facies extends locally for at least 800 yd S.E. of the present margin of the Long Mynd.

The shales and limestone facies of the Pentamerus Beds occupies a narrow outcrop immediately to the S.E. of the grits and conglomerates. Locally the underlying grits are absent and the shales and limestones rest directly on the Longmyndian. This subdivision comprises grey shaly siltstones and mudstones with pale grey sandy limestones, usually less than 3 in thick but occasionally up to 1 ft in thickness. The characteristic fossil Pentamerus oblongus J. de C. Sowerby is abundant in most of the limestones and locally in the siltstones. It is difficult to determine the thickness of the shales and limestones because of an extensive drift cover, but the Geological Survey borehole at Hamperley [SO 4217 8912] proved 425 ft 9 in (true thickness at least 300 ft) of grey siltstone with thin beds of sandstone and shelly limestone (Figure 2). Surface exposures are poor, the best section being in an old quarry [SO 396 876] 950 yd N.W. of Castle Ring, just beyond the W. margin of the map, where 18 ft of grey shales and bluish grey limestones are seen. In addition to abundant P. oblongus these beds contain the coral Streptelasma whittardi Smith and the brachiopods Coolinia applanata (Salter), Rostricellula sp.and Stricklandia lens (J. de C. Sowerby).

A small crop of the shales and limestones occurs in the faulted Brokenstones area, and the facies is also present in the New House outlier.

Hughley Shales

This formation takes its name from the hamlet of Hughley, which is situated on the Main Outcrop in the Shrewsbury district (Pocock and others 1938). It is equivalent to the Purple Shales of Whittard and comprises purple and maroon shaly mudstones, locally silty, with many green bands and patches. Thin calcareous laminae are common and thin beds of shelly and argillaceous limestone occur locally.

In the Western Outcrop the Hughley Shales lie S.E. of the Pentamerus Beds (shales and limestones) and are largely obscured by drift. They are about 300 ft thick with a general S.E. or E. dip of 25 to 30 degrees. Extensive exposures of purple and green mucistone, locally crushed, occur in the stream [SO 4074 8763], [SO 4128 8822] between two faults to the S. and S.E. of Churchmoor Rough, and there is further intermittent exposure for about 600 yd upstream to the base of the formation [SO 4132 8874] a quarter of a mile S. of Old Churchmoor. The Hamperley Borehole [SO 4217 8912] proved 168 ft of maroon mudstones with green patches, the higher part of the formation being cut out by faulting.

Main outcrop

Pentamerus Beds

In the Main Outcrop the shales and limestones facies of the Pentamerus Beds rests unconformably on the Caradocian Onny Shales. The basal grits (known as the Kenley Grit) are only developed N.E. of Plaish, to the N. of the Craven Arms area. The beds are almost entirely drift covered, and their southward extent is uncertain. They appear to be overstepped by the Hughley Shales in the vicinity of Wistanstow. North of the disused railway near Henley their crop is marked by a slight topographical feature. From evidence farther N. they are similar in lithology to the shales and limestones facies of the Western Outcrop.

Hughley Shales

The Hughley Shales crop out from Cheney Longville northwards to Ironmongers Coppice. Near Wistanstow they overlap the Pentamerus Beds and to the S. rest on Ordovician rocks. They are themselves cut out by the transgressive base of the Wenlock Shales near Cheney Longville. Lithologically they resemble the Hughley Shales of the Western Outcrop, and reach a thickness of about 225 ft in the N. part of the area. They have a general south-easterly dip of up to 20 degrees.

The only section in the S. part of the outcrop is that on the N. bank of the River Onny [SO 4262 8528] 720 yd W.25°S. of Wistanstow Church, where 35 ft of green and purple shales with many beds of hard calcareous flags rest unconformably on the Onny Shales (Greig and others 1968, p. 169). The fauna includes the. brachiopods Aegiria [Chonetoidea] grayi (Davidson), Atrypa cf. reticularis (Unite), Clorinda globosa (J. de C. Sowerby), Eopholidostrophia salopiensis Cocks, Eoplectodonta millinensis (Jones) and Glassia compressa (J. de C. Sowerby) and the trilobite Encrinurus onniensis Whittard. North of the Onny valley the Hughley Shales are very poorly exposed.

Wenlock Series

The subdivision of the Wenlock Series adopted in the present work is based essentially on lithological criteria. In the Main Outcrop the basal member of the Series, the Wenlock Shales, is largely drift covered and has not been further subdivided. The overlying Tickwood Beds constitute a transition facies between the Wenlock Shales and the Wenlock Limestone in the northern part of the area. Southwards the Wenlock Limestone dies out near Craven Arms, resulting in strata of Tickwood Beds lithology being directly overlain by the Lower Ludlow Shales. There is insufficient palaeontological evidence to determine whether the Tickwood Beds are diachronous, and as there is no evidence of an unconformity at the base of the Lower Ludlow Shales it is probable that the strata of Tickwood Beds lithology S. of Craven Arms are equivalent to the Wenlock Limestone and Tickwood Beds farther north.

In the Western Outcrop the Wenlock Shales pass up into a group of beds in which layers of nodular limestone are abundant. These beds have been given the local name of Edgton Limestone (Greig and others 1968, p. 150) from the village of Edgton [SO 386 858] to the W. of the present area. The Edgton Limestone resembles the Tickwood Beds of the Main Outcrop, and in the absence of any definite palaeontological evidence its upper limit has been taken as the top of the Wenlock Series.

The Cwm Head Limestone of Dean (1964, p. 286) has not been shown as a separate unit on the map, and is included within the area of Wenlock Shales at Cwm Head. Dean pointed out that it may be the lateral equivalent of the "calcareous beds to the west of F1 near Horderley" (i.e. the Edgton Limestone). Whittard (1952, p. 171) originally described the Wenlock Series of the Cwm Head area as being intermediate in facies between the true graptolitic and shelly facies, and brought into juxtaposition with the graptolitic facies of the Western Outcrop by F1. However, Dean (1964, p. 285) stated that the lower Wenlock beds on the W. side of F1 contain both trilobites and graptolites, and concluded that the apparent change in facies between the Cwm Head area and the Wenlock beds to the W. of F1 is due to a difference in age of the strata. He suggested that the succession at Cwm Head and on the W. side of F t is probably the same, but that this is difficult to prove owing to the paucity of exposure.

Western outcrop

Wenlock Shales

The outcrop of the Wenlock Shales is limited to the S.E. by the Edgton Limestone W. of Horderley, F1 between Horderley and Crosspipes [SO 430 893], and a subsidiary fault to the north. They are poorly exposed because of an extensive drift cover, but appear to have a general south-easterly dip. A small downfaulted area of probable Wenlock Shales (Figure 3) occurs within the Church Stretton Fault Complex S. of Horderley, and there is a second fault-bounded crop S.E. of F1 at Cwm Head.

The Wenlock Shales comprise about 1000 ft of flaggy and shaly grey siltstones and silty mudstones, locally calcareous, with thin nodular argillaceous limestone beds in the upper part and calcareous nodules and bullion towards the base. South of the River Onny the basal beds are reddish brown and purple silty mudstones, with local greenish colouration. These 'Purple Wenlock Shales' are up to 40 ft thick but, because of the extensive drift cover, it is not known if they extend N. of the river.

West of Horderley there are a number of sections on the lower slopes of the Edgton Limestone escarpment and at Horderley itself 50 ft of shale with nodular limestone beds are seen in old river cliffs. These beds, near the top of the formation, contain the brachiopods 'Camarotoechia' nucula (J. de C. Sowerby) and Aegiria [Chonetoidea] grayi and the graptolite Monograptus cf. dubius (Suess).

North of Horderley sections are chiefly confined to small tributary streams which flow S.E. into the Horderley–Cwm Head valley. About 300 yd S.S.E. of Hawkhurst [SO 4139 8776] the Wenlock Shales are faulted against Wentnor Series grits by F (Greig and others 1968, pp. 172–3) and in the streams and old quarry [SO 4224 8877] near Pillocksgreen they are highly contorted and calcite-veined due to the proximity of F1. In the fault-bounded outcrop at Cwm Head, khaki mudstone with many thin beds of grey fine-grained limestone (the Cwm Head Limestone) is exposed at the house and in the nearby quarry [SO 4242 8870]. These beds, which may be the equivalent of the Edgton Limestone (p. 32), contain a varied fauna including the brachiopods Atrypa reticularis, Eospirifer radiatus (J. de C. Sowerby), Leangella segmentum (Lindstrom) and Leptaena rhomboidalis (Wilckens), the trilobites Dalmanites caudatus (Brunnich) and Otarion megalops (McCoy) and the ostracod Hemsiella [Beyrichia] maccoyiana (T. R. Jones).

Edgton Limestone

This formation is confined to a small area between Horderley and the W. margin of the map. It comprises about 200 ft of grey, thinly bedded, fine-grained, nodular, argillaceous limestones (2 to 3 in thick) interbedded with grey shaly mudstone. About 50 per cent of the rock is limestone. The junction between the Wenlock Shales and the Edgton Limestone is transitional, but the contact with the calcareous flaggy shales of the overlying Ludlow Series is more clearly defined.

The Edgton Limestone forms a dip slope in Ridgway Wood and is well exposed in the stream [SO 4000 8643], [SO 4036 8644] S. of the road. Its dip is about 20° to the S. in the W., increasing eastwards to vertical near F1. In the fault-bounded triangle S.W. of Horderley an old quarry [SO 4044 8679] 300 yd E.S.E. of Castle Ring has yielded the trilobites Dalmanites cf. caudatus, Encrinurus sp.and Phacops stokesii (Milne Edwards). Edgton Limestone caps the prominent escarpment between the Castle Ring and Horderley.

Main outcrop

Wenlock Shales

This formation has a broad continuous outcrop from the S.W. corner to the N.E. corner of the area. North of Grove [SO 436 846] it occupies the broad drift-covered valley of Ape Dale and the lower slopes of Wenlock Edge. In the low ground S. of Grove the Wenlock Shales are largely obscured by drift. At the southern end of the outcrop they rest on Harnage Shales. Northwards they rest on successively higher divisions of the Caradoc Series, and N. of Cheney Longville they overlie the Hughley Shales. To the S.E. there is everywhere an upward transition into the Tickwood Beds. They have a general south-easterly dip of 10 to 15 degrees.

The Wenlock Shales of the Main Outcrop are about 1000 ft thick and are similar to those of the Western Outcrop (p. 33). The lower beds contain calcareous nodules and bullions, and yield small fossils, particularly brachiopods. There appears to be no development of basal Purple Wenlock Shales.

In the southern part of the outcrop, between Broome and Grove, there are few sections, though beds near the base of the formation are seen at Sibdon Pool [SO 4141 8331]. North of Grove, in Ape Dale, the Wenlock Shales are largely drift covered, and most exposures are located in streams or on the lower slopes of Wenlock Edge. The upper part of the formation is well displayed in the footpath [SO 4468 8533] a quarter of a mile S.E. of Strefford where about 185 ft of flaggy argillaceous siltstones are seen. Grey argillaceous limestone beds occur, except in the basal 30 ft, and become more common and more nodular towards the top of the section. A very large fauna includes the brachiopods Aegiria [Chonetoidea] grayi, Craniops implicata (J. de C. Sowerby), Dalejina hybrida (J. de C. Sowerby), Resserella basalis (Dalman), R. elegantula (Dalman), Schizocrania striata (J. de C. Sowerby) and Skenidioides lewisii (Davidson), the gastropod Theca forbesi (Sharpe), the cephalopod Kionoceras angulatum (Wahlenberg), the trilobite Dalmanites caudatus and the ostracod Hemsiella [Beyrichia] maccoyiana. The Wenlock Shales are well exposed around Harton.

Tickwood Beds

The crop of the Tickwood Beds extends from the S. limit of the area near Rowton to the N.E. corner, N. of Middlehope. They overlie the Wenlock Shales and are overlain by Lower Ludlow Shales S.W. of the River Onny and Wenlock Limestone to the north-east. Between Row ton and the River Onny the dip is south-easterly and the crop is affected by several faults, the most important of which, the Stokesay Fault, displaces it to the S.E. by about 500 yd. On the side of the River Onny, between the Stokesay and Bache faults, the Tickwood Beds clip E. at about o° and the feature formed by the Tickwood Beds and Wenlock Limestone is poorly developed. North-east of the Bache Fault, the dip direction changes again to S.E. at about to°, and the Tickwood Beds and Wenlock Limestone form the prominent scarp of Wenlock Edge. Tickwood Beds crop out on the upper part of the slope alone, the dip slope being formed by the Wenlock Limestone.

The Tickwood Beds vary in thickness from 100 to 180 ft and consist of alternating beds of grey fine-grained argillaceous limestone and grey calcareous siltstone. The limestones are generally 1 to 3 in thick and tabular in form, and in the main part of the formation they make up about 50 per cent of the rock. Towards the base siltstones become progressively more dominant and there is a passage into the Wenlock Shales beneath. North of the River Onny there is a rapid upward transition into the Wenlock Limestone.

Typical Tickwood Beds are seen in several old quarries between Broome and Paddock. About 120 ft of interbedded limestone and siltstone are exposed in the railway cutting [SO 4332 8212]–[SO 4336 8190] S. of Craven Arms station, and 50 ft of similar beds in the E. bank of the River Onny [SO 4386 8250]–[SO 4387 8224], which section has yielded a sparse fauna including the scyphozoan Conularia sp.and the brachiopods Atrypa reticularis and Brachyprion sp.The series of quarries on the scarp edge W. of Moorwood shows the top part of the Tickwood Beds and the lowest beds of the Wenlock Limestone. The following section is seen in the quarry [SO 4531 8556] 250 yd W. of the farm:

Wenlock Limestone

The outcrop of the Wenlock Limestone parallels that of the Tickwood Beds from the River Onny to the E. limit of the area near Middlehope. South-west of the river the formation is not developed and the Tickwood Beds are directly overlain by Lower Ludlow Shales.

About 30 to 40 ft thick over most of its outcrop, the Wenlock Limestone is a roughly bedded, nodular, finely crystalline argillaceous limestone with thin irregular siltstone beds and partings. Its most southerly exposure is in the E. bank of the River Onny [SO 4386 8221], N. of Stokesay Bridge, where it contains the brachiopods Amphistrophia funiculata (McCoy), 'Camarotoechia' nucula, Craniops implicata, Sphaerirhynchia davidsoni (McCoy) and Strophonella euglypha (Hisinger), and the trilobites Dalmanites caudatus and Otarion sp.North of the Bache fault-belt, the following section is seen in quarries [SO 451 851] S.W. of Moorwood:

This section yields an extensive brachiopod fauna including Aegiria [Chonetoidea] grayi, Atrypa reticularis, 'Camarotoechia' nucula, Howellella elegans (Muir-Wood), Leptaena rhomboidalis, Sphaerirhynchia wilsoni (J. Sowerby) and Strophonella euglypha.

The basal beds of the formation are seen in old quarries [SO 480 876] S. of Harton Hollow Wood, and the highest beds, transitional to the Lower Ludlow Shales, are exposed in the stream N.N.E. of Middlehope.

Ludlow Series

As a result of recent work, particularly in the type area around Ludlow, a re-classification of the Ludlow Series, based mainly on faunal assemblages, has been put forward by Holland, Lawson and Walmsley (i959, 1963). It has been possible to tie in their biostratigraphical classification with the lithological classification used in the present account (see table).

The lower limit of the Ludlow Series of the Main Outcrop has been taken at the top of the Wenlock Limestone (or Tickwood Beds where the Limestone is absent). The base of the Aymestry Group corresponds approximately with the base of the Upper Bringewood Beds. The Lower Bringewood Beds are essentially a transition facies between the shales of the Lower Ludlow and the limestones and calcareous siltstones of the Aymestry Group, and are here considered to be more closely related lithologically to the Lower Ludlow Shales. Elles and Slater (1906) and Alexander (1936) appear to have included these beds within their Aymestry Limestone. The top of the Aymestry Group shows considerable variation in horizon along the outcrop when compared with the biostratigraphical classification. At View Edge [SO 426 807] the Lower Leintwardine Beds are calcareous and have been included within the Group in their entirety. Farther E., on the E. side of the Onny valley, around Park Farm [SO 447 803] and Norton Camp, only the lower part of the Lower Leintwardine Beds is calcareous and the top of the Aymestry Group has been taken within them. As the Group is traced N.E. from Norton Camp it is less easy to define its upper limit as there is a more gradual transition upwards into the Upper Ludlow Shales. At Siefton Batch [SO 477 849] the whole of the Lower Leintwardine Beds are included within the Aymestry Group, and to the N.E. calcareous beds with abundant Shaleria ornatella (Davidson), probably equivalent to the Upper Leintwardine Beds of the Ludlow area, are also included within it.

The Ludlow Series rocks of the Western Outcrop have not been subdivided since the small area so far mapped has not yielded sufficient evidence for satisfactory correlation with the beds of the Main Outcrop.

Western outcrop

Ludlow Series (Undivided)

Ludlow Series rocks, totalling about 800 ft in thickness, occupy a small area on the W. edge of the map S. of Ridgway Wood between the fault F1 and the Edgton Limestone. They are poorly exposed within the present area, but to the W., around Edgton [SO 388 858], they comprise hard grey and olive-green siltstones and calcareous siltstones.

Lower Ludlow Shales

A small area of Lower Ludlow Shales is present within the Church Stretton Fault Complex S. of Cwm Head. The nature of their boundary with the Wenlock Shales to the N.E. is not clear.

Main outcrop

Lower Ludlow Shales

The Lower Ludlow Shales extend from the S. limit of the area, near Rowton, to the E. margin at Middlehope. They occupy most of the scarp slope beneath the Aymestry Group and also the low ground in front of the scarp. North of Lower Dinchope they floor the long strike valley of Hope Dale. The dip S. of the Stokesay Fault is between 5° and 10° to the S.E., swinging to10° to the E. between the Stokesay and Bache faults. North of the Bache Fault the dip is again to the S.E. at about to degrees.

The Lower Ludlow Shales are about 850 ft thick between Rowton and Westhope [SO 475 867], thinning N.E. from Westhope to about 650 ft at Middlehope. They show little lateral variation in lithology. The lowest too or 150 ft are olive-green and grey siltstones, with layers of fine-grained limestone nodules, particularly in the lower part where there is a rapid upward transition from the Wenlock Limestone (or Tickwood Beds). These beds commonly have a speckled appearance due to an abundance of small shell fragments, and are equivalent to the Lower Elton Beds of Holland, Lawson and Walmsley (1959). The main part of the formation (equivalent to the Middle and Upper Elton Beds), about 300 to 500 ft thick, comprises olive-green and grey flaggy mudstones and siltstones with occasional thin limestone beds, especially in the upper part, and a predominantly graptolitic fauna. The topmost 150 or 200 ft (approximately equivalent to the Lower Bringewood Beds) are transitional to the Aymestry Group (p. 42). They are flaggy calcareous siltstones with fine-grained limestone nodules, the nodules becoming more abundant upwards and coalescing into nodular limestone beds as the base of the Aymestry Group is approached.

From the S. limit of the area to Clapping Wicket [SO 421 812] the lower part of the Lower Ludlow Shales is drift covered and exposures are confined to the higher beds on the Aymestry Group scarp. Flaggy siltstone with thin limestone beds, in the upper part of the formation, is exposed in the quarry at Stokewood Cottage [SO 4360 8085] and contains an abundant fauna including the brachiopods Brachyprion arenaceus (Davidson) and Coolinia pecten (Linne), the trilobites Acidaspis cf. coronata Salter and Dalmanites myops (Konig) and the graptolites Monograptus cf. dubius and M. tumescens Wood. East of this section the outcrop is displaced about 500 yd to the S.E. by the Stokesay Fault. In the River Onny the gradual upward transition to the Aymestry Group is seen S.W. of Park Farm (Greig and others 1968, p. 185). Between the river and Lower Dinchope exposure is poor.

There are sections through a large part of the formation between Westhope and the N. end of Siefton Batch. The basal beds, olive-green siltstones with limestone beds, occur in a track [SO 4655 8625] 500 yd W. of Westhope church, and slightly higher beds are seen about 200 yd to the east-south-east. About 300 yd farther S., a stream section [SO 4761 8589] about 150 ft above the base of the formation shows 27 ft of olive-green siltstone with five beds of clay up to 4 in thick. The lower beds contain much comminuted shell debris, and this lithology together with a fauna which includes the brachiopod Aegiria [Chonetoidea] grayi, the trilobite Dalmanites myops and the graptolite Monograptus colones (Barrande) indicates an horizon close to the junction between the Lower and Middle Elton Beds. Farther N., 20 ft of calcareous, silty mudstone with beds and nodules of argillaceous limestone crop out in a track [SO 4857 8694] S.W. of Taylor's Coppice. These beds, which here form the crest of the scarp, are probably very near the top of the formation and contain a fauna which includes the brachiopods Amphistrophia funiculata, Gypidula galeata (Dalman), Isorthis orbicularis (J. de C. Sowerby) and Sphaerirhynchia cf. davidsoni.

Aymestry Group

Between View Edge and the Stokesay Fault the Aymestry Group has an extensive outcrop in which the sequence is partially repeated by faulting. East of the Stokesay Fault it caps the striking feature of Norton Camp. Between Halford Wood and Buck House the outcrop is disrupted by the Bache fault-belt which has a dextral displacement of almost a mile. North of the fault-belt the Aymestry Group has a continuous outcrop to the E. margin of the area, and forms the high ground of Callow Hill (1112 ft O.D.) and Diddlebury Common. The dip is mainly to the E. or S.E. at about 10°, but is locally affected by faults.

The Aymestry Group is about 115 ft thick at View Edge, thinning to about 80 ft at Park Farm. Northwards from Park Farm it thickens to about 100 ft at Norton Camp, 145 ft at Callow Hill and 200 ft on Diddlebury Common. Estimation of thickness is often difficult because of the transitional nature of its base and top.

Unlike the Lower Ludlow Shales, the Aymestry Group exhibits considerable lateral lithological variation. Between View Edge and the River Onny grey nodular limestones, with siltstone beds in the lower part, form the basal 30 ft. These pass upward into massive blue-hearted crystalline limestones, also about 30 ft thick, in the middle of which is a bed, up to 12 ft thick, composed almost entirely of the shells of the brachiopod Conchidium knightii (J. Sowerby).1 The massive limestones lie within the Upper Bringewood Beds. There is a rapid upward passage into grey nodular limestones (Lower Leintwardine Beds) which are about 55 ft thick. Their upper limit is not well defined and there is a gradual upward passage into the Upper Ludlow Shales. The basal beds of the Aymestry Group are exposed 5250 yd W. of Whettleton Bank [SO 4300 8094] where the C. knightii bed, here 5 ft thick, is overlain by 4 ft of crystalline limestone and underlain by 38 ft of similar beds with nodular chert and with thin siltstone partings in the lower part. In the most southerly of the View Edge limestone quarries [SO 4260 8070] about 27 ft of limestone are exposed. The lowest beds contain a varied fauna including the compound coral Favosites sp.,the bryozoan Ptilodictya lanceolata (Goldfuss) and the brachiopod Sphaerirhynchia wilsoni. The C. knightii bed lies between 9 ft and 16 ft above the base and is overlain by beds in which the brachiopod Atrypa reticularis is particularly common. Higher beds, with abundant Dayia navicula (J. de C. Sowerby) are seen at Newfoundland [SO 4272 8029].

East of the River Onny, from Park Farm to Norton Camp, the lower part of the Aymestry Group (Upper Bringewood Beds) is poorly exposed. It comprises about 50 ft of nodular grey limestone with thin siltstone beds, with a 3 in bed containing C. knightii apparently at a similar horizon to the shell bank of View Edge. The Lower Leintwardine Beds include about 8 ft of nodular limestone at the base overlain by massive limestone at least 30 ft thick. Thus the massive limestone facies occurs at a higher 'horizon at Norton Camp than at View Edge. The upper part. of the Lower Leintwardine Beds consists of siltstones with limestone nodules, here included in the Upper Ludlow Shales. Large quarries at Park Farm and in Rotting Wood [SO 4456 8060]–[SO 4466 8100] show a total of about 70 ft of limestone, and there is an almost continuous section along the top of the scarp [SO 4440 8111]–[SO 4470 8256] in Nortoncamp Wood where the sequence is as follows:

'This brachiopod has recently been referred by Amsden, Johnson and Boucot (1967) to the genus Kirkidium.

North-eastwards from Norton Camp the massive limestone facies dies out and the whole of the Aymestry Group consists of grey nodular limestones with siltstone beds and partings, with a transitional base and top. In this sequence there are several ill-defined, possibly lenticular, calcareous sub-groups of beds separated by less calcareous beds. On the N. side of Castle Road, the Aymestry Group is well exposed in a series of quarries [SO 454 828]–[SO 448 838] along the scarp. This section terminates against the Bache fault-belt. Within the fault-belt Aymestry Group limestones are faulted against flaggy siltstones of the Upper Ludlow Shales (top Lower Leintwardine Beds) in the N.E. corner of a quarry [SO 4606 8260] S. of Greenway Cross. Between the Bache and Dinchope faults, 25 ft of grey crystalline nodular limestone with siltstone beds are exposed at Shawbank [SO 4636 8319].

In Siefton Batch the Aymestry Group is well displayed in a number of sections. The basal beds are exposed in an old working [SO 4744 8503] 500 yd S.E. of Hillend, and 45 ft of higher beds comprising greyish green calcareous siltstone and grey limestone are seen in a quarry [SO 4772 8478] about 500 yd W. of Elsich Barn. The topmost beds, 12 ft of coarse-grained siltstone with crystalline limestone beds, are exposed in a track [SO 4780 8468] 150 yd S.E. of the quarry. They contain a fauna which includes the brachiopods Dayia navicula and Shaleria ornatella and the graptolite Monograptus cf. leintwardinensis Lapworth indicating an horizon near the top of the Lower Leintwardine Beds. There are numerous sections in the Aymestry Group along the scarp edge towards Diddlebury Common, and the Aymestry Group–Upper Ludlow Shales transition is well displayed in a number of quarries near Halehead [SO 489 863].

Upper Ludlow Shales

Three small separate areas between View Edge and the Stokesay Fault are occupied by this formation. The western two are fault-bounded to the E. while the third is part of the main outcrop extending S. towards Downton on the Rock [SO 428 733]. East of the Stokesay Fault the Upper Ludlow Shales have a broad outcrop from the S. margin of the area to its eastern limit N. of Corfton. They have an easterly dip of 5° to 10° S. of the Dinchope Fault, swinging to the S.E. farther north. About 340 ft thick in the S., they thin northwards to about 200 ft at Corfton Bache; as they occupy much of the dip slope of the Aymestry Group ridge their thickness is difficult to estimate with accuracy.

Unlike the Aymestry Group, the Upper Ludlow Shales maintain a fairly constant lithology along the outcrop. They comprise olive-green and buff flaggy siltstones, commonly calcareous, with occasional thin platy and nodular limestone and silty limestone beds, especially towards the base. Towards the top of the formation the siltstones become coarser, and locally there are thin beds of fine-grained sandstone. Its upper limit is taken at the base of the Ludlow Bone Bed (p. 49).

Buff flaggy siltstones of the basal Upper Ludlow Shales are exposed in each of the outcrops between View Edge and the Stokesay Fault. A section [SO 4402 8012] 750 yd W.12°S. of Park Farm has yielded an extensive fauna including the brachiopods Aegiria [Chonetoidea] grayi and Shaleria ornatella, the trilobite Calymene neointermedia R. and E. Richter, the ostracod Neobeyrichia cf. lauensis (Kiesow) and the graptolite Monograptus cf. leintwardinensis, indicative of the Upper Leintwardine Beds.

East of the Stokesay Fault the basal 6 ft of the Upper Ludlow Shales are seen, resting on Aymestry Group beds, in the quarry [SO 4473 8006] 200 yd S. of Park Farm. Northwards there is much debris of buff siltstone on the broad dip slope E. of Norton Camp, and the upper part of the formation is well exposed W. of Norton. Within the Bache fault-belt, in the triangular outcrop S. of Upper Dinchope, an old quarry [SO 4578 8318] 250 yd S. of the farm shows 12 ft of olive-green weakly calcareous siltstone. North of the Bache Fault, the Upper Ludlow Shales are seen in a quarry [SO 4705 8188] 100 yd S.E. of Bache where there is a 2-ft bed showing well-developed slump structures which may be equivalent to the Concretion Band recorded by Elles and Slater (1906) from the Norton–Onibury–Ludlow area to the south.

The greater part of the Upper Ludlow Shales succession is seen in the brook and adjacent exposures in Siefton Batch. The basal beds crop out in a track [SO 4776 8466] 350 yd N. 10°E. of Middle Barn. About 32 ft of coarse-grained flaggy siltstone with occasional beds of grey limestone are exposed, the limestone beds being commoner but more nodular towards the top. The brachiopods Dayia navicula and Shaleria ornatella are abundant in the lower part of the section and the brachiopod Atrypa reticularis in the higher and more calcareous beds. This section shows clearly that the upper boundary of the calcareous Aymestry Group is not well defined since the more calcareous beds occur towards the top of the exposure, well within the Upper Ludlow Shales. Beds within the Aymestry Group are exposed farther down the track, towards the road (p. 45). The Aymestry Group–Upper Ludlow Shales transition is also seen in the stream too yd to the E.S.E. with higher beds exposed intermittently for boo yd downstream. After a gap of about too yd, exposure is almost continuous to the main road N. of New House. A total of 118 ft of beds is seen, comprising flaggy greyish green calcareous siltstones with some thin platy limestone beds usually with abundant brachiopods.

Upper Ludlow Shales are well exposed around Corfton Hall and Corfton Bache. A lane section [SO 4969 8522] 350 yd S.W. of Hale Barn shows the highest beds of the formation, buff-yellow shaly calcareous flags, overlain by the Ludlow Bone Bed and Downton Castle Sandstone (p. 50). These flags contain a fauna including the brachiopods Howellella elegans, Protochonetes ludloviensis Muir-Wood and Salopina lunata (J. de C. Sowerby) and the lamellibranch Fuchsella amygdalina (J. de C. Sowerby).

Chapter 5 Old Red Sandstone: Downton Series

Old Red Sandstone rocks occur in the S.E. corner of the area, occupying the low ground of Corve Dale; they have a gentle dip to the E.S.E. or south-east. The stratigraphical sequence is shown on the margin of the map where it will be seen that there is a conformable relationship between the Old Red Sandstone and Silurian rocks. Only the lower part of the Lower Old Red Sandstone (the lower part of the Downton Series) is present within the Craven Arms area, but higher beds occur to the E. towards Brown Clee and Titterstone Clee hills. The lower beds are displaced by the Bache and Dinchope faults and their subsidiaries; these faults appear to die out within the Ledbury Group, but this may partly be due to the difficulty of tracing faults within a sequence with no easily recognizable marker horizons.

In late Silurian and early Old Red Sandstone times, major changes occurred in the palaeogeography of Wales and Central England. The main phase of the Caledonian mountain building movements commenced and raised the sediments of the Welsh Geosyncline into a folded mountain chain. The shallow shelf-sea which covered much of the Midlands and Welsh Borders in Silurian times was converted into a subsiding delta plain fed with sediment from the rising mountains, and there was a gradual transition from a marine to a continental environment. During a pause in sedimentation the Ludlow Bone Bed was formed by the winnowing and concentration of fish remains and other debris on the floor of a shallow sea. The lower part of the Downton Castle Sandstone resembles the Upper Ludlow Shales in lithology but has a fauna of horny brachiopods and fish in place of the marine shelly fauna of the older rocks. These sediments, probably laid down offshore under brackish-water conditions, are followed by the deltaic sandstones of the higher part of the formation. The Temeside Shales show a temporary return to the depositional conditions of the lower part of the Downton Castle Sandstone. The conditions of deposition of the Ledbury Group are still under debate (Allen and Tarlo 1963; Ball, Dineley and White 196 t) It is clear, however, that the general transition from a marine to a continental environment was continuing and that the sediments in this formation were laid down in a deltaic environment at or near sea level. The transition to a continental environment was completed in the overlying Ditton Series which crops out E. of the Craven Arms area.

Ludlow Bone Bed and Downton Castle Sandstone

The Ludlow Bone Bed is a brown sandstone or sandy mudstone with abundant fish remains, some fragments of other fossils and small phosphatic concretions. In the Craven Arms area it is a single bed up to 2 in thick, but farther north (Greig and others 1968, pp. 208, 219–20) it may occur as a series of closely spaced lenticular beds. It usually crops out in a hollow between the dip-slope of the Upper Ludlow Shales and the feature formed by the Downton Castle Sandstone, and as a consequence is rarely exposed.

The Downton Castle Sandstone is generally about 30 ft thick, increasing to 50 ft at the S. margin of the area. It yields a limited fauna including Lingula spp., gastropods, ostracods and fish remains. The basal 3 to 6 ft consist of olive-green shales or mudstones, which are overlain by fine-grained, micaceous, yellowish buff sandstone, locally flaggy and false bedded, but commonly massive. The most southerly exposure [SO 4556 8052], 200 yd W. of Upper Onibury, shows 15 ft of yellow micaceous sandstone. North of Norton the strata are displaced about 1600 yd to the S.E. by the Bache Fault, and there are numerous old sandstone quarries at Burley [SO 475 815]. The old quarry [SO 4744 8151] 550 yd W. of Burley Farm shows about 12 ft of Upper Ludlow Shales overlain by the Ludlow Bone Bed, up to in thick and impersistent, overlain by basal Downton Castle Sandstone consisting of 5 ft of olive-green siltstone and mudstone with a fauna including the brachiopod Lingula cornea J. de C. Sowerby, the gastropods Loxonema gregarium (J. de C. Sowerby) and Platyschisma helicites (J. de C. Sowerby), the lamellibranch Modiolopsis complanata (J. de C. Sowerby) and the ostracod Kloedenia wilckensiana (T. R. Jones). The rocks are displaced by faults N. of Burley, the Downton Castle Sandstone being seen in the roadside [SO 4742 8326] 200 yd E.N.E. of Castle Barn. Immediately to the N. the crop is again displaced about goo yd to the E.S.E. by the Dinchope Fault. The top 4 ft of the formation are exposed in a quarry [SO 4870 8434] at Pedlar's Rest, and the basal beds, to a thickness of 25 ft, are seen above the Ludlow Bone Bed and Upper Ludlow Shales in sections 500 yd N.E. of Pedlar's Rest [SO 4899 8465] and 400 yd S.E. of Hale Barn [SO 4972 8520].

Temeside Shales

The Temeside Shales comprise green and purple blocky mudstones with beds of yellowish green, micaceous, flaggy sandstone. They have a fauna similar to that of the Downton Castle Sandstone. The formation thins northwards from about 150 ft at Upper Onibury to about 40 ft at Siefton. These thicknesses are only tentative since the rocks are very poorly exposed.

Temeside Shales can best be seen in a roadside section [SO 458 798] immediately S. of the present area, about 800 yd N.15°E. of Onibury church, which shows some 35 ft of pale yellowish green blocky mudstone and coarsely micaceous sandstone, commonly with purplish brown and black staining (Greig and others 1968, p. 221). There are no good sections within the present area although individual sandstone beds can commonly be traced by small topographic features and debris of yellowish green sandstone in the soil.

Ledbury Group

The Ledbury Group, present in the S.E. corner of the area, consists of red and purple siltstones ('marls'), commonly unstratified, with beds of purple and dark greenish purple, coarsely micaceous sandstone. The sandstones are locally false-bedded and sometimes contain marl pellets. It has a total thickness of about 1500 ft, but only the basal 1000 ft are present within the Craven Arms area. Almost the only fossils are fish fragments which occur rarely in arenaceous strata in the higher beds. There are few exposures and part of the outcrop is obscured by river gravel and alluvium in the valley of the River Corve.

The most extensive sections are at Holt [SO 4780 8003] and in the lane at Stocking [SO 4785 8012] where purple micaceous sandstone is seen. Ten feet of similar beds are exposed in a pond [SO 4862 8027] 200 yd S.W. of Langley and, farther N., 12 ft of purplish brown micaceous sandstones and marls crop out in a stream [SO 4805 8312] 500 yd W. of Siefton Rectory.

Chapter 6 Structure

The main structural feature of the Craven Arms area is the Church Stretton Fault Complex or Eastern Uriconian Axis (Pocock and others 1938, p. 169) which extends northeastwards across the north-western part of the area from Hopesay Hill to Marshbrook. To the N.W. is a small area of Longmyndian and Silurian rocks forming part of the Longmyndian Ridge of Pocock and others (1938, pp. 9, 169). The main part of the area, S.E. of the Fault Complex, is structurally simple, comprising south-easterly dipping rocks of Ordovician, Silurian and Old Red Sandstone age on the N W limb of the Brown Glee Syncline (Ball, Dineley and White 1961, p. 209).

The Longmyndian Ridge

The greater part of the Longmyndian Ridge lies N. and N.W. of the present area, and its structure has been described by Greig and others (1968, pp. 260–4) and Wright (1968, pp. 65–8). The small part on Sheet 5048 lies on the eastern, normal, limb of the deep isoclinal syncline postulated by James (1956, fig. 4). In general the dip is steep north-westerly, though local south-easterly dips may indicate the presence of minor folds on the limb of the major structure.

North of the Long Mynd, in the Habberley Brook district [SJ 412 046], the Caradoc Series rests with strong unconformity on steeply dipping Longmyndian rocks and so it is inferred that the intense folding in the Longmyndian is at least of pre-Caradocian age. In the Shelve district W. of the Long Mynd there is a continuous stratigraphical sequence from Arenig to Caradoc with no evidence of contemporaneous folding. There is evidence of small scale folding and faulting during Cambrian times in the Comley district [SO 486 964] but no indication of folding of such magnitude as has been suggested for the Longmyndian syncline. It is therefore probable that the great Longmyndian downfold was formed in Pre-Cambrian times.

The Longmyndian rocks are cut by several dip faults. Their dominant trend is W.N.W.–E.S.E., approximately normal to the regional strike, but on either side of Old Churchmoor Hill [SO 409 895] are two faults with a N.–S. trend. The displacements of these dip faults are both dextral and sinistral and are considered to result from mainly horizontal movements since if the movements had been in a vertical sense they would have had to be very large in some cases to displace steep beds by the amounts observed. Within the region of the Long Mynd there are several major faults which have an approximate N.–S. trend. One of these, the Yewtree Bank Fault, extends southwards into the Craven Arms area as far as Mount Gutter [SO 402 883], although the magnitude of the fault within this area is uncertain. Farther N. it can be shown to have a considerable sinistral displacement. The age of the faulting in the Longmyndian has been discussed by Wright (1968, p. 68). From evidence in the Shrewsbury district and by analogy with faulting in the Shelve area W. of the Long Mynd it seems probable that the faults are post-Ordovician and pre-Upper Llandovery in age.

Although founded on essentially the same evidence, the suggested fault pattern in the Silurian rocks between the Long Mynd and the Church Stretton Fault differs in many respects from that postulated by Whittard (1932, pp. 882–3). In particular, the 'Churchmoor Dingle Fault' of Whittard has been replaced by a fault with a northerly downthrow which extends W.S.W. from near Cwm Head [SO 4209 8851] together with a second fault with a southerly downthrow S. of Hawkhurst [SO 4132 8775]. The fault which extends W.S.W. from Horderley appears to have a northerly throw of about 350 ft S. of Castle Ring [SO 4014 8684].

The Eastern Uriconian Axis

The Eastern Uriconian Axis, which in this area consists essentially of the ground within the Church Stretton Fault Complex, extends from the S. end of Hopesay Hill and includes the Uriconian rocks of Wart Hill and the intensely faulted area of the Brokenstones. Development of the structures within the Complex has probably been controlled by movements along three major faults, F (Church Stretton Fault), F2 (Lawley Fault) and F3 (Cwms–Hoar Edge Fault), described by Cobbold (1927, pp. 565–6) in the Comley district [SO 485 964] to the N. of the present map. Movements along these faults appear to have taken place from Longmyndian times onwards.

F1 can be traced easily across the N.W. corner of the map. It follows a generally straight course, and no consistent evidence is given by the line of outcrop as to the direction of hade, though the fault plane is clearly very steeply inclined. From the W. margin of the area to Cwm Head, apart from a short section S. of Horderley, it throws down Silurian rocks to the N.W. against Longmyndian, but farther N. Silurian rocks occur on both sides of the fault.

From Upper Carwood almost to Brokenstones, F2 separates the Stretton Shale Group from Wentnor Series grits. Its course is marked by only occasional exposures of the contrasting rocks, but such evidence is strongly reinforced N. of the River Onny by a prominent broad bench or shallow groove at the foot of a distinct west-facing escarpment. North of the cross-fault at Brokenstones the fault considered to be F2, and marked by a continuation of the topographic feature just described, separates Wentnor Series grits on its W. side from Silurian shales. F1 and F2 converge a quarter of a mile N.E. of Cwm Head church [SO 4262 8900].

Steep and reversed dips in the Chatwall Flags immediately W. and N.W. of Sibdon Carwood are the most southerly indications of the presence of F3. In the valleys S. of Sibdon Coppice its westerly downthrow is shown by the occurrence of Harnage Shales to the E. of eastward dipping Chatwall Flags. North-eastwards, towards Marshbrook, the fault causes repetition of strata at various levels in the Caradoc Series with a consequent widening of the outcrops of the formations affected. The beds W. of the fault dip at high angles towards it and are locally overturned. Immediately E. of the fault the dips decrease to the order of 15°, a change which takes place within 5 yd in the old railway cutting S. of Glenburrell, and is also well displayed at Woolston [SO 424 872]. The westerly downthrow is anomalous and unless the fault is interpreted as a reaction to the release of horizontal compression it is difficult to regard it as a normal fault. It may alternatively be a high-angle reverse fault, hading towards the E., the gently dipping beds on the hanging wall having been pushed upwards by westward pressure against the Pre-Cambrian horst.

Many subsidiary faults are associated with the three main fractures of the Church Stretton Fault Complex. North of Oldfield [SO 406 830] a N.–S. strike fault, parallel to F3, cuts out the Hoar Edge Grit and forms the eastern margin of the Longmyndian rocks of Hopesay Hill. In the vicinity of Wart Hill the distribution of the Uriconian and Longmyndian rocks is ascribed to faulting, but few of the boundaries can be certainly shown to be faulted. As well as the major N.N.E. trending faults there are several on N.N.W. lines, which may be compared with the many cross-strike faults on the Long Mynd. Faults on N.N.W. lines also appear to delimit the small area of Silurian rocks S. of Horderley (Figure 3). The easterly fault is exposed in the bank of the River Onny [SO 4080 8649] 600 yd S. of Horderley where it can be seen to be offset dextrally by F2.

The structural pattern in the Brokenstones area is exceedingly complex and, in view of the scattered nature of the outcrops, the interpretation presented on the map is a hypothesis which attempts to reconcile the many anomalies of outcrop distribution in this small area. A north-westward valley divides the area into two parts which are in many ways geologically distinct. The most striking difference is the absence to the N. of the Hoar Edge Grit which forms very prominent outcrops on the S. side of the valley. In addition the outcrops of rocks of the Stretton and Wentnor series are differently disposed on the two sides of the valley. These effects are ascribed to a N.N.W.–S.S.E. trending fault, approximately along the line of the valley, cutting all the structures between Ft and the parallel fault about 700 yd to the south-east. About 150 yd S. of Blakemoor [SO 425 880] a similar, but smaller, fault on an approximately easterly line cuts the Ordovician rocks lying W. of F3. Their crosscutting relationship to the other structures suggests that these two faults are the results of later movements probably extending into post-Ludlovian times. The other faults in the area run approximately parallel to the main Church Stretton faults and are considered to be localized movements associated with the more persistent fractures. If the rocks of the Wentnor Series are indeed faulted against those of the Stretton Series, and not unconformable on them, as Whittard (1953, p. 247) and Dean (1964, pp. 268–72) believed, then the displacements in these steeply dipping beds must be essentially horizontal. On the other hand the faults which limit the Ordovician and Silurian outliers may have a major vertical component.

An important fault trending N.W.–S.E. cuts off the outcrops of the Chatwall Flags and Chatwall Sandstone about 300 yd S.W. of Whittingslow. It has a downthrow to the N.E., and the displacement in the steeply inclined beds, whether vertical or horizontal, must be at least 600 ft and may exceed 1200 ft.

Faulting in the Ordovician rocks to the S.E. of F3 occurs near Cheney Longville and Marshbrook. Although not strictly within the Church Stretton Fault Complex these faults are probably associated with it. West of Cheney Longville the Ordovician rocks are cut by a fault trending W.N.W.E.S.E. which appears to cause an essentially horizontal displacement. Although the rocks are displaced sinistrally the zones of high and low dips are unaffected, and the folding and the movement along F3 are clearly later phases in the development of the structure. A similar parallel fault cuts the rocks between Cheney Longville and the River Onny but probably dies out E. of F3.

At Marshbrook a fault trending E.N.E.–W.S.W. branching from F3 700 yd N.E. of Whittingslow throws down beds of the Acton Scott Group to the N. against the Cheney Longville Flags. A small parallel fault through Marshbrook station throws down beds at the top of the Cheney Longville Flags on the N. side of the road against beds lower in the sequence to the south. The Acton Scott Limestone W. of Acton Scott is broken up by small scale block faulting, the faults having N.E. and S.E. trends and throws of up to 50 ft. A fault under the alluvium of the Marsh Brook between Marshbrook and Henley has an easterly downthrow of about 100 ft.

The Brown Clee Syncline

The greater part of the area is occupied by gently dipping beds on the N.W. limb of the Brown Clee Syncline. The general dip is to the S.E., with local swings to the E. in the southern part of the area. It is usually about 8° to 10° in the Silurian beds, increasing to 15° or more in the Ordovician and with some irregular variations in direction and amount adjacent to some faults. Significant dip-faults affect the Silurian and Old Red Sandstone rocks S. and E. of Craven Arms, but elsewhere faulting is of little importance.

The distribution of the strata on View Edge demonstrates the occurrence of a number of minor faults. A N.–S. fault passes beneath Viewedge Farm [SO 4246 8066], where it has an easterly downthrow of about 45 ft, and accounts for the absence of the Aymestry Group at the top of the scarp in the W. part of Stoke Wood. In the E. part of the wood the Aymestry Group is repeated by an E.S.E. fault with a northerly downthrow of up to 70 ft. In Church Way the more northerly outcrop is completely cut out by this fault while a N.N.E. fault cuts off the W. end of the southern outcrop about 300 yd to the west. To the S.E., an area of Upper Ludlow Shales is bounded on its S. side by a fault with a northerly down-throw of up to 100 ft.

The S.E. trending Stokesay Fault has a S.W. downthrow of 270 ft measured on the top of the Aymestry Group and the rocks are shifted dextrally by about 600 yd. The River Onny has breached the limestone barrier of the Aymestry Group along the line of the fault.

Between Broome and Craven Arms the crop of the Tickwood Beds is affected by three strike faults. Half a mile E. of Broome the basal beds are thrown down about 25 ft to the west. A second fault, with a westerly downthrow of about 65 ft, follows the line of Park Lane, while the third diverges from the second 450 yd W. of Clapping Wicket and ends against the Stokesay Fault 450 yd N. of Paddock. It has a north-westerly downthrow of about 60 ft N. of Clapping Wicket, increasing to about 100 ft N. of Paddock.

Between Grove and Culmington the Wenlock, Ludlow and Downton series are affected by a number of dip-faults. The total effect of these faults is a south-westerly downthrow of about 700 ft, measured on the displacement of the Aymestry Group outcrop. It is possible that there may be a horizontal component in the throw of some of these faults, but there is no direct evidence of this.

Most important of these faults is the Bache Fault. It has a south-westerly downthrow of 475 ft, measured on the Downton Castle Sandstone at Burley, and the outcrop of the Ludlow Bone Bed is shifted dextrally by 1650 yd. There is a strong feature at Bache, where the Downton Castle Sandstone and the hard siltstones of the Upper Ludlow Shales are thrown against the softer Old Red Sandstone beds. Near Greenway Cross [SO 460 828] the Bache Fault is shifted dextrally 475 yd by a cross-fault. North of this cross-fault it throws Upper Ludlow Shales against Lower Ludlow Shales and there is no feature. About 200 yd N.E. of the Bache Fault there is a sub-parallel subsidiary fault with a north-easterly downthrow of about 100 ft which ends against the cross-fault near Green-way Cross.

Another important fault is the Dinchope Fault, which runs from Siefton to join the Bache Fault between Upper Dinchope and Ireland. This fault has a southerly downthrow, displacing the Downton Castle Sandstone by about 200 ft and the Aymestry Group by 100 ft.

A third fault runs from the cross-fault 300 yd W.S.W. of Greenway Cross to join the Bache Fault 275 yd E.S.E. of Ireland. This fault, which is oblique to the strike, also has a south-westerly downthrow, and causes a repetition of the Aymestry Group outcrop. About 500 yd S.W. of Upper Dinchope there is a small anticline in the Aymestry Group, probably resulting from drag on this fault.

North-west of Ireland the Bache Fault splits into two parts and causes a double repetition of the Wenlock Limestone with a total downthrow of about 600 ft to the southwest. The fault system cannot be traced for any distance in the Wenlock Shales to the N.W. nor in the Old Red Sandstone beyond the Culmington area, owing to lack of marker horizons.

A dip-fault with a north-easterly downthrow of about 50 ft causes a breach in the Wenlock Limestone scarp near Lower Dinchope. A less pronounced break in the scarp is caused by a fault running obliquely across the strike half a mile S. of Harton [SO 481 878] which has a south-easterly downthrow of 75 ft. About 200 yd N.E. of Hillend [SO 4732 8550] a strike-fault of small throw gives rise to minor folds and vertical bedding in the Lower Ludlow Shales exposed in the brook.

Chapter 7 Glacial, post-Glacial and recent deposits

Glacial deposits are almost entirely confined to the N.W. half of the map; large areas are covered by boulder clay, but glacial sand and gravel occurs only locally and in comparatively small amounts. Of the recent deposits the most notable are the river terraces and alluvium of the rivers Onny and Corve.

Boulder clay

Between Horderley and the N. margin of the map there is a general cover of boulder clay, containing much debris of local origin, on the slopes between the Long Mynd and the Church Stretton Fault (F1). These slopes, which are deeply incised by several streams, are a relic of the pre-glacial valley floor. The boulder clay is generally thin, though 20 ft are exposed in a stream section [SO 4010 8785] about 1100 yd N.W. of Horderley. Patches of boulder clay also occur on both sides of the River Onny S. and E. of Horderley.

Boulder clay covers the whole of the col between Broome and Craven Arms, and appears to be at least 20 ft thick. It is generally very stony and locally passes into a clayey gravel. As well as local debris the suite of crratics includes grits and greywackes of possible Welsh origin suggesting derivation from the west. This sheet of boulder clay extends northwards on the W. side of the Quinny Brook valley towards Marshbrook. The boulder clay in this area is thin, and solid rocks are seen locally in the valleys as, for example, around Bushmoor [SO 436 878]. About 1000 yd E. of Wart Hill an outlying area [SO 411 846] of yellowish brown clay, with erratics locally abundant, occurs at about 900 ft O.D.

The boulder clay cover in Ape Dale is also thin, rarely exceeding 6 ft. A 3-ft deep trench [SO 4853 8934]–[SO 4869 8916] 200 yd S.E. of Harton Road Station showed orange-brown clay varying in texture within a few feet from almost stoneless clay to a clayey gravel. Erratics included purple Longmyndian slate and grit, weathered tuff (possibly Uriconian), hard grey limestone and buff mudstone of local origin, micaceous greywacke, fine grey grit and coarse pale grey grit of possible Welsh origin and vein quartz. These erratics suggest a derivation from Welsh ice.

Patches of boulder clay W. of Lower Dinchope indicate that ice penetrated the gap in Wenlock Edge at this locality. This is the only occurrence of boulder clay in the southern part of Hope Dale although there is a small patch of clay with abundant erratics at about 760 ft O.D. on the top of Wenlock Edge [SO 460 864], S.S.E. of Alcaston. A small area of boulder clay at Dunstan's Lane [SO 494 876] may have been deposited by ice spilling through the nick in the scarp S. of Harton. Small areas of boulder clay near Culmington and Siefton in the southern part of Corve Dale show that the Welsh ice penetrated eastwards into this area.

Sand and gravel

Areas of sand and gravel lie on both sides of the valley of the Marsh Brook between Callow Hollow [SO 437 907] (N. of the present area) and Henley Common. The deposits rest on boulder clay and solid rock. They are commonly flat-topped and have the general appearance of high terraces, upwards of 50 ft above the valley floor. They are interpreted as early outwash deposits laid down during the northward retreat of the Church Stretton glacier prior to the cutting of the Marshbrook channel. An old sand pit [SO 4492 8932] about 500 yd W.S.W.of Acton Scott Church shows 4 ft of bedded gravel with beds of pinkish brown sand. The pebbles include purple and grey grits, Acton Scott Limestone, grey quartzite, tuffs and felsite.

Fluvio-glacial gravel

Small areas of gravel occur to the E. and S.E. of Craven Arms. They have a poorly defined terrace-like form and are tentatively classed as fluvio-glacial gravels, laid down by meltwater during the retreat stage of the glaciation. Patches of high level gravel S. of Siefton and Corfton may have a similar origin.

Head deposits

The head deposits are of two types. On the top of the Long Mynd there is a thick mantle of stony debris, derived by weathering of the underlying solid rocks, which has moved little, if at all, from its place of formation. In the bottoms of valleys and on the more gentle slopes, deposits of head have accumulated by solifluxion, the material having been derived from higher ground. The unmoved debris on the Long Mynd plateau may have been formed by frost action during a glacial period when the Long Mynd was surrounded by ice. This frost weathering doubtless persisted on the Long Mynd in the arctic climate which continued during the withdrawal of the ice from the area. The deposits of head on the low ground probably accumulated under the periglacial conditions which existed after the retreat of the ice. No distinction has been made on the map between the solifluxion deposits of the valleys and the in situ stony detritus of the higher ground.

Areas of solifluxion head occur on the slopes around Wart Hill, below Briar Edge, and S.E. of Hopesay Hill, near Oldfield. Patches of head also occur locally below the Aymestry Group scarp, and in the dry valleys of the dip slopes. Some of these latter deposits may have been partly reworked by surface water.

River terraces and alluvium

The two main drainage basins are those of the River Onny and its tributary the Quinny Brook, and the River Corve. A small area in the S.W. corner of the map is drained by the River Clun. Along these rivers there are considerable developments of river terraces above the flood-plain alluvium, and on the map these terraces are numbered in ascending topographical sequence. It is considered that the oldest terraces are the highest, these being succeeded by younger and lower terraces. The numbering of the terraces has been made with regard to the sequence in each river basin so it does not follow that the First Terrace of the River Clun is the exact stratigraphical equivalent of the First Terrace of the River Onny. However, it has been possible to correlate between the basins of the rivers Onny and Corve and the terraces of these rivers are numbered in a common sequence.

River Clun

Four terraces have been recognized within this river basin. The Third and Fourth terraces occur only as restricted spreads of gravel, about 40 and 60 ft above the alluvium, N. of Broome. The Second Terrace, about 20 to 25 ft above the alluvium, is more extensive, but only a small part of it occurs within the present area, S. of Broome. The flood-plain alluvium and First Terrace occur beyond the W. limit of the map.

River Onny and tributaries

Four terraces have also been recognized within this river basin. The Fourth Terrace is developed only along the W. side of the Quinny Brook from just N. of Upper Affcot to Marshbrook Bridge [SO 4469 8880]. South of Felhampton it merges with the Third Terrace.

The Third Terrace is the most extensively developed in the Onny basin within the Craven Arms area. South of Craven Arms it lies at about 30 ft above the flood-plain. Along the River Onny N.W. of Stretford Bridge [SO 4323 8466] this terrace is well developed S.W. of Wistanstow, and there are small areas of gravel on both sides of the river between Glenburrell and Horderley. Along the Quinny Brook it is extensively developed N. of Strefford, and continues northwards on the W. bank of the stream to near Marsh Farm [SO 448 885]. It is less well developed on the E. bank but forms a wide feature in the angle between the Quinny and the Eaton brooks.

North of Craven Arms, on the W. side of the River Onny, there is a spread of gravel up to goo yd wide varying in height from 25 ft to 5 ft above the alluvium. It is probably formed by the deposits of the First and Second terraces which are not separated here by a feature. Boreholes through this terrace in Craven Arms show 45 ft of sand and gravel. West of this terrace a double alluvial fan has been built up by two small streams flowing E. from Hopesay Hill. On the E. side of the River Onny the Second Terrace occurs 15 ft to 25 ft above the alluvium from Craven Arms to Grove and again on either side of the Quinny Brook from Grove to Strefford. It is also developed on the N.W. side of the Eaton Brook between Wolverton and Alcaston. The First Terrace is of limited extent, and occurs between 5 and to ft above the flood-plain.

The alluvial flood-plain S. of Wistanstow is about 300 yd wide, and the deposits are mainly buff silty clay with occasional pebbles. Between Wistanstow and the W. margin of the area the alluvium is narrow and mainly composed of gravel. Above Grove, along the Quinny Brook, the alluvium is generally about 200 yd wide and consists of buff silt with beds of gravel. Upstream from near Upper Affcot the alluvium is mainly gravelly.

River Corve

The Third Terrace is the most extensive of those on the River Corve, and on Ludlow Golf Course, S. of the present area, it is continuous with the Third Terrace of the River Onny. It forms a prominent feature about 20 ft above the alluvium on the W. side of the Corve S. of Culmington. West of Sparchford it is up to 200 yd wide, and rises here to about 30 ft above the alluvium. The terrace deposits comprise coarse gravel with beds of false-bedded sand.

The First Terrace occurs at about 5 ft above the alluvium on the W. side of the River Corve S. of Culmington. It is correlated with the First Terrace of the River Onny, there being no equivalent of the Second Terrace of that river developed along the River Corve. Along a tributary, the Siefton Brook, the First Terrace occurs to the N.W. of Culmington as far as New House [SO 482 838].

The flood-plain alluvium of the River Corve forms a broad flat about 400 yd in width and consists of red silty clay with occasional pebbly layers, exposed at some localities to a depth of 8 ft.

Interpretation of glacial history

With the onset of glacial conditions, glaciers from the ice-sheet in the Shropshire Plain extended southwards along the Church Stretton valley and the northern part of Ape Dale. Possibly at the same time Welsh ice from the Camlad valley advanced around the S. end of the Long Mynd and moved E. into the southern part of Ape Dale. This ice was probaby confluent with a glacier flowing E. from the Chin valley, and the combined stream appears to have advanced eastwards through the valleys N. and S. of Norton Camp into the lower part of Corve Dale. The suggested limits of the glaciers in the Church Stretton and Craven Arms area are shown on Figure 4. It is not certain whether the maxima of the Welsh ice and Shropshire Plain ice occurred at the same time.

At the peak of of the glaciation the major valleys were probably filled with ice, but the intervening ridges were apparently only locally covered. It is probable that most of the Long Mynd remained ice-free (Wright 1968, p. 80), and there is no evidence that the high ground of Wart Hill, Hopesay Hill, Norton Camp and View Edge was ice-covered. Ice locally overrode the Wenlock Limestone escarpment S.S.E. of Alcaston and S. of Harton, and also penetrated for a short distance through the gap N.W. of Lower Dinchope, but otherwise the area S.E. of the Wenlock Limestone scarp and N.E. of Lower Dinchope and Culmington appears to have remained ice-free.

At its maximum it is thought that the Church Stretton glacier advanced at least as far S. as Hamperley and moved S. from Marshbrook towards Henley. It is likely that the hill on which Whittingslow stands emerged from the ice at an early stage of the retreat and then, with the decay of ice in the lower ground of the Wistanstow area, melt-streams from a glacier snout standing on the col N. of Hamperley cut the two dry valleys at Hamperley and Cwm Head, and probably began the incision of the deep gully of Scaldbank Gutter [SO 421 890] and its southward continuation to Horderley. Whittard (1952, p. 184; 1953, p. 244) suggested that the Cwm Head channel was excavated by spill from a lake ponded up by ice standing to the N. in the Church Stretton valley. Such a lake (spilling at about 760 ft O.D. at Cwm Head) would require a confining barrier across the Marshbrook valley. From the present profile of this valley it appears that the pre-glacial valley floor lay at about 650 ft O.D. so that an ice barrier at least 100 ft high would be necessary there to dam up a lake to a height of about 760 ft O.D. The ice forming this supposed barrier would most probably have come from the Plowden gap via Horderley (Figure 4), thus blocking the southward exit of any water flowing from the Hamperley–Cwm Head col. The northward retreat of ice from the Hamperley col led to the abandonment of the Hamperley and Cwm Head channels, while the withdrawal of ice northwards from the Marsh-brook valley was apparently accompanied first by the deposition of spreads of sand and gravel on the higher ground flanking the Marshbrook channel, and later, with the concentration of melt-water into one stream, by the downcutting of the channel which carries the present southward drainage of the Marsh Brook.

Selected references

ALEXANDER, F. E. S. 1936. The Aymestry Limestone of the Main Outcrop. Quart. .7. Geol. Soc., 92, 103–15.

ALLEN, J. R. L. and TARLO, L. B. 1963. The Downtonian and Dittonian Facies of the Welsh Borderland. Geol. Mag., 100, 129–55.

AMSDEN, T. W., BOUCOT, A. J. and JOHNSON, J. G. 1967. Conchidiurn and its separation from the subfamily Pentamerinae. J. Palaeont., 41, 861–7.

BALL, H. W., DINELEY, D. L. and WHITE, E. I. 1961. The Old Red Sandstone of Brown Glee Hill and the adjacent area. Bull. Brit. Mus. (Nat. Hist.) Geol., 5, 175–310.

BANCROFT, B. B. 1933. Correlation-tables of the stages Costonian–Onnian in England and Wales. (Privately printed.) Blakeney, Gloucester shire.

BANCROFT, B. B. 1945. The Brachiopod Zonal Indices of the Stages Costonian to Onnian in Britain. J. Palaeont.,19, 181–252.

BANCROFT, B. B. 1949. Upper Ordovician trilobites of zonal value in south-east Shropshire (ed. A. Lamont). Proc. Roy. Soc., (B), 136, 291–313.

COBBOLD, E. S. 1927. The Stratigraphy and Geological Structure of the Cambrian Area of Comley, Shropshire. Quart. J. Geol. Soc., 83, 551–73.

DEAN, W. T. 1958. The faunal succession in the Caradoc Series of south Shropshire. Bull. Brit. Mus. (Nat. Hist.) Geol., 3, 191–231.

DEAN, W. T. 1964. The geology of the Ordovician and adjacent strata in the Southern Caradoc District of Shropshire. Bull. Brit. Mus. (Nat. Hist.) Geol., 9, 257–96.

DINELEY, D. L. 1960. Shropshire Geology: An outline of the tectonic history. Field Studies, 1, 86–108.

ELLES, G. L. and SLATER, I. L. 1906. The Highest Silurian rocks of the Ludlow District. Quart. J. Geol. Soc., 62, 195–221.

GREIG, D. C., WRIGHT, J. E., HAINS, B. A., and MITCHELL, G. H. 1968. Geology of the Country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee. Mem. Geol. Surv.

HOLLAND, C. H. and LAWSON, J. D. 1963. Facies patterns in the Ludlovian of Wales and the Welsh Borderland. Lpool. Manchr. Geol. J., 3, 269–88.

HOLLAND, C. H. and WALMSLEY, V. G. 1959. A Revised Classification of the Ludlovian Succession at Ludlow. Nature, 184, 1037–9.

HOLLAND, C. H. 1963. The Silurian Rocks of the Ludlow District, Shropshire. Bull. Brit. Mus. (Nat. Hist.) Geol., 8, 93–171.

JAMES, J. H. 1956. The structure and stratigraphy of part of the Pre-Cambrian outcrop between Church Stretton and Linley, Shropshire. Quart. J. Geol. Soc., 112, 315–337.

POCOCK, R. W., WHITEHEAD, T. H., WEDD, C. B. and ROBERTSON, T. 1938. Shrewsbury District. Mem. Geol. Surv.

STRACHAN, I., TEMPLE, J. and WILLIAMS, A. 1948. The Age of the Neptunian Dyke at Hazier Hill. Geol. Mag., 85, 276–8.

WHITTARD, W. F. 1925. Notes on Valentian Rocks in Shropshire. Proc. Geol. Assoc., 36, 378–81.

WHITTARD, W. F. 1928. The Stratigraphy of the Valentian Rocks of Shropshire. The Main Outcrop. Quart.J. Geol. Soc., 83 (for 1927), 737–59.

WHITTARD, W. F. 1932. The Stratigraphy of the Valentian Rocks of Shropshire. The Longmynd–Shelve and Breidden Outcrops. Quart. J. Geol. Soc., 88, 859–902.

WHITTARD, W. F. 1952. A Geology of South Shropshire. Proc. Geol. Assoc., 63, 143–97.

WHITTARD, W. F. with contributions by Ball, H. W., Blyth, F. G. H., Dineley, D. L., James, J. H., Mitchell, G. H., Pocock, R. W. and Stubblefield, C. J. 1953. Report of Summer Field Meeting in South Shropshire, 1952. Proc. Geol. Assoc., 64, 232–50.

WRIGHT, J. E. 1968. The Geology of the Church Stretton area (Explanation of 1: 25 000 Geological Sheet SO49). Geol. Surv. Gt. Brit.

A more comprehensive bibliography is given in some of the above references, in particular in Greig and others (1968) and Whittard (1952)

Other publications dealing with this district

• Books
• British Regional Geology: The Welsh Borderland Geology of the Country around Church Stretton, Craven Arms, Wenlock Edge and Brown Clee (Explanation of One-inch Geological Sheet 166, New Series)
• The Geology of the Church Stretton area (Explanation of 1:25 000 Geological Sheet SO49)
• The Geology of the Wenlock Edge area (Explanation of 1:25 000 Geological Sheet SO59)
• Geological maps
• 'Ten-mile' map of Great Britain (1:625 000) Solid, South Sheet
• Quaternary, South Sheet
• One-inch to one mile (1:63 360) or 1:50 000
• Church Stretton (166) Sheet, Solid
• Church Stretton (166) Sheet, Drift
• 1:25 000
• Church Stretton (SO49)
• Craven Arms (SO48)
• Wenlock Edge (SO59)
• Tectonic Map of Great Britain and Northern Ireland (1:1 584 000 )
• Aeromagnetic maps
• 'Ten-mile' map of Great Britain, Sheet 2 (1:625 000)
• Diagram Edition, Sheet 5 (1: 250 000) English Midlands and Welsh Borders

Figures

(Figure 1) Locality map for the Onny River Sections

(Figure 2) Horizontal cross-section at Hamperley showing the relationships between the Pre-Cambrian and Palaeozoic rocks.

(Figure 3) Map of fault-bounded area of Silurian rocks at Horderley.

(Figure 4) Suggested limits of glaciers in the Church Stretton district.

(Front cover) Cover picture: Callow Hill, near Craven Arms

(Unnumbered table) [Classification of the Ludlow Series by Holland, Lawson and Walmsley (1959, 1963) and the present classification].