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Geology of the country around Chulmleigh. Memoir for 1:50 000 geological sheet 309 (England and Wales)

E. C. Freshney K. E. Beer and J. E. Wright

Bibliographical reference: Freshney, E. C., Beer, K. E. and Wright, J. E. 1979 Geology of the country around Chulmleigh. Memoir Geological Survey of Great Britain, Sheet 309 (England and Wales).

• Authors
• E. C. Freshney K. E. Beer and J. E. Wright
• Contributors
• Palaeontology M. A. Calver
• Petrography J. R. Hawkes R. J. Merriman
• Geophysics A. J. Burley
• Water supply E. A. Edmonds

Geological Survey of Great Britain, Institute of Geological Sciences Natural Environment Research Council

London Her Majesty's Stationery Office 1979. © Crown copyright 1979. Typeset for the Institute of Geological Sciences by Raithby, Lawrence & Company Ltd, Leicester and London. Illustration films by U.D.O. Jenn Ltd, London. Printed in England for Her Majesty's Stationery Office by Staples Printers St Albans Limited at The Priory Press. Dd 0596819 K16 ISBN 0 11 884035 5

• Authors and contributors
• E. C. Freshney, BSc, PhD, K. E. Beer, BSc, FIMM, C Eng and E. A. Edmonds, MSc Institute of Geological Sciences, St Just, 30 Pennsylvania Road, Exeter EX4 6BX
• J. E. Wright, BSc and M. A. Calver, MA, PhD Institute of Geological Sciences, Ring Road Halton, Leeds LS15 8TQ
• A. J. Burley, BSc, PhD, J. R. Hawkes, BSc, PhD and R. J. Merriman, BSc Institute of Geological Sciences, London

(Front cover)

(Rear cover)

Preface

The Chulmleigh district is included in the old series geological map (Sheet 26) issued shortly after the formation of the Geological Survey in 1835. The area was re-examined by Ussher at the end of the last century. Since six-inch mapping of the Carboniferous rocks started in 1962, memoirs relating to nearby one-inch or 1: 50 000 sheets 324 (Okehampton), 322 (Boscastle) and 323 (Holsworthy) have been published and mapping has been completed on sheets 308 (Bude) and 292 (Bideford). The classification of the Carboniferous of the Chulmleigh district reflects the stratigraphy and palaeontology of the coastal section of Sheet 308 (Bude). Six-inch mapping of the district was carried out during the years 1964–68 by Mr K. E. Beer, Dr E. C. Freshney and Mr J. E. Wright, under Mr G. Bisson as District Geologist. Fossils from the Carboniferous were collected by Dr D. E. White and Mr D. E. Butler, and identified by Drs M. A. Calver and W. H. C. Ramsbottom. The palynology of the Palaeogene deposits has been studied by Dr C. Turner of the Open University. Geophysical surveys in the field were supervised by Mr P. J. Fenning, and the report for this memoir has been written by Dr A. J. Burley. Dr J. R. Hawkes and Mr R. J. Merriman have provided some petrographical data on the Carboniferous rocks, including the road-aggregate properties of some of the sandstones. The account of the water supply was prepared by Mr E. A. Edmonds and the photographs taken by Messrs J. M. Pulsford and C. J. Jeffrey. Thanks are due to E. C. C. Ball Clays Ltd and Watts Blake Bearne & Co Ltd for much helpful co-operation. Sheet 309 (Chulmleigh) is in course of publication at a scale of 1: 50 000. The memoir has been compiled by Dr E. C. Freshney and edited by Mr E. A. Edmonds.

Austin W. Woodland Director, Institute of Geological Sciences, Exhibition Road, South Kensington, London SW7 2DE. 3 February 1978

List of six-inch maps

The following is a list of six-inch National Grid maps included, wholly or in part, in Sheet 309, with the dates of survey. The officers are : K. E. Beer, E. C. Freshney and J. E. Wright

Notes

• National Grid references are given in square brackets throughout the memoir. Unless otherwise stated all lie within the 100 km square SS.
• Numbers preceded by A refer to photographs in the Institute's collections.
• Letters preceding specimen numbers refer to Institute collections as follows :
• E English sliced rocks
• MR Museum reserve collection

Geology of the country around Chulmleigh—summary

An area of inland Devon of pleasant rolling relief and few natural resources, the countryside around Chulmleigh lies midway between Dartmoor and Exmoor and attracts less interest than it deserves. The tourist, seeking the solitude of the moors or the grandeur of the coast, tends to pass by; the industrialist, in search of raw materials, power, labour and good communications, is tempted to look elsewhere. But this is both the area's weakness and its strength. Patterns of farming and settlement have developed slowly in response to natural pressures; adjustment to external change has remained traceable, recorded, and in no way overrun by the development which has so often swept aside all obstacles. Thus the control exercised by the geology described in this book is evident : the ill-drained hollows, scooped out of the softer sediments, which afford poor grazing, the drier slopes which support both sheep and cattle, the concentration of arable farming on the free-draining ridges of coarser sandstone, the boggy highland flats mantled with clay, the birth of Forestry Commission planting on land well suited to conifer crops. And the single major mineral resource of the district, the Petrockstow ball clays, whose derivation traces the drainage patterns of long-gone landscapes and whom, preservation is due to a distant echo of the earth movements which produced the Alps. The retention of much rustic charm may reflect a superficial lack of tourist and commercial attraction; the evolution of that charm, and its integration into patterns of living and livelihood, stem in large part from the accidents of geological history and the nature of the rocks beneath the soil.

(Geological succession)

Chapter 1 Introduction

Geography and physiography

The district described in this memoir is shown on the New Series 1: 50 000 Chulmleigh (309) Sheet. It lies almost midway between the uplands of Dartmoor and Exmoor (Figure 1), both of which may be viewed in fine weather from many of its ridge tops. Largely forsaken by naturalist and geologist, both of whom are more attracted to the nearby moors and coastline, the district also lacks sites of popular tourist appeal. In consequence it still retains much of its rural and, locally, rustic charm, despite the disruptive transient effects of the holiday season.

In common with most of the south-west the region enjoys a mild but humid climate which collects some of the precipitation initiated on the high ground to north and south. The average rainfall is about 40 in (1016 mm) a year but in the north-east corner of the area it rises to 45 in (1143 mm). Snow in January is not infrequent; in a normal year it will lie for only a few days, but in especially hard winters may endure for up to six weeks. Carried by winds funnelled between Dartmoor and Exmoor, drifting snow rapidly blocks the narrow, high-hedged lanes, seriously hampering milk collection. Much of the rainfall is dissipated as surface run-off and in periods of excessive rain the open reaches of the rivers Taw and Torridge are prone to extensive flooding. From time to time debris carried down in these torrents has to be cleared from the angling beats, and over longer periods the river channels are seen to have altered their courses.

Physiographically the district offers a pleasant respite from the wild grandeur of Dartmoor and Exmoor. Broad round-topped ridges alternate with small wooded and steep-sided valleys, the general east–west grain reflecting the strike of the underlying Carboniferous rocks. Viewed from higher ground the ridge tops appear deceptively level, but this suggestion of ancient simple peneplanation is not borne out by closer examination (Figure 1). The highest ground lies mainly in the east, around the village of Meshaw, where it rises to 230.7 m OD (757 ft), but there are isolated patches of land above 183 m (600 ft) towards the west around Burrington and Huntshaw Cross. In direct contrast is the well-defined elongate depression of the Petrockstow Basin, in the south-west of the district, with its large expanse of flat ill-drained moorland punctuated by abandoned and working ball claypits.

Drainage of the district is entirely towards Bideford Bay, via the rivers Taw and Torridge. These course northwestwards through deep, locally gorge-like, pleasantly wooded valleys which cut obliquely through the grain of the country commonly following major fractures. The Taw rises on Dartmoor and flows in a northerly or north-easterly direction to its confluence with the River Yeo, where it turns sharply north-west to meander its way to the coast. Near Chulmleigh it is joined by the Little Dart River, whose course follows the regional strike of the beds, and farther north it is fed by the anomalously south-flowing River Mole and its tributary the River Bray, both rising on Exmoor. The Torridge rises near Hartland and enters the district on an easterly course to link up with the River Okement flowing north from Dartmoor. From this confluence to Great Torrington, where it leaves the map-area, it follows a highly sinuous, deeply incised course within a belt of north-westerly transcurrent faults.

Land use and industry

A long tradition of agriculture has largely obliterated any evidence of stone-age settlement in this part of mid-Devon. Scatterings of worked flint have been ploughed up in fields near Parsonage [SS 698 157] and stone hand axes have been found near Parsonage and around Clapworthy [SS 675 240]; an interesting collection of local prehistoric material is housed in the South Molton Museum.

The succeeding Bronze Age culture is well represented by numerous tumuli, most of which are situated on the higher moorland areas. The name Shilstone [SS 656 245] is probably a debased form of 'shelf-stone', suggesting the former presence here of a Bronze Age dolmen. Earthwork fortifications believed to date from the Iron Age are also preserved on the higher ground, typically in strategically dominant situations. The largest is Berry Castle [SS 495 223], north of Great Torrington, and others are known at Castle Hill [SS 522 168], also near Great Torrington, Burridge [SS 742 126] and Brightley Barton [SS 612 229]. The Iron Age earthworks at Stone Barton [SS 713 141] were later incorporated in a Norman structure.

Apart from two small finds, north of Worlington and north of Morchard Bishop, there is little evidence of Roman presence in the district, and at this time the indigenous Celtic population was probably clearing more and more ground for cultivation and establishing the foundations of many present-day settlements. A bronze bowl found in 1958 during the draining of a bog [SS 7662 2206] at Munson Farm has been dated as late BC or early AD; it is of typical Celtic design and was probably made in Somerset. The Long Stone [SS 775 159], prominently situated on a ridge top north of Worlington, may be of Celtic, or even Bronze Age, origin. About 0.3 m square and 2 m high, this ancient monument is of grey, chloritic quartz-schist, a rock-type foreign to the district.

Extensive agricultural settlement probably dates from the Saxon period, but the pattern of early field systems has long since been eradicated. The spread of the English peoples is attested by the predominance of Saxon place-names and the marked paucity of Celtic names, though many of the churches believed to be built on original Saxon sites retain their dedication to Celtic saints. Norman influence shows in some of the churches, and though there were no stone-built Norman castles, smaller earthwork motte and bailey structures are preserved at Great Torrington [SS 498 190], at Croft Castle [SS 631 081] and Court Castle [SS 633 082] near Winkleigh, at Millstone Castle [SS 667 058], in Heywood Wood [SS 679 125] and at Stone Barton [SS 713 141]. Affeton Castle [SS 756 137] was probably a fortified house. The Domesday Book records mills at Nymet Rowland near the site of East Mill [SS 711 091], and at Hatherleigh near Gifford's Hele [SS 531 064].

Succeeding centuries witnessed the slow, quiet development of an agricultural community under the feudal manorial system, the replacement of fortified castles by more manageable and comfortable square-plan bartons, and the growth of villages and small towns. In 1238 Great Torrington had been granted borough status and sent representatives to the King's Justices of Assize; by 1295 it was important enough to send burgesses to Parliament though it soon petitioned out of this expensive obligation. At about the same time Winkleigh, Chulmleigh and Chawleigh were established as manorial boroughs with weekly markets and annual fairs. Celebration of the Chulmleigh Charter and Annual Fair has been successfully revived in recent years.

War and major disturbance affected the district only indirectly, through taxation, land appropriation, deforestation and levies in kind to support military campaigns and garrisons. By Tudor times much of the land was divided among a few large estates. The Civil War made somewhat more impact on the area. In 1642 the Royalists were defeated at Great Torrington but they recaptured the town in August of the following year. After the Battle of Naseby the Parliamentary forces under Cromwell and Fairfax moved on Devon. Eggesford House was stormed in December 1645, and in February 1646 the Royalists were beaten at Torrington. En route there had been a skirmish near Burrington from which King's Hill [SS 657 170] derived its name.

The most significant changes in the area were brought about by improvements in communications which were initiated in the second quarter of the 19th century. The old turnpike road from Crediton to Barnstaple was replaced by a new road along the Taw valley in 1830, and the long route from Crediton to Bideford via Hatherleigh and Great Torrington was succeeded in 1835 by the present shorter one through Winkleigh and Beaford. Great Torrington was also linked to Bideford by the Rolle Canal, built in 1825 and operated until 1871; part of its course was used for the new railway, part became a road, and an aqueduct which carried it over the River Torridge now carries the driveway to Beam Mansion [SS 473 207]. The railway line from Crediton to Barnstaple was completed in 1854 and was converted from broad to standard gauge in 1862.

The advent of easy bulk transportation, combined with urbanisation induced by the Industrial Revolution, resulted in the rapid closure of most of the smaller markets and a sharp decline in rural population. Villages such as Chulmleigh and Winkleigh have seen their populations halved since completion of the railway. In recent years the mechanisation of agriculture has continued this trend, though the effects are balanced to some extent by an influx of elderly retired people into rural Devon.

Distance from raw materials, especially coal, and from urban markets has protected the district from the dereliction which so commonly followed 19th century industrialisation, and despite recent attempts to encourage small firms to move into the area the pattern of manufacture is still mainly related to the pastoral background.

In spite of market fluctuations and ephemeral subsidies the pattern of agriculture in this part of Devon has changed little through the years. For the most part it is based upon mixed crop and stock husbandry and the area retains a reputation for beef production. The heavy, damp and stony soils derived from the Carboniferous rocks are not well suited to growing root crops, and deep dissection of the land limits the areas available for arable cultivation. Nonetheless barley production, encouraged by the introduction of improved disease-resistant varieties and by increasing prices, now ranks second to hay in the cropping economy and has surpassed the previous wheat and oat yields. In recent years intensive farm mechanisation has modified the rural landscape, removing hedges to enlarge fields and encouraging the combination of small farms into larger economic units; several all-grass farms are now run on a ranch-style basis with straight wire fences replacing beech and oak-topped hedge banks.

Apart from agriculture and tourism, employment in the district centres mainly on the ball clay industry and forestry. Ball clay, now used mostly for domestic and industrial ceramics, has been worked by pits and mines in the vicinity of Meeth and Petrockstow since around 1680. Current annual production is running at about 70 000 tonnes. The Forestry Commission's very first plantings were of conifers on the steep valley slopes of the River Taw near Eggesford in 1919; the spot is marked by a plaque unveiled by the Duke of Edinburgh in 1969. The Commission's holdings have spread widely through the nearby valleys, many of which are now uniformly cloaked by dark green foliage; some of the early stands have been felled and replanted. Independent commercial groups, notably Clinton Estates, are also actively engaged in forestry. Most of the felled timber is transported northwards by road, but recently a saw-mill has been established on the disused Winkleigh airfield.

With the decline in local marketing and the pruning of railway services much agricultural produce is now carried by road to its urban destinations. However milk is processed at Great Torrington and North Tawton, abbatoirs are operated at Great Torrington and Winkleigh, and wool is still collected and graded at Burrington. Small market gardens flourish on some of the south-facing slopes; much of their produce sells locally though some is taken to Bristol and Covent Garden. The traditional craft of cider-making is carried on commercially at two small factories, at Winkleigh and Clapworthy Mill [SS 675 240], but the size of these premises belies the wide appreciation of their product. A recent venture is the manufacture of sparkling wine at Dolton.

Cottage industries, fast disappearing from the rural scene, are still represented in the area. Traditionally dressed dolls and angling flies are made in Dolton and farmhouse cheeses at Eggesford. An innovation for the district is the manufacture of high quality glassware at Great Torrington.

Geological succession and geological history

(Geological succession)

Within the Chulmleigh district the following formations can be recognised:

KEB

The oldest rocks exposed in the district are Namurian shales and sandstones of the Crackington Formation lying in the cores of anticlines aligned east–west. These beds were laid down in a basin, probably marine and trending east–west, by turbidity currents which flowed south from deltas which were themselves being built out southwards from a hinterland in the vicinity of Wales. Tectonic pressures at the onset of the Variscan orogeny caused a rise in the hinterland with a consequent increase in sandy detritus early in the Westphalian. This resulted in a general shallowing of the sea south of the deltaic environment, which had by this time migrated southwards into north Devon, and in the deposition of the paralic Bideford Formation. The sediments of the Bude Formation were laid down mainly in brackish to fresh water, although marine incursions brought with them faunas of goniatites and fish. Deposition ended possibly as late as Westphalian C, and the later Variscan movements folded the Carboniferous rocks into a series of periclinal folds parasitic on a number of broader anticlinorial and synclinorial warps trending approximately east–west. During the final stages of the orogeny normal east–west faults developed and were in turn cut by north-west–south-east dextral wrench faults.

It is this youngest group of faults which gave rise to the north-west–south-east trough in which were deposited the fluviatile kaolinitic clays, sands and lignites of the Palaeogene Petrockstow Basin. These deposits were associated with a river system flowing from south-east to north-west and accumulated in the structurally controlled trough to a depth of at least 661 m.

The Pleistocene period is represented by an extensive cover of Head, formed at least in part by periglacial action. Probably most of the river deposits post-date the last glaciation, although some of the Head Clay of the high ground may have been formed during periglacial fluvial activity. The rivers Torridge, Taw and Okement have been involved in reversals of flow and capture, and some parts of these river courses may even have been affected by block faulting in Recent times.

Previous research

Little has been written in the past on the geology of this inland, poorly exposed district. The earliest work is De la Beche's Report on the Geology of Cornwall, Devon, and West Somerset, published in 1839. Subsequently Ussher (1892, 1901) divided the Carboniferous of the district into his Morchard-type Culm and what he considered to be stratigraphically higher, his Eggesford-type Culm. Prentice (1960) studied the Westphalian rocks south-west of Barnstaple.

The Palaeogene deposits of Petrockstow were mentioned by De la Beche (1839) and more fully described by Ussher (1879), who compared them with the Bovey Tracey deposits. Scott discussed the geology briefly in the Ball Clays Memoir (1929) but concentrated mainly on the ceramic properties of the clays. Gravity surveys by Bott, Day and Masson Smith (1958) led them to estimate the depth of these deposits as 600 ft (183 m). More recently the sedimentology of the beds has been studied by Freshney (1970). ECF

Chapter 2 Upper Carboniferous

General account

Introduction

In the Chulmleigh district exposure is poor and the strati-graphical relationships of the Upper Carboniferous formations are obscure. Interpretations are based largely on studies of the coast section between Bude and Westward Ho!, and much is owed to the detailed palaeontological work on goniatite faunas carried out by Dr M. A. Calver and Dr W. H. C. Ramsbottom.

Work on the coast (Freshney and Taylor, 1972; Edmonds and others, 1979) has shown that strata of similar facies to those mapped as Crackington Formation on the Boscastle (322), Holsworthy (323) and Okehampton (324) sheets continue up for about 200 m above the Gastrioceras listeri horizon (Gull Rock Shale), and that massive sandstones and the other lithologies characteristic of the Bude Formation facies are not common below the Gastrioceras amaliae horizon (Hartland Quay Shale). The sequence of turbidite sandstones and shales termed Welcombe Measures by Ashwin (1958), and thought by him to overlie the Bude Formation, belongs in present nomenclature (Edmonds, 1974) to the upper part of the Crackington Formation and underlies the Bude Formation. The Hartland Quay Shale is not a mappable horizon and the base of the Bude Formation inland is taken below the lowest of the massive sandstones characteristic of this formation. Problems arise where folding has brought the Hartland Quay Shale and the Gull Rock Shale into close proximity, and locally it is impracticable to map out the fold-enclosed fragments of one formation within another.

The Bideford Formation, restricted to the northern part of the district, is equivalent to the Bideford Group of De Raaf and others (1965). Work in the Westward Ho! area (Edmonds and others, 1979) has indicated that the succession there generally youngs southwards towards a large strike fault which brings up the Hartland Quay Shale and the Gull Rock Shale. An analogous situation occurs in the northern part of the Chulmleigh district where a succession of paralic beds overlain by a mixed Bideford–Bude facies youngs southwards to a postulated strike fault which brings up the Hartland Quay Shale to the south (p. 39). Lithologies characteristic of the Bideford Formation become less evident eastwards as massive cross-bedded paralic sandstones such as the Cornborough Sandstone (pp. 8–9) give way to the less distinctive massive sandstones of the Bude Formation. In the north-east of the district, south of South Molton, the Carboniferous rocks are all of Bude Formation type. (Figure 2) shows the postulated relationships of the Upper Carboniferous formations of the Chulmleigh district.

Nektonic marine goniatites are fairly widespread throughout the Kinderscoutian (R₁) strata of north Cornwall and Devon. The Clovelly Court Shale (Gastrioceras? sigma Sub-zone), however, of late Marsdenian (R₂) age, represents a more restricted marine horizon, and goniatites higher in the succession are confined to discrete shale bands with no marine fauna in the intervening strata. Probably there was a gradual decrease in salinity from late Marsdenian to early Westphalian times, brought about by influxes of fresh and brackish water from deltas advancing from the north. Sediments on the tops of these deltas are represented by the Bideford Formation, and those on the edges and seaward slopes by the Bude Formation. Dark shales within the Bude Formation, in bands usually less than 3 m thick, point to a diminution of topographic relief with a consequent reduction in the amount of coarser sediment entering the basin. Spasmodic marine incursions introduced short-lived faunas of goniatites and fish. Alternation of turbidite sandstones with the shallow-water, massive sandstones of the Bude Formation suggests that even the turbidites were not deposited in deep water.

Probably the basin in which the Namurian and Westphalian deposits were laid down was closing up structurally during early Westphalian times, and the Bideford and Bude formations represent the final influxes of river-borne material from the north.

Facies changes to east and south from the Bideford Formation of the northern part of the Chulmleigh district suggest that a delta lobe protruded into this area and that the break in gradient between delta top and delta slope followed an arc from north of George Nympton through Yarnscombe and Alverdiscott. Thicker developments of turbidites within the Bideford and Bude formations point to northward regression of the delta and a temporary return to pro-deltaic sedimentation. However, marine incursions even into this deeper water remained transient.

Crackington Formation

Beds of the Crackington Formation crop out in a tract of country between Speccott Barton ( [SS 503 141] and Meshaw Moor [SS 760 180], around Roborough and between Great Huish [SS 525 179] and Yealmacott [SS 701 212]. They are equivalent to parts of the Eggesford-type Culm of Ussher (1901). Several localities, particularly in the southern belt, have yielded goniatites characteristic of the G. listeri horizon (Gull Rock Shale), and it can be inferred from the scattered outcrops and comparison with coastal lithologies and structures that no great thickness of beds below this horizon crops out in the district. Isolated exposures of rocks resembling those of the Bude Formation, and occurrences of the G. amaliae band (Hartland Quay Shale), suggest the presence of small downfolded periclinal masses of this formation which are not distinguished on the map (Beer, 1966, p. 40). Larger infolds are shown south-east of Great Torrington, north of Chawleigh and north-east of Romansleigh.

The Crackington Formation comprises a succession of well-bedded turbidite sandstones, usually less than 0.5 m thick, interbedded with siltstones, mudstones and sporadic black shales (Plate 2). The ratio of sandstone to shale varies, but usually not abruptly. Strata between the Gull Rock Shale and the Bude Formation are dominantly arenaceous, the sandstones probably constituting over 80 per cent of the sequence, while the beds below are probably 50 to 60 per cent shale. Lithologically these latter beds resemble the Wanson Beds described by Mackintosh (1964) from the coast south of Bude.

Two east-north-east-trending belts of localities yielding fossils of the G. listeri horizon, one running through Beaford and the other passing about 1 km to the north of Dolton, may delineate an anticline. However, no traces of the Gastrioceras subcrenatum horizon (Embury Shale) have been found within the structure. Fossils of the G. amaliae horizon occur in close proximity to the clusters of localities yielding G. listeri horizon fossils in the area between Cutland [SS 681 172] and Great Odam Moor [SS 745 185], and the antiform here probably does not bring up beds much lower than the G. listeri horizon itself.

The northern belt of G. listeri localities is very close to the southern boundary of the Little Torrington–Woolleigh Barton outcrop of the Bude Formation. This suggests only a small thickness of strata between the G. listeri horizon and the lowest Bude Formation sandstone, as is the case on the north Cornwall coast (Freshney and Taylor, 1972).

The sandstones of the Crackington Formation commonly display groove casts, flute casts, prod marks and small load casts. They also show lamination, cross-lamination and ripple-drift bedding, particularly towards the top of each band. These sedimentological structures indicate a flow of depositional currents mainly and consistently from a northerly direction. Evidence from the Okehampton district to the south (Edmonds and others, 1968, p. 49) indicates a depositional basin with east–west axial flow during most of Namurian times, but with increasing flow from the north in late Namurian and earliest Westphalian times. This change in the current pattern, together with the increase in sandstones above the G. listeri horizon, probably marks the southward migration of deltaic conditions.

Bude Formation

The largest outcrop of the Bude Formation lies in the south of the district. It was once thought to be overlain by the Welcombe Formation to the north, but evidence from the coast has shown that the Bude Formation overlies all rocks of dominantly turbiditic facies. The Bude Formation forms an east–west synclinal trough whose axis lies just north of the southern margin of the district. More complex synclinal structures of similar trend in the northern part of the district are complicated by a large strike fault which runs eastwards to Atherington. This northern outcrop of the Bude Formation contains interleaved within it a paralic facies, the Bideford Formation, which dies out eastwards.

The difficulty of putting an accurate line around outcrops of the Bude Formation makes it probable that further infolds of Bude Formation occur within the outcrops of Crackington Formation and vice versa.

The formation comprises sandstones, commonly massive, within a sequence of grey mudstones and siltstones (Plate 3) and (Plate 4). Those sandstones less than about 0.5 m thick closely resemble the turbidite sandstones of the Crackington Formation. The more massive sandstones, however, which range up to 20 m in thickness, are generally coarser grained. In coastal exposures west of the district they show coarse lamination, some cross-bedding with foresets up to 0.5 m long, flute casts and large load casts (Plate 5) but few if any strongly directional sole marks such as groove casts. Analysis of grain-size distribution points to shallow-water, fluviatile or deltaic deposition (Freshney and Taylor, 1972) rather than deep-water turbidity currents or underflows (Burne, 1969, 1970; Burne and Moore, 1971). The presence of cross-bedding rather than the smaller scale cross-lamination also suggests a shallow-water origin.

Other lithologies within the Bude Formation have been well documented from the Widemouth and Bude areas by King (1966, 1967) and Williams (in Freshney and others, 1972). In the Chulmleigh district they are represented by scattered exposures of carbonaceous, muddy siltstones and flaggy silty sandstones with a few xiphosurid tracks; some blocky mudstones also occur. Although shales are fairly common it is unusual for any individual bed to exceed a thickness of about 4 m; exceptions occur in the highest part of the succession along the synclinal axis in the southern part of the district, which may equate with the extremely shalt' lithologies found on the coast at and below the horizon of the Warren Gutter Shale (Freshney and Taylor, 1972). Inland exposures of slumped beds are rare.

Bideford Formation

The Bideford Formation crops out in the northern part of the district between Alverdiscott [SS 520 245] and Clapworthy [SS 675 240]. Grey mudstones and shales with some turbiditic sandstones, indistinguishable from much of the Crackington Formation and from parts of the Bude Formation, contain several massive fairly close-grained feldspathic sandstones which commonly show large-scale cross-stratification and wedge-bedding. These are De Raaf and others' (1965) major sandstone facies (facies L), of which the Cornborough Sandstone can be mapped from the coast inland as far as Umberleigh [SS 608 238]. The culm horizon, a highly carbonaceous mudstone found on the coast above the Cornborough Sandstone, has also been located in one or two places near Umberleigh.

The massive cross-bedded sandstones form marked topographic ridges trending east towards Umberleigh, beyond which the features become less striking. At Clapworthy only one sandstone of Bideford Formation type is recognisable. Farther east the facies change from Bideford Formation to Bude Formation is complete.

No fossils have been recovered from the Bideford Formation of the Chulmleigh district, but on the coast near Westward Ho! the Gastrioceras amaliae horizon (Hartland Quay Shale) occurs towards the base of cycle 3 of the formation (De Raaf and others, 1965; Edmonds and others, 1979 ). The same horizon lies within typical Bude Formation sediments near Langridge [SS 572 223], which suggests a rapid southward facies change from Bideford Formation to Bude Formation. ECF, KEB

Petrography

The Crackington, Bude and Bideford formations show no obvious diagnostic petrographical differences. However, certain Bude Formation sandstones weather more freely than similar, though commonly more thinly bedded, sandstones of the Crackington Formation, and the upper grain-size limit of Bideford Formation sandstones may be a little higher than that of other sandstones. Textural features in the sandstones and shales, coupled with the sporadic occurrence of illite and kaolinite in the latter, suggest a regional metamorphic grade lower than the Barrovian Chlorite I of the Carboniferous rocks of the Bideford district. The sources of the rock specimens examined are given in Appendix 3.

Arenaceous rocks

Of 21 siltstones and 25 sandstones of greywacke type examined in thin section, 16 siltstones and 18 sandstones came from the Bude Formation.

The framework components are detrital feldspar (trace to 8 per cent), lithic fragments (trace to 29 per cent) and subangular and subrounded quartz grains (38 to 71 per cent). Many quartz grains show strain shadows between crossed nicols, and in two specimens (E42694), (E42710) scattered quartz fragments exhibit Boehm strain lamellae. These features were probably inherited, since signs of strain are not general. Of more significance is the widespread diagenetic growth of quartz at the margins of grains.

Lithic fragments comprise acid volcanic rock and, in some specimens, basic volcanic material, together with chert and quartzite and also argillite, siltstone and fine-grained greywacke reminiscent of the local Carboniferous strata. In addition, scattered pieces of silty and phyllitic material of Chlorite I and Chlorite II metamorphic grade commonly occur (E31310), (E36331), (E36332), (E36333) (E42687), (E42690), (E42692), (E42694), (E42703), (E42705), while fine-grained quartz-muscovite-schist of Chlorite III grade has been identified in a few specimens (E30489), (E36332), (E42688), (E42698). None of these lithic types points to a specific source. Metamorphic fragments occur in both Crackington and Bude formations, and are unlikely to be of primary derivation from strata equivalent to presently exposed Devonian rocks. Pre-Armorican basement rocks are a more probable source, perhaps a Lower Palaeozoic Caledonian region or some Precambrian (Cadomian) terrain. The consistent presence of small amounts of detrital feldspar (sodic plagioclase, some perthite) suggests a not-toodistant Cadomian region containing exposures of granite.

The rock matrices are chiefly of chlorite and muscovite (sericite), with some finely divided quartz and clay minerals. Much of the chlorite is detrital, and flakes of detrital muscovite are common in most specimens. However, there has been considerable growth of finely divided muscovite (sericite), presumably from matrix clay minerals, while detrital muscovite crystals locally show partial recrystallisation as aggregates ofsericite flakes. In one specimen (E42693) detrital chlorite is partly replaced by muscovite along cleavage traces. Diagenetic features also include marginal outgrowths on framework quartz grains. However, some of the mica growth in the matrix, particularly in the more silty rocks, and the interpenetrative growth of sericite flakes at the margins of quartz grains (E30476), (E30479), (E30486), (E36331), (E42710), (E42703), may be the result of low-grade regional metamorphism.

Accessory minerals include pale blue-green and pale olive-green tourmaline, zircon, apatite, ?sphene, opaque oxide commonly showing hydration to limonite, and carbonaceous material. One specimen (E42692) contains detrital hematite. Both framework constituents and matrix minerals are locally thinly coated with secondary limonite. A few of the sandstones (E42695, (E42701), (E42702), (E42705), (E42710)) bear secondary calcite in the form of aggregates of small anhedral crystals which have replaced matrix minerals and also penetrate quartz grains and lithic fragments. In specimen (E42702), anhedral grains are accompanied by small rhombohedral crystals. The maximum amount of calcite noted was 2 per cent (E42705); its sporadic occurrence may reflect crystallisation from circulating groundwater in Permian times.

The mean grain size of framework constituents in the 46 specimens examined ranges from 0.02 to 0.1 mm, and the BS value of 0.06 mm is taken as the arbitrary dividing line between silt and fine sand. Silt specimens commonly show mean grain sizes around 0.03 mm, with the largest fragments being between 0.06 and 0.15 mm across, and around 0.05 mm with the largest fragments in the range 0.15 to 0.5 mm. The mean grain sizes of framework constituents in sandstones are predominantly in the range 0.06 to 0.07 mm, with the largest fragments between 0.15 and 0.45 mm across, but seven rocks (including the two Bideford Formation samples, (E42686) and (E42710) have a mean grain size of 0.1 mm for framework constituents with the largest fragments ranging from 0.3 to 1.0 mm across.

Modal proportions of constituents were determined for 45 specimens (Table 1). The siltstones contain less framework quartz and lithic fragments, but more matrix material, than do the sandstones. However, neither the average nor the range figures show distinctions between formations.

In (Table 2) sandstone data from the Crackington and Bude formations have been arranged in lithic-poor and lithic-rich groups with the dividing line taken arbitrarily at a lithic fragment content of 10 per cent. The figures suggest an inverse relationship between lithic fragments and matrix minerals, and that at either extreme-lithic-poor/matrix-rich or lithic-rich/matrix-poor-the average content of framework quartz is likely to be around 60 per cent. Data for the two specimens of Bideford Formation sandstone (Table 1) could be in accord with the suggested inverse relationship, which may itself reflect distance from sediment source.

Argillaceous rocks

Powdered samples of three Crackington Formation and six Bude Formation shales were examined by X-ray diffractometry (CuKα; 40 kV, 30 mA; 3–40° 2θ). The results show no systematic differences in composition between the formations, with mica, chlorite and quartz making up at least 90 per cent of each sample. Mica is generally the most abundant mineral and produces a 10Ǻ basal reflection which commonly shows the characteristic sharpness of well-crystallised muscovite (MR32625), (MR32630), (MR32638), (MR32639), (MR32652). Three samples, however, show a subdued 10Ǻ reflection, broadened on the low-angle side, which is characteristic of less well-crystallised clay-mica or illite (MR32628), (MR32629), (MR32647). Iron-rich chlorite is absent from only one sample, which contains kaolinite and appreciable amounts of pyrite (MR 32625). Thin-section examination of this specimen shows it to contain carbonaceous material (E46800). Quartz forms 20 to 40 per cent of the samples, reaching 60 per cent in one silty shale (MR 32639). Albite and subordinate microcline are the only other principal constituents.

The composition of the argillaceous rocks suggests that the mineralogical effects of low-grade regional metamorphism are slight and in general indistinguishable from those of normal diagenesis. Indeed there are samples from both the Crackington and Bude formations retaining some original clay minerals such as illite and kaolinite, and also carbonaceous material, which are clearly unmetamorphosed. However, thin sections of two shales show an imperfectly developed cleavage which crenulates the sedimentary lamination (E46799)–(E46800). This cleavage is defined by the parallel orientation of phyllosilicates, but in both specimens 60 to 70 per cent of orientated minerals remain parallel to bedding. The unmetamorphosed character of these two rocks, coupled with the rather weak cleavage, suggests that the latter developed at comparatively high crustal levels in poorly consolidated argillaceous sediment. JRH, RJM

Details

Crackington Formation

Alverdiscott–Atherington

A stream exposure [SS 5000 2440] shows siltstones with some fissile cross-laminated sandstones, and also hard grey sandstones, disposed in a series of small, tight folds. Flaggy sandstones cross-laminated at the top and with flute casts at the base occur in a small quarry [SS 5701 2260] .

North Healand–Chittlehamholt

Poorly bedded sandstones with slaty shales are exposed in a small quarry [SS 5227 1848] . Farther east flaggy and slaty sandstones with flute casts occur in several small quarries around Stonyford [SS 5423 1842]; [SS 5433 1836]; [SS 5422 1824]. Near Ebberly House a small stream [SS 5560 1834] to [SS 5608 1869] cuts through bedded and flaggy sandstones exhibiting flute marks and cross-lamination and with bands of silty shales as at [SS 5548 1818]. A small quarry [SS 5817 1831] to the east shows massive sandstones. Sporadic outcrops of shales [SS 5923 1870]; [SS 5977 1937] occur in small streams south of High Bickington.

Roadside sections in the valley of the River Taw show 98 m of well-bedded sandstones [SS 6199 1982] and further exposures of well-bedded sandstones and siltstones [SS 6214 2022] to [SS 6206 2004]. The bank of the River Taw shows [SS 6266 2051] to [SS 6252 2028] thin-bedded sandstones and siltstones and alternations of grey sandstone with dark grey smooth shale. Pyritic nodules occur locally within the shales [SS 6268 2044] . A small tributary stream [SS 6452 2000] to [SS 6450 1991] exposes the following section :

Sections in small roadside quarries in the valley of the River Mole [SS 6599 2110] to [SS 6599 2097] show thick sandstones in beds up to 0.61 m with grey shales and some siltstones. A stream section [SS 6531 2106] to [SS 6568 2098] along the strike exposes thinly bedded sandstones with silty shales and mudstones.

Beaford–Dolton–Roborough

A section [SS 5351 1495] alongside a path shows thick grey shales containing dark discoidal nodules, overlain by thin silty sandstones with groove marks and prod marks on their bases. Gastrioceras coronatum, G. cf. listeri and Dunbarella sp.of the G. listeri horizon were obtained from the silty sandstones. Thin sandstones and shales probably near the same horizon are exposed in small quarries [SS 5384 1518] at the top of the slope above the River Torridge.

Many exposures of well-bedded sandstones, siltstones and shales occur along the banks of the Torridge [SS 5396 1500] to [SS 5352 1500]; at one place [SS 5353 1503] dark grey shales with discoidal nodules are interbedded with thin sandstones showing groove marks, prod marks, small load casts and the impressions of indeterminate goniatites.

Exposures in a forest track [SS 5421 1427] to [SS 5410 1452] west of Beaford Bridge are mainly of flaggy sandstones with some more massive sandstones, siltstones and silty shales; flute casts were seen on the base of one sandstone [SS 5410 1452]. A massive sandstone over 3 m thick is exposed in a small quarry [SS 5475 1469]. A stream [SS 5589 1265] to [SS 5612 1316] in the Dolton area has exposed sandstones, commonly flaggy, with some bands of shale [SS 5600 1220]. Silty micaceous shales [SS 5595 1275] have yielded fragments of G. cf. listeri and Dunbarella sp.A road cutting [SS 5645 1345] shows sandstones up to 0.46 m thick in the middle of the section with more shaly beds to the north. Another stream section [SS 5740 1255] to [SS 5776 1297] in shales and thin sandstones shows flute casts and groove casts; nodules within the shales yielded Gastrioceras coronatum and G. cf. listeri. Nodules with Gastrioceras circumnodosum, juvenile anthracoceratids and Dunbarella sp.were found in field brash [SS 5911 1341].

A stream section [SS 5482 1516] to [SS 5542 1574] north-east of Beaford shows much sandstone, mostly well bedded and some flaggy with shales. Folded shales crop out in other streams [SS 5571 1532]; [SS 5574 1536], as do flaggy and well-bedded sandstones with siltstones and some shales near Great Barlington [SS 5685 1639] to [SS 5673 1658]. Another stream section [SS 5776 1502] to [SS 5773 1515] shows thick shales with some thin sandstones, the shales passing from grey to black and carbonaceous towards the southern end of the outcrop. At one place [SS 5777 1505] the shales are much disrupted and contain nodules with Gastrioceras circumnodosum, G. cf. listeri and G. cf. coronatum. Farther east in a small quarry [SS 5945 1535] thin sandstones are interbedded with silty shales and with dark grey banded shales containing thin ironstone seams and also nodules with indeterminate goniatites. Another quarry [SS 5980 1580] shows the following section :

King's Nympton–Great Odam Moor

A quarry [SS 6598 1739] alongside the River Taw shows 30 m of sandstones in beds with silty rippled tops, and a railway cutting to the south-east [SS 6615 1734] to [SS 6618 1714] exposes greyish green sandstones with thin grey shales. Brownish grey well-bedded sandstones with silty tops to the beds crop out in the bank of the river [SS 6602 1636], as do thick purple and brown sandstones [SS 6605 1642].

East of the River Taw, sections close to the strike [SS 6663 1574] to [SS 6688 1583] expose grey laminated silty shales and carbonaceous shales with thin bluish grey turbiditic sandstones. Stream sections show thin grey turbiditic locally-carbonaceous sandstones with abundant shales [SS 6908 1679] to [SS 6871 1719] and siltstones with Gastrioceras cf. listeri and G. cf. circumnodosum [SS 6837 1735]. This G. listeri horizon (Gull Rock Shale) is repeated in another small stream [SS 6974 1769] to [SS 6951 1787] where it yielded G. listeri, G. circumnodosum, anthracoceratids and Dunbarella sp.The rocks in this section comprise black shales and micaceous siltstones, the shales containing bands of calcium carbonate nodules commonly packed with goniatites, together with some thin turbiditic sandstones. Other stream sections include: banded and flaggy sandstones with a few massive sandstones and much shale and shaly mudstone [SS 7069 1726] and [SS 7092 1653]; dark grey shales and mudstones with nodules yielding Caneyella cf. multirugata, Dunbarella cf. papyracea and Gastrioceras sp.,possibly the G. amaliae horizon within infolded Bude Formation strata [SS 7063 1712]; [SS 7080 1656]; dark grey shales and silty mudstones (Gull Rock Shale) with G. listeri, G. circumnodosum, Dunbarella sp.and plant remains [SS 7117 1712]; shaly mudstones with thin sandstones yielding Posidonia sp. nov., anthracoceratids [mainly juv.] and conodont aggregates including Hindeodella sp. [SS 7337 1900]; grey shaly mudstones with scattered sandstone bands (Gull Rock Shale) [SS 7343 1830] to [SS 7346 1771] yielding G. circumnodosum, G. weristerense, a juvenile anthracoceratid and Caneyella sp. [SS 7347 1783]; flaggy and banded sandstones with much shaly mudstone [SS 7435 1829] to [SS 7434 1783] and including several outcrops of the Gull Rock Shale with G. cf. listeri as at [SS 7438 1804]. ECF

Huntacott Water [SS 7003 1473]–Cuddenhay [SS 7614 1542]

Near Huntacott flaggy sandstones with thin bands of mudstone and laminated siltstone dip 60°/170°^‡1  [SS 7003 1473] and sheared black shaly mudstones with 15-cm sandstone bands dip 25°/170° [SS 7050 1370]; the sandstones contain small pyritous nodules in places. At 180 m SE of Huntacott an old quarry [SS 7100 1437] shows flaggy sandstones with thin bands of laminated siltstone dipping 37°/160°; the siltstones are locally cross-laminated, and the sandstones show bottom structures indicating that the beds are right way up.

At a sharp bend in the Little Dart River [SS 7115 1374] banded grey sandstones and grey shaly mudstones dip vertically and strike 070°; groove casts and load casts indicate that the beds young to the south. These rocks are truncated to the east by a fault trending 165°, east of which black shaly mudstones with sandstone bands up to 15 cm thick dip 40°/340°. The mudstones are locally carbonaceous and micaceous, containing some plant debris, and the sandstones are cut by thin quartz veins; there is evidence of much shearing, mainly along bedding planes. Farther upstream banded grey shaly mudstones and sandstones dip 30°/150° [SS 7142 1352].

Small exposures in the farm tracks around Stone Barton show flaggy grey sandstones, locally micaceous silty and laminated, dipping 30°–35°/155°. Sandstone forming the ridge north of the farm is exposed [SS 7126 1410] dipping 40°/150° on the west side of the motte and bailey. North-west of the ridge stream sections show flaggy sandstones dipping 20°–40°/150° [SS 7100 1412], and vertically, strike 060° [SS 7118 1425]. Farther upstream [SS 7151 1448] to [SS 7195 1459] similar sandstones with thin bands of fissile siltstone and thicker bands of shaly mudstone dip 20°–50°/155°. A stream [SS 7189 1399] 450 m E of Stone Barton has grey shaly mudstone with bands of silty sandstone dipping 45°/l60°; cleavage in the shaly mudstone dips 80°/160°. Nodules within the mudstones contain indeterminate goniatites which may represent the G. listeri horizon.

Near Bealy Court soft grey sandy siltstones with thin mudstone bands [SS 7483 1561], dip 20°/170°, are somewhat reminiscent of Bude Formation lithologies.

Thick shaly mudstones with some flaggy sandstones are well exposed in a stream course [SS 7630 1574] to [SS 7716 1500] near Cuddenhay. Nodules within the mudstones contain G. circumnodosum [SS 7669 1529]; [SS 7764 1531], indicating the presence of the Gull Rock Shale. Farther east a more sandstone-rich succession crops out in the Adworthy Brook [SS 7841 1653] to [SS 7816 1587]. JEW

Totleigh Barton

Thick hard buff sandstones, siltstones with plant remains and olive-coloured mudstones crop out at Ditsham [SS 4971 0628]. A nearby stream section [SS 4992 0617] shows olive-green mudstones and shales, and a small pit [SS 5022 0613] contains 0.45-m sandstones with siltstones and shales.

Bude Formation

Yarnscombe–Bishop's Nympton

Yarnscombe–River Taw

Well-bedded sandstones with cross-laminated tops to the beds crop out with shales and siltstones in a stream [SS 5797 2334] to [SS 5793 2298]. A partly overgrown quarry [SS 5920 2320] exposes thick cleaved silty shales along with hard well-bedded sandstones up to 1.22 m thick. Furrow casts on the bases of some of the sandstones trend E–W parallel to the strike.

Sporadic exposures along the banks of the River Taw [SS 6077 2362] to [SS 6056 2269] show mainly well-bedded hard grey to grey-green sandstones with some siltstones and shales. Only locally [SS 6057 2293]; [SS 6056 2285] do the sandstones exceed 0.5 m in thickness. East of the river exposures are few; some ridges carry notable amounts of sandstone brash, which is coarse and feldspathic in places [SS 6359 2315]. Exposures in a stream around [SS 6375 2341] show mainly laminated siltstones.

Valleys of the River Bray and River Mole

Grey, commonly micaceous silty sandstones and siltstones crop out along the west side of the road running down the valley of the River Bray from Clapworthy. Exposures include thick grey sandstones with some thin laminated siltstones [SS 6789 2352], finely laminated silty sandstones [SS 6743 2323], and micaceous sandstones interbedded with grey laminated cleaved slates at Meeth Bridge [SS 6748 2297]. On the eastern side of the valley a small quarry [SS 6758 2379] shows thick beds of purple and brown sandstone with siltstones and grey silty shales; grey siltstones with ovoidal fractures and overlain by soft grey-green feldspathic sandstones are exposed nearby [SS 6761 2368]. On the southern side of the River Mole [SS 6804 2282] grey-brown sandstones are interbedded with finely laminated grey sandstones. Exposures farther east show dark grey thinly bedded shaly sandstones [SS 6856 2286] and grey sandy siltstones with silty mudstones [SS 6913 2284]. Small quarries [SS 6934 2408] alongside a tributary of the River Mole contain 9 m of flaggy sandstones, and flaggy laminated silty sandstones crop out in the stream [SS 6933 2400]. A quarry near Wampford Bridge [SS 6963 2227] shows 12 m of grey-brown feldspathic sandstones with laminated silty sandstones.

Alswear–Mariansleigh

A roadside quarry [SS 7288 2432] exposes 12 m of massive grey-green sandstones up to 3.7 m thick with flaggy pale brown sandstones packed with plant debris. Farther south-southeast another quarry [SS 7260 2338] shows 33.5 m of interbedded dark blue-grey micaceous muddy sandstones and flaggy purplish red fine-grained sandstones. The blue-grey sandstone closely resembles the matrix material of the slumped beds seen in the Bude Formation on the north Cornwall coast. A quarry [SS 7328 2433] on the eastern side of the valley of the River Mole contains thick green coarse sandstones with grey silty shales, and dark grey mudstones with pale grey-green feldspathic sandstones crop out in the river [SS 7281 2328]. Exposures in the Crooked Oak stream hereabouts consist mainly of shales and mudstones [SS 7419 2311]; [SS 7490 2310]. The stream flowing from the south-east to join the Mole at Alswear cuts through massive fine-grained sandstones and flaggy sandstones with shaly mudstones [SS 7327 2125] and [SS 7311 2138]. A quarry [SS 7297 2160] near the stream shows grey fine-grained sandstone in beds up to 1.8 m thick with thinner sandstones up to 0.31 m; flute casts about 2.1 m above the base of the north-east wall of the quarry indicate current flow from the north. In another quarry [SS 7310 2154] 1.5 m of massive green micaceous sandstone are overlain by 0.31 m of greenish grey laminated siltstones.

A quarry [SS 7500 2219] east of Mariansleigh has been opened in massive feldspathic sandstones.

Bishop's Nympton

A quarry [SS 7556 2428] north-north-west of Bishop's Nympton, although now filled in, is marked by debris of greenish grey coarse sandstone, and at the southern end of the village massive greenish grey micaceous sandy siltstone is exposed at the roadside [SS 7587 2361]. The Crooked Oak stream east of Bishop's Nympton cuts through grey silty and shaly mudstones [SS 7673 2304] to [SS 7726 2303], locally interbedded with micaceous sandstones [SS 7806 2321]. A nearby quarry [SS 7828 2332] contains massive dark grey sandstones and siltstones with some laminated sandy siltstones which show spheroidal weathering. Small stream sections to the south show shaly and silty mudstones [SS 7812 2211] to [SS 7805 2238], locally with thin sandstone ribs [SS 7812 2212].

Huntshaw–High Bickington

Huntshaw–Potems Cross

Exposures in the Huntshaw area are mostly confined to a single stream [SS 4973 2309] to [SS 5148 2383]; [SS 5234 2398] to [SS 5252 2342] and comprise shales with thinly bedded sandstones. Scattered groups of thicker sandstones occur locally [SS 4954 2289]; [SS 4989 2314]; [SS 5253 2343]. Sandstones up to 0.61 m thick beside another stream [SS 5021 2277] exhibit load casts and ripple marks. A roadside ditch [SS 5143 2220] shows shales and silty sandstones on either side of a fairly thick band of shales; the beds dip steeply north but young to the south, and a weathered nodule from the shale yielded a juvenile goniatite and Caneyella sp. [ juv.]. Well-bedded silty sandstones with some red staining crop out in a small stream [SS 5242 2202]. A partly obscured quarry [SS 5401 2398] contains thick-bedded sandstones, and silty shales crop out to the north [SS 5408 2402]. Another quarry [SS 5473 2280] shows 1.52 m of grey-green sandstone overlain by 1.22 m of brown-weathering siltstone containing balled-up fragments of sandstone. Exposures of thick banded silty shales are common around Langridgeford [SS 5706 2246]. The following section occurs in a road cutting [SS 5691 2222] to [SS 5702 2231]:

The fauna from the black shales indicates correlation with the Hartland Quay Shale. Nodular shales in a nearby stream [SS 5665 2172] yielded no fossils but fragments of a nodule in surface brash [SS 5661 2166] contained fossils reminiscent of the Hartland Quay Shale.

A band of laminated micaceous grey silty sandstone occurs interbedded with carbonaceous banded mudstone in a stream [SS 5641 2134], and upstream to the south-east thick-bedded sandstones crop out [SS 5645 2127]. Other stream sections show 30 m of shales underlain and overlain by well-bedded sandstones [SS 5827 2226], and well-bedded silty sandstones [SS 5902 2199]; [SS 5906 2186].

Priestacott Moor–High Bickington

A small stream [SS 5006 2100] to [SS 5036 2110] cuts through thick banded shales with some soft micaceous green-brown sandstone [SS 5034 2110]. An old quarry now occupied by farm buildings [SS 5090 2062] shows thick cleaved shales in its southern part overlain by bedded sandstones in its northern part; the beds dip north. The adjacent stream [SS 5094 2057] to [SS 5094 2029] has exposed fairly well-bedded sandstones with some beds up to 1.22 m thick [SS 5085 2073]. Eastwards from Torrington towards St Giles in the Wood exposure is mostly confined to a single stream section [SS 5229 1930] to [SS 5241 1877] in which well-bedded and flaggy sandstones carry flute casts on the bases of some beds; banded silty shales occur at one place [SS 5228 1900]. Quarry sections in this area show well-bedded sandstones carrying load casts and flute casts and interbedded with silty shales containing discoidal nodules [SS 5240 1878], and a thick sandstone bed with flute casts on its base and flaggy sandstones above [SS 5318 1987]. To the north-east around Dodscott Wood, several massive pink-weathering sandstones crop out in the steep banks of a stream valley [SS 5492 1951]; [SS 5499 1956]. Exposures of thick banded silty shales are common to the north-east and east of Dodscott. Banded cleaved shales alongside a forestry track [SS 5483 1987] dip steeply to the south but are probably overturned. Further exposures of thick cleaved banded shales, with sporadic thin sandstones, occur in a small stream [SS 5514 1987] to [SS 5577 1988]. Two disused quarries show 6.1 m of massive sandstone [SS 5413 2041] and hard red-stained massive sandstone with much breccia and quartz veining and some thin stringers of hematite [SS 5478 2043]. Shales with some thin sandstones are exposed in a small stream [SS 5904 2064] to [SS 5952 2085]; the sandstones are usually less than 0.31 m thick and locally exhibit load casts and ripple-drift tops [SS 5915 2075].

River Mole–Meshaw

King's Nympton area

A large quarry [SS 660 185] on the southern side of the River Mole exposes 53 m of thickly bedded sandstones, grey ovoidal shales and carbonaceous siltstones; a feldspathic sandstone occurs low in the succession [SS 6587 1852] and similar sandstones 3.6 m thick crop out farther south [SS 6600 1787]. A small quarry [SS 6766 1867] south-west of King's Nympton shows 2.44 m of brownish grey feldspathic fine-grained sandstones with silty tops to the beds.

Roadside exposures are common in the Mole valley [SS 6614 1990] to [SS 6628 1950]. They include fine-grained sandstones, silty and carbonaceous in places [SS 6617 1984], micaceous sandstones [SS 6619 1981], and 22 m of thick grey sandstones, thinly bedded silty sandstones, silty shales and siltstones [SS 6625 1964]. A small stream known as Colley Lake affords the following exposures : grey sandstones, locally thin-bedded, with shales [SS 6805 2052]; well-bedded micaceous sandstones and grey-green silty carbonaceous sandstones [SS 6818 2024] to [SS 6822 2014]; grey-brown feldspathic sandstone with abundant plant debris [SS 6846 2001]. Carbonaceous sandstones typical of the Bude Formation crop out in a stream east of King's Nympton [SS 6980 1966], as do thick feldspathic sandstones [SS 6992 1935]. ECF

Romansleigh–Whippenscott

Exposures mainly of sandstones, some of them flaggy, are common in a stream [SS 7187 2008] to [SS 7223 2071] west of Romansleigh. At some places the sandstones are hard buff and feldspathic [SS 7184 2041], at others they are thin, dark grey and banded, with grey shaly mudstones [SS 7208 2063]. Other stream sections show grey micaceous sandstones with some shaly mudstone and flaggy sandstone [SS 7317 2002], and flaggy sandstones [SS 7386 2025], and grey siliceous sandstone of a rather massive nature [SS 7484 2028]; [SS 7497 2031]. Stream-course exposures farther east [SS 7537 2038] to [SS 7843 2088] show: flaggy sandstones with shaly mudstones [SS 7542 2035]; massive grey sandstones near Meshaw Mill [SS 7571 2076]; [SS 7578 2050]; shaly mudstones with thin sandstone bands full of plant debris and with some dark grey micaceous sandstone [SS 7644 2063]; shaly mudstones with some grey silty sandstones [SS 7712 2085]; laminated silty mudstones with thin sandstone bands [SS 7798 2083]; shaly mudstones with thin sandstone bands [SS 7823 2088]. A small stream to the south has exposed massive sandstone with bands of flaggy siltstone and weathered shaly mudstone [SS 7803 2016].

Hummacott [SS 7020 1945]–Langley [SS 7392 1922]

In a small quarry [SS 7160 1918] 300 m SSE of Lower Kingstree flaggy sandstones with bands of silty mudstone dip 35°/050°, and a stream in Kingstree Wood [SS 7180 1950] runs across massive and flaggy, grey and greenish grey sandstones with bands of silty and shaly mudstone dipping 40°–80° S. Weathered sandstones and siltstones dip 60°/180° in an old quarry [SS 7252 1940] 320 m ESE of Horridge. Similar rocks, locally massive, crop out in a stream to the east between Ducksford Plantation [SS 7266 1920] and Beara [SS 7269 1966]; in the northern part of this section dips are 15°–25° N, and at the southern end the dip is 80°/170°. A stream [SS 7340 1905] south-south-east of East Rowley cuts through sections in flaggy sandstones and silty mudstones, locally with bands of grey shaly mudstone, dipping 40°–80° N. Exposures in a stream [SS 7392 1922] north of Langley show sandstone, mainly flaggy but locally massive, with bands of siltstone and shaly mudstone; beds dip both north and south at angles between 25° and 90°.

Langley [SS 7392 1922]–Whitestone [SS 7800 1900]

At the roadside [SS 7632 1920] 90 m W of Meshaw Barton massive purplish grey sandstone is associated with silty mudstone dipping 30°/190°. A small quarry [SS 7660 1908] 180 m SSE of Meshaw Barton contains greenish grey sandstone with some plant debris and thin bands of silty mudstone dipping 30°/190°. At 320 m W of Narracott a stream section [SS 7626 1979] shows banded sandstone and shaly mudstone. Another [SS 7717 1917], 730 m E of Meshaw Barton, exposes flaggy sandstones with siltstones and grey shaly mudstones; the beds dip 32°–60° SW, trending obliquely to the regional E–W strike. SEW

Little Torrington–Woolleigh Barton

Extensive exposures are common beside the A386 road, both in quarries and in cuttings [SS 4944 1755] to [SS 4976 1814]; [SS 5004 1842] to [SS 4999 1894]. The strata are mostly well-bedded sandstones commonly with load casts and other bottom structures and cross-laminated towards the tops of the beds. At one locality [SS 5010 1841] 4.57 m of sandstone are exposed. One quarry [SS 4942 1757] contains a sandstone bed with frondescent markings on its base. Some 4.6 m of shales and siltstones in the roadside [SS 4993 1871] dip about 45° S. These roadside sections south of Torrington indicate a general southward younging of about 300 m of strata.

Hard, poorly bedded to massive sandstones crop out in the River Torridge and along its banks [SS 4975 1692]; [SS 5012 1688]. Sandstones up to 4.6 m thick are folded into a syncline [SS 5024 1732]. A roadside quarry [SS 5021 1752] exhibits bedded and massive sandstones in an asymmetric anticline; dark grey shales in the core of this fold contain elongate concretions and may equate with the Hartland Quay Shale or Longpeak Shale near the base of the formation. The following exposures occur in the steep river bank to the south-east [SS 5061 1734] to [SS 5080 1705] : flaggy banded siltstones with silty shales and some hard sandstones [SS 5064 1727]; dark grey carbonaceous sandstones with plant remains common [SS 5067 1724] and rare [SS 5072 1710]; laminated sandstones with load casts up to 0.1 m across [SS 5078 1706]. Farther upstream flaggy sandstones with silty shales carried a faint impression of a xiphosurid [SS 5147 1633], and massive grey-green sandstones crop out in many places [SS 5235 1658]; [SS 5270 1644]; [SS 5322 1624]. The eastern bank of the river near Long Wood [SS 5342 1538] to [SS 5356 1594] shows much flaggy sandstone and siltstone; some of the sandstones are carbonaceous and muddy.

Little Marland–East Worlington

Little Marland

A quarry [SS 4983 1290] near Winscott Farm shows thick brown feldspathic sandstone. Bands of shale, up to 0.6 m thick, crop out near Heanton Barton [SS 4983 1129]; [SS 4995 1129]; [SS 4998 1130]. Exposures to the north show poorly bedded sandstone over 3 m thick in a disused quarry [SS 4983 1206] and well-bedded sandstones with shales in a stream [SS 4977 1230]. A roadside section [SS 5033 1146] to [SS 5045 1133] shows massive buff sandstones in its northern part dipping 70° N and inverted, and bedded and laminated sandstones in its southern part dipping steeply south. Shales crop out in a stream north of Bury [SS 5049 1064] to [SS 5073 1056], and sheared shales farther downstream [SS 5094 1051] lie in close proximity to white silty clays of the Bovey Formation.

Merton–Stafford Moor

A quarry [SS 5264 1150] near Merton exposes many 0.3-m sandstones with pale yellow-grey shales and siltstones; others contain flaggy sandstones [SS 5291 1157] and flaggy sandstones with harder more massive cleaved sandstones [SS 5363 1265]. Larger sections, some of them quarried, alongside the River Torridge [SS 5449 1158] to [SS 5493 1125] show folded flaggy sandstones and flaggy siltstones together with more massive feldspathic sandstones. Sole markings, such as load casts, are particularly common on the south-eastern side of one quarry [SS 5492 1124]. Shale bands are usually less than 1 m thick. Similar exposures occur to the north [SS 5451 1218] to [SS 5494 1191]. Thick cleaved grey-green silty shales with a few thin sandstones crop out [SS 5528 1149] behind an old mill. Both massive and flaggy sandstones are exposed in quarries [SS 5576 1163]; [SS 5580 1148]; [SS 5585 1163]; [SS 5598 1169], the last locality showing massive sandstones up to 3 m thick. Predominantly shaly exposures occur to the south along a farm track [SS 5587 1003] to [SS 5610 1001] and in a stream [SS 5613 1018]. Flaggy sandstones, thick sandstones and shales are common in several streams e.g. [SS 5631 1175] to [SS 5872 1240]; [SS 5626 1194] to [SS 5644 1221]. Another stream section [SS 5786 1100] shows an outcrop of shales 37 m wide. The only exposure seen near Stafford Moor was of thin-bedded sandstones in a well [SS 5988 1103].

Ashreigney

A stream course [SS 6082 1324] to [SS 6258 1493] exposes fine-grained sandstones in beds seldom more than 1 m thick, together with siltstones and shales; a feldspathic sandstone occurs with siltstone at one place [SS 6196 1368].

Streamside quarries [SS 6141 1095] show 15 m of thick-bedded sandstones with layers of softer micaceous carbonaceous sandstone and partings of dark grey silty shale. Another quarry [SS 6283 1078] beside Hollocombe Water contains 16 m of well-bedded grey-green sandstones with some siltstones and dark grey shales. Thick-bedded grey feldspathic sandstones crop out in the Hollocombe Water [SS 6446 1222].

Two western tributaries of the River Taw [SS 6507 1417] to [SS 6573 1406] and [SS 6518 1288] to [SS 6616 1345] contain many exposures of grey silty sandstones, some of them micaceous, with siltstones and scattered shales. A railway cutting [SS 6673 1370] shows thick sandstones with some purple staining. In the Little Dart River [SS 6788 1356] to [SS 6917 1363] grey, brown and purple-stained sandstones, generally thick-bedded with silty tops to the beds, are accompanied by green and brown siltstones and grey silty shales. Railway cuttings farther south show pale brown sandstones with some sooty shales [SS 6804 1264] and grey-green sandstones with thin siltstones [SS 6909 1116] to [SS 6917 1100]. On the eastern side of the Taw valley roadside quarries [SS 6922 1128] to [SS 6930 1107] have been opened in thick-bedded sandstones with silty tops to the beds; current-lamination is visible and a few shale bands are present. A similar quarry [SS 6942 1031] shows 58 m of thick-bedded grey sandstones; the beds are laminated towards their tops and some thin bands of siltstone and shale are present. ECF

Huntacott Water

Flat-lying flaggy sandstones exhibit flute casts [SS 7026 1481]. At Farrier's Bridge [SS 71161548] sandstones with bands of grey shaly mudstone dip 25°/210°. Farther upstream massive sandstones of Bude Formation aspect crop out at [SS 7231 1617] and at [SS 7261 1655]. In the stream at [SS 7353 1673] flaggy sandstones with silty mudstones occur. Some bands of the mudstone are dark and micaceous with much plant debris.

Little Dart River [SS 700 137] to [SS 750 132]

The western part of the river course contains the following sections : at [SS 7009 1371] flaggy sandstones in bands up to 0.3 m thick with thin bands of laminated siltstone, dip 55°/150°; at [SS 7175 1336] thick-bedded and massive sandstones, dip 40°/160°; at Stone Mill Bridge [SS 7200 1310] grey flaggy sandstones with bands of fissile laminated siltstone and some shaly mudstone, dip 35°–55° SE; at [SS 7200 1303] a small syncline with complementary anticline to the south, fold axial trend 070°, dip of common fold limb 60°/340°.

Cheldon Quarry [SS 7241 1315] shows, at its south-west end, 20.4 m of sandstones, siltstones and shaly mudstones. The beds dip 60°/140° in the south-east limb of an anticline whose crest is visible at the top of the north-west wall of the quarry. The north-west limb of the fold dips 30°/320°. The sequence is composed of 68 turbidite units, up to about 0.45 m thick, each having a basal grey sandstone which cuts sharply across the underlying mudstone or siltstone. The sandstones are mainly massive and micaceous and contain comminuted plant debris with scattered plant stems up to 0.15 m long. Some pull-apart structures and load casts were noted, but no good directional sole marks. The sandstones grade up into laminated siltstones and these in turn into shaly mudstones. At some places the mudstones are up to 1.7 m thick and presumably represent a period of muddy deposition without incursions by turbidity currents. At others the turbidite unit lacks a muddy top and sandstone or siltstone is succeeded by sandstone of the overlying unit, pointing to non-deposition of mud or to erosion of a mud layer by the succeeding turbidity current. Thin units and the absence of coarse sandstones indicate distal turbidites deposited at a considerable distance from the place of origin of their currents.

East of Cheldon Quarry the river contains the following sections : at [SS 7260 1324] banded grey flaggy sandstones, laminated siltstones and mudstones dipping 60°–80°/150°; at [SS 7281 1320] a small sigmoid fold in flaggy sandstone striking 070°; between [SS 7306 1307] and Cheldon Bridge [SS 7384 1285] scattered exposures of flaggy sandstones and siltstones dipping south or south-east at 35° to 55°; between Cheldon Bridge and [SS 7495 1324] scattered exposures, mainly of grey flaggy sandstones dipping 40°–70°/340° and 50°/160°.

Stream section [SS 7480 1411] to [SS 7492 1367]

Grey flaggy compact sandstones and shaly mudstones are interbedded locally with some siltstones. The sandstones commonly occur in bands up to 0.6 m thick; in places they are abundant and massive. The beds strike about 080° and dip at 50°–80° to NNW or SSE.

Affeton Barton [SS 7557 1371]–Drayford [SS 7808 1373]

In the Little Dart River micaceous sandstones, banded flaggy sandstones and shaly mudstones are folded [SS 7632 1324] into an anticline trending about 080°. A nearby small quarry [SS 7629 1305], 90 m SSE of Little Witheridge, contains finely laminated sandstones and mudstones showing conspicuous flame structures. The N–S stream 90 m E of West Worlington has exposed massive and flaggy sandstones with some bands of shaly mudstone.

From West Worlington Bridge [SS 7693 1332] to east of Drayford the Little Dart River shows scattered exposures of sandstone, locally coarse massive and purplish brown, with beds of flaggy sandstone and some shaly mudstone. At 220 m downstream from Drayford Bridge [SS 7808 1373] a small E–W symmetrical syncline has both limbs dipping at about 60°. At 64 m W of Drayford Mill [SS 7833 1376] a small open E–W anticline shows limbs dipping at 25° and 30° and passes north into a complementary syncline.

South of the Little Dart River stream sections in Stockham Wood [SS 7758 1325] and Pedley Wood [SS 7784 1304] show flaggy sandstones and siltstones.

A quarry 136 m NW of Drayford Bridge contains an anticline in fairly massive sandstones with subordinate shaly bands; the fold limbs dip 70°/350° and 40°/170°. In the Adworthy Brook [SS 7798 1390] flaggy sandstones and shaly mudstones are disposed in a fairly tight anticline and syncline.

Coombe Wood Quarry [SS 7875 1376] lies 640 m E of Drayford Bridge and just outside the district. An E–W anticline at the northern end of the quarry, in massive green and purplish brown sandstones with subordinate bands of shaly mudstone, is bounded to the south by a fault zone of sheared dark grey shaly mudstone. The east wall of the quarry shows mainly fairly massive sandstone with thin mudstone bands dipping about 45° S. The southern end of the quarry contains flaggy sandstones with three thin bands of shaly mudstone. Xiphosurid trails were noted on a loose block of silty sandstone at the foot of the south wall.

Thorndon [SS 7750 1514]–Lower Adworthy [SS 7825 1531]

Purplish brown sandstone is exposed [SS 7827 1525] 45 m SE of Lower Adworthy and in the stream [SS 7833 1520] to the south-east. Some bands of shaly mudstone are present at the latter locality and the beds are folded into an asymmetrical anticline and syncline whose axial planes dip about 60° S.

Chawleigh-East Hilltown [SS 702 110]–Fiddlecott [SS 712 111]

A small disused quarry [SS 7027 1236] shows 0.15-m bands of grey silty sandstone interbedded with grey silty mudstone in bands up to about 0.05 m thick. The rocks are stained red on joints and dip 55°/150°. To the south and east thick-bedded and massive purplish grey sandstones dip 45°/170° [SS 7035 1224] and flaggy sandy siltstones dip 60°/l70° [SS 7063 1230].

At 320 m S of East Hilltown thinly laminated siltstones contain a 1-m band of sandstone [SS 7021 1062], and a quarry [SS 7077 1102] 320 m NW of Toatley shows fissile laminated siltstones with thin bands of silty sandstone, some rich in carbonaceous plant debris. Ditches [SS 7110 1076] 90 m E of Toatley show dark grey shaly mudstone dipping 50°/350°. In the lane north of Fiddlecott [SS 7118 1116] to [SS 7126 1156] greenish and purplish grey sandstones, locally with bands of fissile laminated siltstone, dip 20°–75° N and 45°–50° S.

East Leigh [SS 736 l23]–Pouncers Cross [SS 7414 1133]

Grey mudstones dip 80°/180° in a lane [SS 7396 1134] leading to Pouncers. A small quarry [SS 7358 1178] 550 m NW of Pouncers shows thick-bedded and massive grey sandstones dipping 80°/350°, and banded flaggy sandstones and fissile siltstones dip 80°/170° in the stream 45 m farther north-west. Another small quarry [SS 7398 1221] at the west end of Arrish Wood contains grey shaly mudstones with bands of sandstone up to 0.6 m thick; the bedding dips 33°/140°, and cleavage in the shaly mudstone dips 60°/l40°. At 45 m SW of this quarry grey flaggy sandstones with laminated siltstones dip 60°/330°. A small quarry [SS 7420 1222] at the east end of Arrish Wood shows greenish grey sandstones with siltstone bands dipping 40°/155°. JEW

Petrockstow–Morchard Bishop

Petrockstow–Iddesleigh

A stream section [SS 4900 0815] to [SS 4944 0797] exposes sheared grey and grey-green shales with thin flaggy sandstones and laminated sandstones, some with small load casts [SS 4935 0801]. The lithologies are somewhat reminiscent of the

Crackington Formation. Farther north another stream [SS 4942 0952] cuts through banded mudstones, soft green siltstones and thin sandstones typical of the Bude Formation, and 3 m of massive brown sandstone containing abundant plant remains are exposed in a quarry [SS 5011 0932]. Other quarry sections show: at [SS 5165 0848] grey-green shales, overlain by hard brown sandstone with ripple marks 7.62 m, and grey-green shales with thin ripple-marked sandstones 1.22 m; at [SS 5216 0843] thick silty shales with plant remains and some thin green sandstones; at [SS 5261 0780] structure-less sandstones, with shale bands up to 4.57 m thick and a massive slumped bed. Boreholes for clay in the area of Woolladon Moor around [SS 531 081] commonly penetrated Bude Formation rocks and proved them to be predominantly shaly.

Brown and green sandstones crop out at Western Town Farm [SS 5467 0822], Meeth, and thick cleaved dark grey shales in a small stream to the north [SS 5469 0990]; [SS 5485 0990]. A large quarry [SS 5528 0964] on the western bank of the River Torridge contains the following section :

A trackside exposure [SS 5540 0930] to the south shows up to 9.14 m of hard green-grey sandstones in units up to 1.22 m thick. The sandstones commonly have large load casts on their bases, and evenly laminated tops. These beds are overlain by hard laminated grey sandstones, some of them having fragments of banded smooth grey shale caught up in their bottom few centimetres. Another section alongside the track [SS 5534 0802], near Eastern Town Farm, is as follows :

Brown flaggy silty sandstones are exposed at the edge of a track [SS 5645 0803] near Westpark Farm, and a large quarry [SS 5665 0819] to the north-east contains massive 3-m sandstones and thick brown micaceous sandstones with plant remains. Stream sections are as follows: at [SS 5667 0824] 3 m of sandstones with large load casts; at [SS 5696 0920] massive brown sandstone; at [SS 5690 0899] massive sandstone with some flaggy sandstone. To the east of Iddesleigh a belt of shales up to about 520 m wide is exposed in a number of N-S streams and in the E–W stream into which they flow [SS 5812 0790] to [SS 5854 0792]; [SS 5802 0798] to [SS 5822 0836]; [SS 5880 0798] to [SS 5917 0831]. The rocks are banded green-grey shales with some banded siltstones and a few green-brown sandstones; they dip 60°–80° N and are inverted.

Northwards from the last-mentioned stream section the shales contain thin sandstones showing small load casts, flute casts and groove casts on their bases and having cross-laminated tops [SS 5917 0831] to [SS 5923 0858]. Similar rocks crop out in a stream to the east [SS 5967 0844] to [SS 5967 0861] together with more massive sandstones [SS 5967 0851].

Bradley [SS 505 076]–Barwick [SS 594 076]

Folded thick sandstones, thin sandstones and siltstones crop out on the south bank of the River Torridge [SS 4979 0656]. The following exposures occur in tributary streams : between [SS 4962 0770] and [SS 5009 0717] mainly sandstones; between [SS 5060 0785] and [SS 5070 0775], [SS 5096 0780] and [SS 5070 0775], and [SS 5070 0775] and [SS 5069 0765], shales, mudstones and siltstones with thin sandstones, some of the sandstones flaggy and cross-laminated; between [SS 5069 0765] and [SS 5064 0710] shales, mudstones, and siltstones with more and thicker sandstones, including one of 3 m [SS 5069 0739]. Brownish green massive sandstones are exposed in the River Torridge [SS 5078 0683]; [SS 5221 0613]; [SS 5250 0602]. North of the Torridge sandstones crop out in a small stream [SS 5202 0694] and, heavily red-stained, in the farmyard at Crocker's Hele [SS 5258 0687]. South of the river greenish brown massive sandstones occur in a farmyard [SS 5284 0557] at Lewer.

Farther east massive sandstones are exposed in the steep banks of the River Lew [SS 5316 0564] to [SS 5336 0590], and a quarry [SS 5352 0576] east of that river has been opened in a massive brown 6.1-m sandstone with siltstones bearing much plant debris and some dark banded shales. The thick sandstone has load casts on its base and ripple marks on its top. A disused railway cutting [SS 5363 0588] to [SS 5469 0788] contains the following exposures : at [SS 5370 0633] sandstones up to 1.22 m thick interbedded with grey shales; at [SS 5387 0647] massive sandstones up to 5.18 m thick, some of them red-stained; at [SS 5436 0685] banded siltstones with some hard thin sandstones. Massive sandstones up to 3 m thick have been quarried [SS 5452 0694] for use in the construction of the railway. Nearby exposures show buff sandstones in beds 0.61 to 0.91 m thick with shale partings; the bases of the sandstones carry ridge marks trending around 360° and load casts. A quarry [SS 5483 0715] east of the old railway exposes a 3-m hard green massive sandstone with some 0.46-m sandstones and shale bands. Pale buff to green massive sandstones and thin pale green sandstones which are exposed [SS 5486 0731] to the north show much red staining and shearing. Farther north-east along the railway track the succession is more shaly, locally [SS 5470 0783] to [SS 5483 0780] consisting of shales and siltstones with only a few 0.08-m sandstones.

Exposures in the River Torridge show thin sandstones and shales [SS 5451 0632] and a 1.52-m sandstone in a succession of 0.31-m sandstones and siltstones [SS 5466 0652]. Sandstones and shales crop out in a small stream to the east [SS 5513 0649] to [SS 5528 0558]. Exposures mainly of sandy siltstones and massive greenish brown sandstones occur in farmyards around Grove's Fishleigh [SS 5548 0578]. Thick grey shales crop out in small streams and ditches [SS 5590 0572] to [SS 5617 0572] and also in the banks of the River Okement [SS 5645 0580] to [SS 5671 0576]. Farther north-east massive greenish brown sandstones are commonly exposed in farmyards and in the foundations of farmhouses, as at Bridge Town [SS 5550 0720] and Nethercott Barton [SS 5627 0647]. Over 6 m of pale greenish grey micaceous nodular sandstones, in beds up to 0.46 m thick separated by shale partings, are exposed in a quarry [SS 5739 0601], and a further quarry [SS 5840 0580] to the east contains massive sandstones overlain by thin sandstones and shales. Grey banded shales alongside a forestry track [SS 5865 0673] and a road [SS 5877 0691], and in a small stream [SS 5964 0699], show pencil cleavage. Gently folded low-dipping red-stained and quartz-veined sandstones are exposed in the farmyard at Bullhead [SS 5878 0725]. Sandstones exposed at Pixton [SS 5826 0768] and South Barwick [SS 5942 0763] dip 25°–45° N and appear to be right way up.

Winkleigh–Brushford

Exposures in a stream section west of Winkleigh [SS 6027 0757] to [SS 6135 0816] show bluish grey and green well-bedded flaggy silty sandstones with siltstones and silty mudstones; in places e.g. [SS 6096 0811] the sandstones are micaceous. A small quarry [SS 6097 0890] contains 7.62 m of grey feldspathic fine-grained sandstones. Dark grey shales with calcium carbonate nodules in a stream [SS 6364 1001] north of Winkleigh have yielded Anthracoceratoides cornubiensis, Caneyella sp.and Dunbarella sp.; they are equivalent to the Sandy Mouth Shale of Freshney and Taylor (1972) on the coast and possibly of the Margam Marine Band of South Wales. A quarry [SS 6290 0915] near Berner's Cross contains finely banded and thickly bedded purple-stained grey sandstones. Another [SS 6415 0841] has been opened in 7.62 m of massive hard fine-grained feldspathic and micaceous sandstones, and the adjacent stream [SS 6409 0839] cuts through 15.42 m of hard green slaty siltstones.

Brushford

Exposures near the confluence of the Bullow Brook and the River Taw south of Brushford indicate the presence of large thicknesses of shale. At one place alongside the River Taw [SS 6731 0683] 45 m of grey laminated silty shales are exposed, and thick shales and siltstones, locally micaceous, crop out farther upstream [SS 6711 0631] to [SS 6704 0629]. It is probable that these thick shales represent the highest part of the Bude Formation succession and are equivalent to the shaly beds around the Warren Gutter Shale on the coast. As on the coast, they appear to lie near the axis of a major synclinorial structure.

A quarry [SS 6779 0940] north of Brushford shows 0.31 m of blue mudstones overlain by ^.8.84 m of massive grey fine-grained sandstones with thin silty bands. Farther west a stream section [SS 6684 0931] to [SS 6732 0988] shows well-bedded fine-grained sandstones. Locally within this section there occur more massive sandstones [SS 6698 0945], and hard blue-grey silty sandstones with abundant plant debris [SS 6695 0945]. To the north-east the same stream has exposed [SS 6769 1007] dark grey shales with Anthracoceratoides cornubiensis, Caneyella sp.and Dunbarella sp.,the Sandy Mouth Shale. The trace fossil Planolites ophthalmoides was obtained from sandstones and shales in the side of a track [SS 6722 0970]. Sporadic exposures in the River Taw [SS 6883 0827] to [SS 6955 0857] consist mainly of thickly bedded fine-grained sandstones, locally stained red or purple. ECF

Chenson Farm [SS 7057 0992]–Filleigh [SS 7445 1053]–Curriton [SS 7803 1072]

The track 200 m N of Chenson Farm crosses purplish grey flaggy sandstones dipping 65°/350° [SS 7056 1019]. The N-S stream west of Cleave [SS 7532 1053] contains scattered exposures of massive sandstone with silty and shaly mudstone; dips are steep to north or south. In the stream south-east of Deneridge [SS 7640 1080] massive and flaggy sandstone with flaggy siltstone and some bands of shaly mudstone dip steeply north or south. Similar rocks are exposed to the east in the stream which runs south-south-west to Brooms Copse [SS 7710 1040].

River Taw [SS 700 085] to junction with River Yeo [SS 710 093]

Red-stained jointed flaggy sandstones dip 20° N in the river bank [SS 7007 0856]. Flaggy micaceous silty sandstones, with bands of shaly mudstone and dipping 40°/355°, crop out farther east [SS 7067 0861]. Immediately upstream of a mill [SS 7111 0903] flaggy sandstones in the river bank dip 60° N; 27 m downstream they dip 5° N. Near the confluence with the River Yeo hard grey sandstones dip 5°/340° in an old quarry.

Coldridge Barton [SS 7022 0723]–Kelland Barton [SS 7254 0663]

A farm track [SS 7027 0724] 27 m E of Coldridge Barton crosses greenish sandy siltstones, dip 60°/180°, and purple-stained sandstone; massive sandstone is exposed in an old pit [SS 7058 0726] 320 m E of the farm. At Fursdon [SS 7148 0658] there are small exposures of greenish grey siltstone. An old quarry [SS 7175 0743] 600 m NE of Cleaveanger contains thick-bedded and massive purplish grey micaceous sandstone dipping 35°/295°; the rock is traversed by scattered thin quartz veinlets and contains carbonaceous plant fragments. Similar sandstone crops out at the roadside [SS 7215 0658] west of Kelland Barton and in the farmyard at Pennycotts [SS 7229 0642], where it dips 25°/203°. Greenish grey silty laminated sandstones dip 40°/175° in a lane [SS 7260 0648] 120 m SSE of Kelland Barton.

Birch Farm [SS 7065 0562]-Stopgate Cross [SS 7257 0556]

A ditch [SS 7029 0568] 365 m W of Birch Farm contains micaceous sandy siltstones, dip 18°/008°, with sideritic nodules up to 0.15 m across. About 45 m to the north banded sandy siltstones and sandstones contain concretions up to 0.3 m across. Greenish grey micaceous sandstones dip 15°/345° in the roadside at Birch Farm [SS 7062 0557]; [SS 7067 0564]. To the east, in Loosebeare Lane [SS 7142 0571], greenish grey sandy siltstones dip 30°/007°.

River Yeo [SS 710 093] to [SS 732 054]

At the west end of Lapford Northern Wood a disused roadside quarry [SS 7166 0920] shows hard massive purplish grey sandstone overlain by flaggy sandstone dipping 48°/170°. Exposures in Heal Wood [SS 7172 0858] include an old quarry in evenly bedded flaggy sandstones with thin flaggy siltstone bands, dip 85°/345°, and green silty sandstones in the river bank [SS 7170 0881] dipping 65°/345°. The railway [SS 724 081] northwest of Lapford Station cuts through banded sandstones and shaly mudstones, mainly vertical, striking 075°. An excavation [SS 7265 0778] on the south-west side of the old Ambrosia factory at Lapford showed greenish grey silty sandstones interbedded with silty mudstones, locally cleaved and sheared; dips of 45°/315° and 40°/160° were recorded. At Bugford Quarry [SS 7390 0720] the vertical east face shows well-bedded greenish grey micaceous sandstones, with bands of flaggy sandy siltstone locally containing much carbonaceous plant debris; the beds dip 2°–6° S. The north-east corner of the quarry is cut by a small NNE–SSW fault, to the north-west of which the rocks are vertical. Grey laminated mudstones dip 35°/190° at the south end of Bury Bridge [SS 7375 0673] and crop out also in the railway cutting [SS 7385 0673] and near Easton Barton [SS 7427 0674]; [SS 7446 0672].

Massive, purplish and greenish grey sandstones in the railway cutting [SS 7389 0660] dip 45°/190°. In Ball Copse [SS 7372 0633] a small quarry shows massive purplish grey micaceous sandstone dipping 45°/1 90°; siltstones and sandy siltstones, forming part of the same E–W ridge, crop out in the river [SS 7362 0634] to the west. Massive hard grey siliceous sandstones dip 50°/180° near the river [SS 7357 0605] 120 m W of Bradiford, close to which farm [SS 7370 0606] siliceous sandstones in bands up to 1.2 m thick are interbedded with micaceous sandy siltstones containing carbonaceous plant debris. Farther upstream [SS 7347 0570] poorly bedded grey silty mudstones with thin sandstone bands dip 20°/195°. Near the southern edge of the district the river has exposed close-jointed fairly massive greenish grey sandy and micaceous siltstones dipping 40°/180° and containing spheroidally weathered concretions from 0.05 to 0.3 m across [SS 7346 0563].

Bowerthy [SS 7265 0970]–Lapford [SS 7320 0830]

At Bowerthy [SS 7269 9072] flaggy greenish grey silty sandstones and laminated silty mudstones dip 45°–60°/350°. In Rensey Lane [SS 7280 0927] flaggy sandstones with thin shaly bands dip 60°/340°. Lapford village contains small roadside exposures of banded sandstones and shaly and silty mudstones dipping 40°/350° and 60°/170°. Laneside exposures at Rectory Farm [SS 7355 0895] show greenish grey sandy siltstones and silty mudstones, and old quarries [SS 7388 0905] to the east contain massive greenish and purplish grey sandstones with flaggy micaceous siltstones and mudstone bands.

River Dalch [SS 7357 0740]–Lewdon [SS 7765 1060]

For about 275 m upstream from [SS 7365 0762] there is an intermittent section in purple and grey sandstones, locally hard and siliceous; successively upstream the beds dip 10°/170°, lie horizontally, and dip 45°/335°. An anticline in hard purplish grey sandstone trends 070°–250° [SS 7390 0775]. Upstream of Calves Bridge [SS 7496 0878] grey shaly mudstones and micaceous sandy siltstones dip 65°/340°. Massive and thick-bedded purplish and greenish grey sandstones [SS 7500 0906] to [SS 7507 0885] dip 25°–70°/170°. Between Eastington [SS 7500 0910] and Cobley [SS 7527 0963] there are a number of exposures in the river, mainly of purplish or greenish grey sandstone, generally fairly massive but locally flaggy and silty, the dip mainly steep to north or south. Farther upstream [SS 7564 0997] grey mudstones and siltstones dip 40°/175°. South of Waterpark Wood [SS 7620 1020] greenish grey sandstones, locally massive, with silty mudstones, dip steeply to about 170° and 350°. From Deneridge Bridge [SS 7677 1010] to south of Lewdon the stream section shows scattered exposures of massive sandstone, flaggy sandy siltstone and some shaly mudstone; dips are mainly steep to about 350° or 170°.

West Northwood [SS 7757 0863]-Bishop's Leigh [SS 7800 0965]

In the lane [SS 7757 0865] immediately north of West Northwood there is a quarry showing an E–W anticline in sandstone with the north and south limbs dipping 45° N and 35° S respectively. To the north the sandstone is overlain by about 6 m of flaggy grey silty mudstones. In the stream [SS 7753 0951] 460 m W of Bishop's Leigh grey shaly mudstones with sandstone bands and some soft black carbonaceous nodules dip 40°–70°/010°. Farther downstream [SS 7746 0963] a 25-mm band in grey flaggy mudstones has yielded Caneyella sp., Anthracoceratoides cornubiensis, and mollusc spat. Ramsbottom (1965, p. 74) has compared this fauna with that of the Sandy Mouth Shale of Freshney and Taylor (1972).

Chillingford [SS 7540 0829]–Morchard Bishop [SS 770 075]

An old quarry [SS 7565 0830] 230 m ENE of Chillingford contains greenish grey laminated siltstones and silty mudstones with some spheroidally-weathered concretions. A 50-mm band near the base of these rocks contains much coaly plant debris including stems up to several centimetres long. The lowest part of the quarry shows underlying hard pale grey siliceous sandstones dipping 15°/060°. At Hirscombe [SS 7624 0817] greenish grey micaceous sandy siltstones dip 10°/200°, and an old quarry [SS 7673 0819] north-west of Tatepath shows massive and thick-bedded greenish grey feldspathic sandstones dipping 35°/180°. This sandstone is exposed farther east in a lane [SS 7709 0821] and a wood [SS 7724 0817]. Between Morchard Bishop and Farthing Park [SS 785 073], coarse purplish grey feldspathic sandstones are exposed in a lane [SS 7757 0739] north of Broadgate and in Beardown Copse [SS 7802 0738]. A roadside exposure [SS 7849 0731] near Farthing Park shows coarse greenish grey sandstone with beds of micaceous siltstone, vertical and striking 103°. Grey laminated mudstones exposed at Broadgate farm [SS 7765 0734] dip 15° N.

Sharland Farm [SS 7518 0560]–Middlecott [SS 7581 0730]

Flaggy sandstone dipping 25°/205° is exposed in Sharland farmyard [SS 7518 0560] and in the lane 135 m to the east. Massive purplish and greenish grey sandstones crop out in the Pepper Lake stream [SS 7484 0643]. On the ridge to the south-east [SS 7539 0602] two small quarries have been opened in massive purplish grey feldspathic sandstone; in some places the rock is close-jointed with iron staining on joints, in others it is greenish grey with patches of purple staining. Old quarries [SS 7580 0607] about 410 m to the east show similar massive sandstone, locally coarsely micaceous. In Slade Lane [SS 7618 0601], south of Weeke, greenish grey flaggy sandstones, locally silty, dip 75°/190°. Immediately north of Woodgate weathered shaly mudstones with sandstone bands up to 1.2 m thick are exposed in a lane [SS 7621 0658]. The road at Middlecott [SS 7581 0731] crosses greenish grey silty sandstones, locally hard and siliceous, and in an old quarry [SS 7574 0728] 64 m to the west massive purplish grey feldspathic sandstones are overlain by flaggy sandstones dipping 25°/175°.

Oldborough Quarry [SS 776 063]

Massive greenish grey and purple sandstones in this quarry were being worked in 1964 for road metal. The rock is cut locally by numerous steep irregular joints, some of which are iron-stained and carry hematite films associated with small quartz prisms. In the north-west part of the quarry two shatter zones trend about NNE–SSW and have several minor branches. JEW

Bideford Formation

Rocks of the Bideford Formation crop out from Alverdiscott in the west to near George Nympton in the east. Characteristic lithologies permit description in stratigraphical order, first the lowest sandstone unit present, then the overlying argillaceous beds, then what is probably the Cornborough Sandstone, and finally the youngest beds, near Atherington.

The lowest sandstones of the formation crop out in the farmyard at Higher House [SS 5769 2471] where well-bedded brown sandstones occur with laminated siltstones. Similar sandstones also occur in a small roadside quarry [SS 5783 2469] and in the banks of a stream to the east [SS 5819 2468]. Another small roadside pit [SS 5851 2463] contains thick poorly bedded brown sandstone. East of the River Taw an old quarry [SS 6412 2461] shows moderately coarse hard grey-green sandstone.

The overlying more argillaceous beds crop out at roadsides as thick banded silty shales [SS 5544 2490] and as similar shales with scattered brown sandstones [SS 5612 2473]. Stream sections show banded siltstones and sandstones with some of the bands cross-laminated or disturbed [SS 5816 2459] and siltstones and silty shales [SS 5800 2451]. Along the banks of the River Taw [SS 6093 2470] to [SS 6101 2423] alternating muddy and sandy siltstones contain a few sandstone bands. A small stream [SS 6413 2433] farther east cuts through grey micaceous sandy siltstones. Sandstones become commoner eastwards and a roadside exposure [SS 6600 2456] shows feldspathic sandstones with flaggy silty sandstones. Micaceous and carbonaceous sandstones are exposed on the western side of the valley of the River Bray [SS 6724 2443], and finely laminated and cross-laminated sandstones in quarries to the south [SS 6725 2424]. These sandy beds mark an eastward passage into Bude Formation lithologies.

Above the argillaceous beds described in the preceding paragraph lies a coarse brown feldspathic sandstone, commonly cross-bedded and wedge-bedded, which is thought to be the Cornborough Sandstone of Prentice (1960). The most westerly outcrops noted occur at the side of a stream [SS 5436 2469] and comprise brown laminated sandstones showing some cross-lamination and cross-bedded units with foresets around 0.09 m long. A little farther east brown and buff massive feldspathic sandstones, much shattered and quartz-veined, crop out in a small quarry [SS 5464 2470]. Another stream section [SS 5581 2462] exposes brown, coarse well-bedded sandstone. A quarry [SS 5690 2454] to the south-east contains 7.62 m of brown feldspathic sandstone showing wedge bedding. A stream course [SS 5790 2436] to the east shows a band of silty shales within sandstones. Sandstones in another stream section [SS 5796 2446] to [SS 5782 2419] are thickly bedded in some places [SS 5796 2447], laminated and cross-laminated in others [SS 5794 2443]. In the farmyard at Bremridge [SS 5863 2417] there is black sooty soil probably derived from a thin coal or culm seam within the sandstone, and in a stream to the east [SS 5985 2387] coarse brown laminated sandstone is overlain by green sandy siltstone and thin silty shales. Thick sandstones in a quarry [SS 6077 2392] near Umberleigh contain some silty beds and exhibit wedge-bedding and ripple-marked tops; a coaly horizon is also present. On the eastern side of the River Taw the Cornborough Sandstone crops out in rough ground [SS 6166 2419] near the top of a hill. It appears to form several distinct ridges carrying abundant debris of coarse feldspathic sandstone e.g. [SS 6222 2384], and this probably reflects repetition by folding. The easternmost exposure of Cornborough Sandstone occurs in a quarry [SS 6731 2381] near Clapworthy where the rock is brown, fairly fine-grained and feldspathic.

Beds higher than the Cornborough Sandstone but of Bideford Formation aspect are exposed in a small quarry [SS 5976 2347] near Atherington; they comprise flaggy laminated and cross-laminated silty buff sandstones containing much plant debris. A similar quarry section [SS 5834 2366] shows fine well-bedded buff feldspathic sandstones with abundant plant remains. ECF

Chapter 3 Palaeogene

Introduction

The Petrockstow Basin, in the south-western part of Chulmleigh district, is mainly fault-bounded, over 660 m deep and trends north-west to south-east (Figure 3). It is occupied by kaolinitic clays, silts, sands, gravels and lignites of Palaeogene age which are correlated with the similar sediments of the Bovey Formation (Edwards, 1976) in south Devon. Smaller deposits at Lower Cadham Farm [SS 584 030], near Sampford Courtenay Station [SX611 988] and in the Sandy Park area [SX713 893], in the Okehampton district (Edmonds and others, 1968, pp. 160–163), are probably also of Palaeogene age. All lie along NW–SE faults in and near the Sticklepath Fault Zone. North-westwards from the Petrockstow Basin, both flint gravels at Orleigh Court [SS 430 220], near Buckland Brewer (Edmonds and others, 1979 ), and an offshore Tertiary basin between Lundy Island and Morte Point (Fletcher, 1975), are associated with this fault zone.

Most workers have agreed that these deposits are of Palaeogene age, and recent work by Dr Charles Turner indicates that the Petrockstow sediments are mainly Eocene with some Oligocene.

General account

The sediments consist dominantly of kaolinitic clays with a highly variable silt and sand content, fine silty sands, subordinate coarse sands and gravels, a little lignite, and concretionary ironstones.

Clays

Gradations from smooth clay to coarse sand or gravel are common. Most sections, in pits or boreholes, are characterised by great thicknesses of rather homogeneous, sandy to very sandy clays, with a few bands of slightly sandy to smooth clays (Plate 6) and (Plate 7). The clays are generally grey to grey-brown, but those containing much fine-grained vegetable matter are darker brown or even black and lignitic.

The junction between the darker organic clays and the grey clays is commonly gradational. In some cases oxidation of brown clay has produced grey clay with some red staining. Many worm burrows are picked out by infillings of clay of a slightly different colour from that of the surrounding clay. Polygonal crack systems are common, the individual polygons being locally up to 2 m across and the cracks themselves up to a metre deep and filled with fine grey silty sand or extremely silty clay. These cracks are exactly similar in appearance to desiccation cracks which develop in the claypits during very dry weather. Some of the clays show considerable evidence of slumping and slurrying, and intermingled masses of sand and silt and clay occur up to 0.5 m across. Borehole sections through many metres of slumped beds show much convolution; juxtaposition of horizontal and vertical bedding suggests the presence of crumpled and even overfolded masses of sediment.

In a restricted area in the northern part of the basin there occur a few lenticular seams of finely laminated 'varved' clays, rich in organic material and red-brown oxidising to black. These clays commonly contain well-preserved leaf remains. The laminae range from less than 1 mm to about 10 mm thick and consist of very silty clays interlaminated with clayey silts. Each lamina has a sharp base and usually comprises clayey silt, locally cut into the silty clay below, grading up through very silty clay into a silty clay with comminuted organic material and leaf remains. Probably these laminated clays were deposited in temporary lakes by the action of small turbid flows and other gentle currents.

(Figure 4) shows the general range of compositions of clay samples taken from IGS deep boreholes Nos. 1, 2 and 3 and from open claypits. These modal analyses were derived by computation from the silica, alumina, potash and soda contents. Quartz in the form of fine sand and silt ranges between 8 and 70 per cent with an average around 43 per cent. Kaolinite ranges between 8 and 55 per cent, with an average around 32 per cent. The range of mica content is from 10 to 38 per cent, the highest values occurring in clays with the highest percentages of clay minerals.

The dominant clay mineral is fine-grained disordered kaolinite but in some clay seams, notably those with fairly high organic contents, small quantities of smectites are also present.

The presence of iron oxides such as goethite and lepidocrocite results in patchy laminated ferruginous staining; yellow stains are probably due to other oxides of iron. Finely divided marcasite also produces bluish grey patchy staining, locally oxidised to yellow, and is common in rootlet networks and in sand-filled and silt-filled cracks within the clays. Another iron mineral common within the clays is siderite, mostly as sphaerosiderite in the form of compact concretions or scattered spherules. The implications of the occurrence of these iron minerals is discussed below (p. 31).

The clays are rich in alumina owing to the high kaolinite content; alkalis, particularly soda, are low. The average A1₂O₃/Na₂O ratio, for samples from IGS boreholes and as quoted in company technical literature, is 56; this compares with 96 for ball clays of the Bovey Basin and 60 for those of Dorset. Corresponding values are 12 for the average of shales throughout the world (Clarke, 1924), 7 for Mississippi delta clays (Clarke, 1924) and 68 for residual clays such as the plastic clays of Ione (Allen, 1929). Pettijohn (1957, p. 509) related this ratio to the maturity of clays, noting the relatively high mobility of soda with respect to alumina. Thus a high alumina/soda ratio indicates high maturity for the Petrockstow sediments, probably reflecting the fact that most of the materials of the basin were derived from the reworking of pre-existing sediments such as the local Carboniferous and Permo-Triassic rocks (p. 32). The titania content measured against silica is also high, the average silicaititania ratio being 48 as compared with a world average for shales and clays of 89 (Clarke, 1924). This may be accounted for by the relative insolubility of titanium minerals.

Sands and silts

Most of the sands contain a good deal of silt and some clay, although some of this clay-grade material consists of free silica. In many cases, such as in the fining-upward cycles (p. 29), there is a complete gradation from sand up into clay. Some of the sands of low clay content are over 99 per cent silica, which suggests they are very mature sediments.

Mechanical analyses of the sands and silts show two distinct populations. One consists exclusively of fairly well-sorted sand of QD φ (Quartile deviation in phi units) 0.25 to 0.6, while the other comprises fine sand, silt and clay, very poorly sorted, of QD φ >2 ((Figure 5)b, c, d). The former was probably transported by saltation, and the latter as a rather turbid suspension in river waters (p. 29). The sand and silt grade material consists entirely of quartz, with no feldspar or lithic material present. The grains are transparent and subangular to sub-spherical. Almost spherical grains are fairly common in the coarser sands towards the base of the deposit.

Since most of the sections occur in pits opened for clay, extensive exposures of sand are rare. Most of the sands and silts are homogeneous with little or no lamination, although this may be due in part to bioturbation. However one or two exposures show laminated and cross-laminated sand. Lamination is locally picked out by brown organic matter, with successive bands of brown and white sand ranging from 1 to 20 mm in thickness. The lamination is locally very irregular and small scour channels are locally present, and the laminae are disrupted by microfaults which probably acted during compaction. The mechanical analysis of these laminated sands gives a complex result (Figure 5), probably owing to mixing of two sets of saltation and suspension populations, each set characteristic of distinct hydraulic conditions.

A group of finely laminated silts related to the 'varyed' clays (p. 21) also comprises two populations ((Figure 5)e). The break between the better sorted saltation population and the very poorly sorted suspension population occurs between 6 φ and 7φ , which is in the finer part of the silt grade, rather than at 3.55 φ which is the norm for most of the sands and silts. This suggests that these silts, which show fine undisturbed laminations and contain well-preserved leaves, were deposited by very gentle currents, probably in one or more temporary lakes.

Some of the sands contain clay pellets 1 to 2 mm across; others incorporate larger masses of clay. The clay pellets probably reflect contemporaneous erosion of unconsolidated clays from river banks, but the larger masses usually occur in association with the slumped clays (p. 21).

Siderite spherules are scattered through some of the sands, particularly those of low permeability due to high clay content. Iron also occurs as solid marcasite concretions which are usually sub-spherical. Finer crystals of marcasite in the coarser sands form a delicate network between sand grains. Ferruginous staining is very rare, and found only in those sands with high clay contents.

Gravels

The gravels consist dominantly of subangular to sub-rounded pebbles of quartz, although flints occur fairly commonly in the basal gravels along with scattered pebbles of very soft green Carboniferous sandstone. The pebbles in the basal gravels range up to 100 mm across, but most do not exceed 20 to 30 mm. All the gravels contain much sand, and some might be better described as gravelly sands. Mechanical analyses showed a population of poorly sorted gravel and coarse sand ((Figure 5)a), another of relatively well-sorted sand and a third of poorly sorted fine sand and silt, possibly reflecting the mixing of bedload, saltation population and suspension population respectively. Few extensive exposures of gravels occur, and most of the samples came from bore-holes. However one exposure, in a stream diversion channel [SS 5138 1169], shows cross-bedding with some wedges and channel structures. Gravels and sands encountered during drilling commonly contain much iron sulphide, probably marcasite, as small crystals.

Lignite

Lignite, which is much less common in the Petrockstow Basin than in the Bovey Basin, occurs as transported fragmental debris, scattered or concentrated in irregular beds, or as lignitic clay in which finely divided lignite debris makes up almost 80 per cent of the sediment. Rootlet beds common below such clays are probably in situ remains.

All the lignite occurrences are fairly thin and impersistent, and none has been worked.

Siderite

Siderite occurs chiefly as scattered spherules within some of the clays and silty sands. In the clays the spherules are locally aggregated into sub-spheroidal concretions up to 1 m across. The margins of these concretions are commonly imprecise, with scattered spherules and attendant red and yellow iron oxide staining extending some distance into the surrounding clay. Individual spherules are usually less than 1 mm across but some are up to 2 mm. The concretions weather rapidly to red and yellow oxides; lepidocrocite and goethite have been detected in the weathering products.

Non-spheroidal siderite occurs as a fine-grained cement in some silty sands, but such developments are generally thin and impersistent.

Stratigraphy

Central trough

The succession proved by IGS Petrockstow Borehole No. 1, approximately in the centre of the basin, is shown in (Figure 6). Brown and brownish grey more or less sandy clays, with some lignites, extend to a depth of 135 m and are underlain by grey rather sandy clays with few brown clays and lignites to 316 m, below which a band of gravel and sand with some clay rests on sands and very sandy clays, intensely red-stained between 564 and 601 m. The lowest beds comprise lignitic laminated sands underlain by sands and gravels; the gravels coarsen towards the base, at 660.5 m.

Correlations between the three IGS deep boreholes (Figure 7), located along the axis of the basin, may be based on heavy mineral analyses or pollen analyses. Dr C. Turner reports that pollen types indicate a junction between Oligocene and Eocene at about 120 m depth in Borehole No. 1, and about 240 m in Borehole No. 2 to the north-west; pollen from Borehole No. 3 was poor and sparse.

Heavy mineral analyses of the sands showed abundant topaz and coarse fragmental reddish brown tourmaline between 305 and 320 m, and in the sands and gravels below 550 m, in Borehole No. 1; above average quantities of topaz occurred at about 12 m. Borehole No. 2, 1500 m to the northwest, showed much topaz between 111 and 133 m. Correlation of this zone with the horizon at 12 m in Borehole No. 1 accords best with the pollen evidence. The topaz-rich sands between 305 and 320 m in Borehole No. 1 probably correlate with similar sands at around 53 m in Borehole No. 3 to the south-east. Similarly the topaz-rich sands and gravels towards the base of Borehole No. 1 may be correlated with similar sediments below 293 m in Borehole No. 3. (Figure 7) illustrates the proposed correlations along the NW–SE axis of the basin and shows that the thickest part of the succession lies between Boreholes Nos. 1 and 2. This agrees fairly well with the evidence of the residual gravity anomaly (p. 61). These suggested correlations indicate overlap at the north-western end of the basin, and this is in accord with the restriction of lacustrine sediments to the northern half of the basin, which indicates more rapid subsidence in the north than in the south.

North-eastern shelf

A typical succession in the shelf area (Figure 6) shows 9 to 18 m of extremely sandy clay, sand, silt and gravel resting on Bude Formation rocks, which commonly show alteration, probably due to Palaeogene weathering, down to about 20 m. The alteration effects are softening of the Carboniferous sandstones, siltstones and shales, production of a stronger green colouration in the grey-green sandstones, and formation of kaolinite at the expense of illite and chlorite which are the normal clay minerals of the Carboniferous shales. In places small fractures in the rocks are filled with veins of white kaolinite. Red and yellow staining is characteristic of the basal Tertiary beds as is spherulitic siderite, both disseminated and as solid concretionary masses. The concretions are common just above the bottom gravels and sands, about 3 to 6 m above the base.

The sequence above the basal sandy beds contains up to nine fining-upward cycles of sands and clays (p. 29); the sand content is variable and the clays are commonly brown and lignitic. The bases of the smooth to slightly sandy brown clays within the cycles are indicated by the letters A to I in (Figure 6). Discontinuous coarser sands and gravels within the cycles are restricted to the south-western margin of the shelf.

Correlation of the shelf succession with the thicker sequence of the central trough is difficult owing to facies changes (Figure 8). Dr C. Turner believes the shelf succession to be wholly Eocene and therefore that it should be correlated with beds below 235 m in Borehole No. 2 and between 110 and 385 m in Borehole No. 1. The sands and gravels at the base of the shelf succession contain no topaz and therefore probably do not equate with similar but topaz-rich sediments proved in boreholes Nos. 1 and 2. Thus the downthrow on the NW–SE fault separating the shelf from the trough near Borehole No. 2 is in excess of 105 m. The earliest sediments in the basin are those proved below 385 m in Borehole No. 1, and lying below the drilled depth of Borehole No. 2. These lowest beds may also occur below about 260 m in Borehole No. 3. It appears that sedimentation overlapped the internal shelf-bounding faults and continued over a wider area at least into late Eocene times. The topmost Oligocene beds and some top Eocene strata are now restricted, as a result of erosion, to the axial trough area; hence it is not possible to estimate their maximum areal extent of deposition.

South-eastern marginal area (Meeth)

The following stratigraphy, based on recent boreholes, has been provided by Mr B. L. Jones of E.C.C. Ball Clays Ltd.

This succession is comparable to that of the north-eastern shelf, although sandier and much less carbonaceous. Its maximum development is in the Stockleigh Moor area whence it attenuates south-eastwards towards Meeth Claypit, partly owing to thinning of the beds but mainly because of erosion of the upper strata and the upper cycles of the ball clay sequence.

Correlation with the Woolladon area has not been established, but the successions show some similarities.

Cutting across the succession in a roughly NW–SE direction are channels filled with grey sands and lignitic brown laminated sands, the laminated sands commonly showing contemporaneous microfaults. Scattered wispy seams of gravel are also present within these channels.

A spur of Carboniferous rocks projects north-westwards into the basin (Figure 15). The sediments which shelve up against this spur are dominantly very sandy and iron-stained; they contain many seams of lignitic sands, and also gravels.

South-western shelf area (Woolladon)

The Woolladon succession was deposited in a small cuvette with a major NW–SE fault along its south-western margin and the projecting spur of Carboniferous rocks to the north-east. The sediments resemble those of the northeastern shelf area, but are much richer in lignitic brown clays and lignites than are those around Meeth and are cut by sand-filled channels. The base of the succession is exposed in the south-eastern face of the Woolladon Claypit; fine quartz gravels rest on intensely weathered Carboniferous shales and sandstones, and the clay minerals of the shales have been partly converted to kaolinite (Bristow, 1968). The presence of a basal gravel is unusual in this area. Boreholes penetrating the junction usually pass from kaolinitic silty and sandy clays of the Palaeogene into pale-weathered kaolinitic shale of the Carboniferous; in some cases the transitional junction extends through a metre or two, particularly where the Carboniferous lutites consist of mudstone rather than laminated shale.

The total thickness of strata, seen mainly in boreholes, within the Woolladon basin amounts to about 65 m. The lowest 5 to 10 m comprise clays, sands, silts and gravels, commonly red-stained and containing scattered spherules of siderite. They are succeeded by 10 to 15 m of more or less silty clays with, particularly in the south-western part of the area, some smoother brown lignitic clays. The overlying beds of sand, locally lignitic, cut down into this sequence and may be seen in places to occupy erosional channels; they contain some brown laminated lignitic sands and a few lignitic clays. The highest strata are mostly very sandy and commonly stained red and yellow.

No correlation is possible between the Woolladon succession and that proved by Borehole No. 3, chiefly because the sediments from that borehole were rather nondescript and lacked good microfloral assemblages. Although the deposits at Woolladon lie just above the base of the Palaeogene in that area, they cannot safely be correlated with basal sediments in Borehole No. 3 owing to the probable southward overlap of strata.

Sedimentology

Cyclical sedimentation

Borehole No. 1 proved an asymmetrical cyclical pattern of sedimentation (Freshney, 1970), a pattern evident also in boreholes 2, 3, 4, 5 and 6 and in many commercial boreholes and pit sections. Cycles typical of different parts of the basin (Figure 9) are all fining-upward cycles (Allen, 1965) with erosive bases. Those in IGS boreholes Nos. 1, 2 and 3, from the axial trough, commonly show the upward transitional sequence sandy gravels–coarse sands, locally brown and lignitic–clayey sands–silty clays–smooth clays. The uppermost part of the cycle is usually brown, particularly in the northern part of the trough, and in places contains lignites. Seatearths with rootlets are common, generally immediately below lignites or brown lignitic clays, and many are associated with red hematitic staining, yellow-brown iron oxide staining or bluish grey staining attributable to finely disseminated marcasite. Spherulitic siderite is also common around these horizons.

Borehole No. 2 ((Figure 9)A), in the northern part of the axial trough, showed finely laminated 'varved' brown clays in the upper parts of the cycles above the seatearths. Similar clays occurred in some of the cycles in Borehole No. 1. Borehole No. 3 (Figure 9), in the southern part of the axial trough, showed few smooth clays at the tops of the cycles, and generally the fining-upward sequence terminated in grey to grey-brown rather silty clays. The bases of the cycles are erosive, and some show contacts dipping at quite high angles with respect to the bedding.

Cycles in the north-eastern shelf area show no basal gravels, and the sequence above the erosive bases is typically fine silty sand–silty clay–rootlet bed–dark brown smooth lignitic clay ((Figure 9)D). In the Woolladon area several cycles are exposed in the working Woolladon Claypit. One shows the upward sequence brown sand, locally lignitic and laminated–extremely silty clay containing rootlets–smooth lignitic clay–grey-brown silty clay. The base of the succeeding cycle is erosive; at one stage during working, a sand-filled channel about 1.5 m across and 1 m deep was exposed, together with the edge of a larger channel alongside.

The cycles in the Petrockstow Basin differ from the fluvial fining-upward cycles described by Allen (1964) from the Old Red Sandstone in that the sands do not show extensive sharp cross-stratification. Rattigan (1966) described fining-upward sequences in the Italia Road Formation of New South Wales, from erosive bases through rather structureless sandstones to kaolinitic shales containing coals and other plant debris. His suggestion that the sandstones represented sand sheets produced by flash floods sweeping over the flood plain may well apply to some of the sands at Petrockstow, particularly those in the shelf areas. The cyclic pattern at Petrockstow may reflect periodic movements in the fault zone, the natural cyclicity of fluvial sedimentation or variations in climate. The frequency of the cycles ranges from about 10 per 100 m of strata in Borehole No. 1 to about 25 per 100 m in Borehole No. 3. Tectonic effects would probably be widespread, and it is most likely that a high frequency of cycles simply reflects close proximity to shifting river channels.

Facies variation

Broad facies changes may be grouped into those of the axial trough (Figure 3) and those of the flanking areas.

The axial trough shows a strong facies variation along its length. Extremely sandy clays, coarse sands and gravels with a little brown clay predominate in the south-east. Farther north-west the clays are less sandy and accompanied by some smooth brown clays and lignites. The 'varved' brown silts and clays with undisturbed leaf remains (pp. 21, 23) were common in Borehole No. 2, in the north-western part of the basin, less so in Borehole No. 1, and absent from Borehole No. 3, in the south-eastern part of the basin, and from the shelf areas.

The sediments of the north-eastern shelf area, in contrast to those of the axial trough, contain a good deal of brown smooth to slightly silty lignitic clays and few coarse sands and gravels. Such coarse material as is present is confined to basal sand and gravel and to tongues of gravel and coarse sand along the south-western margin of the shelf; the latter probably mark an abrupt transition from shelf facies into the adjacent axial trough.

The Woolladon shelf area similarly shows much brown clay and lignitic material, but also some fairly coarse channel sands which are more typical of the trough.

Small-scale facies variations are so abrupt that in places boreholes sunk only a few metres apart in strata of low dip cannot be correlated with certainty (Figure 10). In some cases the use of electrical logging assisted in the correlation. The most variable factors are the amounts of sand and, particularly in the shelf areas, lignitic material. The extent of staining also varies, as does the content of spherulitic ironstone. Boreholes Nos. 2 and 4 showed passages from 'varved' clays into unlaminated brown clays ((Figure 10)a).

Conditions of deposition

The sharp differentiation into a longitudinal zone containing sands and gravels and flanking areas with few sands and gravels, absence of animal fossils, the presence of lignite, the clay mineralogy (p. 21), the distribution of grain sizes in the elastic materials, and the high maturity of the sediments (p. 21), all indicate a continental, probably a fluvial, deposit. Using maximum grain size, and graphical representations of grain size distribution such as CM diagrams (Passega, 1964), it is possible to differentiate the sediment types. The CM plot (Figure 11) for the sediments of the Petrockstow Basin relates most of the sands of the axial trough to the graded suspension within a river channel, and the coarse sands and gravels to the bed load. The sandy clays all fall within the field of the uniform suspension; they form most of the overbank deposits, which characterise the shelf areas but also occur within the axial trough. The fine silty sands near the bases of the cycles in the shelf areas fall within the finer and somewhat better sorted part of the graded suspension field and probably originated in crevasse splays and flash floods. A relatively small number of smooth clays were analysed mechanically.

They fall chiefly within the pelagic suspension field, are commonly carbonaceous and associated with lignite, and were probably deposited in overbank areas farthest from the river, in back-swamps. Particle size analyses of the finely laminated 'varved' silts and clays common in Borehole No. 2 show that they belong to a graded suspension. The finely graded nature of the individual laminae, with their sharp bases (p. 21), suggests very small-scale turbid flows within small standing bodies of water. The common presence of unbroken leaf remains also points to accumulation in the quiet waters of small lakes or ponds. Probably the north-western part of the basin contained a varying number of small ponds rather than a single large lake, and abrupt transitions from finely laminated silts and clays into lignitic worm-burrowed clays reflect the difference between undisturbed sediments lying beneath water and the surrounding bioturbated richly organic muds.

Fining-upward cycles may be related to fluvial activity. Gravels and gravelly sands with erosive bases suggest the establishment of a river channel with lag gravels at its base, and overlying sands may reflect the vertical accretion of point bar sands. The sandy clays above represent the infilling of the channel, with the establishment of a subaerial cover of vegetation. The succeeding smoother clays are overbank and back-swamp deposits laid down some distance from the migrating river channel. In some places subsidence produced semi-permanent lakes and ponds in which finely laminated clays and silts accumulated. The occurrence of silty bands at the tops of some cycles perhaps indicates the approach of the river channel with its suspended load of fine sand and clay. Basal beds of the succeeding cycle, where they comprise coarse sands or gravels, probably indicate the redirection of the river across the area; where they are of fine sands and silts they may represent a crevasse splay and sheet flood derived from the river. Within the shelf areas the basal sands, silts and sandy clays of the cycles reflect crevasse splay material and sheet floods, and fine upwards into more clay-rich overbank deposits. Subaerial surfaces are marked by rootlet beds, and sand-free clays, usually carbonaceous, were deposited in back-swamp environments. Montmorillonite occurs together with kaolinite in the more carbonaceous back-swamp clays, and locally constitutes up to 30 per cent of the clay minerals present; modern swamp deposits in Florida are similar in this respect (Huang and Freshney, in press).

Staining by iron compounds occurs chiefly within the sandy parts of the fining-upward sequences below the rootlet beds. Probably the finely divided marcasite formed in waterlogged sediments in acid and anaerobic conditions and in the presence of organic material. A subsequent fall of the water table, allowing the ingress of air, would permit oxidation, in some cases to yellow oxides of iron, in others to red oxides (Freshney, 1970). According to Curtis and Spears (1968, p. 260), siderite can form within a sediment only in conditions of low Eh, zero sulphide activity and severely restricted water circulation. The Petrockstow sediments reflect swampy conditions with a high water table, and with water circulation restricted by clays; zero sulphide activity would characterise only those horizons lacking organic material.

The more extensive gravels and gravelly sands, such as those at the base of the shelf deposit in the north-east, were probably laid down in a braided river system rather than one differentiated into channel and overbank. To the south-east, at Cadham Farm in the Okehampton district (Edmonds and others, 1968), gravelly sands constitute much of a thin Palaeogene deposit; this suggests a braided river system to the south-east.

Palynological evidence leads Dr C. Turner to suggest a subtropical climate with palms, ferns and heathers and many plants with swamp affinities. Pollen from fagaceous trees was probably derived from forests established on drier ground away from the river.

The presence of kaolinite as the dominant clay mineral in the Petrockstow Basin is also indicative of subtropical to tropical conditions; it is common at present in soil profiles of such areas. Many boreholes penetrating the base of the Palaeogene at Petrockstow pass into intensely weathered Carboniferous rocks. Bristow (1968) showed that this weathered mantle was characterised by kaolinite rather than micas and chlorites, and must have constituted a source of eroded material available for transport in the adjacent river or rivers. Particle size analyses suggest derivation of the Palaeogene sands from Carboniferous sandstones. The gravels are made up almost entirely ofvein quartz, commonly with small fragments of Carboniferous sandstone attached. Clay minerals were also mainly derived from Carboniferous rocks, and Bristow (1968) noted that the sequence fresh Carboniferous shales–weathered Carboniferous shales–ball clays coincided with diminishing contents ofmobile elements, such as iron, magnesium and the alkalis, and increasing contents of such immobile elements as titanium.

Most of the heavy minerals within the sands of the Petrockstow Basin were probably derived from existing sediments. Heavily abraded zircons and tourmalines predominate, and most likely many, especially the zoned zircons, originated in the Cornubian granites but were incorporated within Mesozoic sediments en route to the Palaeogene sands. The abraded staurolite may also have been recycled but its origin is obscure; perhaps it came from a southerly zone of higher metamorphic facies such as occurs in the Brioverian or Pentevrian rocks. Some horizons show abundant topaz and unabraded fragments of red-brown and green zoned tourmaline; locally, as in some of the basal sands and gravels, topaz makes up 80 per cent of the heavy mineral separation. These minerals may have been directly derived from the roof of a granite or from associated granitic intrusions; recycling through Mesozoic rocks would have dispersed the topaz. There can be little doubt that some granite was exposed in early Tertiary times. Topaz is extremely common in the Buller's Hill Gravel of the Haldon Hills and the Bagshot Beds sands and gravels of Dorset. Also much of the kaolinite in the Southacre Clay and Lignite of the Bovey Basin was derived from the Dartmoor Granite (Vincent, 1974). Nevertheless despite the presence of some Dartmoor minerals at Petrockstow, derivation from local sediments remains the most likely principal mode of origin. The crystallographically highly disordered kaolinite came from the Carboniferous shales.

The general fineness of grain of the Palaeogene deposits suggests that the country bordering the Petrockstow Basin was of low relief with sluggish streams. A few influxes of coarser gravel containing topaz and tourmaline from Dartmoor may reflect spasmodic tectonic movements which increased the gradient from Dartmoor towards Petrockstow, resulting in fast-flowing streams near Dartmoor which passed into a braided river running north-west through the Cadham Farm area and the Petrockstow Basin. The occurrence of lignitic and red-stained clays in the Palaeogene basin off Lundy (Fletcher, 1975) suggests that continental fluvial conditions existed in that area. In Palaeogene times there may have been a Cornubian peninsula with rivers flowing north-west and south-east from its spine towards marine areas in the present Celtic Sea, the English Channel and the Hampshire Basin.

Structural control of sedimentation

Facies relationships within the Petrockstow Basin indicate a change from channel deposits to flood plain and finally to back-swamp. All the deposits lie within the compass of the existing fault-bounded trough, and it is unlikely that they ever spread far to the south-west or north-east. More or less constant conditions of sedimentation were maintained by continuous subsidence throughout the period of deposition. The sharp facies change between the north-eastern shelf area and the axial trough is marked by a NW–SE wrench fault (p. 42). The shelf facies of the Woolladon area is limited to the south-west by a NW–SE fault, and to the north-east by a spine of Carboniferous rocks which is probably fault-bounded. Large-scale slumping and brecciation common between 220 and 270 m in Borehole No. 2, and at several horizons in the adjacent Borehole No. 4, were probably brought about by the development of small fault scarps which caused movement of unconsolidated sediments.

Subsidence within the Sticklepath Fault Zone appears to have reached its maximum in the area between boreholes No. 2 and No. 1; thus overlap developed on the northwestern flank of the deposit, and the greater subsidence at this end resulted in lacustrine sedimentation.

The presence of many Palaeogene fluvio-lacustrine deposits along the Sticklepath Fault Zone attests its control of sedimentation across the peninsula and beyond into the Bristol Channel. ECF

Details

Useful sections are limited to four claypits, one man-made river diversion channel and a large number of boreholes : sections in working pits are, of course, generally temporary. Borehole logs given below have been selected as examples of typical sequences in different parts of the basin.

Axial Trough

IGS Petrockstow No. 2 Borehole [SS 5110 1158], topmost 113 m, typical lithologies of the northern part of the trough :

River diversion channel [SS 5124 1180] to [SS 5157 1148] :

IGS Petrockstow Borehole No. 1A [SS 5200 1040], topmost 100 m:

IGS Petrockstow No. 3 Borehole [SS 5278 9033], topmost 61 m:

Meeth area

Adit mouth [SS 5358 0888], strata dip 12°/010°

Meeth Claypit [SS 5386 0856] :

Meeth Claypit [SS 5372 0857] :

North-eastern shelf area

Westbeare Pit, section between [SS 5138 1219] and [SS 5129 1200] :

Courtmoor Pit, section between [SS 5206 1159] and [SS 5195 1156] :

Borehole [SS 5164 1204], lower part of shelf sequence:

Borehole [SS 5158 1176], higher part of shelf sequence:

Borehole [SS 5046 1281], typical northern shelf sequence:

Woolladon area

Pit section between [SS 5296 0778] and [SS 5290 0785] :

ECF

Chapter 4 Structure

General account

Upper Carboniferous

The Upper Carboniferous rocks of the Chulmleigh district were folded and faulted mainly during the Variscan orogeny. They represent a northward continuation of the similar structures described in the Okehampton Memoir (Edmonds and others, 1968), and an eastward continuation of those beautifully exposed in the cliffs between Bude and Hartland Point.

The rocks straddle zones of northerly overturned folds, upright folds and southerly overturned folds (Dearman, 1969, fig. 1; Simpson, 1970, fig. 1; Freshney and Taylor, 1971, fig. 8) and lie within the suprastructure, or blanket, of folds which probably developed in late Upper Carboniferous and early Permian times (Freshney and Taylor, 1971, pp. 241–248). Both folding and faulting are mostly due to horizontal compressive stresses directed approximately north–south within the uppermost structural levels, with some contemporaneous and later adjustment effected by gravitational movement. East–west faulting affected Permian sediments north of Okehampton (Edmonds and others, 1968), and it is possible that similar faults in the Chulmleigh district were active in post-Permian times as a result of relaxation of the north–south pressure.

Folding

A multitude of periclinal folds shows trends approximately WSW–ENE and gentle plunges to east or west. The folds are generally upright, but may be slightly overturned to the north in the northern part of the district and to the south in the southern part. Rare recumbent folds are probably analogous to similar structures within the Upper Carboniferous rocks between Widemouth Bay and Hartland Point which are thought to be the result of gravitational adjustment (Freshney and Taylor, 1971, p. 244). Although complete folds are not commonly seen, the general pattern of folding can be deduced from attitudes displayed in individual exposures (Figure 13) and from stereographic plots of poles to bedding planes (Figure 12). Such good exposures as are present, chiefly in the valleys of the rivers Torridge ((Figure 13)a), Taw, Mole and Little Dart and alongside the main roads running adjacent to these rivers, show that the folds are usually straight limbed and of the same type as those in the coastal sections between Bude and Hartland Point. Larger scale anticlinoria and synclinoria may be traced across the district in the pattern of outcrop of the Carboniferous formations. Thus the disposition of the Bude Formation points to the presence of east–west synclinorial belts from Little Torrington towards Roborough, from King's Nympton towards Meshaw, and just north of the southern margin of the district. The presence of anticlinorial axes is suggested by the lines of outcrop of the Gastrioceras listeri horizon; one such axis runs east-north-eastwards from the area between Dolton and Beaford towards Colleton Mills [SS 657 176] and Meshaw Moor [SS 760 180]. These larger structures are also present on the Bude–Hartland coastline, and it seems probable that they are at least in part contemporaneous with the smaller folds. No overall regional plunge can be detected.

(Figure 12) shows contoured stereograms of poles to bedding planes for each National Grid six-inch quarter sheet of the 1:50 000 map, except that sheets SS 70 NW and NE are combined. The stereograms suggest a general younging from north to south, except for some northerly younging along the southern margin of the district on the south side of the main synclinorial axis of the Carboniferous rocks. Plots for the northern strip of country indicate a sheet dip of 35°–45° S, while those for the adjacent strip to the south show a dip of about 25°S. There is some indication of low to sub-horizontal sheet dips in the area extending east-north-eastwards from Dolton towards Meshaw Moor, an anticlinal area delineated by the Gastrioceras listen horizon. The stereographic diagrams also indicate that the folding shows no appreciable regional plunge.

Fracture cleavage is fairly common in shale bands interbedded with siltstones and sandstones, but is usually restricted to the hinge areas of folds. Within the thicker masses of shale, minor folds with limb lengths of a few metres are common and the cleavage tends to be more pervasive.

Faulting

The presence of a few major east–west faults on the coast between Bude and Hartland Point suggests that similar fractures probably cross the Chulmleigh district. Outcrops of the Hartland Quay Shale at Langridgeford [SS 572 224] (p. 14) lie south of the outcrop of the Bideford Formation, which apparently youngs predominantly southwards, and shales probably equivalent to the Hartland Quay Shale are estimated to lie 900 m below the top of the Bideford Formation in the Atherington area. This suggests the presence of a fault with a minimum northerly downthrow of 900 m. A similar repetition on the coast is attributable to a fault south of Westward Ho! (Edmonds and others, 1979), and although projection of this fracture inland is locally conjectural, it is thought to be affected by NW–SE dextral wrench faults and to pass just south of Atherington. Downthrow is northerly and probably between 900 and 1000 m. Evidence from the coast (Edmonds and others, 1979) suggests that the fault is steep and possibly reversed.

Most important of the dextral wrench faults are those of the Sticklepath Fault Zone, which crosses the south-west corner of the district. Movements within this zone have been responsible for both the sedimentation and the preservation of the Bovey Formation in the Petrockstow Basin. Some of the many NW–SE faults may be little more than joints, with only minimal movement. Others are pronounced and continuous and may be traced across country on the evidence of spring lines, local reddening and the displacement of topographical features, the last being commonly most readily apparent on aerial photographs. One major fault zone enters the district in the north-west, passes a kilometre or so to the east of Great Torrington, and thence southeastwards down the valley of the River Torridge through Beaford Bridge and on to the neighbourhood of Broadwood-Kelly [SS 617 058]. Other major zones can be traced from Huntshaw [SS 506 229] through St Giles in the Wood and Dolton Beacon [SS 590 135] and on towards Winkleigh, and from Alverdiscott through Sugworthy [SS 590 181] towards Ashreigney and Wembworthy. In the vicinity of Atherington displacements of the outcrop of the Cornborough Sandstone indicate a total dextral movement of about 600 m.

A subordinate set of wrench faults trends NE–SW and NNE–SSW. No clear evidence is available of their direction of movement, but if they are complementary to the main set then it is probable that they are predominantly sinistral.

It is likely that the wrench faulting was initiated during the Variscan orogeny, at the same time as or shortly after the development of the folding. It is also known that these faults, and especially those of the Sticklepath Fault Zone, moved dextrally both during and after the deposition of the Palaeogene sediments at Petrockstow. The structure of the Petrockstow Basin suggests not a simple down-faulted rift but rather the transference of dextral movement from one fault to another up to 2 km away, with down-buckling of the intervening ground (p. 33). The axis of the basin runs NW–SE through Petrockstow Station, and it seems likely that the down-warping was accommodated in the underlying more rigid Carboniferous rocks by oblique slip movements along NE–SW faults at right angles to the axis.

Shearman (1967, p. 564) suggested that NW–SE faults in north Devon moved as normal faults during Tertiary times, and it seems likely that normal block faulting also took place later. The Pleistocene terraced river gravels (p. 50) overlying the lower Tertiary beds in the Petrockstow Basin at about 61 m OD must have formed part of a large river valley trending NW–SE and represented in the present landscape by the small valleys of the River Mere and the Little Mere River. Probably the major Pleistocene river valley was disrupted by faulting along NW–SE lines. Similar gravels to the west of the district near Rivaton Farm [SS 458 136] (Ussher, 1879) lie at around 152 m OD in a valley which swings round towards the north-western extremity of the Petrockstow gravels. This suggests that the two may be equivalent, and their difference in altitude due to a fault in the Sticklepath Fault Zone downthrowing 90 m to the north-east. Several small geomorphological flats, locally mantled with clay, in the watershed area between the Taw and Torridge rivers, terminate along the line of known NW–SE faults. It is most unlikely that these features are pre-Pleistocene. Probably therefore the NW–SE faults, and perhaps also the NE–SW faults, remained sporadically active as normal faults until Pleistocene times and later, as evidenced by the small earth tremor beneath Sticklepath in 1956.

Palaeogene

The dominant structural feature of the Petrockstow Basin is the NW–SE Sticklepath Fault Zone, movement within which was responsible both for the deposition of the Palaeogene sediments and for their preservation. The zone consists of a number of individual faults occurring within a band of country up to 1.5 km wide. It appears that activity on different faults has varied from time to time over a long period. Thus as maximum movement on any one diminished, so it was commonly taken up by another fracture running en échelon with the first. Downwarping of the strata between the two faults might have been facilitated at such times. It is possible that movement was transferred from the fault forming part of the north-eastern margin of the Petrockstow Basin near Meeth to the fault forming the south-western margin. At some time movement was also transferred to the internal fault which separates the north-eastern shelf from the axial trough. This transference of dextral wrench movement caused a down-buckle in the intervening ground which amounted to as much as 670 m in the north-western part of the basin. It is unlikely that the relatively rigid Carboniferous rocks folded to produce this buckle, and fairly certain that the movement was accommodated in a series of NE–SW cross faults. The addition of the mass of sediment deposited in the trough tended to perpetuate the subsidence.

Folding

A major syncline, inferred mainly from borehole information, occurs within the axial trough in the northern part of the basin (Figure 8). It trends NW–SE and its axis passes west of Petrockstow Station. Its eastern limb dips 15°–20° SW in the north, and 10°W in the area around IGS Borehole No. 1. The western limb dips 8°–10°E. Little is known of folds in the southern part of the axial trough, except in the immediate vicinity of the Meeth Claypit. There evidence from open pits and old mine records suggests the presence of minor folds trending NW–SE, but the lack of good marker bands such as brown clays and lignites makes delineation of fold structures difficult.

Contours on the base of the Palaeogene and on the base of a seam of brown lignitic clay ((Figure 14)a, b) show rather complex folding in the south-eastern part of the northeastern shelf. One fold (syncline A–B, (Figure 14)b) in an old claypit was recorded on photographs taken during the 1920s and 1930s.

A complementary pericline to the south-west, proved only in boreholes, brings the Carboniferous rocks to within 30 m of the surface near the south-western margin of the shelf. To the north the presence of a minor anticline–syncline pair is indicated by borehole records. This is confirmed by the records of an old mine which worked a seam dipping SW for most of its length but NE at the extremity of the workings. This fold pattern within the Palaeogene probably reflects a network of faults in the underlying Carboniferous rocks, and the apparent folds indicated by contours of the base of the Palaeogene ((Figure 14)b) are more likely small horsts and grabens. To the north-west of this complex zone of folding the strata of the shelf pass into a simple succession of beds dipping 20°–30°SW. There is some suggestion from contours on the base of seam F that it was not affected by the complex structures of the south-eastern end of the shelf and that it may rest unconformably on the strata below, overstepping the folds.

Folding in the Woolladon shelf area was probably controlled by faults and by the NW–SE ridge of Carboniferous rocks (p. 28) which is cut off by the marginal fault. The fold basin, now truncated (Figure 15), may have extended southwest of the marginal fault. Angular differences between the bedding within channel structures and that of the strata below indicate that folding was taking place during the sedimentation.

Faulting

The fault forming the north-eastern margin of the Palaeogene sediments trends north-westwards from west of Meeth to south-west of Huish. Farther north-west the boundary of the Palaeogene appears to be fairly steeply dipping but unfaulted. It is possible either that the bounding fault swings off in a more northerly direction, or that it is displaced westwards, along the line of an E–W cross fault, to become the internal fault separating the north-eastern shelf from the axial trough. This latter fault throws down over 105 m SW in the areas south-west of the present clay workings. Gravity survey (p. 61) suggests that it may dip 45°NE. Clay workers recall an old mine driven south-westwards towards the fault which showed that clay seams dipping steeply south-west 'rolled under' at depth to assume north-easterly dips, thus indicating possible terminal drag on a reverse fault inclined north-east. On balance, however, geological evidence points to a fault dipping steeply south-west. Correlations across this fault between the axial trough and the north-eastern shelf (Figure 8) appear to show a normal movement of about 105 m. The vertical component of movement on this fault diminishes north-westwards. Nor does there appear to have been much horizontal displacement. It seems probable that most of the movement in this north-western part of the basin was transferred to the SW boundary fault.

Geophysical evidence from the south-western part of the basin indicates a fault, probably a splay from the NE boundary fault, which trends WNW towards Petrockstow village. It has displaced the SW boundary fault of the Woolladon area, which runs on north-westwards from the vicinity of Petrockstow towards Peters Marland. This SW boundary fault may have a maximum vertical displacement in the area of greatest downwarp of up to 780 m. Its relationship to the base of the Palaeogene in the Woolladon area (Figure 15) suggests that Woolladon-type sedimentation may have spread beyond the fault to the south-west whence the deposits have since been removed by erosion.

Minor faults are not common in the working pits of the northern part of the basin. In contrast several occur in the Meeth Claypit, trending 060°–070°, dipping 60°–70°SE and throwing down SE. A larger fault, probably a splay of the NE boundary fault, appears to cut the other faults exposed, trends 008°, dips 60°E and throws down over 10 m E. ECF

Details

Upper Carboniferous

Folds

Crackington Formation

St Giles in the Wood–Meshaw Moor

St Giles in the Wood

A small quarry [SS 5227 1848] shows a small fold trending E–W with a northern limb dipping 35°N and a southern limb dipping 50°S. The cleavage in the core of the fold dips 75°S. An anticline trending 088° and overturned to the north is exposed on a steep valley side [SS 5403 1761]; the overturned northern limb dips 85°/178° and the southern limb 50°/178°.

Beaford–Roborough

A stream [SS 5603 1860] exposes a small anticline plunging l2°/246°; the northern limb dips 70°/330° and the southern limb 30°S. In a disused quarry [SS 5661 1858] silty shales with thin sandstones form an anticline which trends 075°; the northern limb dips 67°/345° and the southern limb 45°/165°. Farther south in a stream [SS 5577 1668] a minor syncline and anticline trend approximately 080° and show southerly dips of 45°–50° and northerly dips of about 65°. A small pit [SS 5593 1544] shows siltstones and thin sandstones folded into an anticline which plunges 20°/053°; the northern limb dips 50°/340°, the southern limb 45°/132°. A stream section [SS 5672 1658] to [SS 5686 1638] near Roborough exposes several folds; most are strongly overturned to the north and the beds generally young in a northerly direction. Another small stream [SS 5977 1602] shows a minor anticline and syncline trending 075'; from north to south successive limbs dip 65°/165°, 10°/345°, 10°/165° and 50°/340°.

Burrington

A fold hinge of thick-bedded greyish green sandstone is exposed in a stream [SS 6184 1742]; the fold plunges 2°/067°, the northern limb dips 20°/340° and the southern limb 37°/155°. Farther downstream [SS 6225 1657] a pair of opposing dip directions indicates the presence of a syncline plunging 7°E with the northern limb dipping 30°/165° and the southern limb 85°N.

Colleton Mills–Cadbury Barton

In a small overturned anticline exposed in a stream [SS 6806 1571] the northern (overturned) limb dips 48°/160° and the southern (normal) limb 20°/178°; the axis plunges 10°/240°. Two minor anticlinal hinges in a small ditch [SS 6824 1717] plunge 5°/255° and 5°/257°. A similar hinge [SS 6867 1728] in a nearby stream plunges 10°E.

Area south of Romansleigh

A stream section [SS 7348 1807] to [SS 7354 1755] shows almost continuous exposure of shaly mudstones and flaggy sandstones with alternate steep and shallow southerly dips. This suggests minor folds overturned towards the north. One such fold [SS 7349 1795] plunges 18°/234°, with its northern limb dipping 18°/220° and its southern limb 70°/150°. Another [SS 7351 1757] shows plunge 13°/238°, dip of northern limb 80°/150° and dip of southern limb 15°/205°.

Area south and south-east of Meshaw

An anticline exposed in a stream [SS 7518 1777] shows northern limb dipping 80°/345°, southern limb 20°/172°, plunge 4°/256°. A minor fold hinge in a stream [SS 7686 1535] plunges 18°/070°. To the east-north-east a syncline seen in a ditch [SS 7702 1529] plunges 2°/287°, with the northern limb dipping 20°/170° and the southern limb 15°/007°. About 400 m farther north in the same ditch [SS 7707 1570] an anticline trends 130°; its northern limb is sub-horizontal and its southern limb vertical, strike 130°.

Merton–Dolton

Merton area

A number of minor folds are exposed alongside a forestry track [SS 5420 1429] to [SS 5412 1441] running north-west from Beaford Bridge. Bedding strike is generally about 075°, and most of the folds are overturned towards the north with inverted limbs dipping 65°–80°S and normal limbs dipping 25°–50°S.

A section of the eastern bank of the River Torridge [SS 5499 1363] shows massive sandstones, shales and thinner sandstones disposed in a series of anticlines and synclines trending about 080° and slightly overturned towards the north.

Dolton

Fold hinges, and opposing dips indicating the presence of folds, are commonly exposed in small tributary streams of the River Torridge west and north-west of Dolton. One such stream between [SS 5605 1293] and [SS 5612 1311] shows several tight folds overturned to the south. Another fold [SS 5606 1296] in the same stream has an overturned limb dipping 85°N and a normal limb dipping 6°N. A roadside section [SS 5643 1348] to [SS 5648 1339] displays a series of folds in sandstones and shales. The folds are nearly upright, with limbs dipping 50°–70°N and 50°–60°S, and the general younging direction is towards the south. In small stream and trackside exposures [SS 5657 1456] an anticline in well-bedded sandstones shows a northern limb dipping 60°/345° and a southern limb dipping 45°/160°. An anticline and complementary syncline exposed in a small stream [SS 5773 1392] show, from north to south, limbs inclined 25°/170°, 25°/038° and 80°/165°. The syncline plunges 10°/104° and the anticline 22°/065°.

West Worlington

A number of small folds may be seen in streams to the north of West Worlington. One [SS 7674 1487] has a vertical northern limb striking 080° and a southern limb dipping 65°/176°, thus producing an axial plunge of 10°/260°. In an adjacent stream to the east [SS 7720 1496] an anticline trends E–W with its northern limb dipping 80°/010° and its southern limb 25°/175°.

Bude Formation

Huntshaw–High Bickington

Of the few exposures in this area, that in a stream [SS 5094 2382] near Huntshaw Water shows a small fold plunging 15°/238° with its northern limb dipping 40°NW; the southern limb cannot be clearly seen, but nearby dips indicate that it is probably overturned and inclined about 80°NW. In a stream farther east [SS 5367 2302] successive dips indicate the presence of an E–W fold whose northern limb dips 50°N and southern limb 60°S. Another stream course [SS 5505 2321] shows a small fold plunging 10°/260° with its southern limb dipping 25°S. Near Langridgeford, a trackside exposure of shales [SS 5705 2247] shows a minor fold plunging 20°/252° with its northern limb dipping 45°S.

King's Nympton–Meshaw

King's Nympton area

An anticline trends 095° in the bank of the River Mole [SS 6620 1910]; the northern limb dips 47°/005°, the southern limb 30°/185°. Another in a laneside [SS 6662 1826] plunges 12°/030° with northern limb dipping 70°/315° and southern limb 22°S. Intermittent roadside sections between [SS 6613 1989] and [SS 6627 1950] expose rocks which dip 40°–60°S; some southerly dips as steep as 85° indicate the possible presence of overturned folds.

Almost continuous exposures in a stream north of King's Nympton [SS 6800 2058] to [SS 6825 2011] show dips of 17°–50°S in beds probably all the right way up. This is consistent with a position near the northern margin of the synclinorium containing the Bude Formation.

Romansleigh-Meshaw

A stream section [SS 7317 2003] shows an anticline plunging 8°/083° and a syncline plunging 4°/080°. The attitudes of the limbs are, from north to south, 25°/009°, 45°/164° and 10°/017°. A stream farther east [SS 7546 2033] has exposed a syncline plunging 10°/286°, with the northern limb dipping 60°/185° and the southern limb 15°/330°.

Mudstones with thin sandstones exposed in a stream [SS 7822 2088] on the eastern edge of the district are disposed in a minor fold plunging 20°/065°; cleavage associated with this fold dips 80°/155°.

Little Torrington-Woolleigh Barton

Many good examples of folding can be seen in roadside sections and quarries along the valley of the River Torridge south of Great Torrington. In one quarry [SS 4950 1775] an upright anticline in thick sandstones is exposed; the fold plunges 14°/078° with the northern limb dipping 50°/357° and the southern limb 50°/156°. Immediately to the north, alongside the A386 road, dips indicate the presence of a syncline plunging 10°/080° with limbs dipping 45°/158° and 50°N. A further 350 m NW along the road an anticline plunges 10°/088° with limbs dipping 50°/170° and 70°/356°. An almost continuous section borders this road south of Great Torrington [SS 5000 1900] to [SS 5000 1825] and the structural pattern displayed is illustrated in (Figure 13)a; most of the folds are upright, except that the most southerly is overturned towards the north, and the section indicates a general younging towards the south.

Exposures showing opposing dips occur alongside the B3220 road [SS 5019 1750] to [SS 5071 1745]. An anticline exposed in a quarry [SS 5022 1754] plunges 10°/080°, with the northern limb dipping 20°/024° and the southern limb 40°/155°. About 200 m to the S, in the steep western bank of the River Torridge [SS 5023 1734], a syncline is picked out by a massive 4.5-m sandstone; the northern limb dips 40°/155° and the southern limb 45°/022°. Dips in a roadside section [SS 5137 1707] to [SS 5178 1770] reflect the presence of many smaller folds. Dips are typically 15°–35°/340° and 55°–75°/340°. The deduced pattern of folding indicates younging towards the southeast.

An anticline with a gently undulating crest is exposed in the River Torridge [SS 5150 1608]; its limbs dip 75°N and 30°S. Intermittent exposures on the steep eastern side of the valley [SS 5353 1595] to [SS 5336 1537] point to the fold pattern shown in (Figure 13)c, although no fold closures were seen. This pattern indicates southerly younging, which conflicts with the presence, about 100 m to the S, of the southern margin of the synclinorium enclosing this outcrop of Bude Formation; perhaps therefore this margin is faulted.

Merton–East Worlington

Merton area

A small quarry [SS 5363 1266] contains a minor fold overturned towards the south and plunging 11°/076°. The northern limb dips 15°/032°, the southern limb is overturned and dips 50°/335°.

A trackside section [SS 5444 1168] to [SS 5496 1120], which includes a number of small quarries, exposes sandstones and shales in anticlines and synclines which trend about 070° and are overturned towards the south ((Figure 13)d). One set of limbs is normal and dips 25°–50°N; the other dips at angles between 70°S (normal) and 65°N (overturned). The section is 660 m long, but shows no dominant younging direction.

Ashreigney

In the Mully Brook [SS 6232 1463] an anticline trending 055° has limbs dipping 80°/325° and 64°/145°. In a stream [SS 6368 1080] near Hollocombe grey sandstones and shattered soft shales show opposing dips which suggest the presence of a syncline plunging 8°/235° with limbs dipping 76°/145° and 85°/321°.

River Taw and Chulmleigh

Brown sandstones in a railway cutting [SS 6675 1369] are folded anticlinally; the fold limbs dip 45°/345° and 43°/175°, and the resultant plunge is 5°/250°. A roadside section [SS 6640 1155] shows an overturned syncline; the inverted limb dips 72°/350°, the normal limb 46°/018°, and the resultant plunge is 35°/064°. A quarry about 350 m to the SE exposes an anticline which plunges 14°/086° and whose limbs dip 57°/004° and 65°/170°.

Little Dart River

Fold structures are commonly exposed in the bed and on the banks of the Little Dart River upstream of Chawleigh. A syncline-anticline pair [SS 7200 1304] trends 070° and shows limbs, from north to south, dipping 50°/148°, 60°/340° and 40°/160°. In a small quarry about 400 m to the ENE an anticline trends 050° with limbs dipping 30°/310° and 60°/140°. In the river about 350 m E of the quarry there is a syncline-anticline pair trending 073°. Another anticline seen in the river [SS 7543 1326] trends about 078° and shows limbs dipping 45°/340° and 80°/175°. This anticline has been traced west-south-west for 350 m across several meanders of the Little Dart and is displaced about 100 m dextrally by a NW–SE wrench fault. The hinge of a major syncline about 150 m to the N [SS 7526 1331] shows limbs dipping about 60°S and 15°–30°N and is displaced by the dextral wrench fault. An anticline exposed in a quarry [SS 7643 1239] trends 075° and has limbs dipping 35°/345° and 40°/l65°.

East Worlington

Many folds have been exposed by the Little Dart River and its tributaries. An anticline-syncline pair [SS 7832 1518] trends E–W and shows limbs, from north to south, dipping 35°/180°, 80°N and 50°/180°. The folds are overturned towards the north. Several anticlinal and synclinal axes were noted around Drayford. In the Adworthy Brook [SS 7798 1391] an anticline and syncline trend 080°; the limbs, from north to south, dip 60°/170°, 70°/350° and 60°/170°. A further anticline, exposed in a quarry [SS 7798 1381], trends about 080° with limbs dipping 70°/350° and 40°/170°. In the Little Dart River a synclinal axis is marked by dips of 55°S [SS 7841 1377] and 30°/017°. The axis of the succeeding anticline lies a few metres to the south-west, and the southern limb of the anticline dips 25°/152° [SS 7825 1374]. Farther downstream [SS 7793 1356] a synclinal axis trends 095°, with northern limb dipping 60°/185° and southern limb 65°/005°. An anticlinal axis in the river [SS 7633 1324] trends 070° between limbs which dip 50°–75°/345° and 40°/150°.

Petrockstow–Morchard Bishop

Area west of the Petrockstow Basin

A stream section [SS 5065 0713] to [SS 5068 0747] shows dips of 25°–50°N and about 70°S, indicating folds slightly overturned to the south. In another stream [SS 5158 0862] a small fold in slaty siltstones plunges 15°/232°.

Disused railway track [SS 5365 0638] to [SS 5483 0780]

Several anticlines and synclines containing sandstones up to 1.2 m thick with interbedded shales are exposed in a cutting [SS 5370 0632]. The fold limbs dip 40°N and about 75°S and the section is cut by an E–W normal fault. A syncline [SS 5400 0661] shows limbs dipping 47°/195° and 50°/010°, indicating a plunge of 12°/270°. A minor anticline in sandstones is visible in the bank of the River Torridge [SS 5379 0606] to the south of the old railway; the limbs dip 45°/345° and 40°/190° and the fold axis plunges 15°/262°.

Iddesleigh area

A stream section [SS 5695 0926] in flaggy greyish green sandstones with thick shales and siltstones shows several faulted folds trending between 060° and 080°. The most southerly fold is recumbent with a cleavage dipping 12°/l70°. It is succeeded to the north by a syncline overturned to the south, faulted against a rather crumpled syncline and anticline which are also overturned to the south and show a cleavage/bedding lineation plunging 15°/260°. Inverted mudstones and slates in a stream [SS 5962 0827] to [SS 5966 0848] dip 50°–80°N. To the north the section continues in right-way-up beds dipping 10°–30°S, followed by beds dipping 5°–30°N. Structural interpretation indicates a box fold with some overturning towards the south. A minor fold plunges 10°/080° in the farmyard at Bullhead Farm [SS 5875 0726].

Winkleigh and Broadwood-Kelly

The presence of folds in a stream section west of Winkleigh [SS 6087 0804] to [SS 6040 0769] may be inferred from the alternation of opposing dip directions. Dips of 38°S [SS 6079 0795] are succeeded a short distance to the south-west by dips of 50°–80°N, indicating the presence of a fold overturned towards the north. A more upright fold is suggested by dips of 52°S and 40°N [SS 6055 0778].

Eggesford

A stream section [SS 6702 0951] shows, successively southward, dips of 44°/345°, 35°/165° and 70°/350°. This suggests an upright anticline succeeded to the south by a syncline overturned to the north.

Lapford–Morchard Bishop

Greenish grey mudstones and flaggy sandstones exposed in a bank [SS 7266 0773] show dips of 45°NW and 40°SE,'with cleavage dipping at 80° between south-east and south.

The section is cut by a NE–SW fault which has probably fractured the hinge of the upright fold. An anticline with limbs dipping very gently north and south trends 070° in a stream course [SS 7387 0774].

A quarry [SS 7755 0865] in massive greyish green sandstones with flaggy bands displays an anticline trending 093°; the fold limbs dip 40°/009° and 35°/165°.

Bideford Formation and associated Bude Formation

Yarnscombe–Umberleigh

Dips in the Cornborough Sandstone, and in the strata up to 1 km S of its outcrop, are nearly all southward at angles of between 50° and 80°; it is possible that they occur in a limb of a major fold whose axis lies north of the district.

Intermittent roadside sections in the Taw valley [SS 6078 2400] to [SS 6073 2370] show dips of 65°–85°S in beds which are right way up and probably form one limb of a large fold. Similar sections in the River Taw [SS 6060 2314] to [SS 6056 2368] show dips mainly southerly between 46° and 70°, but a few around 60°N; this suggests the presence of upright folding. In another section in the river [SS 6098 2463] to [SS 6102 2424] rocks dip at between about 80°N and vertical; the N-dipping strata are probably overturned, but it is likely that all these beds lie in the same limb of a major fold.

Valley of the River Bray

Roadside exposures [SS 6733 2380] to [SS 6740 2349] show beds dipping mainly 30°–77°S with a few dipping steeply north; thus N-facing folds are present. In the River Bray [SS 6758 2341] to [SS 6757 2312] the rocks similarly dip mainly 72°–80°S, with only scattered exposures showing dips of 65°–69°N, and here it may be deduced that the folding is upright.

Valley of the River Mole

A syncline plunging 13°/285° is exposed in a quarry [SS 7275 2437]; the fold limbs dip 40°/208° and 25°/342°. Other scattered exposures along the valley occur chiefly in quarries and show dips predominantly southward, as where [SS 7259 2338] dark grey mudstones and red-stained micaceous sandstones dip 52°/190°.

A stream section [SS 7300 2141] to [SS 7325 2126] south-east of Alswear cuts across several fold axes; one anticline has limbs dipping 55°/350° and 40°/175°.

Crooked Oak [SS 7582 2308] to [SS 7831 2339] and Rose Ash

Cleavage-bedding relationships within mudstones and shales interbedded with scattered thin sandstone bands indicate the presence of much small-scale folding. Cleavage dips range from 60°N to 80°S. In one place [SS 7810 2325] closely adjacent dips of 50°N and 80°N probably reflect the normal and overturned limbs of a minor fold. In a stream to the south [SS 7823 2088] a minor anticline plunges 20°/067°.

Faults

Crackington Formation

Torrington–Cadbury Barton

St Giles in the Wood

A small fault striking 190° and dipping 70°W cuts sandstones in a small stream [SS 5378 1860].

Beaford

Evidence suggesting the presence of a NW–SE dextral wrench fault was noted in Beaford Brook, north-east of Beaford : at [SS 5545 1570] the strike of the rocks has been rotated clockwise from east-north-east to between 100° and 120°, and at [SS 5550 1563] a fault crush zone within shales trends 140°. The presence of a feature with spring-line roughly parallel to but south-west of the NW–SE brook suggests that the main fault runs south-west of the brook. In a stream section [SS 5742 1520] east of Beaford a fault trends E–W and dips 85°N; a short distance downstream [SS 5727 1513] a brecciated zone with quartz veining trends 140° through sandstone and shows evidence of dextral movement. A fault trending 135° and dipping 70°SW is exposed in the same stream [SS 5753 1527]; farther upstream [SS 5773 1515] traces of faulting suggest a trend of 050°.

Roborough

Reddening and rotation of strikes in a stream section [SS 5696 1772] suggest the presence of NW–SE wrench faults. In another small stream [SS 5977 1608] a fault trends 040° through reddened shales.

Burrington

Shattered reddened sandstones and siltstones crop out in a stream [SS 6247 1628], and reddened rocks occur upstream to the north-west e.g. [SS 6187 1724]; this suggests the presence of a NW–SE fault.

Cadbury Barton

Smashed shale indicates the presence of a fault trending NE–SW through a stream section [SS 6879 1710].

Merton–Chawleigh

Merton

A small stream section [SS 5071 1526] shows sandstones and shales cut by a small fault trending 010°, and in the same stream just to the north [SS 5070 1534] sandstones and siltstones are cut by a fault trending about N–S. Farther east another small stream [SS 5202 1517] traverses shales with thin sandstones which strike about 025° and contain some crushed shales with quartz veining. A fault trending 040° cuts shales exposed in another stream [SS 5200 1513], and nearby drag folds indicate that the movement along the fault line was sinistral. Further evidence for N–S faulting occurs in a steep bank [SS 5299 1463] near Warham where slickensides were noted and some of the strata dip steeply to 100°. A quarry [SS 5474 1472] shows a 3-m sandstone cut by faults trending 175° and dipping 70°W; the fault surfaces display slickensiding pitching 25°N.

Dolton

Sheared sandstones and shales trend 140° in a small stream [SS 5658 1247]. At the side of a track [SS 5740 1486] red-stained sandstones striking NW–SE probably parallel a fault; to the south-east this same fault, or another of the same narrow zone, crosses a stream [SS 5791 1419] in a patch of bright red-stained soil and rock. Traces of faults trending between 150° and 180° were noted farther downstream [SS 5782 1404]. Further evidence of this fault zone occurs where sandstones and shales strike NW–SE in a stream [SS 5888 1284] to the south-east; reddened rocks and deeply red-stained soil were noted to the east and south-east [SS 5955 1261] and [SS 5942 1240] . Small dextral wrench faults trend 124° [SS 5970 1341] and 120° [SS 5972 1342] through red-stained rocks exposed in a stream.

Chawleigh

A small fault with a 150-mm crush zone trends 175° through grey shaly mudstones in the Little Dart River [SS 7114 1373]. A small tributary stream [SS 7181 1387] exposes beds which appear to have been rotated anticlockwise into alignment with a NE–SW fault.

Bude Formation

Huntshaw–High Bickington

A fault trending 045° is exposed in a stream [SS 5118 2384]. A short distance upstream quartz-veined shales in a shatter zone appear to trend NW–SE and an adjacent exposure of siltstones and shales shows strikes in this direction.

Sandstones and siltstones in a quarry [SS 5471 2279] near Cranford are cut by a small fault trending 040° and dipping 50°NW. Massive sandstone in a quarry [SS 5412 2040] to the south-west is cut by fractures trending between 150° and N–S, and exhibits horizontal slickensides and quartz veining. In another quarry about 620 m to the E [SS 5475 2042] massive red-stained sandstones are cut by faults trending 110°; horizontal slickensides, quartz veins and drusy cavities are present.

Traces of strike faulting trending 080° were noted in shales in a stream [SS 5680 2187] near Langridgeford. In a stream [SS 5823 2246] farther east dextral wrench faults trend 140° through ripple-marked and laminated sandstones. To the south-east in the same stream [SS 5872 2214] vertical strata appear to have been rotated clockwise to trend about 110°; still farther south-east in the stream a fault trends 120° and dips 60°SW [SS 5906 2187]. Further dextral wrench faulting is exposed in a small stream near High Bickington; at one locality [SS 5937 2082] the faults trend 142° and dip 50°SW and at another [SS 5906 2068] the faults trend 160°.

Romansleigh

Movement on a fault trending 005° along a stream valley [SS 7179 1917] to [SS 7188 2045] has caused anticlockwise rotation of the strike; locally [SS 7177 1957] the strike has been rotated to a N–S position and the bedding is vertical. Sheared and broken grey sandstones in a stream [SS 7313 2009] probably reflect the proximity of a NW–SE fault.

Little Torrington–Woolleigh Barton

A 5-m sandstone exposed in a quarry [SS 5008 1843] is cut by a small fault trending 120° and dipping 80°SW. Another small quarry [SS 5344 1739] shows well-bedded sandstones cut by a fault which trends 130° and dips 70°SW, and in a stream to the south-east [SS 5394 1705] a fault and attendant quartz veins trend 140°. A section alongside a forestry track on the steep valley-side of the River Torridge [SS 5275 1558] to [SS 5303 1552] exposes deeply red-stained sandstones and shales which are cut by a small fault trending 175° [SS 5290 1553]; the reddening intensifies westwards, accompanied by considerable disturbance of the strata, and it is thought likely that a NW–SE wrench fault passes near the western end of the section about [SS 5276 1557].

Hollocombe

A stream section [SS 6362 1094] to [SS 6368 1068] shows red- and purple-stained strata which appear to have been rotated to a trend of about 030°. This suggests the proximity of a fault, which possibly trends N–S.

Valley of the Little Dart River

A strike fault trending about 085° cuts massive sandstones in the Little Dart River 17441 1295] south of East Cheldon. An anticline–syncline pair near Affeton Mill Bridge [SS 7542 1325] is displaced by a NW–SE wrench fault. The fault itself is not exposed, but the positions of the folds indicate a dextral shift of between 50 and 90 m. Shaly mudstones with sandstone bands in a stream [SS 7606 1229] to the south-east steepen to vertical and appear to have been rotated anticlockwise to a trend of 042°, possibly the result of movement on a fault trending 012°.

Petrockstow–Morchard Bishop

Petrockstow–Meeth

Exposures at the roadside [SS 5118 0986] near North Town show thick shales which have been rotated to trend between 125° and 175°, probably by movement on the major NW–SE fault which bounds the Bovey Formation in this area.

A strike fault trending 070° and dipping 60°S cuts sandstones in the southern bank of the River Torridge [SS 5250 0602]. The old railway cutting [SS 5365 0628] to [SS 5495 0778] to the east is crossed by several NW–SE faults, all within the Sticklepath Fault Zone and marked by reddened strata: at [SS 5405 0662] a fault trending about 105° and dipping 80°N cuts thick sandstones, and in a farmyard [SS 5385 0688] about 300 m to the NW a vertical sandstone bed has been rotated to a strike of 135°; at [SS 5487 0731] shearing trending 060° is associated with bright red staining; between [SS 5501 0751] and [SS 5495 0773] red staining is particularly strong, and at [SS 5497 0772] shear planes trend 055° and dip 80°WNW.

Iddesleigh–Winkleigh

In a quarry [SS 5663 0819] just west of Iddesleigh massive sandstones with shales are cut by a fault which trends 160° and dips 85°W. A stream near Coombe Farm exposes faults trending between 040° and 055° [SS 5784 0937], and sinistral faults trending 010° through thin sandstones [SS 5799 0931]. Traces of faults trending 040° are common in a stream [SS 5923 0853] southeast of Henacroft; intense shearing occurs and strata have been swung round to strike about 040°. Along this same line of fracture to the south-west further evidence of faulting was noted in a stream [SS 5882 0802]. Another stream section [SS 5844 0619] shows sandstones and shales sheared by faults which trend 130°–140° and appear to throw down to the north-east. A fault trending 030° and dipping 70°NW is exposed in the farmyard at East Lake [SS 5882 0611].

Sheared shales and sandstones exposed in a stream [SS 6050 0588] west of Broadwood-Kelly have been disrupted by a fault trending about 110° along the valley.

A NW–SE fault has sheared and rotated siltstones and sandstones in a stream [SS 6150 0797] west of Winkleigh.

Wembworthy–Morchard Bishop

A fault, rending around 040° up a straight valley [SS 6672 0920] to [SS 6230 0986] east of Wembworthy has rotated strata so that their strike is displaced anticlockwise.

A small fault trending 047° cuts an anticline in grey silty mudstones in a bank near a roadside [SS 7267 0775] south-west of Lapford. A quarry [SS 7389 0722] on the east side of the River Yeo shows well-bedded greyish green micaceous sandstones with siltstones cut by a fault trending 030°. A large quarry [SS 7756 0627] south-south-east of Morchard Bishop shows massive greenish grey and purple sandstones cut by branching faults which trend between 020° and 035°. These faults may be splay faults of a larger fracture which trends 010°–015° along the adjacent stream immediately to the east.

Bideford Formation and associated Bude Formation

Yarnscombe–George Nympton

The outcrop of the Cornborough Sandstone is much disrupted by NW–SE dextral faults in the area north and north-east of Yarnscombe. A stream section [SS 5788 2434] to [SS 5794 2445] shows strata to have been rotated clockwise into alignment with the fault trend.

A small stream [SS 6583 2448] west-north-west of Clapworthy exposes finely laminated silty sandstones which have been rotated clockwise from a normal E–W trend to strike 130°; this suggests the presence of a NW–SE dextral wrench fault. Fractured sandstones, probably near the line of a NE–SW fault, are exposed in another small stream [SS 6937 2176].

Palaeogene

Folds

North-eastern shelf

A gentle syncline was exposed in an extension of Watts Blake Bearne & Company's Courtmoor Pit [SS 5188 1148], and borehole records show it to trend NW–SE. The north-eastern limb generally dips at about 25°. The attitude of the south-western limb is more complex; borehole information from the north-west [SS 5168 1173] points to dips in excess of 60°, possibly associated with faulting. A photograph of a pit section [SS 5198 1147] to [SS 5187 1141] taken in the mid-1940s shows a syncline whose north-eastern limb dips 25°SW; the south-western limb is less steep, and locally [SS 5191 1142] sub-horizontal.

A number of adits driven south-westwards from places along a line between [SS 5131 1203] and [SS 5153 1186] proved seams dipping generally at between 10° and 20° but locally steepening to 45°; this may indicate the presence of monoclines or possibly faults to the north-east of and parallel to the fault bounding the shelf.

Axial trough

The presence of a major syncline within the central part of the basin may be deduced from dips of 15°–20°SW in the river diversion channel [SS 5160 1146]; [SS 5136 1165] and evidence from IGS boreholes Nos. 2, 4, 5 and 6 which indicates easterly dips. A section based on commercial boreholes [SS 5198 1071] to [SS 5170 1057] showed a dip of 10°SW in its north-eastern part, diminishing to 2°–3°SW in its south-western part. Thus it is possible that the axis of the main major syncline within the basin passes close to Petrockstow Station [SS 5167 1052].

Meeth

A study of old mining records of the vicinity of Meeth Claypit indicates the probable presence of folds trending NW–SE. In the Meeth Clay Company's Mine No. 7 [SS 5369 0915] workings driven about 500 m SW showed dips of about 7°SW, as did workings driven south-west from Mine No. 8 [SS 5372 0896]. In contrast Mine No. 4 [SS 5357 0888] was worked in the direction 068° and showed dips in that direction of about 8°. Mine No. 1, with its adit mouth at [SS 5379 0879], was driven in a south-westerly direction down-dip in a clay seam. At around [SS 5370 0875] this dip reversed to northeasterly and the seam was followed to the surface to an adit mouth [SS 5349 0864]. Information from other mines and from boreholes suggests that a syncline passes through Mine No. 1 at about [SS 5366 0875], trending 290°.

Dips in the eastern part of Meeth Claypit [SS 5350 0865] are around 12°SW. Similar inclinations prevail, with slight local disturbances, for about 90 m to the SW, where dips are around 15°–30°E. Despite complications attributable to faulting, a fold trending in a rather sinuous fashion at around 020° can be traced across the pit.

North-west of Meeth Claypit, in the area of Stockleigh Moor, recent drilling has shown the presence of a sequence of beds dipping south-west and exhibiting a monocline of arcuate plan which trends roughly parallel to the NE boundary fault. The steep limb of this fold shows dips of up to 45°SW, and the structure is cut by faults running N–S and WNW–ESE. The monocline is comparable to similar structures in the north-eastern shelf area.

Woolladon

A truncated bowl-shaped fold at Woolladon is reflected within the claypit by a swing in the strike from approximately NE–SW in the south [SS 5289 0777] to almost N–S in the north-east [SS 5297 0790]. Dips are locally up to 35°, but generally less.

Faults

North-eastern shelf

The main evidence for the existence of a NW–SE fault within the basin, bounding the north-eastern shelf, comes from records of old ball clay mines. A number of adits driven south-westwards from places along a line between [SS 5126 1205] and [SS 5149 1179] encountered such steep dips at distances of between 20 and 100 m from their mouths that working had to be discontinued. The extent of this old mining is now reflected in areas of subsidence at the surface. It is probable that the sharp increases in dip were the result of terminal drag adjacent to a fault, and using the south-western limits of subsidence, together with evidence from working mines, the position of this fault can be estimated with some accuracy.

South-western boundary fault

This fault can be located at a number of places. In a stream [SS 5056 1137] south-west of Marland Moor pale grey clayey sands and silts crop out a few metres east of hard grey banded shales of the Upper Carboniferous, and farther south-west in another small stream [SS 5102 1049] rust-stained quartz gravels and sands occur very close to sheared Upper Carboniferous shales to the west. At the roadside [SS 5117 0986] near North Town Carboniferous shales crop out about 40 m SW of a fast-flowing spring, and it is probable that the spring water issues from the boundary fault. Drilling by E.C.C. Ball Clays Ltd about [SS 5271 0778] just north of Woolladon also located the fault.

Meeth area

The NE boundary fault has been located [SS 5382 0886] in the course of drilling for clay, and a mine [SS 5372 0912] to the north-west encountered very steep dips in exploratory workings driven north-east towards the railway line. Steep dips were also encountered at the edge of a nearby claypit [SS 5392 0853]. Small faults noted in Meeth Claypit included one trending 010° from [SS 5370 0852], dipping 60°E and probably throwing down over 9 m E. This fault may be a splay off the main boundary fault. Other faults probably complementary to the NW–SE fractures were also seen in the pit; one [SS 5365 0859] trended 063°, dipped 70°S and had an apparent southerly downthrow. Minor faults showing a similar trend were noted in an extension to Meeth Claypit [SS 5385 0556]. Records of old mines west of the present Meeth pit show that several headings being driven in smooth clays ran abruptly into steep faces of running sand, indicating the presence of faults or sand-filled channels or perhaps both. E C F

Chapter 5 Igneous rocks

Igneous rocks crop out at two localities in the Chulmleigh district. A stream [SS 5843 2059] about 1.5 km W of High Bickington has exposed a dyke 1.2 m thick of rotten pale brown vesicular rock. No fresh material suitable for sectioning was found, but the rock appears to be very similar to the lamprophyre dykes of Permian age found to the south in the Holsworthy and Okehampton districts (McKeown and others, 1973; Edmonds and others, 1968). In the east, about 3km SE of Bishop's Nympton, a scatter of small blocks of biotite minette in the fields [SS 7835 2231] probably marks an extension of the minette known to exist at Rose Ash, just outside the district. This minette was first recorded by Downes (1884), and has been described by Knill (1969, p. 125). ECF

Chapter 6 Pleistocene and Recent

General account

Head

As in recent memoirs relating to south-west England, Head is taken to include weathered debris moved by solifluxion and that remaining in place. It varies considerably in thickness. On the tops of ridges Head may be absent or less than a metre thick, but on many slopes it is 3 m or more. It is also thick on interfluvial areas of low relief. Most of the Bude Formation is mantled by sandy clay or clayey sand with abundant subangular sandstone fragments. However on the higher interfluves the relatively friable Bude Formation sandstones have been reduced to sand and clay particles and the Head is a relatively homogeneous sandy silty clay or clayey sand, depending on the bulk composition of the underlying rocks. Over the more shaly Crackington Formation the Head consists of rather impervious silty to sandy yellow clay containing angular fragments of hard grey sandstone.

Head clay

In the interfluvial area between the rivers Taw and Torridge, at an altitude of between 150 and 180 m OD, a number of geomorphological flats trend E–W within broad valley forms (Figure 16). Many have the form of degraded terraces whose eastward and westward extensions have been removed by the erosive action of present-day tributaries of the Taw and Torridge. The surface of these flats is very clayey and the land poorly drained.

Four pits were dug in such areas, two on Stafford Moor, south-east of Dolton, one south-west of High Bickington and one on Hatch Moor, east of Great Torrington. All reached solid Bude Formation rocks, either massive sandstone or sandstones with some shales. The rock was overlain by Head consisting of rock debris which became progressively more comminuted upwards towards a fairly sharp junction with overlying Head Clay rarely more than 0.5 m thick.

Mechanical analyses were carried out on samples from different levels within each of the trenches. The weathered mantle between the rock and the Head Clay proved to be 30 to 90 per cent fragments. The disaggregated sand and clay of the matrix, which as might be expected in a weathering profile constituted progressively more of the deposit upwards, showed analyses comparable with those of mechanically disaggregated weathered Bude Formation sandstones. All samples of the overlying Head Clay were over 95 per cent of disaggregated material and contained much more clay than did either the solid rock or its weathered product.

In pit 1 on Stafford Moor [SS 599 116] the solid rock contained 2 to 5 per cent clay, the weathered mantle less than 10 per cent and the Head Clay 15 to 20 per cent. In pit 2 [SS 598 119] both solid rock and Head contained 5 to 10 per cent clay and the Head Clay about 40 per cent. In pit 3 [SS 5737 1973], south-east of High Bickington, the Head contained 10 to 25 per cent clay and the Head Clay 65 per cent.

The Head Clay has clearly undergone some sorting, and is unlikely simply to represent the top part of an in situ weathering profile. Furthermore its base is generally sharply defined. It appears probable that the Head Clay was transported into its present position, and partially separated from a sand and silt fraction on the way. Water seems more likely than wind as a mechanism, since some small sandstone fragments occur within the clay, and either a suspension or a high-density mud flow might be envisaged. However, both the degree of sorting and the geomorphological form of the deposits point to sedimentation along E–W valleys of rather sluggish rivers flowing on a peneplained surface at an altitude of about 150 m OD. The absence of basal gravels is not unusual, even in many of the modern river deposits of the district.

The dating of the Head Clay is problematical. It is older than the present river system and therefore possibly Lower or Middle Pleistocene (Edmonds, 1972a). Early Pleistocene westerly drainage has been recognised near South Molton, just north of the district (Edmonds, 1972b).

River terraces

Petrockstow river deposits

Resting upon the Palaeogene deposits of the Petrockstow Basin, and overlapping on to the adjacent Carboniferous rocks, is a series of five terrace deposits. The most extensive is the lowest (1st) Terrace, which covers most of the floor of the basin and mainly comprises gravels of Carboniferous sandstone with scattered flints. Exposures in this terrace are common, and occur mainly in ditches and around the edges of claypits (Plate 6). Most show gravels with thin seams of yellow or brown clayey sand or sandy clay, but in some places sandy clay is dominant. Thickness ranges from 1.5 to 7 m. Horizons cemented with iron and manganese oxides are common. Pebbles generally do not exceed 20 mm across, except that at the base of the terrace some sandstone fragments and flints are up to 130 mm. The base of the 1st Terrace is irregular and in some places the deposits are channelled down into Palaeogene clays.

The top of the 1st Terrace ranges from about 61 to 69 m OD, that of the 2nd Terrace up to 71 m, the 3rd up to 75 m, the 4th up to 77 m and the 5th up to 85 m. The higher terraces contain less gravel than the 1st Terrace and consist mainly of yellowish grey clay with some gravelly layers.

These river deposits occupy the Petrockstow Basin, which is presently drained by what appear to be relatively recent streams. The River Mere is fed by two main tributary streams, one from the north-west and the other from the south-east, and carries the combined flow north-eastwards through a narrow gap to the Torridge. Clearly the terrace deposits of the basin, which are up to 1.5 km wide, have not been laid down by the present small streams; their trend points to deposition by a fairly large river flowing either north-west or south-east. The ends of the valley to both north-west and south-east are fairly steep, and it appears probable that the terraces represent part of a river course which ran through the basin but has since been disrupted, possibly by movements along the Sticklepath Fault Zone.

North-west of the Petrockstow Basin, near Rivaton Farm outside the present district, occurs a topographical flat [SS 457 138] which is underlain by gravels similar to those at Petrockstow. These gravels lie near the headwaters of the River Mere, within a col between that river and the Mussel Brook to the south, and the shape of the deposit suggests a swing south-eastwards towards the north-western end of the Petrockstow Basin. The deposit is eroded at its eastern end by the River Mere and at its southern end by the Mussel Brook. It lies at about 152 m OD, 91 m higher than the terraces of the Petrockstow Basin.

If the Rivaton Farm and Petrockstow terrace deposits are correlated, then a cumulative north-easterly downthrow of 90 m is implied across one or more fractures of the Stickle- path Fault Zone. A continuation of the river course at the south-eastern end of the Petrockstow Basin is less obvious. However, it is possible that the river formed part of an early Torridge (Figure 16) which flowed eastwards to join a reversed River Taw and continued to the Exe (Edmonds, 1972a). These river deposits are at a lesser altitude, and probably younger, than the Head Clay. They cut across cryoturbation structures in the underlying Palaeogene clays and may date from an interglacial or interstadial period within the Middle Pleistocene.

Terraces of the present river system

Ten terraces have been identified as related to the River Taw (Plate 8), although the remains of the higher ones are generally small and insignificant. Sections are uncommon. The soil capping the terraces is usually rather clayey, and locally contains scattered rounded sandstone pebbles. Terraces are developed alongside the River Mole and its tributary the Crooked Oak.

Eight terraces are aligned with the River Torridge and its tributary the River Okemen,t, although below the confluence they are commonly restricted to narrow lunate spreads in the insides of meanders. Few sections occur, and the terrace flats are covered by clays with scattered rounded sandstone pebbles. North-east of Merton, the 6th Terrace forms a cut-off incised meander about 25 m above the present river to the east (Plate 9), and a section shows well-rounded cobbles of Lower Carboniferous rocks which are uncommon in nearby alluvial gravels. This suggests that in 6th-Terrace times the river was more active than at present and was deriving most of its sediment from the River Okement.

The rivers Torridge and Okement occupy fairly wide flood plains from where they enter the district downstream to about Meeth. Farther north the Torridge flows through a series of deeply incised meanders with narrow alluvial tracts (Figure 16). Upon leaving the district at Great Torrington it returns to a more open valley and meanders within a wide flood plain. Traces remain of a broad old incised meander pattern of the River Taw, and it seems possible that there has been differential uplift as between the Taw and the Torridge and between different stretches of the latter. The abundance of NW–SE faults, some of which bound the incised meander tract of the River Torridge, suggests that block fault movement was responsible for the uplift. Such faults are thought also to have affected Pleistocene and Recent gravels within the Petrockstow Basin (p. 40).

Alluvium

The main tracts of alluvium border the rivers Torridge, Taw and Mole with narrower strips along tributaries such as the Crooked Oak, Little Dart River and Yeo. The floodplain of the deeply incised River Torridge is narrow except around the main meander loops.

The alluvium of the larger rivers and tributaries consists mainly of poorly sorted brown or reddish brown sandy clays and clayey sands up to 3 m thick. Basal gravels are of sporadic occurrence and usually less than 0.5 m thick; the pebbles are generally up to 50 mm across, and of Upper Carboniferous sandstone, vein quartz and some Lower Carboniferous limestone, chert and hornfels brought down by the River Okement.

Smaller streams have commonly deposited yellow to yellowish brown clayey alluvium with local peaty patches. Many of them run along or near lines of springs issuing from faults, and the fine-grained alluvium is usually water-logged and swampy with a dense cover of rushes and scrub. E CF

Details

Head

Great Torrington–Meshaw

A roadside bank [SS 5064 1940] near Great Torrington shows 1.83 m of yellowish brown stony sandy clay with some ferruginous cement. A test pit [SS 5171 1968] east of Great Torrington revealed broken silty shale (Bude Formation) 0.8 m, overlain by yellowish brown red-stained slightly stony clay (Head) 1.0 m, and yellow clay with a few stones (Head Clay) 0.6 m. To the north-east a trial pit [SS 5381 2183] near Cranford Moor exposed yellow and green mottled slightly stony clay overlain by 1 m of red stony clay.

A test pit [SS 5733 1978] 2.5 km WSW of High Bickington showed the following section :

A roadside bank [SS 6190 2022] east of High Bickington shows 1.83 m of sandstone debris resting on grey silty sandstone. In a similar bank to the south-east [SS 6383 1906] 1.22 m of stony clay are exposed. In the banks of the River Mole [SS 6573 1855] west of King's Nympton 2.44 m of stony brown clay rest on shattered sandstones. A trench to the north [SS 6562 2455] exposed 2.13 m of slightly sandy clay containing scattered small stones. At Meshaw a ditch [SS 7807 1922] is cut in 1 m of grey stony clay containing much debris of silty sandstone.

Beaford to West Worlington

Auger holes [SS 548 155] near Beaford showed up to 1.37 m of yellow, brown and grey mottled silty clay containing fragments of silty sandstone and shale. A stream [SS 5781 1120] south-east of Dolton has exposed 4.88 m of clayey gravel and silt, and a well to the east [SS 5987 1104], on Stafford Moor, penetrates 3 m of pink stony sandy clay overlying thin-bedded sandstones. The banks of the Little Dart River [SS 6790 1385] near Chulmleigh show 2.13 m of reddish brown clay with angular stones, and a trench farther east [SS 7003 1367] proved 1.22 m of brown sandy clays containing angular blocks of greenish grey sandstone. A ditch [SS 7307 1488] near Cheldon is dug in 0.46 m of yellowish grey clay with scattered stones and small peaty patches.

Morchard Bishop

A pond [SS 7540 0820] near Chillingford exposes 1.22 m of sandy clay containing sandstone debris.

River terraces

Petrockstow river deposits

A river bank exposure [SS 5038 1224] shows buff clayey sand, probably of Palaeogene age, overlain by 1.52 m of brown creamy clayey gravel. The following section was noted in a ditch [SS 5136 1110] :

Excavations for clay [SS 5157 1181] revealed a section in clays and gravels of the 1st Terrace (Figure 17) .

A ditch [SS 5214 1163] south-west of Merton shows :

Augering in another ditch [SS 5356 0838], near Woolladon Moor, proved ferruginous-stained brown gravel containing sandstone and shale pebbles 0.15 m, overlain by yellow gravelly clay 0.61 m, and dark brown soil 0.61 m.

Terraces of the present river system

River Torridge

Near the village of Merton an exposure in the 6th Terrace at the side of a small stream [SS 5369 1277] shows gravel consisting of rounded cobbles of Lower Carboniferous chert and limestone together with some of granite. Rounded pebbles in a field [SS 528 152] near Warham lie on a flat referred to the 4th Terrace. Farther north-west a ditch [SS 5023 1653] cut in undifferentiated terrace deposits, probably related to the Torridge, showed yellowish brown clayey gravel containing subrounded to sub-angular rock fragments 0.46 m, overlain by red-stained soil 0.61 m.

River Taw

Near Taw Bridge an exposure [SS 6738 0690] in the 1st Terrace shows 2.74 m of silt, the lowest 0.61 m being pebbly.

Near Portsmouth Arms the following section [SS 6330 1959] occurs in 1st Terrace deposits:

A trench [SS 6122 2406] near Umberleigh shows 0.61 m of silty clay with rounded cobbles, probably part of the 3rd Terrace. Rounded sandstone pebbles common on geomorphological flats to the west are thought to represent the 2nd Terrace [SS 597 244] and the 5th Terrace [SS 597 241].

Crooked Oak

A ditch [SS 7780 2300] near Rodsworthy shows over 1 m of pale brown and grey sandy clays with angular and subrounded sandstone fragments; the deposits belong to the 2nd Terrace.

Alluvium

River Torridge

Exposures include : at [SS 5013 0686] grey shales overlain by rusty clayey sand 0.15 m, rusty brown gravel 0.76 m, and brown sandy clay 0.46 m; at [SS 5381 0611] grey shales overlain by yellowish grey clay 0.15 m, brown gravel 1.22 m, and brown sandy clay 1.22 m; at [SS 5563 0852] yellowish grey clay overlain by reddish brown sandy clay 3 m; at [SS 5512 1020] coarse gravel 0.61 m, overlain by brown sandy clay 2.44 m; at [SS 5509 1163] clayey gravel 0.61 m, overlain by brown sandy clay 1.52 m; at [SS 5483 1340] coarse gravel with clay matrix 0.2 m, overlain by greyish yellow mottled clay 0.61 m, yellowish brown silty clay with ferruginous concretions 0.61 m, and reddish brown silt 1.07 m; at [SS 5320 1538] grey shales overlain by brown sandy gravel with ferruginous-cemented layers 1.22 m, and brown sandy clay 1.22 m.

River Okement

An exposure in the river bank [SS 5636 0600] shows black shales overlain by 1.52 m of coarse gravel.

Small stream east of Dolton

About 1 m of brownish yellow clayey gravel crops out in the stream bank [SS 5846 1243].

Stream west of Roborough

Ferruginous-cemented gravel is overlain by 1 m of yellowish brown clay [SS 5590 1679].

Small stream north-east of Yarnscombe

Gravel is overlain by 0.61 m of yellowish brown sandy clay [SS 5671 2455].

Mully Brook

The brook cuts through : at [SS 6088 1457] yellow stony clay overlain by grey clayey silt 0.61 m; at [SS 6566 1561] clayey gravel with subangular pebbles 0.46 m, overlain by brown silt 0.46 m.

Hollocombe Water

The stream banks show : at [SS 6130 1032] brownish grey clayey silt 0.76 m; at [SS 6443 1209] clay 0.31 m, overlain by gravel 0.08 m and reddish brown silt 1.0 m.

River Yeo

Exposures show : at [SS 7357 0618] siltstone over lain by coarse angular silty gravel 0.61 m and brown silt 0.76 m; at [SS 7190 0841] stony clay overlain by brown silty clay 1.22 m.

River Taw

The following sections were measured in river-bank exposures of the River Taw: at [SS 6640 0565] gravel 0.75 m, overlain by brown silt 1 m; at [SS 6997 0950] greyish brown silt 2.29 m; at [SS 6920 1041] reddish brown clayey silt 1 m, overlain by greyish brown silt 1 m; at [SS 6798 1161] subrounded gravel 0.46 m, overlain by greyish brown silt 1.83 m; at [SS 6636 1495] gravel 0.23 m, overlain by brown silt with several gravel bands 30 to 50 mm thick in the bottom metre 1.68 m; at [SS 6635 1581] brown silt with thin sandy bands 1.83 m; at [SS 6600 1712] grey clay 0.31 m, overlain by brown silt 1.83 m; at [SS 6468 1921] greyish brown sandy silt and clay with orange-coloured gravelly base 2.13 m; at [SS 6209 2001] coarse sandy gravel with subangular rock fragments 0.61 m, overlain by brown silt 1.52 m; at [SS 6145 2209] coarse sandy partly cemented gravel 0.46 m, overlain by brown pebbly silt 0.38 m, and greyish brown silt 1.37 m.

River Mole

A river-bank exposure [SS 6760 2270] shows coarse ferruginous-stained gravel 0.31 m, overlain by brown silt 1.22 m.

Crooked Oak

Grey and brown clay and silt 1.83 m thick are exposed in the stream banks [SS 7636 2306].

Little Dart River

Riverside exposures include : at [SS 7648 1336] coarse gravel overlain by silt with pebble bands 1.0 m; at [SS 7032 1375] coarse gravel overlain by brown silt 0.61 m; at [SS 6838 1372] grey very silty clay 0.23 m, overlain by greyish brown silt 1.22 m. ECF

Chapter 7 Economic geology

Metalliferous minerals

Only two small mines are known to have been worked in the district, both probably for lead and its associated silver. Yarnscombe Mine about [SS 541 241] is thought to have been about 2 km WNW of Yarnscombe. Dines (1956) reported that Mr H. St L. Cookes had observed crop workings along a NNE–SSW stream in the vicinity and had collected fragments of galena. No traces of workings or dumps remain. The country shales and sandstones contain traces of NNE–SSW faults and an exposure in the stream [SS 5427 2440] shows quartz veining in sandstones and shales. It seems possible that insignificant lead mineralisation occurs along a fault which trends about 022°.

At Brushford Mine (Dines, 1956, p. 760), mentioned in the Patent Rolls of 1498, a flow of water issuing from the western side of a small gully [SS 675 081] just south of Middle Reeve is thought to mark the site of an adit mouth. No dump survives; nor does the stone of nearby walls show any metalliferous minerals, though vein quartz is common. Probably the 'mine' was little more than a trial digging.

It seems possible that isolated galena veins may occur elsewhere in the district, representing late-stage low-temperature (epithermal ?) mineralisation, but none has been found. West of the district, thin veinlets of quartz, aragonite and dolomite with specks of chalcopyrite (commonly altered to malachite) occur sporadically in coastal sections between Welcombe Mouth and Hartland. A few stream sediment samples were checked for base metal content by a rapid qualitative dithizone method, but yielded no worthwhile results. Low values from the River Taw are probably attributable to leaching of the dumps at Ramsley Mine (Edmonds and others, 1968, pp. 204–205). Copper mineralisation around North Molton was not detected in the River Mole sediments. Nor was there any indication of lead near Brushford. KEB

Building stone and roadstone

Throughout the Chulmleigh district rough stone for building, hedging and road-metalling has been taken from small local quarries, usually in sandstones. Of the few pits still used, most provide only material for hedge banks and rough tracks.

Polished stone values (PSV), strength figures (aggregate abrasion value or AAV) and effective porosities (EP) for three greywackes from the Bude Formation are as follows :

The Department of the Environment currently recommend that aggregates used for the wearing courses of 'A' class roads should have a PSV of 62 or more and an AAV of 10 or less; ideally the porosity value should be under 4 per cent. JRH

Crackington Formation

Much of the Crackington Formation is shalt'. Quarries are mostly in the sandier upper beds above the Gastrioceras listeri horizon. In the Taw valley Bridge Quarry [SS 660 174] has a face 24 m high and some 60 m long, and another quarry [SS 634 194] is now used as a sawmill yard.

Bude Formation

In the western part of the Chulmleigh district most of the larger quarries in Bude Formation rocks occur in the valley of the River Torridge, especially alongside the A386 and B3220 roads. Several such quarries south of Great Torrington [SS 4953 1778] to [SS 4943 1751] are all disused. Another [SS 5022 1753], recently worked for roadstone, shows massive sandstones interbedded with shales and siltstones. To the east, just north of Castle Hill, lies another disused quarry [SS 5218 1721]. Massive sandstones and flaggy sandstones in an active quarry [SS 5491 1124] near Newbridge generally dip steeply to the north and are overturned, although the core of an anticline is visible at the northern end of the quarry; the stone worked is sold as hardcore and much is used for constructing tracks within the nearby clay works. A quarry [SS 5259 0775] west-south-west of Meeth, worked by E.C.C. Ball Clays Ltd, shows sandstones and a slumped bed dipping 40°–45°N; the stone provides hardcore for service roads in adjacent clay workings.

Several small quarries between Riddistone [SS 647 057] and Taw Green [SS 664 064] were opened in a ridge formed by massive purple, brown and green sandstones. Similar rocks were worked near Millsome [SS 669 058], north of Oak Farm [SS 680 059], at South Moor Quarry [SS 678 066] and on a small ridge [SS 685 073] some 730 m E of Westacott. Coldridge church is said to include stone from this last source, and also from quarries in the ridge [SS 698 083] west of Higher Park. About 17 m of sandstones dipping steeply southwards have been quarried in Westacott Wood [SS 683 076], on the bank of the River Taw; the stone was probably used in the building of Brushford Barton and Brushford church, to the west beyond the river.

In recent years the Barlinch Quarries [SS 678 094] have been redeveloped by the Forestry Commission as their major source of road-metal in the Eggesford area. A lower bench, formerly flooded, has been filled in and the upper bench, in 9 m of gently dipping sandstones with silty intercalations, has been extended.

Along the Taw valley downstream from its junction with the River Yeo, the larger quarries include Poplar Tree Quarry [SS 693 111] and another quarry at [SS 622 193]. Park Mill Quarry [SS 694 138] on the banks of the Little Dart River has supplied some of the building stone used in Chulmleigh, though the larger blocks used in the churches probably came from another quarry [SS 660 185] about 5 km to the NNW. The latter quarry, which lies alongside the B3226 road, shows thick sandstones dipping at a moderate angle southwards. The steep face is partly benched, over 25 m high and 213 m long.

West of the River Mole and north of Alswear a large quarry [SS 726 234] has been opened in 30 m of interbedded muddy, flaggy and fine-grained sandstones. Farther north two roadside quarries just south-east of Great Hele Barton [SS 721 245] contain more massive beds of cleaner sandstone interlayered with flaggy silty sandstones and disposed in a westerly plunging syncline.

Quarries are common along the valley of the Little Dart River upstream from Chawleigh. Cheldon Quarry [SS 7241 1316] shows 20 m of flaggy sandstones with dark mudstones folded into an anticline.

A fairly large quarry [SS 739 072] south-south-east of Lapford was worked in sheared and fractured sandstones, some of them micaceous, with bands of sandy siltstone. Oldborough Quarry [SS 776 063] shows massive greenish grey and purple sandstones with many steep irregular joints.

Bideford Formation

A number of quarries in the north of the district have been opened in the Cornborough Sandstone and other thick sandstones of the Bideford Formation. These sandstones are rather soft, and commonly massive in their lower parts but more flaggy above. A quarry [SS 569 245] north-west of Yarnscombe, probably worked for local building stone, exposes 6 m of wedged-bedded brown feldspathic sandstone. Two quarries [SS 607 238]; [SS 608 239] farther east, immediately north of Umberleigh, have been opened in the Cornborough Sandstone, which dips steeply south and is similarly wedge-bedded. The northernmost quarry comprises two bays separated by an unworked silty succession which includes a culm horizon. KEB

Ball clay

The ball clays of the Petrockstow Basin form one of three such deposits occurring in south-west England, the others being at Bovey Tracey in south Devon and in the vicinity of Wareham in Dorset. The name ball clay is said to be derived from the old method of working whereby the clays were cut into 10-in (0.25-m) cubes called balls. In the past the term ball clay has tended to be restricted to the smooth clays used mainly in pottery bodies, the more siliceous clays being referred to as stoneware clays or brick clays. Now, however, ball clays include all the kaolinitic plastic clays obtained from these deposits.

Clays at Petrockstow are said to have been worked as early as 1680, and Josiah Wedgwood is reputed to have dug clay from pits in the southern part of the basin about this time. The clays were mentioned in Polwhele's History of Devonshire (1797) and in Lyson's Magna Brittania (1822). Ussher (1879) briefly described the workings. In the past the clay has been chiefly used for pottery, stoneware and the manufacture of clay pipes. Until about 1942 some of the more siliceous clays were worked for the manufacture of bricks, which were yellow, hard and impermeable.

The deposits are now worked in the north-east by Watts Blake Bearne & Co. Ltd who have two pits, Courtmoor and Westbeare, and in the south by E.C.C. Ball Clays Ltd who have three pits, at Meeth, Woolladon and Stockleigh Moor. Both companies at one time mined the clay. In the northern area small shaft mines were operated, the shafts dipping at between 55° and 70°. When a shaft intercepted a clay seam, which generally dipped at between 5° and 30°, a tunnel was driven down-dip in the clay as far as was practicable. Levels were driven out on either side, at first from the end of the tunnel and later successively nearer the foot of the shaft. Tubs ran down the shaft and the tunnel under the influence of gravity and were hauled back by electric winding engines. When all the levels, which usually extended about 6 m to either side of the tunnel, were worked out, the shaft was in some cases deepened and a similar system of levels driven in a lower seam. At some mines three separate seams were dug successively from one shaft. The mine was then abandoned, a further shaft driven from about 12 m along the strike, and the whole operation repeated; sometimes the workings broke into collapsed levels of an older adjacent mine. In this way seams of clay were removed for up to 90 m from the line of shafts. In some places pits up-dip of the shafts worked the same seams nearer surface.

In the southern part of the basin the underground workings resembled coalmine adits. D-shaped adits were driven down at inclinations of about 1 in 3. Where a clay seam was intercepted the drive was carried on within the seam for distances up to 200 m. Levels were driven out to either side for up to 75 m, and small stall workings driven off the levels. Tubs ran down the main driveway under the force of gravity, and were pushed along the side levels. In some cases auxiliary winding gear was installed at crossroads on the main drives to pull loaded tubs from the levels.

About 120 000 tonnes of clay are taken from the Petrockstow Basin each year. The material sold is usually a blend of clays from several different seams, and this in turn may be mixed with clays from south Devon, depending on what the clay is to be used for. Over 50 per cent of the ball clay is exported. It is used mainly for ceramics such as earthenware and tiles and for refractories.

The ball clays of the Petrockstow Basin are characterised by their high green (unfired) strength, which ranges up to 145 kg/cm² for individual seams and is commonly as high as 98 kg/cm² in blended clays. In contrast the green strength of clays from the Bovey Basin is rarely as high as 70 kg/cm² and is usually around 30 to 60 kg/cm'. In general the Petrockstow clays now worked are more siliceous than their south Devon counterparts. In the recent past only relatively low-silica clays were worked, but subsequently the more siliceous material has become saleable, particularly when blended with less siliceous clays.

Although thin seams of low-silica clay are present in the central axial area of the Petrockstow Basin, they are impersistent and constitute a very small proportion of the total thickness of sediment. Only in the areas of flood-plain and back-swamp sedimentation were thick seams of ball clay developed. Facies distribution suggests that in the north the river channel ran either through the central area of the basin or near the south-western flank. Clays containing least silt tend to be restricted to the north-eastern flank, and indeed all economic ball clays in this area lie north-east of the internal NW–SE fault.

In the south-west, around Woolladon, commercial ball clays are restricted to the Woolladon shelf area, and the concealed ridge of Carboniferous rocks separates the region of back-swamp deposition from the sandy and gravelly deposits of the axial trough.

The occurrence of relatively sand-free commercial clays in the south-eastern part of the basin, near Meeth, is unusual in that the presence of many sand-filled channels suggests nearness to the axial area of channel and near-channel sedimentation. The absence of lignitic clays characteristic of a back-swamp environment reinforces this conclusion. Facies changes are rapid in this area, reflecting migration of the river channel. It is possible that while fine flood-clays were being deposited close to the north-eastern edge of the basin near Meeth, the river was flowing close to the concealed Carboniferous ridge farther to the south-west. Lateral shifting of the river channel towards the north-east may then have caused the development of sand-filled channels within the flood-plain clays.

An approximate estimation of ball clay reserves may be arrived at, using only silica content as a simplified criterion of usefulness. The estimate, given below, is based on present-day methods of selective quarrying of ball clay with no beneficiation. Any future technique which allowed extraction of the kaolinitic clay content from sands and silts would enormously increase the reserves. The figures quoted relate to those localities where workings are active, together with immediately adjacent areas. Such evidence as is available from boreholes in the remaining (major) part of the basin indicates that, with present methods of extraction and processing, workings in this large area would be uneconomic.

Soils

The lower part of the Crackington Formation is commonly shalt', and gives rise to soils which are rather poorly drained; the upper part, which occupies most of the outcrop of the formation in the district, is sandier and yields more friable loams. Soils overlying the Bude Formation are generally well drained and fairly stony, but clayey where the underlying rocks include much shale.

On the high interfluves residual clays and Head Clay mantle the solid formations and produce wet rush-covered ground. These clays are usually less than 1 m thick and it is possible that with deep drainage such land could be improved. Tracts of this nature stretch north-west from

Stafford Moor towards Beaford, and occur between St Giles in the Wood and Huntshaw and around Meshaw.

Soils present over the Petrockstow Basin are mainly derived from silty clays in Pleistocene and Recent river deposits. They are naturally poorly drained, but artificial drainage permits the development of rich grassland. However, much of the area is waterlogged and covered by rough scrub and rushes.

The soils bordering the major rivers are derived from generally clayey alluvial deposits. They tend to be wet, sometimes waterlogged in the winter, but provide good pasture land when adequately drained. ECF

Water supply

The South West Water Authority's Division 2 controls public supplies within the Chulmleigh district. Its authority extends to northern Dartmoor and western Exmoor, and much water is gathered from these highland areas. Water is also abstracted from the river systems of the Taw and Torridge, and on a minor domestic scale from numerous small streams.

Annual rainfall varies on either side of 1000 mm, but infiltration into the Carboniferous bedrock is localised and probably nowhere exceeds 100 mm. These sandstones and shales, which underlie most of the district, have been fractured by folding and faulting. Some water may be contained and move within the arenaceous strata, but it seems likely that the generally small grain size precludes the storage of large quantities and restricts migration within such beds. Most of the groundwater in these Carboniferous strata probably lies in small discontinuous fissure systems which, although permitting rapid movement, are too limited in extent to provide large supplies.

Recorded yields from boreholes up to 61 m deep in Carboniferous rocks range from 450 to 4500 litres per hour (100 to 1000 gallons) and commonly lie between 1800 and 2700 litres per hour (400 and 600 gallons). However, the test periods were generally short, drawdowns varied from 6 to 30 m, and it is likely that these initial yields would not be maintained. A combined well and borehole about 1.6 km N of Chulmleigh and some 170 m deep is said to have been pumped at 5550 litres per hour (1200 gallons), but tests indicated a maximum daily yield of 157 000 litres (34 560 gallons). The typically small but varied quantities of groundwater obtained from Carboniferous rocks show no distinction between the Crackington and Bude formations, thus reinforcing the view that the effects of lithological variation are subordinate to and masked by the overriding influence of the presence or absence, and extent, of fracture systems.

Infiltration into the discontinuous unconsolidated silts, sands and gravels which are intercalated with clays in the Palaeogene deposits of the Petrockstow Basin must be considerably more than into the Carboniferous rocks and may amount to about 300 mm per year. However, much of the silt and fine sand has a high clay content and the number and extent of potential aquifers is small. None the less small to moderate supplies might be drawn from sands and gravels at depths of some hundreds of metres, and controlled recharge might be possible. An exploratory borehole sunk in 1966–67 for the Institute encountered artesian water at around 396 m depth, although the sulphurous smell of the water and the abundance of sulphide minerals in the sediments suggested that no potable supply was available.

Broad expanses of alluvium and river terrace deposits occupy the valleys of the rivers Taw and Mole and, generally over lesser widths, that of the River Torridge. Probably their thickness nowhere exceeds 6 m. Although these drift deposits have not been tapped for water, plentiful but individually small supplies could be obtained from shallow wells or tube wells. These supplies would not be protected by any long natural process of filtration and purification and would therefore be liable to contamination from pollutants at surface or even in the river water. In the long term abstraction would be at the expense of river flow.

Thus the Carboniferous rocks are unlikely to yield large amounts of groundwater, although small domestic supplies are generally available. The arenaceous beds of the Petrockstow Basin lie within a predominantly clayey sequence and are probably too limited in extent to constitute a major aquifer. Water resources in the drift deposits of valleys are small and readily polluted and their exploitation would directly diminish river flow.

All except the smallest of future supplies must depend on surface storage and river intakes. Meldon reservoir on northern Dartmoor has a capacity of 3090 million litres (680 million gallons) and came into use in 1972, but it will probably be unable to meet growing demands beyond the early or mid 1980s. Construction of Wimbleball reservoir, on the southern margin of Exmoor, has already started. On completion it will supply 32 million litres per day (7 million gallons) to the Wessex Water Authority, and a regulated river yield of 58 million litres per day (12.8 million gallons). Such high-altitude reservoirs, with necessarily small catchments and capacities, are the quickest and cheapest answers to immediate needs They generally occupy land of high amenity but low agricultural value and can make maximum use of gravity in distribution, but supplements are soon required.

Alternative approaches might be a barrage across the Taw–Torridge estuary or a large reservoir in the middle reaches of one of these rivers. Direct effects of the former, except for any increased restriction on discharge into rivers, would lie mainly outside the Chulmleigh district (Edmonds and others, 1978). However, a large inland reservoir in Devon would most probably lie within it.

Although it is common to think of storage in estuaries as being on a larger scale than storage inland, north Devon could be an exception. A dam near Beaford Bridge [SS 543 142] on the River Torridge, of a height similar to that at Meldon, would impound over 450 000 million litres (100 000 million gallons), perhaps up to twice the quantity which could be held at high water mark in the estuary. But the impact on the district would be enormous–a 34-km² lake (over twice the area of Lake Windermere) and the inundation of Monkokehampton, the lowest part of Hatherleigh and the Petrockstow ball clay workings. Public and commercial decisions on planning and land use would be influenced twenty years or more ahead of such a project.

If we exclude desalination, an overriding wish to conserve land may suggest estuarial storage; economics may point to a site such as Beaford Bridge. But the arguments embrace major economic and sociological issues and lie outside the scope of this book. EAE

Chapter 8 Geophysical investigations

Regional aeromagnetic and gravity surveys have been supplemented by detailed gravity studies of the Petrockstow Basin and other measurements in boreholes sunk in the basin.

Regional aeromagnetic survey

The aeromagnetic survey of the area formed part of an airborne survey of south-west England carried out by Hunting Surveys Ltd under contract to the Geological Survey of Great Britain (now IGS) and the United Kingdom Atomic Energy Authority during 1958–59. A fluxgate magnetometer was used to measure variations in the total field along N–S flight lines nominally 150 m above the ground and 400 m apart. The resulting aeromagnetic map (Figure 18) represents departures from a linear reference field having a value of 47 903 gamma at the National Grid origin, increasing at a rate of 2.17 gamma/km northwards and decreasing by 0.26 gamma/km eastwards.

The magnetic field is dominated by a N–S regional gradient, from a peak of around 100 gamma immediately north of Dartmoor to a minimum of −90 gamma near Barnstaple. To the south of the peak, the field decreases fairly steadily to about −170 gamma in the English Channel. The structure giving rise to this large-scale anomaly is most probably deep-seated and extends under Dartmoor, since the granite itself is relatively non-magnetic. The most interesting feature of the field in the Chulmleigh district is a northward displacement of the contours by about 12 km over most of the southern half of the area. It may be coincidental that this displacement is of the same order as that postulated by Bott and others (1958) for the hypothetical Exmoor thrust.

Small-scale closed anomalies may reflect mineralisation within the Carboniferous rocks. Susceptibility measurements on 20 samples of such rocks (Fenning in Edmonds and others, 1968, p. 223) showed that the four samples with highest susceptibilities, up to 380X 10⁻⁶ c.g.s. units, contained pyrrhotite; the average value for the remaining samples was 20 X 10⁻⁶ c.g.s. units.

Regional gravity survey

A regional gravity survey of the area was carried out by the Institute in 1965. The average density of stations (excluding the area of the Petrockstow Basin) was a little under one per square kilometre. Observed gravity values were related via the Geological Survey base station network to a datum of 981.26500 cm/s² at Pendulum House, Cambridge, but have been adjusted to the IGSN 71 system in which the datum value at Cambridge is 981.25392 cm/s². Conversion to Bouguer anomalies at sea level has been carried out using the 1967 International Gravity Formula and a rock density of 2.67 g/cm³; inner zone (A–G) terrain corrections have been applied.

(Figure 19) shows the Bouguer anomaly contour map for the district. The well-defined negative anomaly over the Petrockstow Basin is described below. In order to separate regional effects from anomalies caused by small or shallow structures a filtering process similar to that described by Griffin (1949) was applied using a computer programme written by Mr I. F. Smith. The equivalent cut-off wave length of the filter used was 54 km. This method is less effective in the neighbourhood of prominent local anomalies such as that over the Petrockstow Basin; hence to avoid distortion of the regional picture the initial filtering in the immediate vicinity of the basin was done manually.

The filtered results show an E–W gradient over most of the district, with an average westward increase of 0.23 mGal per kilometre. This gradient was recognised by Bott and others (1958); it must arise from deep-seated features, such as low-density rocks thinning westwards or possibly thinning of the crust in this same direction.

The E–W ridge evident in (Figure 19) is better defined on a map of residual values (the differences between the smoothed and observed values), which shows it to have an average amplitude of about 1.5 mGal and a range of 1 to 2 mGal. The ridge corresponds very approximately with the outcrop of the Crackington Formation, which being less arenaceous and therefore more dense than the Bude Formation might be expected to produce a positive gravity anomaly. Measurements on rock specimens (Fenning in Edmonds and others, 1968) indicate a density contrast between the two formations of 0.04 g/cm³. The steepest gravity gradient along the south side of the ridge coincides approximately with grid line 112 N, which runs just south of the boundary between the formations.

Gravity measurements over the Petrockstow Basin

(Figure 20) shows the residual Bouguer anomaly map of the Petrockstow Basin. Many boreholes have been drilled northeast of the fault line A–B, and the thickness of Palaeogene sediment there is less than to the south-west of this line. The peak negative anomaly lies north-east of the axis of the main trough, however, indicating the presence of more light material at depth on the north-eastern side of the basin.

In interpreting the results it is assumed that sediment densities are horizontally uniform and show the same variation with depth as was established in two deep boreholes. To account for the apparent mass deficiency north-east of the axis the contact at depth between the Palaeogene and the Carboniferous is assumed to be a reverse fault dipping 45°NE from the line A–B at surface. (Figure 21) shows a simplified structure which would give rise to the anomaly map. The densities of the various layers have been deduced from measurements on samples and from formation density logs of boreholes 1 and 2. A Carboniferous rock density of 2.65 g/cm³ was used. At surface the shape of the model is approximately that of the basin at outcrop. Borehole information about the depth to the base of the Palaeogene has been included, although where much information is available, as in the northeastern part of the basin, the shape has been simplified.

The shape of the model was calculated by a method based on that described by Talwani and Ewing (1960), in which the effects at surface of a series of polygons at varying depths were evaluated. A computer programme written by Mr J. M. C. Tombs was used. The model shows the deep northeastern part of the basin, a steep base on the south-western side and relatively shallow-dipping Carboniferous–Palaeogene contacts to the north-west and south-east. The suggestion that the depth of Palaeogene material in the north-east reflects the presence of a reverse fault ((Figure 20), A–B) is, of course, speculative; other configurations could give rise to the observed anomaly.

Geophysical measurements in Petrockstow boreholes

Apart from the formation density measurements mentioned above, several other logs were run in three boreholes. Sonic velocity measurements showed a gradual increase with compaction, from 2100 m/s at the surface to 2800 m/s just above the bottom of the basin at 658.3 m. The passage into underlying Carboniferous rocks is marked by an abrupt increase in velocity to 3800 m/s. Formation resistivities calculated from laterologs show substantial variations in the upper sediments, but are more uniform at 10 to 20 ohm metre below 300 m. Gamma-ray, neutron, microlog and temperature logs were also recorded. AJB

References

ALLEN, J. R. L. 1964. Studies in fluviatile sedimentation: six cyclothems from the Lower Old Red Sandstone, Anglo-Welsh Basin. Sedimentology, Vol. 3, pp. 163–198.

ALLEN, J. R. L. 1965. Fining upward cycles in alluvial successions. J. Geol., Vol. 4, pp. 229–246.

ALLEN, V. T. 1929. The Ione formation of California. Univ. California Publ. Dep. Geol. Sci. Bull., Vol. 18, pp. 347–448.

ASHWIN, D. P. 1958. The coastal outcrop of the Culm Measures of south-west England. Abstr. Proc. Conf. Geol. Geomorphol. South West England, Vol. 2, pp. 2–3.

BEER, K. E. 1966. P.40 in Summ. Prog. Geol. Surv. for 1965. Pt. 1.

BOTT, M. H. P., DAY, A. A. and MASSON SMITH, D. J. 1958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Phil. Trans. R. Soc. London, Ser. A, Vol. 251, pp. 161–191.

BRISTOW, C. M. 1968. The derivation of the Tertiary sediments in the Petrockstow Basin, north Devon. Proc. Ussher Soc., Vol. 2, pp. 29–35.

BURNE, R. V. 1969. Sedimentological studies of the Bude Formation. Unpublished Ph.D. thesis, University of Oxford.

BURNE, R. V. 1970. The origin and significance of sand volcanoes in the Bude Formation (Cornwall). Sedimentology, Vol. 15, pp. 211–228.

BURNE, R. V. and MOORE, L. J. 1971. The Upper Carboniferous rocks of Devon and north Cornwall. Proc. Ussher Soc., Vol. 2, pp. 288–298.

CLARKE, F. W. 1924. P. 140 in The Data of geochemistry. U.S. Geol. Surv. Bull., No. 770.783 pp.

CURTIS, C. D. and SPEARS, D. A. 1968. The formation of sedimentary iron minerals. Econ. Geol., Vol. 63, pp. 257–270.

DEARMAN, W. R. 1969. An outline of the structural history of Cornwall. Proc. Geol. Soc. London, No. 1654, pp. 33–39.

DE LA BECHE, H. T. 1839. Report on the geology of Cornwall, Devon and West Somerset. Mem. Geol. Surv. G.B.

DE RAAF, J. F. M., READING, H. G. and WALKER, R. G. 1965. Cyclic sedimentation in the Lower Westphalian of north Devon, England. Sedimentology, Vol. 4, pp. 1–52.

DINES, H. G. 1956. The metalliferous mining region of south-west England. Mem. Geol. Surv. G.B.

DOWNES, W W. 1884. On a newly discovered dyke of mica-trap at Rose Ash, South Molton. Trans. Devon. Assoc., Vol._16, pp. 498–500.

EDMONDS, E. A. 1972a. The Pleistocene history of the Barnstaple area. Rep. Inst. Geol. Sci., No. 72/2.12 pp.

EDMONDS, E. A. 1972b. Drainage chronology of the South Molton area, Devonshire. Bull. Geol. Surv. G.B., No. 42, pp. 99–104.

EDMONDS, E. A. 1974. Classification of the Carboniferous rocks of south-west England. Rep. Inst. Geol. Sci., No. 74/13.7 pp.

EDMONDS, E. A. WRIGHT, J. E., BEER, K. E., HAWKES, J. R., WILLIAMS, M., FRESHNEY, E. C. and FENNING, P. J. 1968. Geology of the country around Okehampton. Mem. Geol. Surv. G.B., Sheet 324.

EDMONDS, E. A. WILLIAMS, B. J. and TAYLOR, R. T. 1979. Geology of Bideford and Lundy. Mem. Geol. Surv. G.B., Sheets 292 and others.

EDWARDS, R. A. 1976. Tertiary sediments and structure of the Bovey Basin, south Devon. Proc. Geol. Assoc., Vol. 87, Pt. 1, pp. 1–26.

FLETCHER, B. N. 1975. A new Tertiary basin east of Lundy Island. J. Geol. Soc. London, Vol. 131, pp. 223–225.

FRESHNEY, E. C. 1970. Cyclical sedimentation in the Petrockstow Basin. Proc. Ussher Soc., Vol. 2, pp. 179–189.

FRESHNEY, E. C. and TAYLOR, R. T. 1971. The structure of mid Devon and north Cornwall. Proc. Ussher Soc., Vol. 2, pp. 241–248.

FRESHNEY, E. C. and TAYLOR, R. T. 1972. The Upper Carboniferous stratigraphy of north Cornwall and west Devon. Proc. Ussher Soc., Vol. 2, pp. 464–471.

FRESHNEY, E. C. McKEOWN, M. C. and WILLIAMS, M. 1972. Geology of the coast between Tintagel and Bude. Mem. Geol. Surv. G.B.

GRIFFIN, W. R. 1949. Residual gravity in theory and practice. Geophysics, Vol. 14, pp. 39–56.

HUANG, W. H. and FRESHNEY, E. C. In press. Clay mineralogy of the Westbeare Pit, north Devon ball clay basin, England. Rep. Inst. Geol. Sci.

KING, A. F. 1966. Structure and stratigraphy of the Upper Carboniferous Bude Sandstone, north Cornwall. Proc. Ussher Soc., Vol. 1, pp. 229–232.

KING, A. F. 1967. Stratigraphy and structure of the Upper Carboniferous Bude Formation, north Cornwall. Unpublished Ph.D. thesis, University of Reading.

KNILL, D. C. 1969. The Permian igneous rocks of Devon. Bull. Geol. Surv. G.B., No. 29, pp. 115–138.

LYSON, D. and S. 1822. Magna Brittania. (London.) MACKINTOSH, D. M. 1964. The sedimentation of the Crackington Measures. Proc. Ussher Soc., Vol. 1, pp. 88–89.

MCKEOWN, M. C., EDMONDS, E. A., WILLIAMS, M., FRESHNEY, E. C. and MASSON SMITH, D. J. 1973. Geology of the country around Boscastle and Holsworthy. Mem. Geol. Surv. G.B., Sheets 322 and 323.

PASSEGA, R. 1964. Grain size representation by CM patterns as a geological tool. J. Sediment. Petrol., Vol. 34, pp. 830–847.

PETTIJOHN, F. J. 1957. Sedimentary Rocks. (2nd Ed.). (New York: Harper and Bros.)

POWHELE, R. 1797. History of Devonshire. (London.)

PRENTICE, J. E. 1960. Stratigraphy of the Upper Carboniferous of the Bideford region, north Devon. Q. J. Geol. Soc. London, Vol. 116, pp. 397–408.

RAMSBOTTOM, W. H. C. 1965. P. 74 in Summ. Prog. Geol. Surv. for 1964.

RAMSBOTTOM, W. H. C. 1970. Some British Carboniferous goniatites of the family Anthracoceratidae. Bull. Geol. Surv. G.B. No. 32, pp. 53–63.

RATTIGAN, J. H. 1966. The Balikera section of the Carboniferous Kuttung Facies. J. Proc. R. Soc. New South Wales, Vol. 100, pp. 75–84.

SCOTT, A. 1929. Ball Clays. Mem. Geol. Surv. G.B.

SHEARMAN, D. J. 1967. On Tertiary fault movements in north Devonshire. Proc. Geol. Assoc., Vol. 78, pp. 555–566.

SIMPSON, S. 1970. The structural geology of Cornwall : some comments. Proc. Geol. Soc. London, No. 1662, pp. 1–3.

TALWANI, M. and EWING, M. 1960. Rapid computation of gravitational attraction of 3-dimensional bodies of arbitrary shape. Geophysics, Vol. 25, pp. 203–225.

USSHER, W. A. E. 1879. On the deposits of Petrockstow in Devon., Trans. Devon. Assoc., Vol. 11, pp. 422–428.

USSHER, W. A. E. 1892. The British Culm Measures. Proc. Somerset Archaeol. Nat. Hist. Soc., Vol. 38, pp. 111–219.

USSHER, W. A. E. 1901. The Culm-Measure types of Great Britain. Trans. Inst. Min. Eng., Vol. 20, pp. 360–391.

VINCENT, A. 1974. Sedimentary environments of the Bovey Basin. Unpublished M. Phil. thesis, University of Surrey.

Appendix 1 List of principal fossil localities and faunas

The localities are listed from south to north. Registered numbers of specimens are given after the locality details, and the fossils are housed in the collections of the Leeds Office of the Institute. Stratigraphical horizons are indicated by the following codes: BF (Bude Formation), BfF (Bideford Formation), GRS (Gull Rock Shale), HQS (Hartland Quay Shale), SMS (Sandy Mouth Shale). The fossils were determined by Dr M. A. Calver, except where stated; all are of Westphalian age.

1. Trackside [SS 5867 0680] 562 m N63°E of Hill Farm. DEB 4786–94. Planolites ophthalmoides. BF.
2. Stream [SS 5916 0840] 411 m S17°E of Henacroft Farm. DEB 4828. Planolites ophthalmoides. BF.
3. Stream [SS 7746 0963] 512 m S89°W of Bishop's Leigh. DEB 4522–35; WJ 65–86. Caneyella sp., Dunbarella sp., Anthracoceratoides cornubiensis (det. by Dr W. H. C. Ramsbottom; cited Ramsbottom, 1970, p. 54). SMS.
4. Trackside [SS 6722 0970] 402 m N18°W of Hayne. DEB 3543–50. Planolites ophthalmoides. BF above SMS.
5. Stream [SS 6364 1001] 1152 m S20°E of Hollocombe church. DEB 5020–40; KEB 50–72. Caneyella sp., Dunbarella sp., Anthracoceratoides cornubiensis, mollusc spat (det. by Dr W. H. C. Ramsbottom). SMS.
6. Hayne Valley, at confluence [SS 6769 1007] of two streams 740 m due W of Four Ways. KEB 23–35. Caneyella sp., Dunbarella sp., Anthracoceratoides cornubiensis (det. by Dr W. H. C. Ramsbottom; cited Ramsbottom, 1970, p. 54). SMS.
7. Exposure [SS 5747 1266] 365 m S15°W of Down Farm. CF 196–200. Gastrioceras coronatum, G. cf. listeri. GRS.
8. Exposure [SS 5595 1275] 429 m W18°S of Venton Farm. CF 202–06. Dunbarella sp., Gastrioceras cf. listeri. GRS.
9. Loose nodule [SS 5639 1326] 365 m N of Venton Farm. CF 201. Gastrioceras cf. circumnodosum.
10. Field brash [SS 5911 1341] 731 m N9°W of Wood Farm. CF 189–95. Dunbarella sp.,anthracoceratid [juv.], Gastrioceras circumnodosum. GRS.
11. Stream [SS 7189 1399] 810 m N68°W of Cheldon House. DEB 5018–19. lindet. goniatite. ?GRS.
12. Trackside [SS 5351 1495] 1700 m S88°W of church with spire, Beaford. DEB 4795–827. Dunbarella sp.,anthracoceratid, Gastrioceras coronatum, G. cf. listeri. GRS.
13. Stream [SS 5777 1505] 274 m S42°W of Down Farm. DEB 6717–89. Caneyella multirugata, Dunbarella papyracea, orthocone nautiloids, anthracoceratids, Gastrioceras cf. listeri, G. cf. coronatum, G. circumnodosum, Hindeodella sp.GRS.
14. Stream [SS 7669 1529] 594 m S73°E of Cuddenhay Farm. DEB 5119–212. Dunbarella sp.,anthracoceratid, Gastrioceras circumnodosum [incl. coronate form], orthocone nautiloid. GRS.
15. Stream [SS 7664 1531] 540 m S73°E of Cuddenhay Farm. DEB 5226–34. G. circumnodosum. GRS.
16. Stream [SS 7080 1656] 795 m S24°W of Bycott Farm. DEB 4911–32; WJ 139–140. Calamites sp., Caneyella cf. multirugata, anthracoceratid [incl. juv.], Gastrioceras sp.,mollusc spat. ?HQS.
17. Stream exposure centred [SS 7117 1712] 1170 m N2°E of Challacombe Farm. Bed A, DEB 4835–41: plant remains, Anthracoceratites sp. [juv.]. Bed B, about 3.65 m above Bed A, DEB 4842–52–50: Dunbarella sp., Caneyella or Posidonia? [juv.], Anthracoceratites?, Gastrioceras sp. [prominent nodes], mollusc spat. Bed C, about 0.30 m above Bed B, DEB 4851–73: coalified wood, Dunbarella sp., Ant hracoceratites?, Gastrioceras listeri [coarse ornament], G. sp. [small nodes, fine ornament], mollusc spat. Bed D, about 2.74 m above Bed C, DEB 4874–84: Gastrioceras circumnodosum, G. cf. listeri. Bed E, about 2.26 m above Bed D, DEB 4885–97: Dunbarella sp., Gastrioceras listeri. GRS.
18. Stream [SS 7067 1715] 475 m W16°S of Bycott Farm. DEB 4898–910; WJ 141. Plant debris, Caneyella cf. multirugata, Dunbarella cf. papyracea, anthracoceratid [ juv.], Gastrioceras sp.,mollusc spat. ?HQS.
19. Stream [SS 6837 1735] 292 m N70°E of Cutland Farm. DEB 6228–54. Gastrioceras cf. listeri, G. cf. circumnodosum. GRS.
20. Stream [SS 6973 1770] 470 m S6°W of Waddington Farm. DEB 5047–89. Dunbarella sp.,anthracoceratid, Gastrioceras listeri, G. circumnodosum, G. sp. [fine ornament]. GRS.
21. Stream [SS 7346 1771] 1143 m S70°E of Waterloo Farm. DEB 4994–5011. Anthracoceratites?, Gastrioceras sp.cf. listerilcoronatum group, G. sp. [fine lirae], mollusc spat. GRS.
22. Stream [SS 6965 1776] 434 m S18°W of Waddington Farm. DEB 5042–46; KEB 79–82. Anthracoceratid, Gastrioceras listeri [loose in stream], G. circumnodosum. GRS.
23. Stream [SS 7347 1783] 1124 m S75°E of Waterloo Farm. DEB 4945–93. Caneyella sp. [juv.],anthracoceratid [ juv.], Gastrioceras circumnodosum, G. weristerense. GRS.
24. Loose in stream bank [SS 6951 1787] 393 m S42°W of Waddington Farm. DEB 5041. Anthracoceratid, Gastrioceras circumnodosum. GRS.
25. Stream [SS 7348 1788] 1124 m S78°E of Waterloo Farm. DEB 4933–44. Gastrioceras cf. listeri, G. cf. weristerense, mollusc spat. GRS.
26. Loose in stream [SS 7438 1804] 1252 m S22°E of Langley Farm. DEB 5101–04. Gastrioceras cf. listeri. GRS.
27. Stream [SS 7438 1805] 1243 m S22°E of Langley Farm. DEB 5105–10. Gastrioceras cf. listeri. GRS.
28. Stream [SS 7438 1806] 1243 m S22°E of Langley Farm. DEB 5111–13. Anthracoceratid. GRS.
29. Stream [SS 7438 1808] 1225 m S23°E of Langley Farm. DEB 5114–18. Gastrioceras sp.GRS.
30. Stream [SS 7343 1828] 174 m W20°N of Higher Thornham. DEB 5012–17; WJ 117–38. Caneyella cf. multirugata, Anthracoceratites?, Gastrioceras cf. listeri [coarse ornament], G. cf. weristerense. GRS.
31. Stream [SS 7337 1900] 613 m S70°W of Langley Farm. DEB 5090–100. Posidonia sp. nov. [right valve with prominent ant. ear], anthracoceratid [mainly juv.], conodont aggregates including Hindeodella sp.GRS.
32. Stream [SS 6453 1998] 500 m N36°E of Abbot's Marsh. CF 629–62. Gastrioceras circumnodosum, G. cf. coronatum, G. cf. weristerense. GRS.
33. Stream [SS 5661 2166] 1646 m E12°N of Cranford Cross. DEB 6702–11. Caneyella sp.,anthracoceratid [juv.], Pseudocatastroboceras? ?HQS.
34. Ditch [SS 5143 2220] 484 m W7°S of Waggadon. DEB 6712–16. Caneyella sp. [juv.], goniatite [juv.] ?HQS.
35. Roadside [SS 5700 2229] 510 m S42°E of Court Barton. CF 626; DEB 9663–90. Caneyella sp. [juv.], Anthracoceratites sp.,anthracoceratid [incl. juv. in 'nests'], Gastrioceras sp. [juv.], G. amaliae, conodont assemblages including Hindeodella sp., Ozarkodina sp. [platformed types]. HQS.
36. Stream [SS 6624 2423] 612 m E23°S of Shilstone. DEB 6700–01. Planolites ophthalmoides. BfF.

MAC

Appendix 2 List of Geological Survey photographs

Copies of these photographs may be seen in the library of the Institute of Geological Sciences, Exhibition Road, South Kensington, London SW7 2DE. Prints and slides may be purchased. The photographs were taken by Messrs J. M. Pulsford and C. J. Jeffrey and are available in colour and black and white. They belong to Series A.

Carboniferous

Palaeogene Pleistocene and Recent

Topography and miscellaneous

Appendix 3 Sources of rock specimens

Arenaceous rocks

Crackington Formation

Bude Formation

Bideford Formation

Argillaceous rocks

Crackington Formation

Bude Formation

Figures, plates, tables

Figures

(Figure 1) Sketch-map showing the position of the Chulmleigh district.

(Figure 2) Sketch-map showing the distribution of Upper Carboniferous formations and major structures.

(Figure 3) Sketch-map of the Petrockstow Basin showing positions of cross-sections illustrated in Figures 7 and 8.

(Figure 4) Triangular diagram showing proportions of kaolinite, mica, and quartz in clays from Petrockstow.

(Figure 5) Grain size distribution curves of sediments from the Petrockstow Basin Sample a is from a depth of 560.5 m in IGS Petrockstow No. 1 Borehole, and b, c, d and e are from IGS No. 2 Borehole at depths of 119.5 m, 42.6 m, 145.0 m and 56.2 m respectively. Sample a is sandy gravel, b is sandy clay, c is clayey sand, d is sandy clay and e is laminated silty clay..

(Figure 6) General stratigraphy of IGS Petrockstow Boreholes 1 and 2 and a suggested correlation with the succession in the north-eastern shelf area. For locations see (Figure 3).

(Figure 7) Cross-section of the Petrockstow Basin showing the correlation between IGS Petrockstow Boreholes 1, 2 and 3. For locations see (Figure 3).

(Figure 8) Cross-section of the Petrockstow Basin showing the north-eastern shelf and axial trough. For position of seams in the shelf stratigraphy see (Figure 6). For location see (Figure 3)..

(Figure 9) Typical cyclothems of the Petrockstow Basin A is from 88.4 to 115.8 m in IGS No. 2 Borehole, B is from 103.6 to 131.1 m in IGS No. 1 Borehole, C is from 112.8 to 149.4 m in IGS No. 3 Borehole, D is from 30.5 to 57.9 m in Watts Blake Bearne & Co's Borehole No. 7.

(Figure 10) Facies variation between correlated parts of a. IGS Petrockstow Boreholes 2 and 4 and b. Watts Blake Bearne and Co's Boreholes 7, 8 and 9. For locations see (Figure 3).

(Figure 11) CM pattern for sediments from the Petrockstow Basin showing differentiation into various fluviatile environments.

(Figure 12) Contoured stereograms of poles to bedding planes of the Crackington and Bude formations Stereograms are contoured at 1, 3, 5, 9, 11 and 13 per cent frequencies. Formational boundaries are shown in outline and may be interpreted by reference to (Figure 2)..

(Figure 13) Fold patterns in the Bude Formation of the Chulmleigh district a on A386 road just south of Torrington, b east of Dodscott, c 2 km WNW of Beaford and d trackside section north-west of Newbridge.

(Figure 14) Contour maps of the north-eastern shelf area (a) on the base of Bed D (see (Figure 6)) and (b) on the base of the Palaeogene. Contours are marked in metres above or below Ordnance Datum..

(Figure 15) Contour map on the base of the Palaeogene in the south-western shelf (Woolladon) area. Contours are marked in metres above Ordnance Datum..

(Figure 16) Sketch-map showing possible early river courses in the Torridge, Taw and Exe basins.

(Figure 17) Section in Petrockstow river deposits seen in a pit dug for a new mine [SS 5157 1181] on Grange Moor.

(Figure 18) Aeromagnetic (total field) map of the Chulmleigh district. Contour interval 10 gamma; negative closures stippled.

(Figure 19) Bouguer anomaly map of the Chulmleigh district. Contour interval 1 mGal.

(Figure 20) Residual Bouguer anomaly map of the Petrockstow Basin.

(Figure 21) Model for the Petrockstow Basin derived from gravity data. Contour interval 1 mGal

(Plates)

(Front cover).

(Rear cover).

(Geological succession).

(Plate 1) Scenery on the Bude Formation near Lapford. Well-rounded sandstone features. The low-lying area beyond the buildings is underlain by silty shales. The sandstones were formerly worked extensively from small quarries near the farm for local building purposes (A11491).

(Plate 2) Thinly bedded sandstones with interbedded siltstones of the Crackington Formation on the east bank of the River Taw near the Portsmouth Arms (A11479).

(Plate 3) Well-bedded sandstones, siltstones and shales of the Bude Formation in a quarry on the B3220 road near Rosemoor, St Giles in the Wood. The beds are folded into an asymmetric anticline running at an acute angle to the direction of view (A11492).

(Plate 4) Thickly bedded sandstones with thin laminated siltstones of the Bude Formation at the side of the A373 road near Meshaw (A11489).

(Plate 5) Flute casts and squamiform load casts on the base of a slab of Bude Formation sandstone in Newbridge Quarry, Huish (A11473).

(Plate 6) Courtmoor ball claypit near Merton. Yellow-weathering smooth to sandy kaolinitic clays dip at around 25° to the south-west. The clays are overlain by the Recent to Pleistocene Petrockstow river deposits (A11500).

(Plate 7) A working face in Courtmoor Pit showing yellow-weathering relatively sand-free ball clay, carbonaceous and brown at the base and overlain by 0.6 m of clayey silty sand stained grey by finely divided marcasite. This in turn is overlain by sandy yellow-weathering clay (A11499).

(Plate 8) The alluvial flood plain of the River Taw near King's Nympton Station. On the right in the middle distance can be seen the gently sloping surface of the second terrace (A11502).

(Plate 9) An abandoned meander of the River Torridge about 30 m above the present more incised river which lies over the hill to the right of the picture (A11507).

Tables

(Table 1) Average modes for arenaceous rocks from the Crackington, Bude and Bideford formations

(Table 2) Average modes of lithic-rich and lithicpoor sandstones from the Crackington and Bude formations

Tables

(Table 1) Average modes for arenaceous rocks from the Crackington, Bude and Bideford formations

(Table 2) Average modes of lithic-rich and lithicpoor sandstones from the Crackington and Bude formations