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Geology of the Bristol district: the Lower Jurassic rocks

By D. T. Donovan And G. A. Kellaway

Bibliographic reference: Donovan, D. T. And Kellaway, G. A. 1984. Geology of the Bristol district: the Lower Jurassic rocks. Memoir British Geological Survey.

British Geological Survey England And Wales

Geology of the Bristol district: the Lower Jurassic rocks: Memoir for 1:63 360 Bristol geological special sheet

By D. T. Donovan And G. A. Kellaway

Authors

Professor D. T. Donovan, DSc Department of Geology, University College London, Gower Street, London WC1

G. A. Kellaway, MSc Formerly of the Institute of Geological Sciences, London

Natural Environment Research Council

London: Her Majesty's Stationery Office 1984

Crown copyright 1984 First published 1984

Preface

The Lower Jurassic rocks of the Bristol district were examined by Sir Henry De la Beche, founder and first Director of the Geological Survey, who described and illustrated a number of the well-known localities in Volume 1 of the Memoirs of the Survey, published in 1846. In 1843 a future Director, A. C. Ramsay, and H. W. Bristow had been given their first lessons in geological mapping along the Jurassic outcrop north of Bath, by John Phillips, nephew of the 'Father of English geology' William Smith. These workers together with W. T. Aveline, T. E. James and D. Williams surveyed the area on the One-Inch scale and their results were published on the Old Series hand coloured sheets 19 and 35 in 1845, with various later partial revisions. An explanatory memoir by H. B. Woodward was published in 1876.

The Lower Jurassic rocks of the district were described in greater detail, again by H. B. Woodward, in Volume 3 (1893) of his five-volume work The Jurassic Rocks of Britain. Woodward had a qualified opinion of the value of ammonite zones and adopted a very simple biostratigraphical classification, so that limited progress was made with detailed correlation of the rocks.

Resurvey on the Six-Inch scale of the Chepstow (250) Sheet in the north-western part of the district by F. B. A. Welch, T. R. M. Lawrie and F. M. Trotter was interrupted by the war. The remainder of the area was surveyed between 1943 and 1952 by F. B. A. Welch, G. A. Kellaway and, after 1950, by G. W. Green. R. W. Pocock and H. G. Dines acted as District Geologists. The results were presented so as to include the whole of the Bristol–Somerset Coalfield and the small coal basins to the west on one map — the Bristol District special sheet published in 1962 on the scale of one-inch to one mile. The various component one-inch [including the Bristol (264) Sheet] and six-inch maps were separately published during the next few years. Some revision of the north-western corner of the special sheet area, including the Jurassic rocks, was made in 1960-62 by R. Cave and I. B. Patterson and the results were incorporated in the Malmesbury (251) Sheet.

The IGS borehole on Dundry Hill was drilled in 1962-63 and provided a complete section through the Lower Jurassic rocks, which has provided a frame of reference for the present account. The detailed stratigraphy and palaeontology of the borehole were published by H. C. Ivimey-Cook and D. T. Donovan in 1978 (Bulletin of the Geological Survey of Great Britain No. 69).

The Lower Jurassic of the Bristol district has received much study from local geologists, amateur and professional, and without their published accounts of many long-vanished sections the present account would be much less complete. The authors wish to express their particular gratitude to Mr T. R. Fry of Bristol who has communicated many unpublished records which are incorporated in the present work.

Determinations of fossils are principally by the authors and have been revised by Dr H. C. Ivimey-Cook. The work has been edited by Mr G. W. Green, formerly District Geologist.

The other formations comprised within the area of the Bristol District special sheet are to be covered in a memoir, now in preparation, which is devoted primarily to a detailed description of the Coal Measures rocks.

G. M. Brown Director Institute of Geological Sciences Nicker Hill Keyworth Nottingham NG12 5GG 13 May 1983

Geology of the Bristol district—summary

The Lower Jurassic of the Bristol area has been studied for over two and a half centuries. John Strachey in 1719 recognised the Lias in its proper stratigraphical position, and William Smith, the 'Father of British Geology', adopted in the 1790s the terms White Lias and Blue Lias, which are still in use. Later, the area was the scene of biostratigraphical research by J. W. Tutcher, a Bristol amateur geologist, while the lateral variations of Liassic sedimentation over the Radstock Shelf were studied by Tutcher and A. E. Trueman.

The authors present a new synthesis of the Lower Jurassic of the area, based on the primary six-inch mapping by the Geological Survey and on examination of all available exposures, including many temporary sections. In the Lower Lias the normal mudstone facies of the Avon valley, with the more calcareous Blue Lias at its base, is replaced on the Radstock Shelf by a condensed limestone development. The uppermost part of the mudstone sequence, however passes on to the shelf with hardly any change.

The Middle and Upper Lias were largely removed from the area by erosion before the deposition of the Upper Inferior Oolite; their remains are nevertheless described.

Chapter 1 Introduction

The Lower Lias is the most extensive Jurassic formation preserved in the Bristol District, and crops out continuously in the east with outliers distributed over nearly the whole of the District Sheet. The Middle and Upper Lias and Inferior Oolite are confined to the south-eastern part of the sheet and to the outlier of Dundry Hill south of Bristol.

The District covers a classic area for students of Lower Liassic rocks. In the northern and western parts of the area these rocks are chiefly argillaceous, but in the south-east much of the sequence is condensed, comprising very fossiliferous limestones and clays of no great thickness. This area is the Radstock Shelf (Kellaway and Welch, 1948, p. 54). It is bounded to the south by the Mendip area, where the Lower Lias is even thinner and locally absent. Only part of this last area falls within the Bristol District sheet. These areas of thin and incomplete deposition may be regarded as a westerly extension of the London Platform, which was gradually transgressed by the Lower Liassic rocks (Donovan, Horton and Ivimey-Cook, 1979).

The Lower Liassic rocks are also famous for their rich fossil faunas, especially for their ammonites which have enabled detailed zoning to be carried out, so that the lateral changes could be studied in great detail. These in turn provide evidence for the structural evolution of the area during Lower Liassic time. Analysis of the fossil faunas from the Radstock Shelf by Tutcher and Trueman (1925) led them to postulate contemporaneous movements of the sea floor, and earlier works including studies of the Avon valley at Keynsham (Vaughan and Tutcher, 1903; Tutcher, 1923), enabled comparison to be made between the strata on the Shelf and those of adjacent areas. More recent work on the stratigraphical palaeontology, including the distribution of the ammonite zones (Donovan, 1956a, 1978), is discussed below. The lateral changes over the Radstock Shelf have been re-interpreted by D. T. Donovan for the purpose of the present work.

William Sanders (in Keal and others, 1865) at an early meeting of the Bristol Naturalists' Society pointed out the desirability of carefully measuring the local Liassic sections, numbering the beds and collecting fossils. Fortunately many workers have done this so that, although few good sections now remain, those that once existed have, with few exceptions, been well recorded. The completion by the Geological Survey of the primary six-inch mapping of the Bristol and Somerset coalfield (1938-1952) makes it possible to describe the major changes in thickness and facies of the Lower Jurassic rocks and to relate these both to stratigraphical palaeontology and to the structural evolution of the region.

The term Brockley Down Limestone is here used for the 'littoral facies' but is not employed on the map. The term Harptree Beds is used on the map for 'silicified strata of Rhaetic–Inferior Oolite age'.

The classification in the left-hand column is that printed on the key to the District map. A discussion of the history and meaning of the stratigraphical subdivisions follows.

The following are the principal lithostratigraphical subdivisions of the Lias which have been mapped:

History of the term Lias

The word Lias is probably of Celtic origin (Arkell, 1933, p. 12) and can be traced back to forms in Breton and Old Irish U. M. Dodgson, personal communication). The term refers to platy, well-bedded limestones such as are well developed in the lowest part of the Jurassic System around Bristol and elsewhere. From its use as a quarryman's name it was adopted as a stratigraphical term by Strachey (1719) as Lyas, and by William Smith (MS) in the forms White Lias and Blue Lias. Smith used these terms in a lithostratigraphical sense and their meanings remain unchanged to the present day. The clay above the Blue Lias of the Bath area (the Lower Lias Clay of the present work) was called by Smith the Blue Marl, but Buckland, in a table published by William Phillips (1818), extended the meaning of 'Lias' to include these marls and, indeed, all the rocks between the New Red Sandstone below and the 'Sands of Inferior Oolite' (i.e. Midford Sands of the present area) above.

Base of the Lias

The classification and correlation of the White Lias has given rise to controversy. It was included by Bristow (1864) as the topmost member of his Penarth Beds. This term was widely used on Old Series Geological Survey maps and memoirs (e.g. Woodward, 1876), was then abandoned for a long time, and has recently been revived in the form Penarth Group (Geological Society Special Report No. 13, Triassic, 1980). The term Penarth Beds was replaced in local usage by Rhaetic Series, placed by the Geological Survey in the Trias, but the White Lias is classified on current geological maps of the Bristol District as the basal unit of Lower Lias. This practice follows the recommendations of local workers such as Wilson (1894, pp. 222-226) and Tutcher (in Crookall and others, 1930). Others, including Moore (1861, 1867), Woodward (1893) and Richardson (in Evans and Stubblefield, 1929) considered the White Lias to be part of the Rhaetic beds.

The further development of the lithostratigraphic and chronostratigraphic nomenclature is summarised by Warrington and others (1980) and Cope and others (1980). The modifications to nomenclature recommended are not all followed here as they would not then be compatible with the units used on the Bristol District map. The main differences include new names for lithological units in the Trias and that the 'White Lias' is placed in the Lilstock Formation of the Penarth Group. The Lower Lias is treated as a lithological unit above the Penarth Group and its lowest part (Ostrea Beds, Pre-planorbis Beds as used here)—prior to the appearance of Psiloceras planorbis, is regarded as of Triassic age.

Subdivision of the Lias

The Lias was first formally subdivided by John Phillips (1829, p. 33) with particular reference to the sequence of the Yorkshire coast. He recognised Lower Lias shale, Marlstone series and Upper Lias shale. Since then a three-fold division has been generally used, the sandy, ferruginous and calcareous rocks of the Middle Lias (Phillips' Marlstone series) being distinguishable from the largely argillaceous Lower and Upper Lias. However, the interpretation of the term Middle Lias has varied among different workers (see reviews by Blake, 1885; Donovan and Hemingway, 1963, p. 204). During the present century the tendency has been to follow the definition by H. B. Woodward in the second edition of his The Geology of England and Wales (1887) and in the same author's Lias volume (1893) of the Geological Survey's Jurassic Rocks of Britain. Woodward took the Middle Lias to comprise the ammonite zones of Ammonites (now Amaltheus) margaritatus and Am. (now Pleuroceras) spinatum. In modern terms this was, of course, a biostratigraphical rather than a lithostratigraphical definition, but the distinction was not made in Woodward's time. For this reason the Middle Lias has usually been regarded as synonymous with the Domerian substage (see below). It is for this reason that the Junction Bed, a thin lithostratigraphical unit locally mapped in the Bristol district, is shown as spanning the Middle/Upper Lias boundary in the index to the Bristol District map.

The lower boundary of the Middle Lias as a mapping unit must be taken at the base of the Dyrham Silts, and even this line was difficult to map because of poor exposure, landslipping and hillwash. In any case it is probably a gradational change from the underlying mudstones, so that the base of the unit must be arbitrarily selected. With regard to biostratigraphy, the mapped boundary probably lies within the Prodactylioceras davoei Zone.

The top of the Lias is marked, lithologically, by the change to limestone deposition of the Inferior Oolite. In the Bristol district it is a non-sequence or unconformity. The part of the Lias above the Middle Lias and below the Inferior Oolite thus comprises the Upper Lias. In the present area the higher part of the Upper Lias comprises sandy beds named Midford Sands by John Phillips (1871), part of a diachronic belt of sandy deposition often referred to as the Upper Lias Sands.

Lithostratigraphical subdivisions of the Lower Lias

The only formal units recognised within the Lower Lias are the White Lias, Blue Lias and the Lower Lias Clay, which may be regarded as members (see also Cope and others, 1980). The Blue Lias has been informally divided for the purpose of local description, into four subdivisions which are defined on p. 6. A number of informal local units, many of them very thin, have been recognised in the Radstock Shelf area, and are listed on pp. 14, 17. They are convenient for correlation between sections, but cannot in general be mapped. Subdivisions of the Middle and Upper Lias are further considered under those headings.

Chronostratigraphical divisions

The subdivision of Lower Jurassic rocks on a chronological basis into stages follows, with little modification, that introduced by d'Orbigny in the mid-nineteenth century. The version now generally accepted, and used here, follows the proposal of Arkell (1946). The zonal scheme employed in this work is slightly modified from that proposed and defined by Dean, Donovan and Howarth (1961) as used in Cope and others, (1980). Of historical interest is the fact that many of the subzones in the Hettangian and early Sinemurian are founded on careful collecting in the present area by the Bristol amateur geologist J. W. Tutcher (1858-1951), whose conclusions were published as an Appendix (Tutcher, 1918) to Buckman's first paper of 'Jurassic Chronology'.

Table 1 shows the chronostratigraphical units which have been used as the basis for correlation in the area.

On account of the relative abundance of ammonites, other fossils have been little used in the area for chronostratigraphy. In part this is also due to the lack of any modern palaeontological treatment of the common fossils, with the exception of the rhynchonellid brachiopods (Ager, 1956– 67).

Geological Survey publications

Some areas of Lower Lias situated within or adjacent to the boundaries of the one-inch to one mile Bristol District Special Sheet have already been described in Geological Survey publications. Memoirs include Monmouth and Chepstow (Welch and Trotter, 1961); the Newport memoir (Squirrell and Downing, 1969) and the Malmesbury (251) memoir (Cave, 1977).

Additional information about some localities within the Chepstow district is given below. The present account of the Lower Lias completes the coverage of the whole width of the crop of this formation east of the Severn Estuary including the area between the Coalpit Heath basin and the Cotswold escarpment. Taken with the contents of the Malmesbury (251) memoir this includes all the Lower Lias on the northern margin of the Bristol and Somerset coalfield.

The south-western part of the Bristol and Somerset coalfield, and the adjacent parts of the Mendips, are already covered by the Wells and Cheddar (280) memoir (Green and Welch, 1965). Attention is drawn to a number of isolated outliers of Lower Lias on the northern margin of the western Mendips, e.g. at Banwell, Locking, Uphill and Kewstoke, which affect the interpretation of the facies changes seen in the Lower Lias. Some of these were briefly described in the Old Series memoir (Woodward, 1876). The outliers on the Weston-super-Mare sheet (279) have been surveyed on the six-inch scale and a description appears in the relevant Sheet Memoir, published in 1983. A general description of the Lias of the whole of the Bristol District is given in the Geological Survey's Regional Handbook (Kellaway and Welch, 1948) now in course of revision.

Chapter 2 Lower Lias

White Lias

The White Lias is not distinguished from the Blue Lias on the one-inch map but is differentiated on most of the component six-inch sheets.

The White Lias of the Bristol area consists of pale grey and cream-coloured limestones, marls and clays which assume even lighter hues when weathered, thus giving rise to their name. The thicker sections in the Avon valley and near Radstock show a clear division into two parts; a lower consisting of thin, rubbly limestones with clay partings, and an upper with stronger, more regular limestones separated by clays usually not more than about 5 cm in thickness. The limestones of the upper part are hard, fine grained and uniform in texture, with conchoidal fracture, and are usually bounded by flat bedding planes, in contrast to those of the succeeding Blue Lias. The top limestone bed is known locally as the 'Sun Bed' (Arkell and Tomkeieff, 1953, p. 116) and may reach 0.46 m in thickness. It almost invariably shows vertical U-shaped burrows usually referred to as Rhizocorallum. The upper beds were formerly extensively quarried for building stone, especially in the Somerset coalfield.

A small-scale isopach map by Kellaway and Welch (1948, fig. 15A) showed that deposition of White Lias took place in two basins, respectively north and south of the Mendips. The northern area is shown in more detail in (Figure 2). The thickest sedimentation occurred in a narrow belt extending east–west through Radstock, 19 ft (5.8 m) being recorded in the Hemington Borehole [ST 7247 5296] and 20 ft 1 in (6.12 m) at Buckland Colliery [ST 7446 5079]. The thick belt may continue eastwards at depth, for 27 ft 3 in (8.3 m) of White Lias were recorded in a borehole at Westbury (Wilts.), 8 miles (13 km) E of Buckland (Pringle, 1922). The White Lias is reduced to between 0.3 and 0.6 m at Dundry Hill and north of Bristol, and dies out about 1.6 km NW of Filton.

Much of the upper part of the White Lias is barren of fossils though trace fossils are often conspicuous in the Sun Bed, and there is a restricted fauna of bivalves, gastropods and ostracods in the lower part. Twelve or thirteen genera of bivalves are recorded. Modiolus is locally abundant, and includes Modiolus hillanus and small shells often recorded as M. minimus. A few genera, including Astarte, Plagiostoma and Pleuromya, make their earliest local appearance in the White Lias. More rarely echinoids and simple corals occur. The bennetitalean leaf Otozamites obtusus, represents the land flora. Most of the recorded genera and species of marine organisms occur also in the Blue Lias. The fauna indicates that the sea was very shallow with the sea-bed suffering intermittent emergence, confirming Hallam's view (1960a) of the formation in Devon.

When in a weathered state some of the higher limestones of the White Lias, including the Sun Bed, commonly show very fine lamination and current-bedding. That these deposits had hardened and compacted before the deposition of the Blue Lias took place is shown by a section seen in Harris's Quarry on Marksbury Plain (see p. 30). Here differential compaction of the Blue Lias has taken place over ridges and hollows in the dissected upper surface of the Sun Bed. At Aust Cliff, Gloucestershire (Avon) [ST 564 895] pockets of conglomerate formed of White Lias limestone are seen from time to time beneath the basal Blue Lias limestone. Desiccation or shrinkage cracks are found locally. The geographical extent of the White Lias in southern England is shown by Donovan, Horton and Ivimey-Cook (1979, fig. 1).

Blue Lias

General description

The term Blue Lias was adopted in the Somerset Coalfield by William Smith who distinguished it from the White Lias below and the 'Blue Marl' (Lower Lias Clay of this memoir) above. The Blue Lias comprises alternating limestones and clays or shales, and is typically developed in the Avon valley around Keynsham and Saltford. The typical section includes a predominantly clayey division, the Saltford Shales (Donovan, 1956a, p. 199), separating lower and upper limestone-rich parts of the Blue Lias sequence; the upper part can be usefully divided into two. The resultant four divisions in Saltford cutting are as follows (see also (Figure 10)a):

• Division D The uppermost part of the Blue Lias, above and excluding the Calcaria Bed. Strong, persistent limestones, mostly from 0.15 to 0.30 m thick, interbedded with similar thicknesses of clay. The name Scipionianum Bed has been applied to a thick limestone at the top of the division, with phosphatised fossils on its upper surface (Donovan, 1956a, bed 1, Saltford cutting.)
• Division CThin-bedded, nodular, impersistent limestones, interbedded with clay. A few limestones are thicker and more regular and at the top lies the Calcaria Bed (named by Donovan, 1948, p. 337) in many places 0.30 m thick (bed 14). The base is taken at the base of bed 22. (divisions C and D are shown as a single division of the Blue Lias on the published maps.)
• Division BSaltford Shales. The base is taken at the upper surface of bed 26, the top is at the base of bed 22. Clays or shales with widely-spaced, thin bands of limestone, and septarian nodules. (Shown as a clay intercalation in the Blue Lias limestone on the published one-inch map.)
• Division A Basal Blue Lias limestone. Thinly bedded clays or shales and limestones, often irregular. The base is taken at the change to this lithology from the White Lias. The top is the top of bed 26, in Saltford Cutting. A limestone about half-way up the division, with ochreous galls and abundant, usually poorly preserved Psiloceras, has been named the Psiloceras Bed (Donovan, 1952, p. 635). The term Pre-planorbis Beds (Richardson, 1904, p. 36) is used informally for the thinly bedded limestones and shales below the Psiloceras Bed and above the White Lias.

The lithology of the Blue Lias corresponds to that described for the outcrops of the Dorset and Glamorgan coasts by Hallam (1960a). As in Dorset (Hallam 1960a, p. 7) laminated limestones occur in the Subzone of Caloceras johnstoni, i.e. the part of Division A (beds 26-29) above the Psiloceras Bed. Laminated shales or paper shales occur between the laminated limestones. The limestones of Division C, except at the top, are commonly a few centimetres thick, and variable in thickness; this type of limestone is referred to as 'nodular' in the descriptive text. At the top of Division C (the Calcaria Bed) and in Division D the limestones are massive and persistent, though some have irregular upper or lower boundaries.

The base of the Blue Lias is marked by an abrupt change from the fine-grained, homogeneous sediments of the White Lias to coarser, more variable and more shelly beds. The top is regarded as the top of the highest persistent, massive limestone bed. In the Avon valley around Saltford and Keynsham this is the Scipionianum Bed with phosphatised ammonites on its upper surface, marking a condensed or remanié horizon. The top of the Blue Lias on the Dorset coast is similarly marked by a condensed bed with glauconite and phosphate (Hallam, 1960a, p. 9).

There is another condensed horizon within the Arietites bucklandi Zone (Donovan, 1956a, p. 202). At Willsbridge (p. 26) it consists of clay with fragmentary phosphatised fossils resting on the Calcaria Bed. As well as bivalves and gastropods the clay contains Vermiceras spp., early species of Coroniceras (s.s.) and the schlotheimiid Sulciferites. These ammonites indicate that it represents the upper part of the conybeari Subzone and the lowest part of the Coroniceras rotiforme Subzone. The bed has been found at several localities in the Avon valley (pp. 26, 29). South of the Avon it has not been detected but its place is probably taken by a non-sequence (Donovan, loc. cit.). It reappears at a quarry south-east of Stowey church [ST 603 593] (Donovan, 1956a, p. 192; Green and Welch, 1965, p. 99) where the Calcaria Bed is capped by an 8 in (0.20 m) limestone, with abundant phosphatised Vermiceras scylla, and has an irregular upper surface. The hollows are filled with dark phosphatic material including Vermiceras solaroides, Coroniceras hyatti, Sulciferites sp., Spiriferina and other fossils. This condensed bed probably includes the whole of the rotiforme Subzone, and the rest of the Arietites bucklandi Zone is probably absent, as noted below (p. 12).

North of Chipping Sodbury the upper part of the Blue Lias passes laterally into clay (Cave, 1977, fig. 8). These and other changes in the Blue Lias are further discussed below (p. 11).

Conditions of sedimentation

At the beginning of Blue Lias sedimentation a major area of non-deposition, the London Platform, lay about 40 miles (65 km) away to the north-east (Donovan, Horton and Ivimey-Cook, 1979). Southward thinning from Bristol towards the Radstock area indicates shallower water, and coarse shelly limestones (the 'littoral deposits' of various authors) occur in Glamorgan, on Broadfield Down, and in the central and south-east Mendips. There is no evidence for beach deposits in the Bristol District Sheet. The Brockley Down Limestone (p. 19) marks areas which were too shallow for thick muddy sediments to accumulate. It is noteworthy that there is an almost complete absence of quartz sand from the area; all the sand grade rocks are calcarenites. In view of the short distance in which the transition takes place between the Brockley Down Limestone (p. 47) and Blue Lias, difference in water depth in which the two were laid down is unlikely to have been great. For this reason it appears that the Blue Lias was laid down in relatively shallow water.

The alternation at small vertical intervals of limestone with softer marls or shales has attracted attention since the earliest days of geology, and various attempts have been made to explain it. The subject was reviewed and reconsidered by Hallam (1960a, 1964).

Closely connected with the problem of the origin of the rhythmic alternation is the question whether the limestones are primary, deposited as carbonate-rich layers, or secondary, enriched during diagenesis. Some, at least, of the limestones are secondary: there are limestones which present strongly irregular upper or lower surfaces, and ones which are nodular or laterally discontinuous. Many limestone beds in the Bristol Blue Lias are of this character; they probably form a majority of the beds present. In the case of the nodular limestones the whole bed must be secondary; in the case of thicker beds this is not proved, for there could have been accretion to an existing bed. Hardening occurred before compaction of the clays, as shown by differential compaction around limestone nodules and by the frequent occurrence of ammonites with crushed inner whorls and solid body chambers, the latter having been filled with calcified sediment before compaction. In the Hettangian, specimens of Plagiostoma gigantea more than 15 cm diameter, are abundant and well preserved at Henleaze and other districts of north Bristol. Some of these shells have been distended by the accumulation of calcium carbonate in the interior of the shell, so that the separated shell fragments have retained their relative position on the exterior of the nodule. Similar effects, which have been observed in other areas (Kent, 1936), show that the growth of the nodular limestones took place by accretion of calcium carbonate in muddy sediments beneath the sea floor, before the weight of superincumbent material was sufficient to cause crushing and collapse of the shell. The formation of some nodules was completed within the time taken to accumulate only a part of one ammonite zone. Thus at Broadway Lane clay-pit (p. 42) pebbles of hard grey limestone with gervilliid valves were exhumed from the underlying clay, bored by organisms while on the sea bed, and recemented to form a conglomerate (Brown Bed) in a small part of the time taken to accumulate the deposits of one zone. Kent (1937) has noticed comparable occurrences of nodules encrusted with Serpula or riddled with annelid borings in the Lower Lias of Nottinghamshire.

Lithification was consequent on the migration of carbonate since the limestones and nodules invariably contain significantly more carbonate than the shales. Studies in Dorset and Glamorgan (Hallam, 1960a) and West Somerset (unpublished) show a discontinuity in the distribution of carbonate within the Blue Lias: the limestones contain more than 72 per cent of material soluble in cold 10 per cent HCI, and the marls and shales have less than 55 per cent; no samples fall in the intervening range.

There is evidence for depositional variations in sediment type. Among the clearest is the distinction between laminated, bituminous shales, laid down in the absence of benthonic fauna, and less well-bedded shales and marls which may show bioturbation. Laminated shales occur within the Bristol District in the planorbis Zone, where they alternate with laminated limestones, and in the thin Saltford Shales at Winsford (p. 47): they may be undetected elsewhere due to poor exposure of the shaly beds. In Dorset (Hallam, 1964, p. 159) and West Somerset (DTD, unpublished) laminated shale tends to occur about mid-way between pairs of limestones, and to be less calcareous than the other rock types. The depositional rhythm may thus have been alternation of laminated, carbonate-poor sediments with more calcareous, bioturbated ones. Concentration of carbonate then took place within the already carbonate-rich layers. The depositional differences may have controlled carbonate segregation even in the areas of thinner sequences (Bristol, Dorset) where the laminated shale phase is less in evidence than in thicker successions such as West Somerset and Glamorgan. More than one limestone may be formed in a marl interval.

Such a mechanism would account for much of the Blue Lias. The laminated limestones in the planorbis Zone have planar surfaces, and may be more easily explained as the result of primary accumulation of carbonate. It is uncertain whether the mechanism can account for the thin, alternating, nodular limestones and shales which characterise much of Division C in the Bristol area.

Fauna

The Blue Lias has been a well-known source of fossils for collectors for centuries, but only the ammonites and rhynchonellid brachiopods have been systematically studied (Donovan, 1952; Ager, 1956-67). Other groups are here summarised from available collections and records in literature.

The earliest ammonite is a specifically indeterminate psiloceratid occasionally found in the upper part of the Preplanorbis Beds. The first well-known ammonite fauna occurs in the Psiloceras Bed (Donovan, 1956a, p. 198), a horizon traceable over most of the area between the Avon valley and the Mendips. It contains abundant ammonites previously referred to as Psiloceras plicatulum and a local strongly plicate subspecies P. plicatulum bristoviense (Donovan, 1952, p. 635). They were thought to be distinct from P. planorbis which was founded on crushed material. More recent work has shown that the two species may, in fact, be identical (A. Whittaker, personal communication).

Beds immediately above the Psiloceras Bed yield Caloceras, the earliest species being Caloceras johnstoni. The genus ranges into the succeeding Alsatites liasicus Zone with close ribbed forms such as Caloceras bloomfieldense (Donovan, 1956b, new name for P. (C.) multicostatum Donovan, 1952, p. 638, pre-occupied).

The Saltford Shales yield Alsatites, Laqueoceras and Waehneroceras and the first-named genus is also found in the lowest part of Division C. These ammonites characterise the liasicus Zone.

Schlotheimia occurs throughout the middle part of Division C, and its range indicates the angulata Zone. Subzones have not been satisfactorily differentiated. Species include Schlotheimia similis with smooth outer whorls, giant species with smooth outer whorls which may, or may not, be the same as the last-named species, and small, wholly ribbed, species such as S. angulata and S. lymensis.

Arietitid ammonites with keeled, bisulcate venters are found in the uppermost part of Division C and their appearance marks the base of the Sinemurian Stage. The earliest are species of Vermiceras^‡1 , which indicate the conybeari Subzone of the Arietites bucklandi Zone. The highest Vermiceras believed to be at their original stratigraphical level are large examples of V. caesar on the upper surface of the Calcaria Bed, where they are accompanied by large Sulciferites ^‡2  but derived Vermiceras are found at slightly later horizons in the area south of Bristol (p. 7), in the lowest part of Division D.

The succeeding rotiforme Subzone shows two successive faunas: the lower with Coroniceras (C.) rotiforme and allied species, the upper with C. (C.) caprotinum and C. (C.) hyatti. Coroniceras s.s. was referred to as Primarietites by Donovan (1952). The incoming of the massive-whorled subgenus Arietites marks the base of the bucklandi Subzone which occurs in the middle part of Division D. Then come large, close-ribbed species of Coroniceras such as C. (C.) reynesi and Arnioceras marking the reynesi Subzone at the base of the Arnioceras semicostatum Zone. Finally, arietitid genera with 'degenerate' venters appear just below the top of Division D. Agassiceras marks the scipionianum Subzone, and along with it occur the earliest Euagassiceras, E. personatum (Simpson). The Euagassiceras sauzeanum Subzone is represented in the clays above the Blue Lias.

The chief fossils in the Blue Lias apart from ammonites are the Bivalvia. About 25 genera are recorded in the following superfamilies:

• Arcacea: Parallelodon
• Mytilacea: Modiolus
• Pinnacea: Pinna
• Pteriacea: Bakevellia, Gervillia, Isognomon
• Pectinacea: Camptonectes, Chlamys, Dimyopsis, Meleagrinella, Oxytoma, Pseudopecten, Terquemia
• Limacea: Antiquilima, Ctenostreon, Plagiostoma, Pseudolimea
• Ostreacea: Ggphaea, Liostrea
• Lucinacea: Mactromya, Unicardium
• Carditacea: Myoconcha
• Crassatellacea: Astarte, Cardinia
• Cardiacea: Protocardia
• Pholadomyacea: Gresslya, Pachymya, Pholadomya, Pleuromya, Pteromya

Many genera range through the Blue Lias, although they do not necessarily occur at frequent horizons. Only a few require comment on their vertical ranges. Parallelodon, Gervillia, Protocardia and Pachymya are recorded in the Avon valley only from the Pre-planorbis Beds, which have a slightly more diverse bivalve fauna (about 16 genera) than any of the higher subdivisions. Liostrea ranges throughout but Gryphaea first appears in the upper part of the Saltford Shales (Division B) and is thereafter common. Gresslya seems to be recorded only from Divisions B and C.

Apart from ammonites and bivalves the Blue Lias fauna shows very low diversity. There are a few records of gastropods, mainly Pleurotomaria, a few other species are listed by Vaughan and Tutcher (1903, p. 47) from Division C. Brachiopods appear in the Subzone of Caloceras johnstoni with Zeilleria sarthacensis and Calcirhynchia calcaria, which range through the Blue Lias. The abundance of the small C. calcaria in Division C has been noted by many workers. Spiriferina is found in Divisions C and D, and Piarorhynchia juvenis in Division D.

Echinoid spines are common in some of the clays (cf. Vaughan and Tutcher, 1903, p. 15) and an unnamed new species of Pseudodiadema was described by them (ibid. p. 53). 'Cidaris' is also recorded.

Vaughan and Tutcher (1903, p. 40) noted that belemnites first appear in the Calcaria Bed and become abundant in the upper part of Division D.

The fossil vertebrates have received little attention. A collection of ichthyosaurs and plesiosaurs, including local specimens, in the Bristol City Museum, was destroyed by bombing in 1940. The plesiosaurs had been photographed and were described by Swinton (1948). Specimens seem to have come from the Avon valley, but there is little information as to their stratigraphical horizons. From the Hettangian of the Radstock area Tutcher and Trueman (1925, p. 654). recorded ichthyosaurs, plesiosaurs and four species of fossil fish.

The distribution of trace fossils has not been studied within the area. Diplocraterion and possibly Rhizocorallium have been noted in Division D at Saltford.

Palaeoecology

Pelagic animals are represented by the ammonites, belemnites, fish and reptiles. Although the Keynsham area has long been famous for ammonites, they are not particularly common except at a few horizons, such as the Psiloceras Bed, the upper surface of the Calcaria Bed, and a few horizons in Division D.

In considering the benthonic fauna it may be emphasised that for much, if not all, the deposition represented by the Blue Lias there was a soft muddy bottom. If many of the limestones are not primary (p. 7) there was little, if any, difference between clays and limestones at the time of sedimentation. It is unlikely that a hard bottom was present except at times when a hiatus in sedimentation resulted in a hard ground, for example the top of the Calcaria Bed and perhaps a few other horizons. No study of the faunal assemblages has been made.

The bivalve fauna, with about 25 genera, compares closely with those of the Blue Lias of Dorset and Glamorgan (Hallam, 1960), 18 genera being recorded from both Bristol and Dorset. The limestones and shales deposited prior to the appearance of P. planorbis (the pre-Planorbis Beds) has more diverse fauna in Bristol (18 genera) than in Dorset or Glamorgan (12 genera in each case).

Comparison may also be made with the bivalve fauna of the Lower Oxford Clay (Duff, 1975), with 30 genera in 17 superfamilies. Ten of the Blue Lias superfamilies are also represented in the Lower Oxford Clay; the only ones absent are the Limacea, which include important members of the Blue Lias fauna, and Carditacea. Of the Lower Oxford Clay bivalves absent from the Blue Lias in the Avon valley, the nuculoids appear to be the most significant, for they are present in all ten biofacies recognised by Duff. As they are deposit feeders which often flourish on muddy bottoms their absence is difficult to explain; they perhaps characterised deeper water. The other absentees are probably of less significance; they include the Myacea, which did not exist in the Lower Jurassic, and four other superfamilies which are all burrowing forms.

The Blue Lias bivalve fauna is dominated by epifaunal (byssally attached or free swimming) genera. Sixteen genera (the first seven superfamilies in the list on page p. 8) belong to this category. Infaunal, burrowing genera are in a minority and only four or five genera are found in the whole of the Blue Lias. The significance of this proportion should not be overemphasised because the overall composition of the bivalve fauna at the time was different from that of today: many burrowing groups had not yet evolved. Nevertheless it is of interest in view of the predominance of soft bottoms mentioned above. A soft substrate appears to be no hindrance to some sessile bivalves, shown by the fact that oyster beds characterise muddy estuaries at the present day. In the Blue Lias Liostrea often shows signs of attachment to Cardinia shells, to other Liostrea or to ammonites. Byssally attached genera may have likewise anchored themselves to dead shells lying on the sea floor or to floating weed or driftwood. Bivalve faunas of modern muddy bottoms tend to be dominated by deposit feeders, nuculoids and tellinaceans in the case of temperate seas off north-west Europe. These are absent from the Bristol and Dorset Blue Lias, though Nuculana is recorded from the liasicus and angulata Zones of Glamorgan (Hallam, loc. cit.).

It may be suggested that the bivalve fauna reflects rather shallow, well-aerated conditions, but at intervals the water near the bottom was often quiet and therefore poorly oxygenated and the fauna is reduced or absent. The greatest diversity occurs at the base of the Blue Lias, where shelly limestones indicate a flourishing fauna. The rest of the Blue Lias, was probably characterised by more rapid deposition of mud and deeper water.

In the Radstock facies the Blue Lias differs from the typical development chiefly by reduction of the clay content. The bivalve fauna is very like that of the normal facies. A few additional genera are recorded, including Goniomya and Eopecten. In contrast to typical Blue Lias, the greatest diversity is recorded from the lateral equivalent of the Saltford Shales, according to Tutcher and Trueman (1925, pp. 656-658, 'Angulata Beds', mainly, liasicus Zone). About half a dozen genera of gastropods are recorded from the Hettangian.

Lower Lias Clay

General description

This term covers the remainder of the Lower Lias above the Blue Lias. The beds are seldom exposed in the area, naturally or artificially. The only complete section was provided by the Dundry (Elton Farm) Borehole in which the clay was 123.8 m thick. The topmost 54.1 m are recorded as clays, calcareous clays, and silty clays, with a few thin harder bands. The base of this unit is just below the base of the Prodactylioceras davoei Zone. The next 39.7 m are recorded mainly as calcareous clays, with about half a dozen bands of mudstone or argillaceous limestone, up to 0.48 m thick, in the lower part. The lowest 30 m are recorded chiefly as dark, waxy clays, with rare thin (less than 0.23 m thick) argillaceous limestones. Nodules are recorded at a number of levels in the borehole (Ivimey-Cook, 1978) and at surface exposures (Donovan, 1978).

In addition to calcareous nodules the higher parts of the Lower Lias Clay contain nodules of clay ironstone or ferruginous cementstone which are commonly hollow or septarian. In the interior of these nodules traces of sulphides and sulphates are found, either as crystalline aggregates or as films of minute crystals (some less than 0.5 mm) lining the walls of cracks and cavities. The upper unfossiliferous part of the Lower Lias Clay at Broadway Lane claypit near Welton (p. 42) contained many such nodules in which tiny crystals of pyrite, galena, sphalerite and chalcopyrite are present. Pyrite is abundant in many of the Liassic clays, especially in the ibex and davoei zones, which were probably deposited in relatively still water under reducing conditions.

South of National Grid northing 60 much of the Lower Lias Clay passes rapidly into the condensed facies, chiefly limestones, of the Radstock area, described on pp. 14-19. The uppermost part continues southwards (the Striatum Clays and Capricornum Clays of Tutcher and Trueman, 1925), who estimated their thickness at Timsbury Sleight [ST 654 594] as about 120 ft (37 m). South-eastwards the thickness diminishes slowly, to about 30 m at Clandown and Stony Littleton, and then more rapidly south of Radstock to about 17 m at Haydon Hill [ST 690 534], about 4.6 m at Newbury [ST 701 505] and about 2.75 m north of Upper Vobster [ST 708 501].

In the Dundry Borehole the Lower Lias Clay presents a fairly complete succession of ammonite zones, except for a major non-sequence involving the Eparietites denotatus Subzone, the whole of the Oxynoticeras oxynotum Zone and the Crucilobiceras densinodulum Subzone (Donovan, 1978). In the Radstock area the Lower Lias Clay probably comprises part of the Beaniceras luridum Subzone, and the two lower subzones of the Prodactylioceras davoei Zone.

Fauna

The fauna of the Lower Lias Clay is known chiefly from Broadway Lane clay pit near Welton (p. 42) and from the Dundry (Elton Farm) Borehole (Ivimey-Cook, 1978). Apart from the ammonites, the chief fossils are Bivalvia, thirty genera of which are recorded. The list is similar to that from the Blue Lias, the chief additions being the protobranch deposit-feeders Palaeoneilo, Palaeonucula and Ryderia. Few genera are found in the dark, waxy clays forming the lowest 30 m (p. 9), but diversity of the fauna increases in the calcareous clays above. The upper part of the ibex Zone and the lower part of the davoei Zone (about 108 to 86 m depth) are almost barren of fossils, only Palaeoneilo being frequently recorded. The remainder of the davoei Zone has a more diverse bivalve fauna than the rest of the clays, 21 genera being recorded, as well as crinoids, echinoids, crustaceans and rhynchonellid brachiopods.

Stratigraphy

Bristol outliers

The largest tract of Lower Lias north of the Avon at Bristol is the Horfield Outlier (Figure 1) extending southwards from Patchway and Stoke Gifford to Montpelier, Bristol. There are a number of smaller outliers to the south. Only the lower part of the Lower Lias is preserved. All the rocks are of an argillaceous facies similar in general character to that south of the river at Knowle and Bedminster Down. Argillaceous limestones, slabby at the base, but lenticular, nodular, rubbly and discontinuous at higher levels, are interbedded with, and pass laterally into, thick clays and marls. Similar rocks are also present west of the Severn estuary at Sedbury, near Chepstow, and Bishopston, near Newport (Squirrell and Downing, 1969), suggesting that the greater part of the Severn estuary and the Bristol district north and west of Broadfield Down was formerly covered by Lower Lias of dominantly argillaceous facies.

The succession in the urban and industrial areas of north Bristol is based on information from old records and temporary sections, from feature mapping, and from boreholes. There are no permanent exposures. The sequence is best known on the western side of the Horfield Outlier, in the districts of Henleaze and Southmead (Figure 15). Here the total thickness of the rocks between the base of the clays of semicostatum Zone age and the base of the White Lias is about 18 to 21 m, while on the eastern side of the outlier it is about 30 to 40 m between Ashley Down and Horfield railway station [ST 6060 7750]. The semicostatum Zone is represented by a residual thickness of about 4.6 m of dark grey or black clay with septarian nodules in the railway cutting at Horfield [ST 606 775] and up to 6.0 to 9.0 m between Horfield Common and Horfield Station. Following the primary six-inch survey in 1946-52 the succession and structure of the Lower Lias at Henleaze and Southmead as described by Kellaway (1932) were modified (6-inch ST 67 NE). The sequence at Henleaze and Southmead may be summarised as follows:

Psiloceras planorbis at Henleaze occurs in thin limestones between the Pre-planorbis Beds and the clays and nodular limestone of the johnstoni Subzone containing Caloceras. North of Henleaze, however, Psiloceras planorbis appears to be missing, and was not found in Filton railway cutting where limestone with Caloceras johnstoni rests directly on Pre-planorbis beds (Tutcher, 1908). Sections in the Filton area suggest at least one non-sequence within the johnstoni Subzone. At Cotham Groom-Napier (1868) recorded a conglomerate which again suggests condensation, possibly in the same sub-zone, although it is difficult to interpret his ammonite records.

At the local top of the succession, Stoddart (1868) recorded Am. sauzeanus (i.e. Euagassiceras sp.) from shales with Am. semicostatus (i.e. Arnioceras sp.), and well-preserved, pyritised specimens of Euagassiceras resupinatum recorded from the former 'Ashley Down Station' [ST 600 756] by Donovan (1952, p. 743) presumably come from a similar horizon. These records indicate that the highest beds in this area belong to the sauzeanum Subzone of the semicostatum Zone.

A number of other fossils are useful as local indices. The brachiopods Calcirhynchia calcaria and Zeilleria sarthacensis both appear for the first time in the planorbis Zone. In general, however, the zeilleriids are most abundant in the angulata Zone while Calcirhynchia achieves its maximum development at the top of the angulata Zone and in the bucklandi Zone. Later rhynchonellids, probably to be identified as Piarorhynchia juvenis, are not uncommon in the nodular limestone and shale at the top of the bucklandi Zone and base of the semicostatum Zone, and Ager (1962, p. 121) has pointed out that this species replaces Calcirhynchia at the top of the bucklandi Zone.

South of the Bristol Avon in the Totterdown and Windmill Hill outliers (Figure 1) the White Lias is up to 0.83 m thick and has therefore been separated on the published Geological Survey six-inch map (ST 57 SE). Owing to the northerly attenuation of the White Lias it is impossible to map it separately from the overlying Blue Lias limestone over a large part of the Bristol area north of the Avon, but in this area its presence is indicated by the symbol WBL on the published six-inch maps.

The thin-bedded, shelly and flaggy limestones of the basal part of the Blue Lias were formerly extensively worked, particularly in the Stoke Gifford area.

Between the basal flaggy limestones (chiefly Pre-planorbis Beds) and the shales and clays of the semicostatum Zone, the rocks consist of clay with a greater or lesser proportion of nodular or lenticular limestone.

The following units were distinguished during the course of the primary six-inch mapping:

The limits of these units shown on the published six-inch and one-inch maps are, however, not very well defined due to lateral passage of clay with nodular or lenticular limestone into wholly argillaceous rocks. The limestones of the bucklandi Zone are the most massive and regular in character.

The principal changes of thickness seen in the north Bristol area are shown in (Figure 15) (p. 38) The westerly attenuation of the Lower Lias is not related to the thickness of Triassic deposits between the base of the Lower Lias and Palaeozoic surface. At least 60 m of Trias underlie the very thin Lias at Gotham (Figure 15) whereas the much thicker sequence at Henleaze is underlain by thin Rhaetic beds which rest on a small thickness of Mercia Mudstone and associated rocks and then directly on Carboniferous Limestone. At Ashley Down, Montpelier, and the adjacent parts of Horfield, the excellent preservation of the fossils, such as Oxytoma (Palmoxytoma) longicostata (Stutchbury, 1839), for which Montpelier quarry was noted, the greater range and variety of the ammonite faunas and increased thickness (Figure 15), suggest basinal conditions.

Eastern margin of the Bristol Coalfield

Changes in thickness and facies of the Lower Lias north of Chipping Sodbury have been discussed elsewhere (Cave, 1977) and are only briefly summarised here. Where the Lower Lias outcrop enters the Bristol District Sheet, at Wickwar, the total thickness may be about 120 m, but is difficult to estimate because of strike faulting. The only limestones present are Division A of the sequence farther south. The rest of the Blue Lias in the Avon valley is represented by shales and nodular limestones with Gryphaea arcuata.

At the top of the Lower Lias in the same area the Dyrham Silts include beds with fossils of the davoei Zone but may be in part of Domerian age. The lower part is probably to be correlated with blue silty clays in the upper part of the Lower Lias Clay at Twerton clay-pit [ST 733 643], west of Bath, which yielded poorly preserved liparoceratid ammonites, probably indicating the davoei Zone. The distribution of the silts also suggests that they are linked more closely with Lower Lias Clay than with the overlying Marlstone Rock-Bed, which can only be traced as far south as Horton, and from which the Dyrham silts may be separated by a non-sequence. For purposes of measurement, therefore, the Dyrham Silts have been grouped with the Lower Lias, unless there is (as at Dundry) positive evidence of the presence of Middle Lias fossils.

In the Chipping Sodbury and Dodington area the generalised succession of the Lower Lias may be summarised as follows:

North of Chipping Sodbury the total thickness of the Lower Lias (including Dyrham Silts) is therefore of the order of 90 m but there is evidence for west–east thickening of the strata (Figure 8). South of Sodbury the upper limestones of the Blue Lias thicken and pass laterally into nodular and lenticular limestone with fossils of the angulata and bucklandi zones. The Blue Lias is about 15 m thick at Chipping Sodbury and between Pucklechurch and Dyrham about 18 m.

Within the Lower Lias Clay a band of lenticular and nodular limestone yields fossils of the jamesoni Zone. In the area east of Wick this lies about 30 m above the top of the Blue Lias, i.e. roughly midway between the top of the Blue Lias and the base of the Upper Lias (Cotteswold Sands).

The Avon valley

The Blue Lias is fully developed in the Avon valley where it is about 18 m thick. Southwards and westwards from here it is incompletely developed as described below, and in the southeast of the Bristol District it is replaced by the Radstock facies (p. 14). On Broadfield Down and in parts of the Mendips massive shelly and conglomeratic rocks replace parts of the normal White Lias and Blue Lias (p. 19).

Isopachytes for the Hettangian Stage (Figure 4)." data-name="images/P936352.jpg">(Figure 3) show that from the structural point of view this is intermediate between the White Lias (Figure 2) and the remainder of the Lower Lias. The overall pattern of downwarping seen in the Lower Lias is well established, with thin Lias in the south thickening northwards to the Avon valley, though a reduced remnant of the White Lias basin at Radstock remains.

In the Avon valley and at Dundry the zonal and subzonal succession in the Hettangian is complete (Figure 4), but the Schlotheimia angulata Zone is absent from the whole of the Radstock shelf and the Alsatites liasicus Zone from a smaller area centred on Paulton^‡3  (Figure 6).

The Blue Lias as defined above (p. 6) is typically developed at Bath and in the Avon valley around Keynsham and Saltford. To the south it becomes thinner, and incomplete, while northwards an increase in thickness is accompanied by a preponderance of clay so that the subdivisions tend to lose their identity as noted above (p. 11). The principal changes to the south of the Avon are described below.

Division A doubles its thickness westwards from about 2.6 to 4.9 m between Saltford cutting and the northern flank of Dundry Hill. It is also thick in the Dundry Borehole where its exact limit with Division B cannot be determined, but it may be about 4.2 m thick.

At Burnett, only 2 miles (3 km) SW of Saltford, Division A is only 1.83 m. This thickness is maintained southwards in the Farmborough and Marksbury areas, south of which the division cannot be recognised due to the absence of the Saltford Shales.

The Saltford Shales (Division B) typically developed in the Avon valley, also become thinner to the south, being 2.9 m at Burnett and 0.9 m at Farmborough. Westwards from Keynsham, in the Charlton Field area, they become too thin to be mapped. North-west of Hursley Hill, Whitchurch, they have again been mapped from [ST 618 662], intermittently along the southern side of Dundry Hill. Their behaviour here may be related to the Winford Growth Fold (p. 33).

Division C is 6.0 m thick at Saltford Cutting, and about 5.0 m in the Dundry Borehole. It is absent from the Radstock area. At Chewton Mendip the angulata Zone is present (Green and Welch, 1965, p. 101) but the nodular limestones characteristic of Division C in the Avon valley are not developed. Instead, the zone consists of massive, coarse-grained limestones with a little shale.

Tutcher and Trueman (1925, p. 627) believed that folding had occurred during the later Hettangian along nearly east–west axes, and reconstructed sub-Sinemurian outcrops (their fig. 11). There is no unique interpretation of the data, and an alternative version is shown in (Figure 6), which takes in a larger area than that considered by Tutcher and Trueman. There are too many gaps in the data for the lines to be well controlled, but there appears to be a roughly concentric pattern, successively lower zones being cut out as one approaches Paulton, where at Phyllis Hill Quarry (p. 42) Sinemurian rests on White Lias (Green and Welch, 1965, p. 89). Interrupting this simple concentric pattern is a small north–south belt in which relatively thick liasicus Zone is locally preserved around Radstock, reaching about 4.0 m at Middle Pit Quarry (p. 44).

Much of the variation in thickness of the Hettangian must be depositional, for it affects individual parts of the succession as well as the Stage as a whole. In view of the time-gap in the Radstock area between the youngest Hettangian deposits (liasicus Zone) and the earliest Sinemurian (see below) there was doubtless some erosion during the interval, and probably Tutcher and Trueman were right in supposing that the present distribution of the rocks of each of the Hettangian zones represented their original deposition slightly modified by later erosion in Lower Lias times.

The Lower Sinemurian is 22.25 m thick in Dundry (Elton Farm) Borehole and is probably fairly complete, though little palaeontological subdivision was possible. Around Keynsham and Saltford the subzonal succession in the bucklandi and semicostatum Zones is complete (Donovan, 1956a). Vaughan and Tutcher's (1903) records of Am. turneri [Caenisites] associated with Am. planicosta [Promicroceras] and Am. geometricus [Arnioceras] indicate the presence of the turneri Zone at Keynsham. For several miles south of the Avon valley exposures used to show parts of the bucklandi and semicostatum zones in Blue Lias facies. At Priston Mill (Donovan, 1956a, p. 193, loc. 9) the semicostatum Zone showed a preponderance of limestone over shale, a reversal of conditions in the Avon valley, and the upper part of the zone was in condensed facies.

In the Avon valley the Arietites bucklandi Zone and the lower part of the Arnioceras semicostatum Zone form the uppermost part of the Blue Lias (Division D plus the uppermost foot or two of Division C). At Saltford cutting this Sinemurian part of the Blue Lias is 5.8 m thick. Donovan (1978, p. 16) has shown that Division D disappears to the south and west of the Avon valley, and its approximate limit is shown in (Figure 6). In view of the general southerly thinning, lateral passage into clay is unlikely, and the limit is close to the southern limit of the area within which the bucklandi Zone has been proved. The disappearance is probably due to non-deposition. In the Radstock area the bucklandi Zone is absent and the A. semicostatum and C. turneri zones are partially represented by condensed deposits, described below (p. 14).

The Upper Sinemurian is 19.3 m thick in the Dundry Borehole, where it is incomplete. The upper part of the Asteroceras obtusum Zone, the whole of the Oxynoticeras oxynotum Zone, and the Crucilobiceras densinodulum Subzone are absent. There is no satisfactory evidence for the substage in the Avon valley, due to lack of exposure. It is represented in the Radstock area by thin and remanie deposits, described below (p. 17).

The Lower Pliensbachian is 84.07 m thick in the Dundry Borehole. The only other complete section is at Timsbury Sleight (p. 40), 11 km ESE of the borehole, where the Uptonia jamesoni and lower part of the Tragophylloceras ibex zones are represented by the Jamesoni and Valdani Limestones (see below), and the clays of the upper part of the substage are reduced to nearly half the corresponding thickness at Dundry. Lateral changes in the clays are summarised on p. 9.

The Radstock Shelf

Hettangian

The changes in the Hettangian rocks southwards from the Avon valley have already been described (p. 12 and (Figure 4)). The pattern of the subcrop of the Hettangian ammonite zones beneath the Sinemurian (Figure 6) has resulted from late- or post-Hettangian erosion, presumably on a submarine shoal marking the position of the Radstock Shelf, before deposition was resumed in the Arnioceras semicostatum Zone.

The Hettangian rocks are chiefly thinly bedded, impure limestones, separated by shales or by shaly partings. They are often referred to as the 'corn grits' in old papers.

Sinemurian

Over the Radstock Shelf the Sinemurian is thin and incomplete. Tutcher and Trueman (1925) recognised five subdivisions as follows:

• Raricostatum Clay
• Obtusum Bed
• Turneri Clay
• Spiriferina Bed
• Bucklandi Bed

The bucklandi Zone, together with the angulata Zone of the Hettangian, is absent. There must have been a shallowing of the sea, the effect of which is to be noted further north by a break in deposition which is probably marked by the top of the Calcaria Bed, which often forms an 'ammonite pavement', and in the condensed, phosphatic facies of part of the bucklandi Zone at Stowey and in the Avon valley. The recommencement of deposition marked in the Radstock area by the Bucklandi Bed (of semicostatum Zone age, notwithstanding its name) is manifested in the predominance of clay in the sections in the Avon valley, and, further afield, by the early Sinemurian transgression over the London platform (Donovan, Horton and Ivimey-Cook, 1979).

The Bucklandi Bed is a limestone ranging from 0.05 to 0.75 m in thickness. Its northernmost occurrences are around Timsbury, and it is restricted to this area and the immediate neighbourhood of Paulton and Radstock. Northwards it is presumed to pass laterally into the Blue Lias limestones and shales of the upper part of Division D, but exposure is poor.

The ammonites of the Bucklandi Bed include Arnioceras, Agassiceras and Euagassiceras. They are commonly phosphatised and presumed to be derived (viz. Tutcher and Trueman, 1925, p. 601). The exact horizons of the numerous specimens now preserved in museums are rarely closely recorded. It may not be known whether a fossil is from the Bucklandi Bed or the succeeding Spiriferina Bed, and hence it is impossible to give a definitive list of species. Arnioceras hodderi and A. notatum (Tutcher and Trueman, 1925, pl. 38, figs. la–c and pl. 39, figs. 2a, b respectively), however, were definitely recorded as from the bed. They are probably synonyms of A. bodleyi and A. semicostatum respectively. The same authors figured two specimens which appear to be a late, close-ribbed Coroniceras (pl. 38, figs. 2a, b), Coroniceras (Eucoroniceras) (pl. 38, fig. 2c) was recorded by Tutcher and Trueman (1925, p. 609) from Bowldish.

The rest of the fauna includes Spiriferina, usually recorded as S. tumida and S. walcotti, Piarorhynchia juvenis (usually recorded as P. radstockiensis, however, Ager (1962) restricted the latter name to the form from the Jamesoni Limestone), Squamirhynchia squamiplex, Zeilleria spp.and Terebratula (Lobothyris?). Tutcher and Trueman (1925) recorded 14 genera of Bivalvia, of which all except one or two are also found in the Blue Lias of the Bristol area. They also noted the gastropods Pleurotomaria and Ptychomphalus, nautiloids, and belemnites.

As regards the age of the Bucklandi Bed, the ammonites (? all derived) indicate the Zone of Arnioceras semicostatum. The bed may have accumulated slowly during that zone, but more probably it was laid down towards the end of the time represented by the zone, and perhaps even in early Caenisites turneri Zone time. Some of the specimens of Arnioceras (e.g. A. notatum) have a long initial smooth stage and are likely to be of turneri Zone age.

The Spiriferina Bed has been discussed in detail by Tutcher and Trueman (1925, p. 630). It forms, as they remarked, the basal part of the Turneri Clay, which marks a short-lived period of clay sedimentation in the turneri Zone. The Turneri Clay ranges from a few centimetres to 1.2 m thick in the Radstock area, but reaches 2.4 m at Dunkerton and Camerton, and presumably thickens towards Bath as the normal Lower Lias Clay facies develops.

The fauna of the Spiriferina Bed includes abundant Spiriferina walcotti, usually in black, phosphatic preservation and presumably derived, and derived ammonites of the genera Arnioceras, Coroniceras and Euagassiceras. Moore (1867, p. 473) recorded the bivalves Gervillia, Pholadomya, and Pleuroyma? (as Myacites). He noted that the Turneri Clay at Camerton was 'laminated blue clay' and recorded 28 species of foraminifera from it, also 'Plicatula spinosa', ostracods and fish-scales. Subsequent fossil records from the Turneri Clay are poor, but include the ammonites Caenisites and Arnioceras, derived Agassiceras and Euagassiceras (Tutcher and Trueman, 1925), and bivalves now interpreted as Chlamys, Gryphaea, Mactromya and Pleuromya (Tawney, 1875b). The absence of the ammonites Microderoceras and Promicroceras which accompany Caenisites throughout most of the turnefi Zone, suggests that the lower part of the zone is represented by the Turneri Clay.

After a hiatus during the later part of the turneri Zone, deposition on the Radstock Shelf was resumed in the early part of obtusum Zone. One foot 5 in (0.43 m) of beds, mainly shale, of this age was exposed at Hodder's Quarry, Timsbury (p. 40), but at most localities any clay deposited was winnowed away to leave the fossils of the Obtusum Nodules. Tutcher and Trueman believed that this denudation also removed much of the Turneri Clay. The Obtusum Nodules yield chiefly Asteroceras and Arnioceras, but Promicroceras, Xipheroceras and Cymbites laevigatus were also recorded by Tutcher and Trueman (1925). The fauna probably represents mainly the Subzone of Asteroceras obtusum. Asteroceras stellare, the index species of the next subzone, was recorded by Tutcher and Trueman (1925, p. 636) without any locality, so that the incidence of this subzone is not known.

The Eparietites denotatus Subzone, the whole of the Oxynoticeras oxynotum Zone, and the lowest subzone of the Echioceras raricostatum Zone are absent from the Radstock area, as they are from the thicker clay sequence further north.

Parts of the Zone of Echioceras raricostatum are represented by fossils in the Raricostatum Clay, and by derived ones in the succeeding Armatum Bed (see below, p. 17). The Clay was thickest at Clandown (p. 43). The ammonite fauna of the Family Echioceratidae was revised by Getty, (1973). According to Tutcher and Trueman (1925), who saw exposures in a fresher state than are now available, most of the fossils in the Raricostatum Clay are derived, but they reported Echioceras raricostatoides ('zieteni') succeeded by Leptechioceras macdonnelli at Timsbury (1925, pp. 605, 631). Echioceras and Leptechioceras are now both restricted geographically to a narrow area extending from Timsbury in the north to the southern outskirts of Radstock. Clays of the raricostatoides and macdonnelli subzones were perhaps never very thick. They have in any case been largely destroyed, leaving behind fossils and nodules some of which were incorporated in the Armatum Bed.

The attractive and often well-preserved derived ammonites of the Armatum Bed have been collected for a century or more, especially from Radstock Grove and Kilmersdon Colliery quarries (p. 44). The commonest member of the Echioceratidae is Echioceras raricostatoides. Trueman and Williams (1925) named numerous new species based on Radstock Grove specimens, but their E. crassicostatum, iridescens, modicum, notatum, sparsicostatum and torquatum are now regarded as synonyms of E. raricostatoides. Echioceras quenstedti also occurs. The bisulcate genus Paltechioceras is represented by P. tardecrescens (von Hauer) (a senior synonym of P. aplanatum Hyatt, the index species of the highest subzone of the E. raricostatum Zone) and other species including P. elicitum S. S. Buckman, P. oosteri (Dumortier), P. rothpletzi (Bose); and P. variabile (Trueman & Williams). Several new genera and species, now relegated to synonymy, were set up by Trueman and Williams (1925) for Radstock material. Paltechioceras (Orthechioceras) edmundi (Dumortier) has been recorded from Kilmersdon Colliery Quarry (p. 45). A single specimen of Gagaticeras neglectum (Simpson) from Rockhill (p. 43) indicates the former presence of the oxynotum Zone.

Other ammonites which occur as derived fossils in the basal part of the Armatum Bed include Oxynoticeras and the related, but more involute Radstockiceras, and rare Eoderoceras and Epideroceras. Some of the derived ammonites, especially the Echioceratidae, are nearly perfect, but others are broken and may show exposed septal surfaces in contact with the matrix, showing that they were broken before being incorporated in the bed.

Lower Pliensbachian

The following lithological units in this substage were recognised by Tutcher and Trueman (1925, table facing p. 598):

Clays (the 'Striatum Clays' and 'Capricornum Clays')

Valdani Limestone

Jamesoni Limestone

Armatum Bed

The Armatum Bed is really the lowest part of the Jamesoni Limestone (s.1.), from which it is sometimes divided by a prominent bedding plane. It is uncertain whether the limit between the Armatum Bed and Jamesoni Limestone, as recognised by various authors at different sections, is a constant horizon.

The indigenous, non-derived ammonites from the Armatum Bed are predominantly the giant Apoderoceras, often 0.30 m or more in diameter. Eoderoceras is rare but recurs in a preservation suggesting that it is not derived. Phricodoceras lamellosum (d'Orbigny) also occurs, and reaches a large size. The indigenous ammonites of the Armatum Bed show that it belongs to the Phricodoceras taylori Subzone of the Uptonia jamesoni Zone.

Tutcher and Trueman (1925) recorded about 15 genera of Bivalvia and 11 genera of Gastropoda from the Armatum Bed, all common Lower Liassic forms; in both groups many of their names require revision. The brachiopod fauna will be considered below in connection with the Jamesoni Limestone.

The Jamesoni Limestone consists, like the Armatum Bed, of ironshot limestone. To the naked eye it is a fine-grained, cream-coloured rock with numerous ferruginous grains in the weathered specimen. In thin section the matrix is seen to be partly micrite with sparite patches, with abundant echinoderm remains showing the original stereomic structure, and shell fragments which have been recrystallised. Low-power magnification shows that most of the ferruginous grains are fragments of fossils, around 1 mm in size.

The Jamesoni Limestone is extremely fossiliferous, ammonites, bivalves and brachiopods being noteworthy. Although echinoderm fragments are abundant in thin section, whole fossils are not, only one species of Pseudodiadema being recorded by Tutcher and Trueman (1925, p. 660). The deposit is highly condensed but Tutcher and Trueman (op. cit., pp. 601, 604) recorded a sequence of ammonite faunas from Clandown Colliery Quarry (see p. 43) which indicate that the following zones and subzones occur in sequence:

Other ammonites include Tragophylloceras undulatum, of which the type came from the Radstock area, and T. ibex; Lytoceras fimbriatum; Parinodiceras parinodus and Liparoceras (Vicininodiceras) simplicicosta.

The bivalve fauna of the Jamesoni Limestone, as recorded by Tutcher and Trueman (1925), with emended names, includes:

• Arcacea: Grammatodon, Parallelodon
• Pinnacea: Pinna
• Pteriacea: Parainoceramus
• Pectinacea: Chlamys, Entolium, Eopecten, Oxytoma, Plagiostoma, Plicatula
• Limacea: Pseudolimea
• Ostreacea: Gryphaea, Liostrea
• Crassatellacea: Astarte, Cardinia
• Pholadomyacea: Gresslya, Pholadomya, Pleuromya
• Carditacea: Tutcheria
• Arcticacea: Pronoella

Eight genera of gastropods were listed by Tutcher and Trueman (1925, p. 655).

The brachiopod fauna of the Jamesoni Limestone is profuse at some localities. In the following list the rhynchonellids are taken from the monographic revision by Ager (1956– 67). The other groups have not been recently revised.

Rhynchonellacea: Cirpa fronto, Cuneirhynchia dalmasi, Furcirhynchia furcata, Gibbirhynchia curviceps, Piarorhynchia radstockiensis, Rimirhynchia anglica, R. elevata, Rudirhynchia ?fallax.

Terebratulacea: Lobothyris radstockiensis

Zeilleriacea: Zeilleria waterhousii?, Z. (Cincta) numismalis [numerous other 'species' of Cincta numismaliswere named and described from the Radstock area by Buckman (1907), these are regarded as synonyms by Delance (1974)], Zeilleria perforata.

The area of occurrence and variations in thickness of the Armatum Bed and Jamesoni Limestone are shown in (Figure 7). These units overlap earlier units southwards and come to rest on White Lias in the Hemington Borehole (Cantrill and Pringle, 1914) and on Carboniferous Limestone at Vobster (p. 46).

The Valdani Limestone is the uppermost subdivision of the condensed Radstock facies. It rests on the Jamesoni Limestone but occurs over a smaller area, being restricted to the country between Radstock and Paulton (Figure 7). It is thickest at Old Pit Quarry (p. 43) where it is 0.7 m of hard, coarse-grained, crystalline limestone, ironshot at the base. At Clandown Colliery Quarry, now filled, Tutcher and Trueman (1925, p. 601) recorded it as 'hard splintery limestone'. Usually it is not more than 0.3 m thick.

The ammonite fauna of the Valdani Limestone includes Acanthopleuroceras valdani, Aegoceras (Beaniceras) spp., and Liparoceras (s.s.) (including the type of L. tutcheri Spath (1938, p. 50) from Tyning Colliery Quarry), Tragophylloceras ibex (Quenstedt) and Lytoceras sp.Tutcher and Trueman recorded about ten genera of bivalves, several genera of rhynchonellids, and 'Terebratula', presumably Lobothyris. The fauna appears less diverse than that of the Jamesoni Limestone but this could be the result of fewer exposures and a less tractable matrix for collecting.

Broadfield Down^‡4  and Winford

Brockley Down Limestone

West of a line from Barrow Gurney to Winford there is a rapid change of facies in the Blue Lias, which is replaced westwards by coarsely crystalline, shelly and conglomeratic rocks. This facies is labelled as 'littoral facies' of the White and Blue Lias in the Index to the One-Inch geological map, though it is not strictly littoral, as noted below. It is known locally as Brockley Down Limestone (Woodward, 1893, p. 293).

The base of the Brockley Down Limestone is usually conglomeratic, with angular or slightly rounded pieces of Carboniferous Limestone up to about 0.3 m in diameter. The rock generally consists of coarse bioclastic limestones, in which bands of shell debris parallel to the bedding are often conspicuous. Larger pieces of shell are more infrequent. The rock is typically both porous and cavernous at outcrop, due to solution by groundwater. It is very massive, hardly any bedding planes being visible at exposures. Current bedding is well developed locally as at Winford but is not common in the more massive deposits.

The original texture is difficult to determine due to later re-crystallisation and to the solution already mentioned. Thin sections (E1251), (E7129), (E28088)^‡5  show numerous recrystallised shells, with calcite overgrowths, in a sparite matrix. Small fragments (<1 mm) of Carboniferous Limestone are present. There are rare small, irregular areas of quartz the significance of which is discussed in connection with the Harptree Beds (p. 20). Ransome and Cooper (1848, p. 687) gave a chemical analysis, finding between 4 and 5 per cent of insoluble matter, chiefly silica. Magnesium carbonate was between 0.5 and 1 per cent.

The thickness of the Brockley Down Limestone in the Pottershill–Downside area is unknown but probably not much more than 8 to 10 m. Stone from Felton and Downside has been widely used for building locally, and William Smith recorded that stone from Felton was used in the construction of Old Bristol Bridge (Smith MSS, Oxford University). It is to be noted that the rock known as Downside Stone was named from Downside near Shepton Mallet (Richardson, 1911, p. 58), and not from the hamlet of the same name in the area now under discussion.

The age of the Brockley Down Limestone is not proved, since no ammonites or other diagnostic fossils have been found. Its stratigraphical position, and the fact that it appears to pass laterally into Blue Lias of Hettangian date to the east, indicate late Rhaetian and/or Hettangian age. An upper limit is given by the clays of the semicostatum or turneri zones preserved in fissures at Lulsgate Quarry (see p. 48).

The eastern limit of the Brockley Down Limestone is closely defined and is shown on the Bristol District map. To the north it interdigitates with normal facies north of Barrow Gurney, and it does not extend as far as Ashton Park or Clifton. To the west, the tiny outlier south of Claverham [ST 451 655], shown as 'Rhaetic' only on the One-Inch map, carries a relic of basal Blue Lias of normal type. The limit to the south is defined by normal Blue Lias at Butcombe. Conglomeratic facies of the Lower Lias reappears in the central Mendips around the Castle of Comfort inn, and more extensively at Shepton Mallet, outside the limits of the Bristol District Sheet. Green and Welch (1965), describing the Mendip occurrence, note that the facies is, in fact, sublittoral, actual shorelines not being preserved. As regards the Broadfield Down area, it appears probable that the Brockley Down Limestone accumulated as a shell bank on a shoal formed by the Carboniferous Limestone. The minimum extent of the bank was about 4.8 by 2.5 km.

Harptree Beds

The term Harptree Beds was introduced in the Index to the Bristol District One-Inch Sheet (1962) for 'silicified strata of Rhaetic–Inferior Oolite age' (i.e. Rhaetian to Bajocian stages) on the Mendip Hills (Green and Welch, 1965, p. 94) and on Broadfield Down. The latter outcrops, at Felton, about 11 km SW of Bristol, were discovered during the mapping and have not been previously described. They are believed to be entirely of early Lower Liassic age.

On Felton Hill [around [ST 520 650] blocks of brown siliceous rock up to 1 m in length are common on the surface, but exposures are very rare. The only exposure seen recently (1976) was in the floor of Hay Lane at its western end [ST 5234 6498]. The rock is very hard and the lack of exposure is therefore surprising. It indicates that the hard siliceous rock occurs as doggers in a softer matrix and not as continuous beds.

To the naked eye the rock appears as a uniform chert with occasional moulds of shells. In thin section (E28094)-(E28094A), (E28095) the rock shows a mosaic of quartz crystals, often with interlocking boundaries. Grain size ranges from about 0.005 to 0.15 mm or larger. Grains of different sizes may occur in patches with sharp boundaries between them. The texture clearly shows that the quartz is not a primary depositional mineral. The silicification and decalcification of the Harptree Beds of Felton is of post-Liassic age. Thus, occasional fragments of Carboniferous chert are found in the Brockley Down Limestone, but no silicified material derived from Harptree Beds has yet been seen in the local Jurassic rocks.

The field relations on the Harptree Beds at Felton show a strongly sinuous boundary with the Brockley Down Limestone on all sides except the south. The age relations of the two are difficult to determine. The small outcrop of limestone at [ST 525 653] is situated in a hollow and could underlie Harptree Beds. Elsewhere, lateral transition from unaltered limestone to decalcified sandy loam with residual masses of silicified rock is the best interpretation of the mapped boundaries. At their southern margin the Harptree Beds rest partly on Carboniferous Limestone and partly on thin 'Rhaetic' beds or 'Tea Green Marl', as in the Downside–Pottershill area to the north.

The thickness of Harptree Beds preserved at Felton is not known, and is probably not very great. Palaeontological evidence of age is poor owing to the bad preservation of the fossils. During the war of 1939-45 attempts were made to clear parts of Felton Hill for cultivation. In doing so large masses of siliceous rock, some 1 to 1.5 m in length, were exhumed from the sandy matrix and stacked in heaps before being cleared away. Some of.these, when weathered, revealed poorly preserved moulds of shells resembling pectiniids and Eotrapezium?, others contained veins and streaks of baryte or baryto-celestite. This association suggests that Rhaetic strata may have been incorporated in the silicified material.

As regards the origin of the Harptree Beds, the field relations indicate that they result from local replacement of Brockley Down Limestone by silica. Support for this hypothesis is provided by rare patches of secondary quartz mosaic in thin sections of Brockley Down Limestone (e.g. (E1251)). The amount of quartz present is very small and intermediate rock types have not been found, but this is not surprising in view of the very poorly exposed ground. It may be conjectured that silicification was complete in small patches, such as has been reported from borehole cores in the Mendips (Green and Welch, 1965, p. 108). Over much of the area of the Harptree Beds it was incomplete, and later decalcification gave rise to the sandy ground encountered during the mapping.

Taking a more general view it may be said that all the Palaeozoic and Mesozoic rocks in the NW–SE faulted belt which extends from the central Mendips to Flax Bourton and Wraxall are affected by silicification to a greater or lesser extent. These effects are greatest in hard fissured calcareous rocks such as Carboniferous Limestone, Dolomitic Conglomerate and Lower Lias and Inferior Oolite limestones. They are minimal in soft poorly jointed clay, marl and shale formations such as the Lower Coal Measures, 'Keuper Marl', Cotham and Westbury Beds, Lower Lias and Fuller's Earth Clays. Selective silicification is particularly well seen in the vicinity of Winford Manor [ST 531 642] where the Dolomitic Conglomerate is highly silicified. Here the siliceous Harptree Beds rest on unaltered Tea Green Marl and Westbury Beds (black shale).

The source of the silica, which is commonly associated with baryte and rarely with galena and sphalerite, was thought by Green and Welch (1965, p. 95) to be hydrothermal.Alabaster (1976, p. 81), referring to local silicification of the Inferior Oolite at Whatley, suggested alternatively that silica might have been derived from formerly overlying Fuller's Earth, but this does not seem a likely explanation for the extensive silicification referred to above.

Fissure fillings

The former extension of the Lower Lias over the Carboniferous Limestone plateaux east and west of the Avon Gorge north-west of Bristol is indicated at several localities. In Clifton, close to the eastern end of the Suspension Bridge [ST 5657 7313], Tawney (1875a, p. 164) recorded Liassic septaria dug out when making the bridge approach road and chain anchorages. About 1 km to the north-north-east the former Avenue Quarry in Carboniferous Limestone showed a fissure-filling of yellow clay at its south-western end approx. [ST 5684 7416] (Moore, 1881, p. 75). The zonal age cannot be determined from the generic records of Moore, but a 'sagged-cover' deposit (Robinson, 1957, p. 267) of Lower Lias Clay is the most likely interpretation. Moore (op. cit., p. 74) also recorded derived Liassic fossils in a Quaternary deposit in the same quarry. On the surface of Durdham Down G. A. Kellaway notes that residual patches of late Triassic clay have been seen from time to time, and that Gryphaea and Spinferina have also been found, indicating the former presence of Lower Lias.

Fragments of argillaceous limestone of Lower Liassic type have been found farther west in Ashton Park [ST 5505 7240] at an altitude of over 120 m. These appear to fill fissures in the Carboniferous Limestone, and were probably introduced before the removal of a cover of Rhaetic and Lower Liassic rocks. Alternatively the Liassic material may have been derived from glacial or other drift deposits.

At Lulsgate Quarry (p. 48) a fissure-filling of blue clay with septarian nodules containing Arnioceras sp., Caenisites plotti (indicating the turneri Zone), bivalves and gastropods was exposed in 1957 (Donovan, 1958, p. 133) and showed that Lower Lias Clay of normal facies formerly covered the conglomeratic basal facies of the Lias in this area.

In the Mendip Hills Green and Welch (1965, fig. 14) indicate the sites of Liassic fissure deposits within the area of the Wells (280) sheet. At Lodmore Farm, north of Priddy, Buckland and Conybeare (1824, p. 303) recorded a marl pit 'the organic remains from which are decidedly those of lias' and Woodward (1876, p. 108) reported that 'black shales with Pecten' had been dug up. The site of the marl pit is probably marked by a pond [ST 5315 5390] west of the farm. Around the farm pieces of thin-bedded, coarse limestone with the ammonite Caloceras and bivalves may be found, perhaps remains of the basal beds of the Lias, though the marl or shale is likely to have been of later Liassic date. Clay 'suggestive of the Belemnite Marls' was recorded about 0.4 km to the north [about [ST 532 543] by Green and Welch (1965, p. 106) in debris from old workings. Both there and at Lodmore there is a strong possibility that fissure-fillings, probably of 'sagged-cover' type, were dug out.

More difficult to interpret is the 'Liassic Lead Mine' of Moore (1867, p. 491). The locality is not known; Green and Welch (1965, p. 106) thought that it might be abandoned workings exposing blue clay in the sides and bottom south of Ubley Warren Farm about [ST 513 546]. Moore observed 'a deposit of deep-blue or greenish clay, 12 feet (3.66 m) thick... in places... deposited in thin horizontal layers' at a depth of 270 ft (82 m) below the surface. Washing the clay produced about 95 fossil species which Moore attributed to the Lias as well as a few obviously derived from the Carboniferous Limestone. Moore believed that the clay was deposited in the fissure in Liassic times and had not been reworked. However, doubt was soon cast on this view. Blake (in Tate and Blake, 1876, p. 430) stated the ostracods listed by Moore to be Carboniferous, and Woodward (1893, p. 98) wrote that 'the age of many species is doubtful'. The five genera of land and freshwater gastropods recorded are also difficult to interpret as Liassic. On the other hand, ten of the twenty-one gastropod species were accepted by Hudleston and Wilson (1892) as Liassic species, most of the Bivalvia and at least some of the Brachiopoda recorded appear to be Liassic forms. The most likely explanation is that there was a late Tertiary or Quaternary fissure filling containing much redeposited Liassic clay. In any event the evidence for a former cover of Lias is strong.

The well-known fissure at Gurney Slade [ST 623 500] (Robinson, 1957, p. 267, 280; Green and Welch, 1965, p. 106) contains rock of Triassic and late Hettangian or early Sinemurian date. A fissure near Slab House [ST 597 475] contained material of Lower Pliensbachian age (Green and Welch, 1965, p. 106). About 3 km S of the southern limit of the Bristol District Sheet the fissure at Windsor Hill [ST 614 452] near Shepton Mallet was chiefly remarkable for yielding abundant bones of the therapsid reptile Oligokyphus (Kühne, 1956). Unfortunately, although numerous Lower Liassic fossils were found, the zonal age of the deposit is doubtful; it appeared to include both Sinemurian and Lower Pliensbachian material.

Fissure-fillings at Holwell and Cloford, about 3 km S of the south-eastern corner of the Sheet, have recently been reported as containing microfossils of early Sinemurian date (Copestake, 1982).

Changes in thickness and facies

The total thickness of the Lower Lias before the incidence of Toarcian and Bajocian erosion can only be calculated in the limited areas where the Middle Lias is preserved. In the strongly eroded areas of the eastern Mendips between Frome and Mells, and in the central Mendips around Binegar and Pen Hill, erosion prior to the deposition of the Upper Inferior Oolite has removed most of the Lias (Figure 8). In these areas, however, the former presence of Lower Lias is established by the preservation of Liassic material in fissures.

North of the denuded area lies a belt of ground about 10 km in width extending in a W–E direction and including the plateau areas of the north central Mendips and the Radstock Shelf as far north as Priston. No Middle or Upper Lias or Lower or Middle Inferior Oolite is known to have survived in this area where the Upper Inferior Oolite, often with a pebbly or conglomeratic base, rests with marked unconformity on Lower Lias. On the other hand the rocks preserved locally beneath the unconformity include uppermost Lower Liassic clays (davoei Zone), and it therefore seems probable that up to 30 m of Lower Lias Clay has been denuded from the Radstock area.

North of a line through Stowey and Priston, Upper Lias rests non-sequentially on the Lower Lias Clay. As far north as Burnett and Bath the upper part of the Lower Lias Clay together with any Dyrham Silts which may formerly have been present, has been removed by Toarcian and Bajocian (including Aalenian) erosion. Farther north, in the Dundry basin, at Bitton, and probably beneath Lansdown, the Lower Lias is complete or has undergone only very slight intra-formational erosion.

Notwithstanding the effects of Toarcian and Bajocian erosion a northward directed 'promontory' shown by the 15 m and 30 m isopachytes can be seen on the map (Figure 8) between New Mells and Braysdown collieries. This is due to attenuation of the Lower Lias over a NNW–SSE oriented ridge or horst-like area of the Radstock Shelf. North of the Timsbury Fault a comparable 'promontory', marked by the 45 m isopachyte, lies farther to the west.

The Farmborough Fault Belt, primarily of Variscan age, corresponds approximately to the northern limit of the Radstock Shelf which constitutes part of a larger area of structurally-controlled Liassic sedimentation which may include the Winford Growth Fold and the line of west–east folding and faulting which extends from Compton Dando to Burnett and Bath.

In the area of attentuation extending northwards to Keynsham the 90 m isopachyte is generalised, as part of the Lower Lias has been removed by post-Jurassic erosion. In this central area the presence of Middle Lias at Bitton provides evidence for the original thickness of the Lower Lias which is about 112 m. In the centre of the Dundry basin, where Middle Lias is also present, the thickness of the Lower Lias is 142 m. Beneath Lansdown, on the eastern side of the attenuated central region, there appears to be about 135 m of Lower Lias but pre-Toarcian thickness may have been greater, possibly approximating to that proved at Dundry.

The typical Blue Lias of the Avon valley occupies a belt of intermediate thickness between the Radstock Shelf in the south and the Dundry and Lansdown basins in the north.

More rapid changes in the thickness of the Lower Lias take place between Lansdown and the Hamswell area at the northern end of the Swainswick valley north of Bath, where the Lower Lias is shown by mapping to be sharply reduced to about 60 m thickness.

In very broad terms the main part of the Bristol District corresponds to that in which the Blue Lias is relatively well developed at the base of the Lower Lias and this gives way to both the east and west to areas where the corresponding beds appear to have been dominantly argillaceous.

White Lias—local details

For the area north of the River Avon notes on the White Lias, where present, are included in local details for the Lower Lias.

Dundry Hill

At Grove Farm, Stockwood, a borehole [ST 6267 6870] logged by G. A. Kellaway in 1951 showed 7 ft (2.13 m) of White Lias. Along the outcrop at the foot of the north side of Dundry Hill, Coysh (1927, p. 329) recorded a total thickness of 4.5 ft (1.37 m) at Knowle'near the tramway terminus' [ST 607 705], and at 'Nubbers' (Novers) Quarry [approx. [ST 583 695] Brodie (1845, p. 87) noted 51 ft (1.66 m). A more detailed section was recorded in 1935 by T. R.Fry in a drainage trench on the eastern side of Headley Park estate [approx. [ST 580 693]:

In the borehole north of Winford [ST 538 661] Trueman (1936) recorded a probable minimum thickness of 2.5 ft (0.76 m) of White Lias, noting that there may have been core loss. In the Dundry (Elton Farm) Borehole only 0.50 m is attributed to the White Lias.

Along the southern foot of Dundry Hill no sections have been recorded. White Lias was mapped as a separate unit between Winford and Chew Magna, where it sometimes forms a narrow feature, but it must be thin. The top 3 ft (0.9 m) were extensively quarried along the edge of the scarp south of Whitchurch from north-west of Hursley Hill at [ST 614 660] to Great Wood [ST 644 658]. The last quarry in work was Queen Charlton Quarry at Charlton Field [ST 636 658] after the 1939-45 war.

Avon valley

At Willsbridge in February 1976 a temporary exposure north of the main road showed White Lias at about [ST 667 705]. About 0.90 m of the upper part were seen, beds of fine-grained porcellanous limestone, up to 0.13 m thick, separated by thin shales; the lower part here consisted of about 0.75 m of pale laminated clay. In the railway cutting about 150 m east of this exposure, Moore (1867, p. 498) recorded 4 ft (1.22 m) of the laminated clay.

Good sections in the cuttings of the Great Western Railway at Saltford were recorded by Bristow and others (Vertical Sections sheet 46, No. 9) and Winwood (1910). Their sections, measured in different places (Winwood's was newly excavated in 1909) totalled 9 ft 7 in (2.92 m) and 8 ft 1 in (2.46 m) respectively, the Sun Bed being 2 ft (0.61 m) and 1 ft 5 in (0.43 m). Both show a clear division into lower marly or rubbly beds, accounting for just under half the total thickness, and strongly bedded limestones above. A short distance to the east in the cutting south of Kelston station on the former LMS railway, Winwood (1884) recorded a section [approximately [ST 689 671]. The lower, rubbly part of the White Lias was 3 ft 7 in (1.09 m) thick, and was very fossiliferous. Winwood noted the bivalves (names revised where possible) Dimyopsis intusstriata, Chlamys valoniensis, Modiolus minimus, Lyriomyophoria postera, 'Nucula', Plagiostoma praecursor, 'Pleurophorus' elongatus and internal moulds of gastropods which he thought represented three genera. The upper beds were 5 ft 6 in (1.68 m) and the middle part yielded 'Gervillia' praecursor, Meleagrinella decussata, and Modiolus minimus besides 'Arca', 'Nucula' and gastropods.

At Keynsham Hams (cf. p. 26) 4 ft 3 in (1.3 m) of White Lias were recorded, cut off by a fault. The upper, well bedded, part is reduced to 2 ft 11 in (0.89 m) so that the total thickness may have been about 1.50 m. Near Chewton Keynsham a lane section 230 yd (210 m) west of Chewton Place [ST 654 670] in 1945 showed 6 ft (1.83 m) of White Lias. Farther south at Burnett road cutting [ST 665 655] a temporary section recorded by T. R. Fry in 1923 proved 9.25 ft (2.8 m) of White Lias.

Bath

A complete section through White Lias at Weston was recorded by H. B. Woodward (Vertical Sections sheet 46, No. 11) (Figure 12). The total thickness is 3.66 m; clays with subsidiary limestones are succeeded by 5 ft 3 in (1.6 m) of marls, and the uppermost part, as usual, consists of limestones with clay partings 3 ft 10 in (1.17 m) including the Sun Bed. Various sections have been exposed from time to time at Weston, and another recorded by Winwood (1871) was similar to the above except that the upper beds were 5 ft 1 in (1.55 m). At Batheaston trial shaft 10 ft (3.05 m) of White Lias was recorded by Conybeare and Phillips (1822, p. 262).

Priston and Camerton

Priston Colliery shaft traversed 12.5 ft (3.8 m) of White Lias, but no details were recorded. Across the valley of the Cam Brook the White Lias appears to get thinner before thickening again near Radstock, for at Camerton only 8 ft (2.44 m) is present according to Moore (1867, p. 472) who remarked that 'In this district it is usually very unfossiliferous'.

Paulton

At Newton Quarry massive limestones were exposed to a thickness of 8 ft (2.44 m) overlying fossiliferous clay, correlated with a similar bed at Rockhill (see below) (Tutcher and Trueman, 1925, p. 615). The upper, limestone, division of the White Lias usually accounts for about half the total thickness, so that the whole thickness may have been 5.2 to 5.5 m as at Rockhill. At Paulton Hill Pit [ST 6588 5614] Conybeare and Phillips (1822, p. 262) recorded 131 ft (4.11 m) of White Lias but this record may have omitted the lower, marly division. At Phyllis Hill Quarry a well section is said to have proved a similar thickness to that at Rockhill (Tutcher and Trueman, 1925, p. 616).

Radstock

At Rockhill Quarry [ST 6795 5540], about 1.2 km WNW of Radstock, a complete section through the White Lias was provided by the quarry and a well (Tutcher and Trueman, 1925, pp. 603, 608). The total thickness was 18.5 ft (5.64 m), divided into an upper series of massive limestones 10.5 ft (3.20 m) thick, formerly quarried for building stone, and thin irregular limestones with clay partings, 8 ft (2.44 m) thick, below. Fossils were reported at two horizons: a 7 in (0.18 m) white clay, 7.5 ft (2.29 m) below the top, and a basal blue clay 1 ft 2 in (0.36 m) thick. Fossils from the two beds were not recorded separately. The list includes bivalve genera which can be attributed to Astarte, Cardinia, Chlamys, Dimyopsis, Liostrea, Meleagrinella, Modiolus, Parallelodon, Pholadomya, Plagiostoma, Pleuromya, Protocardia, and perhaps others; three species of gastropods, an echinoid identified as Diademopsis tomesi, a solitary coral and the plant Otozamites obtusus. At Clandown Colliery Quarry [ST 679 558] the White Lias was 18 ft (5.49 m) thick (Tutcher and Trueman, 1925, p. 601) but no details were recorded.

South of Radstock White Lias was exposed in a number of quarries but few details have been recorded. At Tyning Farm Quarry [ST 6850 5328] 91 ft (2.9 m) was still exposed in 1971. The lower part of the White Lias was formerly exposed in the railway cutting east of Chilcompton [ST 653 523]. A detailed section was recorded by Duffin (1980, pp. 255, 256) who noted 0.94 m of beds, the lower half yielding the bivalves Atreta [Dimyopsis] intusstriata, Modiolus loads and M. langportensis, Pleurophorus elongatus, Protocardia philippiana and Pseudomonotis fallax as well as a brittlestar Ophiomorpha sp.White Lias was formerly quarried in pits east of the cutting.

The White Lias is mapped as wedging out along an irregular line within about 1.6 km south and south-east of Kilmersdon, as shown in (Figure 2). In the Hemington Borehole [ST 7246 5296] (Cantrill and Pringle, 1914) it was 19 ft (5.79 m) thick, and was recorded as 'Limestones, compact, whitish, becoming thinner-bedded and marly below' which agrees with the usual sequence in the area.

Lower Lias

Wickwar–Chipping Sodbury–Oldland

White Lias and Basal Blue Lias Limestone

Details of the White Lias and basal Blue Lias limestone of the area extending northwards from Chipping Sodbury to the Berkeley district have been given elsewhere (Cave, 1977, pp. 84-87). The combined White Lias and basal Blue Lias limestone have been mapped continuously as a single formation from the Wickwar area southwards to Doynton where the White Lias becomes sufficiently thick to be mapped separately on the six-inch scale, although the crop is too narrow to be shown on the one-inch scale.

In the Chipping Sodbury railway cutting (Reynolds and Vaughan, 1902; Strahan, 1903) the White Lias is present, though normally not exceeding 0.6 m in thickness. It is overlain by about 1.5 m of basal Blue Lias limestone, of which the uppermost 0.60 m consists of two beds of limestone with Caloceras johnstoni representing the johnstoni Subzone of the planorbis Zone. The planorbis Subzone appears to be missing.

Old shallow workings on a small outlier [ST 693 730] west of the main outcrop suggest that the residual thickness of the basal Blue Lias limestone is about 0.3 m and that the thickness of the White Lias does not greatly exceed 0.6 m.

Sections seen in Toghill Lane, Doynton [ST 7222 7343] showed White Lias 3 ft (0.9 m) overlain by about 5 ft (1.5 m) of hard grey rubbly and thinly bedded Blue Lias limestone capped by clay (Saltford Shales). Near Blue Lodge, Abson [ST 6955 7423] the succession appears to be similar, at least 1.2 m of grey limestone having been formerly worked above White Lias which is about 0.6 m thick. Indications of the zonal age of the Blue Lias in the Pucklechurch–Abson–Doynton area were found by F. B. A. Welch and G. A. Kellaway during the course of the survey. Thus in the tract of folded Lower Lias between Radford Lane and the Boyd valley at Doynton the ammonite Caloceras (indicating the johnstoni Subzone) was found at the top of the basal Blue Lias limestone at several localities, notably west-north-west of Trunkhouse Barn [ST 7143 7554] and in an anticlinally folded inlier of the basal limestone south-west of Rookery Farm [ST 7153 7507]. The greater part of the basal Blue Lias in the area between Wick and Doynton may consist of Pre-planorbis Beds, the planorbis Subzone being absent. The higher johnstoni Subzone, if present, must be confined to a very thin layer at the top of the limestone underlying the Saltford Shales. South of the Wick inlier the thickness of the White Lias in the areas adjacent to Limebrook Farm [ST 714 726] is uncertain, but is not likely to be less than 0.9 to 1.2 m. A laneside section about 300 yd (274 m) west of Cold Harbour Farm [ST 7025 7169] showed about 4 ft (1.22 m) of White Lias limestone and clay, and a ditch by the crossroads [ST 6995 7102] 430 yd (396 m) N of Barton Farm showed 2 to 3 m of these rocks in 1975.

South of Wick, the basal Blue Lias, consisting of thinly bedded grey limestone with thin seams of clay, was formerly worked in a number of small pits. In an old quarry [ST 7100 7160] 400 yd (366 m) SW of Tracy Park the White Lias was overlain by thin interbedded limestones and shales with a 5-in (0.12-m) shale at the base. They yielded Liostrea hisingeri, Pinna, Pleuromya and other bivalves. About 5 ft (1.5 ft) above the White Lias another small exposure showed the Psiloceras Bed with Psiloceras sp.in it and also 10 in (0.25 m) below it, and Caloceras sp.immediately above. The basal Blue Lias in this area is about 3 m thick, of which at least 1.8 m was formerly worked north-east of Grandmother's Rock [ST 7115 7135].

Saltford Shales and Upper Blue Lias Limestone

At Chipping Sodbury the basal Blue Lias limestone is succeeded by about 15 m of clays and shales with thin limestones. The lower part of these beds, which may be equated with the Saltford Shales, yields bivalves and the echinoids Pseudodiadema and 'Cidaris'. The upper one-third contains Schlotheimia spp.indicating the angulata Zone. The uppermost 0.90 m of nodular limestone and clay yields Schlotheimia and Vermiceras, the latter indicating the base of the bucklandi Zone and of the Sinemurian Stage. The total thickness of the Hettangian is about 17 m. At Cliff Farm, Wapley, [ST 715 803], Dr Welch has calculated the thickness of the Saltford Shales as about 12 m. These beds are succeeded by nodular limestones with Gryphaea arcuata. West of Leigh Farm, Pucklechurch, the thickness of the Saltford Shales was calculated by Dr Welch as about 9 m. A pit sunk in the upper part of the Saltford Shales near the contact with the upper Blue Lias limestone at St Aldam's Ash Farm [ST 7048 7727] proved 28 ft (8.5 m) of blue clay. St Aldam's Well, situated 45 m W of this pit, though only 3 ft (0.9 m) deep, is said to have been an unfailing source of water until 1941 when a bomb fell alongside. This well drew its supply from the bottom bed of the upper Blue Lias (Division C), the water being held up by the underlying Saltford Shale. The presence of abundant 'Cidaris' spines just below the base of Division C is reminiscent of the area north of Bristol where the remains of echinoderms are also abundant between the top of the planorbis Zone and the base of the angulata Zone.

The base of the upper Blue Lias limestone was temporarily exposed in a ditch by the footpath leading from Rookery Lane to Snowdown's Lane [ST 7117 7564]. The base of the nodular limestone layer yielded Waehneroceras, while at slightly higher stratigraphical levels Schlotheimia sp.and Calcirhynchia calcaria were observed. In a bomb crater south of Back Lane, south of Pucklechurch [ST 6972 7574] fragments of Schlotheimia sp.and numerous specimens of Calcirhynchia calcaria suggest that the Calcaria Bed (base of bucklandi Zone) may be present. The bucklandi Zone is certainly represented locally in the upper Blue Lias limestone band since fragments of Coroniceras (Arietites) sp.were observed in another bomb crater [ST 7034 7525] 460 m NNW of Abson Church. These grey limestones with coronicerates are overlain by stiff blue and brown clay which forms an outlier of synclinally folded Lower Lias Clay situated midway between Siston and Abson villages. The zonal age of this clay above the Blue Lias is not known for certain but is probably semicostatum Zone. Phosphatic nodules have been observed at a number of localities in the area near the contact of the Blue Lias and the overlying Lower Lias Clay, recalling the situation at Keynsham (p. 6). The total thickness of the Blue Lias in the Abson area is estimated at 21 to 23 m.

About 550 m east of Doynton Church, near the eastern margin of the Bristol District Sheet, stiff clays with nodular limestone containing Arnioceras sp.rest on the Blue Lias. Nodules of limestone with Ggphaea and Arnioceras were also observed in excavations for land drains on the north side of Doynton Lane [ST 7284 7351]. The position of this material is somewhat anomalous and may indicate the presence of a fault or valley bulge in the Lias.

Evidence for the presence of the bucklandi and/or semicostatum zones at the top of the Blue Lias was seen at Beach where hard grey limestones with Coroniceras and Arnioceras crop out at the contact of the Lower Lias Clay and Blue Lias 90 m ENE of Britton's Farm [ST 7075 7080].

Lower Lias Clay

North of Chipping Sodbury the outcrop of the Lower Lias Clay lies east of the Bristol District sheet. In this region the presence of the bucklandi Zone, at the top of the Blue Lias, and of the semicostatum, lumen, obtusum, oxynotum, raricostatum and ibex zones was recorded by Donovan (1947) east of Charfield in the Hawkesbury area. Farther south, in the railway cutting east of Chipping Sodbury station, Reynolds and Vaughan (1902) recorded the presence of semicostatum, tumeri and raricostatum (?) zones and possibly the base of the jamesoni Zone. North–south strike faulting may explain some of the anomalies in the distribution of the zones east of Charfield and north of Chipping Sodbury (Donovan, 1947; Cave, 1977, p. 86).

South of Chipping Sodbury exposures of the clays between the top of the Blue Lias (bucklandi Zone) and the jamesoni Zone are rare, but the semicostatum Zone is represented by blue clay with Arnioceras, fragments of this ammonite having been observed at many localities. The turneri or obtusum Zone, represented by Promicroceras planicosta, has been found near Britton's Farm at Beach [ST 7063 7058]. The ammonites occur in septarian nodules embedded in clay about 9 m above the top of the Blue Lias. Beyond the margin of the Bristol District Sheet, nodular limestone with numerous ammonites including Caenisites sp.group of C. turneri, Cymbites laevigatus, Microderoceras spp.and Promicroceras capricornoides occurs in the Lower Lias Clay, 550 m SE of Hamswell House [ST 7360 7110]. Both the semicostatum and the turneri zones are probably present throughout the area between Chipping Sodbury and Bath, though they are seldom exposed.

Between Chipping Sodbury railway cutting and the Bitton Fault only one relatively continuous band of nodular limestone has been found in the Lower Lias Clay. This is developed in the middle of the clay above the obtusum Zone and below the ibex Zone. It has been traced just east of the Bristol District map area with only minor breaks from a position about 0.4 km N of Fatting House [ST 7456 8095] southwards of Dodington Lane [ST 7417 8042]. Nodular limestone, possibly occupying a similar stratigraphical position, was seen about 119 m S of Dodington Manor. The total thickness of the Lower Lias between Chipping Sodbury and Dodington (assuming no faulting or strong folding) is about 67 to 70 m. The nodular limestone of Fatting House is approximately 33 m above the top of the Blue Lias. Its position relative to the base of the Dyrham Silt may be less significant as the boundary between the Lower Lias Clay and Dyrham Silt is probably affected by facies changes (Cave, 1977, p. 87). A limestone seen 823 m E of Barley Close Farm, and 960 m SE of Court Farm, Codrington [ST 738 781] which has yielded Piarorhynchia cf. rostellata may also represent this horizon, probably within the jamesoni Zone.

From the Chipping Sodbury–Dodington area the nodular limestone can be traced to the low ground south-west of Hinton and thence in a southerly direction to Lower Ledge Farm [ST 733 750], near Dyrham and thence intermittently, near Bowd Farm [ST 730 740] and Woodlands Farm north of Tog Hill [ST 730 733]. At Dyrham and Tog Hill, the crop has been buried by slipped clay and sand, but the limestone feature can again be detected at Toghill Farm [ST 7215 7231] and followed southwards as a band of earthy and nodular limestone cropping out at the foot of Freezing Hill. Fossils, including fragmentary belemnites, become increasingly common south of Toghill and in a small exposure [ST 7179 7176] between Tracy Park and Freezing Hill several specimens of Zeilleria (Cincta) sp.and a poorly preserved ammonite (?Polymorphites) were seen. North of Beach Farm [ST 7145 7072] it has yielded a number of specimens of Piarorhynchia. The fauna suggests that the limestones at Beach, and at Fatting House, are of jamesoni Zone age. Very little is known about the clays above the Fatting House and Beach limestone bands. The top of the Lower Lias Clay was exposed in a trench at Dodington Ash [ST 750 782], east of the boundary of the Bristol District Sheet (Fry, 1951). Here barren clay with ironstone nodules is overlain by about 3 m of clay with limestone bands which yield Aegoceras (Beaniceras) aff. luridum, Liparoceras cheltiense, Astarte obsoleta, Pseudopecten acuticosta, Parainoceramus substriata, Mactromya subglobosa, Pholadomya ambigua and Pleuromya costata. The ammonites indicate the ibex Zone. Grey clays above may belong to the davoei Zone but no fossil evidence was obtained. The succession is likely to have been disturbed by superficial movements, as noted by Fry.

Farther south, about 400 m ESE of the house at Tracy Park [ST 7173 7167], a single specimen of Acanthopleuroceras sp.and one of Cirpa fronto was found about 25 m above the top of the Blue Lias. These fossils suggest the ibex zone and also indicate attenuation of the Lower Lias Clay as compared with Chipping Sodbury and Bit-ton. At 'Oak's Lane', Upton Cheney, a section in a trial shaft was recorded by Moore (1866, p. 154). It showed several 'Pecten Beds' (shelly limestone c. 0.1 m thick) in the Lower Lias Clay. The section is difficult to interpret and may have traversed slipped material. The site has not been located.

Keynsham–Dundry–Farmborough

Bitton

The borehole at the former paper mills in Golden Valley, Bitton [ST 6814 6986] (Winwood, 1895; Richardson, 1930,p. 58) penetrated 216 ft (66 m) of Lower Lias with Blue Lias at the base, and clays with occasional thin bands of limestone above. No fossils were recorded. The record is not very detailed; the lower part is interpreted as:

Extrapolation of the base of the Dyrham Silts, mapped as dipping gently to the south-east on Bitton Hill a short distance to the northwest, indicates a thickness of 104 m for the Lower Lias (from the top of the White Lias to the base of the Dyrham Silts).

Willsbridge

A group of small outliers occurs around Willsbridge, chiefly between the Willsbridge and Bitton faults. The largest outlier, west of Willsbridge, included three quarries (nos. 1-3 of Vaughan and Tutcher, 1903). All exposed Divisions C and D of the Blue Lias but Vaughan and Tutcher did not give detailed sections for individual quarries. North of Court Farm Road (formerly Limekiln Lane) their quarry No. 2 approx. [ST 657 707] exposed the Willsbridge Fault in its northern wall with a calculated downthrow of about 125 ft (38 m) to the south. Quarries No. 1 [ST 654 705] and No. 3 [ST 659 705] lay south of the road. Quarry No. 3, the more easterly of the two, was traversed by an east–west fault, throwing down about 15 ft (4.6 m) to the north, about 150 m S of the Willsbridge Fault. The northern part of the quarry exposed the Calcaria Bed, containing rare belemnites ('Belemnites acutus') and the whole of Division D, overlain by clays with a thin argillaceous limestone with Arnioceras, Promicroceras ('Am. planicosta') and belemnites.

Moore (1867, p. 503) visited a quarry which appears to have been in Division D at the eastern end of this outlier, then known as Stout's Hill. He gave a long faunal list, including 13 species of foraminifera and numerous molluscs.

Four small outliers lie east of the Warmley Brook. The two northern ones were presumably united until the making of the railway cutting between them, and have tappings of Saltford Shales which owe their preservation to a small east–west syncline, not shown on the published six-inch map but recorded by Moore (1867, p. 498). Moore recorded the basal beds of the Lower Lias as 25 beds of limestone, alternating with marl, to a total thickness of 8 ft 10 in (2.69 m). The Saltford Shales were exposed in 1976 at [ST 667 706] as grey clays with limestone bands up to 5 in (0.12 m) thick, with Plagiostoma gigantea and abundant Liostrea hisingeri.

The Bitton Fault brings down Lower Lias Clay on its southern side and the base was exposed in excavations immediately to the north of the A431 [ST 6678 7036] in 1976. Massive limestones at the top of the Blue Lias were succeeded by about 4.6 m of clays with two limestone bands about 0.20 m thick. Arnioceras sp.occurred in some abundance above the upper limestone.

A quarry north-west of Londonderry Farm [ST 662 701] now filled (Vaughan and Tutcher, 1903, p. 31, no. 4); Donovan (1948, p. 337) showed about 16.5 ft (5 m) of Division C and 4 ft (1.2 m) of Division D (Figure 11). The upper part of the section, amended from Donovan (1948) is as follows:

Ammonites from the remanie bed include: Coroniceras schloenbachi, C. sp., Schlotheimia sp., Sulciferites sp., Vermiceras cf. deffneri, V. rouvillei and V. solaroides. These species indicate condensation of the conybeari Subzone, at least part of the rotiforme Subzone, and probably the top part of the underlying angulata Zone.

Charles Moore recorded (1867, p. 502) a quarry 'At a coal-wharf on the banks of the river Avon' which was probably Londonderry Wharf [ST 661 698] south of the last mentioned quarry, and showed lower beds:

Bed 4 is the lower part of the Saltford Shales, septaria having been recorded at the same level by Lonsdale at Lower Weston, Bath (1832, p. 244). From bed 3 Moore recorded Am. planorbis, Am. angulatus and Am. johnstoni. The first and last were doubtless Psiloceras and Caloceras spp.; the second, suggesting a schlotheimiid, is improbable and was perhaps a misidentification.

South-easterly dips have been recorded in these quarries, indicating a monoclinal flexure on the south-eastern side of the Bitton Fault (Bristow in Woodward, 1876, p. 94; Vaughan and Tutcher, 1903, p. 31).

Putting together the information from the last paragraphs we have for the Willsbridge area:

The Blue Lias has a minimum thickness of 12.3 m and a probable thickness of over 15 m.

The last quarry north of the River Avon is Vaughan and Tutcher's exposure 5 (1903, p. 31), presumed to be the disused and overgrown quarry [ST 662 692] 1150 ft (350 m) west by south of Avonside House (K2 on (Figure 9)). Their diagrammatic section, said to have been chiefly based on this quarry, is shown in (Figure 11). It shows about 12 ft (3.7 m) of clays and limestones above the Calcaria Bed, succeeded by a few feet of clays. These yielded 'Am. geometricus'(i.e. Arnioceras sp.) and belong to the semicostatum Zone. There are no modern fossil records from this quarry but the shale with 'Am. rotiformis, Am. charmassei and Am. conybeari', immediately above the Calcaria Bed, corresponds to the Remanié bed at Willsbridge. The record of 'Am. planicosta' (i.e. Promicroceras) about 9 ft (2.7 m) above the top of the Calcaria Bed is difficult to understand, since the genus is not known at this level in the area; it may have resulted from misidentification.

The Lower Lias Clay on the north side of the Avon valley is poorly known. Marls with belemnites were noted by T. R. Fry south of the Bath road between Willsbridge and Bitton [approx. [ST 672 701]. He also noted in January 1946 a temporary exposure 274 m N of the Crown Inn, Kelston [approx. [ST 700 675] published by Donovan (1948, p. 338). Mapping has shown this exposure to lie in foundered strata, but it is interesting on account of the lithologies present:

About 1.6 km to the north, in Pipley Bottom approx. [ST 700 695] Mr Fry in 1961 found rock, which he identified with the lowest bed exposed at Kelston, which yielded Aegoceras maculatum (Young & Bird). The ammonites recorded from these exposures indicate the davoei Zone. These limestones may represent hard bands in the poorly-known Dyrham Silts.

Keynsham

In the past Keynsham was a well-known locality for Blue Lias exposures and fossils. Sections in the Keynsham, Saltford and neighbouring areas are listed in (Table 2) (Appendix p. 59) and their locations shown in (Figure 9). Quarrying was chiefly in the upper limestone division (C and D) of the Blue Lias but most sections have gone.

A detailed section through the lowest limestones of the Blue Lias (Division A) was recorded in excavations (Kla) for Fry's chocolate factory just north of Keynsham village. A diagrammatic section is shown in (Figure 10). Division A is 8 ft (2.44 ft) thick. Thin irregular limestone with shale partings yielded, in the lowest 2 ft (0.61 m) Parallelodon hettangiensis, Pteromya tatei and Protocardia philippiana, and in the next 2 ft Liostrea hisingeri was common. These 4 ft (1.22 m) of Pre-planorbis Beds are succeeded by a 6-in (0.15-m) limestone known locally as the Psiloceras Bed (Donovan, 1956a, p. 198) which can be recognised at most exposures between the River Avon and the neighbourhood of Farmborough to the south. Tutcher recorded Psiloceras planorbis (as P. sampsoni (Portlock)). The remainder of Division A yielded the ammonite Caloceras and belongs to the Caloceras johnstoni Subzone: Tutcher recorded C. intermedium (Portlock). Above Division A Tutcher recorded 21 ft 6 in (6.55 m) of clays with subsidiary bands of limestone, the Saltford Shales. The top of this unit was not exposed. From the lower part Tutcher recorded Waehneroceras spp., and about 2 ft (0.61 m) below the top, Alsatites liasicus, indicating the two subzones of the Alsatites liasicus Zone.

A closely similar section to the above was recorded in great detail by Bristow (Vertical Sections sheet 46, no. 12). Its exact location is not known. Headed 'Keynsham Hams', a name which today refers to the water meadows north and west of the Somerdale factory, it was perhaps somewhere on the factory site. Bristow's section shows the lowest 4.88 m of the Saltford Shales and records the presence of septaria as well as bands of nodular limestone. Septaria were noted at the same horizon at Weston, Bath, by Lonsdale (1832, p. 244) and by Moore (1867, p. 502) at Willsbridge, but have not been recorded by recent authors, presumably because of the generally poor exposure of these shaly beds.

Another section at Somerdale (Klb) exposed virtually the whole of Division C (Figure 10). This consists of thin limestones, often nodular, with shale partings, and a foot or two of more massive limestones at the top, capped by the thick Calcaria Bed. The main band of nodular limestones is divided by a prominent shale bed 1 ft 4 in (0.41 m) thick, recognisable in other sections in the district including the Keynsham bypass, Keeling's Quarry and Saltford Cutting. The greater part of the division falls in the Zone of Schlotheimia angulata although the base may belong to the Zone of Alsatites liasicus. The massive limestone at the top falls in the Vermiceras conybeari Subzone of the Coroniceras bucklandi Zone. This is the 'calcicosta limestone' of Tutcher and the calcaria Bed' of Donovan (1956a). It contains abundant Calcirhynchia calcaria S. S. Buckman (formerly Rhynchonella calcicosta (Davidson)) which also occurs in the beds below. Besides ammonites, Tutcher recorded from Division C Zeilleria sarthacensis, and a handful of species of bivalves. Liostrea hisingeri occurred at the base of the division, and Gryphaea about 4.5 ft (1.37 m) below the top. There were several species of Limidae, Cardinia, Gresslya and Modiolus.

A section duplicating that of the Somerdale sidings was more recently seen in the cutting for the Keynsham bypass (K5) about 0.25 mile (400 m) to the south-west. Hawkins (1966, p. 199) recorded Unicardium cardioides (Division C) and the gastropod Pleurotomaria (Division C, D) as well as confirming some of the fossil records of Tutcher.

The part of the Blue Lias above the Calcaria Bed (Division D) was fully exposed in the bypass cutting. There are 6 ft (1.83 m) of limestones and clays (Hawkins' beds 31-42) which give way at the top of bed 42 to clays with subordinate thin, fissile limestones.

The minimum thickness for the Blue Lias in the Somerdale area is 17 m, assuming no overlap between the two sections (K1 a and b) recorded by Tutcher (1923). The Lower Lias Clay was exposed in temporary sections at Keynsham Sewage Works (K4) and Stidham Farm gravel pit (K17), examined by T. R. Fry. The Sewage Works yielded the ammonite Agassiceras, considered as diagnostic for the middle Subzone of the semicostatum Zone. The gravel pit yielded the ammonites Caenisites and Asteroceras, indicative of the turneri Zone and the lower part of the obtusum Zone.

The Blue Lias was exposed in several places in the village of Keynsham. A quarry (K6) formerly existed north of the Talbot Inn on the Bath Road. A number of houses in this part of the village have large arietitid ammonites built into their walls, and it may be that both the stone and the ammonites came from this quarry. The best recorded, now filled in, was Keeling's Quarry (K8) south of the Talbot Inn, also known as Keynsham Limeworks Quarry. It was first recorded, and stated to have been recently enlarged, by Tutcher (1923) see (Frontispiece 1).

The quarry exposed Division C to a thickness of 14 ft 4 in (4.37 m) below the top of the Calcaria Bed according to Tutcher (1923, p. 272) but he did not record this part of the section. It was recorded by T. R. Fry, and the upper 7 ft (2.13 m) were still visible when the quarry was visited by the writer in 1950. The resulting section is shown in (Figure 11). Schlotheimia similis was collected from bed 23, between about 6.5 and 11.5 ft (2.0 and 3.5 m) below the top of the Calcaria Bed, and examples were figured by Donovan (1952, pl. 23, figs. la, b, 2a, b). Presumably the whole of Division C exposed in the quarry belonged to the angulata Zone except the uppermost 3 ft (0.91 m) in which Vermiceras solaroides and Sulciferites charmassei denote the Zone of Coroniceras bucklandi. A well is said to have penetrated 30 ft (9.0 m) of clay (?Saltford Shales) below the base of bed 23, according to information given by Mr Keeling to Mr Fry, but the thickness stated seems excessive.

Division D at Keeling's Quarry was 11 ft (3.35 m) thick, the top being marked by the Scipionianum Bed, a prominent limestone 1 ft 4 in (0.40 m) thick. Both lithological and palaeontological correlation suggest that the top of Division D, and therefore of the Blue Lias, here lies at a higher horizon than at the bypass cutting (p. 27) about 1 km NW.

A section through Division D closely similar to the one at Keeling's Quarry was recorded about 0.8 km to the south-west by Vaughan and Tutcher (1903, p. 34, exposure 8, and diagram on p. 32) (K10). The top of the Blue Lias at Keeling's Quarry corresponds to the top of the limestone bed with 'Am. obliquecostatus' [Arnioceras sp.]. The section was capped by about 7 ft (2.13 m) of shales with thin impure limestones, forming the base of the Lower Lias Clay.

Manor Quarry (K12) duplicated the middle part of the section at Keeling's Quarry. At one time a large area of the surface of the Calcaria Bed was exposed, covered with large ammonites: numerous Vermiceras caesar and a few Sulciferites sp.

The basal beds of the Blue Lias were exposed in a small quarry west of Chewton Keynsham (K11) (Figure 11). The ammonite Psiloceras occurred in three successive beds of limestone, of which the highest was identified with the Psiloceras Bed. The same beds were exposed in a road widening at Burnett Hill (K14) where a section was recorded by T. R. Fry in 1923. Here the total thickness of Division A was only 6 ft (1.83m) and the Saltford Shales (Division B) were reduced to 9.5 ft (2.90 m) of shales with thin limestone bands. The highest beds exposed were Division C which was 11 ft (3.35 m) thick.

Saltford

The most extensive section through the Blue Lias of the Avon valley was exposed in the cutting of the former Great Western Railway (now British Rail, Western Region) west of Saltford Station, made about 1840. It was recorded in detail by William Sanders (Sanders, 1842; Tawney, 1876) but no copy of his section has been traced. It was a good deal degraded when measured in 1950 (Donovan, 1956a) and the base was not well exposed but Bristow and others (Vertical Sections, sheet 46, no. 9) recorded down to the White Lias. A diagrammatic section is shown in (Figure 10). The total thickness of Blue Lias is about 18 m; Division A is about 2.60 m, the Saltford Shales 4.27 m, both according to Bristow and Etheridge; Division C is about 5.80 m and D is 4.72 m, both as measured in 1950. A summary of Sanders' section published by Wright (1878, pp. 36-37) shows that Saltford Shales include a number of thin limestones, and septaria. The top of the Blue Lias is taken at the top of the Scipionianum Bed.

Wright recorded a number of fossils. As well as the common bivalves from Division C he recorded the gastropod Pleurotomaria similis, Division D yielded the same bivalves and gastropod as Division C, and large nautilids which were also observed by the writer. Wright recorded Spirzferina walcotti J. de C. Sowerby from the upper part of D, and Ichthyosaurus communis Conybeare from the lower part.

The section along Mead Lane, Saltford (K22) was mentioned by Tutcher (1923) but has never been described. It is about 490 m long and appears to be an old quarry. It shows the topmost part of Division C and most of Division D: a section measured in 1977 is shown in (Figure 11). The Calcaria Bed is identifiable by the usual large Vermiceras and schlotheimiids on its upper surface. A group of limestones in Division D is noteworthy for abundant vertical burrows, about 5 mm diameter, usually containing less broken shell in the filling than in the surrounding rock. Some, at least, may be Diplocraterion. More rarely, sloping burrows, possibly Rhizocorallium, may be seen.

Weston

A complete section through Blue Lias at Lower Weston, on the south side of the Upper Bristol Road approx. [ST 725 651] was recorded by Lonsdale (1832, pp. 243-244). A diagram drawn from his detailed measured section is shown in (Figure 12). It may be interpreted as follows:

The Calcaria Bed is probably the Double Bed giving about 17 ft (5.2 m) each to C and D.

About 1.5 km due north of the quarry at Lower Weston the upper beds of the Blue Lias were temporarily exposed (1946) off the High Street in Weston village [ST 727 665] (Donovan, 1948, p. 336). A diagrammatic section is shown in (Figure 12). Division D is about 13 ft 4 in (4.08 m). The chief interest of the section was that it showed the remanie bed immediately above the Calcaria Bed, also recorded from Willsbridge (p. 26). The bed yielded Sukiferites and Vermiceras spp.besides Gyphaea arcuata and other bivalves.

Newton St Loe, Corston, Priston and Marksbury

Twerton No. 2 Shaft [ST 7129 6420] started in Lower Lias Clay and penetrated 138.5 ft (42 m) of Lias (excluding White Lias) (Richardson, 1928, p. 207). The height of the base of the Midford Sands is difficult to estimate due to cambering, but the Lower Lias has a minimum thickness of 80 m and is probably over 90 m.

The Blue Lias is mapped as a single unit and the Saltford Shales are poorly developed. Quarries exposed the upper part of Division C, in the usual facies of predominantly thin, nodular limestones, with the Calcaria Bed at the top overlain by the massive limestones and thin shales of Division D. Pennyquick Quarry [ST 7170 6465] formerly exposed 16 ft (4.9 m) of divisions C+D, and the quarry in Workshop Covert [ST 698 646] (Donovan, 1956a, p. 195) (K25) about 11 ft (3.3 m) of which 6 ft 3 in (1.9 m) belonged to Division D. Corston Field Quarry (also known as Corston Limeworks, or Combe's Quarry; K24) [ST 683 650], west of Corston, worked about 9 ft (2.74 m) of Division C. The surface rubble contained Vermiceras spp., and probably represented the weathered Calcaria Bed. Below this lay thin limestones, none more than 4.5 in (11 cm) thick, of the angulata Zone with Schlotheimia similis, S. lymense and a large species of Schlotheimia with smooth outer whorls, at least 1 ft 6 in (45 cm) in diameter. Gryphaea and Calcirhynchia calcaria were common except in the lowest beds.

West of Englishcombe cementstones with belemnites were recorded, but otherwise there are no exposures in the Lower Lias Clay. At Priston the mapping indicates a minimum thickness of 30 m for the Lower Lias which could be greater if the overlying Midford Sands have been cambered. The base was exposed in a quarry in the village [ST 697 606] (Donovan, 1956a, p. 196) which showed 14 ft (4.27 m) of beds above the White Lias. The sequence may be summarised:

The upper of the two limestones with Psiloceras may represent the Psiloceras Bed of the Keynsham area.

The upper part of the Blue Lias was exposed south-west of Priston Mill [ST 692 613]. The following section was recorded:

The lowest 7 ft (2.13 m) are taken to represent Division C. The Calcaria Bed was not recognised, but most or all of the upper part must belong to Division D. The base of the section may be near the top of the preceding one, and the minimum thickness of the Blue Lias at Priston is about 7.5 m.

The complete thickness of Lower Lias at the outliers of Stanton-bury and Winsbury Hills is estimated at about 40 m, but could have been more if the outliers of Midford Sands and Inferior Oolite which cap these hills are parts of cambers. South-west of Marksbury the basal limestones form a plateau, and are wholly of early Hettangian date. Exposures show 3 or 4 ft (0.91 to 1.22 m) of limestones belonging to the planorbis and johnstoni subzones. A description of Harris's Quarry [ST 655 616] is given in the Wells and Cheddar Memoir (Green and Welch, 1965, p. 97).

A quarry at Stawcombe [ST 664 637], about 365 m WSW of Wansdyke House, showed a good section through the basal part of the Blue Lias, measured by G. A. K. in 1938 and 1944:

The section is remarkable for the very thin development of the Pre-planorbis Beds.

Farmborough

Bloomfield Road Quarry (R4 on (Figure 9) and Appendix) showed a section (Figure 12) similar to that at Priston (p. 30). The shale near the base of the liasicus Zone may represent a thin remnant of the Saltford Shales. A small roadside quarry about 610 m west of north from the last (Tutcher and Trueman, 1925, p. 620,no. 32; Donovan, 1956a, p. 197, loc. 17) duplicated the lower part of the section. Hobb's Wall Quarry (Tutcher and Trueman, 1925, p. 620, no. 33) lay south of the A39 road and exposed White Lias overlain by 4.5 (1.37 m) of planorbis Zone and 7 ft (2.14 m) of liasicus ('Angulata') Zone. In 1976 a face remained on the eastern side showing about 1.8 m of limestones with subsidiary shales probably corresponding with the lower part of the Bloomfield Road section.

Whitchurch

On the spur north-east of Whitchurch the usual succession of two limestone groups separated by clays or shales (the Saltford Shales) is mapped. The Grove Farm Trial Borehole [ST 6267 6869], starting in the Saltford Shales, penetrated 25 ft (7.62 m) of Blue Lias and White Lias. The Saltford Shales may be about 6.0 m thick, as at Keynsham.

North of Whitchurch a narrow fault-bounded tongue of Blue Lias forms a shallow syncline. Excavations in the upper limestone group (C + D) were briefly reported by Leese and Ross (1960). A condensed, phosphatic bed, similar to that at Willsbridge (p. 26), and Stowey (p. 7), yielded among other ammonites Schlotheimia cf. angulata (Schlotheim) and Vermiceras solaroides. The condensed horizon includes part of the S. angulata Zone and the conybeari and rotiforme subzones of the bucklandi Zone.

Sections formerly exposed in the railway cutting east and south of Whitchurch were recorded in summary fashion by Stoddart (1879). His section may be correlated with the divisions traversed by the railway according to the six-inch map (sheet ST 66 NW) as follows. The grid references denote the points at which the stratigraphical boundaries cross the cuttings. (Square brackets indicate ammonite genera interpreted from Stoddart's records of species.)

About 300 m W of St Nicholas Church, Whitchurch, T. R. Fry recorded a temporary section [ST 609 675] of about 7 ft (2.13 m) of clays rich in echinoid spines, with subsidiary limestones, overlain by thin limestones at the base of Division C. Similar sections were seen in a sewer trench in 1977 north-west of Lyons Court Farm [ST 6080 6751] and [ST 6081 6746]. The same trench south of the Farm [ST 6082 6734] showed:

The massive limestone is the Calcaria Bed, or a union of that bed with beds below it, and the impersistent limestone above it may be recognised in sections in the Avon valley (bed 13 at Saltford Cutting, (Figure 10)). A short distance to the west-south-west the trench showed dark laminated shales, barren except for rare ammonite nuclei and bivalves. The junction with the limestones was not seen, but said by the foreman to have been 'disturbed' and was probably faulted, as the two exposures are on about the same level. The exposure of the Calcaria Bed was almost at the top of the upper group of Blue Lias limestones as mapped, and about 340 m to the north-west an earlier section [ST 6056 6755], also almost at the top of that outcrop, showed 10 ft (3 m) of limestone resting on blue clay (Saltford Shale). This evidence shows that the part of the Blue Lias above the Calcaria Bed (i.e. Division D) is here absent apart from one or two of the basal beds of limestone.

Knowle and Hartcliffe

The Blue Lias forms a broad plateau extending for about 8 km along the north flank of Dundry Hill. During the twentieth century the suburbs of Bristol spread over the northern part of the plateau and several temporary sections were recorded, chiefly by T. R. Fry.

At Hengrove Farm a quarry [ST 6030 6955], now filled, in Division A showed about 15 ft (4.6 m) of limestones with shale partings. A bed with Caloceras occurred about half-way up the section. About 137 m SSW of the Farm Zeilleria was found in excavations in the basal beds of Division C, and further south Schlotheimia was found at about the same horizon.

Brodie (1845, pp. 89-90) described a section 'about 2 miles south-east of Bristol' which may have been at Knowle Park about [ST 595 698] although his directions are not clear. The double limestone of 2 ft 8 in (0.81 m) thick (bed 5) is identified as the Calcaria Bed, which is similarly thick at Whitchurch (see above), so that Division C is at least 4.9 m and Division D at least 4.3 m. Brodie described bed 7 as 'Insect limestones' from which he noted Coleia, anaptychi and fossil plants. He recorded a suite of bivalves from Division C comparable to that from Keynsham (p. 26).

At Knowle West temporary sections were recorded about 1930 by T. R. Fry. Thin-bedded clays and limestones underlay the Psiloceras Bed, a limestone 4 to 6 in (0.10 to 0.15 m) thick with Psiloceras. Above this bed were about 3 ft (0.91 m) of clays and limestones with Caloceras. Division A was at least 12 ft (3.66 m) thick. It was overlain by clays at the base of the Saltford Shales, and spoil from trenches yielded ammonites indicative of the Alsatites liasicus Zone including the zonal index species as well as Caloceras aff. intermedium and Waehneroceras megastoma.

Division A was formerly exposed in quarries at Novers, also now built up. The Psiloceras Bed was a hard, crystalline limestone with pyritic galls, 4 to 8 in (0.10 to 0.20 m) thick, with abundant Psiloceras planorbis, fish scales and saurian bones. A quarry in work in 1929 exposed 8 ft 2 in (2.49 m) of thin-bedded limestones and shales with P. planorbis in the uppermost part. These beds yielded Liostrea hisingeri and, in the lower part, Modiolus hillanus and Meleagrinella decussata. A quarry known as the 'east quarry' about [ST 584 696] exposed a similar section. In both quarries the Psiloceras Bed was overlain by about 6.5 (1.98 m) of limestones and shales and the top of the section was formed by limestone, 1 ft 6 in (0.46 m) thick at the east quarry, with Caloceras, Pholadomya, Plagiostoma giganteum, and brachiopods. Division A here has a minimum thickness of about 16 ft (4.88 m).

The basal beds of the Blue Lias at Novers ('Nubber's Quarry') were recorded by Brodie (1845, p. 87). He noted about 2 ft 6 in (0.76 m) of thin-bedded limestones with Liostrea ('Ostrea'), Modiolus and Plagiostoma overlying a thick limestone (his bed 4) which is probably the Sun Bed of the White Lias.

Sewer trenches at Hartcliffe through the Saltford Shales near Croxbottom Farm [about [ST 582 685] yielded Alsatites liasicus and Waehneroceras megastoma. Higher up the slope to the south, about [ST 582 683] about 8 ft (2.44 m) of shales with Arnioceras were exposed near the base of the Lower Lias Clay and yielded also Euagassiceras sp.at the base of the section.

At Headly Park, approx. [ST 578 693] Division A was exposed during building development in 1938, and recorded briefly (Donovan, 1956a, p. 198). The Psiloceras Bed yielded P. planorbis, as at Knowle West and the Novers, and also fish spines. The limestones and shales between the base of this bed and the top of the White Lias were from 6 to 7 ft (1.83 to 2.13 m) thick, and as well as the usual bivalves yielded Oxytoma (Palmoxytoma) longicostata about 6 in (0.15 m) above the base, and Pinna hartmanni Zieten and Ctenostreon tuberculata in the upper part. On the slopes behind about 20 ft (6.1 m) of Saltford Shales were exposed and yielded Alsatites liasicus and Waehneroceras portlocki.

Bedminster Down

The Saltford Shales were exposed to a depth of about 6 m in bomb craters during the 1939-45 war. They included marls and clays with a few thin beds of limestone. A soft limestone crowded with internal moulds of Cardinia was noted at several exposures, and another with Alsatites. Waehneroceras portlocki was also found. The summit plateau of the Down is formed by the upper group of limestones, exposed in Durnford's (formerly Froude's) Quarry [ST 560 691] (Donovan, 1956a, p. 194). Only a small section remained visible in 1976. The beds belong to Division C and were 22 ft (6.7 m) thick (Figure 14). Tutcher (1929, p. 174) recorded Alsatites from the lowest beds, which suggests that the basal part of the division was exposed, this ammonite being recorded from the bottom 2 ft of the division at Saltford Cutting. The massive limestone (bed 1) exposed as soil rubble at the top may be the Calcaria Bed, but this is not proved. The total thickness of Division C is about 25 ft (7.6 m). This is thicker than in the Avon valley and the thickening has occurred chiefly by increase in the proportion of shale present.

Half a mile south-west of Durnford's Quarry, south of Colliter's Brook Farm, a waterfall in the Colliter's Brook [ST 553 684] shows the Calcaria Bed, 11 in (0.28 m) thick with large arietitid ammonites (probably Vermiceras) and schlotheimiids on its upper surface (Donovan, 1978, p. 16), underlain by thinner limestones and shales.

Division D is largely or wholly absent from this area, as the base of the Lower Lias Clay is mapped within a few feet of both Durnford's Quarry and the Colliter's Brook exposure.

North of Barrow Gurney at [ST 534 691] a borehole together with surface levels indicate that the thickness of Division A may be as much as 7.6 m, perhaps including a small thickness of White Lias. The Saltford Shales are mapped but their thickness is unknown. They wedge out south-east of Barrow Gurney at [ST 536 672].

The lower part of the Blue Lias is exposed in an old quarry by the A38 at [ST 5335 6730]. In 1976, 3.2 m of alternating limestones and clays were exposed with the Psiloceras Bed 0.58 m above the base of the section. The beds above this belong to the Subzone of Caloceras johnstoni. The same beds were noted in the angle of the Bridgwater Road and the lane to Barrow Gurney approx. [ST 535 677] by Reynolds (1912, p. 67).

Totterdown and Windmill Hill, Bedminster

In the railway section at Pyle Hill cutting, Totterdown [ST 5967 7184] Wilson (1891, 1894) recorded Cotham Beds overlain by 2 ft 9 in (0.84 m) of White Lias and 7 ft 5 in (2.2 m) of thin bedded Blue Lias limestone and shale. The lowest 5 ft 5 in (1.6 m) of the Blue Lias is equivalent to the Pre-planorbis Beds of adjoining areas at Knowle and Bedminster Down. The uppermost 2 ft (0.6 m) of the section contains Psiloceras planorbis and Caloceras johnstoni.

The maximum residual (combined) thickness of the White Lias and basal Blue Lias on Totterdown is about 4.5 m. On Windmill Hill the succession and thickness in the White Lias and Blue Lias is probably similar to that of Totterdown.

Northern slopes of Dundry Hill

The exposures on these slopes, underlain by Lower Lias Clay, have been discussed by Donovan (1978) with particular attention to the ammonite records. The zonal conclusions are referred to elsewhere (p. 12).

A claypit formerly worked beside Whitchurch Lane [ST 595 679] showed clay with septaria. About 700 m to the south a stream section [ST 5963 6728] showed 5 ft (1.52 m) of pale marl with belemnites, one of the few surface indications of the presence of marls equivalent to the Belemnite Marls (jamesoni-ibex zones) of south Somerset and Dorset.

Higher beds were exposed in a well dug in 1933 and recorded as '500 yards south-east of Hill Farm' about [ST 597 669]. The spoil yielded Aegoceras maculatum denoting a horizon in the lowest subzone of the davoei Zone and therefore higher than that of the beds with belemnites. The site is mapped as in foundered strata but the vertical displacement relative to the Dundry Borehole appears to be small (Donovan, 1978, p. 14), due to the superimposition of cambering on a basinal structure.

T. R. Fry in about 1930 measured a section in the headwaters of the Pigeonhouse Stream between approximately 76 m and 91 m above OD about [ST 586 670] to [ST 584 665]. The ground through which the stream flows was mapped as 'foundered strata' but the beds exposed had the appearance of being in order, though probably part of a slipped mass. At the top of the exposure were nodules with Acanthopleuroceras sp., and below these about 12 ft (3.67 m) of shales with thin limestones, the former yielding belemnites. Below the shales were about 21 ft (6.6 m) of hard marls, with a band of earthy limestone carrying rhynchonellids. Below an unexposed gap were a few feet of clays with nodules and Asteroceras obtusum and Promicroceras. The unexposed gap presumably represents a stratigraphical discontinuity due to superficial movements, since Asteroceras and Acanthopleuroceras were separated by a minimum of 24.4 m in the Dundry (Elton Farm) Borehole, about twice the vertical distance between these two ammonites in the stream section.

The 'contorted beds' probably correspond with those recorded on the six-inch map (sheet ST 56 NE) at [ST 5872 6720] and represent the disturbed strata below the sole of a slip whose base lies in the unexposed interval a few feet higher.

Clays with a few limestone bands and septarian nodules are exposed in The Malago brook between [ST 5642 6767] and [ST 5670 6793]. A section measured by T. R. Fry about 1930, shown in (Figure 14), agrees fairly closely with the record of the Dundry Borehole between 150.06 m and 165.6 m. The specimens from the section are preserved in the City Museum, Bristol.

About 33 ft (10 m) of clays with septaria and thin limestones were measured about 1930 in Colliter's Brook east of the Winford Arms about [ST 5538 6796] to [ST 5539 6830] (Figure 14): This section shows the lowest part of the Lower Lias Clay. The upper part overlaps with the lower part of The Malago section noted above. The section agrees closely with the record of the Dundry Borehole between 158.3 m and 163.36 m (Figure 14), the bed of nodules with Euagassiceras resupinatum (Simpson) in Colliter's Brook being correlated with argillaceous limestone with Euagassiceras at 168.36 m in the borehole. The other ammonites collected from Colliter's Brook were Arnioceras ceratitoides and A. semicostatum. The beds belong to the semicostatum Zone and perhaps to the lower part of the turneri Zone.

The Caenisites turneri Zone was exposed during excavations for the reservoirs by Bridgwater Road at Barrow Gurney during the last century. Specimens in collections appear to have come from reservoir No. 3 lying north of the road about [ST 543 680]. They include Caenisites decoratus, preserved in a septarian nodule (GSM 2299), and the holotype of C. turgescens (S. S. Buckman, 1918, p. 296, pl. 29, fig. 2, p1.31, fig. 5) whilst Microderoceras depressum S. S. Buckman, 1918, p. 305, pl. 28, fig. 3, pl. 29, fig. 1) was also based on a holotype from Barrow.

The Winford fold belt

In the northern part of Broadfield Down there is a series of folds which appear to have affected the Rhaetic and White Lias prior to the deposition of the coarse crinoidal and conglomeratic rocks of Felton and Downside. Other intra-formational movement may have taken place in the central and southern parts of Broadfield Down but the evidence for this is less satisfactory.

A well-marked elongated dome or pericline at Winford Orthopaedic Hospital [ST 535 656] marks the position of the fold belt south of Hartcliff Rocks (Figure 8).

East of the valley separating Broadfield Down from the western end of Dundry Hill the Winford fold belt can be traced by mapping, at least as far east as Blacklands north of Chew Magna. The main structure is responsible for the rise in the base level of the White and Blue Lias along the southern margin of Dundry Hill. Thickness changes were shown by mapping in the area north and north-east of Winford Church. Along the crest of the fold between Winford Orthopaedic Hospital and Greenleigh Farm [ST 565 648] both the White Lias and the Blue Lias are thin, the Saltford Shales cannot be identified and the bucklandi Zone is missing. South of the anticline the White Lias may attain a thickness of about 1.5 m locally and can be mapped as a separate formation at Bitham's Wood [ST 560 644] and Chew Hill. On Chew Hill the succession is approximately as follows:

From Greenleigh Farm the anticlinal structure can be traced to the vicinity of Blacklands where the crest of the ridge crosses the Blue Lias limestone plateau crossing the Norton Hawkfield–Chew Magna road and continuing thence in an easterly direction along the margin of the plateau. South of Norton Hawkfield another anticlinal crest, possibly associated with minor faulting, has an ENE or NE trend.

There have been few exposures in the Blue Lias. At Norton Hawkfield about [ST 594 649] the top of the division was exposed and the following section was seen in 1948. Thicknesses are approximate:

The lower limestone may represent the Calcaria Bed.

The Lower Lias Clay on the south side of Dundry Hill is covered by foundered strata to an even greater extent than on the northern slopes. Nodular limestones and clays were noted during the mapping at a few places west-south-west of Elton Farm, just above the top of the Blue Lias. Blue marl with septaria is noted west of Norton Hawkfield at [ST 5900 6495], as usual in the basal part of the Lower Lias Clay, and Asteroceras was found at [ST 585 648]. The only substantial exposure was in a clay pit [ST 5843 6558] opened about 1950 in the village of North Wick (Donovan, 1958, p. 130). The pit is mapped as in foundered strata and at the time of the survey it was noted that 'all beds show evidence of slipping and other superficial movements', but the beds appear to be in the correct sequence:

Bed 1 is undated. Bed 2 belongs to the valdani Subzone, and the remainder of the succession probably belongs to the luridum Subzone, unless higher beds have been brought in by slipping.

Bristol and the Severn valley

Basal Lower Lias is seen resting nonsequentially on the Penarth Group at the top of Aust Cliff [ST 566 898] and on the west bank of the Severn at Sedbury Cliffs [ST 553 930]. Another outlier lies in the extreme north-west of the Bristol District Sheet at Bishton [ST 392 876]. All these outliers have been described elsewhere (Welch and Trotter, 1961, pp. 123-125). The Hortham, Patchway and North Woods outliers although within the Chepstow Sheet were not described in the sheet memoir and hence are briefly referred to here.

Hortham, Almondsbury, Patchway and North Woods

The Hortham Syncline (Welch and Trotter, 1961, p. 2) trends from north-east to south-west. At the base about 2.7 m of limestones, differentiated as White and Blue Lias on the one-inch map, are overlain by about 12 m of clay. A shallow borehole at Woodlands Wood [ST 6154 8431], near Almondsbury, proved 11.5ft (3.5 m) of shales and clays overlying 9 ft 4 in (2.84 m) of earthy grey limestones with bivalves, resting on Gotham Beds (Whittard, 1949, p. 477). The limestones yielded Mactromya arenacea, Modiolus laevis, Pteromya tatei and Protocardia philippiana, while a nearby borehole yielded Liostrea hisingeri from the same beds. The absence of the White Lias in this area was confirmed during the construction of the M4/M5 motorway interchange [ST 617 837] (Hamilton, 1962; Wyatt, 1965).

At the western end of the Patchway outlier thinly bedded hard, grey shelly limestones (Pre-planorbis Beds) rest directly on Cotham Beds. About 274m S of Patchway Farm [ST 6150 8213] the White Lias is probably from 0.3 to 0.45 m thick, and forms a continuous plateau-like surface, with two small outliers of basal Blue Lias, south of Savage's Wood [ST 622 821] and 320 m W of Wells Wood [ST 621 816]. Both outliers consist of flaggy and shelly grey limestone with Liostrea. The North Woods outlier consists solely of White Lias

Stoke Gifford, Filton and Charlton

Between Great Stoke and Patchway station, the highest beds, consisting mainly of clay, are preserved in a shallow downfold in the headwaters of the Stoke Brook south of Stoke Bridge [ST 618 802]. The eastern flank of the fold is formed by hard grey slabby limestone with Liostrea and Pteromya, which crops out and has been worked for stone at Great Stoke. This basal Blue Lias limestone is underlain by very thin beds of White Lias probably about 0.3 m thick. The strata have a general west-south-westerly dip of about 3°. Old workings continue between Great Stoke and Stoke Gifford. According to Bristow (in Woodward, 1876, p. 94), up to 10 ft (3 m) of hard grey limestone with Liostrea was formerly worked at Stoke Gifford. Beneath the worked stone lay about 3 to 4 ft (0.9 to 1.2 m) of unworked Blue Lias resting on White Lias.

On the western flank of the downfold, basal Blue Lias limestone outcrops at Blenheim Drive and Lower House Crescent near Filton West Junction [ST 6072 7980], and was seen in the railway cutting at Filton North Platform (Tutcher, 1908). Pre-planorbis Beds about 6.5 ft (2 m) thick are overlain by grey limestone and clay of the johnstoni Subzone, of which about 9 ft (2.7 m) was seen in the cutting east of Charlton railway tunnel. As mapped in the area the lower division of Blue Lias includes part of the johnstoni Subzone as well as the Pre-planorbis Beds. Psiloceras planorbis has not yet been found at Filton, where there is a non-sequential contact between the johnstoni Subzone and the underlying beds. The White Lias is only 1 ft (0.3 m) thick in the Filton cutting (Tutcher, 1908, p. 6).

The basal Blue Lias limestone west of Charlton railway tunnel resembles that of Filton cutting east of the tunnel. The White Lias is slightly thicker than at Filton, about 2 ft 1 in (0.63 m) (Tutcher, 1908 p. 9). Extensive excavations south of Filton railway cutting [ST 595 798] proved Cotham Beds faulted against Blue Lias limestone and clay showing the following successions:

Another excavation 105 m W of the above showed the following succession:

The marl at the top of this section may correlate with the highest beds seen by Tutcher (1908) in the Filton railway cutting. No accurately logged borehole has penetrated the full thickness of the Lower Lias at Filton. In a borehole at Filton Laundry [ST 6025 7908] described by Richardson (1930, p. 234) and reassessed by Whittard and Smith (1944) the total thickness of Lower Lias (including White Lias) is of the order of 21 m and the position of the borehole suggests that it started in nodular limestone and clay in the lower part of the angulata Zone. Filton No. 1 Borehole (Whittard and Smith, 1943) commenced at a stratigraphical level estimated to lie about 10 ft (3 m) below the base of the angulata Zone, in the upper part of the liasicus Subzone. The thickness of Lower Lias (including White Lias) is not more than lb m of which 0.6 to 0.9 m is White Lias. The thickness of the Blue Lias proved at Fairlawn House is therefore about 15 m.

Henleaze, Golden Hill and Southmead

In the north-western part of the Horfield outlier temporary exposures were observed in 1929-30 (Kellaway, 1932, pp. 291-297). On this evidence, extending as far east as the junction of Monks Park Road and Filton Road [ST 5977 7772], the generalised succession at Southmead and Horfield (Figure 15) has been calculated. About 393 m NE of the junction of Southmead Road and Monks Park Road the Filton No. 2 (1943) Borehole [ST 5933 7848], (Whittard and Smith, 1944), proved 45 ft (13.7 m) of Lower Lias resting on Cotham Marble, the basal Lower Lias limestone being composed of 2ft 6 in (0.6 m) of White Lias overlain by 1 ft 6 in (0.4 m) of hard grey limestone (Pre-planorbis Beds). The remainder of the Lower Lias is described in the log as 'grey clay with calcareous nodules'. The hole was probably sited about 3 m below the base of the nodular limestone and clay of the angulata Subzone, possibly in the liasicus Subzone. About 1000 m north-west of Filton No. 2 Borehole a trial boring (Pen Park 'E') [ST 5892 7888] made in 1956 by the City Engineer of Bristol Corporation proved 9 ft (2.7 m) of basal Blue Lias and White Lias limestone resting on Cotham Beds. Minor structures (see below) render it difficult to make out the detailed succession and thicknesses in the northern part of the Southmead area.

In the 'Rhaetic' and Lower Lias rocks flanking the Brentry ridge between Pen Park Hole and the vicinity of Greenway School [ST 577 780] there are many small faults and folds, possibly due to adjustment along structures in the underlying Carboniferous Limestone. Woodward (1875, p. 94) stated that 'the tract of Lower Lias upon which stand the villages of Filton and Horfield, overlaps the Penarth beds on the west, resting directly upon the Carboniferous Limestone', and this interpretation was accepted by Kellaway (1932, fig. 3).

Farther south in the district of Henleaze the general succession near the Beehive Inn [ST 585 772] may be summarised as follows:

At Wellington Walk (Kellaway, 1932, section No. 13 of southern area, p. 290) about 137 m N of section No. 5, a similar sequence was recorded. Excavations on gently rising ground east of these exposures showed limestone and clay with Caloceras sp.Here the mapped boundary between the basal Blue Lias limestone and the overlying clay is probably drawn within the johnstoni Subzone. A small section in 'Beehive Lane' (now Wellington Hill), on or near Harbury Road [ST 5843 7720] (Kellaway, 1932, pp. 288-289) proved Cotham Beds overlain by 1 ft 4 in (0.4 m) of White Lias and 2 ft 2 in (0.65 m) of grey limestone and shale (Pre-planorbis Beds). At West Broadway [ST 5872 7688] higher beds consisting of nodular limestone with Calcirhynchia and Schlotheimia extranodosa were proved (Kellaway, 1932, section No. 7 of southern area, p. 289).

Near Kellaway Crescent approx. [ST 5895 7670] Kellaway (1932, p. 289) recorded ammonites proving the two lower subzones of the bucklandi Zone, in ground later mapped as clay. The highest beds proved in the area were exposed at the eastern end of Monks Park Avenue [ST 598 777] where blue clays and shales with septaria yielded Arnioceras sp.and Coroniceras Lyra. North of Wellington Hill several trenches cut near Wellington Walk [ST 5847 7738] and Cherington Road [ST 5847 7726] show that thin limestones, with Psiloceras planorbis, are present in the upper part of the basal division of the Blue Lias. South of Wellington Hill [ST 5967 7716] and north of Henley Grove [ST 5780 7640] the platform of basal Blue Lias continues by way of Eastfield to Henleaze. The section described by Kellaway (1932, p. 287) at Ropers Farm was exposed in a trench situated on the line of Rockside Drive [ST 5792 7695]. This section shows 0.1 m of White Lias overlain by 0.3 m of Pre-planorbis Beds overlain non-sequentially by at least 0.6 m of limestone and clay with Caloceras. Psiloceras planorbis has not been found at Eastfield or in the south-western parts of Henleaze and it is likely that its subzone is missing in this area. In another section south of Holmes Grove [ST 5768 7627] the Cotham Marble is overlain by about 10 in (0.25 m) of White Lias succeeded by Pre-planorbis Beds 2 ft 2 in (0.65 m) thick (Kellaway, 1937). In the southern part of Henleaze, adjacent to Westbury-on-Trym and Durdham Downs, the crop of the Lower Lias forms a fairly level plateau, 3 to 4.6 m of thinly bedded grey limestone and shale being underlain by 0.3 to 0.6 m of White Lias. A number of sections were seen in 1935. Exposures A and B, Henleaze (Kellaway, 1937, p. 225) were recorded in trenches covering a wide area, but mainly on the line subsequently adopted by White Tree Road [ST 5780 7594] and Fallodon Road [ST 5785 7615]. The combined sections may be summarised as follows:

Ashley Down, Montpelier and Bishopston

The most complete succession was logged (G.A.K.) in Borehole No. 33 made in 1956 at Ashley Down by the Bristol City Engineer. A summary of the upper part of the borehole [ST 5968 7553], surface level c. 188 ft (57.3 m), follows (see (Figure 15)):

The boring may have begun at about the same stratigraphical level as Stoddart's (1868, p. 200) No. 1 Quarry, Ashley Down (site unknown) which showed limestones and shales with schlotheimiid ammonites ('Am. Angulatus'), probably equivalent to the nodular limestones of the borehole (Division C, p. 6). Overlying these beds was a 10 in (25 cm) limestone with abundant coroniceratid ammonites (' Am. Bucklandi, Am. Conybeari') on its upper surface which must correspond with the Calcaria Bed of the Avon valley.

At Montpelier an extensive area was quarried in the second half of the nineteenth century, from approximately [ST 5945 7485] to [ST 5929 7512], the northern part of this area now lying in St Andrew's Park. The quarries straddled the outcrop of the second, clayey division of the Blue Lias, Division B, p. 6). A section of these quarries, exact location unknown, recorded by Stoddart (1868, p. 202) showed 33 ft 10 in (10.3 m) of shales and clays with subsidiary limestones. The fossil records indicate that the whole of the section was probably in the liasicus Zone, though the lowest part may have penetrated into the planorbis Zone. The lowest bed was called by Stoddart the 'Avicula-bed' which was probably the source of the long-spined Oxytoma (Palmoxytoma) longicostata (Stutchbury, 1839) (see Cox, 1961).

No sections have been recorded in Bishopston, but the base of the Lias, with Cotham Marble below, was exposed in an excavation at the public baths near the junction of Berkeley Road and Gloucester Road [ST 5902 7529] (personal communication, late J. W. Tutcher). The basal beds were formerly quarried along what is now Logan Road at [ST 5845 7547] and [ST 5870 7515].

Pur Down, Lockleaze and Horfield

South of Harry Stoke the basal limestones of the Lower Lias (Division A) have been worked near Wallscourt Farm [ST 6195 7827] and were intersected at the surface in the entrance to the incline of Harry Stoke Drift Mine [ST 6188 7860].

At Harry Stoke and Wallscourt Farm the basal limestones have a gentle westerly dip, which steepens to 10°-12° at Pur Down and Lockleaze where the strike of the Lower Lias swings to a NE-SW direction. A number of small NNE-SSW-trending folds and monoclines passing locally into faults, were observed in excavations at Lockleaze. These structures are subparallel with the strike of the basal Lower Lias, but individual subsidiary folds and faults can only be traced over short distances. Flexures and small faults were observed in Cotman Walk [ST 6061 7668] and at a point midway between Cotman Walk and the footbridge over the railway [ST 6049 7660]. These small structures are not shown on the published maps. Evidence of the narrow belt of folding and flexuring was observed at Orpen Gardens [ST 6072 7637] and also at Landseer Avenue [ST 6073 7666]. The presence of these subsidiary structures in the eastern limb of the main syncline gives rise to difficulty in assessing the thickness of the subdivisions of the basal Lower Lias at Lockleaze.

Among the sections seen at Lockleaze, the following are worthy of specific mention:

Copley Gardens [ST 6085 7684] Grey nodular limestone with Schlotheimia and Antiquilima succincta seen in trenches. These rocks are included in the nodular limestone and clay which constitute the greater part of the angulata and bucklandi zones.

Orpen Gardens [ST 6077 7682] Section in White Lias consisting of 9 in (23 cm) of hard white limestone resting on 1 ft 6in (0.5 m) of white clay and marl and soft white limestone with bivalves overlying Cotham Beds, including the Cotham Marble. This is almost the full thickness of the White Lias at Lockleaze which averages 2 ft to 2 ft 6in (0.6-0.76 m).

At Landseer Avenue [ST 6067 7703] the following section was recorded:

Intersection of Landseer Avenue and Brangwyn Grove [ST 6073 7666] A section showing soil and clay 2 ft (0.61 m) resting on 5 ft (1.52 m) of alternating limestone and dark grey clay and shale yielded a poorly preserved ammonite, Waehneroceras? This locality is situated near the contact of the basal Division A of the Blue Lias and the overlying clay.

In the southerly portion of the Lockleaze estate sections adjacent to the railway line about 475 m N of Muller Road railway bridge [ST 6032 7628] showed nodular limestone and clay with Schlotheimia. A tract of heavy clay (roughly corresponding with the Saltford Shales) separates the nodular limestones with Schlotheimia from the basal limestones of the Blue Lias and White Lias, which crop out east of Romney Avenue and form the higher slopes extending to the crest of the Pur Down ridge. The grey limestone and clay at the top of the basal limestones of the Blue Lias were proved in numerous excavations at the southern end of Romney Avenue where the strata have a north-westerly dip of about 10°.

The highest beds of the Lower Lias to be preserved in the area were exposed during widening of the railway cutting at the now disused Horfield Station [ST 6060 7750], where about 15 ft (4.6 m) of dark shale with septarian nodules overlay about 15 to 20 ft (4.6 to 6.1 m) of blue-grey limestone bands in shale.

Hallen and Over outliers

Very little is known about these outliers. At Highwood Lane [ST 5873 8131] hard grey shay limestone (Pre-planorbis Beds) rests locally on Cotham Marble, but in a small isolated patch of Lower Lias limestone near the eastern margin of the main outlier, material from a shallow trench [ST 5873 8125] yielded traces of very thin White Lias and Cotham Marble (probably about 0.3 m in combined thickness) underlying the basal Blue Lias. Cotham Marble was recorded beneath Blue Lias in the bed of the River Trym 716 m NE of the bridge at Catbrain [ST 5810 8095]. It appears, therefore, that Pre-planorbis Beds rest non-sequentially on the Gotham Beds over most of the area but that the White Lias and Cotham Marble are preserved locally. An outlier of Blue Lias forms the plateau at Hollywood Tower [ST 5729 8139].

West of Cribbs Causeway an outlier of the basal Blue Lias limestones shows former workings in flaggy limestone at [ST 5634 8062]. Loose fragments show that White Lias may exist locally beneath the Blue Lias. It is doubtful whether the combined (residual) thickness of the White Lias and Blue Lias limestone total more than 0.6 to 0.9 m.

At least one, possibly two, small outliers of Blue Lias limestones are present at Spaniorum Hill, south-west of Compton Greenfield. Blue Lias with Liostrea, Protocardia philippiana and Pteromya tatei rests on a denuded surface of the Cotham Beds.

Redland Green and Westbury Park

Very little is known about the small Redland Green outlier (Figure 1). The thickness of the Lower Lias probably does not exceed 1.5 m resting on Cotham Beds. No measured sections are known.

In the Westbury Park outlier to the north about 0.9 to 1.2 m of hard grey limestone with clay partings rests on about 0.2 to 0.3 m of White Lias. According to Parsons (1901, p. 104) the Lower Lias rests unconformably on Carboniferous Limestone near St Albans Church [ST 5768 7545]. This is an exceptional circumstance, however, and temporary excavations at several points, notably in the vicinity of the White Tree [ST 5727 7579] on the eastern margin of the Westbury Park outlier, suggest that the Penarth Group is generally present in this area. At the eastern margin of the outlier a detailed section in the Penarth Group and Lower Lias was recorded by Wickes (1901) in a road cutting at or near the junction of Linden Road and Cranbrook Road [ST 5806 7562]. The area to the west, formerly known as 'New Clifton' and described by Wickes as 'Red-land', now lies in Westbury Park. The basal Blue Lias limestones were said to yield 'Am. planorbis' (i.e. Psiloceras) and 'Am. johnstoni' (i.e. Caloceras) in addition to Thynchonella calcicosta' (Calcirhynchia calcaria). The Pre-planorbis Beds, if present, may be represented by very thin limestones with Liostrea hisingeri which form the local base of the Blue Lias. This sequence of Wickes is similar to that recorded at Cotham (p. 39) where the Pre-planorbis Beds are also missing or very thin.

On the northern side of the Westbury Park outlier 'Rhaetic' clays could formerly be seen resting on the Carboniferous Limestone in an old quarry at Halsbury Road [ST 580 758] (Parsons (1901) and Vaughan (1905, p. 234)). Parsons (1901, p. 105) states that in the Halsbury Road quarry the dip in the Penarth Group is 10°-12° to the east. Given a similar dip in the Lower Lias the maximum thickness of basal Blue Limestone is unlikely to exceed 3 m; and hence is probably not younger than the johnstoni Subzone.

Cotham and Clifton

Several quarries were worked at Cotham and Redland in the 19th century (Stoddart (1868); Groom-Napier (1868; 1869). The area of working is thought to lie between Gotham Park [ST 5838 7415] and Cotham Grove [ST 5852 7447]. In this northern part of the Cotham outlier a marked re-entrant on the 200 ft (61 m) contour may indicate the position of old limestone workings. According to Stoddart (1868, fig. 1) Cotham Quarry was situated on the northern part of the hill. Here, the rocks are said to dip at about 8° to the NE, which would account for the relatively low altitude of the basal Lower Lias. W. Sanders' map (1864) shows a narrow belt of Lower Lias extending towards Clifton but the extent of the Lower Lias may be somewhat greater than he indicated.

Stoddart (1868, p. 203) records the maximum thickness of beds in Cotham Quarry as 14.5 ft (4.42 m). Groom-Napier (1868) records 16.5 ft (5.02 m) but does not state the thickness of some of the subdivisions. In a further account, however, Groom-Napier (1869, p.99) gives the total thickness of Lower Lias seen in Quarry I as 22 ft (6.70 m), but he adds 'there is a difference of several feet in the thickness of the beds in a space of a few yards'. This may be due to original changes in thickness, or cambering may have played a part. However, it is apparent from the records of Stoddart and Groom-Napier that the lower part of the Lower Lias (White Lias to basal angulata Zone inclusive) is thinner than in the main outlier at Horfield and Henleaze. Trial pits in Lower Lias (about 1934) seen by G.A.K. on the site of Cotham Grammar School [ST 5827 7397] certainly occur above the strata described by Stoddart (1868) and Groom-Napier (1868, 1869) and are preserved in the highest part of the hill on a gently plunging syncline with a SW–NE trend (Figure 15). The maximum thickness of the synclinally folded Lower Lias preserved in the Cotham outlier is probably about 10 to 12 m but no evidence of the presence of zones above the angulata Zone has been found.

It has been suggested, for example by Stoddart (1868, p. 204), that the stratigraphical succession in the Keuper, Rhaetic and basal Lower Lias of Gotham Hill is incomplete, and that some of the formations are missing altogether but this was not borne out by the primary six-inch survey.

At the western end of Cotham Hill, W. Sanders' map of 1864 shows the Lower Lias as a narrow belt continuing westwards to Oakfield Road Pumping Station, Clifton [ST 5761 7372]. Here a section, including 2 ft (0.61 m) of White Lias, was recorded by Tawney (1878a). Austin (1865, pp. 16-18) recorded structurally disturbed Lower Lias (probably White Lias and basal Blue Lias) at a locality described as 'Vittoria Place' which is situated near the junction of Whiteladies Road and West Park [ST 5784 7376].

Area north and east of Radstock

The area around Timsbury, Paulton and Radstock has become famous on account of the condensed facies of part of the Lower Lias, which is often highly fossiliferous. A large number of exposures have been described and these are listed in the Appendix (pp. 60-62). The majority of them are shown in the locality map, (Figure 16).

Glutton and Timsbury

Exposures were described by Tutcher and Trueman (1925, pp. 620, 621) and summarised by Green and Welch (1965, p. 100). The 'Angulata Zone' of Tutcher and Trueman probably refers to beds of Alsatites liasicus Zone age; the Zone of Schlotheimia angulata is not proved in the area.

The whole thickness of the Lower Lias is preserved at Timsbury Sleight capped by an outlier of Midford Sands and Inferior Oolite. A section exposed in pipe trenches was measured by Tutcher and published in diagrammatic form (Tutcher and Trueman, 1925, p. 618, fig. 7). About 12 ft (3.66 m) of limestones at the base are succeeded by about 120 ft (37 m) of clays. The Hettangian is 8 ft (2.44 m) thick. The clays are belemnitiferous below, and micaceous above, as on Dundry Hill (p. 33) and probably fall within the uppermost ibex Zone and the davoei Zone.

Seven quarries (R6, R9-14) have been described around Timsbury. The best-known was Hodder's Quarry (R11), described in detail by Tutcher and Trueman (1925, p. 604) and shown here in diagrammatic form (Figure 17). It is noteworthy for the thickness (6 ft; 1.83 m) of the clays (Turneri Clay, Obtusum Bed and Raricostatum Clay) between the Bucklandi Bed and the Armatum Bed. A section recorded in great detail by Tawney (1875b, p. 182) as 'below Timsbury' is similar and was probably nearby, but cannot now be located. 'Medyeat Quarry' (R14) lay at the hamlet of Meadgate between Timsbury and Camerton. The clays mentioned above were thick enough to be dug for brickmaking and formed the highest beds in the quarry, but Moore noted (1867, p. 474) that 'Marlstone' (i.e. the Jamesoni Limestone) was exposed in a neighbouring field. The section may be summarised as follows, using modern terminology:

The clays are here at least 3.2 m thick, the greatest recorded thickness. On account of this the Jamesoni Limestone was mapped as a separate unit ('Blue Lias' on the six-inch map) which wedges in as an intercalation in the Lower Lias Clay at [ST 689 588] and continues westwards to [ST 671 585] where it is shown as joining the Hettangian limestones, the clays having become too thin to map.

Tunley and Dunkerton

At Priston Colliery No. 1 Shaft (R7: situated at Tunley, not at Priston) E. H. Staples recorded (MS in IGS) the section when the shaft was sunk in 1914. The shaft commenced almost at the top of the Inferior Oolite, a small patch of Fuller's Earth being mapped about 50 m to the south.

The limestones represent the Hettangian limestones and perhaps the Bucklandi Bed. The Jamesoni Limestone is presumed to have wedged out. The total thickness of Lower Lias is 162 ft 4 in (49.5 m).

Dunkerton No. 2 (Withyditch) Borehole (R8) was also recorded by Staples but his record indicates a total thickness of Lower Lias of about 240 ft (73 m). There is no confirmation for this record, and the Lias is not known to attain such a thickness anywhere else in the Radstock district. It seems preferable to regard the record as erroneous.

In the neighbourhood of Dunkerton Colliery quarries and railway cuttings (R16) exposed early in the present century were recorded by Tutcher and published in summary form by Tutcher and Trueman: The section is:

Dunkerton Colliery No. 1 Pit (R15), sunk in 1902 and recorded by C. Heal, cannot be exactly correlated with Tutcher's section and is less precise. Tutcher regarded the lowest 4 ft (1.22 m) of the Hettangian as Psiloceras planorbis Zone, from which he recorded Psiloceras and Caloceras, and attributed the remainder to the Schlotheimia angulata Zone. In accordance with the situation elsewhere in the Radstock district (see p. 12) this is regarded as Alsatites liasicus Zone, the 'Schlotheimia' recorded by Tutcher being probably Waehneroceras.

Tutcher and Trueman's (1925, p. 617) 'Camerton Quarry' stated to lie 0.25 mile (0.4 km) NNE of Camerton church, cannot be located and the literature references given refer to Medyeat Quarry described above (R14).

Lonsdale (1832) gave a section along the 'Descent from the Red Posts to Camerton' (R17), i.e. along Whitebrook Lane:

Modern equivalents are named in square brackets. The Turneri Clay is only a little thinner than at Medyeat, 1.6 km to the northwest, and the Jamesoni Limestone is thicker in accordance with other evidence for slight southerly thickening. The 100 ft for the Lower Lias Clay can hardly be an accurate figure, but is approximately correct and cannot be improved upon. South of Radford about [ST 673 572] the total thickness of Lower Lias appears to be about 30 m, according to the mapping, but this figure is a minimum because of possible cambering of the Inferior Oolite.

Paulton

A number of small quarries (R18-R21, R24, R25) were worked in the nineteenth century. One of the most complete sections was at Ham Quarry (R20) (Figure 17) which showed the whole sequence from the White Lias up to the base of the Lower Lias Clay ("Striatum Clay'). Perhaps the best-known quarry for mid-nineteenth century collectors was Mungar Quarry (R25), which was also once worked down to the White Lias (Tawney, 1875b, p. 178) but where only the Armatum Bed and higher limestones were visible when the section was measured. A detailed section was published by Tate (1875, p. 500):

Present terminology has been added in square brackets. The ammonite genera are inserted on the basis of the species of Ammonites recorded by Tate. Fossil names are taken unchanged from Tate's section, some others are revised with probable current genera (shown in square brackets).

Phyllis Hill Quarry (R24) was on the west side of the road to Midsomer Norton, now called Phillis Hill on the six-inch map. The bearing and distance from Paulton Church given by Tutcher and Trueman (1925, p. 616) are wrong. The quarry was about 0.4 km W of Mungar Quarry and continues the section downwards. The section below is based on a detailed section by Tawney (1875b, p. 179) with modern stratal names added in square brackets:

The Turneri Clay is thinner than at Ham Quarry, about 700 m to the north and it rests directly on White Lias, the Bucklandi Bed and Hettangian limestones having wedged out. This is the most northerly locality where Hettangian is absent in the Radstock area.

The site of the brickyard (R23) in Lower Lias Clay 'a few hundred yards north of the Mungar quarry' (Tawney, 1875b, p. 178) cannot now be identified. It was presumably north of the Winterfield Fault, somewhere along Water Lane. Tawney thought the clays to be Upper Lias but this was at the time when the Jamesoni Limestone was regarded as Middle Lias. He noted septaria and 'nodules of argillaceous iron-ore'. A complete section of Lower Lias was traversed in Paulton Hill Pit (R22) before 1822. A total thickness of 126 ft (38.4 m) was recorded, all 'blue marl' except for the lowest 6 ft (1.83 m) said to be 'grey and blue lias'.

Welton, Clandown, Broadway Lane and Bowlditch

The quarry at Bowlditch (or Bold Ditch) (R27) has been a source of fossils, especially from the Spiriferina Bed, for a long time. It is now very much overgrown. A diagrammatic section is shown in (Figure 17). Tawney (1875b) recorded Echioceras sp. ('Am. raricostatus') from the upper clay horizon of the Raricostatum Clay. Spiriferina was abundant in the Spiriferina Bed and Tawney also records Rhynchonella variabilis', Terebratula punctata' and Gryphaea arcuata.

The Bucklandi Bed, according to Tutcher and Trueman (1925, p. 635) carried Agassiceras scipionianum, Arnioceras spp., Euagassiceras spp., and Coroniceras (Paracoroniceras) sp.Tawney cites the same brachiopods as for the overlying Spiriferina Bed, and Gryphaea arcuata, Plagiostoma giganteum and Pleurotomaria expansa.

Tutcher and Trueman recorded Psiloceras planorbis from the limestones and shales between the Bucklandi Bed and the White Lias. The thickness of these beds is recorded by them as 3 ft 2 in (0.96 m), and by Tawney as 1 ft l0.5 in (0.56 m). They are presumably entirely of Psiloceras planorbis Zone age. Tawney recorded the following species of Bivalvia from these beds: Astarte sp., Gryphaea arcuata, Liostrea sublamellosa, 'Lima tuberculata', Pinna sp., Plagiostoma giganteum, P. punctata, Unicardium cardioides.

Broadway Lane claypit (R26) was opened in 1928 and closed in 1939. The pit is shown on the published six-inch geological map (sheet ST 65 NE) but has since been filled. The section was studied by G. A. Kellaway in 1936 and an account published by Cox (1941, pp. 22-24). A more detailed section is now presented:

The ammonite fauna is important for zonal dating; unfortunately the main collection from the pit did not survive the 1939 war, and the few surviving specimens are not labelled as to horizon. The lowest beds (8) yielded Aegoceras (Beaniceras) (Cox, loc. cit.) and surviving examples (City Museum Bristol, Nos. Cc 272,273) include forms close to A.(B.) dundryi. Tragophylloceras, probably T. losconzbi (City Museum Bristol, Nos. Cc. 275-277), was also found in this bed and ranges up to the base of the Brown Bed.

All the other surviving specimens of Aegoceras (City Museum Bristol, Nos. Cc 266-270) belong to Aegoceras maculatum, and show at least three different types of preservation. This species probably characterised the sequence from bed 7 up to bed 2, inclusive. Liparoceras kilsbiense was recorded by Spath (1938, p. 62) but the horizon is not known. Spath also recorded (1938, p. 65), Androgynoceras naptonense and the Bristol City Museum has two specimens of A. aff. geyeri (Nos. Cc 262,263).

The most likely conclusion as to age is that bed 8 belonged to the luridum Subzone of the ibex Zone, and that the remainder falls into the maculatum Subzone of the davoei Zone. The uppermost 6 ft (1.83 m) is undated, but it is likely that most, if not all of the capricornus Subzone, present in the Dundry (Elton Farm) Borehole, was removed by erosion before the Upper Inferior Oolite was laid down.

As regards the other fossils, belemnites occurred throughout, Passaloteuthis apicicurvata in all beds except 1, and Hastites stonebarroensis in bed 4. Gastropods included Amberleya cyclostoma in beds 5, 6 and 8, Ptychomphalus expansus from bed 6 or 8, and Zygopleura transversa from bed 5. Bivalves comprised 26 species in 20 genera, most of these occurring in the clays below the Brown Bed.

The highest lithological division of the Lower Lias Clay, Division 5 (p. 35), is reduced from 53 m at Dundry to less than 9 m at Broadway Lane.

Bince's Lodge Quarry (R28), about 400 m SW of Bowlditch, was similar to Bowlditch, except that all the subdivisions are thinner, and the Bucklandi Bed is absent altogether:

Tutcher and Trueman recorded a quarry called Welton Hill Quarry (R29) east of Bince's Lodge Quarry. The exact site has not been determined. The quarry showed 2.5 ft (0.76 m) of Hettangian limestones with Psiloceras planorbis ('P. sampsoni'), Liostrea sp., Isognomon cf. infraliasica Quenstedt, and Pinna sp., resting on White Lias.

The whole of the Lower Lias is present at Clan Down, where it is capped by the Inferior Oolite. The total thickness of the Lower Lias is difficult to determine because the base and top are separated by the Winterfield Fault. It may be about 33 m. The basal limestones have been seen at a number of places (R30–R34). At Clan Down Colliery Quarry (R30), the section shown in (Figure 17), these total 10.5 ft (3.15 m). Numerous fossil species were recorded by Tate (1875), Tawney (1875b) and Tutcher and Trueman (1925). Of particular interest is the sequence of ammonites within the Jamesoni Limestone and Armatum Bed noted by Tutcher and Trueman (op. cit., pp. 601, 604):

• Jamesoni Limestone
• d. Tragophylloceras spp., Tropidoceras sp.
• c. Uptonia jamesoni
• d. Platypleuroceras brevispina, P. rotundum, Polymorphites spp., Radstockiceras complicatum S.S. Buckman, Uptonia sp.
• a. Polymorphites lineatus
• Armatum Bed
• upper part: Apoderoceras sp.
• lower part: Apoderoceras aculeatum, Phricodoceras sp.

The Phricodoceras was found by Tutcher and Trueman but was recorded (as 'A. T[a]ylori') from the lower bed of the Armatum Bed by Tate (1875, p.500). Tutcher and Trueman (1925, p.641) recorded Radstockiceras ('Tutchericeras perfiliatum S. Buckman') from the Armatum Bed, but did not say whether it came from the lower or the upper part.

The limestones between the White Lias and the Bucklandi Bed represent only the Pre-planorbis Beds, the higher beds of Hettangian age, with Psiloceras and later ammonites, being absent.

The section in Clandown Colliery shaft (R32) was recorded by Buckland and Conybeare (1824, p. 278) and may be interpreted as follows:

If the record is accurate the basal limestones are even thinner (7 ft, 2.13 m) than at Clandown Colliery Quarry, a short distance to the south-west. It should be noted that the record is transcribed incorrectly on the published geological six-inch map ST 65 NE.

Lonsdale (1832, p. 247) gave a section in the 'Lane leading to Clan Down in the descent of Radstock Hill.' (R33). It shows thicknesses intermediate between Clandown Colliery Quarry and Old Pit Quarry (see below), the basal limestones being in total at least 12 ft (3.66 m). Interpretation of the section is not quite certain since the clay beds were not recorded, but the 'Corn grit' is 3 ft (0.91 m) and the Armatum Bed and Jamesoni Limestone ('Marlstone and Gritty lias') total 7 ft (2.13 m).

Old Pit Quarry (R34) lies about 450 m ESE of Clandown Colliery Quarry. It was recorded by Tate (1875) but ignored by all other workers on the Radstock Lias, a curious fact as it showed the thickest development of the basal beds at Clandown, a total of about 9.5 m. Fortunately it can still be located, behind Old Pit Terrace, and the upper 3.66 m of the section is still (1975) exposed. The section is shown diagrammatically in (Figure 17). The contrast in thickness with Clandown Colliery Quarry is remarkable. Both main groups of limestones are much thicker. The Hettangian limestones ('Corn grits') are at least 15 ft 4in (4.67 m) (the White Lias was not exposed) compared with only 2 ft 1 in (0.63 m) at Clandown Colliery. The Armatum Bed, Jamesoni Limestone anu Valdani Limestone together are 13 ft (3.96 m), twice their thickness at Clandown Colliery. The great thickness of Hettangian recorded by Tate is unconfirmed but is in accordance with the rapid local southerly thickening shown by Middle Pit Quarry (R35) and Wellsway Quarry (R49).

Rockhill Quarry (R31) was briefly recorded by Tutcher and Trueman as follows:

The Hettangian was said to consist of Ostrea Beds (= Pre-planorbis Beds) succeeded by the planorbis Subzone and then by the johnstoni Subzone. It was implied that later Hettangian Beds were absent. The Armatum Bed yielded a derived Radstockiceras ('Oxynoticeras williamsi' Tutcher and Trueman, 1925, p. 644, pl. 41, fig. 3).

Middle Pit Quarry (R35) was already disused and overgrown when Tutcher and Trueman studied it. The total thickness of Hettangian limestones was about 22 ft (6.7 m) of which 9 ft (2.7 m) was put in the Zone of Psiloceras planorbis. The remainder was therefore liasicus Zone ('Angulata Zone' of Tutcher and Trueman).

Valley north of the Wellow Brook

Tyning Colliery Quarry (R37) was recorded briefly by Tutcher and Trueman:

The Armatum Bed was said to include a thin band of sandy marl, and in this twofold division, and its thick development, it resembles Old Pit Quarry. Apoderoceras occurred both below and above the marl, which itself yielded fish scales and teeth, abundant brachiopods, belemnites, and crinoids.

An exposure in Lower Wood (R38), south-east of Woodborough House, is recorded on the six-inch map as follows:

Another exposure recorded on the six-inch map (R40) lies just to the east of Woodborough Farm and showed 3 ft (0.91 m) of ironshot limestone (i.e. Armatum Bed and Jamesoni Limestone) resting on White Lias.

The shaft at Braysdown Colliery (R39) commenced in the Inferior Oolite and provided a complete section through the Lias, but Greenwell and McMurtrie's (1864, p. 26) record is difficult to interpret. It is possible that the beds between the Armatum Bed and the White Lias were absent, as at R40, 450 m to the south, or very thin. The total thickness of Lower Lias at Braysdown would have been about 23 m.

Stony Littleton

Several exposures are still (1976) visible. Stony Littleton Quarry (R43) may be summarised as follows:

Waehneroceras spp.and Schlotheimia similis were found in loose debris, indicating the Alsatites liasicus and Schlotheimia angulata zones respectively. Tutcher and Trueman (1925, p. 614) saw 14 ft 8 in (4.5 m) of Hettangian, of which they attributed the lowest 5 ft (1.52 m) to the Zorte of Psiloceras planorbis. A similar, but more degraded section occurs in an old quarry on the other side of the valley, about 200 m to the south-east (R44). Here about 1.5 m of White Lias is exposed, and the total thickness of the Hettangian can be little more than the 4.5 m seen by Tutcher and Trueman.

Liassic limestones are poorly exposed in the river cliff north-east of Stony Littleton Quarry, and well-exposed in an old quarry northeast of Stony Littleton Farm (R45). A thick limestone at the top probably represents the 0.50-m massive limestone at Stony Littleton Quarry. Below this about 14.5 ft (4.4 m) of mostly irregularly bedded or nodular limestones, with thin seams of shale, of Hettangian age.

The total thickness of Lower Lias at Stony Littleton is estimated from the six-inch map at about 33 m of which about 26 m is Lower Lias Clay.

Foxcote Colliery

The shaft was sited on Inferior Oolite and gave a complete section of the Lower Lias (R46). The records are very old and not easy to interpret. Vertical Section sheet 48, no. 3 (Anstie, 1873), shows a total thickness of 80 ft (24.4 m) for 'Lias and Penarth Beds'. Greenwell and McMurtrie (1864, p.25) recorded:

If the 'Black/blue marls' are the Westbury Beds, the 'Lias beds' must be the White Lias, perhaps including the Cotham Beds. 'Rumples' would then be the limestones at the base of the Lower Lias, and the total thickness of Lower Lias would be 48 ft (14.6 m). The greater thickness for Lias and Rhaetic' given by the Vertical Section may be explained if the Tea Green Marls were included in the Penarth Beds.

Exposures of ironshot limestone were noted during the mapping to the north-east of Foxcote Colliery [ST 7141 5562] and [ST 7134 5536]. An old claypit in the base of the Lower Lias Clay existed at [ST 709 552].

Area south of Radstock

Radstock Grove and Tyning Farm

Radstock Grove Quarry (R52) ((Figure 17)) 410 m S of Radstock church, is still (1975) open, although it has not been worked for a long time. It is registered as a Site of Special Scientific Interest with the Nature Conservancy Council. It shows Jamesoni Limestone and Armatum Bed, the latter separated by a few inches of strata from the Hettangian limestones which are loft (3.05 m) thick. In these Tutcher and Trueman (1925, p. 610) recognised Ostrea, Planorbis, and Caloceras Beds... followed by a thin development of the Angulata Zone.' This indicates that the whole of the planorbis Zone is present, succeeded by thin liasicus Zone. As explained elsewhere (p. 12), the Zone of Schlotheimia angulata is absent from the Radstock area. The beds between the Hettangian limestones and the base of the Armatum Bed are variable. The Bucklandi Bed is a whitish, coarse, massive, shelly limestone up to 1 ft (0.30 m) thick, but locally reduced to about 2 in (0.05 m), chiefly because the upper surface is very irregular. On the north face at [ST 6884 5426] the upper surface has a relief of about 5 cm, and is encrusted with a ferruginous layer which is capped by a dark phosphatic layer. In places on the western face about [ST 6883 5423] the upper surface is flat and the phosphatic crust is succeeded by a limestone with vertical borings or burrows, 10 cm thick. The Bucklandi Bed and the Armatum Bed are separated by 6 to 12 in (0.15 to 0.30 m) of clay, containing lumps or nodules of limestone at two levels. Both levels yield Arnioceras semicostatum and the upper (Obtusum Nodules) yields Asteroceras confusum.

The Armatum Bed at Radstock Grove contains, as usual, derived material in its lower part. Good examples of Echioceras raricostatoides are common, often with the inner whorls replaced by coarsely crystalline calcite; the body chamber, and sometimes part of the septate whorls, are grey or black phosphatic limestone. Species of Paltechioceras include P. tardecrescens (e.g. the holotype of 'Euechioceras nobile' Trueman and Williams, 1925, p. 725; Getty, 1973, p. 20) and have slightly different preservation—a light grey limestone—from Echioceras. Examples of Gleviceras are numerous and several have been figured (e.g. Buckman, 1919, pl. 136A–C; and pl. 137A, B, the holotype of Tutchericeras perfoliatum'). Most of them are to be identified as Gleviceras subguibalianum and G. victoris (Dumortier). They are preserved in sandy limestone and may be contemporary with the deposition of the beds, rather than derived. Phricodoceras lamellosum figured by Buckman (1920, pl. 149A–C) was probably also contemporary with the bed.

The other quarry by Kilmersdon Road (R53) is said to have shown a section similar to Radstock Grove.

The Hettangian limestones thin southwards and are last seen at Tyning Farm Quarry (R62) where the section is:

The thin limestone at the top of the Hettangian yields Cardinia sp., Dimyopsis intusstriata, Liostrea hisingeri, Modiolus cf. laevis, Plagiostoma sp., Pteromya sp.and indeterminate gastropods. A small exposure a short way to the north-west (R60) showed a similar section.

The plateau of ironshot limestone extends some way to the south, but no sections have been recorded. Haydon Hill is capped by an outlier of Inferior Oolite and the Lower Lias Clay is estimated at 18 m, giving a total thickness for the Lower Lias of about 20 m.

Writhlington and Huish

Bird's Quarry (R50) now lies in the gardens of the street known as Vale View. Tutcher and Trueman (1925, p. 610) recorded the following section:

The Hettangian was stated to belong wholly to the planorbis Zone although the evidence for this was not recorded. The section resembles Tyning Colliery Quarry (R37) about 600 m to the north.

Huish Colliery Quarry (R55) was so called by Tutcher and Trueman (1925, p.610) who listed other names by which it had been known. Tawney (1875b, p. 186) described it as 'Section at Huish' and his paper of 1878b (p. 176) makes it clear that this quarry was intended as lying 'on the side of the tram incline'. The tram incline can still be traced and the quarry face extends to the south of it. A photograph was published by Richardson (1910, pl. 10).

The section was measured in 1971 and is shown diagramatically in (Figure 17), supplemented by details recorded by Tawney and by Tutcher and Trueman.

Tawney noted that Psiloceras sp. ('Am. planorbis') was to be found within an inch or two of the top of the Hettangian limestones and this was confirmed by the recent examination. All the Hettangian above the planorbis Subzone is missing in contrast to exposures north of Radstock such as Rockhill Quarry and Tyning Colliery Quarry. Tawney recorded the usual ammonites from the Armatum Bed and Jamesoni Limestone, and the following bivalves Astarte sp., Glyphaea maccullochi, Parainoceramus ventricosus, Chlamys subulatus and Pholadomya ambigua. He noted that the 'top beds of the Middle Lias' (i.e. the Valdani Limestone) were full of a small Pleuromya, and an Astarte possibly allied to A. subtetragona, and also recorded Cardinia concinna and Myacites sp.Greenwell and McMurtrie (1864, p. 25) recorded a total of 84 ft (25.6 m) of Lias and 'Rhaetic', perhaps including the Tea Green Marl, in Huish Colliery shaft (R56). The thickness of the Lower Lias is likely to be about 15 m.

A section 'behind Branch Huish Farm' was described by Ussher (in Woodward, 1876, p. 98), who mistook the Jamesoni Limestone for Inferior Oolite, and by Tawney (1878b) who corrected Ussher's mistake. Their sections refer to the old quarry (R57) north-east of Huish House, Branch Huish Farm being next to the former Huish Colliery and at too high an elevation for any quarry there to expose the base of the Lower Lias. The Huish House Quarry section is similar to that at Huish Colliery Quarry except that the Hettangian is still thinner, ranging from 2 ft 4 in (0.70 m) at the south-western end to 3 ft (0.90 m) at the north-eastern end of the quarry face, which is about 70 m long. The uppermost limestone bed of the Hettangian at the south-western end is 0.12 to 0.20 m thick and contains numerous U-shaped burrows, like Diplocraterion, but some of which have a horizontal portion in the lowest part of the bed. The lowest part of the Armatum Bed consists of two thin beds, variable in thickness but in all about 0.12 m; the lower carries phosphatic nodules, and the upper has a planed, ferruginous top.

Kilmersdon, Babington and Hemington

No sections are recorded south of Huish House but records of 'iron-shot limestone' on the six-inch map show that the Jamesoni Limestone outcrop continues southwards, overlain by a diminishing thickness of Lower Lias Clay. Around Lowerfield Farm [ST 617 513] and south of Cherry Garden Farm [ST 692 507] the Jamesoni Limestone forms a plateau-like feature. Hemington Borehole (R65), 2750 m ESE of Huish House, proved 25 ft (7.62 m) of Lower Lias Clay over 411 (1.22 m) of Jamesoni Limestone, resting on White Lias.

Upper Vobster

Along Charity House Lane [ST 701 505], north-east of Newbury, the total thickness of the Lower Lias appears to be about 6 m. There are no exposures. The basal part was exposed as overburden at the eastern end of the large quarry in Carboniferous Limestone at Upper Vobster, a few yards west of the road to Kilmersdon [ST 707 497]. The section showed about 4 ft (1.22 m) of the Jamesoni Limestone (including the Armatum Bed), resting on a planed surface of Carboniferous Limestone. A list of fossils was given by Donovan (1958, p. 136). The former existence of beds of mid-Sinemurian age is indicated by a phosphatic nodule with Promicroceras sp.(turned or obtusum Zone). As in the area farther north, the Zone of Echioceras raricostatum, or part of it, is also represented by derived material. Specimens of Paltechioceras were abundant at Upper Vobster, and occurred in a 6 to 12 in (15 to 30 cm) basal layer according to Tutcher and Trueman (1925, p. 613). Their preservation is like that of specimens from the quarries south of Radstock such as Radstock Grove (p. 45), with dark grey phosphatic body chambers and gas chambers filled with crystalline calcite. The Subzones of Echioceras raricostatoides and Paltechioceras aplanatum are proved. Large specimens of Apoderoceras and of Gleviceras belong to the lowest part of the Uptonia jamesoni Zone. This is one of the few localities in Britain where Liparoceras (Vicininodiceras), a rare early liparoceratid (Spath, 1938, p. 84), has been found. Tutcher and Trueman (1925, p. 614) recorded Platypleuroceras and Polymorphites indicating the middle part of the jamesoni Zone. The absence of other ammonite genera suggest that higher horizons are absent, or at least have not been exposed.

The fossil list also includes the coral Oppelismilia, bivalves and gastropods similar to those found in the Jamesoni Limestone farther north. Nine genera of brachiopods are recorded and D. V. Ager commented (in Donovan, 1958, p. 137): 'the variety of the fauna is interesting, and is only comparable in the British Lias to that of the Spinatum Zone in the south-western counties. There are in fact many resemblances between the Vobster fauna and that of my Bridport and Ilminster sub-provinces (Ager, 1956a).'

The basal Jamesoni Limestone is mapped as wedging out about 750 m E of the exposure at Upper Vobster. The Lower Lias Clay, now very thin, continues about 1 km farther east and is overlapped by the Upper Inferior Oolite south of Holwell Farm at [ST 715 496].

An extensive plateau south-west of Radstock is underlain by the Jamesoni Limestone and associated beds, which generally have a thickness of about 1.83 m altogether. It is often turned up in excavations. There were a number of quarries of which only a few have been adequately recorded. Wellsway Colliery shaft (R48) penetrated the feather edge of the Lias and Norton Hill Colliery (R58) penetrated about 47 ft (14 m) of 'Lias and Penarth Beds' of which about half was probably Westbury and Cotham Beds and White Lias.

Wellsway and Norton Hill

Wellsway Quarry (R49), long disused, still (1971) shows some exposure of the upper part of the section, in a vertical face difficult of access. The following summary section is modified from that by Tutcher and Trueman (1925, p. 606):

Tawney (1875b, p. 170) recorded a section which cannot be correlated in detail with that of Tutcher and Trueman. The Spiriferina Bed has been inserted on the basis of his account, and he also recorded the White Lias, which must lie just below the base of Tutcher and Trueman's section. A comparatively full development of the Hettangian is present, the Zone of Schlotheimia angulata, as usual in the Radstock area, being missing. The total thickness of Hettangian is at least 7 m as recorded by Tutcher and Trueman, and probably about 7.9 m to judge from Tawney's record. It is of interest to note the partial return to the normal facies of nodular limestones, similar to Division C of the Avon valley, in this relatively thick development of the Hettangian.

Chapel Quarry (R47), about 1100 m W of Wellsway Quarry, was noteworthy for the contrast with Wellsway in the thickness of the Hettangian, only about 2 m, all of which is planorbis Zone, including Caloceras Beds' (i.e. Subzone of Caloceras johnstoni) at the top. Between the Hettangian and the base of the Armatum Bed Tutcher and Trueman recorded 2 to 6 in (5 to 15 cm) of 'Disturbed clay, with Obtusum Nodules and derived Spiriferina and Gryphaea'. The quarry is open but shows no exposure (1976). A disturbed exposure along the east side of the Foss Way north-east of the quarry [ST 6771 5454] shows the local upper part of the Hettangian, with Psiloceras sp., succeeded by about 0.25 m of limestones with derived arietitid ammonites, which may represent the Sinemurian.

A quarry south-east of Westfield Gardens (R51) was 20 ft (6 m) deep showing 16.75 ft (5.1 m) of Lower Lias resting on White Lias. It is now filled. Allowing about 6 ft for the Armatum Bed to Valdani Limestone inclusive, the Hettangian is likely to have been about 10 ft (3 m) thick. Westfield Quarry (R59) showed the same thickness of Hettangian, recorded by Tutcher and Trueman (1925, p. 611) as planorbis Zone. Part of the Hettangian was still exposed in 1971. A hitherto unrecorded old quarry (R61) lay about 640 m S of Westfield Quarry, and showed 12.75 ft (3.89 m) of limestones and clays of probable Hettangian date, between White Lias and Sinemurian. It has been filled and the site levelled.

At Norton Hall Quarry (R63) and at R64 the Armatum Bed was separated from the White Lias only by 2 to 3 M (5 to 8 cm) of clay with derived Sinemurian ammonites and Spiriferina.

Broadfield Down and Winford

Barrow Gurney–Winford area

The succession in the Blue Lias of the plateau area east of the Yeo River at Barrow Gurney is similar to that seen on Bedminster Down and Deep Coombe on Colliters (Colliers) Brook, with White Lias, normally less than 0.5 m thick, at the base. The total thickness of these basal limestones is about 3 m in old workings at Crossgrove Wood [ST 5273 6872]. In some shallow excavations for a pipe trench at Wild Country Lane [ST 5339 6918] thin layers of crinoidal and fragmental shelly or granular limestone seen by G.A. Kellaway (c. 1935) show that this locality, like Crossgrove Wood, lies near the limit of the 'sub-littoral' zone of Broadfield Down. West of the River Yen at Barrow Court [ST 514 684] the wide plateau surface is formed of hard shelly crinoidal and fragmental limestone, resting on Cotham Beds. No evidence for the presence of White Lias has been found here.

East of Yeo River valley between Lower Barrow Mill [ST 523 686] and the Compensation Reservoir near the Bristol–Bridgwater Road [ST 533 678] the succession above the basal Lower Lias limestone has been determined by mapping. Caloceras has been found at the top of the basal Blue Lias limestone in the Compensation Reservoir. Probably occupying a similar stratigraphical position are shelly fragmental limestones with Caloceras of which 3 ft (0.9 m) was observed in an old quarry [ST 5323 6737] 50 ft (15 m) E of Hobbs Lane and 518 mm SSE of the Ich Dien Inn at Barrow Gurney. Between this quarry and the valley at Cold Bath Spring [ST 5283 6767] the Blue Lias consists of shelly or granular limestone (basal Blue Lias) underlain by a thin representative of the White Lias resting on Cotham Beds.

East of the headwaters of the River Yeo, a band of argillaceous limestone and clay corresponding roughly with the Saltford Shales crops out on the plateau surface between Crossgrove Wood and Whistlewind Cottage [ST 5377 6848]. This can be followed in a south-easterly direction towards the Compensation Reservoir but is lost sight of beneath Reservoir No. 2. About 3 m of alternating thin-bedded, laminated limestone and clay in an old quarry on the west side of the main Bristol–Bridgwater road [ST 5338 6729] yielded Psiloceras at the base and are of planorbis Zone age.

A well and borehole at [ST 5387 6614] provided a complete section with 47 ft (14.3 m) of the Blue Lias (Trueman, 1936). The middle part of the section is amplified by an old quarry [ST 540 656] north of Winford village, noted by Reynolds (1912, p. 68) but previously undescribed. The two are here placed side by side:

The 'upper part' of the well section was said to have conglomeratic layers with Carboniferous Limestone pebbles up to 0.5 in (12 mm). No such layers were seen in the quarry.

The lowest 2.4 m in the borehole presumably represent the Psiloceras planorbis Zone in normal facies. The 'clay' of the borehole record may represent the Saltford Shales, being overlain in the quarry by nodular limestones with Liostrea hisingeri, characteristic of the lower part of Division C in the Avon valley. The Saltford Shales, however, may have been largely replaced by the massive limestones seen in the quarry and recorded to a thickness of 5.8 m in the borehole. These are not found in sections to the north or the east, and represent a transitional facies between the normal Blue Lias and the littoral facies to the west (p. 19).

South of the Hartcliff Rocks Fault Lower Lias clays are present at the surface. They appear to be continuous with the blue clays of semicostatum Zone or younger age but in view of the absence of the bucklandi Zone they may also include the equivalent of the Saltford Shales. At Hartcliff Rocks, about 1670 m NW of Winford Church about [ST 532 532], Richardson (1901) noted 'the angulatus zone is exposed in contact with the Carboniferous Limestone' but gave no further details.

An old quarry at the site of the former cattle market in Winford village [ST 5383 6526] showed about 10 ft (3 m) of Lower Lias limestone in which the upper part of the coarse-grained crinoidal and shelly limestone is interdigitated with argillaceous limestone and clay of more normal type. Zeilleria sarthacensis was recovered from loose material in this quarry, suggesting that the basal beds of the angulata Zone are present. About 185 to 275 m N of the old quarry, recent excavations exposed the basal part of the Lias consisting of up to 10 ft (3 m) of coarse, shelly, current-bedded limestone (Brockley Down Limestone) resting on 13 in (0.33 in) of White Lias limestone underlain by the Cotham Marble (Rhaetic). The total thickness of Brockley Down Limestone at Winford and Harpers Batch appears to be 3 to 4.5 m. The change to a more normal argillaceous and fine-grained facies takes place with great 'rapidity at Winford, the transition being completed within a distance of about 280 m.

Felton, Lulsgate and Downside

East of Stanshall's Lane, Felton, the Brockley Down Limestone consists largely of crystalline shelly grey limestone. This rests on Rhaetic clay at Winford Orthopaedic Hospital [ST 5370 6565] and on Carboniferous Limestone at Felton Vicarage [ST 5195 5577]. South of Upper Town, Felton, the lower ground following the dry valley between Lulsgate Bottom and Harpers Batch is composed mainly of Harptree Beds. Two small patches of unaltered Lower Lias limestone including a roadside section at [ST 525 654] showed Brockley Down Limestone with Pteromya and pectinids. South of the dry valley at Felton a much larger area of unaltered shelly limestone and freestone is seen on the northern slopes of Felton Hill. Here about 20 ft (6.1 m) of freestone, which has a NE dip of about 10°, was formerly quarried north of the Congregational Chapel [ST 5207 6530]. Since the Lower Lias at Upper Town has a southerly dip it is apparent that the Harptree Beds forming the floor of the broad valley at Felton are likely to be mainly of Lower Liassic age.

There are no good sections through the Harptree Beds around Felton Hill. They are exposed in the floor of Hay Lane [ST 5235 6497]. Blocks of siliceous rock up to about 1 m in greatest dimension are to be found scattered over the rough pasture land of Felton Hill, and were noted elsewhere during the mapping. Their significance has been discussed on p. 19.

Richardson (1901) recorded a section at Felton Vicarage [ST 5197 6579]. At the junction of the lane to the Vicarage and Stanshall's Lane [ST 5217 6578] a small quarry still exhibits about 3 m of horizontally bedded shelly freestone. Farther west the line of old quarries extending from Felton Vicarage towards Lulsgate Quarry [ST 517 659] formerly showed south-westerly dipping massive shelly limestone and breccia resting on conglomerate and cut by strong ESE–WNW-trending joints. The basal conglomerate was formerly well exposed in Lulsgate Quarry where it appears to have filled a shallow depression in the surface of the Black Rock Limestone of the Carboniferous Limestone. At least 12 ft (3.7 m) of conglomerate, overlain locally by fine-grained breccia and shelly limestone, was seen resting on Black Rock Limestone in the quarry. Most of the clasts consisted of subangular or partly rounded masses of Black Rock Limestone up to about 1 ft (0.3 m) in diameter, embedded in greenish grey fine-grained limestone containing small chips and fragments of Carboniferous Limestone. At [ST 5168 6589] 'coarse greenish grey conglomerate or boulder bed', 15 ft (4.6 m) was noted. An adjacent exposure [ST 5165 6590] yielded Placunopsis sp.and Pteria cf. alfredi (Terquem) (Donovan, 1958, p. 133). Quarrying in 1957 exposed a strong fissure cutting through the conglomerate down into Black Rock Limestone. The fissure was of post-Liassic tectonic origin and contained reddish material, probably of Triassic age, yellow silty rock, and blue clay. There were numerous septarian nodules, some of which were fossiliferous and yielded the ammonites Caenisites plotti, and Arnioceras cf. bodleyi besides Gryphaea sp.and Pseudolimea pectinoides (Donovan, 1958, p. 133). This indicates that normal clays of the Caenisites tureen Zone were formerly present, so that the conglomeratic and shelly basal facies must be of earlier date, probably equivalent to the Blue Lias.

The massive shelly limestone known as Brockley Down Limestone (Woodward, 1893, p.293) was chiefly quarried west of the hamlet of Downside. Downside Quarry [ST 4935 6620] in 1976 showed about 5.3 m of coarse, detrital limestones with lines of shell fragments. The rock is very massive, with many solution cavities. The bivalves Ceratomya sp., Gerrillella cf. hagenowi, Liostrea sp., Modiolus laevis and Parallelodon hettangiensis were recorded (Donovan, 1958, p. 133). Other quarries, long obscured, existed at [ST 492 662] and [ST 493 660]. It was probably at Downside Quarry, then already disused, that Richardson (1901, p. 72) recorded a section showing 'freestone', with shelly bands, totalling 19 ft, (5.8 m). He recorded Cardinia sp.as well as the genera already noted. At one of the quarries, perhaps then in work, Moore (1867, p.504) noted conglomerates of unknown thickness, with Dimyopsis intusstriata, at the base, overlain by 7 ft 8 in (2.38 m) of 'ragstone and rubbly beds' and these by 10 ft 4 in (3.15 m) of 'stone', i.e. the beds worked as freestone. He recorded the coral Thecosmilia.

Old Hill–Corner Post: Goblin Combe

At Oldhill Farm, about 1200 m W of Winford Church, Richardson (1901, p. 72) noted a shallow quarry approx. [ST 531 649], where the Lias contained well preserved 'Zaphrentis phillipsi', Spirifer striata' and crinoid ossicles derived from the Carboniferous Limestone.

The Chew Valley

Butcombe, Hinton Blewett, Ston Easton and Chewton Mendip

Local details of exposures in this area, included in the Wells (280) One-Inch Sheet, are described in the memoir to that sheet (Green and Welch, 1965, pp. 95-107) and are not repeated here.

Chapter 3 Middle and Upper Lias

Middle Lias

Faunal evidence for the presence of the Middle Lias has only been found at Dundry Hill and at Bitton Hill within the area of the Bristol District Special Sheet. The zonal succession (Dean, Donovan and Howarth, 1961) cannot be recognised because of poor exposure and lack of ammonites, but it is unlikely to be complete. Many historical references to 'Middle Lias' of the Radstock area, refer to the Lower Liassic Jamesoni Limestone, the term Middle Lias being then used in its alternative sense to include the whole Pliensbachian Stage.

The lithostratigraphical sequence is:

• Marlstone Rock-bed (in the Index to the Special Sheet this is included in the Junction Bed).
• Dyrham Silts.

The Marlstone was 1.25 m thick in the Dundry Borehole and the thickest development is 5.5 ft, (1.68 m) at East Dundry. Elsewhere it is thinner. It is a coarse, shelly ironshot limestone, usually recorded as oolitic (e.g. Buckman and Wilson, 1896, p. 706). Petrographical studies indicate that both true ooids as well as pseudo-ooids may occur, as well as rolled intraclasts. The 'oolitic' character is particularly well developed in the upper part. Ammonites include Pleuroceras salebrosum and P. solare, Amaltheus margaritatus and A. (Pseudoamaltheus) engelhardti. These suggest the lower half of the Pleuroceras spinatum Zone. The Marlstone is only known at the outliers of Dundry Hill and Bitton Hill. It is probably absent from the main outcrop.

The name Dyrham Silt[s] was first published in the Index to Six-Inch Sheet ST 67 SE (1960) for beds overlying the Lower Lias [Clay], and succeeded by the Junction Bed. The locality of Dyrham [ST 741 758] lies just outside the eastern boundary of the Special Sheet, and from there northwards the Silts are shown on the Bath (265) One-Inch Sheet as well-developed and attaining 30 m in thickness. In the Dundry (Elton Farm) Borehole the Dyrham Silts are 9 m thick and the upper half, at least, belongs to the Amaltheus margaritatus Zone. Elsewhere in the Special Sheet they are mapped only from Upton Cheney to Freezing Hill and Tog Hill. They are absent south of the Avon valley. The outcrop of the Silts is usually covered by hillwash, with hardly any exposure, but in one or two places, such as Dodington Ash [ST 750 782] (outside the boundary of the Bristol District Sheet) (Fry, 1951) temporary exposures within the area mapped as Dyrham Silts have yielded ammonites of the Prodactylioceras davoei Zone and it may well be that the Silts in the Freezing Hill and Tog Hill area are wholly of Lower Liassic age.

Middle Lias—local details

Upton Cheney and Bath

The Dyrham Silts are mapped at Freezing Hill [ST 719 717] and Upton Cheney [ST 690 700], where there is no palaeontological evidence as to their age. South of Upton Cheney the expected position of the Middle Lias, if present, is wholly obscured by landslips. There is no evidence, here or in the Bath area, for the presence of Middle Lias. If the sections at Kelston and Langridge (p. 52) can be trusted (i.e. in the absence of superficial movements) the Upper Lias at these places rests directly on Lower Lias Clay. It is considered probable, therefore that the Middle Lias dies out eastwards between the thin remnant at Bitton Hill (p. 50) and the main outcrop at Upton Cheney about 1.6 km to the east.

Radstock and Mells

In the Radstock area the Middle Lias is absent, the Inferior Oolite resting directly on Lower Lias. Moore (1867, p. 481) recorded Middle Lias with Am. spinatus (i.e. Pleuroceras), overlying Coal Measures, in the coal shaft [ST 712 501] at Holwell Farm, 1.8 km WNW of Melts church, but as the record is unconfirmed it is likely that he was referring to the Jamesoni Limestone of the Lower Lias, which was then usually included in the Middle Lias, and that the ammonite was misidentified.

Bitton Hill

The Junction Bed is mapped around the small outlier of Bitton Hill. A special excavation in the floor of Bitton Hill Sandpit [ST 680 702] by T. R. Fry in 1950-52 (Fry, 1970) proved that the lowest part of the Junction bed is of Middle Liassic age. The section will be found on p. 52. The Marlstone was a brown shelly limestone, 3 ft 1 in (0.94 m) thick with Amaltheus (Pseudoamaltheus) engelhardti. A loose block of Marlstone 200 m NW of the excavation yielded Amaltheus margaritatus. These ammonite species range through the late Amaltheus margaritatus Zone and early Pleuroceras spinatum Zone. The Marlstone rested on clays of unknown age.

Dundry Hill

The only section through Middle Lias was provided by the Dundry (Elton Farm) Borehole, the rocks at outcrop being covered by slipped strata. The section (Cave in Ivimey-Cook, 1978) may be summarised as follows:

The total thickness of Middle Lias is 10.32 m. The upper half only of the Dyrham Silts yielded ammonites (Amaltheus spp.) indicating the Amaltheus margaritatus Zone. The base of this Zone, and of the Middle Lias, was taken arbitrarily at the thin limestone with conglomeratic base which is likely to mark a hiatus in deposition. The Marlstone Rock-bed yielded Amaltheus cf. margaritatus from a level just below the top, and as this species does not occur in the upper subzone of the Pleuroceras spinatum Zone it is probable that only the lower half of the zone is represented.

Dr J. R. Hawkes reports that higher limestones here are coarser than the micrites in the Upper Lias. Specimens (e.g. (E44530)^‡6 ) contain coarse shell debris and rolled intraclasts, some with micritic overgrowths and others resembling ooids, though no true ooids were seen. The micritic matrix contains calcspar. In another specimen (E44531) limonitised ooids and biocalcarenite are set with micrite in an oobiomicrite.

The Marlstone was again proved, forming the basal component of the Junction Bed, in a well about 275 m SSW of Dundry church (Fry in Donovan, 1958, p. 132). The full section is given on p.53. The Marlstone was dark brown ironshot limestone with a few pebbles of calcareous sandstone, about 0.30 m thick. The sandstone pebbles presumably indicate the erosion of beds which formerly existed, as the Marlstone rested on barren clays of unknown date. The Marlstone yielded the ammonite Pleuroceras salebrosum which denotes the Pleuroceras spinatum Zone. Buckman and Wilson (1896, p. 678) believed the Marlstone to be absent from their section north of Castle Farm, about 1000 yd (915 m) NW of the well section, but in view of the prevalence of superficial movements around Dundry Hill this conclusion should be treated with caution.

The Marlstone was recorded from slipped blocks by Buckman and Wilson (1896) in three areas of the eastern part of Dundry Hill: at East Dundry village, on the northern slopes south of Hill Farm, and at Maes Knoll. At East Dundry the Marlstone cropped out in the rickyard of Spring Farm [ST 5737 6621] and was a strongly iron-shot limestone 5.5 ft (1.68 m) thick. The bottom 2 ft (0.61 m) was fossiliferous with belemnites, Pseudopecten equivalvis, oysters and terebratulids. The bed was also seen on the opposite side of the little valley in which East Dundry lies, about 192 m NNE of Watercress Farm [ST 5695 6619], where it yielded Amaltheus margaritatus and Tetrarhynchia tetrahedra.

South-south-east of Hill Farm [ST 596 668] the most complete section was:

At Maes Knoll a special excavation [ST 5984 6610] was made by Buckman and Wilson (1896, p.686) south of Maes Knoll Tump.

The section was similar to the last:

To the west of here Marlstone blocks were seen in the spinney south-west of the Tump, but to the east it was not seen again, and in another special excavation [ST 6029 6620] below the eastern rampart of Maes Knoll Camp, about 460 m E of the previous one, it was believed to be absent, the Upper Liassic part of the Junction Bed resting on greenish clay of unknown date.

Upper Lias

The zonal scheme for the Toarcian Stage is as follows:

Within the area covered by the Bristol District Sheet the greater part of the Toarcian Stage is either absent or represented by thin, condensed deposits of 'cephalopod bed' facies. Only the highest zone is of any substantial thickness.

The lithological sequence is:

The Junction Bed was so named because on the Dorset coast, and in some parts of the Bristol district, it consists of a Middle Liassic part ('Marlstone') succeeded by an Upper Liassic part, forming a more or less continuous sequence made up chiefly of ironshot limestones. Ammonites, belemnites, bivalves and brachiopods are often abundant fossils, and the former show that the bed often spans five or six zones. The Junction Bed has only been mapped in parts of the area and seems to be impersistent. It is difficult to be certain of this because the slopes on which it should outcrop are in many places covered by hillwash or by landslips.

The Junction Bed is highly variable from exposure to exposure, but consists, where fully developed, of three main parts. The lowest is the 'Marlstone' which has been described under Middle Lias. The middle section consists of fine-grained, sometimes argillaceous, pinkish, grey or purplish limestone, seldom ironshot. These beds carry ammonites of Harpoceras falciferum and Hildoceras bifrons zones. It is uncertain whether any of these ammonites are autochthonous. The same mixture of genera of different ages is found, as at New Barn Farm (p. 53) beds 11 and 12, as obviously derived fossils in higher beds. Since it is difficult to understand how existing condensed limestones could be broken up again to yield the derived fossils, it is perhaps more probable that clays with fossiliferous nodules have been destroyed. Clays with Dactylioceras, probably belonging to the Hildoceras bifrons Zone, have been reported at one locality (p. 54).

The uppermost part of the Junction Bed consists of iron-shot limestones, often conglomeratic at the base. The lower part may contain derived fossils from the Harpoceras falciferum and Hildoceras bifrons zones, and the remaining ammonites denote the Haugia variabilis and Grammoceras thouarsense zones. As with the underlying beds, however, it does not seem to be a case of straightforward condensation of the sequence. At Bitton Hill (p. 52) as at New Barn Farm (p. 53) fossils denoting later subzones occur in beds below fossils belonging to earlier ones; at Bitton Hill bed 4, containing fossils as late as the fallaciosum Subzone, is succeeded by bed 3 with fossils of the Haugia variabilis Zone as well as of the next zone. A continuous supply of derived fossils from the destruction of preexisting beds is demanded. The following sequence of events is tentatively suggested:

The thickest Junction Bed (about 1.5 m) occurs at the eastern end of Dundry Hill (Maes Knoll) and at Bitton. Farther east it is reduced to a single bed about 0.23 m thick at Kelston and the northern outskirts of Bath, thickening south of that city to about 0.9 m. About 5.2 km E of the Bristol District Sheet, at Limpley Stoke, a detailed section was recorded by Gorham (1930), but there are no sections on the District Sheet south of the River Avon except at Timsbury Sleight (p. 54). At both these places the Junction Bed was about 1.2 m thick, and the top seems to be uniformly of Pseudogrammoceras fallaciosum Subzone age.

The Midford Sands were named from Midford, at the junction of the valleys of the Cam Brook and Wellow Brook [ST 760 607] 3 km E of the boundary of the Bristol District Special Sheet, by Williams Smith's nephew John Phillips (1871, p. 118). Only the upper part of the formation is seen at Midford and James Buckman's statement (1897, p. 737) that the Sands are 100 ft (30 m) thick is misleading, as the lower part is in fact below the level of the valley floor. The Sands consist of uniform, fine-grained, even silty, sands with occasional layers of doggers, 'sand burrs' or beds of calcareous sandstone. No sedimentary structures have been recorded.

Fossils are rare. The Midford Sands die out southwards at the valley of the Cam Brook. Around the Avon valley they are probably about 20 m thick.

The lower part of the Midford Sands at Lyncombe cutting [ST 747 634], south of Bath (Buckman, 1889, p. 448) yielded Pseudogrammoceras, also found in the underlying Junction Bed. The base of the Sands presumably lies within the fallaciosum Subzone. In the higher parts near Upton Cheney the ammonites Phlyseogrammoceras, Dumortieria and Pleydellia have been found, and suggest that there is a fairly complete representation of the Zone of Dumortieria levesquei, including the highest Subzone of Pleydellia aalensis.

In the west of the area the Midford Sands pass laterally into finer-grained beds. At Maes Knoll, the easternmost extremity of Dundry Hill, sandy beds succeed the Junction Bed, but at the Dundry (Elton Farm) Borehole the Midford Sands are replaced by micaceous mudstones, with silty beds at the base, altogether 23.5 m thick. At the top of the sequence is the Aalensis Bed, a thin (0.17 m) silty limestone with Pleydellia and other fossils. These muddy and silty beds at Dundry Hill are shown by ammonites to be a close age equivalent of the Midford Sands in the eastern part of the sheet.

Upper Lias—local details

Upton Cheney and North Stoke

The Midford Sands are mapped around Tog Hill [ST 732 730] and Freezing Hill [ST 719 717] where they appear to be about 18 m thick. The village of Upton Cheney is situated on Midford Sands, which are exposed in lane sections. Just outside the village [ST 6937 6986] in the lane to North Stoke, silts with bands of siltstone are exposed in the upper part of the formation. The stone bands may be responsible for the platform on which the village stands. They carry abundant bivalves and ammonites of the genera Dumortieria, Grammoceras and Pleydellia. These rocks may be the origin of the record by Tutcher (1903, p. 5) of a 'cephalopod bed' in the road cuttings but the beds are not cephalopod beds in the sense in which the term is normally used for a condensed facies. Sandy limestone with Dumortieria was also noted in surface rubble near Mount Pleasant [ST 701 701]. These ammonites show that the upper part, at least, of the Midford Sands falls in the Grammoceras thouarsense and Dumortieria levesquei zones.

Buckman (in Reynolds and Vaughan, 1902, p. 736) recorded a section at North Stoke about [ST 702 691]; exact location not known] in which about 3.6 m of Midford Sands with Dumortieria, Hammatoceras, Lytoceras and Phlyseogrammoceras underlay Inferior Oolite. The whole area is mapped as foundered strata and so the relationship is suspect, but if it can be accepted it shows that the uppermost part of the Sands here belongs to the levesquei Zone.

About a mile south of the last section T. R. Fry in 1946 recorded a section through the Junction Bed in foundered strata 275 m N of the Crown Inn, Kelston [ST 700 675]. The bed was here a fine-grained, grey marly limestone only 9 in (0.23 m) thick. Within the bed Hildoceras bifrons, Eopecten velata and the gastropod Ataphrus sp.were collected while on the upper surface occurred Grammoceras thouarsense and Pseudogibbirhynchia moorei (Donovan, 1948, p. 338).

The rest of the outcrop along the Avon valley north of Bath is hidden by slipped strata. East of the boundary of the Bristol District Sheet, exposures in disturbed strata were recorded south of Langridge Church about [ST 740 692] in 1925 by T. R. Fry (Donovan, 1948, p. 339). The Junction Bed was like that at Kelston.

Newton St Loe to Priston and Dunkerton

South of Newton St Loe the thickness of the Midford Sands was estimated at about 20 m decreasing southwards to about 14 m in the Englishcombe to Priston area. The Junction Bed is impersistent and has not been differentiated on the one-inch map except at Pendown Hill [ST 684 616]. Loose blocks of Junction Bed were noted north-west of Dunkerton church at [ST 7062 5940]. The Midford Sands here die out southwards on the south side of the Cam Brook. No Upper Lias has been detected south of this line.

Bitton

The only good exposure of Midford Sands in the Bristol District is at Bitton Hill Sandpit [ST 679 702] which in 1975 showed about 12 m of fine-grained, yellow, micaceous sand with a few hard bands. Bedding is not readily apparent but where it is traceable in a few places it appears horizontal. During the survey dip and fault structure was recorded with north-east to east-north-easterly dips from 25° to 30°. The sands are generally barren of fossils, but the ammonite Hammatoceras cappucinum was found about half-way up the section (Fry, 1970, p. 633). The floor of the Sandpit is formed by the Junction Bed in which a section was excavated by T. R. Fry in 1950-52. The section with some of the more important ammonites is given below; a full list of ammonites will be found in Fry's paper.

Bed 6 belongs to the Middle Lias and is discussed on p. 50. The remaining beds are of Toarcian age, a great part of this stage being represented by 1.6 m of beds. Bed 5 may belong to the Harpoceras falciferum Zone. The lower two-thirds of bed 4 contain fossils from the Harpoceras falciferum and Hildoceras bifrons zones but may belong to the Haugia variabilis Zone or even the base of the Grammoceras thouarsense Zone, since it contains a Haugia identified as H. eseri, a species characteristic of the striatulum Subzone. The upper part of bed 4, and bed 3 contain a mixture of fossils from the Haugia variabilis Zone and both subzones of the Grammoceras thouarsense Zone. Bed 2 contains ammonites of the thouarsense Zone including the index fossils of both its subzones.

Dundry Hill

The Dundry (Elton Farm) Borehole (Ivimey-Cook, 1978) provided an apparently complete section through the Upper Lias, although it must be borne in mind that the overlying, much-disturbed Inferior Oolite is part of a camber and the uppermost part of the Upper Lias could be missing. The section may be summarised as follows:

The total thickness of Upper Lias is 24.05 m. The derived fragments in the cephalopod bed indicate the former presence of Lower Toarcian rocks of the Harpoceras falciferum or Hildoceras bifrons zones, presumably in clay facies. The other fossils of the cephalopod bed indicate the Grammoceras thouarsense Zone and the rest of the sequence belongs to the Dumortieria levesquei Zone, the three upper subzones being present.

South-east of the borehole the Aalensis Bed was exposed during the 1939-45 war in a bomb crater 430 m W of Rattledown Farm [ST 5703 6538].

The only other complete section through the Upper Lias of Dundry was provided by a well section at [ST 556 665] about 275 m SW of Dundry church recorded by T. R. Fry in 1921 and published by Donovan (1958, p. 132):

More details of the Junction Bed were provided by surface occurrences collected and recorded many years ago by Buckman and Wilson (1896). All their 'sections' are now known to lie in foundered strata (Figure 18) and the sequence must be regarded with caution. Near Castle Farm they recorded (1896, pp. 677-678):

Bed numbers are those of Buckman and Wilson. It is difficult with this and succeeding sections to determine the actual ages of the beds because fossils may have been derived, but 27 and 28 appear to belong to the two subzones of the Grammoceras thouarsense Zone, and are unusual locally in being separately developed.

On the north-eastern flank of Dundry Hill, above Hill Farm, the Junction Bed was recorded at two sections (A, p. 683 and B, p. 684). The only ammonite recorded was a derived Hildoceras bifrons. At Maes Knoll, at the easternmost extremity of Dundry Hill, two sections were recorded from special excavations. The first '70 yards east of the Spinney below the Tump' at [ST 5984 6610] showed 2 ft 1 in (0.63 m) of iron speckled limestones overlying 1 ft 7 in (0.48 m) of Marlstone. No ammonites were recorded.

The second (1896, p. 686) above New Barn Farm about [ST 6028 6620] showed:

The top of the section is interesting as showing a sandy clay facies at a horizon which elsewhere on Dundry Hill consists of clays. This easterly extremity may be transitional to Midford Sands facies. The Junction Bed is about 1.5 m thick, the thickest record in the area. The pinkish beds (nos. 13 (lower part), 14-16 may be of the falciferum and bifrons zones, though they are doubtless condensed and perhaps reworked. If the identification of Harpoceras strangewaysi is correct this denotes the Harpoceras exaratum Subzone, and would be the only record of this subzone in the district. Beds 9 to 12 may belong to the Haugia variabilis Zone, and beds 7 and 8 probably to the Grammoceras thouarsense Zone. There was no evidence for any beds of Middle Lias age in this section.

Marksbury, Glutton and Timsbury

Midford Sands have been mapped below the outliers of Inferior Oolite on Stantonbury and Winsbury Hills, north of Marksbury. Tutcher (1903, p. 13) noted that they are 'true sands' presumably in distinction from the clayey facies at Dundry Hill to the west.

Between Glutton and Farmborough two small outliers of Inferior Oolite at Barrow Hill [ST 642 600] and a nearby hill [ST 640 601] preserve the whole thickness of Midford Sands, here estimated at only about 6 m. There is no evidence for the presence of the Junction Bed. At Blackberry Hill [ST 638 602] Tutcher and Trueman (1925, p. 620) noticed clays with nodular limestones containing Dactylioceras. These are presumed to underlie the Midford Sands.

Timsbury Sleight, north-west of Timsbury, is also capped by Inferior Oolite and here Midford Sands are estimated at 7.5 m according to the mapping, although Richardson (1907, table iii) recorded only 5.5 ft (1.68 m) in a trial shaft. It is possible that the base was taken too low in the mapping, or that Richardson's section was affected by cambering of the Inferior Oolite. Richardson recorded the detailed section through the Junction Bed summarised below with revised names for much of the fauna:

The ammonites from the upper layer of ironshot marl indicate the thouarsense Zone whilst those from the lower layer of limestone are of bifrons Zone, commune to fibulatum Subzone age with also a remanie specimen of H. falciferum.

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Appendix Lower Lias exposures

Keynsham- Saltford area

Timsbury–Paulton–Radstock area

NOTE: All the localities in the above list are shown on the locality map (Figure 16) except the following: RI, R3, R10, R12, R45, R63, R64.

Index of fossils

In compiling this index qualifications of taxa (e.g. aff., cf., ?) used in text have been disregarded. Taxa referred to in quotations are given in their original form and authors have not been added.

• Acanthopleuroceras
• Acanthopleuroceras valdani (d'Orbigny)
• Alocolytoceras germaini (d'Orbigny)
• Aegoceras
• Aegoceras maculatum (Young & Bird)
• Aegoceras (Beaniceras) dundryi Donovan
• Aegoceras (B.) luridum (Simpson)
• Agassiceras
• Agassicera scipionianum (d'Orbigny)
• Alsatites
• A. liasicus (d'Orbigny)
• Am. angulatus
• Am. bucklandi
• Am. charmassei
• Am. conybeari
• Am. geometricus
• Am. johnstoni
• Am. obliquecostatus
• Am. planicosta
• Am. planorbis
• Am. raricostatus
• Am. rotiformis
• Am. spinatus
• Am. turneri
• Amaltheus
• Amaltheus margaritatus de Montfort
• Amaltheus (Pseudoamaltheus) engelhardti (d'Orbigny)
• Amberleya cyclostoma (Benz)
• Androgynoceras naptonense (Spath)
• A. geyeri (Spath)
• Anaptychi
• Antiquilima
• Antiquilima succincta (Schlotheim)
• Apoderoceras
• Apoderoceras aculeatum (Simpson)
• Arca
• Arietites
• Arietitesbucklandi (J. Sowerby)
• Arietitid ammonites
• Arnioceras
• Arnioceras acuticarinatum (Simpson)
• Arnioceras bodleyi (J. Buckman)
• Arnioceras ceratitoides (Quenstedt)
• Arnioceras hodderi Tutcher & Trueman
• Arnioceras notatum Tutcher & Trueman
• Arnioceras semicostatus (Young & Bird)
• Astarte
• Astarte obsoleta (Dunker)
• Astartesubtetragona Munster
• Asteroceras
• Asteroceras confusum Spath
• Asteroceras obtusum (J. Sowerby)
• Asteroceras stellare (J. Sowerby)
• Ataphrus sp.
• Atreta intusstriata (Emmrich)
• Bakevellia
• Beaniceras sp.
• Belemnites
• Belemnites acutus
• Bivalvia
• Brachiopoda
• Caenisites
• Caenisites decoratus Spath
• Caenisites plotti (Reynes)
• Caenisites turgescens S. S. Buckman
• Caenisites turneri (J. de C. Sowerby)
• Calcirhynchia
• Calcirhynchia calcaria S. S. Buckman
• Caloceras
• Caloceras bloomfieldense Donovan
• Caloceras intermedium (Portlock)
• Caloceras johnstoni (J. de C. Sowerby)
• Camptonectes
• Cardinia
• Cardinia concinna (J. Sowerby)
• Ceratomya
• Charmasseiceras
• Chlamys
• Chlamys calva (Goldfuss)
• Chlamys subulatus (Minister)
• Chlamys valoniensis (Defrance)
• Cidaris
• Cirpa fronto
• Coleia
• Coral
• Coroniceras
• Coroniceras (Arietites) bucklandi (J. Sowerby)
• Coroniceras caprotinum (d'Orbigny)
• Coroniceras hyatti Donovan
• Coroniceras lyra Hyatt
• Coroniceras reynesi (Spath)
• Coroniceras rotiforme (J. de C. Sowerby)
• Coroniceras schloenbachi (Reynes)
• Coroniceras solaroides (da Costa)
• Crinoids
• Crustaceans
• Ctenostreon
• Ctenostreon tuberculata (Terquem)
• Cuneirhynchia dalmasi (Dumortier)
• Cymbites laevigatus (J. de C. Sowerby)
• Dactylioceras
• Dactylioceras commune (J. Sowerby)
• Diademopsis tomesi (Wright)
• Dimyopsis
• D. intusstriata (Emmrich)
• C. tuberculata (Terquem)
• Cuneirhynica dalmasi (Dumoriter)
• Cymbites laevigatus (J. de C. Sowerby)
• Dactylioceras
• Dactylioceras commune (J. Sowerby)
• Diademopsis tomesi (Wright)
• Dimyopsis
• Dimyopsis intusstriata (Emmrich)
• Diplocraterion
• Dumortieria
• Dumortieria levesquei (d'Orbigny)
• Echioceras
• Echioceras crassicostatum Trueman & Williams
• Echioceras irridescens Trueman & Williams
• Echioceras modicum Trueman & Williams
• Echioceras notatum Trueman & Williams
• Echioceras quenstedti (Schaffiautl)
• Echioceras raricostatoides (Vadasz)
• Echioceras raricostatum (Zieten)
• Echioceras sparsicostatum Trueman & Williams
• Echioceras torquatum Trueman & Williams
• Echinoids, echinoid spines
• Entolium
• Eoderoceras
• Eopecten
• Eopecten velata (Goldfuss)
• Eotrapezium
• Epideroceras
• Euagassiceras
• Euagassiceras personatum (Simpson)
• Euagassiceras resupinatum (Simpson)
• Eucoroniceras
• Euechioceras nobile
• Fish scales, spines
• Furcirhynchia furcata S. S. Buckman
• Gagaticeras neglectum (Simpson)
• Gastropoda
• Gervillella hagenowi (Dunker)
• Gervillia
• Gervillia praecursor (Quenstedt)
• Gibbirhynchia
• Gibbirhynchia curviceps (Quenstedt)
• Gleviceras
• Gleviceras subguibalianum (Pia)
• Gleviceras victoris (Dumortier)
• Goniomya
• Grammatodon
• Grammoceras
• Grammoceras striatulum (J. de C. Sowerby)
• Grammoceras thouarsense (d'Orbigny)
• Gresslya
• Gryphaea
• Gryphaea arcuata Lamarck
• Gryphaea maccullochii J. de C. Sowerby
• Hammatoceras
• Hammatoceras cappucinum S. S. Buckman
• Harpoceras falciferum (J. Sowerby)
• H. strangewaysi (J. Sowerby)
• Hastites
• Hastites stonebarroensis Lang
• Haugia
• Haugia eseri (Oppel)
• Haugia variabilis (d'Orbigny)
• Haugia jugosa (J. Sowerby)
• Hildoceras
• Hildoceras bifrons (Bruguiere)
• Hildoceras sublevisoni Fucini
• Hudlestonia
• Hudlestonia serrodens (Quenstedt)
• Ichthyosaurus communis Conybeare
• Inoceramus ventricosus (J. de C. Sowerby)
• Isocrinus jurensis (Munster)
• Isognomon
• Isognomon infraliassica (Quenstedt)
• Laqueoceras
• Leptechioceras macdonnelli (Portlock)
• Lima tuberculata
• Liostrea
• Liostrea hisingeri (Nilsson)
• Liostrea sublamellosa
• Liparoceras
• Liparoceras cheltiense (Murchison)
• Liparoceras kilsbiense Spath
• Liparoceras simplicicosta (Trueman)
• Liparoceras tutcheri Spath
• Lithophaga
• Lobothyris radstockiensis (Davidson)
• Lyriomyophoria postera (Quenstedt)
• Lytoceras
• Lytoceras fimbriatum (J. Sowerby)
• Mactromya
• Mactromya arenacea (Terquem)
• Mactromya subglobosa (Tate)
• Meleagrinella
• Meleagrinella decussata (Munster)
• Metophioceras
• Microderoceras
• Microderoceras depressum S. S. Buckman
• Modiolus
• Modiolus hillanus (J. Sowerby)
• Modiolus laevis (J. Sowerby)
• Modiolus langportensis (Richardson & Tutcher)
• Modiolus minimus (J. Sowerby)
• Montlivaltia haimei Chapuis & Dewalque
• Myacites
• Myoconcha
• Modiolus
• Nautiloids
• Nautilus
• Nucula
• Nuculana
• Nuculoids
• Oistoceras
• Oligokyphus
• Ophioderma
• Ophiomorpha
• Ophiuroids
• Oppelismilia
• Oppelismilia rugosa (Duncan)
• Ostracods
• Ostrea
• Ostrea semiplicata
• Otozamites obtusus (Lindley & Hutton)
• Oxynoticeras
• Oxynoticeras williamsi
• Oxytoma
• Oxytoma inequivalvis (J. Sowerby)
• Oxytoma (Palmonxytoma) longicostata (Stutchbury)
• Oysters
• Pachymya
• Palaeoneilo
• Palaeonucula
• Paltechioceras
• Paltechioceras aplanatum (Hyatt)
• Paltechioceras elicitum (S. S. Buckman)
• Paltechioceras oosteri (Dumortier)
• Paltechioceras rothpletzi (Bose)
• Paltechioceras tardecrescens (Hauer)
• Paltechioceras variabile (Trueman & Williams)
• Paltechioceras (Orthechioceras) edmundi (Dumortier)
• Paracoroniceras
• Parainoceramus
• Parainoceramus substriatus (Goldfuss)
• Parainoceramus ventricosus (J. de C. Sowerby)
• Parallelodon
• Parallelodon hettangiensis (Terquem)
• Parinodiceras parinodus (Quenstedt)
• Passaloteuthis
• Passaloteuthis aptcicurvata (Blainville)
• Pecten
• Pecten lunularis
• Pectinids
• Phasianella turbinata
• Phlyseogrammoceras
• Pholadomya
• Pholadomya ambigua (J. Sowerby)
• Phricodoceras
• Phricodoceras lamellosum (d'Orbigny)
• Phricodoceras taylori (J. de C. Sowerby)
• Piarorhynchia
• Piarorhynchia juvenis (Quenstedt)
• Piarorhynchia radstockiensis (Davidson)
• Piarorhynchia rostellata (Quenstedt)
• Plicatula
• Pinna
• Pinna folium Young & Bird
• Pinna hartmanni Zieten
• Placunopsis
• Plagiostoma
• Plagiostoma gkankum J. Sowerby
• Plagiostoma praecursor (Quenstedt)
• Plants
• Platypleuroceras
• Platypleuroceras brevispina (J. de C. Sowerby)
• Platypleuroceras rotundum (Quenstedt)
• Plesiosaurs
• Pleuroceras
• Pleuroceras salebrosum (Hyatt)
• Pleuroceras solare (Phillips)
• Pleuromya
• Pleuromya costata (Young & Bird)
• Pleurophorus elongatus Moore
• Pleurotomaria
• Pleurotomaria expansa
• Pleurotomaria similis (J. Sowerby)
• Pleydellia
• Pleydellia aalensis (Zieten)
• Plicatula spinosa J. Sowerby
• Polymorphites
• Polymorphites lineatus (Quenstedt)
• Primarietites
• Promicroceras
• Promicroceras capricornoides (Quenstedt)
• Promicroceras planicosta (J. Sowerby)
• Pronoella
• Protocardia
• Protocardia philippiana Dunker
• Pseudodiadema
• Pseudodiadema slateri Blake
• Pseudogibbirhynchia moorei (Davidson)
• Pseudoglyphea etalloni Oppel
• Pseudogrammoceras
• Pseudogrammoceras expeditum S. S. Buckman
• Pseudogrammoceras fallaciosum (Bayle)
• Pseudogrammoceras saemanni (Dumortier)
• Pseudogrammoceras struckmanni (Denckmann)
• Pseudohastites
• Pseudolimea
• Pseudolimea pectinoides (J. Sowerby)
• Pseudomelania
• Pseudomonotis fallax (Pflucker)
• Pseudopecten
• Pseudopecten acuticosta (Lamarck)
• Pseudopecten equivalvis (J. Sowerby)
• Psiloceras
• Psiloceras (Caloceras) multicostatum Donovan non Brandes
• Psiloceras planorbis (J. de C. Sowerby)
• Psiloceras plicatulum (Quenstedt)
• Psiloceras plicatulum bristoviense Donovan
• Psiloceras sampsoni (Portlock)
• Pteria alfredi (Terquem)
• Pteromya
• Pteromya tatei (Richardson & Tutcher)
• Pteromya tatei langportensis (Richardson &Tutcher)
• Ptychomphalus
• Ptychomphalus expansus (J. Sowerby)
• Radstockiceras
• Radstockiceras complicatum S. S. Buckman
• Rhizocorallium
• Rhynchonella calcicosta Davidson sp.
• R. Moorei
• Rhynchonellids
• Rhynchonella furcillata
• Rhynchonella variabilis
• Rimirhynchia
• Rimirhynchia anglica (Rollier)
• Rimirhynchia elevata (S. S. Buckman)
• Rudirhynchia fallax (Eudes-Deslongchamps)
• Ryderia
• Saurian bones
• Schlotheimia
• S angulata (Schlotheim)
• S extranodosa (Waehner)
• Schlotheimia lymensis (Spath)
• Schlotheimia similis Spath
• Schlotheimiids
• Serpulids
• Spiriferina
• Spiriferina punctata J. Buckman
• Spiriferina tumida (von Buch)
• Spiriferina walcotti (J. Sowerby)
• Spirifer striata
• Squamirhynchia squamiplex (Quenstedt)
• Sulciferites
• Sulciferites charmassei (d'Orbigny)
• Sulciferites greenoughi (J. Sowerby)
• Terebratula
• Terebratula numismalis
• Terebratula punctata
• Terebratulids
• Terquemia
• Tetrarhynchia tetrahedra (J. Sowerby)
• Thecosmilia
• Tragophylloceras
• Tragophylloceras ibex (Quenstedt)
• Tragophylloceras loscombi (J. Sowerby)
• Tragophylloceras undulatum (Smith)
• Tropidoceras sp.
• Tutcheria
• Tutchericeras perfoliatum S. S.Buckman
• Unicardium cardioides (Phillips)
• U. arenacea (Terquem)
• Uptonia
• Uptonia jamesoni (J. de C. Sowerby)
• Vermiceras
• Vermiceras caesar (Reynes)
• Vermiceras conybeari (J. Sowerby)
• Vermiceras deffneri (Oppel)
• Vermiceras rouvillei (Reynes)
• Vermiceras scylla (Reynes)
• Vermiceras solaroides (da Costa)
• Vicininodiceras
• Waehneroceras
• Waehneroceras megastoma (Giimbel)
• Waehneroceras portlocki (Wright)
• Xipheroceras
• Zaphrentis phillipsi
• Zeilleria
• Zeilleria lycetti (Davidson)
• Zeilleria numismalis (Lamarck)
• Zeilleria perforata (Piette)
• Zeilleria sarthacensis (d'Orbigny)
• Zeilleria vicinalis (Schlotheim)
• Zeilleria waterhousii (Davidson)
• Zeilleriids
• Zugodactylites
• Zygopleura transversea (Blake)

Figures and plates

Figures

(Figure 1) Generalised map showing the distribution of the Lower and Middle Jurassic rocks of the Bristol district. The NE, SE and SW corners of the Bristol District Special Sheet are indicated.

(Figure 2) Isopachyte map of the White Lias of the area of Radstock, Keynsham and Dundry Hill.

(Figure 4)." data-name="images/P936352.jpg">(Figure 3) Isopachyte map of the Hettangian Stage of the area of Radstock, Keynsham and Dundry Hill. The area covered is the same as that of (Figure 2). Section along line A–B is shown on (Figure 4).

(Figure 4) Diagrammatic horizontal section through the rocks of the Hettangian Stage between Keynsham and Radstock. The line of section (A–B) is shown in (Figure 4)." data-name="images/P936352.jpg">(Figure 3). The local top of the Hettangian has been drawn as a horizontal datum line.

(Figure 6)." data-name="images/P936354.jpg">(Figure 5) Diagrammatic horizontal section (A–B) through the rocks of the Sinemurian and Lower Pliensbachian Stages between Dundry Hill and Vobster. The local top of the Lower Pliensbachian has been drawn as a horizontal line. Inset (C–D): Detail of Sinemurian between Meadgate and Haydon, to same scale. Lines of sections (A–B, C–D) shown on (Figure 6).

(Figure 6) Sketch-map to show the southern limit of the bucklandi Zone, and the wedging-out in turn of the angulata, liasicus and planorbis zones on the Radstock Shelf. Sections along lines (A–B, C–D) shown on (Figure 6)." data-name="images/P936354.jpg">(Figure 5).

(Figure 7) Isopachyte map of the Jamesoni Limestone (including the Armatum Bed), and the limit of the Valdani Limestone.

(Figure 8) Isopachyte map of the Lower Lias of the Bristol district. The full thickness of Lower Lias is preserved in those places where Middle Lias is present. Elsewhere the thickness shown is residual.

(Figure 9) Sketch-map showing localities of exposures in the Lower Lias in the Keynsham and Saltford area. The locality numbers refer to the Appendix, p. 59.

(Figure 10) Graphic vertical sections through the Blue Lias at Saltford Cutting (K20), the railway siding (K1b) and power house site (K1a); the last two sections were formerly exposed during excavation at Fry's factory, Somerdale, Keynsham, and based on Tutcher (1923). Scale 1:150.

(Figure 11) Graphic vertical sections through parts of the Blue Lias at Keeling's Quarry, Keynsham (K8), south of Avonside House, Willsbridge (K2), Willsbridge [ST 662 701], Mead Lane, Saltford (K22) and west of Chewton Keynsham (K11). Scale 1:150.

(Figure 12) Graphic vertical sections through the Blue Lias at Upper Bristol Road, Weston, near Bath approx. [ST 725 651] (after Lonsdale 1832, p. 243), High Street, Weston [ST 727 665], exposed in 1946 (cf. Donovan 1948, p. 336), and Bloomfield Road Quarry, Farmborough [ST 665 600] (amended from Donovan 1956, p. 191). Scale 1:150.

(Figure 13) Section in Harris's quarry, Marksbury Plain [ST 665 616] showing the base of the Blue Lias resting on a channelled surface of White Lias. The lower end of the scale touches the upper surface of White Lias. From a field sketch.

(Figure 14) Graphic vertical sections through the Lower Lias of the Dundry (Elton Farm) Borehole, from a depth of 150 m to the top of the White Lias, with three surface sections for comparison. Locations are shown on the inset map. The Colliter's Brook and Malago sections from MS sections by T. R. Fry. Vertical Scale: 1:150.

(Figure 15) Generalised vertical sections of the Lower Lias at Cothan Henleaze and Horfield.

(Figure 16) Sketch-map showing localities of exposures of Lower Lias in the Radstock area. The locality numbers refer to the Appendix, p. 60.

(Figure 17) Graphic vertical sections through the Lower Lias (lower part) in the Radstock area. Locations are shown on the inset map. Partly after Tate (1875); Tawney (1875); and Tutcher and Trueman (1925). Vertical scale: 1:75.

(Figure 18) Geological sketch-map of Dundry Hill showing Middle and Upper Lias localities

Plates

(Frontispiece 1) Blue Lias at Keeling's Quarry, Keynsham, Avon. The limestone bed behind the man's hat is the Scipionianum Bed. Below it are alternating limestones and shales of Division D of the Blue Lias. Photographed about 1920 by J. W. Tutcher (A7915).

(Frontispiece 2) Lias unconformable on Lower Carboniferous (Dinantian) at Lulsgate Quarry, Lulsgate Bottom, Avon. The Lias consists of Brockley Down Limestone with basal conglomerate. It rests on the Black Rock Limestone (Tournaisian) which dips approximately west. Photographed by J. M. Pulsford in 1965. (A10720).