A Palaeogeological Map of the Lower Palaeozoic Floor below the cover of Upper Devonian, Carboniferous and Later Formations: with inferred and speculative reconstructions of Lower Palaeozoic and Precambrian outcrops in adjacent areas

A P A L A E O G E O L O G I C A L MAP of the P A L A E O Z O I C F L O O R below the PERMIAN and M E S O Z O I C F O R M A T I O N S in E N G L A N D AND WALES with inferred and speculative reconstructions of the Palaeozoic outcrops in adjacent areas as in Permo-Triassic times L. J. WILLS

" I n the foreground you see the smooth sheets of the Old Boy's bed, here and there

rumpled where he has stirred in his sleep; and there (pointing to the Malverns), that's where he's put his toe through the bedclothes". Charles Lapworth (looking south and west from the Lickey Beacon Hill).

MEMOIR NUMBER

PETROLEUM

7

A JOINT PUBLICATION WITH EXPLORATION SOCIETY OF

GEOLOGICAL LONDON I973

THE GREAT

SOCIETY

BRITAIN

Published for The Geological Society London

by

Scottish Academic Press Ltd. 25 Perth Street, Edinburgh EH3 5DW

First published 1973

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form, or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Geological Society, Burlington House, LondonW1N 0JU

O The Geological Society, London, 1973

SBN

7073 0048 7

Printed in Northern Ireland at The Universities Press, Belfast

CONTENTS PREFACE

.

.

I.

INTRODUCTION

II.

THE

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.

3

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7 .

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9

III. REGIONAL PALAEOGEOLOGY AND ITS I N T E R P R E T A T I O N (A) The Northern Province . (B) The Median Province . . . . . (c) The Southern or Armorican Province

.

10

.

11 16 22

IV.

R E P R E S E N T A T I O N OF THE L O W E R P E R M I A N

REFERENCES

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.

.

.

.

.

.

23

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FIGURE 1. PHANEROZOIC TIME SCALE (P.T.S.) SECTION N W - S E . .

12 and 13

FIGURE 2. PHANEROZOIC TIME SCALE (P.T.S:) SECTION S W - N E . . .

14 and

A

15

SEPARATE~PALAEOGEOLOGICAL M A p OF T H E PALAEOZOIC F L O O R BELOW THE P E R M I A N AND MESOZOIC F O R M A T I O N S I1# E N G L A N D AND W A L E S .

CORRIGENDA

TO

THE

MAP

The Map bears the date of completion, 1971 below the Title and the date of printing, 1973 on the southeast margin. Owing to an oversight, the whole outcrop of the Lizard Complex has been coloured in error as Precambrian. National Grid 3827, outcrop eoloured L. P A L should be ORS/D and the lettering UCM/WCM/ORS refers to Baggeridge Colliery (not shown). National Grid 3929, outcrop coloured ORS should be L. PAL. National Grid 409308, for Witlington read Whittington. National Grid 411339, for Chertley read Chartley. SE Corner, 15 lines from bottom, for Oxen read Oxfordshire.

PREFACE The map with the accompanying short memoir represents a notable addition to British geological literature. Professor Wills has compiled from many sources and with many consultants a map showing the surface of England and Wales as it existed in early Permian times, itself a remarkable achievement. In doing this he has developed to a novel degree concepts of the history of erosion surfaces and of the sub-Permian/Mesozoic massifs which are of major importance in considering the structural history of the region. He has suggested that in the English and Welsh highland areas Permian and later erosion modified only slightly much older surfaces, quoting relics of pre-existing peneplains extending back into the Palaeozoic times which were only a short distance below the postulated sub-Permian surface. Thus he lists pre-Upper Coal Measures in North Wales, pre-Lower Carboniferous in North Wales and the Lake District, pre-Late Devonian from Shropshire across into southeastern England and pre-Llandovery in the Welsh borderland. This is a major extension of previous concepts (for example the view that Wales represented either a dissected Jurassic or late Tertiary surface), but a close analogy is available in east-central England. There (as Wills has shown) the Mesozoic sequence becomes progressively more defective southeastwards on to the London platform, whilst the unconformities at the bases of the individual transgressive units converge at very low angles towards the Thames Valley. Finally there is no distinction between surfaces developed mainly during the Jurassic and the sub-Albian surface. This Mesozoic history had its earlier precursor, for the sub-Carboniferous surface, where preserved, lies so close to the main sub-Mesozoic surface as to be not significantly different on a regional scale, and on present evidence the sub-Old Red Sandstone surface, where known on the block, also lies within very few hundreds of feet below the subMesozoic plain. Feather edges of other formations--in particular the Coal Measures--transgressed on to the platform further west and north. Thus by the accident of escaping regional uplift and denudation, the London-Brabant massif in southeastern England provides a close homologue to the concept of the uplifted Welsh area, with its buried multiple nearly-coincident peneplained surfaces extending over a very long time span. One may extend Professor Wills' concept by making a secondary deduction relevant to the history of the Welsh mountain mass. The Mochras boring, on the north coast of Cardigan Bay, proved 1974 feet of Tertiary upon some 2000 feet of Lias in a fine grained facies, faulted down against the Palaeozoic rocks of Harlech. This has led to deductions that the adjoining highland area was submerged by the Lias sea and was the former site of thick Mesozoic rocks, contrary to early concepts such as those of Arkell. By analogy with the London platform it is however possible to envisage a broad massif, maintained close to sea level throughout the Mesozoic, with the various formations thickening off its margins, in some cases probably increasing sharply across submerged intra-formational faults. According to this concept there need never have been more than feather edges of any marine Mesozoic formations across the Welsh Highland area, and these would have been quickly obliterated by erosion following post-Cretaceous uplift. The compilation of this map and memoir by a Professor Emeritus in his 89th year is a unique achievement. In 1966 the Petroleum Exploration Society recognised Professor Wills' prowess as a palaeogeographer by making him an Honorary Member and in addition that Society has carried most of the cost of this map. With them the Geological Society is happy to have the opportunity of organising its publication. P. E. K ~ T Fellow of the Geological Society Past Chairman, Petroleum Exploration Society

PART

I

INTRODUCTION

The Map 1 sets out to show by colours the outcrops of pre-Permian formations on the Palaeozoic Floor (hereafter called 'the Floor') as they were in Permo-Triassic times, with the lower Cover consisting of L. Permian breccias, sandstones and volcanics, shown by means of symbols overprinted in black. The formations are grouped as follows :-Precambrian, undifferentiated (P-C); the Lower Palaeozoic (L. PAL); the Devonian/Old Red Sandstone (ORS/D and DEV); the Lower Carboniferous (Dinantian) (LC or CL); the Namurian (~A and NA CULM); the Lower Westphalian (LWCM and WCM); the Upper Westphalian, the Stephanian plus the lowest Permian, grouped together as UCM. The upper Cover consisting of U. Permian (Zechstein) and later formations is not shown at all, being assumed removed to display the Floor. The scale of the map is the same as that of the Geological Survey's 10 miles to 1 inch Geological Map. The area coincides with that of a map entitled A Structure contour map of the surface of the buried prePermian rocks of England and Wales by Kent (1949) and an unpublished amended version, which was circulated privately in 1964. In this text, as on the map, the letters shown above in brackets are used instead of the full geological terms. The reader is asked to study the Index for a fuller explanation, and to note that there an attempt has been made to assign a time-interval to each of the groups of formations distinguished by a colour, following The Phanerozoic time-scale (Harland et al. 1964, 260-62). The faults indicated on the map are mostly known post-Triassic ones which are thought to have been active in Permo-Triassic times, though the direction of down-throw may have been in an opposite direction to that shown on the map, which is the downthrow side today (see Wills 1948, p. 59). The Map is accompanied by two Phanerozoic Time-Scale or P.T.S. sections Figs. 1 and 2, which have been designed to eliminate the effects of varying original formational thickness and of subsequent folding and faulting (with the exception of a few major dislocations), thereby elucidating the time/space distribution of the gaps in the Geological Column (Tables I, II), and helping to distinguish between the original and the present limits of the formations. With the exception of the two most recent ones, the complicated series of post-L. Permian erosional phases listed in Table I and shown in the London Platform on Fig. 1, each modified to some extent the surface of the Floor that had been produced during the Palaeozoic/Mesozoic interval. This involves full recognition of the principle that the sediments, whether transgressive or localized, that were laid down on one erosion-surface were liable to be quickly removed by the next phase of planation, particularly from areas of positive tendency. In this way parts of erosion-surface No. 1 became incorporated in erosionsurface No. 2--and so on, making it difficult to distinguish an area that perhaps never received certain formations because it remained a land-area during a marine submergence, from another area where the same formations were deposited and later removed. The Midland Barrier in Carboniferous times is an I The Map was completed in December 1971 and demonstrated to the Geological Society by Dr. P. E. Kent on 6th January 1972.

8

L.J.

W-ILLS

example of the first. Yet the Map can show only the final result; for it is this that is revealed by the borings and by surface exposures along the edges of the Cover. The numerous gaps known to occur in the pre-Permian Palaeozoic sequences where they outcrop in different parts of the country are set out in Table II. They have to be taken into account when considering composition of the Floor below the Cover. They are most numerous and conspicuous in sequences that originated in areas that over long periods tended to behave as positive axes or regions. When traced into negative areas the gaps, as it were, fade out in thick unbroken sequences. T A B L E I : - - I M P O R T A N T GAPS I N THE SUCCESSION CONSTITUTING THE COVER

post -Tertiary post-U. Cretaceous post-L. Cretaceous post-U. Jurassic post-Lias post-Bunter post-L. Permian post-UCM

pre- Quaternary pre-Tertiary pre-U. Cretaceous pre-L. Cretaceous pre-M. Jurassic pre-L. Keuper Sandstone pre-Trias (Bunter Pebble~ Beds and Zechstein)~ pre-L. Permian )

TABLE II:--IMI'ORTANT GAPS IN THE SUCCESSIONOF

PALAEOZOIC OUTCROPPING ON THE F L O O R

post-Ammanian WCM post-l:)inantian inter-Dinantian post-L. ORS]D post-U. Silurian post-U. Ordovician post-Llanvirnian post-U. Cambrian (Tremadoc) inter-Cambrian post-Precambrian

pre-UCM pre-NA

Maj or Major Major Local Local ?Major 1 Universal 2

FORMATIONS Major s Local Local Major/4

pre-U. ORS/D and} pre-Dinantian pre-L. ORS/D pre-Silurian pre-Caradoc pre-Arenig

Major) Major Major Major

pro-L. Cambrian

Local Universal

1 ? = Hardegsen Unconformity of Germany 2 These two gaps merge locally and together constitute the only gap within the Phanerozoic succession that is universally present in England and Wales and Western Europe = the 'Post-Variscan' or 'Post-Hercynian' or Tost-Armorican Unconformity'. a Post-Malvcrnian Unconformity 4 Post-Caledonian Unconformity

The nature of the gap between Cover and Floor is strikingly displayed by Figs. 1 and 2 which cross the Midland Barrier almost at right angles. In Fig. 2 the base of the Cover is seen to be consistently Permo-Triassic, whereas in Fig. 1 it is shown to vary from Permo-Triassie to U. Cretaceous where the Mesozoic sequence is modified by the unconformities listed in Table I. Both figures demonstrate how greatly the age of the Floor varies from place to place and how the gap separating Floor from Cover can be a simple unconformity where the base of the Cover is Permian lying on the Floor of U. Coal Measures (as in Lincolnshire, Fig. 2), but a highly composite one (as in the region of the London Platform and Kent, where U. Cretaceous may rest on Silurian, and Lias on Cambrian or Caradocian).

Reliability of boundaries It must be concluded that at any one spot, each gap represents a stage or stages in the production of the Floor, but only the last stage can be dated (by the basal member of the Cover at that spot). However, the Map is intended to show only those outcrops on the Floor that were exposed in Permo-Triassic times. It follows that :--

A P A L A E O G E O L O G I C A L MAP OF T H E PALAEOZOIC FLOOR IN E N G L A N D AND

WALES

(a) only where the basal element of the Cover is known to be Permian or Triassic can the outcrops on the map approach correctness. (b) elsewhere they are at best approximations, as in East Anglia and the London Platform where the basal member of the Cover varies in age from Triassic to U. Cretaceous, and where some parts of the Floor have been exposed to denudation on at least three occasions since Triassic times. Other outcrops shown a r e (c) inferences from indirect evidence, as in the 20 mile strip east of the Malvern Axis, and in areas adjacent to the present limits of the Cover, as in North England, the Welsh Borderland and Cornubia: (d) conjectures where there a r e no relics of the Cover as in North Wales (except at the Mochras borehole) and as in Herefordshire and South Wales, north of the South Wales Coalfield: (e) speculations where the Cover is so thick t h a t no borehole data are available from the Floor, as in south England between Devon and Sussex.

PART

II

THE

REPRESENTATION PERMIAN

OF

THE

LOWER

~

The difficulties in representing the L. Permian arise from the vast interval of time covered by the Period (L. Permian 40 m.y., U. Permian 15 m.y.), during which earth-movements, continental conditions, and desert denudation and sedimentation predominated. There is a general absence of fossils from all but the oldest L. Permian Stages as developed in the Central Midlands. Elsewhere the rocks ascribed to the L. Permian cannot be more precisely dated t h a n as intermediate between the age of the rocks forming the underlying Floor at t h a t spot and t h a t of the overlying Cover of U. Permian (Zechstein) or Trias or later formation. No reliable correlation from place to place is possible. The continental deposits are mainly breccias (desert scree and wadi-distributed scree), sandstones (often dune-bedded aeolian, but also water-laid ones), and water-laid conglomerates, siltstones and mudstones (alluvial fans and temporary lake deposits). I n synclinal (negative) areas of the Floor, the earliest members of the Permian (the Enville Beds) follow conformably upon Upper Coal Measures. For this reason they are classified by the Geological Survey as Carboniferous or Permo-Carboniferous. B u t vertebrate footprints from the lowest member and one well preserved skull (Dasyceps bucklandi Lloyd) from the upper part, at Kenilworth, are certainly L. Permian in age ;3 yet they are with equal certainty part of the Floor in the sense of this Map. As such they are coloured and labelled UCM on the Map with the symbol for Permian breccia over-printed in black on those outcrops where appropriate. I n anticlinal (positive) areas of the Floor, breccias generally accepted as approximately the same age as the above, b u t possibly somewhat newer, rest with marked unconformity on UCM, ORS/D and L. PAL The Map incorporates certain data from a generalized map of the L. Permian of the British Isles by D. B. Smith (1972}. a yon Huene 1910 redeseribed the Dasyceps skull and assigned it to the Permian family Zatracheidae. A few years ago D. M. S. Watson said in a letter that it is "almost indistinguishable from Zatrachys from the Wichita and Clear Fork of Texas", which is proved Artinskian (L. Permian) by ammonites. One other old find is a maxilla of Oxyodon, a Pelycosaurid which is also a Permian Family.

10

L.J.

WILLS

e.g. the Clent Breccia near the Lickey Axis (not obviously unconformable) and the Haffield-Abberley Breccia near the Malvern-Abberley Axis (violently unconformable). In this category also are the Dawlish Breccias of South Devon and their submarine extension to Start Point and The Lizard, if, as is accepted here, the Ugbrook Formation is of UCM age (Scott Simpson 1959). Other breccias (including the so-called brockrams in Cumberland) lying unconformably on the Carboniferous predate the Zechstein, but cannot be more precisely dated. There are also breccias (e.g. the Moira and Hopwas Breccias that probably are of L. Permian age) that lie unconformably on the Floor, but can only be dated as pre-Triassic. There are also in each of the major Mesozoic Basins great expanses of sandstones, likewise not more precisely datable than post-the Second Malvernian movements and pre-Zeehstein or pre-Bunter Pebble Beds, which latter are taken by the author to be the earliest Triassic Formation in Britain (Wills 1948). On this view, which is in line with decisions by the Permian and Triassic working groups of the Geological Society Stratigraphic Committee, the Lower Mottled Sandstone, Bunter (fl) of the Geological Survey, is U. Permian. The following sandstones are now generally accepted as of that age (dune sands being asterisked) :-(i) Those proved to be pre-Zeehstein are the Penrith Sandstone*, the Yellow Sands of Durham*, the Rotliegend Gas Sands of the North Sea Basin and the Collyhurst Sandstone; (ii) Those proved or inferred to be pre-Triassic include part, if not all, of the so-called Lower Mottled Sandstone of Lancashire, Cheshire, Vale of Clwyd* and of the West Midlands (the Bridgnorth Dunesandstone*) and the Clyst St. Mary Dune-sandstone* of the Exeter area.

PART

III

REGIONAL PALAEOGEOLOGY INTERPRETATION

AND

ITS

Three provinces can be recognized in England and Wales: A. T H E N 0 R T H E R N P R 0 VI NC E, with continuations beneath the North Sea and Irish Sea, extends from the Southern Uplands of Scotland to an irregular east-west line running roughly from The Wash through Derby, Stoke-on-Trent, Ruabon, Wrexham to Liverpool. This line approximates to the southern limits of a region with a virtually complete Carboniferous sequence. The Northern Province was rigidified before U. ORS/D and LC times. B. T H E M E D I A N P R O V I N C E extends from the southern limit of the Northern Province to a line running roughly from Calais through Folkestone, Guildford, Reading, Bristol, Swansea, Tenby and the south side of St. Brides Bay. This mainly hypothetical line (shown on the Map by long dashes and small crosses) defines approximately the southern edges of the positive stable belt of St. George's Land, the Midland Barrier, and the Brabant Massif (in Britain the Wales-Brabant Island). This had been rigidified before U. ORS/D times, to form the Armorican foreland. Against this and locally on to it the sediments of the Armorican shelf, foredeep and possibly even those of the geosyncline were pushed in the Variscan orogeny. C. T H E S O U T H E R N OR A R M O R I C A N P R O V I N C E extends from the line just described to beyond the south coast, and continues southwards under the English Channel to the edge of the map. The Southern Province was not rigidified until UCM times. Since the date of rigidification, each province has undergone only gentle open folding with extensive vertical dislocations by faulting, the latter sometimes associated with narrow areas of intense folding and even thrusting. Details follow.

A P A L A E O G E O L O G I C A L MAP O F T H E

PATJAEOZOIC F L O O R I N

A. T H E N O R T H E R N

ENGLAND AND

WALES

11

PROVINCE

Owing to the limited areas of Permo-Triassic outliers to the west of the present Zechstein outcrop, great difficulty arises in attempting to distinguish the amount of movement and denudation t h a t had taken place in pre-Permo-Triassic times from that which has since occurred. However the Author accepts the view of Kent (1949) that the major tectonic features in this westerly area are due to the pre-Permian movements and erosion along persistent positive and negative belts, whereas broad open flexuring and large scale faulting characterizes the post-Permian tectonics further east. The outcrops depicted on the map in the Pennine region can only be said to be conjectural. This is particularly so in the case of the WCM outcrops over the Alston Block (adapted from Shotton 1956) over the Askrigg Block and the present Millstone Grit country between the Yorkshire and Lancashire Coalfields. On the other hand the composition of the Floor where it lies beneath the Cover has been extensively explored by mining, boreholes and geophysical techniques, and the outcrops shown (adapted from Kent 1966 and from Howitt and Brunstrom 1966) can be regarded as broadly correct. THE CARBONIFEROUS SYSTEM. The Carboniferous Stages have been so intensively studied and shown to vary so much lithologically and tectonically that on the Map only a simplified representation of outcrops, as in Permo-Triassic times, has proved feasible. The Map shows that, with the exception of the L. PAL of the Lake District and Howgill Fells, of the U. ORS of the Scottish Border area, and of supposed U. ORS/LC conglomerates (Mell Fell, Roman Fell), the whole of the Floor was composed of Carboniferous Formations. The U. ORS referred to above passes upwards without any break into L. Carboniferous of Tournaisian age--an arrangement comparable w i t h that found in the Bristol Coalfield and at Titterstone Clee, and possibly with the relationship discovered in the Galley B.H. (Northampton) and in the Cambridge B.H. It is now established that LC, NA and WCM in the Northern Province are each developed as virtually unbroken sequences of varying rhythmic facies and thicknesses, reflecting differential rates of subsidence of a region that for the most part received its sediments from a landmass to the north and north-west. There are however minor breaks, especially in thin developments in positive areas; e.g.S.E, of Lincoln (Lees and Taitt 1946). The fourth group, the UCM, is" apparently conformable and unbroken in areas of maximum subsidence, but towards the Midland Barrier it lies in many places unconformably upon WCM (First Malvernian or Symon Fault unconformity) or even on pre-Carboniferous rocks. This relationship is well known in the Median Province, but has also been discovered below the Cover in the area south-east of Lincoln in the Nocton, Dunston and Stixwold B.H's (Lees and Taitt 1946). On the Map the Lincolnshire outcrops of the UCM are shown ending at the coast line. This device is employed to denote that in the North Sea no subdivision has so far been made of the Coal Measures. THE DEVONIAN SYSTEM. Everywhere in the Northern Province where the base of the U. ORS (in the Border Country) or that of the LC has been seen at the surface or in boreholes, it is unconformable on P-C or L. PAL. Since the youngest L. P A L involved in the Caledonian movements is here uppermost Silurian (Ludlovian), both the mountain building and subsequent peneplanation in the Northern Province can be precisely dated as post-Silurian and pre-U. ORS]D (in the Border Country) and pre-LC elsewhere. It may be concluded therefore that it was the destruction of this particular range of the Caledonides with its extension into North Wales that provided most of the detritus of the L. ORS of the southern half of the Median Province as a sort of delta-fan thrown down by rivers draining into a narrow foredeep of the Armorican geosynclinal sea. The final planation was completed by U. ORS]D times in some places and by L. Carboniferous times in others. The general absence of M. ORS from the whole of England and Wales north of a line running from the

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FIe. 2.--PHANEROZOIC

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'till m CIRCULAR 153 1969, THICK HORIZONTAL LINES= PRESENT SURFACE. THICK VERTICAL LINES SEDIMENTS DEPOSITED WITHIN EXTENSIONS SHADED THUS :.:-'i-ii POST--VARISCAN (POST U.CARBONIF.) COVER ( N O T E ~ "-- PERMIAN BRECCIAS AND SANDSTONES |: FORMATIONS: FULL BLACK, PR~AMBRIAN; A, ARVONIAN; C.CHARNIAN; L. LONGMYNDIAN; M~,'~4ALVERNIAN ; M, MONA; U, URICONIAN.

t h e top of the Palaeozoic Floor are indicated b y h e a v y continuous lines. The shor~ thick horizontal line at the top of a thick vertical one denotes t h e present g r o u n d level in relation to t h e stratal age. The inferred present lateral e x t e n t of each f o r m a t i o n or group of formations is, as far as possible, indicated by shading, as follows : - - F o r m a t i o n s of the Cover d o w n to the b ~ e of the U. P e r m i a n are stippled; the L. P e r m i a n Sands a n d Breccias are shown b y the appropriate symbols. The Carboniferous a n d U. ORS/D bear vertical lines, a n d t h e L. ORS/D a n d Lower Palaeozoic bear diagonal lines9

16

L . J . WILLS

Bristol Channel to somewhere north of London shows that the peneplanation extended southwards to the northern shores of the Armorican Sea. T H E L O W E R PALAEOZOIC and PRECAMBRIAN SYSTEMS. On the Map the L. PAL is shown as outcropping only in the Isle of Man, the Lake District and Howgill Fells, because the present-day inliers of Cross Fell, Teesdale and Ingleborough-Craven are held to have been covered by Carboniferous in PermoTriassic times. The composition of the sub-Carboniferous surface has been proved by borings to be : - Precambrian (?Uriconian) at Woodale near Buxton; ?Skiddaw Slates at Crook and Allenheads, both in County Durham; Caledonian granite in Weardale; ?L. Cambrian at Eakring in Notts. and at Bardney and Stixwold in Lincolnshire. Since the Map was completed in 1971 a boring at Eyam, 5 miles N. of Bakewell, Derbyshire has proved L. Ordovician (Arenig) below the L. Carboniferous. B. T H E

MEDIAN

PROVINCE

The Map and the P.T.S. sections (Figs. 1 and 2) show the Median Province in Permo-Triassic times to have been a broad east-west positive region with extensive outcrops of P-C, L. PAL, and ORS/D, the last being confined to the southern haft. In contrast to the Northern Province, outcrops of LC, NA and WCM are almost entirely confined to the northern and southern edges of the Province: the UCM outcrop alone crosses the central part (in the Oxfordshire Coalfield). It must be recalled however that the Palaeozoic Floor of the Median Province east of a line, roughly, from the Wash to Oxford has been remodelled at several dates in post-Triassic times. It follows that outcrops of Carboniferous and Devonian may have been more numerous and extensive in Permo-Triassic times than those suggested on the Map. This is confirmed by the great abundance of pebbles of limestone and chert in conglomerates in UCM--the Corley and 'Calcareous' conglomerates--and in the Bunter Pebble Beds at Bridgnorth and elsewhere. THE CARBONIFEROUS SYSTEM: COAL MEASURES. Along the northern edge of the Median Province the full development of Coal Measures found in the Northern Province thins out southwards against the positive area of the Wales-Brabant Massif. What appear to be embayments and promontories can be detected on the Map, the promontories beingi/the more obvious. They show up as northward directed tongues of L. PAL and P-C near Grantham, Charnwood, Nuneaton, north Warwickshire and the Lickey Hills, ?Kidderminster, The Wrekin, The Longmynd and S. Shropshire, and north-eastern North Wales. Westwards from the Lickey Axis the promontories are partially masked by covering outcrops of UCM. Along the southern edge of the Median Province Westphalian rocks are known only in and near the coalfields of South Wales, Gloucester-Bristol-Radstock and Kent; and in one small WCM Coalfield with associated outcrops of NA, LC and U. ORS/D occurring as an outlier on Titterstone Clee (Salop). On the southern edge of the Median Province only one promontory can be clearly recognized namely the positive area of the combined Usk-Cardiff and Lower Severn axes, in which the WCM and practically the whole of NA fails. (A tiny relic of WCM (Edgehills Coal) occurs below UCM in the Forest of Dean and suggests an original greater extent). There may however have been a second promontory along the Tortworth-Bath axis, and perhaps others now deeply buried between there and Kent. There are some indications of embayments between the southern promontories. In the case of the South Wales Coalfield the NA, WCM and UCM were formed in an embayment between the Towy and Usk-Rumney (Cardiff) anticlines. The WCM and UCM of the Gloucestershire-Bristol Coalfield were laid down in a second one between the Lower Severn and Tortworth-Bath axes, and the UCM may well have extended far beyond their present outcrops and across the two last-named anticlines. In the case of the

A P A L A E O G E O L O G I C A L MAP OF T H E P A L A E O Z O I C F L O O R I N

ENGLAND AND

WALES

17

Kent Coalfield however there is no indication that the present limits of either WCM or UCM bear any relation to their original extent. There was however a break in sedimentation between the WCM and UCM reflecting the First Malvernian movements of other areas. In the central strip of the Median Province between the two areas just described, the Map shows no outcrops of WCM south of Titterstone Clee and the Coalbrookdale, Wyre Forest, South Staffordshire, Coventry and Leicestershire Coalfields; nor any north of the South Wales, Gloucestershire Coalfields and the Westbury (Wiltshire) B.H., and the Kent Coalfield. It has been suggested by Trueman (1947) and Wills (1956) that WCM may have originally extended from Wyre Forest to South Wales via the 'Hereford Straits' but no evidence of this has survived. Large tongue-shaped outcrops of UCM, largely of red-bed facies are shown extending from the Northern Province southwards into Shropshire, Staffordshire, Worcestershire, Warwickshire and Oxfordshire, where they cross the Midland Barrier, and are mapped as merging into the Pennant facies and the paralic facies of Radstock-Bristol, the Forest of Dean, and the South Wales developments. A similar crossing of the Barrier may have occurred along the Abberley-Malvern line as evidenced by the small outliers of UCM in the Wyre Forest, near Abberley and Newent, and in the Forest of Dean, in all of which areas the UCM are unconformable on earlier formations. In L. Permian times it is probable that their outcrops on and west of the Abberley-Malvern line were not very different from the present-day ones (though larger), since there are several small patches of L. Permian (Haffield and Abberley Breccias) on UCM and pre-Westphalian rocks along the line itself and one notable one on the UCM of the Mumble Coalfield five miles west of the line. NAMURIAN. On the northern edge of the Median Province the Namurian is known from surface and borehole evidence to have been a largely marine development (both gulf- and shelf-facies), extending from the Northern Province into Flintshire, Denbighshire and Shropshire to a few miles south of Oswestry. From here eastwards its limits are unknown but they must lie to the north of Market Drayton, for four boreholes between the vicinity of that town and Newport, Shropshire have proved its absence. Nothing is known of its limit to the east until the occurrence in the Trent Bridge B.H. near Rugeley (it is unknown in the nearby Cannock Chase Coalfield). Thence it runs south-east to near Lichfield (Whittington Heath B.H.), to Dosthill near Tamworth (surface exposure). Here it turns north-east round the north side of the S. Derbyshire Coalfield and Charnwood, and thence to Loughborough, south of Long Clawson and Sproxton B.H's; then north-east to Foston, ?Lincoln, and south-east to Stixwold B.H. In all these places, except those on the north side of Charnwood to Sproxton, only thin R and G zones occur, lying unconformably sometimes on CL (Visean) and sometimes on pre-Carboniferous, except where there is a thick development associated with the Widmerpool G u l l In Nottinghamshire and Lincolnshire, NA always lies below Coal Measures, and for this reason does not appear on the Map. On the Map only the North Wales outcrops and two tiny possible outcrops near The Wrekin and at Dosthill are shown. All of these may well have been covered by UCM in Permo-Triassic times. On the southern edge of the Median Province NA is shown in South Wales, with outcrops approximating to those of the present-day. Here NA is strongly unconformable to the CL which it oversteps on to older rocks in Pembrokeshire. In the Swansea-Bridgend area it is developed in a thick marine facies (completely different from the Culm facies of Cornubia), but further north it seems to have been formed as a marine delta in an embayment between St. George's Land to the north-west and the Cardiff-Usk anticline to the south-east. (As the land areas are approached, only the R and G zones are found.) The embayment extended as far as Titterstone Clee (Cornbrook Sandstone, partly Namurian and partly L. Westphalian, George 1958). As indicated on the Map, the outcrops of Namurian were probably more extensive in Permo-Triassic times than the small surviving patch shown at Titterstone Clee. In the Mendips-Bristol and Gloucester Coalfields area NA is developed as the 'Quartzitic Sandstone',

18

L.J.

WILLS

but its outcrops on the Floor are too small to be shown on the Map. It would appear to have been, deposited "in a discrete basin with only occasional access to the sea" (Ramsbottom 1969, p. 228). The basin may have been an embayment on the southern edge of the Median Province between promontories over the Lower Severn and Bath Axes. Eastwards from this area NA has not been encountered in any boring in the Median and Southern Provinces, and it is known to be absent from Kent and the Pas de Calais. LOWER CARBONIFEROUS (DINANTIAN or AVONIAN). On the northern edge of the Median Province the Dinantian is shown on the Map as outcropping on the Floor in North Wales from Llandudno to a spot about five miles south of Oswestry with a small outlier near Corwen. The outcrops as shown on the Map may have been more extensive in Permo-Triassic times, for where the base is seen for some five miles north of Llangollen it rests on a very evenly peneplaned surface of Silurian and Ordovician which probably extended for many miles to the west. On the other hand basal conglomerates from Ruthin to Llandudno would seem to imply nearby cliffs. From Oswestry to a point a little east of Market Drayton a possible boundary of the Visean below UCM is indicated by hachures on the Map. From here the Childs Ercall and Edgemond B.H.s showed that the boundary turns southwards towards Lilleshall and The Wrekin 4 but the present-day outcrops at the two latter places were probably covered by Coal Measures in Permo-Triassic times. From The Wrekin eastwards to Lincolnshire the approximate southern limit of the Northern Visean Sea against the Wales-Brabant Island is known from a few present-day outcrops and a number of boreholes in which very thin developments have been found, but nearly all of these were below the surface of the Floor under a Carboniferous cover of NA and CM. The limit runs from Lilleshall through Rugeley, Lichfield, north of Charnwood, Grantham and Sleaford. South of this line and also at North Creake B.H. in north Norfolk no CL has been found except at Northampton (Galley and Kingsthorpe B.Hs) and at Cambridge. The Tournaisian at Galley and Cambridge, like that of Titterstone Clee has been claimed to belong to the southern or Armorican development. On the southern edge of the Median Province the original northern limit of the Dinantian shelf-sea (the northern part of the Armorican Sea) cannot be defined owing partly to differences between the supposed shorelines in Tournaisian and Visean times, but chiefly to intra-Carboniferous and PermoTriassic denudation, and to the dearth of borehole evidence. The Author would agree with George (1958) as far as the original extent of the Tournaisian is concerned, though prepared to extend the shoreline further north between Titterstone Clee and Northampton now that marine U. ORS/D is known at Merevale near Tamworth. The outcrops of CL shown on the Map near the South Wales, Forest of Dean, Bristol-Somerset Coalfields and the Mendip Hills as far south as the Canningbon Thrust, and as far east as Westbury, Wiltshire, and the Kent Coalfield and Pas de Calais are based on what is known about the geology of these areas, and are reasonably reliable. Some other occurrences, e.g. at Titterstone Clee, Northampton, Cambridge and those shown on borehole evidence in Kent, Surrey, Sussex and ?Faringdon, may have had larger or smaller outcrops than those drawn on the Map. In Permo-Triassie times there may also have been outcrops in other places from which the pebbles of chert and limestone so common in certain conglomerates in the Midlands were derived, e.g. the Calcareous and Corley Conglomerates and the Bunter Pebble Beds. Negative evidence proving the absence of CL from the southern half of the Median Province is provided by the following boreholes :--HamsweU (N. of Bath), Faringdon, Noke, Witney, Little Missenden and all the deep boreholes in the London area. 4 The small outcrop of CL shown on Titterstone Clee is an outlier of Tournaisian belonging to the South Wales d e v e l o p m e n t of t h e D i n a n t i a n (George 1958).

A PALAEOGEOLOGICAL MAP OF T H E PALAEOZOIC ]~LOOR IN E N G L A N D AND WALES

19

THE DEVONIAN SYSTEM. 5 Two large outcrops of ORS/D are shown on the Map. A. The Anglo-Welsh area in south-east Wales, tterefordshire and Shropshire extends eastwards to the Malvern-Bath Axis. A few small outliers of ORS/D (Long Mountain and Clun) lie on the L. PAL to the West. The Mesozoic Cover has been stripped off except from parts of Glamorganshire. It is probable, however, that in Permo-Triassic times there were more outliers of Lower and Upper Carboniferous rocks in the area shown as ORS/D than the two present day outliers of Pen-eerrig-calch and Titterstone Clee to which reference has already been made. B. The second area of ORS/D, the London platform area, lies east of the Malvern-Bath Axis and is separated from area A by a hypothetical 25-mile strip of L. PAL and P-C and by the almost equMly wide outcrop of UCM in the Oxfordshire Coalfield, below the southern part of which the ORS/D is known to be present at Witney and Steeple Aston. In this area the ORS/D is completely concealed by the Cover and obviously our knowledge of the stratigraphy and tectonics is of a very different order of preciseness from that further west. On the Map the limit of the ORS/D outcrop shown towards the west and north has been inferred from records of borings, some proving Devonian in the :Floor, others reaching L. PAL or P-C. On the east it is conjectural, except in the Thames Estuary and Kent. In most cases the dating has been based on spores alone. Only exceptionally has any boring been continued to an appreciable depth below the Floor, with the result that the existence or absence of the Lower and Middle Stages below the Upper can only be conjectural.

The Median Province thus presents a picture of Devonian happenings very different from that found in the Northern Province which, as already described, was a mountainous region undergoing profound erosion throughout the Devonian Period. In Lower Devonian times rivers carried the products of erosion from the mountains in the north and from their extension in north and western Wales towards the Armorican Sea across a subsiding and ever-widening alluvial plain and delta (now the area of Anglo-Welsh development of the Lower ORS): but where the continental Old Red facies passed into the marine Devonian facies is nowhere seen. In the west presumably it lies hidden below the Bristol Channel, for in North Devon the Lower Devonian is marine. Downtonian in ORS facies occurs as far north as the Long Mountain near Welshpool, Bridgnorth, the South Staffordshire Coalfield (in the two latter it is covered unconformably by CARB). To the south-east the farthest north known occurrences of L. ORS/D are at tIamswell BH. near Bath, Witney, Little Missenden and Canvey Island in the Thames Estuary. In Middle Devonian times uplift, erosion and peneplanation were renewed in the MedianProvince. This is shown by the pre-U. ORS/D unconformity which is present everywhere to the west of the Malvern-Bath Axis as far north as Titterstone Clee; and has also been proved in boreholes on the east side of that Axis at a number of places as far north as Lichfield, Tamworth and Wyboston (six miles north of Bedford) where U. ORS/D rests on pre-Devonian rocks, and at Witney where it follows unconformably upon L. ORS/D. The extent of the land in Middle Devonian times is difficult to determine. It extended southwards at least to Pembrokeshire and the present coast of South Wales but not to North Devon where the Middle Devonian is largely marine. The area of the mouth of the Severn, Somerset and the Mendip Hills as far east as the Bath Axis was all land. Farther east the coast line cannot be defined, even vaguely. No undoubted marine macrofossils have been recorded and although M. DEV and M/U. DEV microfossils are widely known (unpublished information from Dr. Margaret Mortimer), the nature of the sediment in 5 In this account the Downtonian is treated as Devonian and conformable to the Silurian.

20

L . J . WILLS

which they occur does not tell us whether the rocks were deposited in the sea or on the coastal shelf or delta. It seems probable, however, that the Armorican Sea had advanced over the peneplaned coastal plain northwards to well north of London and perhaps as far as Cambridge and Norwich by the end of M. DEV times. As the great mid-Devonian peneplanation approached completion in the Northern and Median Provinces, the Armorican Sea advanced northwards in Upper Devonian times. Coastal and deltaic deposits were laid down in places (e.g. North Crop of South Wales Coalfield, Forest of Dean and Bristol Coalfields, with pebbles, ?derived from the L. PAL and P-C of the country east of the Malvern-Bath Axis and also perhaps from the remnants of the Caledonian Mountains in Wales). In other parts sandstones, probably of marine shelf origin were formed, containing the same fishes that occur elsewhere in association with marine benthos, and in yet other places definitely marine strata with benthos alone. These marine rocks record the beginning of the U. D E V - - L . CARB transgression, as is shown occasionally where U. ORS is followed by Tournaisian L. CARB (see below). The farthest west and north that U. ORS has been recorded are S. Pembrokeshire, Caermarthen, Titterstone Clee, Whittington Heath, Merevale, Northampton, Wyboston, Cambridge and ?Attleborough. There is no suggestion of proximity of a coastline in any of the English U. ORS/D east of the Malvern-Bath Axis: the original extent of the transgression remains a mystery. The Map does however suggest that the transgression halted on the west by the Caledonian mountains in West and North Wales and in the north-east by a massif of P-C in the north-east Midlands. Between these two areas, at Titterstone Clee in Shropshire and, on the south of the latter area, at Northampton (Gailey), and at Cambridge, the U. ORS/D is succeeded by probably conformable Tournaisian. On the other hand at Whittington Heath U. ORS/D is succeeded unconformably by Visean, showing that in places the transgressive U. DEV may have originally extended farther north and that its deposits have been destroyed by intra-Dinantian planation. The same explanation could hold for the whole of the Northern Province, though the absence of any ORS/D from any part of it would appear to imply that it was land throughout that period, except on the Scottish border where U. ORS/D occurs. THE L O W E R PALAEOZOIC AND PRECAMBRIAN SYSTEMS. The Map shows six separate outcrops of L. PAL on the Floor of the Median Provinces in each of which there are areas of P-C. Area la. North and West Wales, with P-C of St. Davids, Arvon, Mona and the Longmynd, is roughly coincident with the Welsh Caledonian Geosyncline and is defined on the east by the Church Stretton Fault and the Towy Anticline. Today, in the absence of the Cover, the L. PAL and P-C are fully exposed except where concealed by a few small outliers of ORS and L. CAleB. There may have been larger and more numerous outcrops of these formations in Permo-Triassie times. (See the Tablet Note on the Map). The surface of Area la, had been partially or perhaps completely peneplaned and had suffered pedimentation before L. CARB times (Fig. 1), and probably had been levelled off again before UCIV[ and Permian times. For this reason, the Floor here must not be assumed to have been a rugged mountain tract in PermoTriassic times. Area lb. South Shropshire and the Welsh Borderland between the Church Stretton Fault and the Malvern Axis, including a narrow strip from Abberley to Malvern on the west side of the Malvern Fault, the Lower Palaeozoic inliers, with small areas of P-C, usually Uriconian (some possibly not reaching the Floor in Permo-Triassic times). It is probable that in Permo-Triassic times the main outcrop of the L. PAL was less than that shown on the Map because the L. PAL formations may have been more extensively covered by ORS/D and Lower, and even Upper Carboniferous. Similarly the inliers of Woolhope, Usk and Runmey may have been smaller or non-existent. The L. PAL sequence in Area lb. is in a shelf-facies and differs strikingly from the geosynclinal development in Area l a., in the general absence of the Middle Cambrian and of the Arenig, the Llanvirn and Llandeilo stages of the Ordovician with the resulting great pre-Caradoc composite unconformity.

A

PALAEOGEOLOGICAL

MAP

OF

THE

PALAEOZOIC

FLOOI~ IN E N G L A N D

AND

WALES

21

There is also a great pre-U. Llandovery composite unconformity, followed by a full sequence of the Wenlock and Ludlow stages of the Silurian, the latter passing up into the Downtonian ORS/D (Fig. 1). Area 2. The 25 mile-wide, north-south 'Lickey Strip' of P-C and L. PAL outcrop mapped east of the Malvern-Bath Axis is essentially hypothetical, since these systems are only exposed at the Lickey (at Rubery UCM on U. Llandovery on L. Cambrian; at Barnt Green, P-C), and only one boring has reached the Floor, (at Hamswell near Bath, L. ORS/D). Patches of P-C have been inserted on Acremagnetic evidence as continuations of the Malvern and Huntley outcrops in Area lb, and others farther east. The largish patch south of the Lickey is indicated by the preponderance of Uriconian P-C in the nearby Clent Breccia. The L. PAL succession, where seen in Area 2, resembles in many respects the succession in Area lb near Church Stretton, but differs in the complete absence of any Ordovician. In this respect it matches the succession near the Malvern Axis (here placed in Area lb) and, as will be noted below, with the succession proved in Areas 3 and in East Anglia. In fact, only near Huntingdon in Area 5 and at Bobbing in Kent have Ordovician rocks been proved east of the Malvern Axis. Various authors have pointed to this strip as a probable source of pebbles of P-C (Malvernian and Uriconian) and of Cambrian and Silurian rocks in the numerous conglomerates and breccias found at various levels in the U. Carboniferous, Permian and Trias of the Midlands, and possibly also in the U. ORS/D conglomerates of the eastern part of the South Wales, Forest of Dean and Bristol-Somerset Coalfields. It is noted on the Map near Malvern that this strip of P-C and L. PAL may have included some ORS/D in synclinal outliers. The rocks of this strip pass eastwards under the UCM of the Coventry and Oxfordshire Coalfields to reappear i n - Area 3. The 'Nuneaton-Charnwood' strip extending from N E W to SSE from Tamworth, Nuneaton and Charnwood to Bicester and Bletchley is shown as bounded on the west by a postulated basement-fault continuing the Nuneaton Axis. P-C is known at the present surface at Nuneaton and Charnwood and has been explored by borings in the north and the south, but in between the composition of the floor has not been proved. In the north only L. and U. Cambrian with Caledonian intrusives have been found. In the south only U. Cambrian (Tremadoc) has been proved, together with a poorly documented ?P-C or Caledonian granitic intrusion at Bletchley. The Cambrian succession in Area 3 resembles that in Area lb, but there is no Ordovician or Silurian present. The strip is bounded irregularly in the south-east by the wide outcrop of the unconformable U. ORS/D, and on the north-east lies-Area 4. The large outcrop of P-C extends from Charnwood and the Oxendon and Orton B.H.S. in Northants., to Upwood and Warboys, north of Huntingdon; to Norfolk (North Creake B.H.), with other provings at West Wittering and Glinton near Peterborough. The outcrop shown is adapted from Kent (1967). Area 5. The East Anglian Outcrop of L. PAL. The boundaries of this outcrop against Area 4 and the large outcrop of ORS/D on the London Platform are conjectural, but L. PAL have been proved at Huntingdon and Great Paxton, both Ordovician (Llanvirn). (Extensions of the L. PAL below the ORS/D outcrop on the London Platform are proved by borings at Wyboston, five miles south-west of Great Paxton, U. ORS/D on U. Cambrian (Tremadoc); and at Ware, U. Silurian (Wenlock). :Farther east L. PAL were encountered in old boreholes. Of these Culford near Bury proved ?Silurian; Stutton (part of the core now in Geology Department University of Birmingham closely resembles U. Silurian Denbighshire Grits) and Weeley (both in Essex) both have been shown by acritarchs to be Silurian (?Llandovery). No exact date can be given for the L. PAL at Lowestoft and Harwich. Area 6. Kent and Thames Estuary. There is some uncertainty about several of the older borings, but

22

L.J.

WILLS

the following have been dated :--Bobbing, U. Ordovician (?Caradoc); Chilham, Silurian (U. Llandovery-by graptolites)--Cliffe No. 10, Silurian (Wenlock) ; (Shalford near Guildford, Silurian, U. Llandovery, is in the Southern Province). Others are generally accepted as Silurian:mat Sheerness, Herne, Reculver. An old boring at Brabourne (?ORS/D on Silurian) is in the Southern Province. C. T H E

SOUTHERN

PROVINCE

Where bare of the deep Mesozoic and Tertiary Cover namely in Pembrokeshire, the Gower peninsula, Mendip Hills, Cornubia and the Pas de Calais the Southern Province is floored by so complicated and controversial a sequence of Devonian and Carboniferous rocks and tectonic structures, that the Map shows only present-day outcrops (including the Ugbrook Beds (Simpson 1959), which have been recently defined or generally accepted in the past, that were probably exposed on the Floor. No adjustments have been made for Tertiary tear-faulting; and the granites are shown as outcropping, although it is still a matter of debate whether they and their postulated metamorphic and volcanic aureoles were exposed in Permian times. Important faults and thrusts are shown in Pembrokeshire (Owen et al 1971; Sanzen-Baker 1972), in the Bristol-Radstock Coalfield and Mendips, and in Cornubia, especially the Cannington Thrust (see below) and the Lizard thrust (information kindly supplied by E. M. L. Hendriks), faults and thrusts at Padstow, Tavistock and east of Dartmoor, and the thrusts in Surrey and Sussex (see also Wallace 1968 and Shepherd-Thorn et al 1972). Other very hypothetical outcrops and structures shown and referred to in Tablet Notes on the Map are conjectural inferences. For the Bristol Channel (see Tablet Note) the author accepts the hypothesis of Bott and Scott (1964) calling for the existence of an Exmoor Nappe of DEV overriding on the Cannington Thrust a tectonically thickened lens of LC, NA, and WCM (only Westphalian A) and perhaps some Culm facies of the Carboniferous. Brooks and Scott Thompson (1973) recently have confirmed this, and also demonstrated the probable continuation of the Usk-Cardiff-Vale of Glamorgan anticlinal axis under the sea at least as far west as Lundy Island. This anticlinal area of ORS]D and L. PAL, largely concealed by the Cover, separated the South Wales Carboniferous basin from the tectonically thickened basin in the southern part of the Bristol Channel, and under Exmoor. Eastwards from W. Somerset and E. Devon nothing is known about the continuation of the highly tectonized Devonian and Carboniferous rocks of Cornubia; but eastwards from the Mendip Hills and Frome area (where continental, paralic and marine shelf-developments of U. ORS/D, Dinantian, thin Namurian and thick WCM and UCM are well known) there is for many miles no knowledge of the composition of the Floor. In Sussex, Surrey, Kent and the Pas de Calais scattered borings have established that a fully marine Devonian development (probably all the stages) occurs; that the Dinantian is also present as typical CL deposits (no Culm facies found so far). The Namurian is absent from Kent and Pas de Calais; and there is a development of WCM and UCM resembling that of the Bristol-Somerset Coalfield in Kent, but so far no Coal Measures are known in Sussex or Surrey. Acl~nowledgements.--In addition to those named on the Map, under the Title, the Author wishes to record his deep gratitude to The British Petroleum Company Limited for redrawing his original M.S. map; to the Petroleum Exploration Society of Great Britain and to the East Worcestershire Waterworks Company for their two very handsome contributions towards the cost of reproducing it in colour; and especially to P. E. Kent for his help, advice and encouragement.

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