Geology in Severn Vale and Cotswolds

GEOLOGY EXPLAINED IN THE SEVERN VALE AND COTSWOLDS

Dedicated t oL . RICHARDSON, F . R . S . E . , F . G . S .

w h o , i n the early p a r t o f this c e n t u r y , d i d s o m u c h pioneer w o r k on the geology of the Cheltenham area and whose b o o k The Geology of Cheltenham 1904 remains a classic on this l o c a l i t y .

GEOLOGY EXPLAINED IN THE SEVERN VALE A N D COTSWOLDS

by WILLIAM DREGHORN, B.Sc, F.R.G.S.Illustrations the Author by

D A V I D & CHARLES

:

N E W T O N ABBOT

1967

AcknowledgmentsThe author wishes to express his thanks f o r the help given by Mrs D r e g h o r n i n the p r o d u c t i o n o f this book. He w o u l d also l i k e to express his grateful thanks to the following: T o the e m i n e n t geologist, D r D . V . Ager, w h o s o generously spared t i m e to read and criticise the final script in spite of m a n y pressing c o m m i t m e n t s in m o r e exalted fields of study. To Mr C. E. Leese, B.Sc, r e t i r e d headmaster, past president of Royal Geological Society o f C o r n w a l l , w h o read and criticised the script w h i l e t h e b o o k was t a k i n g shape.

WILLIAM DREGHORN 1967 Printed in Great Britain by W. J. Holman Limited Dawlish for David & Charles (Publishers) Limited Newton Abbot Devon

Contents

i I

iKODUCTION T H E JURASSIC SYSTEM OF GLOUCESTERSHIRE Clays, sands and limestones the L o w e r Lias clay age of the Ichthyosaurus W A I N L O D E CLIFF The Red Cliffs at W a i n l o d e the Tea Green Marls the Rhaetic rocks WESTBURY-ON-SEVERN Rhaetic beds at W e s t b u r y the Pullastra bed fossils o f the 'bone bed' Garden Cliff

7 12

,

18

25

I

' l l l l i SEVERN BRIDGE Structure lines of the L o w e r Severn rocks on the Beachley side the A u s t Cliff side - Aust Rock the bridge approaches I I I I SEVERN BORE A N D H O C K CLIFF : The source of the bore height and speed the fossils at H o c k Cliff

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46

6

T H E SEVERN TERRACES The terrace villages W o o l r i d g e terrace development of a terrace T H E C H E L T E N H A M SANDS The o r i g i n of the sands Cheltenham gravels the Waters of Cheltenham T i l l ; C H U R C H D O W N OUTLIER I lucclecote C h u r c h d o w n H i l l Tinker's H i l l ROBIN'S W O O D H I L L Tuffley B r i c k w o r k s the cliff at Tuffley 'The Age of the A m m o n i t e s ' IIREDON H I L L Oolite limestone scenery of Bredon K i n g and Queen Rocks springs and villages

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I

60

6 >

68 73

10

80

6 11

CONTENTS

LECKHAMPTON HILL The Pea G r i t the Freestone c u r r e n t bedding the Devil's C h i m n e y the Ragstones C h a r l t o n Kings Common CLEEVE H I L L The Postlip valleys N o t t i n g h a m H i l l Cleeve C l o u d B A R R O W W A K E , C R I C K L E Y H I L L A N D BIRDLIP C r i c k l e y H i l l the B i r d l i p anticline v i e w f r o m the Peak T H E COMBES The Vale of W i t c o m b e H i g h Brotheridge W i n c h combe SPRINGS A N D VILLAGES I N T H E G R E A T OOLITE REGIONS The Great Oolite series Fuller's Earth series the 'lost' villages d r y valleys the Rendcomb area C O T S W O L D TILES A N D B U I L D I N G STONES Sevenhampton quarries fossils in slates - slatem a k i n g technique b u i l d i n g stones THE PAINSWICK AREA Geological origins m a i n beauty spots Painswick Beacon THE NORTHERN MALVERNS Rocks of the Malverns Green V a l l e y and I v y Scar the Silurian rocks Herefordshire Beacon Clutters Cave

12 13

14

15

16

17

18

19

T H E SOUTHERN M A L V E R N S The Silurian Pass G u l l e t Q u a r r y M i d s u m m e r H i l l the Bronsil shales origins of the Malverns r o c k specimens M A Y HILL Geological background sandstones and limestones Wilderness Q u a r r y

20

GLOSSARY BIBLIOGRAPHY A P P E N D I X T A B L E OF ROCK S T R A T A

Introduction

M [s an o l d teaching m a x i m that ' i f y o u w a n t to make a subject live, y o u must make it local'. The purpose of this b o o k then, is to m.ikc geology a l i v i n g subject by m a k i n g it l o c a l . Students o f geology w i l l find i t useful b u t as i t has been w r i t t e n I in . i l l w h o are interested in scenery, the maps, diagrams and Illustrations have been simplified so t h a t t h e y are also i n t e l l i g i b l e to ili< layman. Again, the geological expert usually studies rocks b y looking at exposures in quarries, railway-cuttings and boreholes, as Well as by l o o k i n g at scenery, whereas this b o o k attempts to i n t r o ilin c geology t o the l a y m a n i n terms o f scenery, and explains the relationship of rocks to hills, valleys and plains, and the w a y in M I I H I I rock structures have influenced the e v o l u t i o n o f different iypes of scenery. ( i n l y a f e w maps have been included among the illustrations as I h.ivi' assumed that readers w h o are geologists w i l l purchase the relevant one-inch-to-the-mile Ordnance Survey geological maps, the Miiicion-in-Marsh and the Cirencester sheets. Instead of maps, I hive, whenever possible, used 'block' diagrams, w h i c h non|(OlogistS often find easier t o i n t e r p r e t t h a n geological maps. When d r a w i n g a b l o c k diagram the artist selects a 'block' of landscape and cuts t h r o u g h it w i t h an i m a g i n a r y k n i f e to expose the mi ks !' the strata u n d e r l y i n g the scenery, at the same t i m e p u t t i n g in Identifying details of the scenery above. W h e n I used this m e t h o d (0 Illustrate a series of newspaper articles on C o t s w o l d villages w 1111('ii I>y my w i f e , m a n y villagers f o u n d t h a t these b l o c k diagrams li.nl helped them to understand their e n v i r o n m e n t in a w a y no m a p 11.ii I ever been able to e x p l a i n i t . < )nc legitimate c r i t i c i s m w h i c h c o u l d be levelled at this b o o k is ili.it comparatively l i t t l e attention has been paid t o the characterlltli fossils o f the areas coveredparticularly as the geology books w i i i i e n by Gloucestershire's nineteenth-century geologists m a i n l y %isl of lists of fossils. H o w e v e r , l i s t i n g fossils has n o t been 'the

8

INTRODUCTION

object of the exercise' in this case, and the reader w h o requires more i n f o r m a t i o n on this subject is recommended to b u y the B r i t i s h M u s e u m booklets on fossils. The choice of the area covered by this b o o k is explained by the fact t h a t Gloucestershire and its adjoining counties are exceedingly f o r t u n a t e i n h a v i n g a r e m a r k a b l y w i d e v a r i e t y o f rocks a l l w i t h i n easy access. The geology student w h o takes his degree at B r i s t o l U n i v e r s i t y can see almost the entire range of rocks in the B r i t i s h Isles w i t h i n fifty miles o f Bristol whereas, i n areas l i k e Australia o r N e w Zealand, the student m u s t t r a v e l hundreds of miles over the same rocks before a change can be observed. W i t h i n f i f t y miles o f Gloucester, Cheltenham o r B r i s t o l there can be seen the f o l l o w i n g w i d e range of r o c k s : 1. The Pre-Cambrian rocks of the Malverns w h i c h are some o f the oldest rocks i n the B r i t i s h Isles; t h e y f o r m p a r t o f the o r i g i n a l f r a m e w o r k of the B r i t i s h Isles constructed by earth movements m a n y m i l l i o n s of years ago. 2. The Silurian rocks a r o u n d M a y H i l l , the Malverns and Ledbury. 3. The Carboniferous rocks of the L o w e r Severn and the A v o n Gorge. 4. The O l d Red Sandstone rocks of the Forest of Dean. 5. The Coal Measures of the Forest of Dean. 6. The Jurassic rocks of the Cotswolds. 7. The Triassic rocks of the South Midlands. 8. The Cretaceous rocks (the chalk h i l l s of W i l t s h i r e ) . 9. The finest exposures in the w h o l e of the B r i t i s h Isles of the Rhaetic rocks. These are in the L o w e r Severn area at A u s t and Westbury-on-Severn and c o n t a i n the famous 'bone bed', r i c h in the fossil remains of fishes and reptiles.PHYSICAL F E A T U R E S OF T H E AREA

The m a i n physical features of the area covered by this b o o k are s h o w n in the sketch at M a p 1 and include the Severn Vale bordered on the east by the strong escarpment of the Cotswolds and, to the west, the h i l l y regions o f the W e l s h borderlands. These t w o m a i n features guide the River Severn in its south-westerly course to t h e B r i s t o l Channel. To keep the sketch map clear and simple, the numerous valleys of the Cotswolds are n o t s h o w n , b u t east of the m a i n escarpment the drainage is to the Thames.

MAP i

SKetrert m o p o f f t c Coirswolds

Severn Yale

The corresponding geological map (Map 2) shows the c o n t r o l exercised by geology in f o r m i n g those r e l i e f features. In general, the high g r o u n d of the W e l s h borderlands is f o r m e d of older or Palaeozoic rocks w h i c h , being harder and m o r e resistant to erosion, stand o u t as higher g r o u n d . The l o w - l y i n g areas of the Severn Vale, I >n the other hand, consist of the softer clays of the Trias and L o w e r Lias, and clearly demonstrate h o w rivers t e n d to erode t h e i r valleys in the softer rocks. H a r d e r rocks of the O o l i t i c limestones are

10

INTRODUCTION

responsible f o r the h i g h g r o u n d of the Cotswolds and these f o r m a plateau, or 'dip-slope', w h i c h falls a w a y g e n t l y south-eastwards d o w n t o the p l a i n o f O x f o r d . B y c o m p a r i n g the t w o maps i t w i l l be seen t h a t the C o t s w o l d escarpment has been eroded back to leave remnants of the f o r m e r scarp, i n c l u d i n g such conspicuous hills as C h u r c h d o w n and Robin's W o o d , r i s i n g castle-like o u t of t h e Severn Vale. These remnants,

MAP 2 SIMPLIFIED GCOI-QCrY SKE-TCVt MAP Oldev falfteozolc. rooKs on Hie- Wt&t. Younger" M*sozic rocKs on flit- e a s f"ReterfroiUuvfum deposits Loot's*"*"") t ftt- 5/erti. v a l f r .

INTRODUCTION

11

I n o w n as 'outliers', are the subject of a later chapter. The M a l v e r n range, t o o , is a v e r y m a r k e d feature and, composed of ancient crystalline rocks r i s i n g suddenly f r o m the plains of t h e II las, is r e a l l y p a r t of the backbone of England. The A u s t r a l i a n t e r m 11 ii such a range w o u l d be 'jump-up' and, geologically, these h i l l s were pushed up m i l l i o n s of years ago. Looking at the p h y s i c a l map, one m a y ask w h e r e is the best Nccnery to be found? The answer depends u p o n w h a t y o u f a n c y , for (here is c e r t a i n l y no lack of v a r i e t y . The highest parts of the (iotswolds are o n Cleeve H i l l , 1,083 f t above sea level, and these are line scenic areas; f o r m o r e rugged scenery the Malverns are the .inswer and, in parts, resemble the h i g h l a n d zone of the W e l s h interior. Craggy, limestone c o u n t r y is to be f o u n d in the Chepstow .ind W y e valley, w h e r e the massive Carboniferous limestones are deeply cleft by the incised meanders of the River W y e . There are sonic d e l i g h t f u l spots, t o o , along the banks of the Severn, w h e r e the red rocks of the Trias (the Keuper M a r l ) f o r m cliffs at W a i n l o d e , Westbury-on-Severn and at A u s t , f u r t h e r evidence t h a t the v a r i e t y of scenery in this region is m a i n l y due to the great variations in the type o f r o c k . A geological map is rather l i k e Jacob's Coat, a t h i n g of ' m a n y colours', w i t h a different c o l o u r f o r each different p e r i o d of r o c k f o r m a t i o n a n d the prettiest maps w i t h the greatest v a r i e t y o f colours are u n d o u b t e d l y those of the areas in and a r o u n d (iloucestershire.

CHAPTER

1

The Jurassic System of Gloucestershire

M o s t of the rocks in the eastern h a l f of Gloucestershire belong to the Jurassic systemso n a m e d after the rocks in the Jura M o u n t a i n s in France w h i c h are of the same age. There are several m a i n aspects of any r o c k system w h i c h have to be studied in a geological assessment, and these a r e : S T R A T I G R A P H Y , o r descriptions o f the various bands or strata o f r o c k and the relationship between those bands; P E T R O G R A P H Y , w h i c h is the study o f the nature o f the materials i n the bands; a n d P A L A E O N T O L O G Y , the study o f the fossils f o u n d i n the strata. Palaeontology serves s t r a t i g r a p h y in t h a t a bed of r o c k (or s t r a t u m ) can b e identified b y the fossils i t contains. T h e n , b y studyi n g the fossils as complete assemblages and n o t m e r e l y as i n d i v i d u a l fossils, the geologist endeavours to arrive at a conclusion about the l i f e o f the t i m e w h e n they w e r e deposited. This p r i n c i p l e of i d e n t i f y i n g strata by means of fossils was first p o i n t e d o u t b y W i l l i a m S m i t h , t h e son o f a n Oxfordshire b l a c k s m i t h w h o became a surveyor and c i v i l engineer d u r i n g the great days o f canal-building at the end of the eighteenth c e n t u r y . He was engaged i n survey w o r k f o r the b u i l d i n g o f the Somerset Coal Canal f r o m 1 7 9 2 t o 1 7 9 5 and t h r o u g h o u t t h a t p e r i o d h e made a p r o f o u n d study of the Jurassic rocks a r o u n d Bath, h a v i n g been f r o m earliest boyh o o d an observer and collector of Jurassic fossilseven p l a y i n g marbles w i t h t h e m ! I t was h e w h o was the first t o recognise t h a t it is 'a general l a w t h a t the same strata are f o u n d always in the same order of superimposition and c o n t a i n the same peculiar fossils' t h e r e b y l a y i n g the foundations f o r the n e w science of geology. The greatness o f W i l l i a m S m i t h d i d n o t rest u p o n book-learning, b u t came f r o m his powers o f accurate observation, o r i g i n a l i t y o f t h o u g h t a n d his constructive i m a g i n a t i o n . He was less concerned w i t h devising appropriate names f o r his fossils t h a n h e was w i t h perceiving t h e i r place in the ordered sequence of the rocks w h i c h he

T H E JURASSIC S Y S T E M O F G L O U C E S T E R S H I R E

13

,iiievoniri ruclfe 1 H 11") Stale: U:U

STRUCTURE LINES OF THE LOWER S E V E R N

T H E S E V E R N BRIDGE

35

1

I i " i n l i m e to t i m e d u r i n g t h e last 600 m i l l i o n years there have l" i n periods of earth movements w h i c h have taken up alignments liong certain directions. For example, it can be seen in M a p 3 t h a t till Lower Severn A x i s is such a line and this d i r e c t i o n , r u n n i n g Nl'. to SW, is called 'Caledonian'. Other structure lines s h o w n on thlft map are the Chepstow a n t i c l i n e a n u p f o l d w h i c h causes a i"ii;> . I I K I the Beachley-Qanna pericline, w h i c h is a dome-shaped fold, The net result of structure lines is t h a t , v e r y often, older a n d ir irdcr rocks are t h r o w n up near the surface, either as h a r d outcrops in .IN lhe result of the upheaval of rocks l y i n g above t h e m . I m o l i n g again at M a p 3, it can be seen t h a t the L o w e r Severn A n || .mil the Bath A x i s cross at Sharpness. This brings the D e v o n i a n mi ks to the surface and, as t h e y are harder t h a n t h e rocks farther north along the r i v e r , t h e y o u t c r o p in red cliffs at Gatcombe a n d Sharpness. There, in 1874, a r a i l w a y bridge was b u i l t , b u t a f e w i r . ago it was smashed by a barge w h i c h got o u t of c o n t r o l . The Severn Bridge c o u l d have been b u i l t at Sharpness b u t possible i i a l l n congestion and its greater distance f r o m Bristol made i t less convenient t h a n Beachley. A t Beachley t h e t w o structure lines iliown o n the map p l a y some p a r t i n b r i n g i n g t h e h a r d limestones nl i l i r (:.irhoniferous rocks t o o u t c r o p n o t o n l y o n the r i v e r bed b u t III in.ill clill's to the w e s t of Beachley. In fact, there are islands of i l i > \ r limestones almost i n the m i d d l e o f the r i v e r . W h a t better y> " l o g i c a l choice, t h e n , c o u l d be m a d e ! I I H ic was also the added f a c t o r t h a t by b r i d g i n g the Severn ar l\i n l i l i y the appalling traffic congestion at Chepstow w o u l d be " In v c d . Furthermore, t h e Vale of Berkeley f r o m T h o r n b u r y to llom ester is t h i n l y p o p u l a t e d and l i k e l y t o become a 'target area' l"i the overspill populations o f B i r m i n g h a m and L o n d o n . A l l i n a l l , | l f l i hli'v slood o u t a s the obvious place f o r the bridge.T H E ROCKS ON T H E B E A C H L E Y S I D E

f u r t h e r reference t o M a p 4 w i l l s h o w t h a t Carboniferous limeItil11 H " i i t c r o p in a w i d e area to the south-west of the Forest of i Ii HI a n d that the R i v e r W y e has c u t t h r o u g h these rocks at ' In p . l o w . The Chepstow a n t i c l i n e m u s t have some influence on (In outcrop of the limestones on the Beachley peninsula, and so c m i ils convergence w i t h t h e Beachley-Clanna pericline, f o r b o t h 11II ' i "ii I he ancient s t r u c t u r e l i n e of t h e L o w e r Severn A x i s . Along the foreshore west of the f e r r y landing-stage and under

36

GEOLOGY IN T H E S E V E R N V A L E AND COTSWOLDS

the bridge, the h a r d limestone can be seen o u t c r o p p i n g . A l t h o u g h deeply f i s s u r e d i n places and w e l l j o i n t e d , i t was obvious t h a t these rocks w o u l d f o r m a good base f o r the bridge. In fact, the m a i n p r o b l e m in b u i l d i n g the bridge was to understand the nature of these basement rocks. Close inspection of the rocks towards Beachley Point reveals fissures filled w i t h a hard breccia, i.e. angular lumps of the limestone cemented together n a t u r a l l y . This is the D o l o m i t i c Conglomerate w h i c h betrays an ancient landscape surface.MAP 4

FOREST OF JJEAW

'SEVERN ESTUARY

o^lcat sKetck rnarj crj" fkfc "Beachjew "fte. Chepstow Anticline, and 'BeacMev Clanno. pert cU'ne. brt'nj Hit C arban iferou lime stones JD HC ^ur-fitce, < many places ^_ '"4

afCtL


37

11 can also be seen t h a t the limestones are folded i n t o ridges i IflCli lines) and i n t o small valley downfolds (synclines), b o t h factors Which added to the c o m p l e x i t y of the task of finding the basement Wi I-. In w h i c h to sink the piers of the bridge. The limestones are g r e y and are classified as ' d o l o m i t i c ' (containing .1 p r o p o r t i o n of magnesium carbonate as w e l l as c a l c i u m I .n lion.He) and numerous fossils can be seen on the smooth surface, particularly the columnals of ancient sea lilies called crinoids. In I ' l l ' rs, corals can be seen and these are useful in i d e n t i f y i n g the limestones as being of Carboniferous age. I hr geological sketch (Map 5) shows t h a t the Beachley Point l " I I I I I . I I I . I is covered w i t h sands and gravels t o a depth o f some iiiy ilirce feet. These are o f recent geological age (Pleistocene) md were probably related to glacial and i n t e r g l a c i a l periods w h e n i In Severn f l o w e d in other areas and at higher levels. II i . i n . therefore, be seen t h a t the Beachley anchorage concrete i ' i i had to be dug d o w n t h r o u g h these soft sands and gravels in I ' l l to reach the basement rocks. In fact, the steel tubes here go down .some s i x t y feet, and getting t h e m i n t o p o s i t i o n presented no problems for the engineers as the site was on d r y l a n d . M.i11 'i also shows t h a t the areas of limestones have i n t e r v e n i n g is o f the Keuper M a r l , the same r o c k as seen at the base o f

38


the A u s t Cliffs on the opposite shore. These red rocks f o r m cliffs in a f e w places near Beachley Point. A f t e r the Carboniferous limestones w e r e l a i d d o w n some 300 m i l l i o n years ago t h e y became folded, eroded and f o r m e d an ancient l a n d surface in the p e r i o d of the Triassic deserts 200 m i l l i o n years ago. The ancient valleys t r e n d i n g i n a N W d i r e c t i o n were the first t o b e filled u p w i t h Triassic sedimentsthe Keuper M a r l . This presented a p r o b l e m to the engineers w h o bored d o w n f o r the site of the Beachley pier to support the great steel t o w e r . A f t e r going d o w n t h i r t y - f o u r feet, some h a r d mudstones w e r e encountered, b u t n o t the good h a r d limestones. Nevertheless, t h e y p r o v e d to be Carboniferous rocks by the nature of the fossils f o u n d in t h e m and the engineers considered t h a t these steeply-dipping beds presented a strong enough structure f o r the Beachley steel t o w e r f o r the suspension o f the bridge. N o d o u b t these mudstones w i l l p r o v e t o be sufficiently strong b u t i t i s the w r i t e r ' s o p i n i o n t h a t the b o r i n g should have gone even deeper in order to get d o w n to the greater security of limestone rocks. Even the m o n k s in the M i d d l e Ages recognised the greater security presented by using limestone as a f o u n d a t i o n , and there is a r u i n e d medieval chapel, St T w r o g , on t h e limestone r o c k k n o w n as Chapel Rock. I n c i d e n t a l l y , it is r a t h e r r i s k y t r y i n g to reach this p a r t i c u l a r r o c k , as there is a deep channel i n one o f the fissures w h i c h soon becomes impassable w i t h t h e i n c o m i n g tide. Even in m i d w i n t e r , q u i t e a pleasant afternoon can be spent here in one of the m i n i a t u r e coves collecting calcite crystals f r o m the limestones, e x a m i n i n g the s m a l l anticlines and various structures exposed on the foreshore. It is also quite a good place to demonstrate h o w limestones can be folded, f o r it is most difficult to imagine this w h e n m e r e l y c o n f r o n t e d w i t h a h a r d l u m p o f r o c k i n the hand. T h e sketch in Figure 15 shows the Carboniferous limestones p l u n g i n g a w a y under the Severn.T H E AUST C L I F F S I D E O F T H E S E V E R N

These cliffs are visited by geologists f r o m a l l over B r i t a i n because the best display of Rhaetic rocks in the w h o l e c o u n t r y lies t o w a r d s the t o p of the cliffs and contains the h i g h l y fossiliferous r e p t i l e 'bone bed'. The cliffs reach a height of over 140 feet and are m a i n l y com-


39

PIO. IB

Sm3S"ff- down tnroiyh T R I A S and penetrating steeply dipping

Car bor> if erow.s

LirnesTone

PIER

tests an tne limestone 0/Ulversfone

ANCHORAGE

rests oaAust RocK

Carboniferous HvLcLstones

RocK Only 4-ftJown

10 ft down j*issurei limestone.

FIG. 1 9

THE E N G I N E E R I N G G E O L O G Y O F T H E S E V E R N B R I D G E T H E AUST C L I F F SUCCESSION

The f o l l o w i n g is a list of the strata in the order in w h i c h t h e y can be observed f r o m the top o f A u s t Cliff d o w n to the base.LOWER LIAS

UPPER RHAETIC

= shales and t h i n bedded limestones = C o t h a m Marble, 6 in t h i c k = y e l l o w clay w i t h limestone bands, about 4 ft t h i c k = g r e y argillaceous limestone, 2y ft t h i c k = y e l l o w thinly-bedded argillaceous limestone, 4 ft t h i c k = greenish black shales, 1 ft t h i c k = h a r d grey limestone called the U p p e r Pecten bed, 1 ft t h i c k2

T H E S E V E R N BRIDGE LOWER RHAETIC

45

IIUASSIC ROCKS

= b l a c k shales, 8 ft t h i c k = h a r d p y r i t i s e d limestone called the L o w e r Pecten bed, a f e w inches t h i c k = h a r d fissile paper shale, about 8 in t h i c k = the famous 'bone bed', 1 to 4 in t h i c k = G r e y or Tea Green Sandy M a r l , 3 ft t h i c k = h a r d sandy bed, 1 f t t h i c k = Grey or Tea Green Sandy M a r l , 18 ft t h i c k = R e d Marl, 52 ft thick = g y p s u m series, 25 ft t h i c k = R e d Sandy M a r l , 2 0 f t t h i c k

The red beds of the Triassic rocks have n o t revealed any fossils, but the w r i t e r has f o u n d leaf i m p r i n t s in the Keuper M a r l of W.iinlode Cliff, near T e w k e s b u r y . Of the fossil invertebrates f o u n d in the Rhaetic and L o w e r Lias beds, bivalves are the most c o m m o n . Next come arthropods and a f e w p l a n t remains. For the identification of fossils, the best b o o k is Mesozoic Fossils, published by the B r i t i s h Museum of N a t u r a l H i s t o r y , p r i c e 12s 6d.

CHAPTER

5

The

Severn

Bore

and

Hock

Cliff

The one t h i n g t h a t almost everyone k n o w s about the River Severnand often i t i s a l l t h e y d o k n o w about i t i s t h a t i t has a bore. Just w h a t a bore is and w h y the Severn has one w h e n other rivers, i n c l u d i n g even the m i g h t y Thames, do n o t , is less w e l l k n o w n , t h o u g h i t can b e s i m p l y explained. The behaviour o f a r i v e r i s determined b y the n a t u r e o f the rocks i t flows over and between, and the Thames flows over v e r y different rocks f r o m those of t h e Severn. In its t i d a l part, t h e Thames flows over a clay p l a i n w i t h n o h a r d bands o f r o c k t o complicate its manner o f f l o w i n g . The River Severn, too, flows over and alongside a t y p e of clay b u t in t h a t clay and above and b e l o w it are m a n y other bands of different and sometimes harder rocks. Its passage is f u r t h e r complicated, and n a v i g a t i o n u p o n i t made t h e m o r e hazardous, b y h a r d bands o f limestone w h i c h o u t c r o p t h r o u g h the clay, as w e l l as by s h i f t i n g shoals o f sands w h i c h occur i n m a n y o f the estuarine stretches o f the r i v e r . In fact, the Severn p r o b a b l y has m o r e sands and muds t h a n the Thames because the Severn Basin is in softer sedimentary rocks and also receives a h e a v y run-off f r o m the h i g h r a i n f a l l of t h e Welsh mountains. Let us n o w take a closer l o o k at the rocks w h i c h h o l d the 'secret' of the Severn's bore. H o c k Cliff, some five or six miles up-river f r o m Sharpness, is the best place to observe t h e m . Here, near Fretheme, t h e cliffs are of L o w e r Lias clay, and remember that, to a geologist, clays, sands and gravels are 'rocks', just as m u c h as m o r e obvious rocks such as limestone and granite. Remember, too, t h a t the L o w e r Lias is the oldest band of the rocks of the Jurassic system and was l a i d d o w n after the rocks of the Triassic systemso t h a t the rocks at H o c k Cliff, representing a p e r i o d a p p r o x i m a t e l y 170,000,000 years ago, are younger t h a n those of A u s t . H o c k Cliff is o f outstanding geological interest because i t is rare t o find a cliff or quarry-face o f clay w i t h the bands o f strata so c l e a r l y visible. U s u a l l y , c l a y either crumbles a w a y or is q u i c k l y

T H E S E V E R N BORE AND HOCK C L I F F

47

grassed over. We have already learned t h a t clays denote a m u d d y sea, rather deep, b u t i n the case o f H o c k Cliff i t is reasonable t o suppose t h a t the sea there was n o t as deep as in the area of N o r t h Gloucestershire and near Evesham. There the L o w e r Lias clays are 960 feet t h i c k , whereas at H o c k Cliff t h e y are o n l y between 200 t o 500 feet t h i c k . One s h o u l d n o t i n f e r f r o m this, however, t h a t a t h i c k

FIG. 20

I k Hie- Umestene5 aboVCIt i 5 Concrete- made by NATUPyt. "

ExpuANATrOK

OF GULLS.

GULLS AND THE KING A N D QUEEN ROCKS

F I G . 52

'^s^H

wife * ^' Kinj and- Queen rocife eo/ "WestirAOjncotfc

BREDON H I L L

87

e a r t h w o r k and i t m a y w e l l have been t h a t the inhabitants o f this f o r t used the slabs as convenient places f o r assembly. A f t e r s t u d y i n g the K i n g and Queen Stones, the o p p o r t u n i t y should be taken to examine the disused quarries nearby. Between five and ten feet d o w n f r o m the t o p of each q u a r r y can be seen curious c i r c u l a r patterns of r o c k fragments, p r o b a b l y caused by convect i o n a l r o t a r y movements o f r o c k d u r i n g the freeze-thaw conditions of the last Ice Age.

FIG. 53

Tntjc curious structures can, be Seen in many Cotsu/old Under imtensiye- frosts.

* ~

r

above. W e t t m n c o t c , near ttie. K i n } ami. Queen rocKs.

Structures obs*f\/eeL in a, auaYry

SPRINGS AND V I L L A G E S ON BREDON H I L L

M u c h o f the r a i n f a l l o n the h i l l sinks t h r o u g h the fissures i n the limestone to reach the U p p e r Lias clay and, being unable to penetrate the clay, it t h e n travels southwards along its surface e v e n t u a l l y to be released at numerous springs along the southern edge of Bredon H i l l . O b v i o u s l y , the villages along there o w e their o r i g i n t o the existence of these never-failing w a t e r resources. A good place to see the springs in a c t i o n is in O v e r b u r y Park because here the U p p e r Lias c l a y floors the v a l l e y . M a n y springs also issue f r o m the M i d d l e Lias either just b e l o w the Marlstone rock-bed or at the j u n c t i o n w i t h the sandy beds of the M i d d l e Lias (here about 200 feet t h i c k ) and the U p p e r Lias clay. Such springs can be seen near E l m l e y Castle and at A s h t o n - u n d e r - H i l l .

CHAPTER

11

Leckhampton

Hill

Queen of the deserted quarries of the N o r t h Cotswolds is u n d o u b t e d l y t h a t a t L e c k h a m p t o n , t w o miles south o f Cheltenham and n o w o w n e d b y Cheltenham Borough C o u n c i l , w h o acquired L e c k h a m p t o n H i l l as a social a m e n i t y for the c i t y in 1928. Officially opened in 1793 and the source of m u c h of the stone used in the b u i l d i n g of Regency C h e l t e n h a m i n 1810 blocks of dressed stone cost o n l y Id per t o n d e l i v e r e d ! L e c k h a m p t o n Q u a r r y t o d a y attracts student geologists f r o m a l l over the c o u n t r y , m a n y of w h o m t r a v e l l o n g distances at week-ends to see the dramatic exposures it offers of w h a t m i g h t be described as 'the innards' of the Cotswolds. A p p r o a c h i n g L e c k h a m p t o n Q u a r r y f r o m Cheltenham, the first steep p a r t of the road is w h e r e the Marlstone shelf of the M i d d l e Lias begins. A large b u i l d i n g , H i l l House, is perched snugly o n this

Bl-OCK -PMURAK OF l_ec.KHAMPTOfl CJuARRY looKvwj West#

LECKHAMPTON H I L L

89

shelf and the road takes advantage of a s m a l l gap in the rock-bed to c l i m b u p t o the Cotswolds. The official entrance to the q u a r r y is at the car p a r k in Daisy Bank Road and here the steep slopes on the l o w e r sandy beds of the U p p e r Lias are planted w i t h larch trees to stabilise the soil. Here, too, the g r o u n d is everywhere h u m m o c k y , the result of the s l i p p i n g d o w n h i l l of the U p p e r Lias sands and blocks of limestone over the U p p e r Lias clay w h i c h acts as a l u b r i c a t e d surface. F r o m this p o i n t the course of an o l d r a i l w a y line can be f o l l o w e d to the ruins of the o l d lime-kilns w h i c h w e r e in use as late as 1927 (see Figure 56). F r o m the k i l n s , the limestone cliffs rise to impressive heights, a t t a i n i n g 965 feet at their highest p o i n t . F r o m top to base there is a grading of colour f r o m cream to russet b r o w n , i n d i c a t i n g t h a t the rocks become m o r e ferruginous towards the base of the cliff.

SpringBiaoUP

5KET0H rAfiP Of T H E CrECH-OGY OP LECKHAIAPTOM H I L L (ffjuret v% WjttS ftdty

LECKHAMPTON H I L L

91

T H E PEA GRIT

Just b e h i n d the o l d k i l n s are the rather c r u m b l y cliffs made of Pea G r i t , a r o c k composed of o v o i d a l bodies about the size of a pea. This structure is k n o w n as ' p i s o l i t i c ' , and the pisoliths are s i m i l a r to the egg-shaped ooliths in the O o l i t i c limestone above b u t larger. T h e i r o r i g i n is n o t f u l l y understood, b u t one t h e o r y is t h a t t h e y have been f o r m e d by vigorous agitation of nuclei (small grains) in a sea w h i c h was a c t i v e l y p r e c i p i t a t i n g c a l c i u m carbonate. 'Cave pearls' are believed to have been f o r m e d in this w a y . An alternative t h e o r y is t h a t the pisoliths have been f o r m e d by calcareous algae a r o u n d a nucleus. The name Girvanella pisolitica has been applied to this t y p e of algae and recently it has been discovered t h a t if pisoliths are treated w i t h acid, the algal filaments can be seen s t i l l preserved.

S?AGSTONS

ao{4

UPPER. FReEVTONE.

3oft \ OOUlTIC MARL. J 1 Jt 0LOWER, FREESTONE

75-ft

Lfwev t> m est < > *indicate.? tht. *fcSO*oc

L.ECKRAMPTDN

C j u A R X Y FAC .

FIG. 56

92


Fragments of crinoids, corals, echinoids and brachiopods can be f o u n d in this Pea G r i t , w h i c h is here about t h i r t y - t w o feet t h i c k and can b e traced a l l along the h i l l l o w d o w n , b e l o w the Devil's C h i m n e y and a w a y over t o C r i c k l e y H i l l . It is a limestone w h i c h has been f o r m e d in a sea in w h i c h c a l c i u m carbonate was being precipitated, a n d i r o n in the sea w a t e r was p r o b a b l y being precipitated by bacteria. B u t it is n o t a good b u i l d i n g stonefrost a c t i o n can m a k e it disintegrate, the pisoliths f a l l i n g apart f r o m each other.THE FREESTONE

The limestone cliffs above the Pea G r i t belong to the I n f e r i o r O o l i t e d i v i s i o n of the M i d d l e Jurassic system. T h e i r most s t r i k i n g feature is the rather regular spacing of the h o r i z o n t a l bedding planes and the j o i n t s at r i g h t angles to t h e m , g i v i n g a general appearance o f man-made w a l l s . I n fact, this k i n d o f r o c k i s described b y some geologists as ' m u r a l j o i n t i n g ' . A bedding plane is produced by a pause in t h e process of sedimentation on a sea floor. It can also occur if there is a change i n the t y p e o f sediment b e i n g l a i d d o w n . W h e r e there i s a t h i c k mass of r o c k , sedimentation has o b v i o u s l y been continuous f o r a long time. W h e n the bedding planes are far apart the r o c k is called Freestone because n e a t l y rectangular slabs can be r e m o v e d 'freely' f r o m the q u a r r y face, and these m a k e a good b u i l d i n g stone f o r the t y p e of classical architecture f o u n d in Regency Cheltenham. A n o t h e r school o f t h o u g h t , however, holds t h a t the t e r m 'Freestone' derives f r o m the fact t h a t this p a r t i c u l a r stone is so devoid of large fossils and of such an even, fine-grained t e x t u r e t h a t it can be freely sawn i n t o blocks. If the bedding planes are close together, creating t h i n wedges of r o c k , t h e rocks are called Ragstones and these can be seen r i g h t at the t o p o f the q u a r r y . M o r e often, h o w e v e r , the Ragstones are bedded h a r d limestones (often ferruginous) w h i c h break up i r r e g u l a r l y and are w e l l stocked w i t h fossils. I n the past t h e y w e r e extensively quarried f o r l o c a l r o a d m e t a l and at the t o p of the q u a r r y there are m a n y h o l l o w s and banks i n d i c a t i n g w h e r e q u a r r y i n g t o o k place l o n g ago. Hundreds o f people come t o L e c k h a m p t o n Q u a r r y equipped w i t h hammers ready to k n o c k o u t fossils in the Freestoneand t h e y are

LECKHAMPTON FIG. 57 AWUVEJ/ CrEOLQGY OF I N F E R I O R OOLiTE

HILL

93Use f r>)fu'te Grit

-RAGSTONE? - used maid Heo-Cotfjic

dfy sW Wcdlirx,.

W l d i n ^ s (e.a. Sr "Paul's

'College, Cberfenharo). FRESTOHES~ed i'r> GceeH RaviynJ and Crcofgi&o kutdings.

Sf PAULS ccTtcegB';

DRY STONE WALLING-

FREESTONE'

Cr*o* bedding of Carre ntr \x rrvon The 5u|let qwarn Hidsummer Hill Holly .bushQuarry Hill IfcJ Cambrian "Pa*S.1

"Rdcqed. stone

Area coloured- blacfc' or cross hatching denotes Tre-Cavr>trian rooKs. "Tfie Maiv/ervis before the bio saueete

Chase E n d Hill

The^MaJye/ns after the squeeze,

T H E SOUTHERN M A L V E R N S

171

Figure 123 shows h o w Chase End H i l l has been offset f r o m the general n o r t h / s o u t h a l i g n m e n t b u t the t h r u s t i n g has n o t been as pronounced as at Herefordshire Beacon. Compare this w i t h the plans in Figure 124. F r o m the s u m m i t o f Chase End H i l l ( m a i n l y consisting o f h o r n blende gneiss) i t i s w o r t h n o t i c i n g t h a t the m a i n alignment o f t h e M a l v e r n range disappears, and then reappears near the Forest of Dean i n the s u m m i t o f M a y H i l l , w h i c h is a resurgence o f the 'thrust'.CAMBRIAN ROCKS OF T H E SOUTH M A L V E R N S

On the western side of the Malverns f o r about five miles the Silurian rocks o u t c r o p b u t b y the G u l l e t Q u a r r y and southwards the Cambrian series of rocks come to t h e surface. The basal conglomerate is a v e r y h a r d r o c k c o n t a i n i n g pebbles of the Pre-Cambrian and it can be seen in the t r a c k leading to t h e Obelisk f r o m the G u l l e t Q u a r r y . This is one of the proofs t h a t M a l v e r n i a n rocks are Pre-Cambrian because this conglomerate represents the beach in the Cambrian sea w h i c h washed a r o u n d the range o f mountains w h i c h , even a t t h a t t i m e , was v e r y ancient. N e x t comes the H o l l y b u s h sandstone, rather greenish w i t h the c o l o u r i n g of the m i n e r a l glauconite, and this can be seen in a s m a l l q u a r r y by W h i t e Leaved Oak w h e r e there is a gap between Ragged Stone H i l l and Chase End H i l l . As this gap has the H o l l y b u s h sandstone o u t c r o p p i n g , it c o u l d be called the C a m b r i a n Gap to parallel w i t h the Silurian gap further n o r t h . The oblique strips of Silurian sandstone o c c u p y i n g t h e Silurian gap and those o c c u p y i n g the C a m b r i a n gap are related to the set of oblique N W / S E faults w h i c h affected the M a l v e r n range at a m u c h later t i m e t h a n the 'great squeeze'.T H E BRONSIL S H A L E S

On the western side of the Malverns, by Bronsil Castle, is an area , of Cambrian shales w h i c h , being softer rocks, f o r m valleys. These shales are d i v i d e d i n t o the W h i t e Leaved Oak shales ( w h i c h are black) and the Bronsil shales ( w h i c h are grey). Figure 124 shows the o u t c r o p of the C a m b r i a n rocks and, as the H o l l y b u s h sandstone o n l y outcrops over a small area, it can be taken f o r granted t h a t most of the area m a r k e d C a m b r i a n r o c k means t h e presence of shales.

172


Figures 126 and 127 show t h a t the shales are d i p p i n g v e r y steeply and t h a t magma ( m o l t e n r o c k ) has been i n t r u d e d i n t o t h e m . These igneous intrusions f o r m h a r d outcrops so t h e y can easily be recognised as s m a l l knolls c r o w n e d w i t h trees. Here again, nature has d r a w n a convenient geological m a p . L o o k i n g across the W h i t e Leaved Oak v a l l e y in the shales at least half-a-dozen of these k n o l l s can be seen, some about t w e n t y acres in e x t e n t and others m e r e l y a f e w acres. Figure 128 shows a small igneous o u t c r o p standing o u t l i k e a castle in t h e Bronsil area (and previously the site of a medieval castle). The ruins of the castle are n o w occupied by a prosperous f r u i t f a r m , the farmer h a v i n g the advantage o f r i c h soils derived b o t h f r o m the shales and the igneous rocksthe latter, being m o s t l y basic rocks, are r i c h in i r o n . Figure 128 also shows h o w the d o m i n a n t t o p o g r a p h i c a l feature


173

"UetWee-n. CKo.se, E n d H i l l and. High \Jood.. A Woode-d Knoll o igneou* r o c K overshadowing the v a l l e y is the plateau f o r m e d of the massive M a y H i l l sandstone o f the Silurian. I n the Bronsil Castle area the plateau slopes d o w n t o Eastnor Park, t o b e f o l l o w e d f u r t h e r westwards b y the scarps of the W e n l o c k limestone and A y m e s t r y limestone. I s i t possible t o t e l l w h a t k i n d o f rocks are u n d e r g r o u n d w i t h o u t going t h r o u g h the v e r y expensive process o f b o r i n g d o w n i n t o

An igneous intrusion in the t o n s i l Shales. Clipper Cambrian) A prosperous fruit furvn with abundant Water supply is Sift-d on the "Brans!! Shales, at Sronsii C a t i e . . 'S

174


them? The answer isyes. Structures can be revealed and it is possible to decide w h e t h e r rocks are l i g h t or heavy by w h a t is k n o w n as a ' g r a v i t y survey'. G r a v i t y i s a force w h i c h varies w i t h differences i n l a t i t u d e and variations i n t o p o g r a p h y and the densities o f the rocks under the surface. If allowances are made f o r a l l these factors t h e n the last one, i.e. the densities of the rocks, can r e a d i l y be deduced. A g r a v i t y survey of the M a l v e r n s area was carried o u t in 1954. Before it began, careful l a b o r a t o r y checks w e r e made on t h e densities of the t y p i c a l rocks of the region and the subsequent field w o r k o f the survey covered the plains o f Worcestershire and extended on the east as far as the Cotswolds and on the west as far as W a l e s . The f o l l o w i n g w e r e some o f t h e conclusions reached d u r i n g the survey: 1. The Triassic rocks in the Worcestershire basin are about 5,000 feet t h i c k . 2. There is a pronounced 'step' going d o w n steeply at 4 5 t o w a r d s t h e p l a i n o f the Trias. 3. A f t e r the gravity-meter observations w e r e p l o t t e d on a m a p t h e y revealed the great scale of the M a l v e r n upheaval. 4. The l i g h t rocks lie on the heavier ones and it can be assumed t h a t the 'floor' of the Worcestershire basin u n d e r l y i n g the Trias consists of Palaeozoic r o c k s b u t w h e t h e r these are Silurian or C a m b r i a n i t i s n o t possible t o decide. Figure 129 shows t w o aspects of the results of the g r a v i t y survey t h e tremendous thickness of the deposits of Trias close to the Malverns and the Pre-Cambrian r o c k rising l i k e a w a l l f r o m t h e plains o f the Trias, latter-day evidence o f t h a t v i o l e n t upheaval w h i c h o c c u r r e d s o m a n y m i l l i o n s o f years ago.T H E ORIGINS O F T H E M A L V E R N S

Ideas about the Malverns are c o n s t a n t l y changing as n e w exposures reveal n e w clues about t h e i r o r i g i n . Sometimes these clues are quite d r a m a t i c l i k e those b r o u g h t t o l i g h t i n the nineteenth c e n t u r y w h e n the r a i l w a y t u n n e l was constructed a t the W y c h e and the c u t t i n g revealed n e w structures. The most recent c o n t r i b u t i o n t o the great M a l v e r n debate i s the e x c i t i n g n e w exposure o f the S i l u r i a n rocks d i p p i n g off t h e Pre-Cambrian rocks i n the G u l l e t Quarry.


175

The sequence of diagrams in Figure 130 shows t h a t the ancient M a l v e r n range sank beneath the S i l u r i a n sea and the general assumption is t h a t the Silurian rocks once covered the Malverns. Some geologists have f o u n d fragments ('clitter') of M a y h i l l sandstone (Silurian) on t o p of Herefordshire Beacon and there is other evidence w h i c h supports the v i e w p u t f o r w a r d i n Figure 130. A f t e r the submersion of the Malverns in the Silurian sea there Worcestershire Beacon1,384- ft FiC. 129

l^OOO -to

Coo million years old.LIKE A WALL

TRiAS'ltc ROCKSZoo vmltian years o l d FROM THE MIDLANDS

THE MALVERNS RISE

came the 'great squeeze' f r o m the south-east, an upheaval of the rocks w h i c h must have occurred even before the l a y i n g d o w n o f the Triassic rocks because these are n o t i n v o l v e d in the folds. The Palaeozoic rocks are folded, however, so the folds m u s t have o c c u r r e d at some t i m e t o w a r d s the end of the Carboniferous p e r i o d . In adjacent areas, the later Carboniferous rocks are folded and this is s u p p o r t i n g evidence. It is believed t h a t the 'great squeeze' produced a single m o n o c l i n a l f o l d w i t h a steep l i m b t o the west and a flat l i m b t o the east (see Figure 130[b]). D u r i n g the squeeze, the mass of Pre-Cambrian rocks f o r m i n g Herefordshire Beacon w a s pushed over on to the t o p of the Silurian strata. D u r i n g the b o r i n g o f a w e l l near the B r i t i s h Camp H o t e l

176


p r o o f was f o u n d t h a t Silurian rocks are underneath the Beacon H i l l mass, and along the western edge it has been f o u n d t h a t t h e Silurian rocks close to the Malverns are overturned. A f t e r the f o l d i n g effect of the great squeeze, the Pre-Cambrian rocks fractured and w e r e u p t h r u s t i n t o blocks along a general n o r t h / s o u t h l i n e o f dislocation w i t h oblique faults (the cause o f the present-day gaps) setting in later. Figure 130(d) shows h o w t h e M a l v e r n range stood o u t as an island m o u n t a i n (inselberg) in the vast Triassic desert.FIG. 130 Covered the

The "Silurian Sea

ancient MalvernRange (based oglomeraie formed )

, -

Joroe folded- tti rocKs"into ou Vnonoc-Unestraetare TVie- Cheltenham0>we"

A

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Geology in Severn Vale and Cotswolds