Whitsun field meeting, 1936. The Lyme Regis district

30r

WHITSUN FIELD MEETING, 1936. THE LYME REGIS DISTRICT. Report by the Directors: W. D. LANG, M.A., Sc.D., F.R.S., and H. DIGHTON Tnoxxs, M.A., Ph.D.

Saturday, May 30th, 1936. MAKING an early start, the party travelled the short distance to Morcombelake by motor-cars. The lane that leads to the chert-pits on Hardown Hill was followed till fossiliferous sections in the Foxmould (Upper Greensand) were reached. Here it was pointed out that the Albian horizons between the Chert Beds and the Gault (which is not exposed at Morcombelake) are represented by soft brownish-yellow sands, the Foxmould, which yield silicified fossils in places--usually just below the cherty drift-but contain no Cowstones, such as are found on Black Ven, a cliff which could be seen three miles away to the west-south-west across the mouth of the Char. After a search for fossils, of which Exogyra conic a Sow. and broken pieces of Neithea quadricosiata Sow. were the commonest, the ascent was continued to the chert-pits. The cherts were seen to be extensivelv fractured and intimately associated with contorted sandy layers-one such bed was very prominent near the middle of the working-face (PI. 32, A). It was pointed out that, since to the west the Upper Greensand contains numerous thin beds of cherts interbedded with glauconitic sands, the extensive fracturing of the Hardown Hill cherts can be explained by the washing-out of the soft sandy beds by percolating water, with resultant collapse of the resistant cherts and contortion of what intervening sand remains. The whitish weathering of the cherts, which often also produces a pinkish-mauve chalcedonic , rind,' was demonstrated, as well as the fact that much of the chert consists of sponge spicules. The party then made their way on to the plateau above, to a vantage-point whence the surrounding panorama could be viewed, with the Isle of Portland and the Fleet to the eastsouth-east, the Chalk hills (with the Hardy Monument) stretching northwards on the eastern horizon, the nearer storm-swept hills of Pilsdon and Lewesdon to the north, and the Devonshire plateau country and coast beyond Lyme Regis to the west. Below, to the north, lay the Vale of Marshwood, with the valleys of several streams clearly visible; while, to the northwest, the steep-sided Char corridor which opens from it appeared as a deep cleft separating Conegar Hill and its spurs from Hardown Hill. Below, too, lay the Ryall and Morcombelake gaps, while a short distance away, and aligned with the upper part of the valley of St. Gabriel's Water above the easily discernible

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W . D . LANG AND H . DIGHTON THOMAS,

elbow-bend, the deep -not ch dividing Golden Cap from Langdon Hill broke th e sky -line. Thus, spread out before t he pa rty, lay the evidence for the extensive rive r-capture that has ta ken place since the dr ain age was initiated on the gent ly folded postCretaceous peneplane, vesti ges of which remain in the t ops of Golden Cap, Pilsdon, etc . An account of the origin of the Char ri ver-system was given in spite of t he difficulties du e t o a strong , gusty wind, which pr esaged the sha rp sto rm that overtook the part y as they began th e descent t o Morcombel ak e. F rom Morcomb elak e the western side of the St . Gabriel's Valley was followed to it s mouth, excep t for a shor t diversion at St. Gabriel's Farm , sto ps being made a t the Saint's Well and at one or two other points t o demonstrate the waterseepage and the ph ysical feature at the base of the Cret aceous. Th e latter was clearly seen ac ross the valley on the slopes of Golden Cap , where the Cretaceous overstep results in the Gault rest ing on the Middle Lias, of which , however, abo ut 300 ft. are developed there. Th e surface features of the Middle Lias and of the Green Ammonit e Beds were also indic ated. On reaching the beach at St. Gabriel' s Mouth , th e party first exa mined the coast for abo ut i-mile to the eas t. Here the Green Ammonite Bed s" and th e highest bed s of the Belemni te Marls were seen t o form a slight ant icline with its a xis at a bout half-wa y to Golden Cap, so that beds were followed eastwards in descending sequence. At St. Gabriel' s Mouth the base of t he cliff was seen to be at the h orizon 1 2 3 d , a nd t he six feet of sma ll red-nodules (123 m) were visible at about 6 ft. higher, with the Red Band (1 2 6) a t about 2 5 ft. from the beach . In following the cliff-foot eastwards, the lowest 2 5 ft. of the Green Amm onit e Beds were passed bed by bed , including th e Lower Limest one (123 a) an d the two nodule -beds 1 2 2 d and tzz f. Th e body-chambers of Green Ammonites were collect ed at va rious levels. Meanwhile th e platfo rm mad e by the Belemnite St one (1 21 ) had risen across t he beach and begun to mount the cliffface (Plate 32, B). Its surface was seen to be crowded with ammonites (Lytoceras , B ean iceras, L iparoceras) and other fossils, especially Inoceram us ventricosu s. The Crumbly Bed and P yritic Marls (120) formed th e rise of this little ste p , a nd showed a rich fauna of belemnites (especially in 120 c) and py ritiz ed ammonite s of the group of A canthopleuroceras actaeon. The ti de being high, only the highest part of bed II9 was shown, with ammonit es of the A. ualdani-maugenesti group. Returning to St. Gabriel's Mouth, the party walked westwar ds to the Ridge F ault , where they stopped for lunch. On the way the Green Ammonit e Bed sequence was followed from bed 123 d upwards, until the Red Band descend ed t o the I W , D . Lang, "Tbe Green Ammonite Bed s of tb e Dorset Lias," Qllart. [ ourn, Grot. Soc., vol, xcii, 1936, pp , 423 -3 7, pl. xxxii,

PROC. GEOL. Assoc., VOL. XLVII. (I936).

PLATE 32

Photo by Jr. E. Howarth. A.-ALBIAN CHERT-BEDS,

Top

OF HARDOWN HILL.

Photo by L. Bairstou. B.-BELEMNITE MARLS AND GREEN AMMONITE ST. GABRIEL'S MOUTH.

BEDS, EAST OF

[To jure p. 302.

WHITSUN FIELD MEETING, 1936.

beach just before the Ridge Fault. The fault was better exposed than at any time during the pa st 35 years. It is now seen that, instead of consisting of two closely placed step-fault s, each letting down the beds some 50 ft. on the east, as was formerly supposed, the two faults cross each other at nearly a right angle. The second fault strikes obliquely across the cliff to the east and th e foreshore to th e west, and leaves a little patch of bed 123 m of the Green Ammonite Bed s showing on the cliff in the angle between it self a nd the north-and-south fault. On the cliff immediately eas t of the oblique fault, bed 1 27 with Oisioceras is seen t o overlie th e Red Band (12 6 ) in normal sequence. After lun ch the succession in the lower half of th e Belemnite Marls was considered. The Belemnite Stone (121) west of the Ridge F ault caps the cliff, whose base lies in bed III (zone of Polymorphiies polymorphus) . The dip is still westwards, but owing to three small st ep-fault s throwing up the beds on the western side, bed III is as often brought up to cliff-level. The axis of the syncline was reached soon after the waterfall of Ridge Water was passed, and the beds were seen to resume their normal eastward dip. After Westhay Water, bed III rises on to the cliff, and for about a t -mile bed IIO forms the cliff-base. In this bed the long guards of the belemnite Pseudohasiites were noticeable, as well as large Inoceramus and, occasionally, fragments of the zonal ammonite Tetraspidoceras. The lowest beds of the Belemnite Marls, I OS-I09, were well exposed on the foreshore and at the cliff-foot, and the zonal ammonite Polymorphites per egrinu s was discovered in bed 108. Lar ge specimens of Apoderoceras were conspicuous on the surface of bed 105, which formed a marked reef on the fore-shore, while, on the wavewash ed clay of bed 102 below, many partially pyritized Echioceras were very noticeable. Here, too , Hummocky Lime stone (103), lyin g a few inches below 105, could be examined ; but more effectively where it was seen rising on t o the cliff-ba se. There, in one or two pla ces, the cliff was undercut enough for several of the party, by inserting their heads, to see Echioceras sticking t o, and even penetrating, the lower surface . Since the ammonite Epideroceras ha s been found in the upper portion of the limest one; and since the Echioceras, which occurs in it s under parts, is of the rari costaioides gr oup , a nd two Echioceras zones are found elsewhere above E. raricostatoides ; it is a lmost certain that a non-sequence occurs within Hummocky. The next bed to prove of interest was the Watch Ammonite Stone (99), which was found in place at the cliff-foot, and its horizon excavated by the members, who soon found pyritized specimens of Echioc eras as well as a specimen of the oyster, Gryphiea, exactl y lik e the familiar G. incurua of the Blue Lias. This oyster occurs const antly at this one horizon, and has been commonly found associated with E chioceras in the

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D. LANG AND H. DIGHTON THOMAS,

Vale ofMarshwood, but as yet has occurred in no other stratum in this district above bed 72-the Pararnioceras alcinoe bedin the Shales-with-Beef, nor below this until the Blue Lias is reached. Passing westwards and downwards in the Black Mar] sequence, the party noticed bed 95, of little nodules resembling whetstones; and then the Oxynoticeras lymense bed (92) was examined, but the zonal ammonite was not found. The Coinstone (89) was noticed, with its eroded surface indicating a nonsequence above the zone of Asteroceras stellare ; the large and regular stellaris nodules (88) and the Limestone-with-brachiopods (87) were passed and examined in turn; and next the paper-shales with Flat-Stones (83) containing the well-known specimens of Asteroceras obtusum, which, from the rich brown-coloured calcite filling their chambers, have, with Microdcroceras birchi, been locally known as "tortoise ammonites." Below the Flatstones, the Pavior (82) was noticed with the curious stellate nodules below it; and the Lower Cement Bed (80), some IS ft. below the Pavior, was seen to rise on to the cliff and there mark a low anticline-the first of a series of folds of increasing intensity which, as the fault in the Char valley is approached, throw the beds into contortions (better seen on the western side of the fault, PI. 34, B). On the talus-slopes beneath the Lower Cement Bed lay numbers of fallen blocks of the Limestone-with-brachiopods (87) with numerous specimens of a Rhynchonellid, and the party spent some time in breaking up the blocks and collecting the contained fossils. The comparatively barren shales (77-79) of the zone of Promicroceras capricornoides lying beneath the Lower Cement Bed (80) form the broken cliff between this point and Charmouth beach; the zone of Arietites turneri (76) below it being covered by the beach, and the Birchi-bed (75-76a) lying at extreme low-water mark beneath this point. After tea at the "Wayside Tea-rooms," near Charmouth beach, the party took the cliff-path up Stonebarrow to just below the base of the Cretaceous. Thence the more venturesome climbed down over some convenient bluffs and ledges to the narrow undercliff above the Belemnite Stone. The Red Band was located about 22 ft. above the Belemnite Stone, and a successful search made in it for Prodactylioceras mode (Quenstedt). At the next eastern bluff the Upper Limestone was seen apparently only some 18 ft. above the Red Band. It was also evident that the overstep of the Upper Cretaceous was becoming more marked, for between its base and the Green Ammonite Beds there was room for only a very small part of the Middle Lias. Further clambering brought the party to a fairly wide undercliff below a high, vertical, Upper Greensand cliff, in which the beds were seen to be of varied hue, from pale yellow to

WHITSUN FIELD MEETING,

1936.

golden brown and even pink. Near the base of this cliff, Bed 3 of the Gault was discovered, but it was difficult to extract its fossils. Fallen blocks of the overlying Greensand yielded some very good specimens of E xogyra conica Sow. ; while, on the way up the cliff, fossils were found in situ , both as casts in soft, pinkish sands, and as silicified shells. From the top of Stone barrow, the return was made along the cliff-path and Lower Sea Lane to Cha rmout h .

Sunday, May 31st, 1936. Lea ving headquarters at 9-45 a .m ., the party made the journey to Dowlands Farm by coach, noting particularly on the way the old peneplan e, which was especially well seen t o the west and north-west after the ga te s of Pinhay had been passed. The field-pat h was then followed to a point at the to p of the Dowlands Cliffs almost due south of the farm. A striking view of the coas t -line was obtained, to the east towards Portland Bill, and to the west and south-west beyond Berry Head. In the cliff-face, which borders the path leading down into the Dowlands landslip, Chalk of the T erebratulina lata zone is exposed. To the party it yielded a number of fossils, including numerous spec imens of the zonal index. Following the footpath at th e top of the cliffs westwards for some 8 0 0 yards, th e party arrived a t a point above the gre at Bindon landslip of Christmas, 1839. The great chasm (PI. 33, A), flanked by the shee r Green sand cliffs capped with Chalk, is now a riot of trees and t angled undergrowth , a vivid but pict uresque contrast to the ploughed land carried away by the catastrophe of nearly a century ago . The Director gave an a ccount of this, and qu ot ed lively extract s from G. Roberts's " An Account of, and Guide t o, the Mighty Land-slip of Dowlands and Bindon, in the Parish of Axm outh, near Lyme Regis , December 25th, 1839, " (1840). Thence the route descended directly into, and across, the chasm, by way of a path which wound pleasantly through the den sely massed vegetat ion. A pause was made to examine one of the pinnacles of Upper Greensand Chert Beds left isolated in the landslip. Formerly these were very numerous and conspicuous, but weathering has now considerably reduced th eir number and prominence. Joining the main path through the landslip , the party then turned south-westwards for 400 yards, whence a track led to the cliff-edge, over which they scrambled to the shore about halfway between Cor bin Rocks and Culverhole Point. A little to t he east a great mass of fallen Upper Green sand showed the characters of the Cher t Beds (unbroken layers of cherts interbedded with glauconit ic sands) and of the underlying greensands, with a bed full of Exogyra conica Sow. a little

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W_ D. LANG AND H. DIGHTON THOMAS,

below the junction of the two.' Some fifty yards farther to the east blackish, glauconitic, Gault clay was seen underlying grey sands with thick calcareous doggers like Cowstones.s Moving westwards the party then examined the fossiliferous Greensand blocks at Culverhole Point, which lay on the way to the Rhaetic section exposed about 200 yards beyond it. A heavy storm broke as this was reached, and lunch was taken in what shelter the cliffs could provide. Evidence of the Rhaetic Bone-bed was discovered, and the black Contorta-shales yielded their typical fossils, while a specimen of Diademopsis sp. [British Museum (Nat. Hist.), E. 31440J was obtained from the White Lias by Dr. F. Raw. The westward overstep by the Upper Cretaceous was shown in the presence of blue, sandy, Gault clay, with a thin basal pebble-bed, directly overlying these limestoness (PI. 33, B) _ In the 'slip-scarp' running inland a fine section was visible of the overlying Upper Greensand. Passing the neighbouring fault which brings up the Grey Marls to the west, the party then examined the Trias beds seen in the cliffs on the way to Axmouth Bridge, near which the Cretaceous rocks were visible resting on the Keuper Marls. Further rain necessitated a short halt there before the route was continued through Seaton to Seaton Hole, where a big fault brings the Upper Cretaceous down on the west against the Trias. Here again dark bluish-green, sandy Gault was found below the Greensand. The opportunity was taken of examining the large blocks of Cenomanian Limestone and Middle Chalk brought down by a great cliff-fall four-and-a-half weeks previously, which had considerably altered the aspect of White Cliff. Numerous fossils were obtained from the former, as well as from both the RhynchoneZla cuvieri (or Inoceramus labiatus) and Terebratulina lata zones. The return was then made to Seaton, and the coaches rejoined for the short journey to Beer. On the way evidence for the northward continuation of the Seaton Hole fault was seen at some new houses on the main road. A welcome tea at the" Dolphin Hotel," Beer, was followed by an examination of the Chalk- in the northern side of the harbour, but the tide was too high for either the East Ebb or the western side to be closely approached. Finally, the path to Annis's Knob was taken, and the Chalk of that dominating bluff successfully searched for fossils. As the party descended the path to Beer to rejoin the coaches for Charmouth, they looked down to the picturesque I The section is almost identical in its details with that given by Jnkes-Browne in " The Cretaceous Rocks of Britain. Vol. I. The Gault and Upper Greensand of England," Mem, Geol, Surv. United Kingdom, '900, p. I92 (upper half). 2 The section was like that described by Jukes-Browne, ibid., p, I90 (lower part). 3 Also seen in a very large fallen block on the shore. 4 See Rowe, A., " The Zones of the White Chalk of the English Coast. III, Devon," ProoGeol, Assoc.} vol. xviii, 1903, pp. IS-I8.

PROC.

GEOL. Assoc., VOL. XLVII. (I936).

PLATE

33.

Photo by IT'. E. Howarth. A.-THE GREAT CHASM, BINDON LANDSLIP.

Photo by W. E. Howarth. B.-GAULT OVERSTEPPING WHITE LIAS, NEAR CULVERHOLE.

[To face p. 306.

WHITSUN FIELD ~IEETING,

1936.

harbour , wit h the long white Cha lk cliff ru nning out t o t he green-topped promontory of Beer Head .

Monday, June 1st, 1936. A start was made at about 9.30 a .m ., and the party drove through Lyme to Pinh ay gat es on the Sidmouth road. Thence they walked t o P inh ay , where they were met by Maj or and Miss Allhusen, who kindl y escorted the party through their delightful grou nds, pointing out the more remarkabl e plants and othe r features of int erest ; particularl y a gro up of banana trees th at are grown out of doors th rough out the yea r in this shelt ered spot; and the hydrauli c ra ms by which the house, standing on th e Chalk at nea rly 400 ft., is supplied with water from some 150-200 ft. below. This wooded landslip-garden opens out above Pinhay ste ps on t o a sea-sca pe no less beautifula wide bay backed with nearly ver tical cliffs of Blue Lias. Here it was pointed out that, in descending to the beach, one crosses the Blue Lias stone beds from Grey Ledge (49) at the top, to the limestone H . 60 a t the cliff-base, comp rising the zones, from above, downwar ds, Coroniceras bucklandi, C. rotiforme , M~etophioceras conybeari, S chlotheim ia marmorea, A lsatites lia ssicus, A. laqueus, P silophyllites hagenowi and W aehneroceras p ortlocki . On the descent t he higher limest ones of t he series were individually recognised, severa l forming the steps themselves. From the beach it was seen that the steps lie west of a fault which throws the bed s down some 40 ft. on the west ern side; that the fault itself was hidden under vegetat ion and talus ; and that east of the fault the cliff showed the whole of the Blue Lias in sect ion , from Grey Ledge (49) to wh ere the bas e rested on some 20 ft. of White Lias (Pla te 34, A). Thus, while the beach covers the lowest bed s of the Blue Lias on the west side of the fault, on the east side it covers t he lower par t of the Wh ite Lias (horizon of the Cotham mar ble) ; while below the beach a few members foun d the Pteria contoria sha les on the east, ju xtap osed t o the Whi t e Lias on the west , of the fault. Imm ediat ely east of the fault a large portion of the cliffface, comprising horizons H . 18 to H . 42, had slippe d down to the bea ch without displacing the component beds ; a nd members were able t o see the numerous oysters in the Ostrea-beds, H. I 8-H. 24, and the zonal fossil P siloceras planorbis on t he under side of the limestone H. 26. The cliff between Pinhay steps and Seven Rock Point is rendered da ngerous by showers ' of debris which pitch over the top from time t o time. Th e beds here dip regularly t o the east , and t he very finely divided sha les interbedded with minute lamina: of beef which mark the junctio n of the Blu e with the

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D. LANG AND H. DIGHTON THOMAS,

White Lias, were soon brought low enough to be examined. As the beds of the Blue Lias dipped off the cliff and re-appeared {me above another in upward sequence on the reefs, each was examined in turn, and its (generally) meagre fauna sought for. Osirea was found abundantly in the lowest beds; Psiloceras was not again discovered, and, unfortunately,the under surface of H. 26, where this zonal fossil had been seen at Pinhay Steps, was not exposed on the reefs. Caloceras (in very poor preservation) was pointed out on the upper surface of H. 44. The next zonal ammonite, Waehneroceras portlocki, was not met with; but the large pick brought by one of the party made short work of the shales, H. 67, and soon disclosed the zonal fossil, Psilophyllites hagenoiei, but failed to turn out Alsatites laqueus from H. 71 in the zone above, which yielded only Waehneroceras sp. Alsatites liassicus was not found, but at Seven Rock Point a specimen of Schlotheimia angulata, the next zonal ammonite, was seen in place in Upper Skulls (IS), which here reaches the cliff-base. The irregular surface of Mongrel (23) was particularly noticeable, as the wide puddly reef which it forms proved treacherous to the feet, and heavily let down at least one of the party. East of Seven Rock Point the reef formed by Top Tape (29) made a wide, smooth, platform crowded with the zonal ammonite, Metophioceras conybeari and allied forms; and Third Quick (21) was noticed to be full of Gryphaea. Bed 32, Gumption Shales, was seen to mark the reversal of the dip and to lie in the axis of a syncline, so that the party now crossed the reefs in reverse order, noting again especially Top Tape (29) crowded with ammonites. The axis of the anticline was seen to lie at the Devon-Dorset boundary, and the dip again to become normal; but from the syncline, eastwards, the fore-shore was found to become more and more covered, at first with broken and tumbled blocks, which made the going laborious, and later with sand leading up to a pebble beach. Among the tumbled blocks many large Coroniceras and other fossils were noted, and at one point the fragmentary fore-parts of an ichthyosaur. There, as Major and Miss Allhusen were returning to Pinhay Bay, the Vice-President, Mr. C. W. Hobley, expressed the Members' appreciation of their kindness and proposed to them a vote of thanks, which was heartily echoed. The party scattered in Lyme for lunch, re-assembling at 2 o'clock to proceed to the cliffs and fore-shore between Lyme and Charmouth. Here they were able to contemplate the classic ground of the pioneer geologists and collectors. Here Conybeare would be found, far from his parish ofAxminster. Here Buckland would spend his Oxford vacations, and in later days take every opportunity of visiting these cliffs. To Lyme his mother brought De la Beche, while still a boy, from Char-

WHITS UN FIELD MEETING,

1936.

m outh to live ; and t o Lym e R ichard Owen would (as he put it in a letter t o his friend Clift) " t ak e a run down to mak e love to Mar y Arming ." Here, at Church Cliffs, the eccentric Thomas H awkins found scope for his ext ravagant energies. Here Lym e's schoolmaster , R obert s, found time between t enching a nd hist orical research t o observe t he beha viour of limpets on the Cobb rocks and to wri te an acco unt of the landslip of 1839. H ere, above all , was born , lived and died , Mary Arming, who , by her assiduous and intelligen t collecting and th e good use she made of her mater ial , by her warm friendships with t he pioneer geo logist s, and by her honest virt ues, has brought Lyme nearer to t he geological world t han ever did the fict itiou s and giddy Louisa t o the wider circle of J ane Aust en 's a dmirers. Climbing down from Gun Cliff (where a wild Salvia was found in flower ) the party were ju st able to get round th e pro jections of Church Cliffs before th e ti de cut them off. Here the low cliffs, protected beneath Lym e Chur ch by groynes and a cement wall , comprise the top-most beds of the Blue Lia s (36-49), the wid e platforms beneath them being formed of Second Quick a nd Gumpt ion (33-35). Th ey rise in an anticline ea stw ard s, a nd were seen to repeat the st ructure noticed a t Devonshire Poin t , west of Lym e, since the lowest bed shown in the axis of t he an t icline , both east and west of Lym e, is th e same , na mely , Brick Ledge (r ). F rom th is point th e various features of Black Ven were point ed out. E ast of Chur ch Cliffs th e Saurian shales (52) capped by Ta ble Ledge (53) gradua lly descen d t o t he beach ; but through out the whole dist an ce from here t o Charmouth the beach thi s summer reached th e cliff-foot, oblit erating all traces of Tab le Ledge beneath Blac k Ven, and preventi ng the party from exa mining the det ails of t he Sha leswith-Beef. Attent ion was called t o the conspicuous Birchi-bed capp ing the lowest precipice on Black Ven, and above it , the main features of the Black Marls and Belemnite Marls, examined in detail (but on th e east side of Charmouth) on t he Sat urday before, were eas ily recog nised; the Green Ammonite Beds on the eas te rn sho ulder of Black Ven, as well as th e Cretaceous bed s crowning a nd overst epping t he whole, were also pointed out. As the fault in the Char va lley was approa ched, the Shales-with-Beef were seen to become wavy , an d then progressively more cru mpled , until, just before the groyne at Charmouth beach, a sma ll section showed them conto rt ed in great detail and looking lik e a Palseozoic slate rath er than a Liassic shale (Plate 34, B). After tea, taken at the " Wayside Tea-rooms," the party followed the cliff-path towards Black Ven from the old cementworks to the abandoned Lyme coas t-road , which it joins near the Devil's Bellows. There t he yellow Foxmould, overlain by remnants of the Chert Beds, is ad mira bly displayed . The

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W. D. L A NG AND H. DIGHTON THO~AS,

old coast-road was followed west wards, the evidence of recent land-slipping being every where visible. Not only had the road foundered ex te nsively in places, but newly-formed cra cks and holes were als o frequ ently not ed . The grea t coombe -like hollow where a big lan dslide has carried away the rocks right up to the edge of th e road was viewed from abo ve, and t hen a t rack on it s easte rn side followed till the Gault a t the southeas te rn corne r of the "amphit hea t re" was reac hed . Th e several beds of the Gault were indicated , then the overlying 30 ft . of Cowstones, and the higher 60-80 ft. of Foxmould. After a successful search for fossils in Bed 3 of t he Ga ult, the party slightly descended in order to view, at about 20 ya rds across a vertical drop, the un conformable juncti on of the Gault on the thin remnant of the Green Ammonite Beds, the base of which was shown by a piece of Belemnite Stone projecting from the cliff about two feet below the base of the Gault. Here a lucky stroke revealed fossils (as internal casts) for the first time in the sandy Bed 2 of the Gault on Bla ck Ven , and the rest of the time was spent in searching for mor e. A large number of specimens was obtained and included about half-a-dozen ammonites, bu t the fragile matrix rendered it difficult to transport th em . The ammonit es, identified by Dr. L. F. Spath as A nahoplites praecox Spa th I and A nahoplites sp. ju v.. indi cate a Lower Gault age (intermedius zone) . Th e other mollusca have been exa mi ned by Mr. L. R. Cox, who reports that th ey include Panopea gurgitis (Brongn.) a nd Th etironia laevigata (Sow.) as common form s, in addit ion to Pecten (Syncyclonema) orbicularis Sow. and Aporrhais sp. Attempts were made by digging to reach the Gault peb ble-bed, but these had to be a bandoned, while the lateness of t he hour forced the party relu ctantly to return to Cha rmouth.

Tuesday, June 2nd, 1936. Start ing at 9.15 a .m ., the party drove to a point bet ween P enn Cross a nd the tunnel on th e Axminster Road , where they were met by some members of the Dorset Field Club. From here the members looked eastwards down the Gwyle to where t he 'Wootton Stream, running from left to right across the middl e distance, is backed by the Conegar-Nutcombe spur. In the corridor lying between this spur and the long ridge formed by th e hilltops of Stonebarrow a nd Chardown, th e river Cha r runs a straight course from Whitchurch on the left-hand, where it emerges from the Vale of Marshwood, to Charmouth on the right. In the distance behind Stonebarrow, the hills of Golden Cap and Langdon were seen to form a further ridge, bre ached I This ammo nite bad previou sly be en ob tained in tbe equivalent bed on Stonebarro w (Proc. Gt Ql. A ssoc., vol. xxv i, 1915, p . 115 ; redetermined by Spath, L.F.," A Monograph of the Ammonoidea of tbe Gault," vol. i, pt. 3. Mon . Pal. Soc., 1925. pp , I3D-I33).

PROC. GEOL. Assoc., VOL. XLVII. (1936).

PLATE 34.

Photo by W. F. Howarth, A.-JUNCTION BETWEEN BLUE AND \VHITE LIAS,

PINHAY BAY.

B.-FoLDING IN SHALES-WITH-BEEF, CHARMOUTH.

[To face p.

3IO.

"VHITSUN FIELD MEETING,

1936.

3I I

by the Golden Cap gap. The long, flat line of the StoncbarrowChardown ridge could be followed northwards, across the gap at Morcombelake, to the flat top of Hardown, and, across the Ryall gap, to the hill called Coppet, Hardown Hill and Coppet form the southern rim of the Vale of Marshwood-a saucer-shaped expanse here mostly hidden from view by the hills forming its western rim, namely, the conical, wooded hill of Conegar, and, to the north of Conegar and across a gap at the head of the Wootton Stream, the much higher hills of Coney Castle and Lambert's Castle. The steep, and western, side of Coney Castle was pointed out as a good example of a , slip-scarp '-a cliff left by the whole hillside sliding away; a smaller example was that on the edge of which the party was now standing, and overhanging the Axminster road; while a third lay in the fore-ground above Hogchester Farm and Badger Covert." It was such a scene that de Luc described when, on one of his geological travels, he viewed the country" adjacent to Charmouth," on the 6th September, 1805. "I found myself," he writes," "on an eminence higher than any other within a considerable distance around. It would be difficult to find a country which afforded more agreeable walks. All the paths are through grass fields, crossing dales, or obliquely ascending small hills; so that the objects, distant as well as near, are continually changing their aspect. From the top of the eminence on which I now stood, I saw branches of the large hills around this lower space advancing into it, broken into hillocks; the whole of this cheerful landscape being interspersed with houses surrounded by trees." The outstanding feature of such a view is the flat tops of the hills, the remnants of the peneplane out of which the existing valleys ha \'C been carved; and from this spot were noticed the four gaps which are supposed to mark the courses, at their respective levels, of the consequent streams which originally drained the peneplane from north to south, and began to dissect it. The highest, and therefore oldest, of the four is that behind Golden Cap; and a line drawn north-westwards from this gap to the next in height, namely, that at Morcombelake, coincides with the present course of the upper part of St. Gabriel's Water. A subsequent stream, now represented by the lower part of St. Gabriel's Water, but then at a much higher level, is supposed to have cut back, and beheaded the consequent at a point vertically above the present bend in St. Gabriel's Water. This capture caused the gap at Golden Cap to go dry. Later, a subsequent stream cutting back along the course of the present Char, beheaded the same consequent near Whitchurch, and the I

J. A. de Luc,

" GeologicalTravels," vol. ii, r8u, p, 88.

Paoc. GEOL. Assoc., VOL. XLVII., PART 4, 1936.

20

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W. D. LANG AND H. DIGHTON THO:lIAS,

Morcom belake gap also became deserted. Meanwhile another stream, now represented by that part of the Winniford which flows between Chideock and Seatown, had cut back and captured a consequent stream that flowed across the Vale from north to south, over the Ryall Gap, and past Chideock, thus causing the bend in the Winniford just below that village. Then a subsequent cutting back along the present course of the Char above Whitchurch beheaded this consequent near Middle Brook Farm, and the Ryall gap went dry. Finally the Char-subsequent cut back and captured the higher waters of the Simene, leaving the low water-shed which at present divides these two rivers, and accounting for the bending in the present Char from a southerly to a westerly direction at Borden Bridge. After this sequence of events had been demonstrated, the party drove through Penn, up Greenways Hill, and along the main road behind Lambert's Castle, to Marshwood church. At several points on the way the view opened out northwards across the Blackwater valley and past Hawkchurch to the main valley of the Axe, showing Albian greensand occupying all the high land, and Middle Lias in the valley slopes. At Marshwood church the coaches were left, and the party made their way to the meadow by the churchyard, whence a view over the Vale was obtained. Here, from its north-western rim the basin-like shape of the Vale was apparent, the spur formed by Lambert's Castle, Coney Castle and Conegar (last viewed from the western side) was seen to form its western, rim, and Hardown Hill and Coppet its southern rim; while the Char escaped through a breach at its south-western corner. The watershed between the Char and the Simene was seen to form its low eastern rim, while on the north-east the land rose towards the high gap between Lewesdon and Pilsdon. The road b.etween Marshwood church and Pilsdon runs along the northern nm. Jukes-Browne, in 1897, described how this basin shape arose. By plotting the level of the base of the Cretaceous from east to west and from north to south, and then continuing these base-lines across the Vale, he ingeniously showed that the Cretaceous beds formed a slight dome over the Vale, and that this dome was already formed when the Cretaceous surface was planed, and before the peneplane was finally uplifted. (This dome must not be confused with the previously-formed doming of the Liassic beds beneath the Vale). When, therefore, the peneplane began to be dissected, and the Cretaceous and sandy Middle Lias were cut through, the Liassic clays were first exposed in the middle of the Vale, and the consequent streams would first have flowed over sand, then over clay, and then have re-entered the sand. Owing to springs being thrown out at the saud-on-clay junction, and owing to the consequent

WHITSUN FIELD MEETING,

1936.

Iandslipping, those parts of the valleys which were cut ill clay widened more rapidly than those which were cut through sand. Thus the Vale tended to be hollowed out in the middle, where the clay was first met with, and to form the present clay basin with sandy sides and rim. The drive was then resumed as far as Low Gate (where some more members of the Dorset Field Club were met), whence the party climbed the remaining 200ft. to the summit of Pilsdon Pen (909 ft.). Although the atmosphere was fairly clear, the northern distance was somewhat blurred by rainy mists; moreover, the wind was too cold for full advantage to be taken of the view, which extended from the Mendips on the north to the sea on the south. From here the Vale was seen in its fullest extent and the same features observed as were noticed from Marshwood church, but from a new angle-a slightly undulating expanse of innumerable hedgerow and pasture, with patches of copse and woodland, and here and there a farmstead or a hamlet-the whole surrounded, except on the east, by high, generally bare, flat-topped hills separated by gaps. After listening to a short discourse on what was visible from this point, the chilled party returned to the coaches and began their exploration of the Vale by descending to the village of Bettiscombe. Here a halt of half an hour was called, during which many of the party took the opportunity of walking across the hay-fields to Bettiscombe Manor to see the Screaming Skull (of which a full account is gi\'en by J. S. Udal, Proc. Dorset Field Club, vol. xxxi, I9IO, pp. 176-2(3) and, what was more worth seeing, the Manor itself, "an early Georgian restoration of a much earlier building." It happened, however, that the farm had changed hands and was not then open to visitors. Meanwhile one or two of the party had found in the road-side bank of Bettiscombe Coppice the brachiopod Tropiorhynchia thalia, which indicates the top of the Belemnite marls. The bank opposite the Coppice, which used to show a section in the Green Ammonite Beds, now formed the approach to the drive of a newly-built house, and had been overlaid with masonry. The coaches next took the party past Marshwood Manor to the long-abandoned Marshwood brick-works. Raricostatusclays had been dug here in the past, and numerous pieces of Raricostatus-limestone lay about, which had clearly worked out of the grass-covered slopes. In the field above one of the party found Adder's-tongue fern, while Dyers Green-weed-Genista tinctoria-abundant -in this district wherever on the surface the clay is more evident than the drift, was commonly observed. Here lunch was taken, and soon afterwards the clinking of hammers on stone was heard on all sides as the pieces of limestone were broken up and the contained ammonites

314

W. D. LANG AND H. DIGHTON THOMAS,

extracted. The search yielded no Gryph.ea, such as was found on Saturday associated with Echioceras in the coast section, although it has occurred in this pit. Time did not permit an examination to be made of the numerous sections along the course of the Middle Brook, near Brick House and Oakford Farm, and the next stop was made at Sansom's Cross. Here, at the end of Batt's Lane is a road-bank in the ibexmaugenesti beds; and the party soon had the lane filled with clay dug from the bank in their search, which was quickly rewarded with Acanthopleuroceras raldani and other fossils characteristic of this horizon. The coaches were again left at Paddock's Cross, and the party walked down to Stoke Mill, and along the leat, to below a sluice, where there was a high section in the [amesonimasseanum-maugenesti beds. Though easily reached by wading, this vertical section is difficult to work owing to the slippery ledge at its base and the deep pool below. The route was resumed through Shave Cross and over the Char, at Bluntshay, to the road-side banks south of that spot. Here, recent road-widening had left heaps of clay, which yielded Uptonia, while the banks were soon found to be in the same beds as those just examined at Sansom's Cross, and Acanthopleuroceras 111 augen esti was again discovered. Time now demanded a prompt return, and, after a few minutes spent at Whitchurch in viewing the " Hwitan Cyrican "-white church-mentioned in King Alfred's will, the party, wry reluctantly leaving this noble building, reached Charmouth in time to take tea and catch the evening train to London. In the evening of Monday, rst June, the party gathered at headquarters, where the Vice-President, Mr. Robley, proposed to the Directors and ~Ir. Gibbons, the Secretary for this Field Meeting, votes of thanks, which were heartily endorsed. Sixty-five members of the Association attended the meeting, and twenty-four members of the Dorset Field Club were also present on the Tuesday. EXPLAKATION OF PLATES 32-34. PLATE 32.

A. B.

PLATE 33.

A.

Albian Chert pit, Hardown Hill. The fractured Chert Beds are divided near the middle by a bed of contorted greensand. Foreshore of Belemnite Marls E. of St. Gabriel's Mouth, with cliff of Green Ammonite Beds. The strong limestone near the cliff-foot is the Belemnite Stone. The bottom of the rod near the right centre is at the horizon of Cceloceras pettos ; while in the picture its top appears level. with the lowest beds of the Acanthopleuroceras maugenesti zone. The Bindon Landslip-the Great Chasm viewed from the cast, with Beer Head in the distance.

\YHITSC~ FIELD MEETING,

B. PLATE

34.

A.

B.

1936.

315

Section near Culverholc, showing Gault overstepping horizons in the White Lias. Cliff at Pinhay Bay, showing junction of Blue Lias and White Lias. In the triangular mass on the left of the picture, a line of vegetation beneath the thinly-bedded limestones about half-way up the block separates the darker Blue Lias above from the paler White Lias below. Shales-with-Beef sharply folded against the Mouth Fault, at Charmouth beach.