293 THE GEOLOGY OF THE CHARMOUTH CLIFFS, BEACH AND FORE-SHORE. By W. D. LANG, M.A., F.G.S., F.Z.S. CONTENTS. PAGE, INTRODUCTION 294 THE LIAS- I. COMPOSITION AND CONDITIONS OF DEPOSIT 296 I I. STRUCTURE. 299 Ill. FAULTS ., • 302 IV. THE GENERAL SEQUENCE- (a) Primary Divisions. (b) Stages. (c) Series. (d) Zones 3°4 V. THE DETAILED SEQUENCE- (a) Angulatus and Bucklandi zones. .• 308 (b) Tuberculatus zone. r , Subzone. of Arnioceras. 2. Sub­ zone of Mtcroderoceras birchi . 3II (c) Obtusus zone- r. Subzoue of A rietites brooki. 2. Subzone of Xtpheroce1'as planicosta. 3. Subzone of A stero­ ceras stellare 315 (d) Oxynotus zone • 320 eel Raricostatus zone 321 (I) Armatus zone . 322 (g) :Jamesoni zone. 324 (h) ibex zone. 324 (i) Daorei zone 326 (j) Margarttatus zone 329 VI. HISTORICAL SUMMARY. 330 THE CRETACEOUS- I. GENERAL CONSIDERATIONS­ (a) The Cretaceous Overstep. 333 eb) The General Character of the Beds 333 II. STRATIGRAPHICAL SEQUENCE- (a) General sequence, (b) Detailed sequence. 334 III. SECTlONS- (a) General remarks, (b) Details of the Gault Sections. 338 IV. COMPARISON OF THE GAULT BEDS ON BLACK VEN, STONEBARROW AND GOLDEN CAP-":- (a) The Pebble Bed. (b) Beds 1-2. (c) Beds 3-10. 347 V. STRATIGRAPHICAL SUMMARY . 350 THE POST-CRETACEOUS DEPOSITS­ I. THE CHERT-BED REMAINS 35° II. THE PLATEAU DEPOSITS. 35r III. THE CHAR VALLEY DEPOSITS 352 I V. THE BEACH 353 BIBLIOGRAPHY- A LIST OF TITLES OF WORKS REFERRED TO IN THE PAPER AND CONCERNING THE GEOLOGY OF THE DISTRICT 354 CONCLUSION. 359 PROC. GEOL. Assoc.. VOL. XXV, PART 5, 1914.J 20 294 W. D . LA NG ON INTRODUCT IO N . I SI T ORS to Charmouth and Lvme who come with the V intention of finding fossils are 'often disappointed at the results of their collecting. Those to whom the fossils appe al chiefly as specimens of form and shape, having heard much of Lyme as a collecting ground, seen in museums fossils f rom the neighbourhood in numbers and in fine preservati on, and read their descriptions in the works of De la Beche, Buckland, Sowerb y, and others, lea rn by experience that specimens such as they expect to find are commonest in the cottages of fishermen and in the shops of Lyme. Those, on the other hand, whom the evolutionary history and , consequently, the ord er of succes­ sion of the fossils interests, learn that for one specimen found in place in its bed, twenty are picked up loose on the cliff-slopes and on the beach. It is true that a large amount of time and labo ur are necessary, whether for obtaining fine specimens or a succession of forms whose exact horizon is known; and really to learn their evolutionary sequence it is essential eith er to live in the neighbourhood for some time or regularl y to revisit the locality for a great num ber of years. Thus it is 'that, although hundreds of species are known and described from the Lyme district, our knowledge of their exact horizons, so necessary for unravelling their evolution, is very small. And to help towards this end has been the chief motive of the work resultin g in these notes. The first requisite for collecting fossils in pl ace is to know the exact geological hori zon of the spot whence the fossil is being taken ; and for convenience in determining this, it is necessary to map the cliff and fore-shore in detail. H ence arose the maps.* In drawing them man y points of stratigraphical interest incidentall y were involved. And so it appears how intimately one asp ect of geology is connected with another. F or, sp ringing from a pa lreontological aim, the results appear at first sight main ly stratigraphical. The maps, record ing geological structure and stratigraphical sequence, are the outcome of an attempt to find fossils in place as a means towards unravelling their evolution. It would appear ungracious not to acknowl edge here the help received from various sources. First, from those who already have written on the geology of the district, references to whose works will be found scattered throughout the text of the se notes; particularly from the author of the two last Survey Memoirs dealing with this district, the late Mr. H. B. Woodward, who directed the excursi ons on both the former occasions when the Geologists' Association visited these parts, and, in the Memoirs, has given a detail ed account of the geology and a summary of what • Publisb ed as Par t 6 of thi s Volume of tb e P ROCEEDINGS. THE GEOLOGY OF THE CHAR:\IOUTH CLIFFS. 295 has been formerly written. With these authors must be remem­ bered the local collectors, fr om Miss AIming onwards, who so largely supplied the various authors with their fossil material. Secondly, acknowledgment must be made of help from many pr ofessional colleagues and friends, who by identifying speci­ mens and by advice in other technical points have rendered valuable assistance, especiall y Dr. A. S. Woodward, Dr. F. A. Bather, Mr. R. B. Newton, "Mr. G. C. Crick, Dr. C. W. Andrews, 1I1r. W. Campbell Smith, Mr. R. H. Rastall, Mr. L. F. Spath, Mr. S. S. Ruckman, 1I1r. D. ~. S. Wats on, Mr. C. D . Sherborn, 1\lr. H . L. Hawkins, :'dr. C. P. Chatwin, and others. Thirdly , help has been received from the Charmouth and Lyme residents, who, during frequent visits extending over a period of some fift een years, have always been ready with local information and advice. T o the late Miss Harriot Templer, of Charmouth, I am ind ebted for sketches that show" differences in the former physiography of the neighbourhood. Mr. Thomas Hunter, fisherman,· has imparted much information that is made use of in the following notes. Finally, my wife, by constant help in the field and at home, has rendered possible the work resulting in these notes, that otherwise would have remained unwritten. To und erstand rightly the salient features of Charmouth geology, ifis necessary to consider the three periods of denuda­ tion that from time to time ha ve influenced the district. The evidence of these periods is clearl y marked on the cliffs. The most casual observation will show that the cliffs consist of clays below and of sand above. The cla ys (with occasional lime­ stones), which extend f rom the base to more than half -way up the cliff, are of Liassic age, and are thrown int o gentl e folds, though , on the whole, they dip in an east-south-easterly direc­ tion. At the height of a little over 300 ft. a nearly hori­ zontal line" truncates these Liassic clays, and the cliff con­ tinues upwards in beds of loamy Cretaceous sands, with no appreciabl e dip, passing into the weathered remains of Chert ­ beds, also Cretaceous in age, which in their turn are capped with Cretaceous and post- Cretaceous drift. The hor izontal line separating the Liassic clays below from the Cretaceous sands above marks a plane of denudation that was formed in late Jurassic and early Cretaceous times. t That is to say, when the visible geological history of Charmouth began, the whole of this area was und er the sea, and the Lias was being deposited as a more or less homogeneous, calcareous mud derived from the denudation of limestones (probably Carboniferous Limestone) t in some neighbouring land (possibly the Mendip •Very clearly shown In the ske tches by Buckland (Buckland, 1822, PI. xiv), als o In tbe sections of the Survey Memoir (Wo odward and Ussher, 19 I1, p. 2 2 ). t De Ran ce. 1874. PP.248-9. ~ Possibly Devonian also. See II . B. Woodward, I88g. p , xxxv l, W . D . LANG ON Hills). * The whole of the Lower Lias was thu s deposited, and on it, as sediments of various kind s, the rest of the Lias, the Lower and the Middle Oolites, and , prob ably, the lower pa rt of the Upper Oolites. The sea-bott om with all these sed iments was then elevated , and the resultant folding is shown in the und ula­ tions now seen in the Liassic beds of the Charmouth cliffs. As soon as the f ormer sea-bed became dr y land, denu dation began a nd proceeded dur ing la te Jurassic and early Cretaceous times, until , by the end of the Lower Cretaceous period, all the land in the Charmouth distr ict (fo rmed of Middle and Upper Lias and Oolite) had been worn away to sea-level, and , subsiding, had formed a new sea-bottom ready to receive the U ppe r Cretaceous deposits. This is the plane that is shown by the horizont al line on the cliffs between the Li as clays and Cretaceous sand, and it marks the end of the first period of denudation. During Upper Cretaceous times, the Gault, Upp er Green­ sand, and Chalk were deposited in the sea which had again occupied the Charmouth district ; and during the next period of elevation they suffered denudation, until, in this area, all the Chalk was removed, and again the land was reduced to sea­ level. Thus a second plane of marine denudation was form ed. This plane is shown by the flat -topped hills of the district, of which the Char mouth Cliffs, Black Ven, and Stonebarrow are sections. If the present valleys were filled in to the level of lines draw n fr om hill-top to hilI -top, a nearl y flat surface would result ; and thi s would be the plane of post-Cretaceous denudation.