Anyone standing on the beach between Charmouth and Lyme, and looking up at the bluffs and precipices of Black Ven, is struck by the contrast between the pale, blue-grey colour of the third, and highest, Lias precipice and the deeper shade of the underlying Black Marl. These pale marls, about 75 feet thick, soon pass off the eastern shoulder of Black Ven, but are readily picked up again as the eye follows the line of stratification eastwards, and across the valley, to Stonebarrow Cliff. There the pale marls make the second precipice, but soon dip to the beach to form most of the long, low cliff reaching from Westhay to the Ridge fault. These 75 feet of the pale marls are the Belemnite Beds of Day and of the Geological Survey. No adequate mention of them is made before 1863, when Day wrote his classic account of the Middle and Upper Lias of the Dorset Coast. although they are included in the ‘Upper Lias Marls’ of De la Beche's earlier account, and form the middle part of the eighth bed from the bottom of the section there described. In De la Beche's later account they occupy the middle of the lower part of his bed ‘a’: ‘Marls and slaty marls with several beds of indurated marl and earthy limestone in the lower part, micaceous in the higher—350 ft.’ In both accounts De la Beche particularly notes that belemnites are numerous, and other fossils occur in ‘marls on the shore, between
Shales-with-‘beef’ was the name given to some 70 feet of Lias on the Dorset coast, lying above (53) Table Ledge and below (76 a) the Birchi-Tabular. The beds consist of paper-shales, marls, indurated bands, and limestone nodule-beds, with numerous, more or less impersistent, interbedded seams of fibrous calcite, called ‘Beef’ by the Officers of the Geological Survey. Descending to the beach at Charmouth, and there forming reefs on the foreshore, the Shales-with-Beef are the most accessible Lias of that place. Yet they are, perhaps, the least known of all the beds. This is doubtless because of the generally unsatisfactory condition of the fossils found in them, and their consequent worthlessness on the one hand to the native, who finds no sale for such fragmentary and friable remains as the fossils present; while, on the other hand, the geologist seldom finds specimens more than approximately identifiable, and generally obtains completely satisfactory examples from but three or four horizons. Sir Henry De la Beche described the Lias of this coast nearly a century ago; and, although he generally under-estimated their thickness, the subdivisions that he then made can be approximately correlated as follows, at any rate those five which lie above the Blue Lias limestones:— (5) Irregular bed of Limestone, with nodular concretions, frequently containing ammonites; 2 feet=Stellaris beds (87–89). (4) Slaty marls, with several thin beds of indurated marl; 67 feet = Black Marl series above Birchi-Tabular and below Stellaris beds (77–88). (3) Slaty marls containing small crystals of selenite; 32 feet Shales-with-Beef (54–76).
The Black Marl series of the Dorset coast was so named by the officers of the Geological Survey to include the shales and limestones lying above the stone-beds of the Blue Lias and below the pale marls of the Belemnite Beds. I have already given a detailed account of the Shales-with-‘Beef’—the lowest part of the Black Marl series. But a detailed account of the sequence above the birchi bed is much needed; and, although our knowledge of the contained fossils and their order of occurrence is still incomplete, yet a particular description of these horizons as represented on the Dorset coast should help to unravel the sequence in the corresponding Lias of other areas, especially in view of the amount of work which is now being done on the Lias, and of the use already made of the Dorset sequence in such work. In this connexion much has been heard of non-sequences. It is hoped that this paper will show the importance of this principle in interpreting the Lias; yet to what error it may lead if rashly applied. I need hardly indicate how barren the detailed observations and descriptions of the paper would be without the accurate (even if approximate) determinations of the ammonites. Indeed, except from the view-point of lithology, detailed description of the beds is only necessary in order that the exact positions of the contained fossils may be recognized. The experience and the knowledge reuired to determine specimens as poorly preserved as most o those collected
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.