138 (iF.OLOGY OF THE ISLE OF WICillT. 



Ledge the base of the marine beds falls to the level of the beach, 

 and from this point nearly to Linstonc Chine continuous sections 

 are generally exposed, for there is little talus, and the lower part 

 of the cliif is so full of fossils that it presents a vertical face. 

 The thickening o£ the Oyster Bed, and the way in which it cuts 

 into the underlying clay full of Cytherea, are very noticeable in 

 this part of the cliff. 



We have now reached the section which all geologists visit, and 

 from which the majority of the marine Headou fossils have been 

 obtained. It may therefore be well to stop for a moment to point 

 out that even this most ])urely marine i)ortion of the Headon 

 series is full of freshwater shells. A few minutes search is sure to 

 yield several specimens of Limncea and Cyrena mixed with the 

 Oysters. The underlying clay full of Cytherea is more thoroughly 

 marine, but it also contains a good many valves of Cyrena. 

 -However there is a decided and essential difference between these 

 marine beds with drifted freshwater shells, and the beds full of 

 Potamomya, Melania, and Potamides, which lie above and below 

 them. These fossils probably point lo deposition in brackish-water 

 lagoons and not in the open sea. Like all accumulations formed 

 in such conditions, therefore they contain abundance of individuals 

 belonging to very few species, instead of a wonderfully varied 

 molluscan fauna like that of the Middle Headon Beds. 



The How Ledge limestone, which underlies the marine bed, is 

 another well-marked horizon. This stone is a band, from 3 to 

 5 feet thick, of freshwater rather tufaceous limestone full of well 

 preseived Liinncea and Planorbis, belonging to many species. The 

 perfect preservation of the fossils, the softness of the matrix, 

 and the ease with which the bed can be examined, render this 

 the favourite bed from which to obtain these shells. The rock is 

 always visible between How Ledge and Warden Point, and can 

 be traced continuously southward to the Coast Guard Station. 

 Here it passes inland, but Messrs. Keeping and Tawney identify 

 it with the Limniean limestone at the top of the Lower Headon 

 Beds at the north-eastern end of Headon Hill (see section p. 136). 

 A section of the lower part of the cliff near Colwell Chine, given 

 at p. 242, shows the small reversed or overthrust faults developed 

 in this limestone by lateral pressure connected with the formation 

 of the great uniclinal fold of the Isle of Wight. 



A short distance below the How Ledge limestone is a mass of 

 calcareous concretionary sandstone and sand, forming Warden 

 Ledge. This sand is traceable at intervals for about a mile. 

 South of Warden Ledge other thin limestones form a minor ledge 

 on the foreshore. These limestones, full of Chara and Limnaa^ 

 can be traced nearly to Widdick Chine. 



The sections of the Headon Beds near Cliff End are, un- 

 fortunately, somewhat obscure at present (1889), and the thinning 

 out of the thick U[)per Headon limestone renders it difficult to 

 trace the noithwurd limit of tlie Headon Beds. Messrs. Keeping 

 and Tawney identify the thick limestone of Headon Hill with a bed 



