2 6 4 PREHISTORIC EUROPE. 



ice-sheet formed during the climax of glacial cold. It is this 

 boulder-clay which has been traced south to the valley of the 

 Thames. Eesting upon it occur sands, gravels, and loams of 

 much the same character as those that immediately overlie the 

 Cromer boulder-clay. These deposits are the flood-accumulations 

 formed during the subsequent dissolution of the mer de glace, 

 which was brought about by another change of climate. To 

 these deposits succeeds a third boulder-clay, that which is known 

 as the purple clay, — the presence of which points to a third 

 advance of the ice-sheet. Overlying the purple-clay, again, we 

 encounter a series of sands and gravels which in the valley of 

 the Humber have yielded remains of Pleistocene mammalia, 

 together with many shells, conspicuous amongst which, by 

 reason of its abundance, is Cyrena jluminalis — a shell which no 

 longer lives in British waters. Lastly, these beds are covered 

 in their turn by a fourth sheet of boulder-clay, the Hessle 

 boulder-clay. Such is the general succession of the drift de- 

 posits which are exposed in the sea-cliffs and other sections in 

 Holderness. It proves that these deposits were accumulated 

 under very variable physical and climatic conditions. The great 

 chalky boulder-clay is the moraine profonde of the mer de glace 

 which flowed south as far as the valley of the Thames. By and 

 by a change of climate ensued, and the ice-front retreated to- 

 wards the north. To what extent this great covering of ice 

 melted away in Britain before the incoming of the succeeding 

 mer de glace which deposited the purple boulder-clay we cannot 

 tell. There are certain patches of shelly clay that occur above the 

 great chalky boulder-clay which lead us to believe that, after 

 the retreat of the ice underneath which that till was formed, the 

 North Sea was tenanted by an arctic fauna. Kecently, Mr. 

 Lamplugh has described the occurrence in a till near Bridlington 

 Harbour of patches of freshwater beds, with peat and many shells 

 of a variety of Lymncea peregra, which appear to be intermediate 

 in age between the great chalky boulder-clay and the purple 

 boulder-clay. 1 But whether these freshwater beds be older or 



1 Geological Magazine, Dec. ii. vol. vi. p. 393. 



