266 PREHISTORIC EUROPE. 



of the Irish Sea corresponds to the Hessle boulder-clay of 

 Holderness. Bearing this correlation in mind, let us now 

 attempt to sum up the general physical and climatic changes 

 which obtained in the British area during the last interglacial 

 and concluding glacial epochs. 



The dissolution of the mer de glace underneath which the 

 purple boulder-clay accumulated was followed by the appearance 

 of a wide land-surface in England. The British area formed at 

 that time a part of the Continent, and the Pleistocene mammalia 

 — horse, mammoth, ox, deer, etc. — invaded the land. Eventually 

 a gradual submergence ensued, and the sea by and by overflowed 

 wide regions. Traces of this ancient submergence have been 

 met with up to a height of over 1200 feet in Ireland, of more 

 than 1300 feet in Wales, and of 500 feet in Scotland. During 

 the earlier stages of that submergence the climate was mild and 

 genial, as is shown by the presence of Cyrena fluminalis and other 

 shells in the estuarine beds near Hull. But the temperature of 

 the sea fell as the submergence continued, the general facies of 

 the fossils which occur in the north-west of England and in 

 Ireland indicating upon the whole colder conditions than now 

 obtain in the adjacent waters. This conclusion is borne out 

 by the character of the shells in the high-level Scottish beds 

 which have yielded Tellina calcarea and Cyprina islandica, the 

 former a shell which does not now live in British seas, but 

 ranges north from the Danish shores of the Baltic to Spitzbergen. 

 It is evident then that during the deposition of the "middle 

 sands " of England and Ireland, the British Islands must have 

 formed an archipelago of islets. Although no marine deposits 

 occur in Scotland above a height of 500 feet, we must not 

 assume, for reasons that will presently appear, that the depres- 

 sion of the land was not so great in that direction. But the 

 submergence appears certainly to have decreased towards the 

 south to about 40 feet in the Fenland, and some 20 to 60 feet or 

 so along the borders of the English Channel. In Cornwall, 

 Devon, Dorset, etc., and at various points on the opposite French 

 coasts, occur deposits of marine gravel and sand, extending from 



