INTERGLACIAL EPOCHS. 265 



younger than the clays with arctic shells cannot be well made 

 out. They tell us, however, that between the accumulation of 

 the two tills referred to, a land-surface existed in Yorkshire, and 

 this of itself implies a very considerable lapse of time, not less 

 than a great change of climate. Plants once more crept north, 

 and molluscs found their way into the streams and pools. Again, 

 however, the climate changed, and another vast ice-sheet over- 

 flowed the country, and ploughed out marine and freshwater 

 deposits, which we now find confusedly commingled with the 

 lower part of the purple boulder-clay. This third ice-sheet went 

 south as far at least as Lincolnshire. The beds which succeed 

 to the bottom-moraine of this ice-sheet afford very convincing 

 evidence of a complete change of climate. They have yielded 

 remains of the Pleistocene mammalia and estuarine and marine 

 shells — the general facies of which implies climatic conditions as 

 favourable as those of the present day. Yet once more those 

 glacial conditions vanished, and a fourth and last ice-sheet over- 

 whelmed the land, flowing south into Lincolnshire, but perhaps 

 not extending so far as that of the third glacial epoch. 



Thus we have evidence in these English sections of no fewer 

 than four glacial epochs separated by intervening epochs of 

 mild climatic conditions. During the mild interglacial epochs 

 the Pleistocene mammalia made their appearance, and Palaeo- 

 lithic man was likewise an occupant of English soil. 



In the north-west of England and in the east of Ireland there 

 occurs a triple series of drift-deposits, as was first clearly indicated 

 by Professor Hull, consisting of a lower and an upper boulder- 

 clay, with an intervening group of marine deposits, which attain 

 in some places a thickness of several hundred feet. These, as I 

 believe, are the equivalents of the upper part of the glacial 

 series as developed upon the Yorkshire coast. The lower 

 boulder-clay of Lancashire and Cheshire and Ireland corresponds 

 to the purple boulder-clay of Yorkshire ; the middle sands and 

 gravels of the north-west of England and the northern and 

 central districts of Ireland are represented in Yorkshire by the 

 Hessle estuarine beds ; and the upper boulder-clay on both sides 



