INTERGLACIAL EPOCHS. 269 



of the latest boulder-clay of Scotland, Ireland, England, and 

 Wales. Of the southern extension of the North Sea mer de glace 

 I will speak farther on. 



The final dissolution of the latest ice-sheet in Britain was 

 followed as usual by the accumulation of vast quantities of sand 

 and gravel, boulders and angular de'hris. Flooded rivers, tor- 

 rents, and inundations, spread sand and gravel in wide sheets 

 over the low grounds, which reach to such heights and are dis- 

 tributed over so wide an area that they have sometimes been 

 attributed to the sea, although they have no particular resem- 

 blance to marine deposits, and are quite destitute of marine 

 organic remains. It cannot be denied, however, that some of 

 the gravel-deposits are with difficulty to be accounted for by 

 mere flood -action. They occur upon valley- slopes, and are 

 spread over the intervening plateaux between valleys in such a 

 way as to suggest that some other explanation of their origin 

 must be forthcoming, and such an explanation has been furnished 

 by Mr. Darwin (see supra, p. 141). Doubtless the phenomena 

 described by him were reproduced more or less extensively with 

 every return of glacial conditions, all through the Ice Age. One 

 can readily understand how, during the latest cold epoch, the 

 floods and torrents would frequently undermine and redistribute 

 alluvial deposits of interglacial age — how they would sweep 

 together all relics lying loose upon the surface, and again scatter 

 these broadcast — so that flint implements and the bones both of 

 arctic and southern mammals might come to be commingled in 

 those pell-mell accumulations of angular gravel which after- 

 wards gradually settled down as the frozen snow with which 

 they had been interbedded melted slowly away. 



While such changes were taking place in the low grounds of 

 Southern England, the northern ice-sheet continued to retire 

 towards the heights, strewing hill-slope and valley-bottom with 

 its superficial moraines and erratics. Eventually a time came 

 when it ceased to invade the Lowlands, withdrawing, as it were, 

 to the mountains, where it broke up into a series of local 

 glaciers, and ere long finally vanished. In Scotland the closing 



