CHAP. XIII.] PLEISTOCENE EPOCH, 297 



extremity and joined that which traversed the Bristol 

 Channel. 1 



The great lake in the Irish Sea was not the only one 

 which existed on the great tracts of undulating ground 

 which surrounded the more hilly districts of Great Britain 

 and Ireland. Many other lakes existed off the west coast 

 of Scotland, the sites of these being indicated by the areas 

 of deeper soundings. 2 Others appear to have existed in the 

 English Channel and in the bed of the North Sea, but 

 some of these are probably portions of the ancient river 

 valleys, and are now hollows because of the unequal 

 distribution of detritus over the sea-floor, or because they 

 are exposed to the scour of currents. 



How long the land remained at such a high level we 

 cannot say, but there are reasons for thinking that physical 

 changes of considerable magnitude occurred in the northern 

 districts, while little change took place in the physical con- 

 dition of southern England ; the record of these changes, 

 however, has not yet been satisfactorily deciphered. The 

 union of Ireland to England and of England to France and 

 Belgium seems to have continued throughout the later 

 Pleistocene period ; it lasted long enough for Palaeolithic 

 man to be supplanted by Neolithic man, and for a large 

 number of mammalia to become extinct. 



There is, however, good reason to believe that during 

 the immigratioQ of the existing fauna into Britain a 

 gradual subsidence of the whole region was taking place, 



1 In the map given by Professor B. Dawkins ('-'Early Man in Britain/' 

 p. 150) the Irish Sea is made by some oversight to drain northward 

 instead of southward ; but an inspection of the reduced Hydrographic 

 Chart of the British Isles shows that there is a continuous ridge 

 between Mull and Malin Head where the soundings are under 40 

 fathoms, and which must have formed a watershed under the conditions 

 assumed. 



a See map in J. Geikie's " Great Ice Age," and chap, xxiv., second 

 edition. 



