CHAP, v.] THE ICEBERGS A PERIOD OF DEPRESSION. 117 



Ingleborough, in Yorkshire. The tough clays with 

 scratched stones, sometimes so hard to work that it is 

 necessary to employ gunpowder, are considered by Dr. 

 James Geikie to be the debris underneath the ice-sheet 

 accumulated on land, and termed by the Swiss geologists 

 " moraine profonde." These are met with chiefly in Scot- 

 land, but they have been observed by Mr. De Eance in 

 South Lancashire, and at the Little Ormes Head in North 

 Wales, 



The climate must have been arctic in its severity 

 during this period of glaciation, and this may have been 

 partially due to the fact of the land standing at a 

 higher level above the sea, and being lifted up into the 

 colder regions of the atmosphere. It cannot, however, 

 be wholly so explained, since it was the culmination 

 of a series of changes by which the tropical climate 

 of the Eocene passed into the warm Meiocene and 

 temperate Pleiocene climates. 



2. The Icebergs a Period of Depression. 



Then followed a period of depression beneath the 

 sea. The glaciers, which had before carried their bur- 

 dens of sand, clay, and stone far away from the present 

 seaboard of Britain, 1 now ended at the retreating shore- 

 line, giving rise to icebergs, which deposited the lower 

 boulder clay as they melted, and drifted as far to the 

 south as the valley of the Thames. The mountains were 



1 The English boulder clays, as a whole, differ from the moraine 

 pro/oiide in their softness and the large area which they cover. Strata of 

 boulder clay at all comparable to the great clay mantle covering the lower 

 grounds of Britain north of the Thames are conspicuous by their absence 

 from the glaciated regions of central Europe and the Pyrenees, which 

 were not depressed beneath the sea. 



