80 STUDIES, SCIENTIFIC AND SOCIAL chap. 



mostly over level ground or up slopes, and near its termi- 

 nation at Soleure the ground is actually nearly 200 feet 

 higher than it is at the head of the lake of Geneva. There 

 remains as a cause of motion only the slope of the upper 

 surface of the glacier, the ice slowly flowing downward, 

 and, by means of its tenacity and its viscocity on a large 

 scale, dragging its lower portion still more slowly over the 

 uneven or upward-sloping surface. This mode of motion 

 will be discussed later when dealing with the origin of 

 lake-basins. 



No doubt at this epoch of maximum glaciation the ice- 

 sheet extended over the whole country between the 

 Bernese Alps and the Jura, and the downward tlow of the 

 lateral glaciers along the valley of the Sarine, Aare, and 

 other rivers flowing towards Soleure greatly assisted the 

 general onward motion. But the fact remains, and it 

 cannot be too strongly insisted on, that here we have a 

 veritable ice-sheet moving over hill and valley, carrying 

 on its surface quantities of erratic blocks, rounding, 

 striating, and polishing the rocks over which it passed, 

 and with the material thus crushed and ground away 

 forming great deposits of boulder-clay, much of which 

 still remains, although enormous quantities must have 

 been carried away by the rivers to the lowlands of Europe 

 and to the sea. The fact is therefore demonstrated, and is 

 implicitly admitted by the most conservative of glacialists, 

 that in this case an ice-sheet has moved onward over a 

 hilly plateau for more than 100 miles, even when its 

 terminal moraine is at as high a level as its exit from the 

 mountain valley where it had its origin. 



Erratics in Great Britain. 



It will now be well briefly to sketch the distribution of 

 erratic blocks in Great Britain, and the conclusions to be 

 drawn from them as to the former existence of an ice- 

 sheet under which the greater part of our islands was 

 buried. 



Every mountain group north of the Bristol Channel was 

 a centre from which, in the earlier and later phases of the 



