106 STUDIES, SCIENTIFIC AND SOCIAL chap. 



probably be no erosion at all. Some writers maintain that 

 the lakes were all filled up with alluvium previous to the 

 glacial epoch, and that the ice cleared out this incoherent 

 matter; but it is almost certain that no such clearance 

 would have taken place, because the glacier would pass 

 over such a surface, the stones temporarily furrowing it, 

 while the sub-glacial water would cut for itself one or 

 more deep channels, and there would thus be no water 

 flowing over the whole surface of the basin, which must be 

 so great an aid to erosion in solid rock. 



These considerations apply to the equally common ob- 

 jection, that the great masses of boulder-clay left behind 

 by the ice-sheet, and over which it must have passed, 

 prove that it could have had little eroding power. The 

 product of the erosion of irregular rock-surfaces in an 

 undulating tract of country, where not carried away by 

 water, would necessarily, by the pressure of the ice, be 

 forced into the more or less sheltered or land-locked 

 hollows, thus tending to equalise the surface-contours and 

 facilitate the onward motion of the ice. In such hollows 

 it would be pressed and compacted by the weight of the 

 ice, but would be neither eroded nor forced away until, by 

 the continued process of rock-erosion, it became exposed 

 to unequal lateral pressure, when it would be gradually 

 removed to some other sheltered hollow, perhaps to again 

 undergo the same process of removal at a later period, and 

 finally rest in the positions in which we find it. During 

 the later stages of the ice age when, notwithstanding the 

 onward motion of the middle portions of the glacier, the 

 lower portion was melting away both above and below, 

 and the terminal ice-cliff was permanently retreating, 

 almost the whole of the eroded matter except what was 

 carried away by the sub-glacial torrents, would remain 

 behind ; and it is this final product of glacial erosion that 

 forms the huge deposits of boulder-clay which encumber 

 the surface of the lowlands in most highly glaciated 

 countries. When, however, the moving ice changed its 

 direction, as it often did during the varying phases of the 

 ice age, it sometimes acted most energetically in crushing, 

 dragging, and contorting both the boulder-clay and other 



