THE ICE AGE AND ITS WORK. 49 



face, the stones temporarily furrowing it, while the subglacial 

 water would cut for itself one or more deep channels, and there 

 would thus be no water under pressure acting 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 objection, 

 that the great masses of bowlder 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 landlocked hollows, thus 

 tending to equalize the surface contours and facilitate the onward 

 motion of the ice. In such hollows it would be pressed and com- 

 pacted 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 melt- 

 ing 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 subglacial torrents, would 

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

 forms the huge deposits of bowlder 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 

 bowlder clay and other superficial beds, often causing the wildest 

 confusion in the deposits and sometimes imbedding huge sheets 

 of Tertiary strata or chalk in the midst of the bowlder clay. But 

 this is a very different mode of action from that by which hard 

 rocks are ground down or lake basins eroded. 



In reply to the continual assertions of Prof. Bonney, and of 

 most of the Alpine explorers, that the action of glaciers is entirely 

 superficial, and that they actually preserve the surfaces they cover 

 from denudation, a few facts may be here given. From a large 

 number of gaugings by Dollfus-Ausset, Dr. Penck has calculated 

 that the solid matter in the torrent which issues from the Aar 

 Glacier annually amounts to six hundred and thirty-eight cubic 

 metres for each square kilometre of the surface of the glacier, a 

 quantity sufficient to lower the bed of the glacier one metre in six- 

 teen hundred and sixty-six years, or one foot in five hundred and 



