46 THE POPULAR SCIENCE MONTHLY. 



Hispar Glacier (forty miles long) are " entirely covered with a 

 mantle of moraine." If these glaciers extended to over a hundred 

 miles long, as did the Rhone Glacier when it reached the Lake of 

 Geneva, much of this debris would probably have found its way 

 to the bottom, and thus supply the necessary grinding material 

 and the abundant stones of the " till " found everywhere in the 

 tracks of the old glaciers. 



Again, although ice is viscous and can slowly change its shape 

 to almost any extent, yet it takes a considerable time to adapt 

 itself to continually changing outlines of the valley bottom. 

 Hence, where great inequalities occur portions of the rocky floor 

 might be bridged over for a considerable space, and where a val- 

 ley had a narrow V-shaped bottom the subglacial stream might 

 eat away so much of the ice that the glacier might rest wholly on 

 the lateral slopes, and hardly touch the bottom at all. On a tol- 

 erably wide and level valley bottom, however, the ice would press 

 with its fullest intensity, and its armature of densely packed 

 stones and rock fragments would groove and grind the rocky 

 floor over every foot of its surface, and with a rate of motion 

 perhaps greater than that of the existing Greenland and Alaskan 

 glaciers, owing to the more southern latitude and therefore higher 

 mean temperature of the soil and the ice. At the same time sub- 

 glacial streams, forced onward under great hydrostatic pressure, 

 would insinuate themselves into every vacant groove and furrow 

 as each graving tool successively passed on and the one behind it 

 took a slightly different position ; and thus the glacial mud, the 

 product of the erosion, would be continually washed away, finally 

 escaping at the lower extremity of the glacier, or in some cases 

 getting embayed in rocky hollows where it might remain perma- 

 nently as masses of clayey " till," packed with stones and com- 

 pressed by the weight of the ice to the hardness of rock itself. 

 The continual lubrication of the whole valley floor by water 

 forced onward under pressure, together with the ever-changing 

 form of the under surface of the glacier as it slowly molded itself 

 to the varying contours of the rocks beneath, would greatly facili- 

 tate the onward motion. Owing to these changes of form and the 

 great upward pressure of the water in all the hollows to which it 

 gained access, it seems probable that at any one time not more 

 than half the entire bottom surface of the glacier would be in 

 actual contact with the rock, thus greatly reducing the friction ; 

 while, as the process of erosion went on, the rock surfaces would 

 become continually smoother and the inequalities less pro- 

 nounced, so that even when a rock basin had been ground out to 

 a considerable depth the onward motion might be almost as great 

 as at the beginning of the process. 



If, now, we consider that the erosion I have attempted to describe 



