102 STUDIES, SCIENTIFIC AND SOCIAL 



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 " or sub- 

 glacial moraine-stuff, found everj^where 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 the continually changing outlines of a 

 valley bottom. Hence, where great inequalities occur 

 portions of the rocky floor might be bridged over for a con- 

 siderable space, and where a valley had a narrow V-shaped 

 bottom the sub-glacial stream might cat 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 tolerably 

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

 downwards 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 tem- 

 perature of the soil and the ice. At the same time 

 sub-glacial streams, sometimes forced onward under hydro- 

 static pressure, would insinuate themselves into every 

 vacant groove and furrow as each graving tool succes- 

 sively 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 permanently as masses of clayey " till," packed 

 with stones and compressed by the weight of the ice to 

 the hardness of rock itself. The continual lubrication of 

 the whole valley floor by water, together with the ever- 

 changing form of the under surface of the glacier as it 

 slowly moulded itself to the varying contours of the rocks 

 beneath, would greatly fticilitate 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 



