THE GRINDING ACTION OF GLACIERS. 293 



glaciers in forcing along bowlders or in producing striation of the underly- 

 ing rock-surface should remember that their observations were confined to 

 the melting extremities of glaciers when the temperature of the ice was at 

 its highest possible point and when the hydrostatic pressure had almost 

 vanished. The latter circumstance enables the ice to gradually rise and ride 

 over the till, while the softened and comparatively warm ice, on the point 

 of melting, would offer the least resistance to bowlders or any other solid 

 objects. It should be further remembered that when these objects have 

 become exposed so as to be visible to the eye they must constantly absorb 

 heat from the air in summer and thus, as it were, thaw their way into the 

 glacier as fast as it advances towards them, producing grooves in its sub- 

 stance just as a stone will sink into ice by gravitation. If, on the contrary, 

 bowlders and finer debris be incorporated in very cold and hard ice thou- 

 sands of feet beneath its surface and firmly held in their places by the enor- 

 mous pressure from all sides, there can be no doubt of their acting as most 

 powerful abrading agents. In places w r here the ice- sheet was from one to 

 two miles in thickness, as some geologists reasonably enough believe it to have 

 been, its weight would exercise not only an abrading but a tremendous 

 crushing and bruising effect on the surface of the rock beneath. At times 

 this would slightly displace great sections of rock exposed to its force and 

 gradually break them up and wear them into bowlders, some of which might 

 still remain of large size at the close of the drift period ; or if the whole, 

 mass should happen to settle into a protected situation, or if the ice should 

 disappear before breaking it up, the greater part of the mass might remain 

 till the present day. The crevices or spaces between the rock in situ and 

 the displaced mass would become packed with drift material, and the fact 

 that the displacement had occurred at all could only be discovered in the 

 side of a cliff, or by landslides or artificial cuttings. 



A case of this kiud appears to have occurred at Wine harbor, Nova 

 Scotia, where a part of the area mined for gold seems to have been slightly 

 displaced en bloc, as a layer of hard gravel and mud was found separating 

 the upper hundred feet or so of rock from that below. In the cuttings 

 along the Canadian Pacific railway, north of lakes Huron and Superior, 

 seams or crevices filled with till are occasionally seen in the apparently 

 solid crystalline rocks. When building the line at Rossport, on Lake 

 Superior, a part of the mountain side including many thousands of cubic 

 yards, slid bodily into the lake in consequence of one of these openings. 

 It is probable that these crevices often act as reservoirs of water which 

 feed the springs among the Archean rocks. 



When we think of the enormous weight of the ice-sheet with its abrading 

 materials beneath it, the only wonder appears to be that the evidence of its 

 crushing and gouging effects is not greater than we see. These forces are 



