n8 POPULAR SCIENCE MONTHLY 



the rate of erosion. There can be no question, however, that the nature 

 of the valley rock is of profound importance, some weak rocks being 

 eroded with relative rapidity by small glaciers, other rocks resisting 

 the erosion of even large, powerful glaciers. Two causes, the size 

 of the glacier and the nature of the enclosing rock, are, in all proba- 

 bility, of most importance in the modification of the height of valleys 

 left hanging by more rapid erosion along the main trough. 



An argument which has been advanced against the power of 

 glaciers to erode, is the fact that rock islands sometimes rise from the 

 floor of valleys through which powerful glaciers have passed. It 

 has been claimed that such protuberances should have been erased 

 if the glaciers were really eroding greatly. When the operation 

 of glaciers as agents of erosion is truly understood, however, this 

 argument seems to favor rather than to oppose glacial erosion. It 

 is not to be supposed that glaciers would erode everywhere at the same 

 rate. There is naturally a variation in the rate of erosion of a valley 

 bottom dependent upon at least two important influences — nature of 

 rock and rapidity of ice currents — both of which are liable to vary in 

 any valley and thus necessarily give rise to irregularities in the ice- 

 eroded valley bottom. Once an obstacle arose in the path of a power- 

 fully moving glacier, it would have the tendency to split the ice current 

 around itself, much as a sand bar spilts the current of a river. By 

 interfering with the ice current in line with the obstacle, and by caus- 

 ing a concentration of movement on either side of it, the size of the 

 obstacle would naturally increase. Eock knolls, islands and nunataks 

 (Fig. 14) are such characteristic features in glacially eroded valleys 

 that, when the full significance of glacial erosion is understood, I be- 

 lieve they will be found to constitute one of- the distinctive evidences 

 of glacial erosion, to be -classed with hanging valleys, truncated spurs, 

 steepened slopes and U-shaped profiles. 



In discussions on glacial erosion much attention has been paid to 

 rock basins, — basins with rock rims in the bottoms of glaciated velleys, 

 and oftentimes holding lakes. Such basins also occur on the fiord 

 floors of the Inside Passage. Irregularities in erosion, due to dif- 

 ferences in rock resistance and in ice currents, readily account for these. 

 As Andrews has shown in his remarkable papers on glacial erosion 

 in the ISTew Zealand fiords, one important cause for such basins, and 

 other forms of vigorous erosion, is the convergence of ice currents in 

 a valley of smaller cross section, causing acceleration of motion. 

 Rock basins must be added to the land forms resulting from and hence 

 indicative of profound glacial erosion. 



Another feature at first apparently opposing glacial erosion is that 

 hanging valleys, truncated spurs, and steepened slopes are at times well 

 developed on one side of a main trough and either absent or poorly 

 developed on the other. This, however, seems a perfectly normal result 



