ERODED RIVER BEDS. 65 



surfaces in the valleys or ravines were very little above those now exposed. 

 It was the high exposures, peaks and cliffs, which furnished the moraines to 

 be fouud at lower altitudes. 



Thus the relation between the ancient and modern stream beds is that rep- 

 resented in the following diagram. 



Figure &— Erosion by modern rivers. 



Erosion influenced by Ice Cap. — The climatic conditions affecting erosion 

 during the Pliocene were very different from those which influenced it during 

 the period of glaciation, and for this difference an allowance must be made. 

 During the Pliocene there were no glaciers and probably little snow, if any. 

 The rain must therefore have flowed down the mountain sides very rapidly, 

 eroding them as it went. During the glacial period, on the other hand, the 

 flats and valleys from 5,000 feet or less to the summits of the passes were 

 deeply buried in ice and snow. The upper half of the range was thus pro- 

 tected from direct rain erosion, though subjected to the action of ice. 



Some few geologists still believe that glaciers not only sweep their beds 

 clear of loose rocks and soil and polish the solid underlying mass, but that 

 they vigorously erode solid rock. In my opinion this theory is maintained 

 in opposition to overwhelming evidence. Reference has already been made 

 to some of the many facts indicating a trifling amount of erosion since a 

 preglacial date in the higher part of the Sierra, and long before my exam- 

 inations Professor Whitney reached the conclusion that the solid rock had 

 been scoured rather than eroded by glaciers. It is more satisfactory, how- 

 ever, to observe what erosion modern glaciers actually effect than to infer 

 what ancient ones accomplished. On this question Professor Albert Heim 

 has no equal as an authority. A few of his conclusions, based upon his own 

 fully described observations and confirmed by those of others, are as follows : 

 "Advancing glaciers often leave even loose detritus undisturbed." This 

 happens, he explains, especially where the glacial valley widens. " Under 

 other circumstances, particularly in constricted valleys or where there are 

 obstructions, the advancing glacier ploughs up the ground down to the solid 

 rock;" but " it is only loose masses standing very much in the way which are 

 pushed along by the glacier." •' A glacier is unable to scour away even a 

 small crag standing in its path." I have myself seen, in Switzerland, illus- 



X— Bull. Geol. Soc. Am., Vol. 2, 1890. 



