Smith.] 



Some Aspects of Erosion. 



'73 



conditions in the two regions;" for the breadth of the valleys in 

 their lower reaches implies that the bases of the slopes have been 

 cut back by lateral corrasion, and the effectiveness of this method 

 of reduction of interstream uplands has been indicated above. 



According to the explanation of beveling given here, it might 

 be expected in mature regions even of horizontally bedded rocks, 

 which may give more or less flat-topped ridges, that the strata will 

 be beveled going down stream. In fact, there should be beveling 

 of the edges of the strata in all cases where the streams and ridges 

 cut across the bedding of the rocks, except where the dip of the 

 rocks is in the direction of the flow of the stream, and at an angle 

 corresponding closely to the grade of the channel. 



Soil. — In regions of active erosion, depth of soil appears to be 

 more largely dependent on slope, climate, character of the sub- 

 jacent rocks and of the vegetation, than on elevation. I have seen 

 many instances of very thin soils among the mature forms of the 

 Coast Ranges of California, at very low altitudes. On the other 

 hand, I have recently had occasion to observe, in the Sierra 

 Nevadas, a striking instance of deep soil at a comparatively high 

 altitude — about 4,500 feet. The area is on a gentle slope, and in 

 such a position that the surface could not well be assumed to be a 

 part of an old peneplain, although in a region of generally even 

 sky-line. In a shaft which had been run to a vertical depth of fifty 

 feet below the surface, no solid rock had been reached. The soil is 

 due to decomposition of the rock (diorite ?) in place. This instance 

 goes to show that a considerable depth of soil may occur elsewhere 

 than at low altitudes, and without peneplanation of the surface. 



It remains true, however, that deep soils widely distributed over 

 the uplands, and more or less independent of rock character, may 

 be taken as evidence in favor of a peneplain, when there is other 

 sound testimony to the same effect. ^ In the actual peneplain, river 

 gravels should be widespread, and it would seem that their evidence 

 should persist for a long time after the elevation and beginning of 

 the dissection of the peneplain. 



Vegetation. — There can be no question as to the effectiveness of 

 erosion, even on vegetation-covered areas. If the streams continue 

 to cut down their beds, the slopes must follow, sooner or later, no 



