7i 6 ARTHUR C. TROWBRIDGE 



aided in the upper parts of the canyons by glaciers. There is every 

 evidence of immature weathering of the materials. At the time 

 the fans were being deposited, the mechanical processes of weather- 

 ing greatly exceeded the chemical, and transportation was free and 

 rapid. 



TEXTURE 



The fans contain all textural grades from pieces the size of small 

 sand grains and even clay particles, to bowlders more than 20 ft. 

 in diameter, but pieces less than an inch in diameter predominate. 



The most striking and surprising feature of these fans is the 

 extreme coarseness of some of its materials. Innumerable large 



Fig. 6. — Bowlders on the surface of the Sierra bajada. Their size may be esti- 

 mated from the horse. Picture taken on the fan of Sawmill Creek about a mile from 

 the foot of the mountains. 



and small bowlders appear on its surface, on the sides of the shallow 

 channels, and on the walls and floors of the canyons. On the 

 unaltered surface, they occur in radiating lines and belts, roughly 

 parallel with the radiating channels. They are practically confined 

 to the higher parts of the surface, where the main streams flowed. 

 None appears in the inter-fan depressions. More bowlders are 

 scattered near the heads of the fans than toward the outer edges 

 but they are not noticeably larger here. They are arranged in 

 belts or low ridges along the borders of the shallow channels and 

 are scattered more sparsely on the side slopes. Bowlders are 

 numerous on the walls and bottoms of the canyons, but without 

 definite arrangement. The beds of the streams are everywhere 

 choked with them, and they occur in and along the braided channels 



