712 ARTHUR C. TROWBRIDGE 



of mere divides between channels and of lines of bowlders bordering 

 the channels. 



In keeping with their origin, the depressions and ridges are 

 radiate in their arrangement. At the head of each fan, these fea- 

 tures are few; toward the outer edge they are numerous; but at 

 the extreme edge they are again rare. Three miles from the edge 

 there are probably 50 channels, for one close to the head, and some- 

 thing like that proportion between the same three miles from the 

 edge and the outer edge itself. Channels which, near the head of the 

 fan, are close together, diverge outward, and each may break up 

 into other channels, each less deep and less broad than the one 

 from which it springs. Quite commonly these channels lead to 

 depressions between fans and disappear. 



It is clear that these channels on the fans mark the courses of 

 the distributaries from the fan-making streams. The streams 

 branched again and again, some of the distributaries reaching the 

 inter-fan depressions and flowing off through them. It is equally 

 clear that some of the elevations are merely inter-distributary 

 divides. The origin of the ridges bordering the depressions is not 

 so clear. Possibly they are in principle natural levees, built as the 

 waters overflowed their channels. It is understood that these are 

 the streams which deposited last on the surface of the fans. In the 

 building of the bajada, the channels undoubtedly shifted frequently, 

 those of one time being filled up and a new set formed during periods 

 of greater deposition following heavy rains or the rapid melting of 

 snow in the mountains. 



THE STREAM CANYONS 



The streams of the bajada do not now distribute over the fans, 

 but flow in deep, steep-sided, canyon-like valleys; that is, the 

 bajada is being dissected (Fig. 2). This is true to a greater or less 

 extent of all the streams which have played a part in the deposition 

 of the plain. 



The canyons in the bajada vary in depth from 20 ft. to 250 ft., 

 and average about 200 ft. in width. The depth is determined by 

 the size of the stream, the height of the fan, and the position of the 

 stream on the fan. The most pronounced canyons are those of 



