720 ARTHUR C. TROWBRIDGE 



lower zone of bowlders is a fairly well-defined bed of gravel, the 

 constituents of which average 3-4 in. in diameter. Between this 

 gravel layer and the upper zone of bowlders, the material differs 

 in different parts of the cut, the structure being decidedly pockety. 

 In one place the fine gravels grade into the coarse bowlders above; 

 in another, there is a body of clay between the two; in another, the 

 gravel layer does not appear, and clay separates the two beds of 

 bowlders (Fig. 9). 



2. A six-foot cut a mile from the mountains shows a matrix 

 of clay and sand in which there are angular fragments averaging 



Fig. 10. — Roughly sorted materials of the Sierra bajada on George Creek 



10 in. through, with occasional bowlders 3 ft. in diameter. The 

 bowlders are angular or subangular, and none are well rounded. 

 They are not abundant enough to touch one another. There 

 is apparently no sorting. 



3. A hundred yards above the last section, the material is rudely 

 but distinctly sorted. At the bottom there is a fairly uniform layer 

 of angular gravel, averaging 4 ft. in thickness, of which the upper 

 i| ft. give place at the east end to a projection downward of a 

 pocket from the coarse layer above. There is little clay in the cut, 

 and the fragments of rock are abundant enough to touch one another. 

 The pores are filled with coarse, arkose sand, loosely packed. 



4. The last section gives place, within a few feet, to unsorted 

 material entirely similar to that of section 2. From here to the 

 mountains the sorted and unsorted materials occur with about 



