746 ARTHUR C. TROWBRIDGE 



correct, the explanation of the dissection of these deposits and the 

 later deposition of fans is not complex. 



A lake existed in Owens Valley, probably in Pliocene times, and 

 deposits were laid down in it on the flanks of the mountains. The 

 lake was drained or dried up, and the mountains were probably 

 uplifted. Dissection of the deposits thus exposed and uplifted 

 followed. After erosion had removed a large part of the lacustrine 

 deposit, deposition began at the foot of the mountains, and the 

 present fans have been built up among the remnants of the old 

 lacustrine deposits. 



CRITERIA FOR DISTINGUISHING ALLUVIAL FAN MATERIALS 



In conclusion we may bring together the distinguishing features 

 of the materials of fans, as seen in this region. The region affords 

 especially good facilities for the drawing of such conclusions, as it 

 contains both running-water and standing-water deposits of similar 

 ages. 



Deposits on alluvial fans may be distinguished from those in 

 still water, either lacustrine or marine, as follows: 



i. In alluvial fans, coarse material has a wide distribution as 

 against confinement to a narrow zone near shore in standing-water 

 deposits. 



2. Textural range in single exposures is large in fan materials. 



3. Fan materials are not in general so well sorted as deposits 

 in standing water. 



4. The beds and surfaces of fans are likely to have slopes of 

 6-1 8°, as against 0-3 in standing-water deposits. 



5. Fan materials are likely to have fewer and different fossils 

 than deposits in standing water. 



6. Fan material has a lens and pocket stratification, as against 

 a sorting into more or less uniformly thick horizontal layers, as in 

 lakes or seas. 



7. Huge bowlders widely distributed vertically and horizontally 

 in a deposit indicate that it was deposited by running water, and 

 with a large proportion of fine material; that is, they indicate that 

 the material is part of an alluvial fan deposit, except in cases where 

 glaciers have affected it, or where standing waters could have 



