742 ARTHUR C. TROWBRIDGE 



ting in front, and then by gravity and the push of water and sedi- 

 ment from behind, the large piece is pulled and pushed forward 

 into the depression prepared for it. This process takes place over 

 and over again, the large piece being moved down the low gradient 

 in a halting fashion. The depth of the depression into which the 

 piece falls is never so deep as the diameter of the fragment moved. 

 Once started in motion, the piece is sometimes carried many times 

 its own length by its momentum, and by the force of water and 

 gravity. The motion is usually one of sliding rather than rolling. 



It is conceivable that these same methods might operate on 

 the surface of an alluvial fan, on a scale large enough to transport 

 bowlders even 20 ft. in diameter distances of several miles. The 

 bowlder starts from the head of the fan in company with a rela- 

 tively large amount of fine material. The volume of the stream 

 varies greatly from time to time, with great differences in preci- 

 pitation in the mountains, and with daily and seasonal ranges in 

 the rate of melting of glaciers. Material is deposited and rehandled 

 time and time again. When the volume is great, fine material is 

 removed from the front of the bowlder and from beneath its front 

 edge, while other material is piled against its upper side, and the 

 bowlder falls, or is pushed, or rolled over into the depression. As 

 the flood subsides, the bowlder may be almost or completely buried, 

 but the next flood uncovers it, and the process is completed. With 

 sufficient time, sufficient variation in volume of water, and sufficient 

 rehandling of material, huge bowlders may thus be transported 

 great distances. 



Would the slope of the fans be sufficient for such transportation? 

 In the roadside rill the piece of rock moves a distance several times 

 its own length, while dropping less than its own diameter. Suppose 

 the bowlder 20 ft. in diameter moves 40 ft. horizontally, with a fall 

 of 15 ft.; this would require a gradient of 1,980 ft. per mile. If it 

 moves 60 ft., with a 10-foot drop, the gradient would be 880 ft. 

 per mile. The average slope of the bajada is about 400 ft. per mile. 

 This requires that the bowlder west of Lone Pine, 10X20X30 ft., 

 move about 120 ft., or four times its own length, in dropping 10 

 ft., or about its own smallest diameter, if the proportions observed 

 hold. 



