GRAVEL AS A RESISTANT ROCK 51 
gradient near the mountain core probably existed; first, because 
such an increased gradient is normal, and second, because there is 
in this entire region strong evidence of post-Pleistocene tilting 
(due to faulting) in the north. 
Next, the diagram, Fig. 5% (used as evidence in the argument 
that the gravels never extended farther mountainward), is mis- 
leading. By actual outcrop on the map it may be shown that 
the gradient of the gravel base, measured from a ridge top to a 
© QBs 5 
=] 150 FE. to/mile 
the 
Gradcent less than 
= to / mile 
i!  Aemllr 
Pleistocene Gravel 
Fic. 1.—Showing irregular edge of gravel sheet and outliers north of it 
valley bottom (the most favorable measurement), in places does not 
exceed 200 feet to the mile. The diagram shows a gradient of 
1,500+ feet to the mile. The long thin outlier north of Silver 
City has a basal gradient approximating too feet to the mile. The 
outlier on the extreme west has apparently a still lower gradient. 
The diagram is misleading because, with the low gradient that is 
shown above to exist, and with a gravel plain the top of which has 
-an increasing gradient, the gravel might well have extended farther 
mountainward. 
Further to cast doubt on the hypothesis that the mountain 
front once stood at the present position of the gravel edge and 
LOP. cit., p. 50. 
