GRAVEL AS A RESISTANT ROCK 501 



may have worn back and the lowland developed by differential 

 erosion since the deposition of the gravels. 



Opposed to the first of these alternatives is the fact that the 

 gravel plateau ends abruptly along a relatively straight line. There 

 are no outliers of gravel between this general line and the mountains. 

 It is highly improbable that streams flowing nearly parallel and not 

 more than a mile apart should strip all signs of the gravels from the 

 upper four miles of their course, while in their lower course, where 

 they flow across the gravel plateau, they should be in relatively 

 narrow valleys with almost no tributaries and should have done 

 little more than to cut their way through the plateau without 

 having, been able to widen their valleys to any great extent (Fig. 2). 



A second objection is the fact that the line of contact between 

 the gravels and the underyling rock slopes upward toward the moun- 



Fig. 5. — -Sketch showing the relation between the gravels of the plateau and the 

 underlying rock which indicates that the gravels never reached much nearer the moun- 

 tains than now. , 



tains at such an angle that it would intersect the projected line 

 of the plateau surface at a point not far within the present limit of 

 the gravels (see Fig. 5). In other words the gravels thin toward 

 the mountains at such a rate that they would wedge out within a 

 short distance from their present limit, and the lowland is accord- 

 ingly developed in the bed-rock. 



A third objection is raised by the fact that in the gravels of the 

 plateau there are aggregations or nests of huge lava bowlders, 

 some of them 15 feet in diameter, which indicates that the moun- 

 tians must at one time have been closer, for bowlders of such size 

 are too large to be carried far by water, particularly by water 

 flowing on a slope of 100 ft. per mile, which is approximately that of 

 the plateau surface. 



The second alternative, faulting, seems highly improbable, for 

 there is no evidence whatever of the presence of faults along the 



