5°° 



JOHN LYON RICH 



general line of the gravel plateau. Such a projection may be 

 assumed to be a minimum original slope, for it makes no allowance 

 for an increased slope of the plateau surface nearer the mountains, 

 as must have been the case if the gravels once covered the lowland. 

 This feature is illustrated in the two profiles shown in Fig. 4, 

 drawn to scale from two different points well out on the plateau 

 northward across the lowland to the base of the mountains. 



-ital <Scaie m Milt 



Vertical ScaU in Feet 



Fig. 4. — Profiles across the gravel plateau and the lowland from points on the 

 desert to the foot of the mountains, showing the projected gravel surface and the 

 relations of the gravels to the mountains. 



With topographic relations as they are at present the nearest 

 possible source of the gravels of the plateau is separated from it by 

 a lowland averaging 4 miles in width. It will be at once evident 

 that, at the time of the formation of the plateau, the lowland could 

 not have existed in its present relation. Any one of three things may 

 have happened to bring about present conditions: (1) The gravels 

 may have been removed by erosion from the area between their 

 present limit and the mountains; (2) there may have been faulting 

 by which the lowland was relatively lowered ; or (3) the mountains 



