GRAVEL AS A RESISTANT ROCK 503 



country. With respect to the drainage to the south, the extent to 

 which this process can be carried is limited by the base level of the 

 interior basin to which the streams are tributary. 



A factor which must profoundly affect the topographic develop- 

 ment of the whole region is the Gila River with its tributaries which, 

 passing within 20 miles of Silver City, to the northwest, drains 

 a large proportion of the mountain area. The Gila drains directly 

 to the sea, and being a good-sized permanent stream, whose valley 

 is some 1,600 ft. lower than the gravel plateau, it is actively pushing 

 its headwaters southeastward into the drainage area of the interior 

 basin in which the plateau is situated. The divide, in one place, 

 now lies only 6 miles from the gravel plateau and is only 400 ft. 

 higher. The Gila affords opportunity for the free removal of the 

 waste from the mountains. Short and steep slopes combine to 

 increase its effectiveness. 



Eventually the normal outcome of processes now in operation 

 should be that the mountains would become lowered; the interior 

 lowland between the plateau and the mountains would, by capture, 

 become tributary to the Gila; and the plateau itself, remaining 

 higher on account of its superior resistance to erosion, would ter- 

 minate in a scarp overlooking the lower lands to the north. 



SIMILAR FEATURES IN OTHER REGIONS 



Other areas are known where gravel deposits of a nature similar 

 to those on the plateau east of Silver City occupy a similar topo- 

 graphic position and seem to show much the same history. 



A good example, with which the writer is familiar, is the Bishop 

 conglomerate of southwestern Wyoming and northeastern Utah. 

 This represents a Piedmont gravel accumulation derived from the 

 Uinta Mountains, and at one time skirting entirely round their 

 base. Subsequent erosion has so lowered the mountains that over 

 considerable areas, particularly at the eastern end, they are actually 

 lower than the tops of the gravel-capped plateaus which represent 

 the eroded remnants of the Piedmont gravel deposits. This condi- 

 tion has been described by the writer in an earlier paper. 1 Between 



1 "The Physiography of the Bishop Conglomerate, Southwestern Wyoming," 

 Jour. Geol., XVIII, No. 7 (1910), 601-32. 



