NEW EROSION CYCLE IN GRAND CANYON DISTRICT 753 



the history of the region. Indeed, if the supposition is as correct as 

 it appears to be that the ancestors of the Colorado River, several 

 generations removed, had their courses originally determined by the 

 configuration of the peneplain, it is evident that the peneplain must 

 have embraced practically the entire area of the Colorado plateaus, 

 and consequently this problem presents a very wide interest. 



A comparison of the extent of erosion, taken in connection with 

 the conditions under which it was accomplished, during the post- 

 peneplain and canyon cycles furnishes added reason for separating 

 them. The figures which follow are, of course, the roughest approx- 

 imations, but they may serve to give some idea of the amount of 

 material eroded. And first it may be noted that the period of the 

 great denudation is distinctly in a class by itself. Taking Button's 

 estimate of an average thickness of 10,000 feet of strata removed 

 over an area of 13,000 square miles, the volume eroded is equal 

 to 25,000 cubic miles; or a more conservative estimate, with the 

 thickness of strata placed at 6,000 feet, is 16,000 cubic miles. 

 The amounts of material eroded during the post-peneplain and 

 canyon cycles were very much smaller; it is estimated that their 

 combined volume is only about 5 to 10 per cent of that removed 

 in the period of the great denudation. 



The amount of material eroded during the post-peneplain cycle 

 is placed at 800 cubic miles. This is based on the removal of an 

 average thickness of 500 feet of Permian and Triassic strata from 

 an area of some 8,000 square miles. The latter figure shows how 

 widespread was the stripping of the soft strata overlying the resist- 

 ant upper Aubrey limestone during this cycle. In view of so exten- 

 sive a denudation ending, as it did, with the development of a 

 mature topography of only slight relief, the conclusion seems justi- 

 fied that the region during this time must have stood at no great 

 elevation above the sea. 



The amount of material eroded during the canyon cycle is 

 considered as equal to the volume of the various canyons of the 

 region, which practically means the Grand Canyon and its tribu- 

 taries, and in addition some volume of soft strata removed by 

 stripping. This latter process has been confined to limited areas 

 and the volume of material thus removed cannot well be calculated ; 



