754 S. H. ROBINSON 



it is, therefore, arbitrarily placed at 50 cubic miles, which allows for 

 the removal of over 100 feet of strata from an area of more than 

 2,000 square miles. Including this, the total volume of material 

 eroded during the canyon cycle is placed at 600 cubic miles. 



A contrast may thus be drawn between the removal of 800 cubic 

 miles of Permian and Triassic strata over an area of some 8,000 

 square miles with the development of a mature topography on the 

 stripped resistant limestone — the post-peneplain erosion — while 

 the land stood at a fairly low elevation, and the erosion of 600 cubic 

 miles of material as the result of canyon cutting — the canyon 

 erosion — with the land at, or rising to, a high elevation. If it is 

 asked whether the erosion above described occurred in a single 

 cycle, the answer must be in the negative. For erosion in the can- 

 yon cycle has been proceeding at an extremely rapid rate and con- 

 sequently the 600 cubic miles of material removed in the cutting 

 of the youthful canyons indicates much too short a time to permit 

 the stripping of the 800 cubic miles of Permian and Triassic 

 strata from the surface of the plateau and the development of the 

 mature topography on the underlying Hmestone. To consider the 

 post-peneplain erosion as occurring in the same cycle as the canyon 

 erosion is to include the greater within the lesser, thus producing 

 an anomalous result. If, on the contrary, a thousand cubic miles 

 of material had been eroded in the canyon cutting and but a few 

 hundred stripped from the surface of the plateau, and especially 

 if the mature topography had been developed on weak instead of 

 resistant strata, it might be supposed to have entirely occurred 

 in a single cycle. But in view of the evidence furnished by the 

 mature valleys, the differences in direction between the mature and 

 canyon systems of drainage, the relative amounts of erosion in the 

 post-peneplain and canyon cycles, and the differences in the resist- 

 ance of the strata eroded in the two periods, the conclusion is fully 

 justified that the erosion of the post-peneplain and canyon cycles 

 could not well have taken place in a single cycle and consequently 

 the post-peneplain cycle must be given an independent rank. 



An important consideration in separating the post-peneplain 

 cycle of erosion from the preceding peneplain cycle and especially 

 from the succeeding canyon cycle lies in the fact that the mature 



