NEW EROSION CYCLE IN GRAND CANYON DISTRICT 755 



topography of the intermediate cycle was developed on what was 

 evidently, under the circumstances, a highly resistant formation, 

 namely, the upper Aubrey cherty limestone. This formation is as 

 resistant, for instance, as the basalt which was erupted after the 

 development of the peneplain at the close of the period of the great 

 denudation and is very much more resistant than the overlying 

 Permian and Triassic strata remaining after the development of the 

 peneplain. A comparison of the amount of erosion during the post- 

 peneplain cycle in the extreme southwestern part of the plateau, 

 where the limestone and basalt were present, and the remainder of 

 the district which was covered, for the most part, with Permian and 

 Triassic strata, is very instructive upon this point. The basalt in 

 the former locality was only moderately dissected, and the lime- 

 stone only slightly, as the result of erosion during the whole of the 

 post-peneplain cycle, while elsewhere Permian and Triassic strata up 

 to a maximum thickness of over 1,000 feet were removed. The 

 difference suggests how erroneous might be an idea of the erosion 

 based on incomplete observations. The point to be noted is that 

 the development of a thoroughly mature topography, even of a 

 low degree of relief and representing no great removal of material, 

 on so resistant a formation as this limestone is indicative of an 

 erosion interval as long in itself, perhaps, as that marked by the 

 canyon cutting. And when to the time required for the develop- 

 ment of the mature topography on the resistant limestone is added 

 that necessary for the previous extensive and widespread removal 

 of the overlying Permian and Triassic strata an interval is indicated 

 which was probably much longer than that covered by the cut- 

 ting of the youthful canyons. Moreover the strata involved in 

 the latter process are not all so resistant as the upper Aubrey 

 limestone on which the mature topography was developed. 

 Several important formations are distinctly weaker, as, for ex- 

 ample, much of the lower Aubrey red sandstone, and the presence 

 of these weaker beds tends to increase the difference between 

 the lengths of time required for the canyon cutting and post- 

 peneplain erosion in favor of the latter. The interval covered 

 by the post-peneplain cycle of erosion was very much shorter, of 

 course, than that of the preceding period of the great denudation, 



