NEW EROSION CYCLE IN GRAND CANYON DISTRICT 761 



Quaternary/ while the present one throws it still farther forward 

 into the latter part of that period. Granting the correctness of the 

 assumptions previously made, the reasonableness of this conclu- 

 sion seems evident when the relative amounts of erosion during 

 the post-peneplain and canyon cycles, and the wide difference in 

 the conditions under which it was accomplished are considered, 

 although the correctness of the results in general remains for future 

 demonstration. It is suggestive of the truth of the conclusion 

 here reached, however, that Lee in his bulletin entitled "A Geo- 

 logical Reconnaissance of Western Arizona"^ describes, at a num- 

 ber of localities, a thick unconsolidated gravelly formation that 

 has experienced marked faulting. In speaking of this formation 

 as it occurs in the Chemehuevis Valley, he says : 



The older gravels are horizontally bedded in some places but in others are 



faulted and highly inclined In general appearance they resemble the 



Temple Bar Conglomerate and are provisionally correlated with it.3 



The suggestive point is that the Temple Bar Conglomerate is 

 considered as being of Quaternary age and deposited after a last 

 period of pronounced faulting at the opening of the Quaternary, 

 so that it should be comparatively undisturbed. If one may 

 judge, however, from Lee's descriptions and cross-sections, the 

 formation which he provisionally correlates with the Temple Bar 

 Conglomerate has been very strongly faulted and tilted at a num- 

 ber of localities. This is easily explained, and Lee's correlation 

 strengthened, if the major faulting which introduced the canyon 

 cycle of erosion occurred during instead of at the beginning of the 

 Quaternary. For with the faulting occurring during the middle or 

 latter part of the Quaternary there would still have been ample 

 time in the earlier part of that period for the deposition of heavy 

 alluvial deposits upon the lowlands bordering the plateau country. 



There are three other points which may receive further mention. 

 One is that on the basis of the erosion cycles in the Grand Canyon 

 district the mature topography of the Basin Ranges of southern 

 Nevada and of Arizona should be considered as the result not only 

 of the peneplain but also of the post-peneplain cycle of erosion. 

 The former was much the more important, but the latter may have 

 introduced significant modifications. Whether they would be 



I Ibid., III. 2 U.S.G.S., Bull. 352 (1908). 3 Ibid., 43-44. 



