90 



SCIENCE 



[N. S. Vol. XXXIV. No. i 



by Dutton." This conelusion does not appear 

 to have received the attention that it merits, 

 probably because Button's monograph has for 

 so long been the accepted source of informa- 

 tion concerning the region. But as more re- 

 cent work has shown, Dutton's conclusions in 

 regard to the history of the Grand Canyon 

 District are subject, in general, to important 

 modifications. The writer would like to em- 

 phasize, therefore, some of the data support- 

 ing Davis's conclusion as to a single cycle of 

 development as they impressed themselves 

 upon him during a recent revisiting of the 

 eanyou after a lapse of several years. 



One of the most striking features in the 

 development of the canyon is the perfect cor- 

 relation that exists between the topography 

 and the different rock formations. This is 

 seen throughout the canyon, but is more 

 easily comprehended in small areas. On a 

 large scale two distinct canyon forms may be 

 noted depending upon the character of the 

 eroded strata. One is the comparatively wide- 

 bottom type which is developed where the 

 strata are soft, as in the Unkar and Chuar 

 area in the vicinity of the mouth of the Little 

 Colorado River. The other is the more com- 

 mon gorge type as developed, for example, in 

 the Kaibab and Toroweap sections of the 

 canyon where the river is cutting the more 

 resistant granite, Tonto and Eedwall forma- 

 tions. In any limited view of the canyon 

 walls the relationship is even more impres- 

 sive; the resistant formations always give rise 

 to cliffs and the softer ones to graded slopes, 

 so that the resulting topography is of wonder- 

 ful constructive beauty and possesses an al- 

 most infinite variety of detail. 



Attention may be called particularly to the 

 resistant cliff-making formations and the part 

 they have played in bench-making through the 

 stripping of the overlying soft strata. It 

 may help to visualize the facts here presented 

 if the idea is kept in mind of the Colorado 

 River cutting into the plateau and succes- 

 sively exposing the resistant formations on 

 which benches have been developed by the 



= ' ' The Tertiary History of the Grand Canyon 

 District, ArizoBa," Monogr. II., U. S. G. S., 1SS2. 



removal of overlying soft strata; the develop- 

 ment and dissection of each bench progressing 

 as the river has cut more deeply into the 

 plateau. 



In the Kaibab section of the canyon benches 

 may be seen at several different horizons. 

 There are traces of one at the summit of the 

 upper Aubrey cross-bedded sandstone near the 

 top of the canyon walls. It has probably 

 never been of any extent, since the overlying 

 upper Aubrey cherty limestone is in itself too 

 nearly of the same resistance to be easily 

 stripped off. There are numerous indications 

 of a bench at the summit of a group of heavy 

 sandstone members near the top of the lower 

 Aubrey red sandstone. The upper one third 

 of this formation is, on the whole, more uni- 

 formly soft than the lower two thirds and 

 must have been eroded with comparative 

 rapidity. The next bench, a very noticeable 

 one, is situated at the summit of the Eedwall 

 formation, which is the most prominent cliff- 

 maker found in the canyon walls. The lowest 

 bench is the Tonto, located immediately above 

 the granite gorge. This bench is the latest 

 one that has been formed and is, therefore, 

 the least dissected. 



Throughout there is, then, a striking de- 

 pendence of the benches upon the character of 

 the strata; they are found at the suimnits of 

 resistant beds which in all cases are overlain 

 by soft ones. If, therefore, there is any good 

 reason for considering the Tonto bench in the 

 Kaibab section as indicating a base-level of 

 erosion and a halt in the uplift of the region, 

 as is necessary on the assumption of two 

 cycles of development, there is equally good 

 reason for supposing that base-levels of ero- 

 sion also occurred at the summits of the other 

 resistant formations. It is hardly reasonable, 

 however, to expect such a nice adjustment of 

 base-levels, three or four in number, to such 

 definite structural horizons. It must be con- 

 cluded, rather, that the benches are simply 

 what they appear to be — the stripped surfaces 

 of resistant formations which have been suc- 

 cessively exposed in the progressive down-cut- 

 ting of the Colorado River through the pla- 

 teau. Consequently the uplift of the region. 



