744 S. H. ROBINSON 



deciphered by Button, and omitting many details of an admittedly 

 speculative character, comprised the following events: 



1. A period of great denudation during which a thickness of 

 strata averaging 10,000 feet was removed from over an area of 

 13,000 to 15,000 square miles. This period ended somewhere 

 about the close of the Miocene. 



The Grand Canyon platform then may have lain near sea-level, and the 

 remnants of Mesozoic beds .... were gradually obliterated, and the entire 

 region was planed down to a comparatively smooth surface [p. 119] . . . . 

 and was at a base-level of erosion [p. 120]. 



2. A canyon-cutting cycle. This was initiated by an epoch 

 of faulting at the beginning of the Pliocene which elevated the 

 region from 2,000 to 3,000 feet above the level it occupied at the 

 close of the period of the great denudation. 



At the epoch when the cutting of the present Grand Canyon began, no 

 doubt the region at large presented a very different aspect from the modern 

 one. While the greater part of the denudation of the Mesozoic had been 

 accomplished, there were some important remnants left which have been 



nearly or quite demolished in still more recent times [p. 223] The 



uplifting forces suspended operations for a time, and the drainage system sought 

 a new base-level. During this paroxysm of upheaval the outer gorge of the 

 Grand Canyon was cut, the river corrading down to the level of the esplanade 

 in the Kanab and Uinkaret divisions, but below that horizon in the Kaibab 



[p. 226] The process of erosion during this second period of base-level 



was occupied in the only possible work under the circumstances, viz., sapping 

 the newly formed cliffs of the canyon. The cliffs, thus attacked, receded 

 away from the river, gradually developing the broad avenue of the outer 



chasm [p. 121] We now come to the final upheaval which brought 



the region to its present condition A new paroxysm of upheaval set 



in ... . amounting probably from 3,000 to 4,000 feet. The narrow, inner 

 gorge of the Toroweap was swiftly cut and it is in this respect a type of the 



lower depths of the entire canyon The epoch at which this latest 



upheaval took place is no doubt a very recent one in the geologic calendar. It 

 began most probably near the close of the Pliocene [p. 228]. 



The history of the region, as worked out by Button, may be 

 tabulated as follows: 



I. The period of great denudation lasting until the close of the Miocene. 



II. Uplift by folding (?) and faulting at close of Miocene. 



III. The canyon cycle of erosion. 



a) Cutting of outer gorge of Grand Canyon during the Pliocene. 



b) Uplift by faulting at close of Pliocene. 



c) Cutting of inner gorge of the canyon during the Quaternary. 



