280 W. T. LEE GEOLOGY OF THE LOWER COLORADO RIVER 



lowed southward to the sea. After the inauguration of the Grand can- 

 yon, six distinct epochs are recognized, as follows, and more detailed 

 examination may reveal others. 



CANYON CUTTING (1) 



The first epoch was one of regional uplift and active erosion in the 

 plateau province. During this epoch the Colorado plateau rose to some- 

 thing like its present altitude and the Grand canyon was cut to a depth 

 of ahout 6,000 feet. The area west of the plateau remained at a lower 

 altitude, but was probably elevated to some extent, since the river lowered 

 its channel 1,400 feet or more beneath the previously graded surface in 

 Iceberg and Virgin canyons. The amount of erosion within the Detrital- 

 Sacramento valley was probably correspondingly great, but little can be 

 said of this on account of the later alluvial filling of the valley which has 

 not been removed. 



GRAVEL DEPOSITS AND LAVA FLOWS 



After the canyon had been cut to its present depth near the western 

 border of the Colorado plateau, but probably not to so great a depth 

 farther upstream, some change occurred which caused the river to deposit 

 sand and gravel along its course from the Grand canyon to the gulf of 

 California. Near the mouth of the canyon gravels were deposited in a 

 narrow valley and only small remnants of the beds remain at the present 

 time, as those in Grand Wash trough previously referred to as the 

 younger of the two detrital formations found there; but in the broad 

 Detrital-Sacramento valley immense deposits accumulated to a depth of 

 2,000 feet or more. Similar deposits were formed in the other valleys of 

 the Southwest and the low-lying parts of the interstream areas were built 

 up by accumulations of angular. rock debris derived from neai-by moun- 

 tains. Plate 33 is a photograph taken at a point where the detrital plain 

 meets the rock slope of Hualpai mountains, and illustrates the character- 

 istic relation of the detritus to the mountain slopes throughout western 

 Arizona. 



The extensive aggradation caused some of the rivers to wander from 

 the valleys formerly occupied by them, and their gravel-filled channels 

 were in some cases covered by upland wash. A notable occurrence of 

 river gravels beneath a desert plain and the diversion of the river which 

 deposited them has been described by the writer in a paper on the under- 

 ground water conditions of Salt Eiver valley,* in which Salt river, now 

 flowing north of Salt Eiver mountains, is shown to have formerly passed 

 to the south of those mountains. 



* U. S. <:eoloj. v ieal Survey. Water Supply and Irrigation Paper no. 136, 1905. 



