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



where the only hard rock to be eroded was the volcanic dam, it eroded four 

 rock gorges, namely, Boulder, Black, Needles, and Aubrey canyons, and 

 crossed four debris-filled basins, namely, the Las Vegas wash, Cottonwood 

 valley, Mohave valley, and Chemehuevis valley, before returning to its 

 former course in Great Colorado valley. The excavation of the detritus 

 in the basins was naturally more rapid than the work in the hard rock of 

 the ridges separating them, and the result is the curious alternation of 

 short, sharp canyons and basin-like valleys characteristic of the Lower 

 Colorado river. 



Plate 34 is a photograph, taken in Boulder canyon, showing walls about 

 2,000 feet high and illustrating the youthful character of the canyons 

 eroded during this epoch. 



A satisfactory explanation for the passage of the Colorado river through 

 Boulder canyon is yet to be found, as the mountain ridge is apparently 

 higher than the volcanic dam supposed to have caused the diversion of 

 the river. Several possible explanations might be offered, such as stream 

 capture, overflow through a low pass, or a rise of the mountains across the, 

 river's course. The latter seems to be the more probable, from the fact 

 that the Black mountains are in a region known to have been affected by 

 recent faulting and block tilting. 



DEPOSITION OF GRAVELS 



Some influence not certainly known brought the second period of ero- 

 sion to a close and caused the river for a second time to fill its valley 

 with sand and gravel. The debris-filled basins in which broad valleys 

 had been excavated while the canyons were being cut in the harder rock 

 were again filled to a depth of about 700 feet. These deposits occur in 

 the terraced bluffs on either side of the river more or less continuously 

 exposed from the mouth of the Gravel canyon to the gulf of California. 



CANYON CUTTING (3) 



At the beginning of this epoch of erosion the river was flowing within 

 narrow limits over an aggraded surface as it had done over a much wider 

 aggraded surface at the close of the second epoch described (epoch 6 of 

 the following table). Some change of condition, the cause of which is 

 not definitely known, caused the river to again erode its bed, reexcavating 

 for the most part the old channel, but in a number of places cutting rock 

 gorges at one side, as at Bulls head near Fort Mohave and at Big bend 

 in the Needle mountains. 



FORMATION OF FLOOD-PLAINS 



After the channel had been cut considerably lower than the present bed 

 of the river, sand and gravel for the third time accumulated, the action 



