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W. T. LEE GEOLOGY OF THE LOWER COLORADO RIVER 



geologic history of the Southwest may not be here recorded. It is 

 thought/ however, that the Colorado river, flowing as it does through the 

 midst of the region where the Quaternary deposits have such great devel- 

 opment, is likely to furnish the best field for the study of these deposits. 



Tabular Resume 



Deposition epochs. 



f 10. Formation of flood-plains ; ac- 

 cumulation still in progress. 



8. Deposition of 

 and gravel. 



i00 feet of sand 



6. Widespread aggradation and 

 volcanic eruption ; Colorado 

 river deposits 2,000 feet or 

 more of sand and gravel. 



Pliocene ; local deposition ; fill- 

 ing of Grand Wash trough; 

 erosion of Detrital - Sacra- 

 mento valley. 



Erosion epochs. 



Rejuvenation of streams; Colorado 

 river lowers its bed about 500 

 feet. 



Rejuvenation of streams ; Colorado 

 river, flowing west of Black 

 mountains, lowers its bed 2,000 

 feet or more and cuts Aubrey, 

 Needles, Black, and Boulder can- 

 yons. 



Grand canyon eroded to a depth of 

 about 6,000 feet ; Colorado river 

 flowing in Detrital-Sacramento 

 valley. 



Rise of Colorado plateau and dis- 

 placement at Grand Wash fault ; 

 origin of Grand canyon. 



Great volcanic activity ; eruption of 

 andesite and rhyolite. 



Miocene ; general degradation. 



