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



period of detrital accumulation. Briefly stated, the facts bearing on this 

 subject are as follows : 



The sedimentary formations of the Colorado plateau, still represented 

 by a thickness of several thousand feet at its western margin, origi- 

 nally extended over a large part if not all of western Arzona. These were 

 removed by erosion, the underlying crystalline rocks extensively exposed, 

 and great quantities of andesite and rhyolite outpoured over a large part 

 of the denuded area, to a maximum depth of about 3,000 feet. These 

 lavas were later extensively eroded and broad valleys cut through them 

 to a depth of 3,000 feet or more. Detrital-Sacramento valley, 10 to 15 

 miles wide, is the most conspicuous of these old valleys in western Ari- 

 zona and was probably formed by the ancient Colorado river. Entering 

 Arizona from southern Nevada where it is now occupied by Virgin river, 

 this valley extends southward across the present course of the Colorado 

 river east of the Black Mountain range (see plate 32) to the mouth of 

 Williams river. The detrital basin still farther south, known as Great 

 Colorado valley, through which the river flows for a distance of about 

 75 miles, is the southward continuation of this ancient valley. 



Sometime during the period in which Detrital-Sacramento valley was 

 formed, a displacement of several thousand feet occurred at Grand Wash 

 fault, accompanied by the tilting of a large crust block* and the forma- 

 tion of Grand Wash trough. This trough was later filled with detritus, 

 while the upturned edge of the block exposed now in Iceberg canyon was 

 planed off by erosion. The graded surface thus formed, consisting in 

 part of the beveled edges of upturned strata and in part of the detrital 

 filling, now forms a shelf 1,400 feet above the river near the mouth of the 

 Grand canyon. 



In order that deposits of different age may not be confused, a brief ex- 

 planation may be in place regarding the detrital filling of Grand Wash 

 trough. 



Two formations occur. The older one, consisting of angular unas- 

 sorted fragments, mainly of granitic rock, varying in size from sand 

 grains to boulders 10 feet in diameter, closely resembles the detrital 

 masses accumulating at the present time throughout the Southwest along 

 the bases of the mountain slopes. This formation contains a large 

 amount of carbonate of lime near the top, occurring in part as a cement- 

 ing substance in the detritus and in part as a sheet of travertine 200 feet 

 or more in thickness and nearly free from detrital matter. The younger 

 detrital deposit is several hundred feet thick near the mouth of the Grand 

 canyon and consists of water- worn gravel and boulders of limestone, 



* G. K. Gilbert: U. S. Geological Survey West of the 100th Mer., vol. 3, p. 54. 



