DESCRIPTION OF THE LOWER COLORADO 277 



took place the streams were sometimes depositing sand and gravels and 

 sometimes carrying away those previously deposited, while at other times 

 the fluviatile material was buried beneath mountain wash. 



Means of Correlation 



The beds are unfossiliferous and for the most part are difficult of 

 access. Filling as they do the ancient depressions, they occur chiefly in 

 the lowlands and over un dissected plains. It is only where an uplift of 

 recent date has occurred or where some stream has cut a canyon in very 

 recent geologic time that the deposits are well exposed. For these 

 reasons the subdivisions of the detritus in one region can not be definitely 

 correlated by ordinary means with those of a neighboring region. It is 

 therefore of the greatest importance in studying these deposits that some 

 means of correlation be found. In the course of my investigations in 

 western Arizona, which included several overland excursions and a river 

 trip from the mouth of the Grand canyon southward to Yuma, it became 

 evident that an investigation of the detrital formations would be greatly 

 aided by a physiographic study of the Colorado river. A brief summary 

 of the physiographic history of the lower part of the Colorado is here 

 offered. 



General Description of the lower Part of the Colorado River 



The Colorado river emerges from the Grand canyon at the edge of the 

 Colorado plateau and passes in succession across a debris-filled valley, the 

 Grand Wash trough ; through a rock gorge known as Iceberg canyon ; 

 across a second debris-filled trough near Hualpai wash; through a second 

 rock canyon cutting the Virgin mountains ; across the debris-filled Detri- 

 tal-Sacramento valley ; through a third rock gorge — Boulder canyon — 

 cutting the Black Mountain range; across Las Vegas basin; through 

 Black can}'on (see plate 3l), and thence southward through a succession 

 of less conspicuous rock canyons and detrital basins. In selecting its 

 course the river seems to have shown little consideration for the easiest 

 lines of erosion. It has disregarded mountain and valley alike. From 

 casual observation, it would seem to have chosen about the roughest 

 course possible. These facts are best shown in the accompanying map, 

 plate 32. 



Early geologic Events 



Xo attempt is here made to write the early history of the region 

 through which the lower Colorado river flows, further than is necessary 

 to indicate the geographic conditions existing at the beginning of the 



