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



notably in Salt River valley in central Arizona, Owens valley in eastern 

 California, and in the Eio Grande valley in New Mexico. 



Geographic Distribution of Detritus 



Over a large part of the Southwest, extending from New Mexico to 

 the Pacific ocean, detrital material fills the low places generally and 

 transforms into broad detrital plains districts which might otherwise 

 have very uneven surfaces. In Arizona, where the writer has observed 

 the deposits most widely, the detritus occupies the lowlands to the south 

 and west of the Colorado plateau. 



There are three general topographic provinces in Arizona — the high 

 Colorado plateau, which occupies the northeastern part of the Territory : 

 the low-lying desert plains, which occupy the southwestern part; and an 

 intervening mountainous region. The mountains are in some cases rem- 

 nants of erosion and in others faulted and tilted crust blocks. In the 

 mountain province the valleys are narrow, but broaden away from the 

 plateau and merge finally into a detrital plain which surrounds the com- 

 paratively small and more or less isolated rock mountains of the plains 

 region. 



Character of the detrital Formations 



The detrital material varies greatly from place to place in physical 

 character, thickness, and general field relations, according to the manner 

 in which it was accumulated. Along stream courses and in regions for- 

 merly traversed by streams, it is composed of sand, silt, and water-worn 

 pebbles. In the alluvial cones and slopes at the base of the mountains 

 it is composed of angular rock fragments. In still other places it is com- 

 posed principally of clay, suggestive of flood-plain or lake deposit. Owing 

 to the arid climate of the Southwest, there are few permanent streams, 

 and nearly all of the rock waste from the mountains accumulates at the 

 present time as alluvial cones and slopes or as a general film of "wash" 

 over the surface of the plains. The climate, however, has not always been 

 arid, and the angular material usually found at the surface is no indica- 

 tion that like material occurs beneath. In many places wells sunk in the 

 detritus to depths of 1,000 to 1,300 feet have penetrated beds of very 

 diverse character. No uniformity in the kind of material or in the 

 arrangement of constituent parts is found by which neighboring de- 

 posits can be definitely correlated. I have described at some length the 

 composition of the detrital beds within a limited area. Salt River 

 valley,* and have shown that during the period in which accumulation 



♦Willis T. Lee: Underground waters of Salt River valley. Arizona. U. 8. Geological 

 Survey, Water Supply and Irrigation Paper no. 13G. 1905. 



