DETAILS OF VALLEYS. 33 
material ends abruptly against the sedimentaries at the fault line, 
and is locally disturbed by movements apparently of recent date. 
The somewhat complicated relations in the Grand Wash Trough 
may perhaps be made clearer by a brief statement of the events in 
the order of their occurrence. The formation of the Grand Wash 
fault and the tilting of the crust block were followed by a long period 
of erosion and deposition, which filled the trough with debris and 
planed off the upturned edge of the block, bringing the surface to a 
graded condition. The Colorado Plateau to the east was apparently 
at the level of this graded plain. Then followed a second period of 
faulting, in which great displacements occurred at the Grand Wash 
fault and minor displacements at several smaller faults to the west, 
accompanied by further tilting of the crust block and the elevation 
of the Colorado Plateau. This uplift of the plateau resulted in the 
inauguration of Grand Canyon, the river cutting directly across the 
Grand Wash Trough. The erosion proceeded until the canyon was 
cut to a depth practically the same as that it has at the present time. 
This was followed by deposition of gravels and of the Temple Bar 
conglomerate, and by eruptions of basalt, after which occurred re- 
newed faulting and tilting of the limestone block, the Greggs breccia, 
and the Temple Bar conglomerate. Erosion again became active, 
and a second time the canyon was eroded to a depth greater than that 
it now has. 
Later events, which are important farther downstream, are not 
conspicuously recorded in the Grand Wash Trough. 
HUALPAI WASH. 
Hualpai Wash enters Colorado River from the south and occupies 
a structural depression between, the western edge of the limestone 
block on the east and the crystalline rocks of the White Hills on the 
west. This trough extends northward across the river and is there 
filled with gravel deposits, but their relation to other formations 
*was not determined. 
In Hualpai Wash a limestone several hundred feet thick occurs. 
In some places it is a massive, pink, compact rock resembling the 
Redwall limestone of the canyon section. Unlike that limestone, 
however, its character varies within short distances from a compact 
rock to a comparatively soft earthy substance, weathering with 
rough cavernous surface. In Hualpai Wash it rests directly upon 
an irregular surface of granite. In other places it abuts rather 
abruptly against previously existing granite cliffs, and contains frag- 
ments of the granite embedded within it. In still other places it 
apparently rests upon beds of sand and clay, although this observa- 
tion was not satisfactory. 
19964— Bull. 352—08 :j 
