DETAILS OF VALLEYS. 
31 
of a large crust block. It derives its name from Grand Wash, which 
here enters the Colorado from the north. Its western limb consists of 
strata of the tilted block, and its eastern limb is the fault scarp of the 
Grand Wash Cliffs. 
Not only are all the rock formations composing the Grand Wash 
Cliffs found in the tilted block in Iceberg Canyon, but strata younger 
than those in the cliffs are also present and pass beneath the river 
level with an eastward dip of 60° to 80°. Making no allowance for 
dip and considering only the altitude of corresponding strata in the' 
walls of Grand Canyon and in Iceberg Canyon, the downthrow at the 
Grand Wash fault is 
about 5,000 feet. The 
strata of the block 
pass beneath the river 
level several miles 
west of the fault line. 
If the downthrow be 
computed from the 
dip of the strata and 
the distance from the 
fault line at which 
they pass beneath the 
river, the displace- 
ment at Grand Wash 
fault appears to be 
several times greater 
than the 5,000 feet in 
evidence at the sur- 
face. 
The magnitude of 
the Grand Wash fault 
has long been recognized. Gilbert a states that the most profound 
dislocation of the plateau region has occurred at this fault, and Dut- 
ton, 6 in describing it, writes that "the plateau had its origin in a 
great fault along the crest of which the country east of it has been 
hoisted several thousand feet above the country on the west." Again 
he writes "It [the Grand Wash Cliffs] is a feature of the highest 
importance, since it is the boundary not only of the Grand Canyon 
district, but of the Plateau province itself. It drops the country on 
the west about 6,000 feet at a maximum." Huntington and Gold- 
thwaite c have recently shown that the fault is traceable northward to 
a Gilbert, G. K., Final Repts. U. S. Geog. Survey W. 100th Mer., 1873, pt. 1, p. 54. 
&Dutton,C. E., Tertiary history of the Grand Canyon district: Mon. U.S. Geol. Survey, voi. 2, 1882, 
pp. 12, 19. 
'•Huntington, Ellsworth, and Goldthwaite, J. W., The Hurricane fault ; n the Toquerville district, 
Utah: Bull. Mus. Comp. Zool. Harvard Coll., vol. 42, 1904. 
Fig. 3.— Diagrammatic sections across Colorado River. A, At 
mouth, of Grand Canyon; B, in Iceberg Canyon. 1, Cambrian; 
2, Redwall limestone; 3, travertine; 4, granite; 5, Greggs breccia. 
