290 = JI. C. Russell—Subaérial Deposits of North America. 
are supplied are usually less easily determined, however, than in the 
case of sand hills which accumulate along the borders of oceans, 
lakes and rivers. 
Talus Slopes—These occur in abundance at the bases of nearly 
all rock escarpments, and are usually composed of coarse angular 
material. They owe their accumulation to the falling of detached 
blocks from the faces of rocky cliffs. Their surface slopes are 
usually high, depending on the angle of stability of the material of 
which they are composed, and are much greater than the correspond- 
ing slopes of alluvial cones, next to be described. Unlike alluvial 
cones, too, they occur quite uniformly along the bases of steep 
mountains instead of being confined to the mouths of canons. 
The talus slopes of arid regions are of essentially the same character 
as those occurring in humid climates, and farther notice of them at 
this time seems unnecessary. 
Alluvial Cones.—The smooth, even slopes of these deposits are 
a noticeable feature about the bases of all the principal mountains in 
the Far West. ‘They are formed of both coarse and fine material 
which has been swept out of gorges in the mountains and deposited 
in the valleys. In many instances they have a great superficial - 
extent, and where the conditions for their formation are most favour- 
able, may be hundreds and possibly thousands of feet in thickness. 
The alluvial cones of the region of interior drainage in the 
western part of the United States, known as the Great Basin, have 
been described by Mr. G. K. Gilbert, and we cannot do better than 
quote his description :— 
“The sculpture of a mountain by rain is a twofold process; on 
the one hand destructive, on the other constructive, The upper 
parts are eaten away in gorges and amphitheatres until the inter- 
vening remnants are reduced to sharp-edged spurs and crests, and 
all the detritus thus produced is swept outward and downward by 
the flowing waters, and deposited beyond the mouths of the moun- 
tain gorges. A large share of it remains at the foot of the mountain 
mass, being built into a smooth sloping pediment. If the outward 
flow of the water were equal in all directions, this pediment would 
be uniform upon all sides; but there is a principle of concentration 
involved, whereby rill joins with rill, creek with creek, and gorge 
with gorge, so that when the water leaves the margin of the rocky 
mass, it is always united into a comparatively small number of 
streams, and it is by these that the entire volume of detritus is 
discharged. About the mouth of each gorge a symmetric heap of 
alluvium is produced—a conical mass of low slope, descending 
equally in all directions from the point of issue; and the base of 
each mountain exhibits a series of such alluvial cones, each with 
its apex at the mouth of a gorge, and with its broad base resting 
upon the adjacent plain or valley. Rarely these cones stand so far 
apart as to be completely individual and distinct, but usually the 
parent gorges are so thickly set along the mountain front that the 
1 Contributions to the History of Lake Bonneville, in Second Ann. Rept. U. S. 
Geol. Surv. 1880-81, p. 183. 
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