GLACIAL DEPOSITS OF THE CONTINENTAL TYPE 293 
Except near the mountains, no rock outcrops are known. The 
topography is wholly glacial. The material is till, gravel, sand, 
silt, clay; and loess. 
Thickness of deposits—The observed thickness of the drift is 
500-700 feet’ in places, and may exceed 1,000 feet. This great 
thickness, in flat plains at a considerable distance from the moun- 
tains, forms a striking contrast with the drift of the region south 
of the Great Lakes, where it rarely exceeds 400 feet in thickness 
and averages 115 feet in Illinois, as determined by Leverett, and 
40 to 250 feet in southeastern Wisconsin, as determined by Alden. 
Plains topography.—The plains topography dominates the upper 
Copper River valley, the level of the country rising from 600-800 
feet, at the southern edge near the lowest outlet of the basin across 
the Chugach Mountains, to 3,600 feet near the Alaska Range 
from which the northern portion of the glacial drift was derived, 
and 5,000 feet on the slopes of the Wrangell Mountains to the east. 
The broad area of monotonously even plains is shown along the 
route traversed in 1898 by Mendenhall, from Cook Inlet to the 
Alaska Range, a distance of over 100 miles, and our own route 
across this basin in rgr1t from the head of the Copper River canyon 
at Chitina to the Delta Pass, 160 miles. 
Streams have cut deeply (500-800 feet) into the outwash 
plain, opening out wide valleys, either because of recent uplifts, 
or, as we think much more probable, because of increased ability 
to erode because the retreating glaciers have retired into the moun- 
tains, are no longer excessively overloaded, and have replaced 
aggradation by degradation. Within the bordering mountain 
valleys this degradation results in the leaving of lateral terraces 
of thick bench gravels. 
Dominance of outwash.—Outwash gravel, sand, and silt are 
the chief materials making up the surface of this plain, many of 
the beds being of alternate weakness and resistance. Lake deposits 
also make part of the flat topography. 
Till of the normal sort.—There is also much till of the normal 
sort, covering large areas and 400-600 feet thick, but in much 
*W. C. Mendenhall, Prof. Paper 41, U.S. Geol. Survey (1905), p. 63; F. H. 
Moffit and S. R. Capps, Bull. 448, U.S. Geol. Survey (1911), p. 49- 
