292 R. S. TARR AND LAWRENCE MARTIN 
The Tanana—Kuskokwim valley area is clearly one of glacial out— 
wash extending far outside the limit of glaciation, as the silt-laden 
streams from existing glaciers testify. Whether the Yukon Flats 
area is entirely glacial outwash is not absolutely clear. Russell 
thought it a flood-plain deposit and Spurr a lake bottom. The 
narrow strips of Pleistocene silts, sands, and gravels along the 
rivers, and. the broad expanse of the Yukon-Kuskokwim delta 
have not been specifically considered in this paper, although 
distant existing glaciers are still supplying much of the sediment 
even to these regions. 
GLACIAL DEPOSITS OF THE COPPER RIVER BASIN 
The upper Copper River flows through an intermontane basin 
which it shares with the headwaters of the Susitna River. This 
basin is walled in by the Alaska Range and the Nutzotin, Talkeetna, 
Wrangell, and Chugach mountains. It is clear that the area of 
Pleistocene in the upper Copper River valley is wholly of glacial 
origin. The nature of this intermontane basin is such that prac- 
tically none of the glacial débris escaped down the stream outlets. 
Here the topography and the glacial material have been 
described in more or less detail by Hayes, Schrader, Mendenhall, 
Spencer, Moffit, Maddren, Capps, and others,’ and here the junior 
author of this paper in 1910 and both of us in rg11 made the obser- 
vations which form the basis of the present discussion. 
Area covered by drift——Throughout over 15,000 square miles 
(Fig. 1), an area at least equal to the portion of Illinois covered by 
drift deposits of the last glacial epoch, the basin of the upper Copper 
and Susitna rivers has glacial deposits which dominate the region. 
tC. W. Hayes, Nai. Geog. Mag., Vol. IV (1892), 135-36; W. C. Mendenhall, 
20th Ann. Rept., U.S. Geol. Survey, Part VII (1900), 282-84; F. C. Schrader, zbid., 
Pp. 384-86, 410-12; F. C. Schrader, and A. C. Spencer, House Doc. 546, 56th Cong., 
2d sess. (1901), pp. 29-30, 58-61, 62-75; Oscar Rohn, 2rst Ann. Rept., U.S. Geol. 
Survey, Part II (1900), 408-9; W. C. Mendenhall and F. C. Schrader, Prof. Paper 
15, U.S. Geol. Survey (1903); W.C. Mendenhall, Prof. Paper 41, U.S. Geol. Survey 
(1905), Pp. 19-22, 62-74, 79, 88-90; F. H. Moffit and A. G. Maddren, Bull. 374, 
U.S. Geol. Survey (1909), pp. 37-42; S. R. Capps, Bull. 477, U.S. Geol. Survey (1910), 
pp. 36-42; S.R. Capps, Jour. Geol., Vol. XVIII (1910), 38-30; F. H. Moffitt and S. R. 
Capps, Bull. 448, U.S. Geol. Survey (1911), pp. 43-52; S. R. Capps, Jour. Geol., Vol. 
XX (1912), 420-21, 428-30; F. H. Moffit, Bull. 498, U.S. Geol. Survey (1912), pp. 
39-44, 51-53- 
