36 THE UNITED STATES GEOLOGICAL SURVEY. [bdll.227. 
and innumerable stream valle}'s have been incised below it, but from 
the northern base of the St. Elias and Alaskan ranges to the southern 
foothills of the Rocky Mountain system, and throughout their length, 
the remnants of this ancient level are to be seen. In height it varies 
from about 5,000 feet, close to the bases of the mountain systems, to 
less than 3,000 feet in the vicinity of the main lines of drainage, and 
slopes gradually toward the north. 
There is an area about 160 miles in width between the northern base 
of the Rocky Mountains and the Arctic Ocean. A portion of this area 
next to the mountains, and extending 80 miles northward, has a 
southern extension along the eastern base of the northern Canadian 
Rockies, and is comparable with the Great Plains region of the United 
States. There is a gradual slope from an elevation of 2,500 feet at 
the base of the mountains to 800 feet at the outer margin. The sur- 
face is gently rolling and diversified by only a few minor ridges 
parallel with the main range. Beyond this plain a nearly flat coastal 
plain 80 miles in width extends to the shores of the Arctic Ocean. 
The main surface features of Alaska, then, from south to north, are 
the Pacific Mountain system, the Yukon Plateau, the Rock} T Mountain 
system, and the Arctic slope. The general trend of these from the 
eastern boundary to Bering Sea is at first northwest and then south- 
west. These features and their trend are the visible expression of 
important structural lines intimately associated with the geologic devel- 
opment of the country. 
THE RIVERS. 
The development of the surface features of Alaska is inseparably 
connected with the history of the deeply incised drainage systems. 
The rivers fall naturally into three groups — those of the south and 
southeast coasts, those of the west coast, and those of the north coast. 
The rivers of the south and southeast coasts, from southeastern 
Alaska westward, include the Stikine, Taku, Chilkat, Alsek, Copper, 
Matanuska, and Sushitna. They drain the southern slopes of the 
Pacific ranges and a portion of the plateau region lying north of the I 
mountains. The Stikine, Taku, and Alsek have their sources beyond 
the mountains and reach the coast through deep-cut canyons in the 
Coast and St. Elias ranges. The Chilkat finds its way to the coast 
between the Coast Mountains and the St. Elias Range. The Copper 
drains a portion of the large irregular area between the Alaskan Range ■ 
and the mountains to the east, and reaches the coast through a canyon 
cut through the Chugach Mountains. The Matanuska and Sushitna 
occupy the area between the Copper River drainage basin and the 
eastern slope of the Alaskan Range, and are tributary to Cook Inlet. 
As they How more or less parallel to the main structural lines, the 
necessity of a canyon through a coastal range is avoided. 
Most of these rivers are swift, powerful streams which attain a 
