39. 



ALASKA AND THE YUKON 



GEOMORPHIC PROVINCES OF ALASKA 



The principal geomorphic provinces of Alaska are, from north to south, 

 the Arctic Coastal Plain, the Brooks Range, the Central Yukon Plateau and 

 Lowland, the Alaska and associated Coast Ranges, the Alaska Peninsula, 

 the Aleutian Archipelago, and the Alexander Archipelago. See map, Fig. 

 39.1. They are part and parcel of the continent's great western Cordillera. 

 The generalized tectonic divisions are shown in Fig. 39.2. 



The Brooks Range stretches east-west across northern Alaska and in- 

 cludes several smaller ranges, such as the De Long, Baird, and Endicott 

 Mountains. They support an extensive upland erosion surface, whose 

 higher elevations reach from 5000 to 6000 feet above sea level. The Brooks 

 Range in its central portion is a sharply defined mountain mass that rises 



conspicuously from the Yukon Plateau on the south and above the foot- 

 hills of the Coastal Plain on the north. The Colville River drains much of 

 the northern slopes of the range and the piedmont of the Arctic Coastal 

 Plain. The Brooks Range is covered for the most part with perennial 

 snow fields and contains a number of glaciers. The erosional features are 

 described as distinctly youthful, and presumably very little erosion has 

 occurred there since the once far greater ice fields and valley glaciers of 

 the Pleistocene have disappeared. The Arctic Coastal Plain from the air 

 appears as a bleak, flat wasteland of frozen lakes and rivers and snow- 

 covered flats. 



The Yukon or Central Plateau in the central and upper Yukon drainage 

 is a broad dissected plateau bounded on the north bv the Brooks Range 

 and on the south by the Alaska and Coast Ranges. The two great ranges 

 are about 300 miles apart. The plateau loses definition in the lower Yukon 

 drainage, where it is characterized by the flat-topped interstream areas 

 separated by broad and low-lying, estuarine-like embayments. A few 

 minor ranges and peaks rise above the general level of the upland sur- 

 face. The Yukon River has eroded a meanderbelt 35 miles wide in places, 

 but with several narrows along its coarse. Near its mouth, a very low 

 alluviated portage separates it from the Kuskokwin River, and both rivers 

 are in the process of building large deltas in the Bering Sea. 



The Central Plateau was dissected and then, during the maximum gla- 

 ciation, heavily alluviated, chiefly with silt. The Yukon and tributaries 

 have since been engaged mostly in clearing out the silt. 



The Seward Peninsula is a geological entity in itself and will receive 

 special mention later. It is generallv included in the Central Plateau prov- 

 ince. 



The Coast Range of southeastern Alaska and British Columbia extends 

 northwestward by way of the St. Elias Range, Wrangell Mountains, and 

 Nutzotin Range into the Alaska Range, which together form a great arc 

 approximately parallel to the margin of the Gulf of Alaska. The Alaska 

 Range continues southwestward to the Aleutian Range, which forms the 

 backbone of the Alaska peninsula. Mt. McKinley in the Alaska Range 

 (20,300 feet) is the highest mountain in Alaska. The St. Elias Range and 

 the Chugach Mountains support the greatest ice field in North America; 

 several peaks rise above 14,000 feet, including Mt. Logan, the highest at 



605 



