252 proceedings: geological society 



Alfred H. Brooks: The physiographic provinces of Alaska (Illus- 

 trated). 



Five principal physiographic provinces, each divisible into sub- 

 provinces, are recognizable in Alaska. These are (1) Pacific Mountain 

 system, (2) Central Plateau region, (3) Rocky Mountain system, (4) 

 Arctic Mountain system, and (5) Arctic Slope region. 



The Pacific Mountain system is made up of a number of parallel 

 ranges forming a rugged highland of crescentic outline sweeping around 

 the Gulf of Alaska. Its central part is upwards of two hundred miles in 

 width, but the system narrows to the southeast and to the southwest. 

 It is continued to the southeast by the Coast Range of British Colum- 

 bia and to the southwest by the rugged Aleutian Islands. Several sub- 

 provinces of lesser relief are included within the Pacific Mountain 

 system. In most places the inland slope of this system falls off abruptly 

 to the Central Plateau region, though the line of demarcation between 

 the two provinces is not everywhere well defined. 



The Central Plateau region is characterized by flat-topped inter- 

 stream areas separated by broad valleys and lowlands and broken by 

 minor ranges and peaks that rise above the general level. The plateau 

 feature is best developed in the upper Yukon basin, for it loses its 

 definition on approaching Bering Sea. Here the characteristic topog- 

 raphy consists of low rounded highlands rising island-like from broad 

 lowlands. 



The Rocky Mountain system maintains its northwesterly trend 

 through western Canada to within about 400 miles of the Arctic 

 Ocean and then bends to the west and enters Alaska as a single range 

 (Ogilvie Mountains). Crossing the boundary just south of the 66th 

 parallel it loses its definition and soon merges with the fiat summits of the 

 Central Plateau region. The Crazy and White Mountains of the 

 Yukon-Tanana region that stand above the plateau level lie in the 

 continuation of the Rocky Mountain axis. 



A new name, Arctic Mountain system, is proposed for the east and 

 west trending mountain system of northern Alaska formerly regarded 

 as part of the Rocky Mountain system. Recent investigations by 

 Canadian and American geologists have shown that this is a distinct 

 system from the Rocky Mountains, although they are connected by the 

 flat-topped Richardson Mountains forming the Mackenzie-Porcupine 

 divide. The Arctic Mountain system stretches westward from the 

 International Boundary to the Arctic Ocean north of Kotzebue Sound. 

 It is not everywhere sharply differentiated from the plateau region 

 to the south, for in many places the dissected plateau remnants merge 

 with the foothills of the ranges. In its western part the northern limit 

 of the lowland of the Kobuk Valley affords a definite line of demarca- 

 tion. On the north the mountains, so far as known, everywhere fall 

 off abruptly to the Arctic Slope. This scarp affords a definite boundary 

 line between the two provinces. The system is made up throughout 

 its extent of two or more parallel ranges and includes some broad low- 

 lands. These lowlands are specially striking topographic features in 



