THE BACKGROUND 9 



Yukon Valley system, which provides a continuous valley across 

 northern Alaska and Yukon Territory extending from the western 

 coast to within 50 miles of the Mackenzie River without significant 

 obstruction to the distribution of animals. McDougall Pass, at an 

 elevation of 1,200 feet through the Richardson Mountains, is the 

 lowest and shortest pass through the western mountains of North 

 America. 



Arctic Climates Present and Past 



The northermnost treeline in America is in arctic Yukon. East 

 of the Mackenzie River the forest retreats from the arctic circle and 

 its distribution roughly follows the June isothermal line of 50° F. 

 mean temperature, which reaches the Arctic in the interior of Alaska 

 (fig. 2). In winter, interior northern Alaska and Yukon include the 

 coldest part of North America, but in summer warm air from the 

 north Pacific tempers the northwestern interior climate as far north 

 as the Brooks Range. North of the Range the cold summers of 

 arctic shores prevail. Many birds and mammals can endure any 

 degree of arctic cold, but their existence depends upon the provision 

 of food which is ultimately the product of vegetation. Many plants 

 also endure cold but their growth occurs in summer warmth. The 

 vegetation that results from these climatic conditions in the Yukon 

 Valley enables the woodland animals there to reach their northern- 

 most limit in America. 



Since summer temperatures are cold north of the mountains and 

 warm on the southern slope, in passing northward across the Brooks 

 Range a rapidly cooling gradient of summer temperature is experi- 

 enced. This steep northward gradient is peculiar to western arctic 

 America, for east of the Mackenzie Valley summer warmth equaling 

 that in the northern Yukon Valley reaches only as far north as James 

 Bay. The northward extension of summer warmth into western arctic 

 America is the result of conditions in the earth's circulation of heat 

 which have been long in operation. This is shown by the southern 

 limit of extensive permanently frozen ground in North America, which 

 in Alaska lies north of Anchorage (lat. 61° N.), while in the middle 

 of the continent around James Bay it extends farther southward by 

 about 8 degrees of latitude. Near the arctic coast of Alaska explora- 

 tory drilling for petroleum shows that permanently frozen conditions 

 extend for 1,000 feet below the surface. (Black, 1951.) 



Relics of the great American ice fields of the past now exist only in 

 southeastern Alaska although evidence for recent, more extensive 

 glaciation is shown in the gradual invasion of vegetation into valleys 

 from which it is apparent that glaciers have recently melted. The 

 western limit of Alaskan glaciation extended only as a shelf into the 



