54 



producing thereby a very irregular topography. The 

 summits of the unreduced hills and ridges, lying between 

 the waterways, mark a gently rolling plain which slopes 

 toward the north and northwest. The plateau is best 

 seen from a summit that stands at about the level of the 

 upland, where the observer will be impressed with the even 

 sky-line, sweeping off to the horizon and broken only 

 here and there by isolated, residuary masses rising above 

 the general level. This plain, however, bears no relation 

 to rock structures, erosion having bevelled the upturned 

 edges of the hard as well as the soft strata. The surface, 

 consequently, is entirely discordant to the highly contorted, 

 metamorphosed rocks that make up much of the plateau. 



Along the northern portion of the Coast range, the 

 general summit of this terrane merges into that of the 

 Yukon plateau in a manner suggesting the synchronous 

 planation of these two provinces, — a view that is held 

 by Brooks [6, pp. 286-290, 293], Spencer [67, p. 132], and 

 others. The various vertical movements that have affected 

 these terranes, however, whether the Coast range was 

 planated or not, have been such that the uplift has been 

 greatest along the axis of the Coast range and least along 

 that of the Yukon plateau, so that the latter possesses 

 the contour of a huge flaring trough whose median line 

 is in a general way, marked by the course of Yukon river 

 from its headwaters in Northern British Columbia to 

 Bering sea. 



As topographic features are often to a certain degree 

 merely expressions of the bedrock structure and composi- 

 tion, it might be expected that the same general geological 

 terranes would extend through Alaska, Yukon, and British 

 Columbia, following the general trend of the coast line, 

 and to a limited extent this has been found to be true. 

 In Yukon, however, this parallelism, and to some extent 

 conformity of geological formations to the physiographic 

 provinces, is most apparent when the entire territory 

 is considered. The Coast range everywhere consists 

 almost entirely of the granitic materials composing the 

 great Coast Range batholith, and the various geological 

 terranes of the Rocky Mountain system have a decided 

 general trend parallel to its physiographic boundaries. 

 In the Yukon plateau, however, the different geological 

 formations are somewhat irregularly distributed and 

 have no marked trend parallel to the borders of the plateau 

 province. 



