338 L. D. CAIRNES EROSION AND EQUIPLANATION IN ALASKA 



graphic province, which in Alaska, Yukon, and northern British Colum- 

 bia, stretches from the western edge of the Rocky Mountain system 

 westward to the ranges of the Pacific Mountain system. This terrane 

 trends northward through central northern British Columbia and con- 

 tinues northwesterly through Yukon and westerly through Alaska to 

 Bering Sea, following, in a general way, the contour of the Pacific Coast 

 line. This plateau province has been described by a number of geolo- 

 gists/ including Dawson, McConnell, Brooks, Spurr, Spencer, and 

 Hayes, all of whom unite in the opinion that it represents a penepla- 

 nated and subsequently elevated surface ; but it is to Dawson that we are 

 indebted for the earliest recognition of the baselevel character of the 

 Yukon Plateau region, as well as of the Interior Plateau of British Co- 

 lumbia, to which it is closely related. 



Xearly everywhere throughout the Yukon plateau there is a decided 

 uniformity of summit level ; and in various portions of the province 

 numerous hills and ridges occur, with flat or gently undulating tops, 

 which evidently once constituted portions of a widespread surface having 

 only slight relief. This surface formerly extended over the entire prov- 

 ince; but extensive portions of the upland have now been destroyed, and 

 in their place the valleys of the present erosion cycle exist. 



The plateau is best viewed from a summit that stands at about the 

 level of the general upland. From that viewpoint the observer will be 

 impressed with the even skyline sweeping off to the horizon, broken only 

 here and there l^y isolated, residuary masses, which rise above the general 

 level. This plain-like upland, however, bears no relation to rock struc- 

 ture, erosion having beveled the upturned edges of the hard as well as 

 the soft strata; in fact, this surface is entirely discordant to tlie liighly 

 contorted metamorphic rocks Avhich make up much of tlie plateau : and 

 for this reason is considered to be an uplifted and dissected peneplain 

 produced by long continued sul^aerial erosion, during a period of crustal 

 stability. The exact date of the uplift which terminated this long ero- 

 sion cycle is somewhat in doubt, and is bv different writers considered 



5 G. M. Dawson : Trans. Roy. Soc. of Can., vol. 8, sec. 4. 1890. p. 1.3. 



R. G. McConnell : "Report on the Klondike jjold fields." Ann. Uep., Geol. Surv.. 

 Can., vol. xiv. 1903, p. 6B. 



A. H. Brooks : "Geography and geology of Alaska." U. S. Geol. Surv.. professional 

 paper No. 45, 1906, pp. 36-41, 286-290. 



J. E. Spurr: "Geology of Yukon gold district, Alaska." Eighteenth Ann. Kept.. V. S. 

 Geol. Surv., pt. iii, 1898, p. 260. 



Arthur C. Spencer : "Pacific mountain system in British Columhia and Alaska." 

 Bull. Geol. Soc. of America, vol. xiv. April. 1003, pp. 117-132. 



C. W. Hayes : "Expedition through the Yukon district." Nat. Geog. Mag., vol. Iv, 

 1893, p. 129. 



