59 



then became elevated, and the streams of the district 

 were thus given renewed life and erosive powers, and con- 

 sequently immediately commenced sinking their channels 

 in the uplifted peneplain. Soon, numerous, deep incisions 

 were carved, which intersected the region in various 

 directions. The interstream areas became more and more 

 individualized, and assumed gradually the aspect of 

 separate mountains and ridges. 



The uplift of the Yukon plateau and adjoining tracts 

 was of a differential character, and so conditioned that the 

 resultant topography had the contour of a broad shallow 

 trough, the approximate axis of which is marked by the 

 present position of Yukon river and its main tributary 

 the Lewes, while the Coast range lies along its western or 

 southwestern rim. 



The higher tracts, including the ranges of the Coastal 

 system, during the Pleistocene, became the gathering 

 grounds for glaciers, and huge tongues of ice moved down 

 the sides of the Coast range both seaward and inland. 

 These valley-glaciers accentuated the topography produced 

 by uplift and subsequent erosion, and deepened and 

 broadened the depressions they occupied, steepened the 

 valley walls, and sculptured the land forms in a manner 

 characteristic of ice action. Vast amounts of morainal 

 and other materials were carried southward to the Pacific , 

 and northward on the way to Bering sea. The floors of 

 the main valley bottoms of Southern Yukon are deeply 

 covered with these deposits. Distinct ice markings occur 

 along the valleys of Lewes and Nordenskiold rivers nearly 

 to Tantalus, and are claimed to have been found a few 

 miles below this point. All traces of the presence of glacial 

 ice vanishes, however, long before Dawson is reached. 



After the retreat of the ice the topography was 

 virtually that of to-day. The master-streams have been 

 since engaged in removing the burden of glacial sands, 

 gravels, clays, and silts from their valleys and have not as 

 yet succeeded in trenching their channels to bedrock. 



A thin veneer of Recent materials forms the surface 

 nearly everywhere. This consists mainly of sands, 

 gravels, clays, and silts of the present waterways, ground- 

 ice, muck, volcanic ash and soil. The volcanic ash is an 

 interesting feature and occurs as a layer of pumiceous sand 

 ranging in thickness from less than an inch (25 mm.) to 

 over 2 feet (-6 m.) This material is noticed as far south as 

 Lake Bennett, where near Caribou it is about an inch 



