PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
7 
and Coast ranges; to the east the Columbia system ; on the north 
it is bounded by a group of irregular ranges, which lie between 
the Coast range and the Rocky Mountain system and begin near 
the 56th parallel of latitude; on the south it ends in wedge- 
shaped form against the Hozameen, Skagit, and Okanagan ranges 
of the Cascade system, and the Colville mountains of the 
Columbia system. 1 
In the southern part of the Plateaus the uplands lie from 
4.000 to 6,000 feet above sea-level. They decrease in general 
elevation to the north. The main valleys lie from a few hundred 
to over 4,000 feet below the upland, and a few rugged hills rise 
several hundred feet above the surface. The total relief within 
areas of a few hundred square miles is probably nowhere over 
5.000 feet, and is generally less than 4,000. 
The greater part of the Interior plateaus drains south and 
west into the Pacific by way of the Fraser and its tributary, the 
Thompson. A portion at its northern end drains into the Skeena 
river. The southern and southwestern end is drained by the 
Similkameen, Okanagan, and Kettle rivers, whose waters all 
flow into the Columbia. 
The deep valleys which separate the Interior plateaus into 
irregular blocks have walls upon which the slopes are much 
steeper than those prevailing on the uplands between them. 
The junction of upland and valley wall is generally abrupt, and 
the plateaus are thus divided into upland and valley portions 
which are topographically distinct (Plate I). By valleys we 
mean those valleys which have steep sides and whose bottoms 
lie below the general upland surface. Shallow valleys occur upon 
the upland; but are here considered as forming a part of that 
surface. The areal ratio of upland to valley is about three to one. 
The following more detailed description of the Interior 
Plateaus region applies particularly to that part south of the 
52nd parallel of latitude. A conception was gained of the general 
character of the topography by a comparison of the topographic 
l “The nomenclature of the North American Cordillera between the 47th 
and 53d parallels of latitude,” by R. A. Daly. The Geographic Journal, 
vol. 27, No, 6, June 1906, pp. 586-606, 1 map. 
