Cairnes — Some Suggested New Physiographic Terms. 83 



writer has found this process to be quite active in places on 

 the ' upland surface of the Yukon plateau province. This 

 process is facilitated in Yukon by the conditions of almost per- 

 petual frost, with the result that the tine material that collects 

 is but slightly disturbed by running water. This process 

 grades into that described previously as being effective in 

 other portions of Yukon and Alaska, and differs from it largely 

 in that, to the south, nivation has acted in conjunction with 

 other disintegrating and dissolving agencies. To the north, 

 however, the process is really the more effective and is more 

 truly equiplanating in character, as practically no material 

 escapes from the upland areas of accumulation, whereas to the 

 south the climate is less rigorous and running water is an erod- 

 ing and transporting factor even on the plateau surface. 



Although snow-drifts have this tendency to reduce the relief 

 in gently undulating upland areas, they have a tendency to 

 accentuate the topographic features when the snow gathers 

 along valley walls and on steep slopes. The drifts excavate 

 year by year, increasing in size as their fostering basins are 

 enlarged, and thus tend eventually to be transformed into ice, 

 and to become in time the cirque heads of valley glaciers. 

 This process has been described in a very lucid and instructive 

 manner by Matthes.* 



Equiplanation is thus a process by which levelling occurs 

 without reference to sea-level, and is in this respect funda- 

 mentally different from peneplanation and marine planation. 

 The surfaces affected by equiplanation in the majority of 

 regions, however, are above sea-level, but there are districts such 

 as the Salton sink,f north of the Gulf of California, in which 

 the central basin is below sea-level. 



Deplanation. 



All topographic terranes having plain-like surfaces;}: that 

 have been produced dominantly by deplanation, may be 

 grouped in one of the three following categories : 



1. Peneplains. 



2. Plains of marine denudation. 



3. Glacial plains. 



The chief agents capable of producing these forms, and of 

 assisting in deplanating processes, include land-ice, the sea, and 

 all subaerial erosive, disintegrating, and transporting forces, in 

 regions having exterior drainage. In regions having interior 



* Matthes, F. E., "Glacial sculpture of the Bighorn Mts., Wyo.," 

 Twenty-first Ann. Rep., U. S. Geol. Surv., Pt. II, 1899, pp. 179-190. 



f Davis, A. P., ''The new inland sea," Nat. Geog. Mag., Jan., 1907, 

 pp. 37-49. 



% This here is intended to include plateaus as well as high and low plains. 



