344 L. D. CAIRNES EROSION AND EQUIPLANATION IN ALASKA 



upland constitutes throughout its extent a gently undulating plateau, 

 with only occasional summits or ridges rising above the general level, 

 and the plateau surface truncates equally the various geological bedrock 

 terranes, regardless of their composition, structure, and other physical 

 characters. 



Following the uplift of this planated tract, erosive agencies were re- 

 juvenated and began their work in the elevated terrane with renewed 

 energy. Certain belts, in which the bedrock is composed, dominantly, 

 of limestones and dolomites, have withstood erosive forces much better 

 than other adjoining areas, where the bedrock formations consist pre- 

 vailingly of slates, phyllites, quartzites, and related rocks, with the result 

 that, in the limestone-dolomite belt, the topography is still in a youthful 

 stage, and the plateau surface is so well preserved as to constitute the 

 outstanding topographic feature of the area ; whereas in the adjoining 

 tracts the original upland has been either wholly or nearly destroyed 

 and the topography is in a mature stage and rapidly approaching old age. 



The reason for this differential erosion appears to be largely that the 

 limestones and dolomites are relatively massive, compact materials; 

 whereas the slates and associated rocks prevailingly contain innumerable 

 seams, cracks, and spaces of various sorts, containing water and even 

 allow of its circulating through them ; and this water not only erodes the 

 rocks, but when frozen causes them to split readily into innumerable thin 

 slabs, rendering them an easy prey to all ordinary erosive and weathering 

 activities. Further, since the slates, phyllites, etcetera, are thinly cleav- 

 able rocks, they are readily broken, due to temperature changes. In the 

 limestone-dolomite belt or belts the waters either tend to run off rapidly 

 through a few trunk channels or are held in a nearly dormant condition 

 by the frost and more or less frozen debris of the upland. 



Equiplanation 



GENERAL 



In the limestone-dolomite belt there still exist occasional well .rounded 

 monadnocks or rock residuals that rise above the general level of the 

 upland surface: these represent the principal topographic relief that 

 remained to break the monotony of this portion of the old peneplanated 

 tract just previous to the uplift, which, as mentioned previously, is vari- 

 ously claimed to have occurred in late Miocene, Pliocene, and early 

 Pleistocene time. The general upland bordering these residuals is in 

 most places covered by accumulations of more or less frozen superficial 

 deposits, the surfaces of which are either flat or but slightly inclined, 



