340 L. D. CAIRNES EROSIO>' AND EQUIPLANATION IN ALASKA 



portions, in which the Mesozoic rocks dominate, the change in topog- 

 raphy is so abrupt as to suggest faulting. A close investigation, how- 

 ever, revealed no evidence of any extensive movements, but on the 

 contrary resulted in the finding, in a number of places, of individual, 

 unbroken, limestone and dolomite beds, which are traceable from the pla- 

 teau upland out into adjoining thoroughly dissected Mesozoic portions 

 of the area, where they underlie the slates, phyllites, quartzites, etcetera, 

 thus proving conclusively that the fault theor}^ does not account for the 

 rapid change in topograjDhic t}^es or stages. 



Moreover, wherever the Ordovician-Silurian limestones and dolomites 

 occur at all extensively, the topography possesses the same striking pla- 

 teau characteristics: but in all localities, where the dominant bedrock 

 consists of Mesozoic beds, the thoroughly dissected upland topography is 

 exhibited. 



An examination of the resistance which these two classes of sediments 

 offer to the attacks of the various eroding and disintegrating forces to 

 which they are subjected, and of the rate at which they succumb to these, 

 discloses the fact that erosive agencies work much more rapidly in the 

 slates, plwllites, quartzites, etcetera, than in the limestones and dolo- 

 mites, although the Mesozoic beds consist frequently of much harder and 

 more indurated materials; and further demonstrates that this differential 

 erosion quite sufficiently accounts for the differences in topography. 



It is not intended in this short paper, however, to attempt a detailed 

 consideration of all the various subaerial destructive forces that have 

 combined to produce the present topography, but to deal only with a few 

 striking points concerning the leading agencies, and especially those that 

 have been mainly effective in accentuating the differences in the two 

 types of topography in the area. 



AVind action, foi- iust-ance, altliough so powerful an eroding agent in 

 some districts, and effective 1o some extent even in this northern, arctic 

 region, has had its influence reduced practically to a minimum, since the 

 greater part of the land surface is* covered with forest, moss, or tundra, 

 and all the superficial geological materials are frozen during all or the 

 greater part of the year. The greatest extent of exposed bedrock occurs 

 in the limestone and dolomite areas — and even there the uplands are 

 largely covered with superficial deposits; but the rugged, jagged, valley 

 walls are bared to the wind, which in such places accomplishes by abra- 

 sion a certain definite, although relatively small amount of erosion. 



Chemical action is also more effective in areas of limestones and dolo- 

 mites than in localities where the slates, phyllites, quartzites, etcetera, 

 dominate, as the former materials are much the more soluble. Ordinarv 



