FEATURES OF ICE-CAP EROSION 723 



certainty. The fact that the sea once reached nearly^ if not quite, to 

 this locality, as indicated from old shorelines in the near vicinity, may 

 account in part, at least, for the peculiar location of these deposits, 

 rhey deserve detailed study. 



Features of Ice-cap Erosion 

 ice-cap beveling 



During the Glacial epoch, ice not only filled the valleys, but covered 

 the entire plateau country and many mountain tops, leaving only isolated 

 peaks here and there exposed. In Iceland its thickness over the upland 

 area was from 500 to 800 meters less than in the river valleys, and con- 

 sequently its pressure and power to abrade the uplands were greatly 

 diminished, especially as the highland country lacked the gradient and 

 directive influence of the river bed. In a water flood, the main work of 

 erosion is accomplished in the river bed itself, and, likewise, in the 

 glacial ice-cap the chief down-cutting action of the moving ice is con- 

 fined to and concentrated in the submerged river valleys; but a glacial 

 ice cap or flood is more permanent than a water flood, and preserves its 

 level over a long period of time. Its surface acts, therefore, like a water 

 surface (lake or ocean), as a plane of reference toward which exposed 

 masses tend to be reduced. Above the ice-sheet all cliffs and peaks 

 break down rapidly as a result of rapid temperature changes and conse- 

 quent freezing and expansion of included moisture, and in a compara- 

 tively short time become reduced to the surface of the ice-sheet. An 

 ice-sheet compares favorably with a body of water, in that its action is 

 most intense along the coast and margins of the exposed land-masses. 

 Its surface, moreover, is determined by the same general law of gravita- 

 tion, but instead of being horizontal, as a water surface, it is, because of 

 its greater viscosity or plasticity, gently inclined, and slopes away from 

 the upland areas toward the sea or other lower level. 



From the foregoing it is evident that upland rock-masses not far be- 

 neath the ice surface suffer little abrasion, while the exposed masses 

 above the ice tend to be reduced rapidly to its level, with the result 

 that if such ice action were continued long enough the mountain peaks 

 in the area would be reduced to about the same general elevation and 

 conform roughly in altitude to- that of the ice-sheet as datum plane. 

 The changes which would take place under such action are illustrated 

 diagrammatically in figures 1 and 2. On recession of the ice, the upland 

 surface, which before may have been exceedingly irregular, would re- 

 semble an old uplifted baselevel of erosion, sloping gently toward the sea 



