I92I. No. II. THE STRANDFLAT AND ISOSTASY. 23 



of this erosion must also have varied very much; and no plane could be 

 developed in that manner. 



It has been maintained that the many low skerries of the strandfiat 

 may have been planed off to their present level by the inland ice. There 

 is, however, no reason why this erosion should stop at sea-level. 



It has been a quite common mistake amongst geologist to assume 

 that the erosive power of a glacier should be reduced by a partial sub- 

 mergence of the glacier into the sea, because the pressure of the ice on 

 the sea floor, as compared with that on dry land, should bs reduced bv 

 the boyancy of the submerged volume of the glacier. But, as was pointed 

 out by G. K. Gilbert [1904, pp. 10 f.] and myself [1904, p. 163, footnote], 

 the pressure of the ice on the underlying ground will naturally be the same 

 whether the ice be submerged or not, and provided that the velocity of its 

 movement be the same, there can be no difference in the erosive power of 

 the glacier, as long as it actually rests on th? ground. The difference will 

 come in the moment that the glacier begins to float in the sea, and a layer 

 of liquid water actually intervenes between the under side of the glacier 

 and the sea floor. The chief effect of submergence will otherwise be 

 that the outward pressure in the mass of the glacier will be counteracted 

 by the pressure of a layer of sea-water equal in depth to that to which 

 the glacier is submerged, and its velocity may thus be reduced. 



Provided, however, that the velocity of its motion remains unaltered, 

 a glacier will have the same erosive effect upon the underlying ground, 

 whether above or below sea-level, down to a depth below the latter equal 

 to nearly nine tenths of the tlrckness of the ice, at which depth the glacier 

 begins to float and is lifted from the sea floor. 



This erosion cannot therefore be limited by any definite level, except 

 that determined by the depth at which the glaciers float. But as this depth 

 must have varied much, owing to the varying thickness of the glaciers, 

 no horizontal level of erosion, like that so distinctly exhibited by the sker- 

 ries and islets of the strandfiat, could possibly be produced by this glacial 

 erosion. 



On the whole, it is hardly conceivable that an inland ic2 can have a 

 planing effect, even where it moves over a fairly flat land surface. It may 

 break away projecting rocks, and more or less smooth the rock surface; 

 but it will soon move with greater velocities along certain lines than along 

 others, especially along the initial fluvial valleys, even though they may be 

 ever so broad and shallow. As the erosive power of the moving ice in- 

 creases so much with its velocity, this will lead to a deepening of these 

 depressions or valleys, with an increasing rate as time goes on. The final 

 result will be that the initially flat land surface is dissected by deep glacial 

 valleys and channels. 



In the region of the Norwegian strandfiat a planing effect of the 

 moving inland ice seems to me to be especially impossible. The inland ice 



