1 92 1. Xo. II. 



THE STRANDFLAT AND ISOSTASY 



301 



Fig. 1 70. Diagram showing, with much exaggeration, the depression of the crust under an 



ice sheet, and the upheaval on the sides. The vertical arrows indicate the vertical movement 



of the crust, the horizontal arrows the "undertow" in the plastic substratum. 



dicated in Fig. 170, I. As, however, this wave will not represent a state 

 of equilibrium, it will gradually extend outwards, and will be flattened 

 down as it becomes wider and wider. 



If now the ice-cap increases in thickness and in extent, the crust will 

 continue to be pressed down, and the surrounding upheaval wave will 

 increase somewhat in height, while it will be moved outwards by the 

 advance of the ice-cap (Fig. 170, IT). 



The depression of the crust will continue a long time after the ice-cap 

 has ceased to increase, and will onlv very slowly and asvmptoticallv ap- 

 proach its level of isostatic eiiuilibrium. The wave of upheaval surrounding 

 the depressed area, will continue to widen, and the real level of isostatic 

 equilibrium will not be fully reached, before this surrounding wave is 

 entirely flattened out. and the depression of the ice-covered area is fully 

 compensated for by the upheaval of the floor of the Ocean, as was men- 

 tioned on p. 233, but this is a state which is never reached. 



\\'hen the ice-cap begins to decrease towards the end of the glacial 

 period, the underlying crust will probably not yet have been fully depressed 

 to the level of equilibrium conditioned by the weight of the ice. The sub- 

 sidence will, therefore, continue inside the area which is still covered by 

 the retreating ice-cap, and it will not stop as long as the weight of the 



