CHAPTER XXIV 



THE UPTILT OF THE LAND AT THE CLOSE OF THE 



ICE AGE 



The response of the earth's shell to its ice mantle. There 

 is now good reason to believe that the earth's outer shell makes 

 a response by oscillations of level due to the loading by ice, on the 

 one hand, and to the removal of this burden upon the other. We 

 know, at least, that both in northern Europe and in North America 

 areas which have undergone depression during and elevation after 

 the ice age, correspond closely to the regions which were ice cov- 

 ered. Wherever in these regions there was high relief before the 

 advent of the ice, river valleys were drowned at the land margins 

 and were also gouged out into troughs through erosion by the 

 outlet tongues upon the margin of the ice sheet. Such furrowed 

 and half -submerged valleys have a characteristic U-shaped sec- 

 tion, so that their walls rise precipitously from the sea. From 

 their typical occurrence in Scandinavian countries the name fjord 

 has been applied to them. 



It is now no less clear that the removal of the ice blanket brought 

 from the earth a relatively quick response in uplift, which began 

 before the ice front had retired across the present international 

 boundary of the United States, and that this uplift continued 

 until the final disappearance of the ice. A far slower elevation of 

 a somewhat different nature has continued, even to the present 

 day. 



It is obvious that at the time of their formation all shore lines 

 referable to the work of waves must have been horizontal, and 

 hence any variations from a perfect level which they reveal to-day 

 must indicate that a tilting movement of the ground has occurred 

 since the waters departed from their basins. We have thus 

 provided for us in the positions of these ancient water 'planes, 

 particularly because of their wide extent, a complete record the 

 refinement of which is not easily overstated. Interpreting this 



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