BELT ON THE GLACIAL PERIOD IN NORTH AMERICA. 101 



Baltimore and Ohio was slowly and successively worked over. 

 Many of its old valleys would be deepened and many new ones 

 excavated. Lines of faults, of fractured or of softer strata, would 

 be worn into valleys and lake basins. Everywhere the ice would 

 find out the weakest points of the rock masses and work deepest there. 

 The moving margin of the ice flow and especially the glaciers 

 thrown out in advance, would be the great producers and carriers of 

 drift. The stones borne along upon a glacier are from the cliffs and 

 peaks that rise above it and not from the rocks beneath, over which 

 it passes. The latter are only smoothed and rounded, but the cliffs 

 that bound a glacier are eaten into like a river bank. Masses of 

 rock are undermined and fall down upon and are carried away on 

 the ice, to be deposited in terminal and lateral moraines. 



3. Culmination. — At last the ice reaches its limits. Mount 

 Washington is glaciated nearly to its summit, and at the time when 

 there was most ice only its top could have stood out — a desolate 

 island in a frozen sea. To the north the whole continent must have 

 been covered without a single peak rising above the universal pall. 



It is probable that during the greatest development of the ice, 

 most of the drift that had been produced would be destroyed by 

 being ground to powder under the mighty moving mass, and carried 

 away in the water which we know flows turbid from beneath every 

 glacier. The time of thickest ice was not that of the production of 

 drift, but of the rounding, polishing and grooving of mountain masses. 

 Could the icy covering have been lifted the rocky skeleton of the 

 countiy would have been exposed, with scarcely a patch of gravel 

 or soil upon its bare, scarred frame. 



The scratchings on the highest peaks show that the main body 

 of the ice moved south-easterly. Here we see the action of two 

 forces — one, from the north, was the accumulation of ice in that 

 direction ; the other, from the west, was the slope of the continent 

 towards its eastern sea board. 



4. Retreat. — If then drift was not formed when the ice was at 

 its height, and that which had been produced during its advance was 

 then destroyed ; whence the heaps of gravel and the transported 

 blocks that now cover the face of the country ? They were dis- 

 tributed during the slow retreat of the ice, when again every part of 

 the country was subjected to the action of the moving margin. Just 



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