THE OLD ICE-FLOOD 



rock>^ floors under the soil which look as if they had 

 been dressed down by a huge but dulled and nicked 

 jack-plane. The reason is that the line I have 

 indicated marks the limit of the old ice-sheet which 

 more than a hundred thousand years ago covered 

 all the northern part of the continent to a depth of 

 from two to four thousand feet, and was the chief 

 instrument in rounding off mountain-tops, scatter- 

 ing rock-fragments, little and big, over our land- 

 scapes, grinding down and breaking off the protrud- 

 ing rock strata, building up our banks of mingled 

 clay and stone, changing the courses of streams and 

 rivers, deepening and widening our valleys, trans- 

 planting boulders of one formation for hundreds of 

 miles, and dropping them upon the surface of an- 

 other formation. When it began to melt and re- 

 treat, it was the chief agent in building up our river 

 terraces, and our long, low, rounded hills of sand 

 and gravel and clay, called kames and drumlins. 

 In many of our valleys its flowing waters left long, 

 low ridges, gentle in outline, made up entirely of 

 sand and gravel, or of clay. In other places it left 

 moraines made up of earth, gravel, and rock-frag- 

 ments that make a very rough streak through the 

 farmer's land. All those high, level terraces along 

 the Hudson, such as that upon which West Point 

 stands, were the work of the old ice-sheet that once 

 filled the river valley. The melting ice was also the 

 chief agent in building up the enormous clay-banks 



159 



