THE OLD ICE-FLOOD 
rocky 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 
