GLACIAL EPOCH. 
81 
direction of the ocean currents. But so far no hypoth¬ 
esis advanced seems adequate to explain the origin of 
the ice sheet, and so only a few of the more important 
results of the Cllacial period will be added here. 
One effect was the destruction of some of the great 
animal monsters. The appearance of man at a later 
time must have caused the passing away of the rest, for 
man, with his inventive genius, doubtless proved more 
than a match for even the most ferocious of them. 
Another result of the ice was the preparation of 
suitable rich farming areas for the use of man. The 
boulder till was spread over the best land, and it now 
produces the finest wheat and other cereals for man’s 
subsistence. 
A third effect was the formation of lakes and the 
changing of river basins. Before the ice came, many 
streams of this continent found outlet in the Arctic 
Ocean. As the sheet moved forward, pushing along 
the ground moraine, it dammed up these rivers and 
forced them to cut their way through the Alleghanies. 
The debris carried by the ice also enclosed bodies of 
water in Wisconsin and other Northern States ; when 
the ice retreated by melting, the G-reat Lakes, which 
had thus far found outlet through the Wahash, Miami, 
and Mohawk valleys, turned eastward through the St. 
Lawrence. All along the shores of these lakes are 
evidences that the water in the lakes once stood much 
higher than it does at present. 
There is proof that there were two Glacial periods, 
with a warmer epoch between when the continent was 
plunged into the sea. 
