THE ICE AGE 
characteristic rounded or prismatic shapes. Ultimately 
many of them are reduced to dust. All this material, both 
coarse and fine, together with the earth that falls from 
the sides of the glacial valleys, combines to form what is 
known as “till,” or ‘“‘bowlder clay.”” The occurrence of 
this substance is another evidence of the former presence 
of a glacier. 
Often, too, after the ice has melted away, it leaves be- 
hind it bowlders of all sizes, usually with rounded contours, 
called ‘erratic blocks.’”’ When these are composed of rock 
unlike that of the country around them, they can often 
be traced back, sometimes for vast distances, to the region 
whence they originally came. 
The glacier, as it slowly travels downhill, eventually 
reaches a level where it melts and forms a “glacial stream,” 
which carries along with it bowlders, gravel, and sand, 
automatically sorting them out, as it goes, according to 
size. This material forms heaps called terminal moraines, 
often of crescent shape, with the hollow side toward the 
glacier. Wherever, owing to stability of climate, the melt- 
ing end of a glacier has remained stationary for centuries, 
these appear as huge mounds quite unmistakable to the 
trained eye. 
Through the careful study of these and other traces of 
former glaciation, particularly in western Europe, geolo- 
gists have been able to learn a great deal about the nature 
and history of the last great Ice Age. They have found 
that the latter, far from being confined to western Europe, 
extended over the world, with glaciers forming in both 
the Southern and Northern Hemispheres apparently about 
the same time, and through their gradual extension en- 
veloping large portions of the globe. In this way much 
of North America, of northern Europe, and of southern- 
most South America was buried under enormous fields 
of ice during untold thousands of years. Asia, owing to 
a drier climate, seems largely to have escaped such visita- 
tions. For we must remember that to bring on an ice age 
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