MAN FROM THE FARTHEST PAST 
for ages until another slow change in climate causes them 
gradually to melt. In rugged country, however, these 
vast accumulations of ice tend to move downhill, seeking 
ever lower levels (Plate 17). In this way the valleys are 
occupied by slowly moving rivers of ice, partly carried 
along by their own weight and partly pushed on by that 
of the ever-growing masses behind them. Usually glaciers 
move down the valleys very slowly, although in the polar 
regions they may advance as much as fifty feet per day. 
The rate depends on the temperature, the slope of the 
ground, and the volume of ice involved. It is a little 
greater by day than by night and in summer than in 
winter. 
Now the passage of millions of tons of ice over the sur- 
face of the earth produces many interesting effects. For 
one thing, it naturally exerts a tremendous grinding and 
scouring force which steadily wears away the sides and 
bottoms of the glacier-filled valleys. Their cross-sections 
thus become changed in time from the typical V shape, 
produced by ordinary stream erosion, into one resembling 
a capital letter U. Where valleys of this type occur, we 
may be sure that glaciers have once passed. 
Much as the current of a river often undermines its 
banks and causes them to cave in, so a glacier, in spite 
of its vastly slower movement, produces in the same way 
falls of rock, gravel, and earth. These lie on its surface 
and gradually form long lines of débris, known as lateral 
moraines, which, sinking slowly to the bottom of the 
glacier, become frozen solidly in the ice; being thus dragged 
along irresistibly over the underlying rock surface, they 
score parallel grooves running in the same direction as that 
of the moving glacier (Plate 18). After the latter has dis- 
appeared with the return of a warmer climate, these marks 
remain to afford unmistakable proof that the region was 
once ice covered. 
In time the bowlders carried along at the bottom of the 
glacier are worn by their contact with the ground into 
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