160 ELEMENTARY GEOLOGY 
or whirled along, they wear one against the other, and 
against the bed of the river, all the time grinding off 
other fragments. By this means, angular pebbles have 
their corners worn off and rounded, and after a short 
journey, become greatly reduced in size, perhaps being 
entirely ground to sand and clay. In the course of 
this reduction, the rocks in the river bed are battered 
away, and the valley deepened. Even the finer par- 
ticles of sand and clay rasp at the rock, so that in 
time, by this cause also, the valley bottom may be 
dug deeper. 
The supply of these cutting tools comes in part 
from the softer rocks over which the river flows; but 
it is chiefly furnished by the crumbling cliffs and valley 
sides, and by the washing action of the rain. A river, 
ordinarily clear, may become a muddy torrent after a 
heavy rain, or in the spring when the snow quickly 
melts. So the supply of cutting materials varies from 
day to day, and one river may be heavily laden while 
another is clear; or one may transport pebbles while 
another carries only fine mud or sand, the size depend- 
ing upon the velocity of the water and the source of 
the materials. 
In some cases rivers are furnished with so much sed- 
iment that they cannot remove it all, but must deposit 
some and build up their valleys instead of cutting them 
deeper. The Platte, for instance, because of this over- 
