CHAPTER IX 
RIVER EROSION 
Rain Erosion. — The work of erosion begins at the 
very moment when the raindrop strikes the earth, 
provided it touches the soil, We may see this on any 
summer day when a few drops of rain are falling upon 
a dusty road. ‘The drops strike the dust and send a 
little of it into the air, thus doing some mechanical 
work. As the rain increases, small rills begin to form, 
and the water of these runs off, and finally becomes 
part of a stream. Between the raindrop and the river 
no distinct line can be drawn. 
In forest-covered regions the soil is protected from 
the action of rain at the surface; and unless the 
rains are very heavy, little work of erosion is done 
until the drops have gathered into streams; but on 
unprotected ground, as we may see on a ploughed 
field, or a road, or anywhere in dry countries, where 
there is little vegetation, the rain cuts into the earth 
even before it has gathered into rivulets. 
The very drops, and then the little rills, remove 
151 
