RIVER EROSION 
1738 
EROSIVE WORK. OF RIVERS 
CHEMICAL. 
MECHANICAL. 
UNDERGROUND 
WATER. 
Dissolves mineral sub- 
stances, broadens and 
lowers the valley, and 
furnishes the river with 
mineral matter in solu- 
tion. 
A very slight effect. 
RAIN WATER. 
Supplies the under- 
eround water. Here, and 
also at the surface, does 
some chemical work. 
Carves soft rocks. 
Slowly wears the harder 
strata. Furnishes’ the 
streams with sediment. — 
River WATER. 
Slowly dissolves the 
rocks of the stream bed, 
particularly the more 
soluble. 
Moves the cutting tools 
supplied it, and wears 
them down, at the same 
time eating into its bed. 
By meandering, broadens 
its valley somewhat. 
Sometimes unable to re- 
move all the load. Works 
intermittently according 
tovelocity. Rate of work 
also varies with nature of 
the rock. Forms water- 
falls, gorges, cafions, and 
even broad valleys. 
By means of the intimate codperation of these two 
processes, valleys are deepened and broadened, and 
most river valleys of the earth are thus formed. 
In 
connection with the valley formation much material 
is removed from the land and carried to the sea. 
