RIVER EROSION 165 
opposite bank, and slowly broadens its valley (Figs. 71 
and 79). When confined within steeply rising banks of 
hard rock, this broadening action of the river is limited. 
Codperation of Weathering. 
river were at work, the val- 
ley would be narrow and 
trench-like (Fig. 80). <A 
gorge would necessarily be 
the result as the river cut 
deeper and deeper. Here is 
where river erosion and 
weathering go hand in 
hand. The cliffs crumble 
in the air, the soil is washed 
into the streams, and slowly 
the sides of the gorge wear 
back, and the banks change 
from angular cliffs to rounded 
slopes. If the stream is deep- 
ening its valley, it may cut 
its channel more rapidly than 
the slow action of weathering 
Still, if nothing but the 
é : 
Pie. £0. 
A very narrow gorge cut so rapidly 
by the stream that it has not 
widened greatly. ‘The cutting is 
being done without much broad- 
ening by river meandering. 
can broaden it; and then the river valley remains 
a gorge (Fig. 80); but if the stream has been long 
at work, the action of weathering will have oper- 
ated to widen the valley until the gorge condition is 
destroyed. 
