172 ELEMENTARY GEOLOGY 
across, we see that to form a wide, gently sloping 
valley would require a-much longer time than to 
carve out a steep-sided gorge, even a Colorado. In 
the gorge the river has cut the rocks apart; m the 
broad valley, not only has this been done, but the 
slow action of weathering has been able to melt back 
the valley sides and form rounded slopes (Fig. 87). 
> TE 
a a ee = 
lene 8 Se ae 
<P es eS SSS SS SS Ny 
New => SSS S 
Fic. 87. 
Diagram to show difference in amount of rock removed in narrow and broad 
valleys. 
River work of valley deepening is rapid compared 
with that of weathering which, because of the slow- 
ness of its action, escapes common notice. But even 
the relatively rapid work of river erosion was long 
overlooked, and it was thought that rivers had little 
share in the formation of the valleys that they occupy. 
It was believed that they represented great rents in 
the earth or else the action of powerful ocean waves. 
Now, we know that most of the gorges and deep 
valleys have been formed by the rivers. 
