THE WORK OF RUNNING WATER 



cuts into each, thus opening out its valley and forming valley 

 plains. 



Old and young rivers and valleys. As we travel through 

 the country we see valleys of all kinds : narrow valleys with 

 steep banks (Fig. 238) ; wide valleys with gently sloping banks ; 

 wide, nearly level valleys bounded by low, gently sloping hills. 



FIG. 238. A valley in which deepening has progressed faster than widening. 



The narrow, steep, high-banked valleys are young, and erosion 

 has not been going on long enough to level their beds and 

 banks. The wide, more gently sloping valleys with lower 

 banks are older, and erosion has been going on long enough 

 to lessen the gradient of the valley floor and to reduce the 

 steepness of the slopes. The wide, almost level valleys bounded 

 by low, sloping hills are old, and erosion has been in progress 

 long enough to level down the land nearly to flatness. Small, 

 narrow, and steep valleys are called gorges ; large, narrow, and 



