308 LAND SCULPTURE 



Rivers cut down and deepen their channels so long as their 

 beds have sufficient slope and fall. The banks also are under- 

 mined, as the current swings from side to side, and frequently fall, 

 thus widening the channel. The sides of the trench, unless re- 

 moved by other agencies, will be as steep as the nature of the 

 rock material will allow. Unassisted river action will, therefore, 

 cut nearly vertical trenches, which are continually deepened, until 

 the base-level is reached. Examples of such river-cut trenches 

 are the Au Sable Chasm (see Fig. 32) and the inner gorge of the 

 Grand Canon of the Colorado (see Frontispiece). 



The trench-like valley, with nearly vertical sides, is, however, 

 not the usual form of river valley. The atmospheric agencies, the 

 undermining and sapping of springs, landslips, and the like, are 

 continually wearing away the sides of the excavation, the waste 

 thus produced being readily carried away by the stream. As the 

 upper part of each hillside and cliff is that which has been long- 

 est exposed to the denuding agencies, the valley will be widened 

 at the top more than at the bottom, and will gradually be- 

 come widely open, unless the alternation of hard and soft strata 

 be such as to favour the retention of the cliff-like form by under- 

 mining. 



The rapidity with which the deep and narrow trench is widened 

 into the broad, gently sloping valley will depend upon two sets of 

 conditions. (1) Upon the climate, which is as much as to say, 

 the intensity with which the denuding forces operate. Canons 

 and narrow gorges are much more frequent in arid regions than 

 in those of abundant rainfall. (2) Upon the resistant power of 

 the rocks. If the valley sides are composed of rocks which yield 

 readily to weathering, the trench will be speedily broadened, while 

 if the rocks offer great resistance to chemical and mechanical dis- 

 integration, the gorge-like form will be retained very much longer. 

 This is illustrated by almost any considerable stream, such as the 

 Delaware or the Potomac. In certain places the valley is widely 

 open, while in other parts, of the course are deep gorges, as at the 

 Delaware Water Gap and Harper's Ferry. The gorges occur in 

 the places where the stream cuts across hard, resistant rocks, and 



