THE WORK OF STREAMS 



95 



out the length of 

 which one cannot see 

 the polished surfaces 

 and regular curves 

 which are the traces 

 of more or less com- 

 plete potholes. 



Canyons. — Can- 

 yons are deep valleys 

 with steep sides. 

 They are formed 

 where the down-cut- 

 ting of a stream 

 (corrasion) greatly 

 exceeds the weather- 

 ing back of the slopes. 



The conditions favoring the formation of such valleys are (i) a rock 

 capable of maintaining a steep face, such as resistant rock on which 

 the trickling water cannot act quickly, or a firm, permeable rock 

 into which a large part of the water soaks, leaving little for erosion ; 

 and (2) a rapidly cutting stream. (3) An arid climate is more 

 favorable than a moist one, since the work of the weather will be 

 at a minimum in the former. Canyons are nevertheless ~formed-4n 

 regions of heavy rainfall (Fig. 64). When a stream approaches base 

 level and ceases to corrade its bed rapidly, the walls of its canyon 

 will be weathered back until in time they form a broad, open valley. 



Fig. 72. — Potholes in gneiss, Shelburne Falls, 

 Massachusetts. 



Fig. 73. — A generalized block diagram of the Grand Canyon of the Colorado. 

 The youthful stage of the region is shown in the fact that the streams have as yet 

 accomplished but a small part of the work to be done. The Colorado valley is a young 

 valley. The cliffs of the canyon are formed of resistant beds, while the slopes are of 

 weaker beds. 



One of the grandest canyons in the world is the Grand Canyon 

 of the Colorado in Arizona (Fig. 73), which was formed under condi- 

 tions most favorable for steep-sided valleys. The river flows through 



CLELAND GEOL. — 7 



