THE WORK OF STREAMS 



85 



Fig. 6i. — Bowlders in a stream bed. Here the bowlders 

 form a pavement which hinders the erosion of the valley. 



bedded rocks, as is apparent on the brink of Niagara Falls where 

 the thousands of tons of water which pour over them hourly are 

 unable to remove the 

 soft algae which cover 

 the rocks, as the 

 water is filtered by 

 Lake Erie. (3) When 

 swift streams are 

 supplied with tools 

 (Fig. 61) in the form 

 of sand and pebbles, 

 their erosive power 

 becomes greatly in- 

 creased. 



Weathering and 

 Vertical Erosion. — 

 Even very young 

 valleys are usually 

 wider at the top than 



at the bottom. This is due to the fact that while the valley is 

 being deepened by erosion it is also being widened in several ways. 

 The rock is loosened by the various agents of the weather and 

 carried to the stream by rainwash and wind. Normally, valleys 

 are most rapidly widened in temperate regions, since there the 

 soil freezes and thaws frequently so that " creep " (p. 73) plays 

 an important role. Valleys cut in sand or clay are often widened 

 to a considerable degree as a result of the pressure of the overlying 

 sediment, which forces the unconsolidated sand or clay at the base 

 to " flow out," causing a slumping of the upper portion. Animals 

 walking on the slopes, falling trees, the cutting of the stream against 

 its sides are among the agents which help to loosen the material 

 of the valley sides and thus tend to widen the valley. If erosion 

 is very rapid as compared with the work of the agents of the 

 weather, steep-sided gorges barely wide enough to accommodate the 

 stream will result : such are the gorge of the Aar at Meirengen, 

 Switzerland, the picturesque gorges of Watkins Glen and Ausable 

 Chasm, New York, and the canyon of the Virgin River in Arizona. 

 Usually, however, young valleys are V-shaped, the wearing back 

 of the sides more than keeping pace with the deepening of the 

 valley. 



