176 CONTROL OF EROSION ON RANGE AND PASTURE 



Gully Erosion. — On lands which are somewhat steep and 

 roUing. well-defined trench channels are readily formed where 

 the plant cover has been destroyed. A large portion of the 

 water from heavy rains finds its way into the channels, and 

 the water, rushing rapidly down, carries away much of the 

 loose soil over which it passes. Gully erosion is much more 

 noticeable than sheet erosion and usually progresses more rap- 

 idly. Occasionally numerous small, more or less parallel gul- 

 lies are formed which are popularly known as " shoestring 

 guUies." They vary in form from sharp-bottomed V-shaped 

 cuts with steep sides to broad depressions with gently rounded 

 sides. 



Sheet Erosion. — Sheet erosion is marked by a more or less 

 uniform washing away of the soil over the surface as a whole. 

 The water that is not taken up by the soil runs downward more 

 or less uniformly over the land. With a heavy rainfall the flow 

 down the slope may remove much of the surface soil. This 

 form of erosion not uncommonly causes very serious losses of 

 soil fertility and is difficult to control. Sheet erosion, however, 

 is not confined to the action of water alone, but is caused by 

 wind as well, a stiff wind often depositing considerable loose 

 soil at the base of the slope. 



Landslides. — The slipping down of surface soil, usually of 

 a large solid mass, is known as landslide erosion. Such shdes 

 occur as a rule where the soil rests on a smooth wet substratum. 

 The soil is usually saturated with water and slips ofT suddenly 

 from the underlying rock formation. Fares which destroy the 

 deeply ramifying roots that bind the surface soil to the sub- 

 soil contribute to the action of landshdes. Landslides usually are 

 confined to small areas and, except in restricted localities, are 

 seldom an important factor in the destruction of soil fertility. 



River-Bottom Erosion. — Much highly valuable valley land 

 has been destroyed by the wearing away of the river banks or 

 by the formation of new channels. The historic Mississippi 

 floods of almost annual occurrence afford excellent examples of 

 this form of destructive erosion. The enormous amount of 

 debris carried down by the swollen river may be deposited in 



