286 SOIL EROSION 



Fire a Causal Agent. — The effect of fire in its relation to soil 

 erosion is similar to that of clearing. Fires have burned not only 

 the tree growth but the leaf mold as well, leaving the ground 

 exposed to the beating of the lains and thus causing surface run- 

 off and hence soil washing. 



Destructive Lumbering. — Clear cutting of timbered lands has 

 commonly been practiced even on steep slopes where a part of the 

 younger growth, at least, should have been allowed to stand. 

 Fires have been permitted to do much damage, especially on cut- 

 over areas. Roads were so laid out as to result in serious erosion 

 and skid tracks were gouged out and left improtected only to 

 wash out after every heavy rain. 



Overgrazing. — ^A hillside well grassed is fully protected from 

 soil washing. During ordinary rains the grass protects the soil 

 from the beating effect of the falling water, and also causes the 

 water to soak into the soil and not pass away as surface run-off. 

 The physical condition of the soil commonly present imder a 

 good grass covering has much to do in facilitating the entrance 

 of the rain water into the ground. During heavy rains the long 

 grass blades form a protective mat over which the flood waters 

 pass with little or no harm to the soil underneath. When rolling 

 lands are pastured or grazed too severely this protective grass 

 covering is destroyed and severe erosion results. 



KINDS or EROSION 



Gully erosion is characterized by the formation of V-shaped 

 cuts or ditches. Sometimes these gullies are small, numerous and 

 more or less parallel to each other. At other times they are few 

 in number, but deep and broad. Under certain conditions gullies 

 form which have vertical, cliff-like sides which keep caving in as 

 the water undermines them. When once started, such gullies 

 rapidly become deeper, wider and longer with every storm until 

 they develop into ugly chasms. 



Sheet erosion is the washing away of the soil more or less 

 uniformly over the entire surface without the formation of gullies. 

 This erosion usually forms on fields of only moderate slope. Every 

 rain washes away some of the finer particles; but since the process 

 of washing is usually slow, results are not particularly noticeable. 



Landslides. — ^A landshde is the sliding down of a large quantity 

 of soil. They are usually caused by the soil becoming saturated 

 with water and then slipping in a large body from the under- 



