192 SEEDING AND PLANTING 



8. Overcoming Soil Instability on Slopes. In regions of 

 heavy precipitation over short intervals of time, exposed soil on 

 steep slopes becomes very unstable because of land slipping and 

 surface erosion. This is particularly true when shallow soils are 

 underlaid by impervious rock. The harm done is not only in re- 

 ducing the productive power of the land, but also in covering the 

 soil lower down with accumulations of gravel, sand, and other 

 debris. Where the soil is likely to become unstable when exposed, it 

 should be kept permanently in forest growth. Only selection cutting 

 should be permitted, and the reproduction attained while a crop 

 is still on the ground. As a rule, grazing should not be allowed. 

 Where excessive soil movement has taken place, regeneration 

 often becomes extremely difficult. It is necessary to fix the soil 

 that still remains before any attempt is made at seeding and plant- 

 ing. Soil instability is frequent in mountainous regions where the 

 forest has been over-cut, repeatedly burned, or over-grazed. The 

 reclamation of denuded and excessively eroded slopes can be at- 

 tained only through large expenditures in erecting protective 

 works for the control of surface water. Such works have been 

 constructed on an extensive scale in denuded portions of the 

 mountainous regions of southern Europe, and under their pro- 

 tection successful artificial regeneration has been accomplished. 

 The lack of proper protective measures in many of the mountain 

 forests of the United States is already bringing about condi- 

 tions of soil instability which will ultimately call for vast expendi- 

 tures to control the surface flow of water before regeneration can 

 be attained. 



The control of surface water on barren slopes in order to in- 

 crease soil stability can be attained only by checking the velocity with 

 which it flows over the surface. This can be accomplished by 

 either of the following two methods or by both combined. 



1. By the construction of various types of revetment works or 

 retaining walls to hold back earth, gravel, and boulders. These 

 obstructions are built across the beds of streams and their tribu- 

 taries at suitable intervals. They are constructed of masonry, 

 timber, and earth. Trees are often felled into the bed of the 

 stream and fences of one sort or another thrown across. The 

 cutting of banks is usually controlled by planting willows or cot- 

 tonwoods. By means of trenches and other conduits, the water 

 is distributed over as wide a surface as possible. 



