148 CONTROL OF EROSION IN THE MOUNTAINS 



as a general rule the precipitation increases with the altitude. Accord- 

 ing to an official report: 



"While atmospheric precipitation gives birth to torrents in the mountain regions, 

 one must not overlook the fact that this same moisture enables the creation of forest 

 stands to diminish the violence of floods." 



It is clear, therefore, that the rains have a double effect. From one 

 viewpoint they damage the mountains; and from the other they make 

 forests possible and thus prevent disastrous erosion. 



A part of the abundant rainfall is absorbed by the ground, a part is 

 evaporated and a part rims off in streams. The quantity of water 

 absorbed naturally depends on the character of the soil and on its vege- 

 tative cover. The runoff depends on the slope for its speed, and the 

 evaporation (which is often very great) depends upon the water being 

 available. Natural vegetation plays an important part. On impervious 

 rocks practically all the water either runs off or is evaporated. On per- 

 meable ground, especially if it is wooded, most of the water is absorbed. 

 The desirabihty, therefore, of having the slopes forested is apparent. On 

 the other hand, one must not lose sight of the fact that on impermeable 

 surf aces " certain ground sometimes . . . absorbs water to the satu- 

 ration point and slides according to the degree of slope." 



After prolonged rains or storms the water erodes the surface of the soil 

 and forms ravines. While this form of erosion is dangerous it is not 

 nearly so difficult to control as landslides, which occur where the per- 

 meable soil is saturated and slides over an impermeable surface. Snow 

 frequently causes damage; it forms avalanches which erode ravines by 

 tearing away the surf ace»of the soil. Hail acts mechanically in detaching 

 little particles of rock and in facilitating the movement of large rocks 

 down steep slopes. Variations of temperature and wind all assist in 

 erosive action. "The wind is a denuding agent which is often very 

 active." According to my notes : 



"In the Maritime Alpes on a windy day thousands of particles of rock and soil are 

 moved by the wind; this makes a peculiar noise which struck me as being especially 

 mournful." 



Sometimes a considerable mass of water accumulates under a glacier 

 and when it breaks out erodes a ravine with great rapidity. 



Rocks and SoUs Easily Eroded. — In the Alps the soils which disinte- 

 grate the more readily are the marls, the schists, the gypsums (including 

 the so-called "terres noires"), and, finally, the detritus and the glacial 

 and alluvial sediment. The very great abundance of land of this 

 nature explains the intensity of the damage done by torrents in this 

 region. Most of the slopes are steep, and the water of storms or heavy 

 rains concentrates rapidly in the arroyos and is laden with d6bris of 



