152 CONTROL OF EROSION IN THE MOUNTAINS 



by fire and grazing. On the west side of the valley there are belts of 

 forest that extend from the foot of the slope almost to the top. On the 

 east side, which faces the west or the south, the forest belt is narrower. 

 At the head of the valley there is hardly any forest at all, because the 

 slopes are rocky and very steep. Above and between the present forest 

 zones grazing by sheep had started erosion and landslides. The soil, 

 permeated with water, loosens and slides down, the slides producing great 

 gullies. This is a good example of how small torrents begin. 



At Barcelonnette it was pointed out that the beginning of ravines on 

 bare grass-covered slopes was usually due to grazing by sheep and par- 

 ticularly to permanent bedding grounds at the heads of high mountain 

 valleys. According to my field notes: 



"The Ravine de Roche Noire (Basses- Alpes) 50 to 60 years ago was merely a cow 

 trail or a path where wood was sledded down the slope. The soil, formed of a schist- 

 marl, has now been eroded to a depth of 30 to 40 feet. As no dams were built when the 

 slopes of this ravine were reforested the erosion has continued, and it is now necessary 

 to build low, dry stone dams to prevent further damage. Without these dams, which 

 are built without masonry binder, the erosion will eat in 20 inches after a single bad 

 storm. It was explained that this erosion was not caused by grazing but was merely 

 due to the falling of rocks, through natural causes, from the precipices above. This 

 raises an interesting question : What would the natural erosion in the Alps have amounted 

 to without the advent of man! The local inspector said that while sheep grazing is 

 still doing great damage it cannot be forbidden or even summarily restricted because 

 it is the sole industry of the mountain villages, and drastic restrictions would mean the 

 depopulation of the countryside." 



Damage Caused by Torrents. — We have already seen that " a tor- 

 rential wash" is a fluid mass which carries material of all sizes and whose 

 speed is dependent upon the slope and upon the amount of rainfall. 

 This material comes from a number of sources. It may consist of debris 

 from the disintegration of rocks, carried by gravity, by water, or by 

 avalanches; (2) it may be due to simple erosion by rainwater, to slides, 

 or to the movement of glaciers; (3) it may include trees, branches of 

 trees, and debris from fellings, which, even if small, are dangerous ele- 

 ments in washes because, on account of their light weight, they do not 

 stop until the slope becomes very gentle and they form temporary dams 

 behind which the water accumulates only to break forth and inflict greater 

 damage lower down. Each year torrents and ravines cause material and 

 even great damage and, only too often, loss of human life. Such acci- 

 dents direct public attention to their danger. The direct losses during 

 17 years are estimated in Table 17, and it is very noteworthy that the 

 damage was always less in the areas where erosion betterments had been 

 started: 



