150 CONTROL OF EROSION IN THE MOUNTAINS 



the Central and Western Pyrenees. In the Eastern Pyrenees and in 

 the Cevennes and the Central Plateau there is not much year-long snow. 

 Even after deducting the areas protected by snow in the Alps and Pyre- 

 nees, vast areas of erodible ground remain. The north and northeast 

 slopes remain covered with snow for considerable periods in each year 

 and consequently suffer less than the other slopes. On these slopes the 

 variations in temperature are less sudden and of less extent, the rains 

 are even less intensive, and the vegetation is habitually more vigorous 

 than on south slopes. Therefore most of the torrent gorges are formed 

 on south and west slopes. A French definition of a torrent gorge says 

 it is: 



"A temporary water-course in which the water concentrates after heavy rainfall 

 and acquires, because of its mass and because of the slope of the stream bed, a con- 

 siderable live force. The characteristic trait of torrent gorges is the faculty which they 

 possess of reuniting in a single flood all the water falling within a certain time on an 

 extensive area. This faculty is due to the configuration of the ground and the principal 

 feature of a torrent gorge, that which gives it being is a collecting basin which favors 

 the rapid concentration of rainfall. The basin of reception of a torrent, called also the 

 funnel, is a more or less complete circle on whose steep slopes falls the water of heavy 

 rains. . . . The Gavarnie 'circus' in the Pyrenees is a good example. . . . The 

 torrents run in very short valleys, which cut the mountains at right angles to the slope 

 just as in simple depressions. Their slope may exceed 6 per cent for their entire length 

 but it varies a great deal and is never less than 2 per cent. They have an entirely 

 special characteristic in that they flow in areas determined by their courses, resting 

 upon one another and diverging because of their deposits. According to Scipiongras, 

 a torrent is a water course whose rise is swift and violent, whose slopes are considerable 

 and irregular and which often raises certain parts of its bed because of the deposit of 

 material; it is this which makes the water diverge at the time of floods." 



Torrent gorges may be the courses of temporary or permanent streams 

 whose beds are not yet fixed and which perform the work of carrying 

 away the mountain for deposit in the plain. The following definition 

 is also of value: 



"A torrent gorge is a temporary or permanent water course in which the water 

 concentrates with extreme rapidity after heavy rains and by its energy of movement 

 digs out its bed, which is considerable because of the mountain slope and because of 

 the increase in density of the material transported. The soil and d<5bris of all kinds 

 eroded by the waters are deposited on the plain." 



These different definitions support each other. Ravines have the 

 same character as torrent gorges but are less strongly marked. "A 

 ravine is a branch of a torrent gorge in process of formation. . . . " 



Formation of Torrent Gorges. — The energy of movement or force of 

 water that flows down a steep slope is greater at the base of the slopes 

 than at the summit and erosion is therefore greatest at the base. It 

 also follows that the lower part of the slope is often erodible ground 



