1912] STEVENSON— THE FORMATION OF COAL BEDS. 495 



slide, a level clearing through which Mad River flows, was covered 

 with great heaps of logs brought down by the slide and swept away 

 by the freshet attending it. They were broken and shattered, 

 though some were 60 feet long. They were piled up confusedly to 

 a height of 15 or 20 feet, stripped of foliage and most of the 

 smaller branches. Farther up the stream, no trees were visible; 

 they had been buried in the coarse debris of the slide. 



The terrific discharge of Lake Mauvoisin has been cited as evi- 

 dence that blocks of the surface could be transported with standing 

 trees and be deposited in the normal position. Knowledge re- 

 specting this great debacle is derived from the description by Escher 

 von Linth, of which a synopsis was published in English.^* The 

 Val de Bagnes is drained by the river Dranse, whose progress had 

 been impeded for several years prior to 1818 by blocks of ice and 

 by snow avalanches from the glacier of Getroz. At length the river 

 was dammed and a lake was formed, which became 10,000 to 12,000 

 feet long, 700 feet wide at top, 100 feet wide at bottom, with an 

 average depth of 200 feet. The content equalled at least 800,000,- 

 000 cubic feet. A gallery, 600 feet long, was cut to drain the lake, 

 but this enlarged quickly in the ice, so that before one half of the 

 water had passed off, the dam gave way and the mass of ice, water 

 and debris was precipitated into the valley below. The whole lake 

 was emptied in less than half an hour and the author well says that 

 one cannot describe the violence of the flood. The passage of the 

 water was checked by a narrow gorge, where it tore away a bridge, 

 90 feet above the preexisting stream ; beyond that, it entered a 

 wider part of the valley, only to be banked by another gorge beyond. 

 Thus passing from one basin to another, it acquired new violence 

 and carried along forests, rocks, houses, barns and the cultivated 

 surface. The flood seemed to contain more debris than water and 

 it moved at the rate of 18 feet per second. The acquisition of these 

 materials made the current more efifective and, when it entered the 

 narrow valley leading from Saint Branchier to Martigny, it con- 



^* " Account of the Formation of Lake Mauvoisin by the Descent of a 

 Glacier and of the Inundations of the Val de Bagnes in 1595 and 1818," 

 Edin. Phil. Journ., Vol. I., 1819, pp. 187-191. 



