74 
tion of the bursting of a lake in his Principles of Geology . 
The upper portion of the valley of Bagnes was converted into 
a temporary lake by the damming up of a narrow pass by 
avalanches of snow and ice, precipitated from an elevated 
glacier into the bed of the Dranse. The lake was half a 
league in length, 700 feet wide, and 200 feet deep in some 
places. Half the contents of this lake were quietly drained off 
by an artificial channel,* 
“ But at length, on the approach of the hot season, the central portion of 
the remaining mass of ice gave way with a tremendous crash, and the residue 
of the lake was emptied in half an hour. In the course of its descent, the 
waters encountered several narrow gorges, and at each of these they rose to a 
great height, and then burst with new violence into the next basin, sweeping 
along rocks, forests, houses, bridges, and cultivated land. For the greater 
part of its course the flood resembled a moving mass of rock and mud, 
rather than of water. Some fragments of granitic rocks of enormous magnitude, 
and which from their dimensions might be compared, without exaggeration, 
to houses, were torn out of a more ancient alluvion, and borne down for a 
quarter of a mile. One of the fragments moved was sixty paces in circum- 
ference. The velocity of the water, in the first part of its course, was thirty- 
three feet per second, which diminished to six feet before it reached the Lake 
of Geneva, where it arrived in six hours and a half, the distance being forty- 
five miles. 
“ This flood left behind it, on the plain of Martigny, thousands of trees 
torn up by the roots, together with the ruins of buildings. Some of the 
houses in that town were filled with mud up to the second story. After 
expanding in the plain of Martigny, it entered the Khone, and did no 
further damage. 
“ Now,” continues Sir C. Lyell, “ if part of the lake had not been gra- 
dually drained off, the flood would have been nearly double, approaching in 
volume to some of the largest rivers in Europe. It is evident, therefore, that 
when we are speculating on the excavating force which a river may have 
exerted in any particular valley, the most important question is, not the 
volume of the existing stream, nor the present levels of its channel, nor even 
the nature of the rocks, but the probability of a succession of floods at some 
period since the time when the valley may have been first elevated above 
the sea. 
“For several months after the debacle of 1818, the Dranse, having no 
settled channel, shifted its position continually fi om one side to the other of 
the valley, carrying away newly-erected bridges, undermining houses, and 
continuing to be charged with as large a quantity of earthy matter as the 
fluid could hold in suspension.” 
* Lyell’s Principles of Geology , vol. i. p. 364, 6th edition. 
