Occasionally we may have also to distinguish between all these deposits and the _debris_ brought down by land-slides, or by sudden freshets transporting to a distance a vast amount of loose materials which are neither ice-worn nor water-worn. At Rossberg, for instance, in the Canton of Schwitz, the land-slide which buried the village of Goldau under a terrific avalanche, and filled a part of the Lake of Lauertz, spread an immense number of huge boulders across the valley, some of which even rolled up the opposite side to a considerable height. Many of these boulders might easily be mistaken for erratic boulders, were not the aggregate of these loose materials traceable to the hills from which they descended. In this case water had no part in loosening or bringing down this mass of fragments. They simply rolled from the declivity, and stopped when they had exhausted the momentum imparted to them by their weight. In the case of the _debacle_ of Bagnes, above Martigny, in a valley leading to the St. Bernard, the circumstances were very different. A glacier, advancing beyond its usual limits and rising against the opposite mountain-slope, dammed up the waters of the torrent and caused a lake to be formed. The obstruction gave way in the course of time, and the waters of the lake rushed out, carrying along with them huge boulders and a mass of loose materials of all sorts, and scattering them over the plain below. Such an accumulation of _debris_ differs from the pebbles and loose fragments found in river-beds. The comparatively short distance over which they are carried, and the suddenness of the transportation, allow no time for the abrasion which produces the smooth surfaces of water-worn pebbles or the polished and scratched surfaces of glacier-worn ones. In the latter case, we have seen that the pebbles, being so set in the ice as to expose only one side, may be only partially polished, while others, more loosely held and turning in their sockets, may receive the same high polish on every side. In such a case the lines will intersect one another, in consequence of the different position in which the stone has been held at different times. No such appearances exist in the water-worn pebbles: their blunt surfaces, smoothed and rounded uniformly by the action of the water in which they have been rolled or tossed about, present everywhere the same aspect.