GLEN ROY, IN SCOTLAND. 
49 
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 mate¬ 
rials traceable to the hills from which they 
descended. In this case water had no part in 
loosening or bringing down this mass of frag¬ 
ments. They simply rolled from the declivity, 
and stopped when they had exhausted the mo¬ 
mentum imparted to them by their weight. 
In the case of the debdcle of Bagnes, above 
Martigny, in a valley leading to the St. Ber¬ 
nard, 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 mate¬ 
rials 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 
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