OF THE PARALLEL ROADS OF LOCHABER. 
703 
the bare surface not yet protected by vegetation, would be the result on that surface, 
—placed under water by the temporary conversion of certain valleys into lake basins, 
—by the sudden discharge of successive portions of the water through cols at different 
lower levels ? 
In the absence of any conspicuous detrital remnants on Glen Glaster col, we may 
assume that the barrier was formed wholly or chiefly of ice—a remnant of the old ice- 
sheet due to the accumulation of ice by glaciers descending the glens of Ben Mheirlich 
(2,994 feet), and other hills immediately at the head of Glen Glaster. The barrier 
must have been one of considerable magnitude—high enough not only to dam the pass 
when the lake stood at the level of No. 2 “ road” and flowed into Glen Spey, but also 
to have barred the passage when the lake, before it escaped over the col at the head of 
Glen Roy, had the yet higher level required on our hypothesis. 
After a time, and for the reasons and in the manner before mentioned, this barrier— 
unsupported on the Glen Spean side, where the thaw had already begun to form the 
Spean Valley lake and had removed much of the ice in the lower levels—gave way, 
I conceive, suddenly, and was followed by a rapid partial discharge of the lake waters. 
If we consider the condition of the surface of the ground at the time this took 
place, we find it recently relieved from its mantle of snow and ice, and the hills covered 
with a coating, several feet thick, of local angular fragments mixed with sand and clay, 
the result of decomposition, and, here and there, of moraine matter, sand and gravel, 
the product of the great ice-sheet and of the intense cold on the underlying rocks. 
As the ice disappeared this debris remained bare and exposed, and without any layer 
of vegetation to protect it from shifting from very slight causes. 
Under these circumstances the detritus on that portion of the slopes, over which 
the water had risen in the enclosed valleys, became saturated with water, forming a 
soft yielding muddy and stony mass. As the lake fell rapidly, and exposed this 
detrital bank, the saturated mass of debris would part with its water, which, as it 
drained away, must have caused a loss in volume that led to the displacement of 
its diverse component parts, and thereby set the whole semi-liquid mass in motion, 
and caused it to slip or slide down after the falling waters of the lake, with a velocity 
in proportion to the angle of slope and momentum of the mass. The slide would con¬ 
tinue so long as the escape of the lake waters continued, or until their level fell to 
the level of the Glen Glaster col. As these waters came gradually to rest, their 
inertia, opposed to the momentum of the detrital mass, combined with the absence of 
any fresh exposure and the cessation of the original cause of movement, more or less 
suddenly arrested the sliding mass, deflecting it in a direction more horizontal and 
outwards from the face of the hill, and thereby causing it to form a shelf parallel with 
the water-line :—the extent both of the deflection and the slide depending on the 
relation between the volume of the detrital matter and the angle of slope, and 
according as these varied so would the inclination of the shelf (or “road”) to the 
horizon, and its precise parallelism with the horizontal water-line, also vary. 
MDCCCLXXIX. 4 X 
