
D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 611 
there was a strong stream or river flowing through Glen Glaster from the 
lakes occupying Glen Roy and Glen Gluoy. Then there were streams descend- 
ing from the Glen Roy hills on both sides. On the south side there was the 
Bohina Burn, and on the north side the river from Glen Collarig. These 
numerous streams would act on and annihilate the blockage which kept up the 
Glen Roy Lake to the middle shelf. The result would be an entrance of lake 4 
into Glen Collarig by its south end; and accordingly it will be seen from the map, 
(Plate XLII.) that Shelf 4 goes up Glen Collarig, but stops on both sides at a 
point which is only a few hundred yards from the place where the middle shelf came 
to, from the north. What was it which prevented Shelf 4 reaching further north? 
The ground plan on Plate XLII., taken from the Ordnance Survey, shows 
where this: blockage must have been situated. It must have been between the 
points where Shelves 2 and 3 terminate, and the points where Shelf 4 terminates. 















































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Fig. 10. 
Section showing in Glen Coilarig the termination of Shelves 2 and 3, viz., at B and C, and 
of Shelf 4 in a lower part of the glen, viz., at D, with the supposed detrital blockage A 
which separated the lakes. 
The intervening space must evidently have been occupied by a blockage (A in 
fig. 10), which answered the double purpose of keeping separate the waters of 
Shelves 2 and 3 (viz., B C), and the waters of Shelf 4 (D). This intervening 
space, when measured on the ordnance map, gives an average thickness of 
blockage or barrier of about 660 yards. The width of the blockage (crossing 
the glen) need not have been more than 800 yards; and the depth of the block- 
age, for the highest of the shelves B, even supposing that the glen was as deep 
then as now, would not have been more than 368 feet. But making allowance 
for the erosion of the valley, since the time of the Parallel Roads, the probability 
is that a blockage of considerably less height was sufficient, and existed. 
The detrital mass A (fig. 10), at first must have been in such quantity as to 
reach northwards to where the highest shelf, No. 2, stops, viz., B. It then was 
scooped out or was undermined on that side for 50 or 60 yards, so that when 
VOL, XXVII. PART IV. 7Y 
