— 
© 
the Parallel Roads of Glen Roy. 33 
soft soil and stones would be washed away and carried into Glen 
Glas Dhoire. Thus, while the waters of Roy rush up that glen, 
the tongue of land between the two lakes would be steeply and 
abruptly cut off, and the materials washed away and packed into 
the convenient gulf hollowed out of the dam by the Glas Dhoire 
Lake. We may see that this has happened. The sketch of Glen 
Glas Dhoire shows Meal Dubh steeply cut off; while just opposite, 
at a level a little below the second ‘‘road,’’ is the big rounded 
bank, Cruaidh Bheinn, whose size, shape, height, and position 
show that it remains as a cast of the gulf which existed there 
at the time of the débacle. ‘The stones in this drift-bank are 
water-worn, which is very unusual in these valleys. (Fig. 4.) 
After the waters of Glen Roy had thus overflowed through 
Glen Glas Dhoire and settled to the level of the middle ‘‘road,” 
the united lake would continue to eat its way into the ice-dam. 
At the same time the lake in the upper part of Glean Spean 
would similarly continue to hollow out its gulf for another three 
or four miles. Then in the position of the second ice-margin 
drawn in the map the two lakes would communicate over the 
shoulder of Creag Dubh, and again form a drift-bank in the end 
of the advancing Spean gulf. which remains to-day as Meall 
Dhoire, a big bank, height 800 ft., on the east side of the entrance 
to Glen Roy. Thus the water in the glens sinks to the 850 ft. 
level, and there forms the lowest ‘‘road”’ round its edge. 
But at this time there must be a small lake left in the upper 
part of Glen Collarig, for it communicates with Glen Roy as yet 
only by its ‘‘col,”’ height about 950 ft. Until its gulf joins the 
advancing Spean gulf, this little lake ought to be forming a less 
marked ‘‘road”’ at that level. I am not aware if that road in 
Glen Collarig does exist, but expect it does, since opposite Glen 
Collarig there is a mound, height 800 ft., due north of Roy Bridge, 
which I take to be the drift-bank formed when this Collarig lake 
emptied itself into the Spean. Anyhow, the enlarged Spean lake 
would continue to advance into the diminishing glacial dam until 
it reaches the position of the third ice-margin drawn, and shaded 
vertically, where the ice is seen hugging the mountain, and 
sheltered under its northern slopes, while still stretching across 
to the other side of the valley. Then what probably happened 
was that the dammed-up waters found, or melted, a channel or 
erevice under the ice, and emptied all the waters of this huge 
lake, thirty miles long, and at this place 600 ft. deep, by this 
subglacial channel. 
When such a large body of water at such tremendous pressure 
was discharged through a narrow passage at one place, it would 
naturally make some permanent mark on the locality. And there 
is here a striking phenomenon. The small river Spean, in its 
meandering course through a broad flat valley, at this point runs 
D 
