32 Dr. T. Archibald Dukes on 
Gloy undermines the ice-mass, eats its way through to freedom, 
and empties its waters in a northern direction into Loch Lochy. 
Note.—To this day the River Gloy turns north at this point and 
runs into Loch Lochy, instead of continuing its course south- 
west in the direction of the valley. 
— 
Contour Map 
BEN NEVIS GLACIAL DAM 
Moentacns an~ asoor a 
Hille mn 1250 1 
Glacial Dow afin mg 
Rritcl Roads -- ++ 
YD 
© 
pe ROY = 
Glacier dn Pee 
Fie. 6.—Map showing the successive positions of the shrinking ice-dam 
which blocked up‘the valleys, so making lakes, the shores of which are the 
parallel “roads.” Hills over 1250 ft. shaded lightly. Mountains over 
2500 ft. are black. The lower edge of the map shows the northern slopes of 
Ben Nevis and his big neighbours, with peaks ranging from 4400 to 3500 ft. 
high. In other parts a few peaks reach 2500 ft. The parallel ‘‘roads’’ are 
seen as thin lines bordering the valleys. The margin of the ice is indicated 
by a thick wavy line, and is shaded horizontally, diagonally, and vertically 
in the first, second, and third positions respectively. 
In the same way the lakes in Glen Roy and Glen Glas Dhoire 
eat their way into the icy dam till they reach the position drawn 
as the first ice-margin. A little more of this undermining, and 
these two lakes finally intercommunicate sideways, deep down 
under the ice, by a hole which would at first be small, but, since 
the greater pressure on the Roy side of that hole corresponds to 
a column of water 85 ft, high, it would be rapidly enlarged. The 
