EEMAEES OK GLEN EOT. 13 



flauk would have spread eastward some distance beyond the point 

 where the Parallel Eoad finally stops. This shows that, although 

 the Corry N'Eoin glacier cut off the western extension of the lake 

 at this side of Glen Spean, it nevertheless played but a very sub- 

 ordinate part in the formation of the ice-dam at its final stage. 

 It was the heavy bed of ice in the valley of the Caledonian Canal 

 that seems to have constituted the main barrier. This ice probably 

 filled the whole valley from Loch Ness down to Corran Perry, being 

 powerfully reinforced on its eastern side by the glaciers descending 

 from the ravines of Ben Nevis. The lake was nearly four miles 

 broad where the ice-dam stood across it at the mouth of Glen 

 Spean ; and in the centre of the valley the water must have been 

 some hundreds of feet in depth. This shows what a powerful 

 barrier was required, and gives an excellent idea of the mass of ice 

 that still lay over the Western Glens, and how slowly it melted 

 away. One of the heaviest beds of all was probably in Glen 

 Arkaig, a glen which has a very capacious basin situated in the 

 region of maximum precipitation. 



yil. Subsidence oe the Lake : teaces oe a debacle. 



A question of some interest arises in connexion with these lakes, 

 namely, did the ice-dam give way suddenly, causing a debacle, or 

 did the water drain off quietly by finding an escape underneath the 

 glacier ? Both methods are known to occur in the case of glacier- 

 lakes in the Alps and Himalayas. 



When the water dropped 80 feet from the higher to the middle 

 parallel in Glen Eoy there was probably a rush into Glen Glaster 

 — a very heavy rush if the drop was at all sudden. The water 

 thus let off would first flood Glen Glaster and then come pouring 

 over its col down the hollow at the back of Craig Dhu into the 

 Kough Burn. As in some places there are traces of intermediate 

 shelves between the two upper ' roads,' it is probable that this 

 drop did not take place all at once. Now, at the mouth of the 

 Hough Burn and to the east of it I fancied that I saw some traces 

 of a debacle. The moraines of the Treig glacier have been broken 

 down and scattered ; clusters of great stones are to be seen on the 

 east or lee sides of the granite rocks, looking as if they had been 

 thrown together by a powerful gush of water, being better packed 

 than is usually the case in a moraine, and with the sandy matter 

 washed out of them. Heaps of these big stones may be noticed 

 near the 7 J milestone from Roy Bridge, and also on the moor along 

 the opposite bank of the Spean. The large gap in the lines of 

 moraine, where they should have crossed the Spean near Gorstan, 

 may therefore have been caused in some measure by the rush of 

 water which took place when the lake subsided from the highest to 

 the middle line, or from that to the lowest. On these two occasions 

 the water would sweep eastward and go out by the col at Makoul. 

 Its course, however, would depend on how far the Treig glacier 

 extended at the time, because if the ice lay across the valley it 



