404. MR MILNE ON THE PARALLEL ROADS OF LOCHABER. 
have seen that, whilst Glen Glaster is exempt from shelf 2, it is well marked on 
both sides by shelf 3. 
To explain these facts, I assume that there was a blockage of some sort, in 
Glen Roy, which filled the lower part of the valley up to the level of shelf 2, and 
which blockage extended a little farther east than the mouth of Glen Glaster. I 
assume also a similar blockage in Glen Collarig, which filled the lower part of the 
valley, and as far eastward as the place where shelf 2 stops in that glen. This 
blockage may have been gravel, clay, or any other detrital matter. 
Such is the supposed state of things, whilst the waters stood at shelf 2 in 
Glen Roy; at which period, it will be remembered, they were discharged to the 
eastward. 
Former writers have assumed, that when the waters sunk from shelf 2, the 
amount of sinking must have been 82 feet, the distance of shelf 3 below shelf 2: 
and that this sinking had been one act, caused by an earthquake, or other violent 
operation, which all at once lowered the barrier by that number of feet. But 
this is a mistake. MacCuttocn takes notice of a shelf faintly marked on Tom- 
bhran hill, between shelf 2 and shelf 3, though he expresses afterwards some 
uncertainty about it. In fact, there are two intermediate shelves visible there : 
and they are also discernible, at precisely the same level on Ben Erin, and also 
more distinctly near Achavaddy, on the south side of Glen Roy; the one being 
about 14 feet below shelf 2, and the other about 36 feet lower down.* These two 
intermediate shelves clearly indicate, that the water which filled the valley, did 
not all at once sink from shelf 2 to shelf 3. They prove that the water first sunk 
down 14 feet, and was stationary at this level for some time; that it then sunk 
down other 36 feet, and continued at this level for some time; and that it again 
sunk other 32 feet, at which level it remained for a much longer period, till it 
formed shelf 3. 
It is evident, from these facts, that the lowering of the barrier (of whatever 
material composed) which confined the water in Glen Roy, was a process of a 
more gradual and ordinary description than what former writers, and especially 
Mr Darwin, suppose. It is plain, also, that the barrier which kept in the waters 
was less rapidly worn down, when they stood at shelves 2 and 3, than at either 
of the intermediate levels. We see that at shelves 2 and 3 the waters flowed over 
rocky ledges, in the one case into Spey valley, in the other case by Glen Glaster. 
Is it not fair from this to infer, that at the intermediate shelves, the water flowed 
over a blockage of such a nature as was capable of being more easily worn down 
and obliterated, such as detrital matter? It is, at all events, obvious, that when 
* There are hummocks or knolls of stratified gravel and sand in Glen Glaster, the tops of which 
are all about 36 feet above shelf 3. It is probable that they were deposited when the lake stood at one 
or other of the intermediate points last mentioned. 
