4<)4 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 ( ilaster. I 

 assume also a similar blockage in Glen CoUarig, which filled the lower part of the 

 valley, and as far eastward as the place Avhere 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 Avi-iters 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. MacCulloch 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. 



