PARALLEL ROADS OF LOCHABER. 97 
must therefore have stopped there. What hindered the lake extending farther 
down the Glen? Some blockage extending across the Glen must have existed. 
It is also worthy of observation that, when the lake subsided from Shelf 2 
to Shelf 3 (a fall of 81 feet), the lake was enabled to extend farther down Glen 
Collarig by about 40 yards. This is evident from the circumstance that Shelf 3 
can be distinctly seen, and it is marked on the Ordnance Survey as terminat- 
ing 40 yards beyond where Shelf 2 terminates. 
This state of matters will be better understood by referring to fig. 10, on 
page 611, and to Plate XLII. of my former Memoir. 
2. If it be established, as I venture to think it is, that the blockage in Glen 
Collarig, which kept in the lakes of Shelves 2 and 3, and separated these from 
the lower lake represented by Shelf 4, was detritus, and not ice, there should 
be the less hesitation in accepting a similar blockage for Glen Roy. 
Sir Henry JAMES, in his one-inch Ordnance Map, has indicated the position 
of two lake barriers in Glen Roy, calling them “ Jce-Barriers.” 
One of these barriers crosses Glen Roy a little above Cranachan, where the 
valley is about a mile wide, and the bottom of the valley about 800 feet below 
Shelf 2. 
The other barrier Sir Henry JAMES marks on his map as crossing Roy 
Valley between Cranachan and Boheenie. To reach Shelf 3 at its two extremi- 
ties, this barrier must have been 14 mile long. The bottom of the valley is here 
about 700 feet below Shelf 3. 
In my last Memoir, I observed that the first of these barriers need not 
necessarily be at the place indicated by Sir Henry James. I suggested that 
it might have been at the head of Glen Glaster, where the col reaches a height 
of 1075; so that at this col, a blockage of only 81 feet in height instead of 800 
feet, and a quarter of a mile wide instead of one mile, would be sufficient. This 
spot, therefore, is the more probable for the required blockage of Shelf 2. 
I farther then stated, that the necessity of adopting this position instead 
of Cranachan would be established, were it ascertained that Shelf 2 extended 
into Glen Glaster. On my last visit, I discovered traces of Shelf 2 on both sides 
of Glen Glaster, so that there is now no room for farther question on this point. 
With regard to the blockage for Shelf 3, between Cranachan and Boheenie, 
I admit that the difficulty of magnitude remains. But that difficulty any 
theory of barriers must encounter ; for as to the fact of there Heaney: been a 
blockage here, of some kind, all are agreed. 
The only question is—whether it was detritus or ice ? 
That there is, even yet, on the south side of the valley, an enormous 
accumulation of muddy detritus, must be perceived by any one who examines 
Shelves 3 and 4 in this locality. The facility with which this detritus 
is cut through and removed by streams is indicated in many places. It 
VOL. XXVIII. PART I. 2B 
