OF THE PARALLEL ROADS OF LOCHABER. 
711 
the pass of Makoul, where I presume it to have been blocked by a minor barrier, so 
that at first the level of the lake was considerably above that of the pass. 
My hypothesis requires that “road” No. 4 should be formed simultaneously in Glen 
Roy and in Glen Spean, and for that purpose the removal of the main barrier at the 
entrance of Glen Roy previously to, or at the time of, the formation of that “ road” is 
essential, as otherwise the “roads” in the glens would not have been on the same 
level. If the displacement of the barrier had taken place previously to the fall of the 
water, it could only have been that the waters in the Spean Valley, having yet no 
outlet, gradually accumulated until they reached the same level in both valleys. 
Then, on the gradual decay and waste of the main barrier, the waters of the two 
valleys intermingled in one sheet, so that when, later, the Makoul barrier burst, the 
fall of the united lakes would be simultaneous. 
If, on the other hand, the Glen Roy main barrier gave way soon after the destruction 
of the Glen Glaster barrier, we may suppose that the sudden large accession of water 
to the Spean lake resulted in the immediate bursting of the Makoul barrier, when the 
fall of the two lakes would proceed in conjunction, until the level of the pass was 
reached, and “road” No. 4 projected simultaneously in Glen Spean and Glen Roy, with 
probably a slight break on the site of the barrier. 
On the first supposition the barrier at Makoul should have exceeded 200 feet in 
height: on the second it need not have exceeded 80 to 100 feet. Around this pass 
the drainage of the mountains between Loch Laggan and Loch Ericht (a prolongation 
of the Ben Nevis range) centres, and consequently a large volume of ice must have 
issued from the glens or valleys of the Pattack and the Mashie. These streams, 
joining with the glacier coming from the Spean Valley, here met the ice of the valley 
of the Spey, together with that coming from an opposite direction down Glen Markie. 
All, therefore, tended to a large accumidation and remainder of ice at this particular 
spot, on the removal of which the lake was suddenly lowered to the level of No. 4 
“road.” Chambers, like Jamieson, speaks of this pass as confined by wall-like rocks 
which seem water-worn, and as “ having the appearance of an ancient watercourse.”* 
The difficulty respecting the origin of No. 4 “road” is, however, no less on the 
other hypothesis, which requires first the pre-existence of the Spean lake, inasmuch 
as that valley is supposed to have received the overflow of the Glen Roy lake over 
Glen Glaster col, and secondly the subsequent extension of the Spean Valley waters 
into Glen Roy. In that case, the greater antiquity of “road” No. 4 in Glen Spean 
(for that greater age must have been equal to the time required for the formation of 
the second Glen Roy “road,” No. 3, by slow wear and erosion) should show in some 
difference of form or dimensions of No. 4 road in the two glens ; but there is none. 
Again, the removal of the Glen Roy barrier and the extension of the lake into the 
two valleys involves the continuity of “road” No. 4 from one valley to the other; 
whereas the line is interrupted in Glen Collarig, and on one side of Glen Roy (though, 
* ‘ Ancient Sea Margins,’ p. 112. 
MDCCCLXXIX, 4 Y 
