668 
PROFESSOR PRESTWICK OK THE ORIGIN 
height, the water filling Glen Roy would have to escape by the col at the top of that 
glen into the head of Strath Spey. This col, therefore, would determine the level of 
the lake, and keep it at the upper line as long as this state of things lasted. 
“ Now let the Glen Treig glacier shrink a little. This would open the Glen Glaster 
col, and let out all the water above its level. That watershed would now determine 
the height of the lake, and therefore keep its surface at the middle line so long as this 
second state of matters lasted. 
“ Then let the Glen Treig glacier shrink again, until it withdrew out of Glen Spean. 
That valley being now clear, the water would escape by the outlet at Makoul, which 
would then determine the level of the lake, and keep it at the lowest line so long as 
the ice-stream across the mouth of Glen Spean maintained itself of sufficient height. 
When this latter finally gave way, Glen Roy would at length be emptied. 
“ Grant, then, these two ice-streams, one in the Great Caledonian Valley and the 
other at Glen Treig, and the problem of the parallel roads can be solved, provided 
we allow that glaciers have the power to dam such deep bodies of water as must have 
occupied Glen Gluoy and Glen Roy.” 
In support of this view, Mr. Jamieson shows that Glen Arkaig is glaciated to the 
height of 700 feet and more, and that at the entrance to Glen Treig the glacial striae 
reach a height of 1,800 feet above the level of the sea. He also found ice-striae at the 
entrance of Glen Gluoy at a height of 800 to 900 feet, which he takes to indicate a 
movement of the ice from Glen Arkaig across Loch Lochy, in the same way that the 
striae on the north side of the Spean Valley indicate the protrusion of the Treig 
glacier across that valley (see p. 684). 
§3. Objections to this Exposition —1, the Barriers ; 2, the Cols. 
The Barriers .—Loch Arkaig is only 140 feet above the level of the sea, and, although 
the glen is 18 miles long, the cols at its head are of no great elevation—one by Glen 
Dessarry being under 800 feet, and another by Glen Pean under 500 feet high. The 
mountains at that end of Glen Arkaig attain a height of from 2,800 to 3,200 feet, 
whilst those nearer its entrance do not exceed 2,700 feet in height. With mountains 
of this height, and with these low cols forming channels of outlet to the west coast, 
the eastward ice-stream could hardly have attained such dimensions as to traverse 
Loch Lochy, here 456 feet deep, and ascend the opposite hills to a height of at least 
1,200 feet: still less so if an ice-stream had existed in the Great Glen. 
The Glen Treig glacier, rising in the higher mountain chain of Ben Nevis, would 
have had greater power, but still it seems to me incompetent to the task assigned to 
it. This glen, although 10 miles long, is occupied for a length of 5-§- miles by the loclp 
which is 784 feet above the sea. To block Glen Glaster col the glacier would have to 
cross Glen Spean, and then to travel 2 miles with a rise of not less than 500 feet;'"' 
and if it reached the top of the pass what was there to stay its further progress ? 
* The bottom of the Spean Valley is here about 600 feet above the sea level, 
