708 
PROFESSOR PRESTWICH OK THE ORIGIN 
section I took on the slope of Tombrahn, at the junction of Glen Roy and Glen 
Turret, at a point where the upper “road” passes over a bare rounded ice-worn surface, 
r, of rock. At this place there are also faint traces of a higher road, u. 
Fig. 17. 
Section of the upper two “ roads ” in Glen Roy where they pass a protruding rock. 
To return to the three “roads,” Nos. 1, 2, and 4. We do not know what the conditions 
immediately antecedent to their formation were. In each case there is the essential col 
over which the water could escape, and in each case there is a like admissibility of a 
temporary barrier, such as the hypothesis requires, to dam back the waters of the lake 
until they rose to a height sufficient to overflow and destroy the barrier. Until this 
took place and a fixed water level was definitely established, the level of the lake would, 
for the reasons before assigned, be constantly varying, and although it might have left 
slight horizontal marks or lines on the hill sides, these lines would not have had the dis¬ 
tinctness and regularity of a detrital shelf formed in the way we have suggested. Nor 
are there wanting indications of such subordinate lines, though they are in general 
very faint. In Glen Gluoy there are two such lines at about 150 and 200 feet above 
the parallel road. They are shown in a sketch by Captain White, R.E. He says of 
them : “ The two upper tracks, although well marked to the eye from this point of 
view and parallel to the lower one (‘road’ No. 1), show nothing when you reach the 
ground to indicate roads”—“an observation,” adds Sir Henry James, “which is 
equally applicable to other places.”* 
These more or less indistinct lines and terraces have been particularly noticed by 
Mr. Milne Home and Dr. Chambers.! They describe two shelves intermediate to 
“roads” 2 and 3, at three several places in Glen Roy, namely, on the front of 
Tombrahn, on the face of Ben Erin, and on the hill opposite Achavady farm. The 
higher one was 14 feet below the No. 2 shelf, and the other 36 feet lower. A 
second shelf was observed in Glen Gluoy, of the same character with No. 1 “road,” 
but considerably fainter. This line was found to be 200 feet below the “ road,’ and 
was traced for several miles in the upper part of Glen Gluoy. 
* ‘Notes on the Parallel Roacls of Locliaber,’ by Sir Henry James, p. 3 and pi. 2. 
t ‘Ancient Sea Margins,’ p. 115. 
