722 
PROFESSOR PRES TWICE ON THE ORIGIN 
resist, and when they gave way there would be deposited lower down the valley, or in 
the next basin, great beds of debris and shingle carried down by these debacles, to he 
in their turn buried at a later period; for eventually all the watercourses were stopped, 
and ice overspread the whole land. 
Admitting, therefore, under certain modifications, with most of the Scotch geologists 
that we have in the Till (and its associated beds of gravel and sand) the sub-glacial 
product of the great ice-sheet, we have further to consider the detritus scattered on 
the surface of that great mass of ice and distributed by the torrents and floods to 
which it gave rise in its decay. For this purpose the evidence furnished by the 
Lochaber lakes is of singular value, for it is evident that what we see there is only 
part of a great general phenomenon extending in mountainous districts as far as the 
ice-sheet itself extended. 
Instances are not wanting of traces of Parallel Roads in other valleys, but they are 
so faint that they often escape notice. Besides the special glacial conditions, a suffi¬ 
cient slope, and a sufficient covering on the slopes of peculiar detrital debris, are 
essential for the perfect development of the shelves. Often apparent at a distance, 
they fade on approach. This is not surprising when, as occasionally happens in Glen 
Roy, the inclination of the “roads” so nearly approaches the angle of hill-side slope. 
It is possible that some of the many higher lines and shelves noticed elsewhere by 
Chambers may be referable to this class of phenomena; but besides these more 
general cases,* Darwin mentions a particular instance in Glen Kilfinnin—a glen 
which runs into the Great Glen 8 miles north of Glen Gluoy.t This Gien Kilfinnin 
“road” is about 40 feet above the upper “road” of Glen Ptoy, and extends about 
2 miles on the face of the surrounding mountains. Mr. Milne Home has also recently 
directed attention to distinct traces of similar lines, but having apparently a wider 
range, in a glen on the opposite side of the Caledonian Valley. These lines which 
are near the head of Glen Doe, form four horizontal terraces, the highest being about 
085 feet above the sea, and the lowest about 895 feet.j 
Although in an ice-sheet formed of confluent glaciers from converging glens and 
valleys, and of opposing glaciers from other distinct mountain and valley systems, the 
ice may often have been heaped up and have formed barrier-dams, yet without the 
secondary conditions just named, and without regulating cols, no “roads” could have 
been formed. At the same time, from the frequent rough and hummocky character 
of the ice produced by these causes, its surface could not fail to have been covered, on 
the first melting of the ice and snow, with pools and tarns or small lakes, such as, at 
the present day, are frequent where, as before described (p. 694), the lower part of a 
glacier occupies a warm valley with a small gradient. 
* ‘ Old Sea Margins,’ pp. 119-124. 
t Phil. Trans, for 1839, p. 42. 
X Proc. Roy. Soc. Edinb., Session 1877-78, p. 23. 
