J. F. Campbell Collection > Additional memoir on the parallel roads of Lochaber

t^XAt^^^^
112 MK DAVID MILNE HOME: MEMOIR ON THE
then they have been cut through by the ISIouse water, and the lake has thus
l)een drained. But its site is still visible " (p. 312).
Looking to these passages, and others, in Professor Geikie's writings, it
is difficult to uuderstanil how he should have given it as his opinion, that
had detrital barriers existed in Glen Roy to dam back the lakes, they
" would undoubtedly have been able to maintain themselves, and be still
extant."
^ , There would, of course, be a greater probability of removal by rain and
^ViA*_C4.iu*v streams, if the detritus forming the blockage consisted of soft sandy mud. Now,
as previously noticed, at and near the places where the Glen Eoy and Glen
CoUarig blockages occurred, there is great abundance of such kind of detritus.
To that circumstance is owing the great breadth of Shelf 3 on the N.E. shoulder
of Craig Dhu, extending to 100 yards, as indicated even on the Ord-
nance 6-inch Map. To show how easily detritus of a soft muddy character
may be eroded by rain and small burns, reference may be made to
the circumstance tliat Shelf 4, which must have existed round the west
shoulder of Craig Dhu and Meall Dherry, is for more than a mile not traceable,
it, however, must have at one time existed there. As the lake reached to this
part of the valley, a beach line must have been formed here as elsewhere, and
the only explanation is, that the beach line at this place was washed away ])y
the stream descending the sides of the hills.
The same remark applies to the discontinuance of Shelf 4 on the N.E. side
of Ben Chlinaig. It will be seen from the map of Lochaber, that for more than
a mile, the shelf is not traceable on this hill-side. But it must at one time
have existed there also. The materials which composed it have been removed
by streams and rain.
I have in the foregoing remarks assumed the correctness of Dr Tvndall's
statement, that not a trace is to be now seen of the detrital barrier by which
the lake in Glen Roy was kept \\\) at its two successive levels. But regarding
the correctness of this assumption, some doubt exists. In my previous Memoir
(p. 620) I mention that, at the places where Shelves 2 and 3 terminate iTi
(lien Roy, there are banks of detritus in a direction transverse to the valley,
which greatly resemble the renniants of a barrier.
I w(juld only add, that I observe in Dr Tvndall's lecture, with satisfaction,
a fidl admission of the enormous extent of detritus abounding in the Lochaber
district. He si)eaks of the " vast masses of detritus in certain portions of Glen
Spean " (p. 5); and of "the friable drift over-speading the vioitntains" (p. 4).
He, moreover, explains (p. 2) that " the Parallel Roads are terraces formed in
the yieldiwj drift, which here rovers the slopes <fllie mountains."
Having made tiiese admissions, Dr Tvndall must at all events allow that
the matciials wen; amj)le for forming the re(piired blockages.