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To the Editor of the Scotsman.—
Sir
As the parallel roads of Glen Roy have lately been again brought into notice by
Mr Milne's able & interesting Memoir, & as you have always shown much interest on this
& allied subjects, I venture to request a little space, for a few remarks,
chiefly for the purpose of calling attention to some points, which appear to me to
require further examination. My opinion regarding the great improbability of there ever
having existed barriers either of detritus or rock at the mouths of the valleys, so as
to form lakes, remains quite unaltered: I still think it clear from the facts stated in
my paper in the Philosophical Transactions that the whole
country emerged slowly from beneath the waters of the sea: I believe that this is
admitted by Mr Milne. Formerly I thought that if it could be shown that
such barriers had had not existed the shelves must have been formed by the waters of the
sea. Since my observations, were published, M. Agassiz &
Dr Buckland visited the site, & advanced (Eding. New. Phil.
Journ. vol 33. p. 236) a most ingenious theory namely that lake had been formed
by great ancient glaciers having blocked up the mouths of the valleys. When this theory
was first propounded, I mentally rejected it from my belieff that when the water stood
at the middle shelf of Glen Roy, there was no outlet for drainage except over the ice
itself; & this being so, it appeared to me highly improbable, that the water
could have been banked up to the same exact level for the great length of time necessary
for the formation of the shelf. but this prominent objection falls to the ground, since
Mr Milnes important discovery of an outlet at the head of Glen
Glastig.) (Mr Milne is strongly opposed to
the ice-lake theory & believes that glaciers could not have descended from Ben
Nevis in the course suggested by Agassiz & Buckland. But from the published
accounts of ancient glaciers in many parts of the world & from what I have
myself seen (London Phil. Magazine Vol XXI. p. 184) in North Wales, I can feel no difficulty in admitting that the whole valley of
the Spean was formerly filled up with ice: Mr Milne's curious
details on the state of the surface near Loch Treig strikes me as corrobarating the
former existence of glaciers. To form lakes at the level of the shelves in Lochaber, a
glacier must have blocked up Glen Collarig and been pushed up Glen Roy above the mouth
of Glen Glastig: at this period the water of the lake would have stood at the upper
shelf & its drainage escaped at the head of Glen Roy, down the Spey. As the
climate became more temperate, the glacier may be supposed to have retreated a little
towards Ben Nevis & then the water would have escaped by Glen Glastig &
stood at the level of the middle shelf. Subsequently the glacier must have retreated
greatly, a portion of it (or a distinct glacier) still remaining at the mouth of the
Spean, & the lower shelf would have been formed, the drainage flowing through
the Pass of Muckull. Glen Gluy apparently must have been
closed by an independent glacier. The blockage in these cases, may be supposed to have
been by the ice itself, as in the famous case in the valley of Bagnes, & not by a gigantic terminal moraines. As in North Wales, I could in
parts, distinguish iceberg action, from that of the ancient glaciers, so here I suspect,
that the scored rocks described by Mr Milne at the heads of several glen,
will turn out to have been effected by tide-driven icebergs grating over the surface,
during the emergence of the country from beneath the sea.) (The ice-lake theory explains
beautifully the obvious difficulty of the non-extension of the shelves over the whole
country, & the remarkable coincidence in height of the the three shelves of the
Roy & Spean,—of the upper shelf in Glen Gluoy—& of
that of Kilfinnin, with outlets, over which the drainage-waters of the successive lake
might have escaped. The middle shelf of Glen Roy, extends further than the upper one at
the mouth both of this Glen & of Glen Collarig, & both shelves at their
terminal points die away in the most insensible manner: this further extension of the
middle shelf agrees well with the theory of a retreating glacier, & the very
gradual dying away of the extremities of both shelves, which always struck me as most
remarkable & opposed to the former existence of any barrier, might be explained,
as was suggested to me by Mr Lyell (with whom I have discussed this
subject, & at whose instigation I have written these few remarks) by the lake
having having been generally frozen near the great icy barrier, littoral action having
been thus prevented. This theory explains the number of erratic boulders on the shelves,
(with which fact I was much struck,) for charged icebergs might have floated off from
the glacier when they entered the lake. Finally ice-lakes explain,, as well as lakes
formed by barriers of detritus, all the appearances at the outlets & elsewhere,
considered by Mr Milne as incompatible with the marine theory;
furthermore it would, perhaps, account for the very considerable bodies of water, which
Mr Milne describes, as having formerly escaped at the outlets; for the
accumulated rain of the whole year, in the form of snow, would flow away only during the
summer.) (Opposed to the ice-lake theory, there appear the following facts (1). The
existence on Tomhbran of a shelf described by me, between the
upper & middle shelf & which Mr Milne discovered in two
other parts of Glen Roy; & again of a second shelf in Glen Gluy, extending,
according to Mr Milne for several miles on both sides of the valley; for
it is believed that at the level of these shelves there are no outlets in the
surrounding mountains; & if so, the drainage-streams must have flowed in both
cases for a very long period, over the ice itself without deepening their channels, so
that the lakes were retained at the same level: I say for a long period, for the shelf
on Tomhbran, is as plainly marked as that, for instance, in Glen Spean, & the
number of buttresses, composed of rounded pebbles, on all the shelves, plainly
shows that a long lapse of time was requisite for their formation. We know in the case
of the ice-lake in the valley of Bagnes (Lyell's Principles of Geology
7th Edit. p. 198) that the water running over the ice did
rapidly deepen its channel: to suppose that the onward movement of the glaciers exactly
coun- teracted the deepening the drainage channels, appears highly improbable.
(2nd) The existence of shelves, one above the other, in the other
districts described by Mr Milne, & apparently with no
drainage-outlets at their levels except over the ice itself. (3rd) The
improbability of a glacier having blocked up the short & very wide valley or
rather hollow of Kilfinnin; the Great Glen of Scotland, being low in front of it.
(4th) The apparent existence of a shelf
(noticed by Macculloch & not fully examined by myself) at the head of the Spey,
beyond the limits of Glen Roy; if this be seen a true shelf, there must have been a
blockage at both ends of Glen Roy, which on any lake theory, seems improbable.
(5th) The manner in which the hard & solid rocks have been worn
at the mouth of L. Treigh at the level of the lowest shelf, & cut off
at the head of Lower Glen Roy;—the large accumulations of rounded pebbles in,
the buttresses on the shelves, & more especially those round once insular little
points of rock (as on Meal Roy) & therefore disconnected with any running
stream,—all which facts seem to me to indicate greater destroying power in the
waves or currents, than can well be attributed to any lake. (6th) The
similarity of the “haughs” in Glen Spean
to those in other vallies in various countries, which in many cases can be proved to
have been of marine origin. And lastly the huge buttresses, projecting from the
hill-sides at every successive level beneath the lowest shelf in Glen Spean and built of
stratified rounded pebbles, for these afford good evidence that water receded very
slowly from this valley, standing apparently for centuries at every successive foot in
height, so slowly that I cannot believe the recession was caused by the waters of a lake
escaping over a barrier of ice.) (Weighing, as well as I am able, these difficulties on
both hands, I am still inclined to abide by the marine theory. I do not think my remarks
on the non-extension of the shelves over the entire country, & their coincidence
in height with the outlets quite so unsatisfactory as Mr Milne does. With
respect to this coincidence in height, I would add one hypothetical remark, namely that
it has long appeared to me probable, that a slowly rising surface becomes stationary,
not at once, but by the movements becoming excessively slow; & if this be the
case, it would greatly favour, the cutting down rocky isthmus between islands, when
brought within the tidal action in a rising archipelago, to nearly the level which the
land held during the quite stationary periods. On the other hand, the outlets at head of
Glen Roy & at Kilfinnin appeared to me, (but perhaps I was mistaken) to be
brought up to the exact level of the shelves, by having been silted up. If the marine
theory be found to be erroneous, then I believe that the theory of Agassiz &
Buckland will prove the true solution, & that Macculloch, Sir Thomas Dick
Lauder, & Mr Milne are all so far right that the shelves were
formed round the shores of lakes & not of arms of the sea.) (The points which
appear to me to require further examination or corroboration (& I hope I shall
be excused the presumption in enumerating them), are (1.). The nature of the apparent
shelf by Loch Spey & beyond Glen Roy: what is its height relatively to the upper
one in Glen Gluoy, & to a mound in Glen Turret, mentioned by Macculloch, above
the main upper shelf. (2.) In Glen Glastig, are the rocks simply waterworn, or also
scratched or polished as if by a glacier or by drifting icebergs: are such worn surfaces
confined to the actual bed of the valley or do they reach some way up the sides? Can
they be observed in the bed for the whole 212 vertical feet of fall in this
valley? Similar points to be attended to at the heads of Glen Roy, Gluoy & the
pass of Muckul. Mr Milne has described knolls of rock, at the outlet of
Glen Gluoy, waterworn at their bases & rough above: can this be accounted for by
their bases alone having been protected by detritus from disintegration: this appears to
me most important. Knolls, such as those described by Mr Milne, low down
in the bed of the valleys, which forms the outlet of Glen Glastig, would offer
convincing evidence that the valleys had been occupied by river & not by a tidal
strait. Can the worn rocks at the entrance of Loch Treig, on a level with the shelf, be
accounted for by glacier action: they appeared to me, (at that time totally ignorant
regarding glaciers) to be the effect of a strong eddying current. (3rd)
Is there any trace of a terminal moraine (though partially obliterated by the supposed
ice-lake) or of glacier action, such as perched boulders &c at the mouths of
Glen Roy, Collarig, Glen Gluoy & Kilfinnin, & likewise in the valleys,
where Mr Milne discovered shelves. (5th) Is there any gap
in the mountains near the mouth Glen Gluoy, which might have served as an outlet for the
waters, when they stood at the level of Mr Milne's lower shelf
in the valley? The same observation with regard to the intermediate shelf of Tomhbran.
(6th) M. Agassiz believes that he saw three shelves on the
south side of the Spean in front of Glen Roy: do such exist? I saw lines there,
& believed that they were sheep-tracks.
7th The nature & height of the shelves
mentioned by me (p. 45) on the authority of Sir David Brewster, low down the
valley of the Spey. (8th) The height of the
shelves in Lochaber above the level of the Sea (for my Barometrical measurements made me
doubt the accuracy of those by Macculloch) & of the successive shelves, one
above the other)—for the sake of comparison with those described as existing
elsewhere by Mr Milne. (9th). The horizontality of the
shelves, taken in a line at nearly right angles to the Great Glen of Scotland, on
probable axis of elevation of the country: I believe the levels have hitherto been taken
exclusively in Glen Roy, in a line approximately parallel to the Great Glen. I may
remark, that if the shelves should ever be proved to deviate from the horizontal, it
would not demonstrate their marine origin, as the advocates of the lake theories, might
argue, that Scotland within late times had been uplifted unequally. On the other hand,
if they shd be proved strictly horizontal, the vast plains, which in many
parts of the world have been upraised above the level of the sea, to my mind render it
not improbable that some districts 20 or 30 miles across should have
been uplifted with perfect equability.) Charles Darwin
