More Letters of Charles Darwin Volume II

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In following your suggestion in drawing out something about Glen Roy for
the Geological Committee, I have been completely puzzled how to do it. I
have written down what I should say if I had to meet the head of the Survey
and wished to persuade him to undertake the task; but as I have written it,
it is too long, ill expressed, seems as if it came from nobody and was
going to nobody, and therefore I send it to you in despair, and beg you to
turn the subject in your mind. I feel a conviction if it goes through the
Geological part of Ordnance Survey it will be swamped, and as it is a case
for mere accurate measurements it might, I think without offence, go to the
head of the real Surveyors.

If Agassiz or Buckland are on the Committee they will sneer at the whole
thing and declare the beaches are those of a glacier-lake, than which I am
sure I could convince you that there never was a more futile theory.

I look forward to Southampton (518/2. The British Association meeting
(1846).) with much interest, and hope to hear to-morrow that the lodgings
are secured to us. You cannot think how thoroughly I enjoyed our
geological talks, and the pleasure of seeing Mrs. Horner and yourself here.
(518/3. This letter is published in the privately printed "Memoir of
Leonard Horner," II., page 103.)

[Here follows Darwin's Memorandum.]

The Parallel Roads of Glen Roy, in Scotland, have been the object of
repeated examination, but they have never hitherto been levelled with
sufficient accuracy. Sir T. Lauder Dick (518/4. "On the Parallel Roads of
Lochaber" (with map and plates), by Sir Thomas Lauder Dick, "Trans. R. Soc.
Edinb." Volume IX., page 1, 1823.) procured the assistance of an engineer
for this purpose, but owing to the want of a true ground-plan it was
impossible to ascertain their exact curvature, which, as far as could be
estimated, appeared equal to that of the surface of the sea. Considering
how very rarely the sea has left narrow and well-defined marks of its
action at any considerable height on the land, and more especially
considering the remarkable observations by M. Bravais (518/5. "On the
Lines of Ancient Level of the Sea in Finmark," by M. A. Bravais, translated
from "Voyages de la Commission Scientifique du Nord, etc."; "Quart. Journ.
Geol. Soc." Volume I., page 534, 1845.) on the ancient sea-beaches of
Scandinavia, showing the they are not strictly parallel to each other, and
that the movement has been greater nearer the mountains than on the coast,
it appears highly desirable that the roads of Glen Roy should be examined
with the utmost care during the execution of the Ordnance Survey of
Scotland. The best instruments and the most accurate measurements being
necessary for this end almost precludes the hope of its being ever
undertaken by private individuals; but by the means at the disposal of the
Ordnance, measurements would be easily made even more accurate than those
of M. Bravais. It would be desirable to take two lines of the greatest
possible length in the district, and at nearly right angles to each other,
and to level from the beach at one extremity to that at the other, so that
it might be ascertained whether the curvature does exactly correspond with
that of the globe, or, if not, what is the direction of the line of
greatest elevation. Much attention would be requisite in fixing on either
the upper or lower edge of the ancient beaches as the standard of
measurement, and in rendering this line conspicuous. The heights of the
three roads, one above the other and above the level of the sea, ought to
be accurately ascertained. Mr. Darwin observed one short beach-line north
of Glen Roy, and he has indicated, on the authority of Sir David Brewster,
others in the valley of the Spey. If these could be accurately connected,
by careful measurements of their absolute heights or by levelling, with
those of Glen Roy, it would make a most valuable addition to our knowledge
on this subject. Although the observations here specified would probably
be laborious, yet, considering how rarely such evidence is afforded in any
quarter of the world, it cannot be doubted that one of the most important
problems in Geology--namely, the exact manner in which the crust of the
earth rises in mass--would be much elucidated, and a great service done to
geological science.

LETTER 519. R. CHAMBERS TO D. MILNE-HOME.
St. Andrews, September 7th, 1847.

I have had a letter to-day from Mr. Charles Darwin, beseeching me to obtain
for him a copy of your paper on Glen Roy. (519/1. No doubt Mr. Milne's
paper "On the Parallel Roads of Lochaber," "Trans. R. Soc. Edinb." Volume
XVI., page 395, 1849. [Read March 1st and April 5th, 1847.]) I am sure
you will have pleasure in sending him one; his address is "Down,
Farnborough, Kent." I have again read over your paper carefully, and feel
assured that the careful collection and statement of facts which are found
in it must redound to your credit with all candid persons. The suspicions,
however, which I obtained some time ago as to land-straits and heights of
country being connected with sea-margins and their ordinary memorials still
possesses me, and I am looking forward to some means of further testing the
Glen Roy mystery. If my suspicion turn out true, I shall at once be
regretful on your account, and shall feel it as a great check and
admonition to myself not to be too confident about anything in science till
it has been proved over and over again. The ground hereabouts is now
getting clear of the crops; perhaps when I am in town a few days hence we
may be able to make some appointment for an examination of the beaches of
the district, my list of which has been greatly enlarged during the last
two months.

LETTER 520. TO R. CHAMBERS.
September 11th, 1847.

I hope you will read the first part of my paper before you go [to Glen
Roy], and attend to the manner in which the lines end in Glen Collarig. I
wish Mr. Milne had read it more carefully. He misunderstands me in several
respects, but [I] suppose it is my own fault, for my paper is most
tediously written. Mr. Milne fights me very pleasantly, and I plead guilty
to his rebuke about "demonstration." (520/1. See Letter 521, note.) I do
not know what you think; but Mr. Milne will think me as obstinate as a pig
when I say that I think any barriers of detritus at the mouth of Glen Roy,
Collarig and Glaster more utterly impossible than words can express. I
abide by all that I have written on that head. Conceive such a mass of
detritus having been removed, without great projections being left on each
side, in the very close proximity to every little delta preserved on the
lines of the shelves, even on the shelf 4, which now crosses with uniform
breadth the spot where the barrier stood, with the shelves dying gradually
out, etc. To my mind it is monstrous. Oddly enough, Mr. Milne's
description of the mouth of Loch Treig (I do not believe that valley has
been well examined in its upper end) leaves hardly a doubt that a glacier
descended from it, and, if the roads were formed by a lake of any kind, I
believe it must have been an ice-lake. I have given in detail to Lyell my
several reasons for not thinking ice-lakes probable (520/2. Mr. Darwin
gives some arguments against the glacier theory in the letter (517) to Sir
Charles Lyell; but the letter alluded to is no doubt the one written to
Lyell on "Wednesday, 8th" (Letter 522), in which the reasons are fully
stated.); but to my mind they are incomparably more probable than detritus
of rock-barriers. Have you ever attended to glacier action? After having
seen N. Wales, I can no more doubt the former existence of gigantic
glaciers than I can the sun in the heaven. I could distinguish in N. Wales
to a certain extent icebergs from glacier action (Lyell has shown that
icebergs at the present day score rocks), and I suspect that in Lochaber
the two actions are united, and that the scored rock on the watersheds,
when tideways, were rubbed and bumped by half-stranded icebergs. You will,
no doubt, attend to Glen Glaster. Mr. Milne, I think, does not mention
whether shelf 4 enters it, which I should like to know, and especially he
does not state whether rocks worn on their upper faces are found on the
whole 212 [feet] vertical course of this Glen down to near L. Loggan, or
whether only in the upper part; nor does he state whether these rocks are
scored, or polished, or moutonnees, or whether there are any "perched"
boulders there or elsewhere. I suspect it would be difficult to
distinguish between a river-bed and tidal channel. Mr. Milne's description
of the Pass of Mukkul, expanding to a width of several hundred yards 21
feet deep in the shoalest part, and with a worn islet in the middle, sounds
to me much more like a tidal channel than a river-bed. There must have
been, on the latter view, plenty of fresh water in those days. With
respect to the coincidence of the shelves with the now watersheds, Mr.
Milne only gives half of my explanation. Please read page 65 of my paper.
(520/3. "Observations on the Parallel Roads of Glen Roy, and of other
Parts of Lochaber in Scotland, with an Attempt to Prove that they are of
Marine Origin." "Phil. Trans. R. Soc." 1839, page 39. [Read February 7th,
1839.]) I allude only to the head of Glen Roy and Kilfinnin as silted up.
I did not know Mukkul Pass; and Glen Roy was so much covered up that I did
not search it well, as I was not able to walk very well. It has been an
old conjectural belief of mine that a rising surface becomes stationary,
not suddenly, but by the movement becoming very slow. Now, this would
greatly aid the tidal currents cutting down the passes between the
mountains just before, and to the level of, the stationary periods. The
currents in the fiords in T. del Fuego in a narrow crooked part are often
most violent; in other parts they seem to silt up.

Shall you do any levelling? I believe all the levelling has been [done] in
Glen Roy, nearly parallel to the Great Glen of Scotland. For inequalities
of elevation, the valley of the Spean, at right angles to the apparent axes
of elevation, would be the one to examine. If you go to the head of Glen
Roy, attend to the apparent shelf above the highest one in Glen Roy, lying
on the south side of Loch Spey, and therefore beyond the watershed of Glen
Roy. It would be a crucial case. I was too unwell on that day to examine
it carefully, and I had no levelling instruments. Do these fragments
coincide in level with Glen Gluoy shelf?

MacCulloch talks of one in Glen Turret above the shelf. I could not see
it. These would be important discoveries. But I will write no more, and
pray your forgiveness for this long, ill-written outpouring. I am very
glad you keep to your subject of the terraces. I have lately observed that
you have one great authority (C. Prevost), [not] that authority signifies a
[farthing?] on your side respecting your heretical and damnable doctrine of
the ocean falling. You see I am orthodox to the burning pitch.

LETTER 521. TO D. MILNE-HOME.
Down, [September] 20th, [1847].

I am much obliged by your note. I returned from London on Saturday, and I
found then your memoir (521/1. "On the Parallel Roads of Lochaber, with
Remarks on the Change of Relative Levels of Sea and Land in Scotland, and
on the Detrital Deposits in that Country," "Trans. R. Soc. Edinb." Volume
XVI., page 395, 1849. [Read March 1st and April 5th, 1847.]), which I had
not then received, owing to the porter having been out when I last sent to
the Geological Society. I have read your paper with the greatest interest,
and have been much struck with the novelty and importance of many of your
facts. I beg to thank you for the courteous manner in which you combat me,
and I plead quite guilty to your rebuke about demonstration. (521/2. Mr.
Milne quotes a passage from Mr. Darwin's paper ("Phil. Trans. R. Soc."
1839, page 56), in which the latter speaks of the marine origin of the
parallel roads of Lochaber as appearing to him as having been demonstrated.
Mr. Milne adds: "I regret that Mr. Darwin should have expressed himself in
these very decided and confident terms, especially as his survey was
incomplete; for I venture to think that it can be satisfactorily
established that the parallel roads of Lochaber were formed by fresh-water
lakes" (Milne, loc. cit., page 400).) You have misunderstood my paper on a
few points, but I do not doubt that is owing to its being badly and
tediously written. You will, I fear, think me very obstinate when I say
that I am not in the least convinced about the barriers (521/3. Mr. Milne
believed that the lower parts of the valleys were filled with detritus,
which constituted barriers and thus dammed up the waters into lakes.):
they remain to me as improbable as ever. But the oddest result of your
paper on me (and I assure you, as far as I know myself, it is not
perversity) is that I am very much staggered in favour of the ice-lake
theory of Agassiz and Buckland (521/4. Agassiz and Buckland believed that
the lakes which formed the "roads" were confined by glaciers or moraines.
See "The Glacial Theory and its Recent Progress," by Louis Agassiz, "Edinb.
New Phil. Journ." Volume XXXIII., page 217, 1842 (with map).): until I
read your important discovery of the outlet in Glen Glaster I never thought
this theory at all tenable. (521/5. Mr. Milne discovered that the middle
shelf of Glen Roy, which Mr. Darwin stated was "not on a level with any
watershed" (Darwin, loc. cit., page 43), exactly coincided with a watershed
at the head of Glen Glaster (Milne, loc. cit., page 398).) Now it appears
to me that a very good case can be made in its favour. I am not, however,
as yet a believer in the ice-lake theory, but I tremble for the result. I
have had a good deal of talk with Mr. Lyell on the subject, and from his
advice I am going to send a letter to the "Scotsman," in which I give
briefly my present impression (though there is not space to argue with you
on such points as I think I could argue), and indicate what points strike
me as requiring further investigation with respect, chiefly, to the ice-
lake theory, so that you will not care about it...

P.S.--Some facts mentioned in my "Geology of S. America," page 24 (521/6.
The creeks which penetrate the western shores of Tierra del Fuego are
described as "almost invariably much shallower close to the open sea at
their mouths than inland...This shoalness of the sea-channels near their
entrances probably results from the quantity of sediment formed by the wear
and tear of the outer rocks exposed to the full force of the open sea. I
have no doubt that many lakes--for instance, in Scotland--which are very
deep within, and are separated from the sea apparently only by a tract of
detritus, were originally sea-channels, with banks of this nature near
their mouths, which have since been upheaved" ("Geol. Obs. S. America,"
page 24, footnote.), with regard to the shoaling of the deep fiords of T.
del Fuego near their mouths, and which I have remarked would tend, with a
little elevation, to convert such fiords into lakes with a great mound-like
barrier of detritus at their mouths, might, possibly, have been of use to
you with regard to the lakes of Glen Roy.

LETTER 522. TO C. LYELL.
Down, Wednesday, 8th.

Many thanks for your paper. (522/1. "On the Ancient Glaciers of
Forfarshire." "Proc. Geol. Soc." Volume III., page 337, 1840.) I do
admire your zeal on a subject on which you are not immediately at work. I
will give my opinion as briefly as I can, and I have endeavoured my best to
be honest. Poor Mrs. Lyell will have, I foresee, a long letter to read
aloud, but I will try to write better than usual. Imprimis, it is
provoking that Mr. Milne (522/2. "On the Parallel Roads of Lochaber, etc."
"Trans. R. Soc. Edinb." Volume XVI., page 395, 1849. [Read March 1st and
April 5th, 1847.]) has read my paper (522/3. "Observations on the Parallel
Roads of Glen Roy, etc." "Phil. Trans. R. Soc." 1839, page 39. [Read
February 7th, 1839.].) with little attention, for he makes me say several
things which I do not believe--as, that the water sunk suddenly! (page 10),
that the Valley of Glen Roy, page 13, and Spean was filled up with detritus
to level of the lower shelf, against which there is, I conceive, good
evidence, etc., but I suppose it is the consequence of my paper being most
tediously written. He gives me a just snub for talking of demonstration,
and he fights me in a very pleasant manner. Now for business. I utterly
disbelieve in the barriers (522/4. See note, Letter 521.) for his lakes,
and think he has left that point exactly where it was in the time of
MacCulloch (522/5. "On the Parallel Roads of Glen Roy." "Geol. Trans."
Volume IV., page 314, 1817 (with several maps and sections).) and Dick.
(522/6. "On the Parallel Roads of Lochaber." "Trans. R. Soc. Edinb."
Volume IX., page 1, 1823.) Indeed, in showing that there is a passage at
Glen Glaster at the level of the intermediate shelf, he makes the
difficulty to my mind greater. (522/7. See Letter 521, note.) When I
think of the gradual manner in which the two upper terraces die out at Glen
Collarig and at the mouth of Glen Roy, the smooth rounded form of the hills
there, and the lower shelf retaining its usual width where the immense
barrier stood, I can deliberately repeat "that more convincing proofs of
the non-existence of the imaginary Loch Roy could scarcely have been
invented with full play given to the imagination," etc.: but I do not
adhere to this remark with such strength when applied to the glacier-lake
theory. Oddly, I was never at all staggered by this theory until now,
having read Mr. Milne's argument against it. I now can hardly doubt that a
great glacier did emerge from Loch Treig, and this by the ice itself (not
moraine) might have blocked up the three outlets from Glen Roy. I do not,
however, yet believe in the glacier theory, for reasons which I will
presently give.

There are three chief hostile considerations in Mr. Milne's paper. First,
the Glen [shelf?], not coinciding in height with the upper one [outlet?],
from observations giving 12 feet, 15 feet, 29 feet, 23 feet: if the latter
are correct the terrace must be quite independent, and the case is hostile;
but Mr. Milne shows that there is one in Glen Roy 14 feet below the upper
one, and a second one again (which I observed) beneath this, and then we
come to the proper second shelf. Hence there is no great improbability in
an independent shelf having been found in Glen Gluoy.

This leads me to Mr. Milne's second class of facts (obvious to every one),
namely the non-extension of the three shelves beyond Glen Roy; but I abide
by what I have written on that point, and repeat that if in Glen Roy, where
circumstances have been so favourable for the preservation or formation of
the terraces, a terrace could be formed quite plain for three-quarters of a
mile with hardly a trace elsewhere, we cannot argue, from the non-existence
of shelves, that water did not stand at the same levels in other valleys.
Feeling absolutely convinced that there was no barrier of detritus at the
mouth of Glen Roy, and pretty well convinced that there was none of ice,
the manner in which the terraces die out when entering Glen Spean, which
must have been a tideway, shows on what small circumstances the formation
of these shelves depended. With respect to the non-existence of shelves in
other parts of Scotland, Mr. Milne shows that many others do exist, and
their heights above the sea have not yet been carefully measured, nor have
even those of Glen Roy, which I suspect are all 100 feet too high.
Moreover, according to Bravais (522/8. "On the Lines of Ancient Level of
the Sea in Finmark." By A. Bravais, Member of the Scientific Commission of
the North. "Quart. Journ. Geol. Soc." Volume I., page 534, 1845 (a
translation).), we must not feel sure that either the absolute height or
the intermediate heights between the terraces would be at all the same at
distant points. In levelling the terraces in Lochaber, all, I believe,
have been taken in Glen Roy, nearly N. and S. There should be levels taken
at right angles to this line and to the Great Glen of Scotland or chief
line of elevation.

Thirdly, the nature of the outlets from the supposed lakes. This appears
to me the best and newest part of the paper. If Sir James Clark would like
to attend to any particular points, direct his attention to this:
especially to follow Glen Glaster from Glen Roy to L. Laggan. Mr. Milne
describes this as an old and great river-course with a fall of 212 feet.
He states that the rocks are smooth on upper face and rough on lower, but
he does not mention whether this character prevails throughout the whole
212 vertical feet--a most important consideration; nor does he state
whether these rocks are polished or scratched, as might have happened even
to a considerable depth beneath the water (Mem. great icebergs in narrow
fiords of T. del Fuego (522/9. In the "Voyage of the 'Beagle'" a
description is given of the falling of great masses of ice from the icy
cliffs of the glaciers with a crash that "reverberates like the broadside
of a man-of-war, through the lonely channels" which intersect the
coast-line of Tierra del Fuego. Loc. cit., page 246.)) by the action of
icebergs, for that icebergs transported boulders on to terraces, I have no
doubt. Mr. Milne's description of the outlets of his lake sound to me more
like tidal channels, nor does he give any arguments how such are to be
distinguished from old river-courses. I cannot believe in the body of
fresh water which must, on the lake theory, have flowed out of them. At
the Pass of Mukkul he states that the outlet is 70 feet wide and the rocky
bottom 21 feet below the level of the shelf, and that the gorge expands to
the eastwards into a broad channel of several hundred yards in width,
divided in the middle by what has formerly been a rocky islet, against
which the waters of this large river had chafed in issuing from the pass.
We know the size of the river at the present day which would flow out
through this pass, and it seems to me (and in the other given cases) to be
as inadequate; the whole seems to me far easier explained by a tideway than
by a formerly more humid climate.

With respect to the very remarkable coincidence between the shelves and the
outlets (rendered more remarkable by Mr. Milne's discovery of the outlet to
the intermediate shelf at Glen Glaster (522/10. See Letter 521, note.)),
Mr. Milne gives only half of my explanation; he alludes to (and disputes)
the smoothing and silting-up action, which I still believe in. I state:
If we consider what must take place during the gradual rise of a group of
islands, we shall have the currents endeavouring to cut down and deepen
some shallow parts in the channels as they are successively brought near
the surface, but tending from the opposition of tides to choke up others
with littoral deposits. During a long interval of rest, from the length of
time allowed to the above processes, the tendency would often prove
effective, both in forming, by accumulation of matter, isthmuses, and in
keeping open channels. Hence such isthmuses and channels just kept open
would oftener be formed at the level which the waters held at the interval
of rest, than at any other (page 65). I look at the Pass of Mukkul (21
feet deep, Milne) as a channel just kept open, and the head of Glen Roy
(where there is a great bay silted up) and of Kilfinnin (at both which
places there are level-topped mounds of detritus above the level of the
terraces) as instances of channels filled up at the stationary levels. I
have long thought it a probable conjecture that when a rising surface
becomes stationary it becomes so, not at once, but by the movements first
becoming very slow; this would greatly favour the cutting down many gaps in
the mountains to the level of the stationary periods.

GLACIER THEORY.

If a glacialist admitted that the sea, before the formation of the
terraces, covered the country (which would account for land-straits above
level of terraces), and that the land gradually emerged, and if he supposed
his lakes were banked by ice alone, he would make out, in my opinion, the
best case against the marine origin of the terraces. From the scattered
boulders and till, you and I must look at it as certain that the sea did
cover the whole country, and I abide quite by my arguments from the
buttresses, etc., that water of some kind receded slowly from the valleys
of Lochaber (I presume Mr. Milne admits this). Now, I do not believe in
the ice-lake theory, from the following weak but accumulating reasons:
because, 1st, the receding water must have been that of a lake in Glen
Spean, and of the sea in the other valleys of Scotland, where I saw similar
buttresses at many levels; 2nd, because the outlets of the supposed lakes
as already stated seem, from Mr. Milne's statements, too much worn and too
large; 3rd, when the lake stood at the three-quarters of a mile shelf the
water from it must have flowed over ice itself for a very long time, and
kept at the same exact level: certainly this shelf required a long time
for its formation; 4th, I cannot believe a glacier would have blocked up
the short, very wide valley of Kilfinnin, the Great Glen of Scotland also
being very low there; 5th, the country at some places where Mr. Milne has
described terraces is not mountainous, and the number of ice-lakes appears
to me very improbable; 6th, I do not believe any lake could scoop the rocks
so much as they are at the entrance to Loch Treig or cut them off at the
head of Upper Glen Roy; 7th, the very gradual dying away of the terraces at
the mouth of Glen Roy does not look like a barrier of any kind; 8th, I
should have expected great terminal moraines across the mouth of Glen Roy,
Glen Collarig, and Glaster, at least at the bottom of the valleys. Such, I
feel pretty sure, do not exist.