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Letter 3260

Darwin, C. R. to Lyell, Charles

22 Sept [1861]

    Summary Add

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    Additional discussion of Jamieson's theory that the roads of Glen Roy were formed by a glacial lake. Suggests the possible marine origin of the Glen Spean terraces. Comments on the power of lakes to produce pebbles. Discusses elevation of Wales and Scotland during the glacial period.

Transcription

Down Bromley Kent

Sept. 22d

My dear Lyell

I have read Mr Jamieson's last letter, like the former ones, with very great interest. What a problem you have in hand! It beats manufacturing new species all to bits.— It would be a great personal consolation to me, if Mr J. can admit the sloping Spean Terraces to be marine; & would remove one of my greatest difficulties, viz. vast contrast of Welch & Lochaber valleys. But then,, as far as I dare trust my observations, the sloping terraces run far up the Roy valley so as to reach not far below the Lower shelf. If the sloping fringes are marine & the shelves lacustrine, all I can say is that nature has laid a shameful trap to catch an unwary wretch.— I suppose that I have underrated power of lakes in producing pebbles: this, I think, ought to be well looked to. I was much struck in Wales on carefully comparing the glacial scratches under a lake formed by moraine, (& which must have existed since glacial epoch) & above water, & I could perceive no difference. I believe I saw many such beds of good pebbles on level of Lower shelf, which at the time I could not believe could have been found on shores of Lake. The Land-straits & little cliffs above them to which I referred were quite above the highest shelf—: they may be of much more ancient date than the shelves. Some terrace-like fringes at head of the Spey strike me as very suspicious.

Mr. J. refers to absence of pebbles at considerable heights: he must remember that every storm, every deer, every Hare which runs tends to roll pebbles down hill, & not one ever goes up again.— I may mention that I particularly attended to this on S. Ventana in N. Patagonia,—a great, isolated, rugged Quartz mountain, 3000 ft high, & I could find not one pebble, except on one very small spot, where a ferruginous spring had firmly cemented a few to face of mountain.—

If the Lochaber Lakes had been formed by a ice-period posterior to the (marine?) sloping terraces in the Spean; would not Mr. J. have noticed gigantic moraines across the valley, opposite opening of L. Treig? I go so far as not to like making the elevation of the land in Wales & Scotland considerably different with respect to ice-period; & still more do I dislike it with respect to E. & W. Scotland. But I may be prejudiced by having been so long accustomed to plains of Patagonia. But the equability of level (barring denudation) of even the Secondary formations in Britain, after so many ups & downs, always impresses my mind, that, except when crust cracks & mountains are formed, movements of elevation & subsidence are generally very equable.—

But it is folly my scribbling thus.— You have a grand problem, & Heaven help you & Mr Jamieson through it. It is out of my line now a-days, & above & beyond me.—

Ever my dear Lyell | Yours truly | C. Darwin

(This is dreadfully untidy & useless, but I am too tired to rewrite it.—)

    Footnotes Add

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    f1 3260.f1
    The reference is to Thomas Francis Jamieson's letter to Lyell, 19 September 1861 (see Correspondence vol. 9, Appendix IX).
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    f2 3260.f2
    Jamieson had written: `I see no difficulty at present in allowing that the sea might have occupied Glen Spean &c after the era of these lakes up to 500 or 600 feet above the present coast line, in which case these accumulations might be partly of marine origin.' (see Correspondence vol. 9, Appendix IX). CD discussed this point in his paper, `Notes on the effects produced by the ancient glaciers of Caernarvonshire, and on the boulders transported by floating ice', published in the Edinburgh New Philosophical Journal 33 (1842): 352--63 (Collected papers 1: 163--71). Andrew Crombie Ramsay espoused the view that there had been a period of glacial activity in Wales after the deposition of marine drift (Ramsay 1852, p. 376).
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    f3 3260.f3
    See CD's Glen Roy notebook, pp. 31--2 (Notebooks), in which he noted that the sloping terraces of Glen Spean were `difficult to explain on <formation> deposition in lake'. In his paper on the `parallel roads' of Glen Roy, CD described the accumulation of `alluvial' gravels and other debris that formed flat-topped `buttresses' below the level of the lowest `road' in Glen Spean. He argued that if the valley had been filled with an arm of the sea, into which the river transported its debris, and if the land was rising in a succession of short steps, the river would have deposited its material in deltaic form. Therefore, as the sea retreated from the valley, the river would erode a gorge through material previously deposited and, at the same time, deposit more material further downstream at the point where it entered the sea at its new, lower level. With this account, CD believed he had demonstrated the case for a gradual fall of the level of water in Glen Spean from the lowermost `road' towards the present sea level. See `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' (Collected papers 1: 89--137); see also Rudwick 1974, pp. 121--4.
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    f4 3260.f4
    CD mentioned this observation in his paper on the ancient glaciers of Caernarvonshire, North Wales, p. 354 (see n. 3, above).
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    f5 3260.f5
    See South America, p. 79.
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    f6 3260.f6
    In his work on Glen Roy and other geological phenomena, CD based his ideas on Lyell's theory of crustal elevation and subsidence as the vera causa of the phenomena. For a discussion of CD's work on Glen Roy within the context of his wider conception of natural history, see Rudwick 1974, pp. 153--74.
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