Discusses the role of ice in determining the geological features of the Jura. Mentions view of Agassiz. Objects to idea of "a [sea of ice] carrying rocks". Notes Agassiz's earlier view of "ice expanded in the line of the Great Swiss Valley". Comments on Pentlands.
My dear Lyell
I find I do not know enough of case to enter in your difficulty about subsidence.— Do you suppose the Jura had its present form, when the terrestrial quadrupeds were embedded? if not, the subsidence need not have been so very great, & the Jura might have partly assumed its present form during that movement & its subsequent elevation when scratched by ice.— Do you make too sure of the period when ice floated being cold— pray remember in very latitude of Lake of Geneva, I forget how many miles of side of mountain is covered with ice, & the sea-channels are choked with icebergs—& the forests more resemble those of Brazil, than those of England.—
I dont look at bridge of ice, (or the subsidence, or the absence of shells, for I think I out-Lyell Lyell) as any great difficulties.— but the whole turning point of the case appears to me to be, that he who knows most of glaciers, says most distinctly that the Jura erratics are totally distinct from those in valleys of Alps, which are moved in moraines. It strikes me as monstrous the expansion & contraction of a sea of ice carrying rocks from a small central point; surely the expansion would rather break up edges at foot of the peaks,—one can clearly see there is some limit,—to this action, if a point of rock a yard square projected out of enormous field of ice, no one would pretend that the ice would travel away on all sides from it.— Indeed I well remember in some of Agassiz's earlier papers he says the ice expanded in the line of the Great Swiss Valley & hence the scratches had this direction but I see nothing about this in present work.—
I hope there are no perched rocks on Jura, as the more I think of that the stronger the argument appears for sheet of ice.— Do you know certainly whether Agassiz actually found the ‘creux’ or caldron under the existing glaciers in Alps— I can only remember, & could only find on casual look, that he found furrows— These caldrons appear to me far the most inexplicable part of case under every hypothesis—
I quite agree with you about putting the difficulties on both sides—
I dont know what reasons there are for supposing the Pentlands were dry land within any reasonably short time anterior to the elevation during ice time.— I presume you would not object to half a dozen oscillations since the secondary period—
Ever yours | C. Darwin
- f1 592.f1This letter continues discussion of the passage in the manuscript of C. Lyell 1841 which disputed Louis Agassiz's interpretation of the Jura erratics. Agassiz argued that a great terrestrial ice sheet had covered northern Europe, including the Alps and the Jura, at the same time that large terrestrial mammals were imbedded in ice in Siberia (L. Agassiz 1840, pp. 306–14).
- f2 592.f2Lyell was concerned that Miocene fossils were the most recent to be found in the valley between the Alps and the Jura. This suggested a warm climate in which icebergs were unlikely (C. Lyell 1841, 1: 252–3). In reply CD alluded to the presence of icebergs and glaciers in southern South America, where the vegetation seems almost tropical by European standards. See Journal of researches, pp. 268–79.
- f3 592.f3Agassiz had observed that the erratics of the Jura were scattered irregularly, in contrast to those of the Alps which were aligned in recognisable moraines. Accordingly, he argued that blocks had not been carried to the Jura by normal glaciation (L. Agassiz 1840, pp. 268, 299–300).
- f4 592.f4Agassiz had reported that evidence of polished surfaces, striations, rolled rocks, and moraines ‘leave no doubt as to the former existence of a layer of ice which covered all the great Swiss valley, and moved towards the north-east, in the direction of the declivity’ (L. Agassiz 1839, p. 390).
- f5 592.f5Lyell argued that erratic blocks had also been carried from the Grampians to the Pentland Hills in southeast Scotland by icebergs (C. Lyell 1841, 1: 254–5).