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Darwin Correspondence Project

From James Geikie   20 December 1880

Perth

Dec. 20. 1880

My dear Sir

I am much pleased to hear that “Prehistoric Europe” has interested you, and that you are of opinion that the establishment of “interglacial epochs” is of some importance.1 Evidence on this head is continually increasing: only a few days ago I heard from Dr. Penck of the Geol. Survey of Saxony that the well-known lignites of Imberg in Bavaria are interglacial.2 The section he gives as follows:

mètres

Ground-moraine — — 20.

Conglomerate — — 3.

Lignite with clay— — 0.5

Conglomerate — — 2.

Lignite — — 1

Conglomerate — — 10. to 15.

Ground-moraine — — 10.

The section is exposed for some 400 mètres—so that there is no doubt about the matter.

Mr. Mackintosh’s paper I read with much interest, but without being convinced that any of his erratics have been floated by sea-ice.3 The “intercrossings” of boulders upon which he rests his belief, are not unknown in Scotland in our till. Thus over a belt of country extending from Ayrshire in the west to the coast of Berwickshire in the east we find similar & even more remarkable intercrossings of boulders than any of those cited by Mr. Mackintosh. Boulders from the Northern Highlands are here mixed with erratics which have come from the Southern Uplands. The belt of land is simply the debateable grounds which mark the meeting of the ice-flow from North & South respectively, and over which now the one and now the other ice-stream prevailed. Then we have evidence in Forfarshire & Aberdeenshire of a similar crossing of boulders upon the low maritime tracts—and these we explain by the pressure exerted by the Scandinavian ice-sheet which now & again [pushed] back the Scotch ice.

Similar intercrossings of boulders are common in Scandinavia and in North Germany, and to some of these I refer in a Note in Appendix of Prehistoric Europe.4

The origin or cause of such intercrossings is clearly revealed in the glacial phenomena of the Rhone valley between Bourg & Lyons etc. In the beautiful map of M. M. Falsan & Chantre, we see that boulders have crossed each other at all angles.5 Nay, in some cases it is proved that the boulders have travelled in exactly opposite directions! Thus in the Valromey glacial striae & boulders indicate the passage of a glacier down the valley—while many erratics occur in the valley even up to its head which indicate a flow of ice up the valley! Falsan & Chantre show that before the Great Rhone glacier had reached its greatest development numerous glaciers came down from the hills of Savoy & Dauphiny, but by & by the great ice-streams of the Rhone overwhelmed these local glaciers—grinding over their moraines & commingling them with its own. In many cases the flow of the Rhone glacier was at acute & right angles to the course of the local glaciers, & even in some cases flowed up the smaller valleys & passed across dividing cols. Of course when the Rhone glacier decayed in importance, the local glaciers again became independent, and a new series of “intercrossings” of moraine-material & erratics took place.

In some such way I believe the “intercrossings” of the boulders referred to by Mr. Mackintosh must be explained. The objections that might be urged to the “iceberg” origin of the English erratics are many and strong, but I hesitate to bore you with them; and they are probably already present in your mind. During the growth or increase and subsequent gradual disappearance of the great mer de glace that filled up the basin of the Irish Sea, there must have been many modifications in the ice-flow of the contiguous tracts in north-west of England—quite enough I can conceive to give rise to the crossing of boulders again & again.

I may mention that I spent some time this past Autumn in in studying the drifts of South Wales & the borders of the Bristol Channel. I could find no trace of marine action anywhere, altho’ I was constantly on the outlook for it among the drifts. I traced boulder-clay from the hills right down to the margin of the recent marine flats. In the hill-valleys the boulder-clay is covered with much angular débris, some of which appeared to be truly morainic, while much seemed to me to point rather to the action of névé, frost & thaw etc.— in fact to be of the same nature as the “Head” of Cornwall. In the low maritime tracts the till is overlaid with morainic or “diluvial” gravel & sand, of the same character as the similar deposits in Scotland & Sweden etc.

Enclosed is a paper by Mr. Kerr which may interest you, if you have not already seen it.6 Pray do not trouble to return it, as I have another copy.

I hope you will excuse this long & I fear somewhat incoherent letter—and with highest regards, believe me | Sincerely yours | James Geikie

Footnotes

See letter to James Geikie, 13 December 1880 and nn. 1 and 2. CD had just read Prehistoric Europe: a geological sketch (Geikie 1881) and was particularly interested in the evidence Geikie provided for the existence of intervening warmer periods during the Ice Age, known as interglacial epochs.
Albrecht Penck was an assistant geologist for the Land Survey of Saxony (Landesaufnahme in Sachsen). Amberg is in the Upper Palatinate, Bavaria.
Daniel Mackintosh suggested that the intercrossing of routes taken by boulders was a result of currents changing course with the rising and falling of the seabed (Mackintosh 1879, p. 427).
See Geikie 1881, pp. 565–6.
The map of the Bourg region was one of six maps made by Albert Falsan to accompany the monograph by him and Ernest Chantre, Monographie géologique des anciens glaciers et du terrain erratique de la partie moyenne du bassin du Rhône (Geological monograph on the ancient glaciers and erratic terrain of the middle part of the Rhone basin; Falsan and Chantre 1875–80, Atlas: map 1).
CD’s copy of Washington Caruthers Kerr’s ‘Gold gravels of North Carolina’ (Kerr 1880) is in the Darwin Pamphlet Collection–CUL.

Bibliography

Falsan, Albert and Chantre, Ernest. 1875–80. Monographie géologique des anciens glaciers et du terrain erratique de la partie moyenne du bassin du Rhone. 2 vols. and atlas. Lyon: Imprimerie Pitrat Ainé.

Geikie, James. 1881. Prehistoric Europe: a geological sketch. London: Edward Stanford.

Kerr, Washington Caruthers. 1880. The gold gravels of North Carolina—their structure and origin. Transactions of the American Institute of Mining Engineers 8 (1879–80): 462–6.

Mackintosh, Daniel. 1879. Results of a systematic survey, in 1878, of the directions and limits of dispersion, mode of occurrence, and relation to drift-deposits of the erratic blocks or boulders of the West of England and east of Wales, including a revision of many years’ previous observations. [Read 26 March 1879.] Quarterly Journal of the Geological Society of London 35: 425–55.

Summary

Discusses Prehistoric Europe; establishing the existence of interglacial periods; iceberg vs glacier transport of erratic boulders.

Letter details

Letter no.
DCP-LETT-12929
From
James Murdoch (James) Geikie
To
Charles Robert Darwin
Sent from
Perth
Source of text
DAR 165: 32
Physical description
ALS 7pp

Please cite as

Darwin Correspondence Project, “Letter no. 12929,” accessed on 28 March 2024, https://www.darwinproject.ac.uk/letter/?docId=letters/DCP-LETT-12929.xml

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