Ɛpsilon

To Charles Lyell   5 March [1869]

Down. | Bromley. | Kent. S.E.

March 5

My dear Lyell

Your point seems to me a very good one.1 When reading Croll, I thought of the perfectly preserved glaciation of the rocks in Scotland & N. Wales, & of the preservation of the turf-covered shelves of Glen Roy & of all turf-covered surfaces; but I vaguely supposed that Croll would say that the wear & tear was confined chiefly to the valleys & ravines & was much less than average when rocks hard.— But when the wear & tear is put into figures, as you have done, it seems scarcely possible that the water channels could supply detritus enough.—2

It wd be most interesting to visit Glen Roy for this sole purpose, & estimate how large an area, consisted of naked rock or naked detritus. Certainly my impression is strong that the shelves themselves are now, except when crossed by a rill, just as they were when they bordered the lake or water.—3 One is led to suspect that Crolls & Geikies estimate of the amount of surface degradation, is much too high, even supposing that the amount of loose matter now carried away by the rivers is correct.—4 Somehow I cannot persuade myself that the Glacial period was much more recent than hitherto supposed. I shd like to hear what Croll wd say to your line of argument—5

I am sorry to hear about the Amazonian shells.—6

My dear Lyell | Yours most sincerely | Ch. Darwin

Endorsement: ‘1869’

1
The letter from Lyell has not been found.
2
In ‘On geological time’ (Croll 1868, p. 381), James Croll discussed the factors influencing the power of river systems, their ability to create detritus, and their influence in forming soil or barren rock under different conditions. In Earthworms, p. 233, CD cited this work in a discussion of how rivers carrying detritus downstream deepened river valleys, particularly when moving through softer stone. CD had travelled in Scotland and North Wales doing fieldwork; for more on his studies, see Herbert 2005.
3
The steep sides of Glen Roy, Scotland, are marked by a series of parallel shelves; CD once believed they indicated that Glen Roy had originally been formed by the sea, with each shelf indicating a point at which the water level had remained for some time. For more on his work on Glen Roy see ‘Parallel roads of Glen Roy’ and Rudwick 1974.
4
Both Croll (Croll 1868, p. 379) and Archibald Geikie (A. Geikie 1868, p. 183) had estimated that land lost a foot from its general surface in 6000 years. In Earthworms, p. 233, CD referred to these sources in the context of air and rain, aided by streams and rivers, being the most powerful agents of denudation.
5
In the tenth edition of the Principles of geology, Lyell had calculated possible glacial periods based on, among other things, Croll’s calculation of the eccentricity of the Earth’s orbit (Lyell 1867–8 1: 293). In Croll 1868, p. 385, Croll discussed Lyell’s calculations and the evidence for defining the last glacial epoch as the period from 240,000 to 80,000 years ago, rather than 850,000 years ago.
6
James Orton had informed CD of his discovery of fossil shells at Pebas, Peru, above the valley of the Amazon river in his letter of 4 January 1869 (Correspondence vol. 17). CD saw this as important evidence against Louis Agassiz’s theory of the glacial formation of the Amazon (see letter to James Orton, 23 January [1869]). In 1871, Orton sent Lyell shells from the Amazon field trip (see Correspondence vol. 19, letter from J. D. Hooker, 22 December 1871).

Manuscript Alterations and Comments

1.2 perfectly preserved] interl
1.5 & was … hard.] interl
1.7 channels] above del ‘currents’
2.5 Crolls] after del ‘the’
2.5 surface] interl after del ‘s’
2.6 by the rivers] interl

Please cite as “DCP-LETT-6692,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-6692