To Charles Lyell [9 March 1841]
[12 Upper Gower St]
My dear Lyell
I have just received your note— it is the greatest pleasure to me to write or talk Geolog. with you— Your objection to objection against upheaval, in favour of Glaciers (as explaining Glen Roy) about elevation (you will understand what I mean) is quite new to me, & seems very sound.— I think I have thought over whole case without prejudice & remain firmly convinced they are marine beaches.— My principal reason for doing so, is what I have urged in my Paper,1 the buttress-like accumulations of stratified shingle on sides of valley, especially those just below the lowest shelf in Spean valley.
2d I can hardly conceive the extension of the glaciers in front of the valley of Kilfinnin2 where I found new road, where sides of Great Glen are not very lofty.—
3d the flat water-sheds, which I describe in places where there are no ‘roads’, as well as those connected with ‘roads’3 These remain unexplained
I might continue to add many other such reasons, all of which, however, I daresay would appear trifling to any one who had no visited the district. With respect to equable elevation, it cannot be a valid objection to any one who thinks of Scandinavia or Pampas.—4
With respect to Glacier-theory the greatest objection appears to me, is the following though possibly not a sound one. The water has beyond doubt remained very long at the levels of each shelf.— this is unequivocally shewn—by the depth of the notch or beach formed in many places in the hard mica Slate, & the large accumulations or buttresses of well rounded pebbles at certain spots on the level of old beaches.— (The time must have been immense, if formed by lakes without tides). During the existence of the lakes their drainage must have been at head of the valleys & has given the flat appearance of the watersheds.— All this is very clear for four of the shelves. (viz upper & lower in Glen Roy—the 800 ft one in Glen Spean, & the one in Kilfinnin) & explains the coincidence of “roads” with the watersheds more simply than my view, and as simply as the common lake theory.
But how was the Glen Roy lake drained when the water stood at level of middle ‘road’? it must (for there is no other exit whatever) have been drained over the glacier.—5 Now this shelf is full, as narrow in vertical line & as deeply worn horizontally into mountain-side, and with as large accumulations of shingle, (I can give cases) as the other shelves.— We must, therefore, on Glacier theory suppose, that the surface of the ice remained at exactly the same level, not being worn down by the running water, or the glacier moved by its own movement, during the very long period, absolutely necessary for a quiet lake to form such a beach, as this shelf presents in its whole course.—
I do not know whether I have explained myself clearly— I should like to know what you think of this difficulty—
I shall much like to talk over Jura-case with you—6
I am tired, so good | Bye. Ever yours | C. D
Defends his theory [in "Parallel roads of Glen Roy" (1839), Collected papers 1: 87–137] against the view that the "roads" were formed by glacial action.
Please cite as
Darwin Correspondence Project, “Letter no. 594,” accessed on 19 February 2017, http://www.darwinproject.ac.uk/DCP-LETT-594