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.
[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, the buttress-like accumulations of stratified shingle on sides of valley, especially those just below the lowest shelf in Spean valley.
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.—
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.— 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—
I am tired, so good | Bye. Ever yours | C. D
- f1 594.f1‘Observations on the parallel roads of Glen Roy’, Collected papers 1: 87–137. Glen Roy empties into Glen Spean, and the lowest shelf or ‘road’ of Glen Roy also extends throughout Glen Spean.
- f2 594.f2A valley to the north of Glen Roy which opens directly into the Great Glen of Scotland, as does Glen Spean. Kilfinnin is marked by a single ‘road’. In 1840 Louis Agassiz visited the area with William Buckland, and they concluded that a glacier blocking Glen Spean had created a lake which had produced the beaches (see L. Agassiz 1842).
- f3 594.f3Channels at the heads of the valleys marked by the roads. Agassiz pointed out that water must have passed over them when the lower outlets were blocked by ice (L. Agassiz 1842, pp. 237–8).
- f4 594.f4Agassiz had argued that the perfectly horizontal roads could hardly have been subjected to repeated elevation (L. Agassiz 1842, p. 237). However, Lyell in Scandinavia, and CD in South America, had observed numbers of uniformly elevated formations.
- f5 594.f5In September 1846 David Milne discovered a watershed at the head of Glen Gaster on the level of the middle road (Milne 184747a and 1847b). Though shaken by this discovery, CD continued to defend his theory of marine beaches until 1861. See his letter to David Milne, 20 [September 1847] (Correspondence vol. 4). See also Barrett 1973 and Rudwick 1974.
- f6 594.f6See letter to Charles Lyell, [12 March 1841].