Extract from Letter from C. Darwin to R. Chambers 11 Sept 1847²

I hope you will read the first part of my paper before you go (to Glen Roy)³ & attend to the manner in w^h. the lines end in Glen Collarig.⁴ I wish M^r. Milne had read it more carefully. He misunderstands me in several respects, but suppose it is my own fault, for my Paper is most tediously written. M^r. Milne fights me very pleasantly & I plead Guilty to his rebuke about “demonstration”.⁵ I do not know what you will think: but M^r. Milne will think me as obstinate as a Pig, when I say, that I think any Barrier of Detritus at the mouth of Glen Roy, Collarig & Glastig⁶ more utterly impossible than words can express. I abide by all that I have written on that head—

Conceive such a mass of detritus hav^g been removed, without great projections being left on each side, in the very close proximity to every little delta⁷ preserved on the lines of the shelves, even on the Shelf 4, w^h. now crosses with uniform Breadth, the spot where the Barrier stood with the Shelves dying gradually out &^c. To my mind it is monstrous. Oddly enough, M^r. Milne’s description of the Mouth of Loch Treig (I do not believe that valley has been well exam^d in its upper end) leaves hardly a doubt that a Glacier descended from it,⁸ & if the Roads were formed by a Lake of any kind, I believe it must have been an Ice Lake. I have given in detail to Lyell my several reasons for not thinking Ice-lakes probable;⁹ but to my mind, they are incomparably more probable than detritus of rock barriers.

Have you ever attended to Glacier action? After having seen N. Wales, I can no more doubt the former existence of gigantic glaciers, than I can the Sun in the Heaven. I could distinguish in N. Wales to a certain extent Iceberg from Glacier Action—¹⁰ (Lyell has shown that Icebergs at the present day score rocks,)¹¹ & I suspect that in Lochaber the two actions are united, & that the scored rock on the water sheds, when tideways, were rubbed & bumped by half stranded Icebergs. You will no doubt attend to Glen Glastig. M^r. Milne I think does not mention, whether Shelf 4 enters it,¹² w^h. I sh^d. like to know; & especially he does not state, whether rocks worn, on their upper faces, are found on the whole 212 vertical course of this Glen, down to near L. Laggan or whether only in the upper part:—nor does he state whether these rocks are scored, or polished, or moutonnés,¹³ or whether there are any “perched” Boulders there or elsewhere. I suspect it w^d. be difficult to distinguish between a River Bed & Tidal channel. M^r. Milne’s description of the Pass of Mukkul expanding to a width of several hundred yards 21 feet deep, in the shoalest part, & with a worn Islet in the middle sounds to me much more like a tidal channel than a River Bed.¹⁴ There must have been in the latter view plenty of fresh water in those days.

With respect to the coincidence of the shelves with the now water sheds, M^r. Milne only gives half of my explanation Please read p 65 of my paper. I allude only to the head of Glen Roy & Kilfinnin as silted up. I did not know Mukkul Pass; & Glen Gluy¹⁵ was so much covered up, that I did not search it well as I was not able to walk very well. It has been an old conjectural belief of mine, that a rising surface, becomes stationary, not suddenly but by the movement becoming very slow. Now this would greatly aid the tidal currents cutting down the passes between the mountains just before & to the level of the stationary periods. The current in the fiords in T. del Fuego in a narrow crooked part are often most violent; in other parts ⁠⟨⁠    ⁠⟩⁠ they seem to silt up¹⁶

Shall you do any levelling?¹⁷ I believe all the levelling has been in Glen Roy, nearly parallel to the Great Glen of Scotl^d. For inequalities of elevation, the valley of the Spean, at right angles to the apparent axes of Elevation, w^d. be the one to examine. If you go to the head of Glen Roy, attend to the apparent Shelf above the highest one in Glen Roy, lying on the South side of Loch Spey, & therefore beyond the Water shed of Glen Roy. It would be a crucial case.¹⁸ I was too unwell on that day, to examine it carefully, & I had no levelling Instrument. Do these fragments coincide in level with Glen Gluoy Shelf?

Macculloch talks of one in Glen Turret above the Shelf 1.¹⁹ I c^d. not see it— These would be important discoveries. But I will write no more, & pray y^r. forgiveness for this long ill written outpouring. I am very glad you keep to y^r. subject of the terraces— I have lately observed, that you have one great authority (C. Prevost)²⁰ that authority signifies a Getting)²¹ on y^r. side respecting y^r. heretical & damnable doctrine of the ocean falling.²² You see I am orthodox to the burning pitch—