To Amy Ruck 24 February 
9 Devonshire St | Portland Place
My dear Amy
I want you to observe another point for me; so you see that I treat you as my geologist in chief for N. Wales.1
The late Prof. Henslow,2 who was a very accurate man, said that he had often observed on very steep slopes, covered with fine turf, (such as may be found in mountainous countries & no where else) that the surface was marked by little, almost horizontal, sometimes sinuous & bifurcating ledges; or as he called them, wrinkles. These are commonly attributed to sheep walking in nearly horizontal lines along the sloping surface; & they are undoubtedly thus commonly used by the sheep; but Henslow convinced himself that they did not thus originate. Dr Hooker, to whom Henslow made these remarks, has since observed such little ledges on the Himmalayah & Atlas ranges, in parts where there were no sheep & few wild animals—3 Henslow speculated that the earth beneath the turf was in some manner gradually washed away; & he compared the wrinkles on the turf to those on the face of an old man whose face is shrunk. I cannot possibly believe in this notion.
Would you look at any very steep grass covered slopes near you, & if you can find any appearance such as I have described, will you make for me an eye sketch on some approximate scale of the relative appearance & distance of the ledges. I have been speculating whether the ledges can possibly be due to the washing down of the worm castings, & their union into little ridges, on nearly the same principle that when the wind drifts loose sand, it makes numerous little ridges at right angles to the course of the wind.
You must not give yourself much trouble on this subject, but I shd be very much obliged for any observations or remark.
yours very sincerely | Ch. Darwin
If you find anything of the kind observe whether there are worm-castings on the slope.— Also whether the earth is bare & exposed beneath the little ridges or ledges.—
Wants AR to make any observations she can on the occurrence of little ledges on the side of steep, turf-covered slopes. Feels they may result from the washing down of worm-castings.
Please cite as
Darwin Correspondence Project, “Letter no. 8224,” accessed on 21 February 2017, http://www.darwinproject.ac.uk/DCP-LETT-8224