WCP1686

[1]

9, St. Mark's Crescent

Regent's Park, N.W.

Nov^r. 23^rd. 1869

Dear Sir^{1, 2}

Many thanks for the paper on "Denudations"³ which I have read again with great interest. The only remaining difficulty, & one to which you do not allude, is the very perfect moraines in Scotland & Wales. They could never have been protected by a covering of clay or drift ^{like the scratches} & it is hard to see why they have not all disappeared standing as they so often do upon slopes in valleys.

I am glad you have [2] some direct evidence of glaciers forming lake basins. I have been trying hard to convert Sir C. Lyell⁴ & often think I succeed, but he suffers continual relapses. I cannot get him to realize fully that a lake basin can only be produced by the differential action of a glacier of great variation in thickness. A glacier of tolerably uniform thickness however large, might deepen its valley all along but form no lake; which another glacier of less bulk, which owing [3] to confluence of lateral streams at one spot and a narrowing of the valley below, had the ice heaped up at that place to double the thickness of the rest of the valley, must inevitably grind out a hollow.

The most important datum wanted is the amount of denudation effected by a glacier effected over its area, as this denudation will be almost all by rock grinding. This must be very easy to measure[;] has no one ever done it? Then if the thickness of the glacier in different parts of its course [4] can be approximately determined, the amount of denudation would be very nearly in proportion to the thickness (combined with the rate of motion) & thus the time required to grind out a basin of a given depth under existing conditions might be determined.

Hoping you know some resident Swiss Geologist who will work at this subject

I remain | Yours very faithfully | Alfred R. Wallace [signature]

P.S. An article of mine on Geological Time will shortly appear in "Nature".⁵