WCP1686

[1]

9, St. Mark's Crescent

Regent's Park, N.W.

Nov^r. 23rd. 1869

Dear Sir

Many thanks for the paper on "Denudation"¹ 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 or in valleys.

I am glad you have² [2] some direct evidence of glaciers forming lake basins. I have been trying hard to convert Sir C. Lyell³ & 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".⁴