To Charles Lyell   1 April [1862]¹

My dear Lyell.

I am not quite sure that I understand your difficulty, so I must give what seems to me the explanation of the glacial-lake-theory at some little length.² You know that there is a rocky outlet at the level of all the shelves.— Please look at my map:³ I suppose whole valley of Glen Spean filled with ice; then water would escape from outlet at Loch Spey & the highest shelf would be first formed. Secondly ice began to retreat, & water would flow for short time over its surface; but as soon as it retreated from behind hill marked Craig Dhu, where the outlet on level of 2^d shelf was discovered by Milne, the water would flow from it, & the second shelf would be formed.⁴ This supposes that a vast barrier of ice still remains, under Ben Nevis, along all the lower part of the Spean. Lastly I suppose the ice disappeared, everywhere along L. Laggan, L Treig & Glen Spean, except close under Ben Nevis, where it still formed a Barrier, the water flowing out at level of lowest Shelf, by the pass of Muckul at head of L. Laggan.— This seems to me to account for everything. It presupposes that the shelves were formed towards close of Glacial period.—

I come up to London to read on Thursday short paper at Linn. Soc.^{y 5} Shall I call on Friday morning at 9$\frac{1}{2}$ & sit half an hour with you? Pray have no scruple to send a line to Q. Anne St to say “no”—if it will take anything out of you.—⁶ If I do not hear, I will come.—

Yours affect | C. Darwin