From Amy Ruck to Horace Darwin [1 February 1872]1
Will you tell Mr. Darwin that in the case in which the furrows run down the hill & are 4in at the top & 1in at the bottom—the slope is 15o and faces North East. The furrows are about 7 ft apart & on the level ground above are 4in 3in deep, although it is difficult to measure these on account of the mole hills—40 paces long.
On another slope of 15o facing S. West the furrows were scarcely perceptible but on level ground at the bottom the same furrows were 3 2in deep. The length of the slope was 80 paces— On another short slope of 10o the furrows at the top were 3 at the bottom 1 1in deep.2
Observations on earthworm activity in old furrows on a hillside.