English: Worm castings, Bwlch Mawr These tiny coils of digested soil are deposited on the surface of lawns and grassy areas by earthworms. Although annoying to fussy gardeners, they form a highly nutritive compost. Charles Darwin's last book, The Formation of Vegetable Mould through the Action of Worms with Observations on their Habits (1881), was the result of 40 years' research on these humble creatures which, he explained, have been responsible for sifting and turning the entire earth's surface. He calculated that in the course of one year, in an acre of land, they bring to the surface in the form of castings, 18 tons of earth, enriching and aerating the land in the process.