1) The earth cast up by a mole. It was common practice to break up and spread the earth.
1446-58 et pro Moldhyls iijd, Fountains Abbey. This makes sense of the earlier entry: et petit pro prostracione monticulorum xxd. It later became mole-hill by association: 1785 Had G. Town & C. Crowther Spreading Mole hills & dung, Ovenden.