Dorset possesses 3 onshore oil fields in the southeast of the county, at Wareham, Kimmeridge and Wytch Farm. Wytch Farm lies between the Purbeck Hills and Poole Harbour; the Purbeck Monocline defines the field's southern limit. Wytch Farm has been producing oil since 1979. A few miles to the southwest at Kimmeridge, a nodding donkey has been pumping up oil from beneath the shale cliffs there since the late 1950s, although yields are declining.