Purbeck ball clay is composed of kaolinite, mica and quartz. It is of sedimentary origin, having been formed during the Eocene epoch in the particular depositional environment of the then subtropical Wareham Basin. It is thus found today on the northern half of the Isle of Purbeck, to the north of the Purbeck Hills, and is used in the production of fine pottery. It has been quarried on a large scale since the middle of the 18th century; Dorset's first railway was constructed in 1806 in order to transport the excavated material. It is still being excavated today.