Purbeck Beds

Purbeck Beds, the highest series of the Jurassic rocks in England, typically developed in the Isle of Purbeck, in Dorsetshire; but occurring also in the Isle of Portland; at Swindon; near Aylesbury; and near Battle, in Sussex. It passes, with but little break, downward into the marine Portlandian, and upward into the fresh-water Wealden series (q.v.); but itself combines marine and fresh-water rocks. It is divided into three parts, the lower, mainly fresh-water limestones or Purbeck marble, made up of shells of mussels (TInio) and of snails ( Viviparus), with shales, and "dirt-beds" or ancient soils; the middle, mainly marine; and the upper, again fresh-water. A bed of oyster-shells (Ostrea distorta), 12 feet thick, is one indication of marine life, and considerable beds of gypsum point to the evaporation of inland waters. Many insect remains occur at some horizons, and chelonian and crocodilian remains ( Goniopholis) are frequent. Near the base of the Middle Purbeck, at Swanage, the lower jaws of various rat-kangaroos have been discovered, as if dropped by carcases floating in an estuary. The Purbeck marble of Dorsetshire was much used in the mediaeval architecture of southern England and the gypsum discovered in the Sub-Wealden boring near Battle is now worked.