PURBECK BEDS

uppermost division of Jurassic rocks in Great Britain. (The Jurassic Period lasted from 208 to 144 million years ago.) The Purbeck Beds overlie rocks of the Portland Beds and characterize the Purbeckian Stage, a worldwide standard division of Jurassic rocks and time. Rocks of the Purbeck Beds are highly varied and indicate major alterations in the geographic conditions of the Jurassic Period. Limestones, marls, clays, and old soil horizons occur and may attain a thickness of about 170 m (560 feet). Many of the varied rock types of the Purbeck were deposited in lagoons and some in freshwater. Ancient land soils in the Lower Purbeck Beds include the fossilized remains of coniferous trees and cycads, primitive palmlike trees. The Middle and Upper Purbeck consist of freshwater limestones that are quarried for use as building stone. Marls and shales are interbedded with the limestones; included are the famous Insect Beds that contain a remarkably well-preserved insect fauna. The Cinder Bed consists of enormous quantities of oyster shells, trigonids (rare Mesozoic clams), and fragments of echinoids, or sea urchins. The Purbeck Beds have provided a wealth of Jurassic vertebrate remains as well.