trees grew around the low coastal areas and were, from time to time, subjected to drowning from the sea. The fallen trees and their stumps became covered by stromatolites which trapped sediments. The limestones at this level, which vary greatly in colour, are composed of densely packed shells of the freshwater snail Viviparus. Their ability to take a very high polish has led them to become known as Purbeck Marble. Like the older Forest Marble, it is a sedimentary marble rather than a true metamorphic one. The Purbeck Limestone Group has yielded a diverse selection of fossilised remains, including rare reptiles, amphibians, and mammals, for which the formation has become internationally important. Wealden Group The Wealden Beds are confined to the south and increase in thickness from 107 to 716 m, between Upwey and Swanage. By the latest part of the Berriasian, the waters covering Dorset's landmass had subsided, leaving the area almost completely terrestrial save for the large rivers that dominated it. These rivers drained the upland areas to the north and west, and were responsible for depositing much of the succeeding sediment across southern Dorset. Comprising clays, silts, sands, and grits, they cover the floors of the valleys between Lulworth and Swanage. The sands and grits are also exposed in the cliffs there, and form ridges along the valleys. Traces of granite from Cornubia, the landmass that later became Cornwall, were also washed down. The Wealden Beds contain a great deal of plant matter and charcoal, indicating periodic fires in the region. At Mupe Bay, some of the sands have been bound together by oil and formed into sandstone boulders, suggesting that oil generation had already begun when these sands were deposited. Lower Greensand & Selborne groups The marginal marine incursions that occurred in the previous period increased in severity until Dorset was completely under water by the Aptian. The geology from this period indicates a gradual flooding of the county from the south-east to the north-west. The earliest deposit, the Lower Greensand, covers the preceding Wealden Beds in the south, and the Oxford and Kimmeridge clays in the north. It is so called because of its green colour, which is caused by the presence of the mineral glauconite. In the west, overlying the eroded Lias, is a thin layer of Gault Clay of Albian age, which decreases in thickness from 27 to 7 m the further west