formation were caused by coccoliths following periodic blooms of algae. Many of the fossilised remains are squashed flat, indicating that the Kimmeridge beds were heavily compacted, perhaps to an eighth of their original thickness. This was not sufficient to produce oil reservoirs like those beneath the North Sea, yet the oil-rich shales have, in the past, been economically important as a local fuel. Portland Group Whether the formations created between the Upper Jurassic and Lower Cretaceous were deposited across the whole of Dorset like the preceding beds, and subsequently eroded, or only formed in the areas in which they are now found is unclear. The Portland sands were created in a deep marine environment, whereas the Portland stone was a near-shore deposit. The lower Portland ,and formation, usually greyish-blue in colour, consists of dolomite beds best viewed on the West Weares, Gad Cliff, and Hounstout. The overlying Portland stone is a distinctive white oolite containing thick-shelled fossils and marine algae. The marine algae, which formed large reefs, are responsible for the occasional stacked and arched bands in the formation and are best viewed in the Roach, an extremely fossiliferous limestone confined to the north of Isle of Portland. To the north of the oolite is a fine-grained limestone, which was deposited in calm, shallow water. It contains flint-like rocks known as chert, were it not for the presence of very large ammonites, might be mistaken for an early chalk. Previously thought to be around the middle of the Purbeck Strata, modern research suggests that the boundary between the Jurassic and Cretaceous periods lies within the basal few metres of the overlying Lower Purbeck. Latest Jurassic to Cretaceous Purbeck Group The Purbeck Limestone Group is a succession of limestones, shales, and clays named after the Isle of Purbeck, laid down in the latest Tithonian to late Berriasian mainly in very shallow lagoons and freshwater lakes, with only occasional marine influence at the lower levels where halite, gypsum, and other evaporites are found. The muddled sediments around Lulworth are thought to be caused by a thick layer of these evaporites dissolving. Evidence also exists of uplift and erosion of earlier formations. The climate around this time had changed from a tropical to a subtropical one, with pronounced wet and dry seasons as indicated by the growth rings in the fossilised trees also found in the lower levels of the Purbeck strata. These