Age : 142 to 205 million years ago
The British Isles continued its northwards drift. As global sea-level began to rise in the Early Jurassic the consequent marine transgression replaced the low-lying desert plains and marginal seas by more permanent, warm, shallow shelf-seas, which occupied much of Britain. However, only southern and eastern England remained submerged throughout the Jurassic, with the south-west and northern England, Scotland, Wales emerging as land areas from the Lower Jurassic onwards. An isolated island, the Anglo-Brabant landmass, occupied the area around London.
The warm, tropical Jurassic sea teemed with life including marine reptiles such as plesiosaurs and relatives of squids known as ammonites and belemnites. Reptiles also flourished on land and evolved into many forms including the dinosaurs and birds.
Marine Jurassic rocks comprising mudstones, limestones and sands run right across the country from Dorset to North Yorkshire. Lower Jurassic rocks are superbly exposed on the Dorset and North Yorkshire coasts, while Middle Jurassic oolitic limestones deposited in warm shallow seas and over tidal flats now form the broad ridges of the Cotswolds and their continuation through Northamptonshire and into Lincolnshire.