The sea levels remained more or less unchanged until the end of the Cretaceous period and despite Dorset having moved even further north to 40 degrees, the climate had reverted to a tropical one. During this period, the sea deposited an enormous amount of chalk across the county, perhaps as much as 300 metres more than there is currently. The first chalk deposits contained sands and grits thought to have originated in Cornubia. This gritty chalk was deposited across central Dorset and can clearly be seen in the sides of Eggardon Hill. As the water deepened, plankton, coccoliths and foraminifera became the main source for the calcereous soup which would later form the Dorset chalk. The chalk forms a narrow strip inland between Studland and Worbarrow Bay when it reaches the coast once more. A much wider strip continues westward along the coast until it meets the wide swathe of chalk that cuts north-west across the county and forms the south-east point of the Hampshire Basin. Generally the Dorset chalk is too soft to be used as a building stone except along the Ridgeway where faulting and folding produced a particularly hard variety popularly used by the Romans in their mosaics. The chalk often contains larger fossils including ammonites, belemnites and brachiopods. Flint occurs throughout in both nodule and tabular layers.