Age : 65 to 142 million years ago
The Cretaceous in England can be divided into two parts. In the Early Cretaceous, much of Britain was above sea-level and in southern England, the earliest Cretaceous strata were deposited under continental lagoonal, lake, and fluvial conditions. Further north, the Lower Cretaceous is entirely marine in character. However, only a few millions of years into the Cretaceous, there was a global rise in sea-level and the sea rose over most of Britain, thereby flooding most of the early Cretaceous landmass. Marine conditions then prevailed until the Palaeogene.
Warm, tropical conditions prevailed throughout the Cretaceous.
Reptiles were the main large predators and herbivores of the Cretaceous world and dominated both terrestrial and marine habitats. Many species of dinosaurs occurred and their remains are commonly found in the Lower Cretaceous river deposits of the Weald Group.
The Chalk is mostly made of the remains of microscopic plankton called coccoliths but the fossils of sea-urchins (echinoderms), belemnites, ammonites and bivalves are also found in the Chalk. Large marine reptiles such as ichthyosaurs, crocodiles and mosasaurs were the top marine predators.
The end of the Cretaceous was marked by a global catastrophic event, probably a large meteorite impact, which contributed to the extinction of the dinosaurs, marine reptiles, ammonites and many species of plants.
During the Lower Cretaceous thick deposits of fluvial and lacustrine muds and sands accumulated in southern England (the Wealden Group), which now outcrop in Sussex, the Isle of Wight and east Dorset. These are succeeded by marine sands and clays (the Greensand and Gault Clay) associated with the rise in sea-level and establishment of marine conditions over much of Britain. Deepening of the sea in the later Cretaceous marked the onset of the deposition of large amounts of calcareous ooze on the sea-floor. These deposits later formed the Chalk, which originally covered much of Britain, but now outcrops in eastern and southern England.