The two parts of the Cretaceous represent the sea level where the divisions were first defined. In the Lower Cretaceous, the sea level was much lower than in the Upper Cretaceous. In the Wealden, Southern England was a huge tropical river delta; in the Greensand the same areas were just offshore. However, in the Upper Cretaceous the deposits were made in deeper water, on what was then the continental shelf. Sea level then stood at its highest level in the Mesozoic.