- Lower, the Wealden Beds, laid down in freshwater or brackish conditions. Sands came from the flood plain of a huge tropical braided river. Clays came from a coastal lagoonal plain.
- Upper, the Greensands, an olive-green sandstone deposited in the sea; and the Gault Clay, a formation of stiff blue clay deposited in a calm, fairly deep water marine environment. It overlays the Lower Greensand formation.
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.
References[change | change source]
|Precambrian (4.567 gya – 541 mya)|
|In the left column are Eons, bold are Eras, not bold are Periods. gya = billion years ago, mya = million years ago|
|Hadean (4.567 gya – 4 gya)||Chaotian||Zirconian||Archaean (4 gya – 2.5 gya)||Eoarchaean (4 gya – 3.6 gya)|
|Proterozoic (4 gya – 2.5 gya)||Palaeoproterozoic (2.5 gya – 1.6 gya) Siderian (2.5 gya – 2.3 gya) Rhyacian (2.3 gya – 2.05 gya) Orosirian (2.05 gya – 1.8 gya) Statherian (1.8 gya – 1.6 gya)|
|Phanerozoic (541 mya – today)|
|In the left column are Eras, bold are Periods, not bold or italics are Epochs, Italics are stages. kya = thousand years ago, mya = million years ago|
|Palaeozoic (541 mya – 252.17 mya)||Cambrian (541 mya – 485.4 mya)|
|Mesozoic (252.17 mya – 66.0 mya)||Triassic (252.17 mya – 201.3 mya) Lower Triassic (252.17 mya – 247.2 mya) Middle Triassic (247.2 mya – 237 mya) Upper Triassic (237 mya – 201.3 mya)|
|Cainozoic (66.0 mya – today)||Palaeogene (66.0 mya – 23.03 mya) Palaeocene (66.0 mya – 56 mya) Eocene (56 mya - 33.9 mya) Oligocene (33.9 mya – 23.03 mya)|