The Cretaceous period was the longest period in the Phanerozoic eon, lasting 80 million years from 145 to 66 million years ago (mya). It followed the Jurassic period, and is the latest period of the Mesozoic era. After it came the Cainozoic era.
Though famous for its land fauna, the Cretaceous sea level rose much higher than today. North America was partly covered by an epicontinental inland sea, and half of Britain was under water. In the Upper Cretaceous, chalk, a type of limestone, was laid down in warm shallow seas. Chalk is 95% to 99% made up of coccoliths, the calcium carbonate (CaCO3) plates which tiny single-celled algae (coccolithophorids) produce. At its end came the famous meteorite strike which, with the volcanic flood basalts spewed out in the Deccan Plateau (India), ended so many of the dominant life-forms: see K/T extinction event.
The Cretaceous period has just two very long epochs:
- Levin, Harold 2006. The Earth through time. Wiley, Hoboken N.J. Chapters 13 & 14
- One account goes as follows: "During the Late Cretaceous (94–64 Ma) sea level was up to 650 m higher than at the start of the Albian, when it was perhaps about the same as it is today. Facies analysis in both the Western Interior of the USA and in NW Europe reveals simultaneous major oscillations in sea level that controlled the principal transgressions and regressions except in regions of rapid contemporaneous tectonics. The peak transgressions of the Late Albian, Early Turonian, Early Coniacian, Middle Santonian and Late Campanian, probably recognisable world-wide, involved rises of sea level at rates of about 10–90 m per m.y. The peak regressions are more difficult to measure, but were usually faster and involved falls of sea level of about 95–170 m per m.y". Hancock J.M. & Kauffman E.G. 1979. Journal of the Geological Society of London 136, 175/186. 
|Precambrian (4.6 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 (4.6 gya – 4 gya)||Cryptic (4.6 - 4.533 gya)|
|Archaean (4 gya – 2.5 gya)||Eoarchaean (4 gya – 3.6 gya)|
|Proterozoic (2.5 gya – 541 mya)||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)|