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The 'Cryogenian Period' (from Greek ''cryos'' "ice" and ''genesis'' "birth") is a geologic period from 850 million to 630 million years ago, marked by two heavy glaciations, the Sturtian and the Marinoan, which were the worst ice ages known to have occurred, and may have covered the entire planet in ice. It was the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran, which marked the first development of multicellular life.
The name is derived from the glacial deposits characteristic of the period, indicating that at this time, the Earth suffered the most severe ice ages in its history, with glaciers extending and contracting in a series of rhythmical pulses.^[1] It is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 to 700 Ma and the Marinoan/Varanger glaciation terminated at ca. 635 Ma. The tillite deposits occur also in places which were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of the deeply-frozen planetary oceans called "Snowball Earth".
Also, during the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form.

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References

References

1. Hoffman, P.F. 2001. Snowball Earth theory, accessed 15/Jun/2007, http://www.snowballearth.org


★ Cryogenian Period

★ Status on Divisions of the International Geologic Time Scale, James G. Ogg, , , Lethaia, 2004