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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. The greatest ice ages known to have occurred on Earth, the Sturtian and Marinoan glaciations, may have covered the entire planet in ice during this period. 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 refers to the very cold global climate of the Cryogenian: characteristic glacial deposits indicate that Earth suffered the most severe ice ages in its history during this period. Glaciers extended and contracted in a series of rhythmic pulses, possibly reaching as far as the equator.^[1] It is generally considered to be divisible into at least two major worldwide glaciations. The Sturtian glaciation persisted from 750 million years ago to 700 Ma, and the Marinoan/Varanger glaciation terminated at circa 635 Ma. The deposits of glacial tillite also occur in places that were at low latitudes during the Cryogenian, a phenomenon which led to the hypothesis of deeply-frozen planetary oceans called "Snowball Earth".^[2]

During the Cryogenian, the supercontinent Rodinia broke up, and the supercontinent Pannotia began to form.

1. ^ Dave Lawrence, "Microfossil lineages support sloshy snowball Earth" in Geotimes, April 2003. http://www.agiweb.org/geotimes/apr03/WebExtra041803.html, accessed Sept 18, 2007.
2. ^ Hoffman, P.F. 2001. Snowball Earth theory, accessed 15/Jun/2007, http://www.snowballearth.org

• Cryogenian Period. GeoWhen Database. Retrieved on January 5, 2006.
• James G. Ogg (2004). "Status on Divisions of the International Geologic Time Scale". Lethaia 37: 183–199.