The Cryogenian Times

	Cryogenian Time: Times in the past when the planet cooled and there were more glaciers over the surface of the planet. Some theories suggest these were ice ages similar to what Earth experienced recently (in geologic time), but perhaps a bit colder. Other theories suggest the world froze over solid like a giant snowball. The best evidence for these cold periods indicate that there were long periods of cold climate (or perhaps frequent periodic ice ages) about 2.2 billion years ago and then again around 710 millions years ago and then finally around 640 million years ago. The Cryogenian period is officially set as the time from 850 millions years ago to 630 million years ago. It ended at the close of the Baranger (Marinoan) glaciation when there was a change in the number of Carbon-13 isotopes left in rocks. The Cryogenian period was a late part of the Precambrian Proterozoic Eon, and it preceded the Ediacaran Period.
	Links about the Cryogenian: Susan Truelove’s article about the Cryogenian glaciations. An article suggesting the Cryogenian period was one of waxing and waning glaciers rather than a time of the entire planet feezing over. The Palaeos page on Snowball Earth scenarios. The Snowball Earth website.

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