High Quality Content by WIKIPEDIA articles! The Cryogenian (from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from 850 to 635 million years ago. It forms the second geologic period of the Neoproterozoic Era, preceded by the Tonian Period and followed by the Ediacaran. The Sturtian and Marinoan glaciations, which are the greatest ice ages known to have occurred on Earth and may have covered the entire planet, occurred during this period. These so-called `snowball earth` events are the subject of much scientific controversy. The main debate involves whether these glaciations were truly global or merely localised events.