What is "cryogenian"

The Cryogenian (, from Greek cryos "cold" and genesis "birth") is a geologic period that lasted from . 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, occurred during this period. These events are the subject of much scientific controversy.

The main debate contests whether these glaciations covered the entire planet (the so-called ' Snowball Earth') or if a band of open sea survived near the equator (termed 'slushball Earth').