Cryogenian

The Cryogenian is the second period of the Neoprotozoic era, from 850 million to 630 million years ago. It followed the Tonian and preceded the Edicarian. It is subdivided into 2 epochs: the Sturtian 850 to 650 million years ago and the Varangian 650 to 630 million years ago.

It is thought that this period on Earth suffered a gigantic glaciation extending at least to the tropics. This is the "snowball Earth" theory; The debate is continuing on this subject and sometimes the "slushball earth" theory is proposed in which the Earth was not completely covered in ice.