Scientists believe that the last major Ice Age occurred during The Cryogenian Period (approximately 630 to 850 million years ago). During this time the earth passed through four separate Ice Ages. This period is also sometimes referred to as the period of "Snowball Earth".

One website points out that it was possible to ski from one pole to the other during this time.

This is possibly the worst Ice Age that the Earth has ever endured.

There is a website that is dedicated solely to the theory of "Snowball Earth".