The glacial history of the Earth is complex and extends back in geological time to the Proterozoic and possibly the Archean. Global glaciations have occurred during every geological period except the Jurassic. This persistence of global glaciation, as well as repeated and continuing glaciation in high mountainous areas, wherever altitudes exceed the local snowline, is so consistent that one might argue that the Earth is essentially a glacial planet. Evidence suggests that glacial epochs have repeatedly occurred almost every 100,000 to 150,000 years.