Permafrost

Permafrost
Permanently frozen ground, typically persisting metres or tens of metres below the ground surface. Normally there is a metre-thick active layer that melts each summer and, if on a slope, tends to flow downhill (see head).

PLEISTOCENE: The time period of the ice ages as seen in the northern hemisphere. Refers also to deposits formed between 2.6 million and 11,700 years ago.