ice sheet 
noun
A layer of ice covering an extensive tract of land for a long period of time.Several scientists have proposed that the southern part of the Greenland ice sheet disappeared during the period of maximum interglacial warmth....
- Younger cited evidence for extensive ground freezing in SE England during the Devensian, beyond the limit of the Devensian ice sheet.
- Later, as the permafrost melted and the ice sheet warmed as a result of climatic amelioration, the ice would have become wetbased.