Huge ice sheets, today restricted to the polar areas, periodically expanded to cover areas farther south. Most of northern Europe was covered by a mile-high sheet of ice during periods of maximal glacial advance. There were several periods during the long span of the Upper Paleolithic when the ice receded to nearly its present range. During glacial retreat, more northerly areas were available for habitation by prehistoric people, but subsequent readvances of ice destroyed most traces of these early occupations.