Earth 119- chapter 22: glaciers and ice ages. Erratics: a boulder or cobble that was picked up by a glacier and deposited hundred of kilometers away from the outcrop from which it detached. Glaciers: a river or sheet of ice that slowly flows across the land surface and lasts all year long. Ice sheets: a vast glacier that covers the landscape. Ice age: an interval of time in which the climate was colder than it is today, glaciers occasionally advanced to cover large areas of the continents, and mountain glaciers grew; an ice age can include many glacials and interglacials. Pleistocene ice age: the period of time from about 2 ma to 14,000 years ago, during which the earth experienced an ice age. Sublimation: the evaporation of ice directly into vapor without first forming a liquid. Firn: compacted granular ice (derived from snow) that forms where snow is deeply buried; if buried more deeply, firn turns into glacial ice.