Ice sheets are large, continental-scale (greater than 50,000 sq. km or 19,305 sq. mi in area) ice masses that accumulate in extensive, high elevation catchment areas. They are drained by smaller outlet glaciers or ice streams that flow out from them in different directions.
Today, the only ice sheets on Earth are the massive ice bodies in Antarctica and Greenland. However, during the last ice age (approximately 20,000 years ago), two ice sheets covered much of northern North America. These ice sheets shaped much of the landscape there, including a few of our parks.