Glacial Handiwork in USA

Advancing and retreating over a span of hundreds of millions of years, mantles of ice reached farthest into the U.S. 18,000 years ago. By the time the last ice age ended 8,000 years later, glacial handiwork had created some of the country's most distinctive features: the Great Lakes, Puget Sound, the bogs of northern Minnesota, and the bulldozed flatness of the Central Lowland. There ice ground rock into the fine soil in which Iowa, Illinois, and adjacent states grow 40 percent of the world's corn crop. Sand and gravel deposited by glaciers also helped shape the profiles of Long Island and Cape Cod.

Glacier is an enduring accumulation of ice, snow, water, rock, and sediment that moves under the influence of gravity. Glaciers form where the temperature is low enough to allow falling snow to accumulate and slowly transform into ice. This accumulation is most common in the polar regions, but can also occur at high altitudes on mountains even near the equator. Glaciers are complex systems that grow and shrink in response to climate. At the present, glacier ice covers about 15 million sq km (5.8 million sq mi), or 10 percent, of Earth’s land area.

Glaciers occur on all continents except Australia. Antarctica and the North American island of Greenland have the largest continuous ice masses. Ice covers their land areas almost completely. About 80 percent of the fresh water on Earth is frozen in ice sheets and glaciers. If all of this ice melted, sea level would rise by about 60 m (200 ft). Much of the world’s population resides in coastal areas that would be underwater. Glaciers are an intriguing part of Earth’s natural environment and their majestic beauty in wild and inaccessible mountain settings is unparalleled.

Glaciers occur in many different forms and locations, from the big ice sheet that covers the entire continent of Antarctica to the small valley glaciers that are present in many parts of the world. They are generally divided into several categories depending on their size and location. Glaciers categorized by size include ice fields, ice caps, and ice sheets. Glaciers categorized by location include alpine, valley, and piedmont glaciers. (Adapted from National Geographic and Encarta Encyclopedia)