Iowa is the 26th largest state in the Union. It has a total area of 145,744 sq km (56,272 sq mi), including 1,041 sq km (402 sq mi) of inland water. The state has a maximum extent from east to west of 534 km (332 mi) and a maximum distance from north to south of 344 km (214 mi). The mean elevation is about 340 m (1,100 ft).
The physical features of present-day Iowa are the result of widespread and repeated glaciation during the last Ice Age and the subsequent changes brought about by wind and water erosion. Few of the sedimentary rock formations underlying the state are visible on the surface, for they are covered by a thick mantle of glacial deposits.
During the Ice Age, which began about 2.5 million years ago and lasted until about 10,000 years ago, great ice sheets from the north advanced and retreated successively across Iowa. As each ice sheet, or glacier, advanced across the land, it planed off existing hills and filled in valleys, picking up rock material as it went. As it retreated, the ice sheet left behind layers of clays, sands, gravels, and boulders, which together are called drift, or glacial drift. The drift included clays and boulders that were deposited directly by the ice sheet and are called till, or ground moraine. In addition, streams of meltwater flowing out of the retreating ice deposited a variety of other rock material. Not all of the ice sheets covered all of Iowa. Although the earliest ones did extend across nearly the entire area, the subsequent ice sheets generally covered only the north central parts of Iowa.
Consequently, the materials left by the more recent ice sheets masked some of the older drift deposits. As a result, the age and relative degree of erosion of the mantle of glacial drift differ from section to section. These differences are reflected in the division of the state into natural regions. "Iowa" © Emmanuel BUCHOT, Encarta, Wikipedia
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