Iowa lies entirely within the natural region, or physiographic province, called the Central Lowland, which in turn forms part of the Interior Plains. The Central Lowland can be divided into several subregions, or sections, four of which extend into Iowa. These four sections are the Till Plains, the Dissected Till Plains, the Western Young Drift section, and the Driftless section. In order to distinguish clearly among them, the four sections will be treated as separate natural regions in this article.
The Till Plains, in Iowa, cover a narrow section of the east along the Mississippi River. They are the westernmost extension of the areas of till that cover much of Illinois and Indiana. They are distinguished from the adjoining Dissected Till Plains by being relatively newer in age and, consequently, less eroded. The flat surface of the Till Plains is broken only by a few hummocks that are formed of glacial drift.
The Dissected Till Plains occupy most of southern and western Iowa. Glaciated only during the early part of the Ice Age, the till plains of this section have been more dissected, or eroded, than in more recently formed drift or till areas. The terrain varies from almost flat prairie east of Ottumwa to distinctly hilly areas elsewhere in the region. In general, the hilliest areas are in the south and west. Much of the Dissected Till Plains, as well as the adjoining Till Plains, is covered by deposits of loess, a wind-carried form of glacial silt laid down during the last part of the Ice Age. Along the major rivers, especially the Missouri, the loess deposits were piled up by wind action to form steep-sided bluffs that rise as much as 46 m (150 ft) above the river surface.
The Western Young Drift section occupies most of north central Iowa. Leveled by the most recent ice sheet, it remains a generally flat area little altered by erosion. Glacial boulders still lie scattered over its flat, little-eroded surface.
Small lake-filled depressions were once numerous in the western part of this region, but most of them have been filled in or drained to provide excellent farmland. This western area, called the Iowa Prairie, is the most recently glaciated part of the state. It is marked on the east and west by a series of pronounced ridges or rises, which represent the terminal moraines left by the last ice sheet. The Western Young Drift section lacks the loess deposits common to southern Iowa, but fertile soils have developed on the thick layer of recent glacial drift. Agriculturally this is the most productive crop-growing section of the state. It also, perhaps, typifies the commonly held image of Iowa, with its flat fertile farmlands so well suited to large-scale farming.
The Driftless section, most of which lies in southern Wisconsin and is known as the Wisconsin Driftless section, occupies a small part of northeastern Iowa. It was glaciated by only the earliest of the ice sheets. Erosion has long since removed most of the early glacial drift and has exposed the underlying rock formations. This section, with its many ridges, cliffs, springs, and steep-sided valleys, is generally higher, more rugged, and less fertile than the other sections of Iowa. "Indiana" © Emmanuel BUCHOT, Encarta, Wikipedia
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