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TheDissected Till Plains are physiographic sections of the Central Lowlands province, which in turn is part of theInterior Plains physiographic division of theUnited States, located in southern and westernIowa, northeasternKansas, the southwestern corner ofMinnesota, northernMissouri, easternNebraska, and southeasternSouth Dakota.[1]
The Dissected Till Plains were formed during thePre-Illinoian Stage.Glacial scouring and deposition by theLaurentide Ice Sheet and the later accumulation ofloess during theWisconsin Stage left behind the rolling hills and rich, fertile soils found today in the region.
The region is also the western edge of theCorn Belt.
As a part of the Central Lowland geomorphic province. It is a glacier till plain from flat to rolling plain that slopes towards either theMissouri orMississippi rivers. It is moderately dissected. Local relief is 20 to 165 feet (6.1 to 50.3 m). The region is pocketed by small human landform, i.e., strip-mines among a hummocky or ridge-swale topography. Streams drain and erode the area, moving soils and depositing them downstream. Elevation ranges from 600 to 1,500 feet (180 to 460 m).[2]
Loess (unconsolidated aeolian silt), as much as 25 feet (7.6 m) thick thins toward the east, covers most uplands.Pleistocene (pre-Illinoisan)till lies beneath the loess, covering the bedrock up to 300 feet (91 m) deep. Along the edges, it thins to less than 30 feet (9.1 m). The Mississippi and Missouri floodplains have up to 150 feet (46 m) of unconsolidatedTertiary and Quaternaryalluvium (gravel, sand, silt, and clay) over the bedrock, thinner in the river valleys.[2]
Bedrock appears along the margins.Cretaceousshale andsandstone occur in the northwestern corner, the Nebraska, Minnesota and Iowa boundary section.Permian sandstone is along the western margin or the Missouri Valley.Pennsylvanian shale,limestone, andcoal is beneath nearly the entire geologic unit.Mississippian,Devonian andOrdovician shale and carbonate are the bedrock along the eastern and southern areas. Bedrock is visible along the deeper drainage's and in eroded "windows" of the unconsolidated surface soils.[2]
The Dissected Till Plains is a sub-unit of theCentral Lowlands in theInterior Plains ofNorth America. It is centered on theIowa-Missouri state line. The eastern border is theMississippi River and bounded on the south by the Missouri River Valley across central Missouri. Its western boundary is about 100 miles (160 km) west of the Missouri Rivers border along the Kansas/Missouri – Nebraska/Iowa state line. Its northern border is a line dipping from the Sioux River valley of South Dakota and Minnesota, south into Iowa along a line demarking the Missouri valley from the interior lowlands of Iowa, then curving northward again on a line demarking the interior lowlands of Iowa, from the Mississippi River valley on the east.[3]
