Text Box:   Central Lowlands Introduction | General Description | Physical Features | Great Lakes | More Features | Resources                                                     The Central Lowlands province, which covers 585,00 square miles, including water bodies, is the largest province in the conterminous United States (when the continental shelf portion of the Atlantic-Gulf Coastal Plain province is excluded from the area of that province).  It extends from central New York, east of Lake Ontario, westward to western North Dakota, and stretches southward as far as central Texas.  The province also extends far into Canada.  Boundaries along much of the province are definitive.  In New York, Pennsylvania and most of Ohio the boundary is the prominent escarpment at the western edge of the Appalachian Plateaus.  From southern Ohio (some researchers use central Ohio) to central Missouri the border between this province and the Interior Low Plateaus and the Ozark Plateaus provinces coincides with the southernmost extent of the continental glaciers;  this line very closely coincides with the Ohio River between Cincinnati and Louisville, and with the Missouri River from St. Louis to central Missouri.  The western and southern edges of the province border the Great Plains.  The boundary here is often not as well defined as other portions, although there are a few prominent features along parts of the border.  This boundary, and differences between the Central Lowlands and Great Plains, are discussed in detail in the Great Plains chapter.    General Characteristics Two of the main distinguishing characteristics of the Central Lowlands are relatively low elevation and low relief throughout most of the province.  In the very eastern part, near central New York, the province is about 1,000 feet above sea level, although along the shores of Lake Ontario elevation is  less than 300 feet.  At the Mississippi River, near the middle of the province, altitudes average approximately 500 feet.  Westward the province rises slowly to about 2,000 feet along much of its western boundary.  The second main characteristic–low relief that rarely exceeds 600 feet–has been caused primarily by two factors.  First, the Central Lowlands, along with the Great Plains to the west, compose nearly all of the United States portion of the North American craton.  The platform is a nearly stable region where basement rocks are covered with a relatively thin layer of sedimentary strata; the rocks are primarily sandstone, shale, limestone and conglomerate.  There has been considerably less diastrophism here than in nearly all other parts of the country, and most of the rock layers are nearly horizontal or gently dipping.  There are a few broad domes in the province, such as the Sioux Uplift near the northwestern corner of Iowa, the Wisconsin Dome (Arch) in the central portion of that state, the Kankakee Arch and La Salle Anticline in northern Illinois, the Findlay Arch in western Ohio, and the Ontario (Algonquin) Dome between Lake Huron and Lake Erie, but very limited areas can been considered even relatively mountainous.  The Wichita Mountains in southwestern Oklahoma are rugged but low, consisting mostly of hills and ridges scattered over an area extending 60 by 25 miles.  Some parts of the area, composed mostly of granite with some limestone and dolomite, rise 1,400 feet above the surroundings.  The Arbuckle Mountains in south-central Oklahoma are more plateau-like than mountainous, for maximum relief is less than 500 feet.  The exposed granite here consists of three relatively small anticlines.  The few large basins in the province, such as the Illinois Basin, and Michigan Basin between Lake Michigan and Lake Huron, are not significantly deep depressions.  A second cause of the low relief is continental glaciation, which affected a considerable portion of the province.  Although the glaciers deepened the basins of the Great Lakes and gouged out many smaller areas that are now filled with lakes, the glaciers primarily  planed off high features within the province and deposited significant amounts of material in low places.  Glacial drift several hundred feet thick in valleys is common, and from central Ohio westward across Indiana and Illinois into Iowa drift averages at least 100 feet thick over much of the area and 200-300 feet over some buried valleys.  A thickness of 1,100 feet has been measured in northern Michigan.  Loess, much of which was transported by wind from the Great Plains, has also filled in many low places.    Central Lowlands Introduction | General Description | Physical Features | Great Lakes | More Features | Resources