The Central Lowlands of the Midwest and Great Plains farther to the west are composed of sedimentary rock from material eroded from the Appalachian Mountains, the Rocky Mountains, and ancient upland areas to the north and west of the Great Lakes. As with the coastal plains, the Midwest and Great Plains were once covered by a vast inland extension of the Gulf of Mexico. The Central Lowlands and Great Plains are older, higher in elevation, and consist of harder sedimentary rock than along the coastal plains. Waterborne erosion has carved some gently rolling hill areas in the Midwest and Great Plains. In the north, continental glaciation from the ice ages has influenced the surface physiography.
Winter temperatures are cooler here than in the South, but summer temperatures can still be very warm. This difference is due to the greater degree of continentality experienced in the central part of the continent. The rainy season is in the summer, when moist tropical air moves into the region from the Gulf of Mexico. This moisture is very predictable for the Central Lowlands, which relies on it for intensive agriculture and animal husbandry. Summer rains are much less predictable for the Great Plains, where periodic drought is common. The Central Lowlands receive an average of over 30 inches of precipitation a year, while the Great Plains receives only 20 inches. Precipitation amounts under 20 inches (which is common west of 100 degrees W. Longitude) are considered drought conditions.
Soils on the Central Lowlands are primarily Alfisols, which are produced under the mixed deciduous and evergreen forests that used to cover Ohio and large portions of Indiana and Illinois. The soils of the Great Plains is Mollisols, which develop under grasslands. Mollisols are one of the richest soils and are well suited for nutrient demanding wheat. Alfisols are less nutrient rich but do well with corn and soybeans, the two crops which dominate the Central Lowlands.
Interior Low Plateaus
The eastern transition belt between the Appalachian Mountains and the Central Lowlands is sometimes called the interior low plateaus. It is the result of slight folds which dip gently to the west. It is, for the most part, sandstone underlain by limestone creating a karst topography of limestone caves, such as Mammoth Cave in Kentucky. When the sandstone cover is removed by erosion, limestone basins are created, such as the Blue Grass Basin of Kentucky and the Nashville Basin of Tennessee. Both have rich soils and good farm lands.
The Great Lakes
The Great Lakes section of the Central Lowlands is a true structural depression consisting of a large, bow-like feature with numerous layers of different rock. Continental glaciers carved out the weaker layers of rock in a circular pattern (centered on central Michigan). Glacial action has also affected the river drainage pattern of the Great Lakes and upper Mississippi, Ohio, and Missouri Rivers.
The Great Lakes have a significant impact on the regional climate of the upper Midwest. Winds that cross the lakes pick up moisture, which increases precipitation (including snowfall) on the eastern shores of each lake. The lakes also moderate air temperatures (as the oceans do), creating longer growing seasons on the eastern lake shores. The Niagara Peninsula, north of Lake Erie and surrounded by three of the Great Lakes, is the major fruit-growing region in Canada.
The Coal Age
During the Pennsylvanian (or Carboniferous) Period, 300 million years ago, the Interior Lowlands of the Midwest were covered with vast swamps, shallow seas, and thick, tropical-like vegetation. Dead plants in the swamps were covered by river-borne sediments. Under pressure and over time, this organic material was turned into coal, oil, and natural gas. This period in the earth's history is known as the "Carboniferous Period" and is the main source of the fossil fuel deposits throughout the world. It resulted in rich coal deposits under Missouri and in the western plateaus of the Appalachian Mountains, as well as oil and natural gas in Texas, Oklahoma, and Arkansas.
Settlement Impacts of Midwest Flatness
The entire Midwest and most of the Great Plains are drained by the Mississippi River and its tributaries, including the Missouri and Ohio Rivers. Good soils, flat land, and the navigable rivers made the Midwest an easy region to cross and settle in. Large grain shipments, coal, and iron ore (on the Canadian Shield) could also be transported easily on these waterways. These attributes contributed to making the Midwest both the agricultural and the industrial core of the US and Canada.