The topography and soils of the Grand Prairie are the result of its geological history. The fine-textured soils with high clay content are poorly drained in many places, resulting in extensive wet prairies and marshes. Trees are present along streams and in scattered groves, savanna, and upland forest on moraines and other glacial hills. Natural Divisions of Illinois Midewin NTP in Red Grand Prarie Division in Royal Blue Most of the Midewin National Tallgrass Prairie lies within the Grand Prairie Section of the Grand Prairie Division and shares the characteristic features described above. The southwestern corner of Midewin is in the Kankakee Sand Area Section which is covered by outwash sand deposited by pulses of glacial meltwater like the Kankakee Torrent.
Sandy soils support different vegetation and wildlife.
They drain rapidly and support vegetation better adapted to dry conditions. The probability of fire may be greater because the vegetation dries more rapidly, but on the other hand there is less fuel to feed a fire. This type is the most widespread in Illinois. It has deep, fine-textured soils formed in silt and clay, ranging from dry to wet. The kinds of plants vary based on the amount of moisture.