Illinois State Museum


Central Prairie


The Grand Prairie in 1673 had a temperate climate. The prairie's average high temperature was 17º C (62º F), and average low was 6º C (42º F). There were 170–180 frost-free days annually, and 94 centimeters (about 37 in) of average annual precipitation.


Pleistocene glaciers left behind a flat or gently rolling terrain. Grand Prairie soils are relatively young and highly organic, originating from loess, glacial till, glacial outwash, and lakebed sediments. Bedrock is deeply buried by glacial sediments. It is exposed along the larger rivers.