Illinois State Museum

Pennsylvanian Period graphic

Upland Tropical Forest: Climate and Geology

The climate was generally warm and wet, typical of tropical regions close to the equator. Periods of heavy rain alternated with dryer periods. Rain forest covered the tropical regions. To the north and south in the areas of present-day Hudson Bay and Florida, arid deserts bordered the rain forest. In the area of the South Pole, ice caps expanded and contracted, causing sea levels to rise and fall as water was alternately locked into, or released from, the ice. In the balance, Earth's climate was cooling down.

The clear seas that had covered Illinois during the Mississippian Period gradually turned muddy. They became shallow and retreated as tectonic forces slowly uplifted the area during the Pennsylvanian Period. Land was higher to the northeast, and rivers ran into Illinois from southeastern Canada. River deltas formed, shifted back and forth, and were flooded as sea levels rose and fell.