Read More About It - Tropical Upland Forest:
During the early Pennsylvanian, the plates carrying North America and Europe
were colliding with Gondwana, the supercontinent to the south. The force
of the collision caused Illinois to be shifted up and down. As the shallow
seas that had covered Illinois for almost 230 million years drained away,
They were replaced by lowland swamps. The swamps were periodically flooded
as sea levels continued to rise and fall.