As the North American landscape became drier (and warmer) during the Carboniferous period (355 - 300 Million years ago), land plants invaded the soggy soil.

Huge swamps were dominated by giant sphenopsids (horsetails or scouring rushes such as these calamites), tree ferns, and leafy conifers.

Remains of these plants were buried in sediment, eventually forming deposits of coal in Iowa, Missouri, Kansas.