The Carboniferous is divided into two sections: the Lower (Mississippian) and the Upper (Pennsylvanian). During the Upper Carboniferous about 306 million years ago coal swamps covered the land. The environment at this time was terrestrial and marine. Fauna, particularly four legged vertebrates evolved near the coal swamps. With the transgression and regression of the seas caused by glaciation, the late Paleozoic sedimentation occurred in cycles. These cycles are called cyclothems, a sedimentary cycle associated with landward and seaward oscillation. The significance of the cyclic sedimentation is that it happened in rapid succession, thus not allowing for plant remains to decay, but to help produce coal.

In level swampy land or shallow water these environmental conditions with the vast amount of plant material provided by the extensive coal forests, allowed for the production of coal. As each generation came and went their remains were preserved against decay. The plant debris (peat) developed and accumulated to become many feet thick. Clays, sand, or lime mud (which contained seashells) were deposited on top of the peat. The weight of the sediments caused the peat to become more dense and eventually converted to coal. This cycle was repeated several times because the land reemerged and new layers of peat were formed. Plant material did not decay when the sea covered them and pressure and heat eventually built up over the millions of years to transform the plant material to coal.

Carboniferous Forest: The Carboniferous Period if famous for it vast coal swamps, such as the one depicted here. Such swamps produced the coal from which the term "Carboniferous", or "carbon-bearing" comes. (see web site below)