Lepidodendron and Stigmaria are two dominant fossils of this area. Lepidodendron, one of the dominant tree-like plants of either the Carboniferous Period or the late Pennsylvanian Period, grew to heights of over 120 feet, taller than Cedar Falls, and is identified by the scale-like patterns on its outer skin. Stigmaria is the root system of Lepidodendron. These tree-like plants grew in swampy areas. When the sediment here was deposited, Petit Jean Mountain was neither a mountain nor an ocean basin. The sandstone was deposited in fluvial channels on a deltaic plain similar to the Mississippi Delta, where swamps and marshes formed.