The Ridge and Valley region of Alabama, where the Cahaba River begins, was formed when the African Plate collided with the North American Plate in the Paleozoic era. The valley soils consist of gravel, sand, and clay, while the ridges consist of chert and sandstone. The upper Cahaba region contains Cenozoic-era gravel, clay, and sand. It is filled with different finds from the Pennsylvanian or Carboniferous Age, often referred to as the Coal Age because of the large pure coal deposits found in the shale. They include tiny imprints of ferns to large pieces of Calamites, the extinct ancestors of modern horsetails that have a bamboo-like appearance.