The fifth period of the Paleozoic Era. The Carboniferous Period spanned from about 355 million years to about 295 million years ago. The rocks that formed during this time interval are known as the Carboniferous System; they include a wide variety of sedimentary, igneous, and metamorphic rocks. Sedimentary rocks in the lower portion of the Carboniferous are typically carbonates, such as limestones and dolostones, and locally some evaporites. The upper portions of the system are usually composed of cyclically repeated successions of sandstones, coals, shales, and thin limestones.