Baned iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed around three thousand million years before present (3 Ga), include banded iron layers, and the banded layers are a common feature in sediments for much of the Earth's early history. Banded iron beds are less common after 1.8 Ga, although some are known that are much younger.