The giant bird's foot delta, featuring a large middle toe, that terminates the lower Mississippi River in southern Louisiana marks the seaward growth of land into the Gulf of Mexico. The Mississippi-Missouri River system collects eroded debris from the entire central half of the U.S. Upon reaching the Gulf the river's velocity slows, abruptly reducing its capacity to carry suspended mud and sand, and the sediment is deposited in vast alluvial fans not visible on the map. These submarine fans are major sources of petroleum, natural gas, and sulfur.