Endangered sea turtles nest on white sand beaches. The quaternary sediments consist of clay, silt, and several grades of sand and gravel. This is especially important in areas such as sand dunes. Beach nourishment often involves excavation of sediments from riverbeds or sand quarries. There are about fifteen streams that have watersheds with accumulation of sand and gravel. There are also rocky beaches along with those with fine sand. The Crati transports coarse sand and pebbles in its channel. This beach is very wide with plenty of sand. Magnetite is sometimes found in large quantities in beach sand. They lie under of waterlogged muskeg, of clay and barren sand. Modern stucco is made of Portland cement, sand, and water. Traditional stucco is made of lime, sand, and water. Fluctuating sea levels compressed numerous layers of calcium carbonate, sand, and shells. Concrete is made from aggregate, sand and cement. There are several beaches of dark or "black" sand.