In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as greywackes, mudstones, volcanic sediments, and banded iron formations. Greenstone belts are typical Archean formations: they are made of alternating high and low-grade metamorphic rocks. The high-grade rocks come from volcanic island arcs, and the low-grade metamorphic rocks come from deep-sea sediments. These sediments eroded from the island arcs and ended in a forearc basin. In short, "greenstone belts show where protocontinents were stuck together".