Shale gas is natural gas produced from shale. Shale gas is natural gas that is found trapped within shale formations. Shale oil is extracted by pyrolysis, hydrogenation, or thermal dissolution of oil shale. American petroleum refining largely grew out of oil shale refining. The rock types include marble, chert, limestone, shale, and sandstone. It is very common in sedimentary rocks such as sandstone and shale. There are also deposits of peat, bitumen, shale and oil. The sandstone has alternating layers of shale and marl-pebble conglomerate. It consists mainly of silty brown shale and fine-grained sandstone and siltstone. The Paleozoic sediment of shale and sandstone folded and faulted. The underlying surface consists of sandstone and shale. Chennai's soil is mostly clay, shale and sandstone. The region contains large deposits of oil shale. The Canadian Rockies are composed of shale and limestone. The western valley is floored with more recent shale and sandstone.