The sedimentary layers underneath Houston ultimately extend down some , with the oldest beds deposited during the Cretaceous. Between and below the surface is a layer of salt, the primary source of salt domes which dot the metropolitan area. Since salt is more buoyant than other sediments, it rises to the surface, creating domes and anticlines and causing subsidence due to its removal from its original strata. These structures manage to capture oil and gas as it percolates through the subsurface; Pierce Junction is a notable example of a heavily drilled salt dome oil field in Houston.