A plug-shaped body of salt that has intruded into a sedimentary rock overlay, a salt dome is formed when salt, which is relatively buoyant, is forced upward through a fault in overlying sedimentary strata by subterranean pressure. A typical salt dome is roughly circular in shape and has a one- to two-mile diameter. There are over ﬁve hundred known salt domes in the Gulf Coast region alone, stretching from Mexico to the Florida Panhandle. The tops of most domes are commercially mined; they are an excellent source of nearly pure salt and often have oil and gas reserves along the ﬂanks. In the West, subsequent erosion of the upthrust layers of a salt dome can produce dramatic fracturing along its edges, ultimately resulting in steep hogbacks.