dome, in geology, any large or elliptical structure formed by the fractureless upwarping of rock strata. It is a type of anticline that lacks clear-cut elongation and that slopes outward in all directions from the highest point. Typical examples of such a dome can be found in the Black Hills of South Dakota in the western United States. Where strata plunge more or less uniformly toward surrounding lowlands, erosion may produce a series of concentric ridges with their steep slopes facing inward toward the centre. In some areas domes in which a layer of relatively impermeable shale overlies a layer of permeable sandstone are structural traps for oil and natural gas. The oil and gas migrate upward, becoming trapped against the shale at the uppermost part of such anticlinal formations.