Northern Plains

Northern Great Plains

The northern plains are interrupted by several small mountain areas. The Black Hills, chiefly in western South Dakota, are the largest group. They rise like a large island from the sea, occupying an oval area of about 100 mi (160 km) north-south by 50 mi (80 km) east-west. At Black Elk Peak, they reach an altitude of 7,216 ft (2,199 m) and have an effective relief over the plains of 2,000 or 3,000 ft (610 or 910 m) This mountain mass is of flat-arched, dome-like structure, now well dissected by radiating consequent streams. The weaker uppermost strata have been eroded down to the level of the plains where their upturned edges are evenly truncated. The next following harder strata have been sufficiently eroded to disclose the core of underlying igneous and metamorphic crystalline rocks in about half of the domed area.