Rapid City is located on the eastern edge of the Black Hills, and has developed on each side of the Dakota Hogback. Rapid City's "Westside" is located in the Red Valley between the foothills of the Black Hills proper and the Dakota Hogback, so named for the red Spearfish formation soils and the way the valley completely encircles the Black Hills. Rapid City has expanded into the foothills, with developments having been built on both ridges and in valleys developed, especially in the last 20 years. This arid edge area has a higher risk of wildfire, as shown by the Westberry Trails fire in 1988.