Abstract

Castlewood Canyon is one of the most distinctive landforms on the Colorado plains—a geomorphology that developed as Cherry Creek and its precursors incised into the Eocene Wall Mountain Tuff and overlying Castle Rock Conglomerate (CRC). Outcrops of the CRC in Castlewood Canyon State Park (CCSP) contain boulders of the Wall Mountain Tuff that are up to two meters in diameter, and the conglomerate itself is composed of large (up to 0.5 m), diverse clasts of Precambrian granite, gneiss, quartzite, and other lithologies eroded from the Colorado Front Range that is 25 km to the west and as much as 100 kilometers to the northwest. These clasts and other evidence suggest transport and deposition by a sequence of flood events. Such flooding events, albeit smaller in scale, continue to occur in modern times, including a catastrophic flood caused by the failure of the Castlewood Dam in 1933, and a canyon-scouring event in 2023. These events and the geologic history of this canyon are described in this paper, illustrating that nature, mild though it may be for millennia, is still shaping the Castlewood Canyon system.