After leaving Douglas County and the town of Sedalia behind, BNSF 5735 (ES44AC) runs through Castle Rock, Colorado, which gets its name from the unique rock outcrop that lies in the center of the City. The butte's caprock consists of rhyolite, rock which is strongly resistant to erosion. About 37 million years ago, a volcanic eruption took place that covered the area around Castle Rock with 20 feet of rhyolite. The rock formation was used by Native Americans, explorers and early settlers as a lookout and reference point during their travels. The butte looked to many travelers like a castle on a hill. The name is credited to Dr. Edwin James, a botanist on Long’s Expedition of 1820.