A geologic phenomenon known as a glowing avalanche formed Castle Rock and the other buttes of Douglas County 36.7 million years ago, when a volcanic eruption near Mount Princeton, about 95 miles southwest of here, sent a frothy, gleaming cloud of lava across the landscape. When the liquid rock hit the ground, it hardened into a glassy-textured layer 15 to 30 feet thick now known as Castle Rock rhyolite. Over time, wind and water scoured the valley and eroded the lava flow, leaving the rock-capped mesas seen from Sedalia to Greenland on either side of East Plum Creek. These remnants supported a major Douglas County industry, rock quarrying, from the 1870s onward. In 1872 Silas Madge removed gray, buff, and pink stone from a prominent mesa on his ranch and sent it to mineral experts in Denver, who found it to be weather resistant, exceptionally strong for its weight, and easily cut. Madge opened a profitable but backbreaking quarry just south of Castle Rock, and when two other quarries opened nearby, the town and county also benefited. Builders used the rhyolite for prominent commercial and residential buildings in Denver, Colorado Springs, and Pueblo, and as far away as Kansas City, Cheyenne, and Omaha. Within Douglas County, rhyolite was used for the old Douglas County Courthouse, railroad depots, schools, churches, businesses, and houses. In the first decade of the 1900s, concrete replaced stone as the region's preferred building material and the quarries' heyday ended, although the rock is still quarried for road and construction projects.