A geologic phenomenon known as a "glowing avalanche" formed Castle Rock and the other buttes of Douglas County 36.7 million years ago. A volcanic eruption near Mount Princeton, about 95 miles southwest of here, spewed a frothy, gleaming cloud of lava across the landscape. As soon as the liquid rock hit the ground it hardened into a glassy-textured layer some 15 to 30 feet thick, known today as Castle Rock rhyolite. Throughout time, powerful wind and water forces scoured this valley and eroded the lava flow, leaving behind the rack-capped mesas seen today from Sedalia to Greenland on either side of East Plum Creek. These remnants supported a major industry in Douglas County-rock quarrying-from the 1870s onward.

Gray Gold
In 1872 Silas Madge pried loose some of the gray, buff, and pink rocks from a prominent mesa on his ranch and sent them to mineral experts in Denver. The stone proved to be a superlative building material-weather resistant, exceptionally strong for its weight, and easily cut. Madge capitalized on the discovery with a profitable but backbreaking quarry just south of Castle Rock. When two other quarries opened nearby, the town and county profited from the industry. Builders used the rock for prominent commercial and residential buildings in Denver, Colorado Springs, and Pueblo, and as far away was Kansas City, Cheyenne, and Omaha. Closer to home, rhyolite graced the old Douglas County Courthouse, railroad depots, schools, churches, businesses, and houses throughout the county. In the first decade of the 1900s, concrete surpassed stone as the preferred building material in the region and the heyday for the quarries ended, although the rock is still quarried for road and construction projects.

Captions:
The Rockies Explode by Jan Vrieson. An artist's rendering of a cloud of superheated ash spewing toward Castle Rock from a distant, prehistoric volcanic eruption. All rights reserved, Image Archive, Denver Museum of Nature and Science.In the early decades of the twentieth century, motorists enjoyed touring Perry park Ranch to view the dramatic hogback formations.
Photo courtesy Denver Public Library.
Colorado Historical Society

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