Rhyolite – All that glitters is not gold…

About 37 million years ago, I was startled awake from a lengthy geologic nap. A huge explosion shook the rocks around me. Somewhere in the mountains about 137km southwest of Morrison, a volcano many times bigger than Mount Saint Helens had erupted, spewing a column of volcanic ash miles into the air.

A cloud of incandescent volcanic ash, hundreds of metres thick, rushed across the landscape destroying everything in its path. When it reached Castle Rock, about 40km south east of Morrison, the ash covered the ground. It was so hot that it welded into 10 metre thick rock that today is referred to as the Castle Rock Rhyolite.

You might think all that devastation led to no good. But you should know that rhyolite, which is Greek for ‘streaming rock’, was given that moniker because of its beautiful streaming bands of colour. Castle Rock Rhyolite lives up to its name.

The discovery of rhyolite is actually what put the town of Castle Rock on the map. During the 1870s, Silas W Madge came to Castle Rock searching for gold. He had a sample of rhyolite tested, and the assayer’s report said the rock contained no precious metals, but could be used as building stone.

Madge, realising that ‘all that glitters is not gold’, opened a quarry and began producing dimension stone (stone trimmed to specific dimensions) in 1872. Workers using hand tools drilled shot holes about six metres deep, filled them with black powder, and blasted the rock. The useable pieces were squared off by stone masons. The quarrymen were paid $2.50 a day for their labours.

The Castle Rock Rhyolite was shipped by rail to nearby cities including Denver, Colorado Springs and Pueblo, and was used in foundations as veneer and as decorative trim such as sills, lintels and archways. The rhyolite even made its way to more distant locations including Cheyenne, Kansas City and Omaha.

At the peak of production, during the 1880s and early 1890s, the Castle Rock Rhyolite was being cut, dressed and shipped from three Castle Rock area quarries. The Castle Rock Journal estimated that the quarries produced 1800 railcars of stone per year worth about $10 per railcar. Pretty good money for the 1890s.

Production decreased after the 1893 Silver Crash, but the rock continued to be quarried into the 20th century. Sadly, the Madge Quarry closed in 1902.

But the story doesn’t end there. The dimension stone industry has experienced resurgence, and a local aggregate producer is currently quarrying dimension stone from Castle Rock Rhyolite. Once again, buildings, ranging from discount department stores to upscale restaurants, are being covered with beautiful “streaming rock”. Not too shabby for a 37-million-year-old whippersnapper!

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