Castle Rock has been a landmark for millenia, long before the town of Castle Rock began. How has the rock lasted so long when most sandstone erodes quickly? The answer was recently uncovered by research associates from the Denver Museum of Nature and Science and published in the scientific journal The Mountain Geologist. They found that the cement between the grains of sand that make up the sandstone contains microscopic amounts of opal, the precious gemstone often used in jewelry. Opal was discovered throughout the Castle Rock conglomerate in 2023.
Image credit: Denver Museum of Nature and Science
Opal is a form of cryptocrystalline quartz called chalcedony, a mineral species which also includes agate. The result is sedimentary rock that is harder than concrete, the reason that the butte and mesa structures around Castle Rock remain today despite abrasive Colorado weather.
36 million years ago, volcanic activity near Mt. Princeton spread ash and debris across the state. The deposited silica slowly dissoved and percolated through cracks in the existing conglomerates, leaving the opal behind. These silica-rich rocks are known as the "Wall Mountain Tuff." Over millions of years, most of the area surrounding Castle Rock has collapsed, leaving only buttes and mesas behind.
This Earthcache is located on the trail around the bottom of the capstone of Castle Rock. There is no need to climb to the top.
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To get credit for this cache, include answers to the following questions in a message to the cache owner. Do not put the answers in your log.
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Examine the capstone from the listed coordinates. You will see two distinct layers, one at trail level and another above that. Which layer is the one that contains opal?
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How do the two layers differ in terms of color, hardness, and texture?
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Which of the layers is more prone to erosion and what do you think will happen in the future as this layer continues to erode?
In your log, please include a photo of yourself or a personal item at the listed coordinates.