Yesterday was a perfect day for hiking. In the afternoon I ventured south to the Castlewood Canyon State Park. Here is a glamour shot of the canyon from the Geological Society of America.
The park's geology is complex. Here is a brief description from the Castlewood Canyon State Park Geology pamphlet:
Millions of years ago, a tropical rainforest covered what is now Castlewood Canyon State Park. How do we know? Because plant and animal fossils from those tropical forests have been found in the oldest visible rocks in the park, called Dawson Arkose.
Although the rocks above the Dawson Arkose cannot speak, they tell the story of a tremendous volcanic eruption that occurred precisely 36.7 million years ago. The eruption, which happened about 90 miles away near present-day Salida, filled the air with a glowing cloud of 2,000 degree molten rock, ash and poisonous gases. It reached the area of the park in just a few minutes. The liquid rock and superheated ash welded into a thick layer of solid rock as they hit the ground cooled, and were buried. This rock has several names: ignimbrite (Latin for “fiery cloud”, Wall Mountain Tuff (named for the mountain northeast of Salida where it was first discovered), and rhyolite. You can find pieces of this once liquid rock laying all over the park. Look for rocks with sharp angles and edges, tiny air holes, and shiny specs. It can be pink, purple, gray or brown in color.
The “icing” on the park’s rock layer cake of Dawson Arkose and rhyolite, and its most distinguishing geologic feature, is Castle Rock Conglomerate. These 34 million-year-old rocks, washed down from the eroding Rocky Mountains, form the park’s canyon walls and caprock. Conglomerate rocks are easy to identify – they’re like cookie dough with bits of chocolate chips sticking out. The “dough” is sedimentary rock and the chips are pebbles and boulders that are smoothed and rounded in ancient rivers and cemented into the rock by the water’s high concentration of silicates – nature’s concrete.
I saw a layer of purple rhyolite that looked something like this.