Castle Rock Rhyolite
Skilled quarry workers use simple hammers and chisels to make square building blocks from the welded tuff.

Castle Rock Rhyolite is a volcanic rock that erupted from the Mount Princeton area about 36.7 million years ago. Its elongated pumice fragments and glassy textures indicate that this lava accumulated as a welded flow of frothy, incandescent material. These types of eruptions are known as "glowing avalanches" because the volcanic material crosses large areas extremely rapidly, and they cause more destruction than typical eruptions. The Castle Rock Rhyolite is generally about 15–30 feet thick, although considerably thinner outcrops are recognizable due to the distinctive character of the volcanic rock.
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