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 1530 feet thick, although considerably thinner outcrops are
recognizable due to the distinctive character of the volcanic rock.
Skilled quarry workers use simple
hammers and chisels to make
square building blocks from
the welded tuff.