The "Flatirons" of nearly vertically tilted layers of red rock front the green tree-covered foothills of the Rocky Mountains south of Boulder, Colorado. The slabs of red rock belong to the Pennsylvanian-age (and possibly earliest Permian-age) Fountain Formation, which was deposited on the slopes of a much more ancient mountain range, the Ancestral Rockies, roughly 320–290 million years ago.
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Jerry D. Harris 🦕👣
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