About the St. Francois Mountains

Formed by volcanic and intrusive activity 1.5 billion years ago, the St. Francois Mountains are the oldest in North America. Made from granite, perthitic potassium, feldspar, biotite, and magnetite, they are ancient compared to the Rockies which were created 140 million years ago, and older even than the Appalachians, which began forming around 460 million years ago.

These ancient mountains may be the only in the American Midwest that were never submerged, existing as an island in the Paleozoic seas. Fossilized coral, and the remains of old reefs can be found at the flanks of the mountains.