Saint Francois Mountains
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The Saint Francois Mountains, a range located in southeast Missouri, is an outcrop of Precambrian igneous rock mountains rising over the Ozark Plateau. This range is one of the oldest exposures of igneous rock in North America. The range is often misspelled as St. Francis to match the local pronunciation.
Formed through volcanic and intrusive activity over 1.4 billion (1.4 x 109) years ago, nothing is left of these mountains but the roots. By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 70 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.
Unlike the rest of the mountainous areas in the Ozarks, the Saint Francois Mountains were formed by true volcanic activity. The localized vertical relief observed in most of the Ozarks, a dissected plateau, was caused by erosion. The volcanic activity that formed this mountain range is also thought to be the geological cause of the uplift of the Ozark Plateau. Geologists talk of the "Ozark dome" wherein elevations and stratigraphic inclines generally radiate down from the Saint Francois mountains. These elevations may be the only area in the American Midwest never to have been submerged, existing as an island archipelago in the Paleozoic seas. Fossilized coral, the remains of ancient reefs, can be found among the rocks around the flanks of the mountains. These ancient reef complexes formed the localizing structures for the mineralizing fluids that resulted in the rich ore deposits of the area.
The St. Francois Mountains are the center of the Missouri mining region yielding; iron, lead, barite, zinc, silver, manganese, cobalt, and nickel ores as well as granite and limestone quarries. Historic Mine La Motte near Fredericktown was the site of lead mining activity by the French as early as 1720. An old granite quarry lies on the edge of Elephant Rocks State Park which is a spectacular outcropping of huge weathered granite boulders.
Mountains in this range include; Taum Sauk Mountain, Bell Mountain, Proffit Mountain, Pilot Knob Mountain, Hughes Mountain, Goggin Mountain, and Lead Hill Mountain. The elevations range from 500 feet to 1,772 feet (167 to 540 meters). Taum Sauk Mountain is the highest peak in the range, and the highest point in the state, with an elevation of 1772 ft. (540 m).
Hughes Mountain contains a good example of columnar jointing in igneous rhyolite, the same process that formed Devils Tower. The columnar jointing in this area is called the Devil's Honeycomb.
A part of the Ozark Trail winds through parts of the St. Francois Mountains, including a popular section that crosses Taum Sauk and Proffit mountains.
See also
References
Unklesbay, A.G; & Vineyard, Jerry D. (1992). Missouri Geology — Three Billion Years of Volcanoes, Seas, Sediments, and Erosion. University of Missouri Press. ISBN 0-8262-0836-3.