Saint Francois Mountains

Saint Francois Mountains
A view towards the St. Francois range from Knob Lick Mountain, a granite and rhyolite knob on the edge of the range
Highest point
Peak	Taum Sauk Mountain
Elevation	1,772 ft (540 m)
Coordinates	37°34′13″N 90°43′40″W / 37.57028°N 90.72778°W / 37.57028; -90.72778Coordinates: 37°34′13″N 90°43′40″W / 37.57028°N 90.72778°W / 37.57028; -90.72778
Geography
A relief map of Missouri showing the St. Francois Mountain region (red)
Country	United States
State	Missouri
Geology
Age of rock	Precambrian
Type of rock	igneous

The St. Francois Mountains in southeast Missouri are a range of Precambrian igneous mountains rising over the Ozark Plateau. This range is one of the oldest exposures of igneous rock in North America. The name of the range is spelled out as Saint Francois Mountains in official GNIS sources, but it is sometimes misspelled in use as St. Francis Mountains to match the anglicized pronunciation of both the range and St. Francois County.

The name of the range derives from the St. Francis River, which originates in the St. Francois Mountains. The origin of the river's name, which also was originally spelled "Francois" in the French manner, is unclear. The area, as part of the Louisiana district of New France, is near some of the earliest French settlements in Missouri where many French place names survive. Some sources conjecture that the name honors St. Francis of Assisi, the patron saint of the Franciscan order, but none of the region's early explorers were Franciscans. Others propose that Jacques Marquette, a French Jesuit, named the river when he explored its mouth in present-day Arkansas in 1673. Before his voyage down the Mississippi Marquette had spent some time at the mission of St. Francois Xavier, named for the Jesuit missionary Francis Xavier. The spelling of the river's name shifted from "Francois" to "Francis" in the early 20th century.

The St. Francois Mountains were formed by volcanic and intrusive activity 1.485 billion (1.485 x 10⁹) years ago. By comparison, the Appalachians started forming about 460 million years ago, and the Rockies a mere 140 million years ago. When the Appalachians started forming, the St. Francois range was already twice as old as the Appalachians are today.