Bonneterre Formation, Geological formation in St. Francois Mountains, Missouri, US.
The Bonneterre Formation is a geological layer in the St. Francois Mountains of southeastern Missouri made up mainly of dolomite and limestone. Sand and conglomerate deposits form the base of this sequence, creating a distinctive rock profile.
Geologists identified this formation in the early 1900s near Bonne Terre and classified it as late Cambrian in age. Earlier work by state geologists helped establish the understanding of regional rock sequences.
Research institutions and geology students conduct regular field studies at the formation to examine mineral compositions and fossil evidence.
This formation is found at depths ranging from about 375 to 400 meters near the surface and extends to 1580 meters beneath Pemiscot County. Visitors can explore the best exposures in the St. Francois Mountains where the layers are easily viewed.
The rocks contain fossils of late Cambrian invertebrate animals, including several trilobite species such as Coosella and Tricrepicephalus. These fossils provide insight into the ancient sea life that inhabited this region.
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