The Bonneterre Formation is an Upper Cambrian geologic unit exposed in the St. Francois Mountains and widespread beneath much of Missouri. Composed mainly of dolomite with some limestone, it varies from about 375 to 400 feet thick in outcrop to as much as 1,580 feet in the Missouri Bootheel. It is especially important as a major host rock for the lead ores of the Missouri Lead Belt. Lying above the Lamotte Sandstone or older igneous rocks and below the Davis Formation, it records sedimentation around Cambrian islands and reeflike shallow marine settings. Its fossils include stromatolites, echinoderms, microfossils, trilobites, and monoplacophorans.