While relatively low in terms of elevation at compared to , Taum Sauk and the St. Francois range are true mountains, being the result of a volcanic orogeny. Whereas vertical relief in the rest of the Ozarks region is the result of erosion of sedimentary strata, the St. Francois are an ancient Precambrian igneous uplift several times older than the Appalachians. Geologists believe that Taum Sauk and its neighbors may be among the few areas in the US never to have been submerged in ancient seas. The peaks of the St. Francois range existed as islands in the shallow seaway throughout most of the Paleozoic Era as the sandstones, limestones, and shales typical of the Ozarks were deposited. Weathering and erosion of these ancient peaks provided the clastic sediments of the surrounding rock layers.