The igneous and volcanic rocks extend at depth under the relatively thin veneer of Paleozoic sedimentary rocks and form the basal crust of the entire region.A major unconformity in the region attests that the Ozarks was above sea level for several hundred million years from the time of the volcanism in the Precambrian until the mid-Cambrian with an erosionally produced relief of up to 1500 feet.By the early 20th century, the Ozarks had become a generic term.
The Saint Francois Mountains in the northeastern Ozarks are the eroded remnants of an ancient range which form the geological core of the highland dome.
The igneous and volcanic rocks of the Saint Francois Mountains are the exposed remains of a Proterozoic mountain range hundreds of millions of years old.
The remaining hills are the exposed portion of an extensive terrane (the Spavinaw terrane in part) of granitic and rhyolitic rocks dating from 1485 to 1350 mya that stretches from Ohio to western Oklahoma.
The core of the range existed as an island in the Paleozoic seas.
Drainage is primarily to the White River, with the exception of the Illinois River, although there also is considerable drainage from the south slopes of the Boston Mountains to the Arkansas River.
Major streams of this type include Lee Creek, Frog Bayou, Mulberry River, Spadra Creek, Big Piney Creek, Little Piney Creek, Illinois Bayou, Point Remove Creek, and Cadron Creek.
Although the Springfield formation's surface is primarily Mississippian limestone and chert, the Salem Plateau is older Ordovician dolostones, limestones, and sandstones.
Both are rife with karst topography and form long, flat plains.
This carbonate formation, the Bonneterre now mostly dolomite, is exposed around the St.