The surface rocks of the southern Ozarks are of Ordovician, Silurian, Devonian, and Carboniferous ages. Those of Silurian age are absent over a large portion of the region, as a result of unconformities, and those of Carboniferous age are locally absent, as a result of erosion during the present erosion cycle.
In southern Missouri and northern Arkansas the streams have in many places cut far below the Carboniferous rocks, leaving the older rocks exposed on the hillsides. The rocks of Devonian age consist of a thin stratum of sandstone, or of sandstone and shale, and as those of Silurian age are usually absent, practically all the rocks exposed below those of Carboniferous age belong to the Ordovician.
Near the Arkansas-Missouri line the Ordovician rocks exposed along the sides of the rather deep ravines consist of manganesian limestone containing several beds that are more or less . . .