Concerning the Subcarboniferous, or Mississippian Series in Part I., Vol.  IV., Missouri Geological Survey, Dr. C.R.  Keyes says:  “In the great interior basin of the Mississippi the basal series is exposed more or less continuously over broad areas, extending from northern Iowa to Alabama, and from Ohio to Mexico.”

While this broadly extended series of limestone is honey-combed in many places and all directions by wonderful caverns, those of the Ozark regions in Missouri, although comparatively little known, are well worth knowing, and are possibly the most ancient limestone caves in the world.  Of the region in which they occur, Dr. Keyes, in the volume last quoted, says:  “The chief typographical feature of the state has long been known in the Ozark uplift, a broad plateau with gentle quaquaversal slopes rising to a height of more than one thousand five hundred feet above mean tide, and extending almost entirely across the southern part of the district.  On all sides the borders of this highland area are deeply grooved by numberless streams flowing in narrow gorges.  Against its nucleus of very ancient granites and porphyries the Ozark series of magnesian limestone was laid down.  Then the area occupied by these rocks was elevated, and around its margins were deposited successively the other members of the Paleozoic.  The Ozark region was thus the first land to appear within the borders of the present state of Missouri.”  He further says:  “Although it has long been known that the Magnesian Limestones are older than the Trenton, and that they lie immediately upon and against the Archaean crystallines unconformably, their exact geological age has always remained unsettled.  There seems to be but little doubt, however, that part of the series is equivalent to the Calciferous of other regions.  It is also pretty well determined that certain of the lower beds, all below the ‘Saccharoidal’ Sandstone perhaps, are representatives of the Upper Cambrian or Potsdam.  These conclusions appear well grounded both upon stratigraphical and faunal evidence.  The rocks of the Ozark region have not as yet received the necessary detailed study to enable the several lines of demarkation to be drawn with certainty.  This investigation is now being carried on as rapidly as possible, and promises very satisfactory and interesting results in the near future.”

“The early geological reports represent the Magnesian Limestone series as made up of seven members.  Following Swallow, these may be briefly described in the present connection.  Beginning at the top, they are: 

First Magnesian Limestone. 
First, or Saccharoidal Sandstone. 
Second Magnesian Limestone. 
Second Sandstone. 
Third Magnesian Limestone. 
Third Sandstone. 
Fourth Limestone.”

“The Fourth” Magnesian Limestone, or lowest number of the Ozark series recognized, has its typical exposures along the Niangua and Osage rivers in Morgan and Camden counties.