Text Box: Ozark Plateaus Introduction | Physical Features | Resources                                                       The Ozark Plateaus province covers most of the southern half of Missouri and extends into northwestern Arkansas and slightly into northeastern Oklahoma; a very thin section of southwestern Illinois is also included.   The total area of the province is approximately 50,000 square miles.  The province is bounded on the north mostly by the Missouri River, on the east by the Mississippi River Basin and the Black River, and on the south by the Arkansas River Valley.  The boundary on the west is less distinctive, coinciding with part of the Neosho River and the flat land of the Central Lowlands province.  The Ozark Plateaus province and the Ouachita province to the south sometimes are referred to collectively as the Interior Highlands because this is the only extensive significantly elevated region between the Appalachians and the Rocky Mountains (excluding the relatively small Black Hills in the Great Plains).  However, the two provinces are distinctly different enough to be treated separately.    Formation and General Structure   Many rock types and structures in the Ozark Plateaus province, as with the Ouachita province, are a westward continuation of Appalachian geology; much of the Ozark Plateaus province resembles parts of  the Appalachian Plateaus province.  The event mostly responsible for raising the Ozark Plateaus was a westward extension of the final orogeny–the Allegheny orogeny–that raised the Appalachians.  This orogeny uplifted much of the Ozark Plateaus into a broad, relatively gentle asymmetrical dome.  The apex of this Ozark Dome is in the northeastern corner of the province at the St. Francois Mountains.  This asymmetry has produced very gentle slopes west of these mountains and steeper slopes to the east.  Although portions of this province are mountainous, the rugged nature of these areas is due more to erosional dissection, as there was generally only mild folding and faulting in the province during the orogeny.  The main mountains are the St. Francois Mountains and the Boston Mountains.  The term Ozark Mountains is sometimes used as a general name for any mountainous area in the province, and the term Knobs is used locally to refer to some of these more rugged areas.  Limestones and dolomites are the primary rock types in much of the province.  Exceptions occur in the St. Francois Mountains where igneous rocks, especially granite, are most common, and in the Boston Mountains where sandstones and shales are dominant.  Chert forms a layer over much of the central portion of the province, especially in Missouri.  Originally in the limestone and dolomite, the relatively insoluble chert has weathered out to form a coarse residual layer, in places accumulating to as much as 150 feet thick.  Some river valleys are essentially choked with chert, which has altered drainage patterns.  General drainage on the Ozark Dome is radial.  Rivers on the northern flank of the dome flow toward the Missouri River, those on the east toward the Mississippi River, and those flowing southward eventually enter the Arkansas River; rivers on the western flank mostly empty into the Neosho River.  Several rivers meander significantly, and some are notably incised.  The Ozark Dome has been more deeply dissected than the nearby Nashville Dome to the east.  Many deep and steep-sided valleys punctuate the province, especially on the southern flank of the dome.   Fluvial erosion has been active enough to have stripped off several hundred feet of rock strata.  Rock layers of Pennsylvanian age probably extended across the dome at one time before being eroded away nearly completely.     Ozark Plateaus Introduction | Physical Features | Resources