Home | Geology & Ecology | Rivers, Wetlands, Precipitation & Aquifers | Wildlife - Terrestrial | Wildlife - Aquatic | Wildlife & Nature Photos of the Ouachita Region | Rare & Endangered Species | Herpetofaunal Biogeography of the Ouachita Region | A Photographic Atlas of the Herpetofauna of the Ouachita Region | BIG Trees of Lake Ouachita! | I really "Lichen" them! | Contact, More Information, Acknowledgments & Guestbook | Links

Geology & Ecology

Example of folding & faults, Ouachita Mt. orogeny.
Photo - Blakely Mt. Dam, Lake Ouachita, AR.

  The geology and ecology of the Ouachita Mountain ecoregion is very unique, interesting and complex.  The geological processes that built these mountains began approximately 300 million years ago during the Paleozoic era in the Pennsylvanian period.  The enormous force that uplifted and built the Ouachita Mountains was caused by the South American plate drifting northward and slamming into the denser North American plate, which resulted in fold-mountains being formed.  This mountain building process is called the Ouachita orogeny. 

  The current Ouachita Mountain ecoregion is a smaller physiographic section of the larger Ouachita province that stretches southeast to the Black Warrior basin in Alabama and southwest to nearly the Big Bend region of Texas.  The southeastern and southwestern areas of the Ouachita region were later buried beneath Mesozoic and Cenozoic sediments leaving the current region today stretching from Arkansas into Oklahoma.  The current Ouachita physiographic province includes both the Ouachita Mountains and the Arkansas Valley.  Although the Ouachita mountains parallel the Ozark Mountains they are not part of them and they were created by different processes at different periods in the earth’s history, and are comprised of different materials.  

  The Ouachita Mountains are unique in that they run from east to west, rather than north to south like most mountain ranges in the United States.  The current Ouachita Mountain region covers 47,523 mi² and stretches approximately 225 miles from east to west and 60 miles from north to south.  The highest peak in the Ouachitas and Arkansas is Mount Magazine at 2,753 ft.  In the geologic past the Ouachitas reached heights of 10,000 ft. before mountain building processes ended and erosion started wearing them away over millions of years.   


Map of the Ecoregions of Arkansas.

Map of the Ecoregions of Oklahoma

                      Specific Ecoregion Information of the Ouachita Mountains

                                                of Arkansas & Oklahoma:

The Ouachita Mt. Ecoregion in Arkansas.

The Ouachita Mt. Ecoregion in Oklahoma.


Ecoregion 36 Information. Ouachita Mountains (Arkansas & Oklahoma)

Arkansas Information

The Ouachitas are made up of ridges, hills, and valleys formed by the erosion of folded and faulted Paleozoic sandstone, shale, and chert, known locally as novaculite.  They are a continuation of the Appalachians, formed during the late Paleozoic Era when an ocean closed and continents collided, causing marine sediments to be folded, faulted, and thrust northward.  The Ouachitas are structurally different from the Boston Mountains (38), more folded and rugged than the lithologically distinct Ozark Highlands (39), and physiographically unlike the Arkansas Valley (37), South Central Plains (35), and Mississippi Alluvial Plain (73).  

  Potential natural vegetation is oak–hickory–pine forest; it contrasts with the oak–hickory forest that dominates Ecoregion 39 and the northern part of Ecoregion 38. Today, loblolly pine and shortleaf pine grow in a distinctive mix of thermic Ultisols and Inceptisols. Logging and recreation are major land uses, and pastureland and hayland are found in broader valleys.

  Regional water quality is influenced by lithology, soil composition, and land use activities.  In most reaches, water quality is exceptional; typically, total phosphorus, turbidity, total suspended solids, and biological oxygen demand values are lower whereas dissolved oxygen levels are higher than in Ecoregions 35, 37, and 73.  Water hardness varies by level IV ecoregion; Ecoregions 36d and 36e tend to have the lowest hardness values while progressively higher values occur in Ecoregions 36a, 36b, and 36c.  Stream substrates are made up of gravel, cobbles, boulders, or bedrock; they contrast with the fine-grained substrates of lower gradient streams in Ecoregions 35 and 73.  The fish community is dominated by sensitive species; minnows and sunfish along with darters and bass are common.

: The low ridges and hills of the Athens Plateau are widely underlain by shale in contrast to other parts of Ecoregion 36.  Rocks are less resistant to erosion than in higher, more rugged Ecoregions 36b, 36d, and 36e but are more resistant than the unconsolidated rocks of the coastal plain in Ecoregion 35.  

  Today, pine plantations are widespread; they are far more extensive than in the more rugged parts of Ecoregion 36 in Arkansas.  Pastureland and hayland also occur.  Cattle and broiler chickens are important farm products.  

  Water quality values are distinct from Ecoregion 36c.

: The Central Mountain Ranges are dominated by east-west trending ridges that are characteristically steep and rugged and underlain by resistant sandstone and novaculite (chert).  Igneous intrusions occur along with associated hot springs.  Rock outcrops, and shallow, stony soils are widespread.  Novaculite glades occur.  

  Potential natural vegetation is oak–hickory–pine forest.  Perennial springs and seeps are common and support diverse vegetation.  Constricted valleys between ridges have waterfalls and rapids.  The surface waters of Ecoregion 36b have very low nutrient, mineral, and biochemical water quality parameter concentrations and turbidity.  

  Logging is not nearly as common as in the less rugged Athens Plateau (36a).

: The Central Hills, Ridges, and Valleys ecoregion is lower, less rugged, and more open than neighboring Ecoregions 36b and 36d. Ecoregion 36c is underlain by folded and faulted sandstone, shale, and novaculite (chert); the lithologic mosaic is distinct from the Athens Plateau (36a).

  Its forests are codominated by loblolly pine–shortleaf pine and upland oak–hickory–pine forest types.  Pastureland is also common, much more so than in Ecoregions 36b and 36d.

: The Fourche Mountains are the archetypal Ouachita Mountains.  Ecoregion 36d is composed of long, east-west trending, forested ridges composed of sandstone.  Intervening valleys are cut into shale. Ridges are longer, habitat continuity is greater, the lithologic mosaic is different, and the topographic orientation is more consistent than in other parts of the Ouachita Mountains (36).  

  Differences in moisture and temperature between north- and south-facing slopes significantly influence native plant communities; they are products of the prevailing topographic trend.  Forests on steep, north-facing slopes are more mesic than on southern aspects; grassy woodlands are found on steepest, south-facing slopes.

  Pastureland and hayland are restricted to a few broad valleys.  Logging is not nearly as intensive as in the commercial pine plantations of the less rugged Athens Plateau (36a).  

  Nutrient, mineral, and biochemical water quality parameter concentrations are low in the surface waters of Ecoregion 36d but turbidity can be higher than in other mountainous parts of the Ouachitas.

: The Western Ouachitas ecoregion is composed of mountains, hills, and narrow valleys.  In Arkansas, Ecoregion 36e is confined to Round Mountain in western Polk County, where it is underlain by sandstone and shale; novaculite (chert) is absent in contrast to the Central Mountain Ranges (36b).  Ridge top elevations exceed 2,300 feet in Arkansas; both elevation and precipitation decrease westward into Oklahoma.  Ecoregion 36e in Arkansas is higher and more rugged than the lithologically distinct Athens Plateau (36a).  

  Today, pine and upland oak–hickory–pine forest types codominate.  Ecoregion 36e in Arkansas and Oklahoma contains, perhaps, the greatest concentration of critically-imperiled and imperiled species in mid-North America.

Oklahoma Information:

  The forested low mountains of Ecoregion 36 are characteristically underlain by folded, sedimentary rocks of Paleozoic age.  In Oklahoma, mean annual rainfall in this humid ecoregion is 43 to 57 inches.  Oak–hickory–shortleaf pine forest is native on uplands; it contrasts with the oak–hickory forest of Ecoregions 38 and 39 and the oak savanna or prairie of drier ecoregions to the west.  

  Ecoregion 36 remains mostly forested, but pastureland and hayland occur in wider valleys.  Logging and recreation are major land uses.  

  Most streams have gravel, cobble, boulder, or bedrock substrates but a few have sandy bottoms.  Stream gradients are steeper than in Ecoregion 35.  Turbidity, total phosphorus, total suspended solids, and biological oxygen demand values are lower, and dissolved oxygen levels are higher, than in the streams of Ecoregions 35 and 37.  

  Common fishes include the longear and green sunfishes, yellow bullhead, brook silverside, blackstripe and blackspotted topminnows, largemouth

bass, smallmouth bass, redfin darter, suckers, and the bigeye, Ouachita Mountain, and ribbon shiners.  Orangebelly darters, grass pickerels, and tadpole madtoms are also found in Ecoregion 36, but are absent from Ecoregions 38, 39, and 40.

: The broad Western Ouachita Valleys are etched into Mississippian Stanley Shale and veneered with terrace, alluvial, and colluvial deposits.  

  Ecoregion 36f includes the Kiamichi River, one of the few tributaries to the Red River that has not been heavily impacted by water diversion or major land use changes.  The Kiamichi River system is home to more than half of Oklahoma’s mussel species, including eight imperiled or vulnerable species.  

  Natural vegetation is oak–hickory–pine forest on uplands and bottomland forest on floodplains and low terraces.  Prairies occurred in Ecoregion 36f prior to the 20th century, but were lacking from the other, more mountainous parts of Ecoregion 36.  Today, pastureland, woodland, and hayland are common; poultry, cattle, and hogs are the main farm products.


Ecoregions of Arkansas.  U.S. EPA. 

Ecoregions of Oklahoma.  U.S. EPA. 

A small cave along a stream in the Ouachita Mts

Despite the rich and varied geological history of the Ouachita Mts, few natural caves occur in the region.  The geological sub-regions of the Ouachitas and their associated rock types, simply are not conducive to form caves such as those in karst regions like the Ozark Mts that are largely dominated by Limestone.  The small cave pictured in this photo is a "Fracture" cave.  Fracture caves are formed when layers of more soluble minerals dissolve out from between layers of less soluble rock.   


Pg. 3: Rivers, Wetlands, Precipitation & Aquifers