Oklahoma’s landforms fall into seven broad regions: the High Plains of the Panhandle, crowned by Black Mesa; the red-bed plains, gypsum hills and salt flats of the northwest; the granite Wichita Mountains in the southwest; the Cross Timbers and prairies of the centre; the ancient Arbuckle Mountains in the south-centre; the Ozark Plateau in the northeast; and the folded Ouachita Mountains in the southeast. The state covers about 181,030 km² (69,898 sq mi) and tilts gently from west to east, from its highest point on Black Mesa at 1,516 m (4,973 ft) to its lowest on the Little River at 88 m (289 ft) near the Arkansas line. Few states pack so many different surfaces into one rectangle and a handle.
Black Mesa and the Panhandle High Plains
The Panhandle is the western end of the tilt, a strip of semi-arid shortgrass plain where yearly rainfall drops below 430 mm (17 in). Most of it is flat to gently rolling, cut by intermittent canyons, but its far northwest corner in Cimarron County holds the state’s summit. Black Mesa runs about 45 km southeast from Mesa de Maya in Colorado, crosses a corner of New Mexico and ends in Oklahoma on the north bank of the Cimarron River near the hamlet of Kenton. Its Oklahoma high point reaches 1,516 m (4,973 ft); the Colorado end climbs to 1,739 m (5,705 ft).
The mesa is an inverted valley. Between 3 and 5 million years ago, basalt lava from the Raton–Clayton volcanic field in New Mexico and Colorado poured into a stream channel cut into the Ogallala Formation. The softer sediment around that channel has since been stripped away, leaving the hard lava-filled valley standing as a long ridge, with a 61 m (200 ft) escarpment along the Dry Cimarron. Below the basalt lie Cretaceous Dakota Sandstone and the Jurassic Morrison Formation, which since 1935 has yielded more than 18 tonnes of dinosaur bone, including stegosaurus and diplodocus, plus a trackway of footprints near Carrizo Creek. Nearby Black Mesa State Park, with Lake Carl Etling dammed on South Carrizo Creek in 1959, has the darkest night sky of any state park in Oklahoma.
Red Beds, Gypsum and Salt in Northwestern Oklahoma
East of the Panhandle the ground turns red. Permian sandstones and shales, between 300 and 1,400 m thick, underlie much of western Oklahoma, and where they are capped by harder gypsum they erode into mesas and buttes. The best known are the Glass Mountains in Major County, a line of flat-topped hills along U.S. Route 412 that belong to the Blaine Escarpment. They are not true mountains: they rise only 46 to 61 m above the plain, and their highest ground is about 490 m above sea level. The explorer Thomas James saw them glittering in 1821 because their slopes are strewn with selenite crystals; a mapmaker’s transcription error in 1875 produced the alternative name, Gloss Mountains.
The same evaporite beds, laid down when a Permian inland sea dried out more than 200 million years ago, explain three other northwestern oddities. Near Freedom, streams have tunnelled through a gypsum bed to form Alabaster Caverns, the largest natural gypsum cave in the world open to the public, with pink, white and rare black alabaster. In Alfalfa County the Great Salt Plains cover about 45 km² (11,000 acres) of bare salt crust, renewed whenever a saline aquifer rises and evaporates; the Sibley expedition of 1811 called them the Grand Saline, and they are the only place in the world where hourglass selenite crystals can be dug. South of Waynoka, Little Sahara State Park protects sand dunes 7.6 to 23 m high, built from old terrace deposits left when the Cimarron River wandered across the area.
Farther south, in a bend of the Canadian River in Roger Mills County, the Antelope Hills rise abruptly from the prairie. Francisco Vásquez de Coronado mentioned them in 1541, and Captain Randolph Marcy, surveying the California Road, described flat-topped sandstone hills about 150 ft high near the 100th meridian.
The Wichita Mountains of Southwest Oklahoma
The Wichitas are the product of a failed continental rift called the Southern Oklahoma Aulacogen. Many of their rocky knobs are capped by granite about 500 million years old, which was exposed and rounded by weathering during the Pennsylvanian and Permian periods. The range trends northwest to southeast, and at its eastern end it gives about 305 m (1,000 ft) of relief above rolling grassland. Mount Scott, reached by a paved road of about 5 km, stands at 751 m (2,464 ft); Mount Pinchot is within a few feet of it, and sources disagree over which is higher.
Most of the eastern range sits inside the Wichita Mountains Wildlife Refuge, founded in 1901 and covering 238.8 km² (59,020 acres), where bison, elk and a herd of longhorn cattle graze among boulder fields. Fort Sill, home of the U.S. Army Field Artillery School, occupies much of the southeastern end, and the western outliers reach Quartz Mountain north of Altus.
The Cross Timbers and the Central Prairies
The middle of the state is a transition rather than a range. The Cross Timbers, a belt running from southeastern Kansas across central Oklahoma into Texas, mix prairie, savanna and woodland on coarse sandy soils. Their stunted post oaks and blackjack oaks were useless as timber, which is why about 3,600 km² (890,000 acres) of old-growth woodland survives across the region, with post oaks 200 to 400 years old and red cedars older than 500 years. The belt marks the western edge of the forested east, and Interstates 35, 40 and 44 all cross it; the western suburbs of Tulsa and the northeastern suburbs of Oklahoma City lie inside it.
West of the timbers lie mixed-grass and shortgrass prairie; to the north, a tongue of the Flint Hills enters Osage County, where the Tallgrass Prairie Preserve protects about 160 km² (39,000 acres), the largest protected tract of tallgrass prairie in the world. Two great drainage systems, the Arkansas River in the north and the Red River along the Texas border, carry most of this water east. Natural lakes are scarce, limited to oxbows and playas, but the Dust Bowl drought that began in 1930 prompted a wave of dam-building that has given Oklahoma more than 200 reservoirs, more lakes created by dams than any other state.
The Arbuckle Mountains Near Davis
South of the Canadian River, in Murray, Carter, Pontotoc and Johnston counties, the Arbuckle Mountains expose the oldest rocks between the Appalachians and the Rockies. Their core is Tishomingo granite about 1.4 billion years old, covered by roughly 460 m of Cambrian rhyolite about 525 million years old and then by some 4,600 m of limestone, dolomite, sandstone and shale deposited in shallow seas between 515 and 290 million years ago. Folding and faulting during the Wichita orogeny bent the stack into a west-northwest-trending anticline, and erosion has since planed it down to a low range whose top reaches only 430 m (1,412 ft).
The limestone makes the Arbuckles a karst landscape. The Arbuckle–Simpson aquifer beneath them feeds springs that keep local creeks flowing, and six miles (9.7 km) south of Davis, Honey Creek drops 23 m (77 ft) over Turner Falls into a natural pool. The falls are locally billed as Oklahoma’s tallest, though one at Natural Falls State Park matches their height.
The Ozark Plateau in Green Country
Northeastern Oklahoma, marketed as Green Country, holds the western edge of the Ozarks, a broad dome covering about 120,000 km² that is the most extensive highland region between the Appalachians and the Rockies. In Oklahoma the plateau is dissected into wooded hills rather than peaks, with the Boston Mountains reaching across from Arkansas. The Illinois River, 233 km long, crosses this terrain from Arkansas and is protected by the state as a Scenic River; since 1953 its lower course has been held back as Lake Tenkiller, and the 70 miles upstream flow through the Cookson Hills. For more on the whole upland, see this guide to the Ozark Plateau.
The Ouachita Mountains and the Little River Lowlands
South of the Arkansas River, the Ouachita Mountains are built very differently from the Ozarks. They are a thick pile of Paleozoic strata crushed into the Ouachita Fold and Thrust Belt, so their ridges run east to west in long parallel lines, the Winding Stair Mountains among them. Together the Ouachitas and Ozarks form the U.S. Interior Highlands, one of the few mountainous regions between the Appalachians and the Rockies. The Talimena Scenic Drive follows the crest for 86.9 km from Talihina to Mena, Arkansas, past 22 vista pull-outs; opened in 1969, it climbs over Rich Mountain, whose 817 m summit rises in Arkansas close to the state line.
On the northern fringe near Poteau stands Cavanal Hill, whose roadside sign claims the title “World’s Highest Hill” at 1,999 ft, one foot short of a common definition of a mountain. The summit is actually 727 m (2,385 ft) above sea level, about 600 m above the Poteau River 5 km to the north. The southeast is also the wettest corner of the state, with up to about 1,420 mm (56 in) of rain a year, and the forests turn to shortleaf and loblolly pine, then to cypress swamps. The Little River rises above 610 m in Le Flore County, runs 210 km through the Choctaw Nation, gathers the Glover River and Mountain Fork in McCurtain County, and leaves the state at 88 m (289 ft), Oklahoma’s lowest ground. The largest of the state’s 62 oxbow lakes, 1.1 km² (272 acres), lies near the Red River in the same county.
From the basalt of Black Mesa to that cypress lowland is a drop of more than 1,400 m across the full width of the state, and the rock underfoot changes several times along the way. If the flat-topped hills of the west interest you most, this explainer on what a mesa is covers how they form, while the grassland that frames most of the state is traced in this guide to Great Plains geography.