Arkansas has six natural landform divisions. In the northwest are the Ozark Plateaus, including the Springfield and Salem plateaus and the higher, deeply cut Boston Mountains; south of them the broad Arkansas River Valley holds the state’s highest point, Mount Magazine at 839 m (2,753 ft); below that the east–west ridges of the Ouachita Mountains surround Hot Springs. The southern and eastern half of the state is lowland: the pine-covered West Gulf Coastal Plain and the flat Mississippi Alluvial Plain, known locally as the Delta, broken only by the narrow hills of Crowley’s Ridge. The lowest point is 17 m (55 ft) on the Ouachita River where it leaves for Louisiana. Across 137,733 km² (53,179 sq mi), each of these surfaces records a different chapter of geologic time, and the story runs in order from the oldest.
1.4 Billion Years Down: The Granite Beneath the Ozarks
The oldest rocks in Arkansas are never seen at the surface. Deep wells in the Ozarks and the northern Mississippi Embayment have struck igneous granites dated to about 1.4 billion years, which are thought to have formed soon after the breakup of the supercontinent Columbia and now make up part of the stable basement of North America. The whole state is believed to rest on this crystalline foundation.
In southeastern Missouri, Precambrian igneous rocks of similar age break the surface in the St. Francois Mountains, the eroded core of the Ozark dome. That core stood as an island in the early Paleozoic seas, and the sedimentary layers of the Arkansas Ozarks were laid down around and over it. Everything visible in the Arkansas uplands is younger rock draped over this ancient block.
Paleozoic Seas and the Ozark Plateaus: Dolomite, Boone Chert and the Buffalo River
From the Cambrian, about 542 million years ago, through the Devonian, most of Arkansas lay under shallow seas and rivers. In the Ozark region the water was near-shore and warm, and it left thick beds of carbonate: dolomite and limestone, interrupted in the Ordovician by periods of erosion. In the Mississippian, cherty limestones known locally as the Boone chert were deposited; today they underlie the Springfield Plateau and erode into steep hills, valleys and bluffs. At the start of the Pennsylvanian, a rush of sand and mud buried the carbonates.
Those sandstones now cap the Boston Mountains, the highest and most rugged part of the Ozarks. The Boston Mountains are a plateau rather than a folded range: their Paleozoic layers still lie nearly flat, with ridge tops capped by Pennsylvanian sandstone and shale above valleys cut down through Mississippian limestone into Ordovician dolomite. Rivers did most of the sculpting. The Buffalo River, 246 km (153 mi) long, rises in the highest Boston Mountains, flows out onto the Springfield Plateau near Erbie and crosses a corner of the Salem Plateau before joining the White River. On 1 March 1972 Congress made it the first national river in the United States, ending plans by the Army Corps of Engineers to dam it; its lower 217 km are managed by the National Park Service.
Water moving through the soluble carbonates has also hollowed them out. Blanchard Springs Caverns, north of Mountain View in Stone County, is a three-level cave system whose Discovery Trail descends to 112 m underground along the stream that created it; the caverns opened to the public in 1973.
The Ouachita Orogeny: When South America Folded Western Arkansas
While the Ozark shelf collected carbonates, central and southern Arkansas sat over a deep ocean trough on the southern margin of the ancient continent Laurentia. There, from the Ordovician to the Mississippian, sand, clay and siliceous ooze settled in deep water, including the silica beds that became novaculite. As the ocean floor to the south was subducted, South America closed in. During the Pennsylvanian the continents collided, and the thick pile of trough sediments was faulted, folded, slightly metamorphosed and thrust upward. The result was the Ouachita Mountains, and the same collision raised the Ozark Plateau to the north.
The Ouachita fold and thrust belt crops out for about 350 km (220 mi) across western Arkansas and southeastern Oklahoma. Unlike most American ranges, its ridges run east to west, separated by broad valleys where softer shale has worn away. Hard layers stand up as the ridges: the Fourche Mountains are a long chain of weather-resistant Jackfork Sandstone running from Pinnacle Mountain near Little Rock into Oklahoma, and Rich Mountain near Mena reaches 817 m (2,681 ft). Novaculite, so hard it has been quarried since prehistoric times for points and later for whetstones known as Arkansas stones, forms ridges of its own. The deformation and later uplift also filled cracks with numerous veins of milky quartz.
The folded rocks explain Hot Springs. Rain falling on the mountains to the north and northeast soaks down through fractured chert, novaculite and sandstone at about 30 cm a year, reaching depths of at least 1,400 to 2,300 m, where it heats up. The trip down takes about 4,000 years. The water returns along faults and fractures in about a year and emerges between two thrust faults at about 62 °C (143 °F), in a gap between Hot Springs Mountain and West Mountain. Congress set the springs aside as the Hot Springs Reservation on 20 April 1832, the first time federal land was reserved for recreation; it became a national park in 1921.
Arkansas Valley Coal Swamps and the Mesa of Mount Magazine
Between the rising Ouachitas and the Ozark Plateau, the collision opened a trough bounded by new faults. In the Pennsylvanian it held shallow near-shore swamps, and their plant debris became the coal beds of the Arkansas River Valley. Through the Permian the new highlands were uplifted and worn down, and the Ouachitas lost much of their height between the Permian and the Jurassic.
Today the valley is a lowland of farms and lakes, drained by the Arkansas River, which enters the state near Van Buren and flows southeast past Little Rock. Three isolated mountains rise out of it: Mount Magazine, Mount Nebo and Petit Jean Mountain, the Tri-Peaks. Mount Magazine is a flat-topped mesa capped by hard rock and rimmed by cliffs, with two summits, Signal Hill at 839 m and Mossback Ridge at 823 m (2,700 ft). It is the highest point of the U.S. Interior Highlands, and French hunters are said to have named it for its barn-like shape, magasin.
Cretaceous Seas and Volcanoes: Crater of Diamonds and Magnet Cove
In the Jurassic a shallow, very salty sea spread over southern Arkansas and left thick anhydrite in the subsurface. Around 100 million years ago the crust began to sag into the Mississippi Embayment, a great downwarp that let the Gulf of Mexico reach far inland, and marl, sand and chalk piled up across the south. The same period brought volcanoes.
In Pike County, the Early Cretaceous Prairie Creek diatreme punched lamproite magma up from mantle depths of more than 150 km, carrying diamonds with it. Its eroded pipe is now Crater of Diamonds State Park, a 369-hectare park with a 15.2-hectare ploughed field where the public may search for stones. Diamonds have turned up there continuously since 1906, including the Uncle Sam of 1924, at over 40 carats the largest diamond ever found in the United States. The site became a state park in 1972. Other Cretaceous intrusions surface at Magnet Cove and Potash Sulphur Spring, southeast of Hot Springs, with about 80 small dikes noted near the Ouachita River.
Paleogene Islands and the Gulf Coastal Plain: Bauxite, Lignite and Oil
After 66 million years ago the embayment stayed flooded. Reefs and dark clays formed along its shores, and islands of Cretaceous syenite near present-day Little Rock weathered under a tropical monsoon climate into laterite, leaving pea-sized pellets rich in aluminium. That was Arkansas bauxite, found in Pulaski and Saline counties; at one time it supplied 90 per cent of US aluminium demand, and mining ended in 1982. In the Eocene, swamps returned and formed lignite, a low-grade coal.
These young, soft, gently tilted sediments form the West Gulf Coastal Plain of southern Arkansas, the Piney Woods or Timberlands, drained by the Ouachita and Saline rivers and, in the southwest corner, the Red River. The south also holds oil: a 1921 gusher at El Dorado in Union County launched a boom that peaked in 1925 with 3,483 wells.
Ice-Age Loess, Crowley’s Ridge and the New Madrid Delta
The last 2.5 million years built the youngest landforms. During Pleistocene cold spells, patches of alpine snowpack, like small glaciers, formed on the Ouachita and Boston mountains. Far more important was meltwater: glacial outwash poured down the Mississippi and Ohio rivers, spreading sand and silt over eastern Arkansas and shaping the Eastern and Western Lowlands. Winds blew silt off the braided rivers and dropped it as loess.
Crowley’s Ridge is the result. Built mostly of that windblown loess, it runs about 240 km (150 mi) from southeastern Missouri to the Mississippi near Helena, between 0.8 and 19 km wide, and rises up to 170 m (550 ft) above sea level near its northern end. It is the smallest of the state’s six natural divisions, and most of the Delta’s larger towns, including Jonesboro, Paragould, Forrest City and Wynne, sit on it. It was once explained as an island left between the Mississippi and Ohio rivers when they changed course, but seismic studies point to uplift along faults at its edges, possibly linked to the nearby New Madrid Seismic Zone.
That zone produced the 1811–1812 New Madrid earthquakes, the most powerful recorded in the contiguous United States east of the Rockies, and in northeast Arkansas it has left sand dikes in the alluvium. Over the past 11,000 years Mississippi floods have kept adding silt to the Delta, the flat plain drained by the White, St. Francis, Cache and Black rivers.
Arkansas’s landscape is therefore a stack of eras: Paleozoic plateaus and folds in the northwest, Mesozoic and Cenozoic sediments in the south, and Ice Age silt in the east. The plateau chapter continues in our guide to the Ozark Plateau, the thermal springs have their own Hot Springs National Park guide, and the river that divides the highlands is followed in Arkansas River geography, while Arkansas geography sets all six regions beside the state’s towns and climate.