ARArkansas Geography

Look at the northeast corner of Arkansas and there is a rectangular bite taken out of it. The state's northern boundary should run along 36 degrees 30 minutes latitude all the way to the Mississippi River. For the final stretch it drops to the 36th parallel instead, handing Missouri a block of Delta farmland now called the Bootheel. A planter named John Hardeman Walker held much of that land and lobbied to be attached to Missouri when its boundaries were fixed. He got his way, and Arkansas has carried the notch ever since.

The only real mountains between the Appalachians and the Rockies

Northwestern Arkansas holds the Interior Highlands, which is the only substantial upland between the Appalachian chain and the Rocky Mountains. It comes in two halves that are geologically nothing alike. The Ozark Plateaus in the north are not folded mountains at all but a dissected plateau of flat lying limestone and dolomite that streams have cut into hills, which is why the region is riddled with caves and springs. The Ouachita Mountains to the south are genuinely folded rock, crumpled during a continental collision roughly 300 million years ago.

The Ouachitas run east to west. Almost every other major range in North America trends north to south, so this one reads wrong on a map until you know the collision came from the south rather than the east. Between the two sits the Arkansas River Valley, and standing alone in the middle of that valley is Mount Magazine, whose Signal Hill at 2,753 feet is the highest point in the state. It belongs to neither range: it is a flat topped remnant that survived while the rock around it wore away. The lowest ground is where the Ouachita River crosses into Louisiana at about 55 feet.

Crowley's Ridge, an island of loess in a flat half of the state

Southeast of a diagonal running from the northeast corner toward Texarkana, Arkansas flattens into the Gulf Coastal Plain. Most of the eastern strip is the Mississippi Alluvial Plain, known locally as the Delta, some of the flattest and richest farmland in the country.

One feature interrupts it. Crowley's Ridge runs about 150 miles from Helena north into Missouri, never more than a few miles wide, standing 100 to 200 feet above the surrounding flats and capped with wind blown loess. It is an erosional remnant left between two ancient river courses, and it is the only reliably dry ground for miles. Jonesboro, Forrest City, Wynne and Helena all sit on it for that reason, and so does the highway that links them.

South and west of the ridge lies the Grand Prairie, where a dense clay pan a couple of feet below the surface stops water from draining away. That single soil layer is why Arkansas grows roughly half of all the rice produced in the United States, far more than any other state, after the first successful commercial crops went in during the early 1900s.

Diamonds, hot water and brine

Near Murfreesboro, Crater of Diamonds State Park sits on the weathered top of a lamproite volcanic pipe roughly 100 million years old. It is the only diamond bearing site in the world open to the public where finders keep what they dig. The 40.23 carat Uncle Sam, pulled out in 1924, remains the largest diamond ever found in the United States.

Forty seven springs on the flank of Hot Springs Mountain deliver hundreds of thousands of gallons a day at about 143 degrees. The water fell as rain roughly four thousand years ago, sank thousands of feet through fractured rock, warmed at depth and came back up. Congress set the springs aside as a federal reservation in 1832, four decades before Yellowstone was established, making it the oldest protected area of its kind in the country.

Underneath southern Arkansas, the Smackover Formation holds brine that supplies most of the bromine produced in the United States, and the same brine is now being drilled for lithium. Up in the Ouachitas around Mount Ida, the quartz crystal veins are among the best developed anywhere.

Rivers that keep moving the state line

The Mississippi forms the entire eastern boundary, and it does not hold still. Every time the river cuts across a meander neck it strands land on the wrong bank, so Arkansas still owns parcels east of the current channel while Tennessee and Mississippi own parcels on the Arkansas side. Oxbow lakes across the Delta mark abandoned loops.

The Arkansas River crosses the whole state from Fort Smith through Little Rock to the Mississippi, and since the McClellan Kerr navigation system opened in 1971 barges have run it as far up as the Tulsa area. The Buffalo River, cutting down off the Boston Mountains for 135 undammed miles, was designated the first National River in the United States in 1972. The western boundary with Oklahoma was surveyed as a line running due south from Fort Smith, which is why the city's western edge is the state line itself.

Records, floods and the fault under the Delta

Ozark recorded 120 degrees on August 10, 1936. Pond, near Gravette in the northwest corner, recorded 29 below zero on February 13, 1905. The whole state runs on Central Time with no exceptions.

The New Madrid Seismic Zone reaches down into northeast Arkansas, and the earthquakes of 1811 and 1812 dropped whole tracts of land along the St. Francis River, creating the sunk lands and Big Lake that still sit there flooded. Marked Tree lies near the southern end of the zone. Water has done more lasting damage than shaking: in the Great Mississippi Flood of 1927 roughly two million acres of Arkansas went under, more flooded ground than in any other state.

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