Missouri’s landforms fall into three physiographic regions: the rolling, glacier-smoothed Northern Plains north of the Missouri River (with the Osage Plains wrapping into the west-central part of the state), the Ozark Plateau that covers a little less than two-thirds of the state, and the flat Mississippi Alluvial Plain of the southeastern Bootheel. The highest point is Taum Sauk Mountain in the St. Francois Mountains, at 540 m (1,772 ft); the lowest is the St. Francis River where it leaves the state at the Arkansas border, at 70 m (230 ft). Across its 180,533 km² (69,704 sq mi), Missouri holds more than 7,300 recorded caves, some of the largest springs in the United States, and bluffs up to about 150 ft (46 m) high where the Ozarks meet the Mississippi.
North of the Missouri River: the Dissected Till Plains
The Missouri River is the line that divides the state’s geology in two. It crosses Missouri for 690 km (430 mi), entering at Kansas City and joining the Mississippi just north of St. Louis, and from its mouth westward to about Columbia it also marks the southern limit of the Pre-Illinoian glaciers. Ice sheets pushed down from the north, flattened the older plateau that once extended here, and left a blanket of till behind. South of the river the ancient Ozark surface was never touched by ice, which is why the two halves of the state look so different on a relief map.
Geographers call the northern half the Dissected Till Plains, part of the Central Lowland. Streams have cut into the till so thoroughly that very little of the original flat surface survives; what is left is a country of rounded ridges and wide valleys. Local relief is usually 6 to 50 m (20 to 165 ft), but the valleys of the bigger rivers run about 90 m (300 ft) deep and can be up to 32 km (20 mi) wide. The land tilts gently from northwest to northeast: the extreme northwest corner stands about 370 m (1,200 ft) above sea level, the northeast corner around 150 m (500 ft).
Loess, a wind-blown silt, caps most of the uplands and is as much as 7.6 m (25 ft) thick near the Missouri River, thinning eastward. In the far northwest, in Atchison and Holt counties, the silt piles up into the Loess Hills. These bluffs rise about 60 m (200 ft) above the Missouri River floodplain in a band that runs some 320 km (200 mi) north into Iowa, built after the last ice age from glacial silt that westerly winds lifted off the river’s drying mud flats.
The Osage Plains along the Kansas border
The second slice of the plains lies south of the Missouri River along the Kansas border. From Columbia the boundary of the Ozarks runs southwest toward Joplin, and west of that line is the Osage Plains, the northeastern end of a physiographic section that continues through southeastern Kansas and central Oklahoma into north-central Texas. It is the southernmost of the three tallgrass prairie regions of the Interior Plains.
The change from the Ozarks is abrupt, and over time the line between prairie and woodland has shifted back and forth across this transition zone. Together the Dissected Till Plains and the Osage Plains cover a little more than a third of Missouri.
The Salem and Springfield Plateaus of the Ozarks
South of the rivers the land rises onto the Ozark Plateau, which geologists describe as a low dome with minor faults and folds. A belt of highland runs across it from near Ste. Genevieve on the Mississippi to McDonald County in the southwest corner, and crest elevations along it range from about 335 to 520 m (1,100 to 1,700 ft). Few places anywhere in the Ozarks top 430 m (1,400 ft); the scenery comes from how deeply the rivers have cut, not from height.
A line of cliffs called the Burlington Escarpment splits the Missouri Ozarks into two plateaus. In places it stands 76 to 91 m (250 to 300 ft) high, marking the edge of the Mississippian limestones. East of it is the Salem Plateau, built of older Ordovician dolomite; west of it, around Springfield and Joplin, is the Springfield Plateau, a flatter surface of cherty Boone and Burlington limestone. The second-highest point in the state, Lead Hill at 532 m (1,744 ft), stands near the community of Cedar Gap in Wright County, in the south-central Ozarks.
Along the southern edge of both plateaus, headward erosion by tributaries of the White River has created a second escarpment running through McDonald, Barry, Stone, Christian, Douglas and Howell counties. South of it the Ozarks become far more rugged, including the Shepherd of the Hills country around Branson. Rivers across the plateau, notably the Gasconade, the Osage and the White, twist through tight loops that sit on the upland rather than on floodplains. These are incised meanders, which formed as the rivers kept their winding courses while cutting downward after the old land surface was uplifted. Bagnell Dam, begun in 1929, turned one set of Osage loops into the Lake of the Ozarks, with about 1,850 km (1,150 mi) of shoreline.
Big Spring, Grand Gulf and Missouri’s 7,300 caves
The dolomite and limestone of the Ozarks dissolve slowly in rainwater, and the result is karst: sinkholes, losing streams, caves and springs. Missouri has more than 7,300 recorded caves, second only to Tennessee, which is why it is nicknamed the “Cave State”. Perry County has both the most caves and the longest one, and Marvel Cave, west of Branson, has one of the largest entrance rooms in North America.
The water that carves the caves comes back to the surface at springs. The Current and Jacks Fork rivers, protected since 1964 as the Ozark National Scenic Riverways, flow through the largest concentration of first-magnitude springs in dolomite in the United States. Each of these springs averages more than 2.8 m³ (100 cu ft) per second. The biggest is Big Spring, about four miles below Van Buren, where roughly 13,000 litres (470 cu ft) of water a second boil out from the foot of a limestone bluff at about 14 °C (58 °F) and run some 300 m (1,000 ft) to the Current River.
Sometimes a cave roof gives way entirely. Grand Gulf, near Thayer on the Arkansas line, is a forked canyon nearly 1.6 km (1 mi) long and up to 40 m (130 ft) deep, the remains of a collapsed dolomite cave system. A surviving piece of the old roof spans 76 m (250 ft), one of the largest natural bridges in the state. Water that drains into a cave at the gulf’s eastern end reappears one to four days later at Mammoth Spring in Arkansas, 14 km (9 mi) away.
The St. Francois Mountains and Taum Sauk
In the eastern Ozarks, in Iron, Madison, Reynolds and St. Francois counties, a cluster of knobs pokes through the sedimentary layers. The St. Francois Mountains are the exposed igneous core of the Ozark dome,: rhyolite and granite formed by volcanic and intrusive activity about 1.485 billion years ago, one of the oldest exposures of igneous rock in North America. In the late Cambrian these summits stood as islands in a shallow sea.
Taum Sauk Mountain is less a peak than a long ridge running north-northwest to south-southeast, and its summit rises only 159 m (522 ft) above an already high base. Black Mountain in Madison County has the greatest local relief in Missouri, climbing from 160 m (540 ft) on the St. Francis River to 458 m (1,502 ft), and Mudlick Mountain rises 211 m (693 ft) to 400 m (1,313 ft). On the flank of Taum Sauk, Mina Sauk Falls is the highest waterfall in the state, though it flows strongly only in wet weather.
The rounded pink boulders of Elephant Rocks State Park show how these old rocks weather. The Graniteville Granite there cooled from magma about 1.4 billion years ago and cracked along nearly vertical joints. Groundwater rotted the granite along those cracks, leaving hard corestones inside crumbly weathered rock, and when runoff stripped the loose material away the corestones were left standing on the surface in a line resembling a train of circus elephants. At Johnson’s Shut-Ins, on the East Fork Black River, the same resistant rhyolite squeezes the river into a narrow channel of chutes and potholes.
The Bootheel and the Mississippi Alluvial Plain
A line from Cape Girardeau on the Mississippi southwest to the Arkansas border just past Poplar Bluff marks the end of the Ozarks. Beyond it lies the Southeast Lowlands, about 7,800 km² (3,000 sq mi) of the Mississippi Alluvial Plain and the northernmost tip of the Mississippi Embayment. It is the lowest and flattest part of Missouri. The Bootheel itself, made up of some or all of Dunklin, New Madrid and Pemiscot counties, pushes south of 36°30′ between the St. Francis and Mississippi rivers, and it is on the St. Francis in southwestern Dunklin County, near Arbyrd, that the state reaches its low point of 70 m (230 ft).
Big Oak Tree State Park near East Prairie, established in 1938, protects 1,029 acres, including a rare remnant of the bottomland hardwood forest that once covered this plain. The only real relief in the lowlands comes from Crowley’s Ridge, a narrow ridge built largely of wind-blown loess that reaches 170 m (550 ft) above sea level near its northern end and runs about 240 km (150 mi) from southeastern Missouri to Helena, Arkansas. Beneath the flat ground lies the Reelfoot Rift and the New Madrid Seismic Zone, source of the great 1811–1812 New Madrid earthquakes, which struck the town of New Madrid in the Bootheel.
Where the Ozarks meet the Mississippi River bluffs
Along the entire eastern side the Mississippi is edged by bluffs and uplands standing roughly 120 to 180 m (400 to 600 ft) above sea level. They are highest between the mouth of the Meramec and Ste. Genevieve, where the river slices through the end of the Ozark ridge and rock walls climb as much as 46 m (150 ft) straight up from the water. On the Illinois bank the same ridge continues as the Shawnee Hills. Near Hannibal, Mark Twain Cave, in soft limestone about 350 million years old, has given tours since 1886.
Every drop of rain in the state eventually reaches the Mississippi, directly or by way of the Missouri, the St. Francis or the White. For more on how these rivers compare in length and flow, see whether the Missouri River is longer than the Mississippi, and for the uplands that reach into Arkansas, Oklahoma and Kansas, read our guide to the Ozark Plateau as a whole.