Iowa’s landforms fall into eight regions, and almost all of them are the work of ice, wind and rivers over the last 2.5 million years: the steep Loess Hills along the Missouri River, the rugged Paleozoic Plateau (Iowa’s share of the Driftless Area) above the Mississippi, the young, pothole-dotted Des Moines Lobe in the north-centre, the rolling Southern Iowa Drift Plain, the Iowan Surface and Northwest Iowa Plains, and the two flat alluvial plains of the border rivers. The state covers 145,746 km² (56,273 sq mi) and rises from 146 m (480 ft) where the Des Moines River meets the Mississippi near Keokuk to 509 m (1,671 ft) at Hawkeye Point, a farm field north of Sibley in Osceola County. Under that surface lies rock more than a billion years old, and the story reads best from the bottom up.
Pink quartzite and a failed rift: Iowa’s Precambrian basement
The one place Iowa’s Precambrian basement reaches the surface is an outcrop of Sioux Quartzite, a hard, ruddy-pink stone that breaks the surface in the far north-west of Lyon County, on the corner where Iowa meets South Dakota and Minnesota. It began as sand spread by braided rivers between about 1.76 and 1.64 billion years ago and was later cemented into quartzite some 3,000 m thick across the region. Its toughness made it a favourite for railway and road beds.
Around 1.1 billion years ago the North American continent tried to tear itself apart. The Midcontinent Rift System, about 2,000 km long, opened from Lake Superior south-west through Minnesota and Iowa towards Kansas before the rifting stopped. In Iowa the rift lies deep beneath younger rock and has no surface expression at all, yet it shows up clearly on magnetic anomaly maps of the state. It is not seismically active.
Paleozoic seas, the Decorah crater and the bluffs above McGregor
From the Cambrian onward, shallow seas repeatedly flooded the continental interior, laying down the sandstones, shales, limestones and dolomites that make up most of Iowa’s bedrock. Across the state the bedrock generally gets younger from east to west: Cambrian strata about 500 million years old crop out in the north-east, while north-west Iowa sits on Cretaceous rock roughly 74 million years old.
One event from those seas is hidden directly under a town. About 470 million years ago, in the Middle Ordovician, a meteorite struck what is now Decorah in Winneshiek County, blasting a crater about 5.6 km (3.5 mi) across. The sea then filled it with the Winneshiek Shale, which today lies more than 15 m (50 ft) below the bed of the Upper Iowa River, so nothing is visible at the surface.
What is visible is the Paleozoic Plateau, the north-eastern corner of the state that geographers also treat as Iowa’s share of the Driftless Area. Here glacial deposits and loess are thin or missing, and the Mississippi and its tributaries have cut straight into the old sea-floor rock. The result is a landscape of bluff-forming dolomite, deep V-shaped valleys, springs, sinkholes and caves. At Pikes Peak State Park near McGregor a bluff stands about 150 m (500 ft) above the river, opposite the mouth of the Wisconsin River; Zebulon Pike climbed it on 5 September 1805 while scouting a site for a fort. Further south in Jackson County, Maquoketa Caves State Park holds more caves than any other state park in Iowa, among them Dancehall Cave, carved by water moving through soluble limestone.
Devonian reefs at Coralville and the coal swamps of the Des Moines valley
In the Devonian period, roughly 419 to 359 million years ago, warm, clear water covered eastern Iowa and colonial corals built reefs on the sea floor. Those reefs survive in the limestone bedrock near Iowa City, where until recently they lay hidden beneath soil. That changed in the Great Flood of 1993. On 5 July 1993 water poured over the 217 m (712 ft) emergency spillway of Coralville Lake, the flood-control reservoir the U.S. Army Corps of Engineers built on the Iowa River between 1949 and 1958. The torrent stripped away the soil and loose rock below the spillway and exposed a floor of fossil reef, thick with the honeycomb coral Hexagonaria. The site is now the Devonian Fossil Gorge, and the June 2008 floods, which crested at 219 m (717 ft), widened the exposed fossil bed further.
Later, in the Pennsylvanian period, the sea gave way to swampy coastal lowlands whose buried plant matter became coal. Iowa was once a significant coal producer, especially along the Des Moines River valley from Coalville southward; much of the modern city of Des Moines sits on old workings, and 20th-century mines clustered around Albia, Centerville, Lucas and Oskaloosa. The coal was high in sulfur, and the last commercial mine closed in 1994.
The Manson impact, 74 million years ago
By the Late Cretaceous, the shore of the Western Interior Seaway ran across north-western Iowa. About 74 million years ago an asteroid or comet struck near the present town of Manson, in the south-east corner of Pocahontas County, in one of the largest known impact events in North America. It measures about 38 km (24 mi) across and lies buried 20 to 90 m (66 to 295 ft) below a flat farm landscape, so nothing on the surface hints that it is there.
Well drillers noticed strange, deformed rock in 1912. Robert Dietz proposed an impact origin in 1959, and Nicholas Short confirmed it in 1966 by finding shocked quartz. For a time the Manson crater was a suspect in the extinction of the dinosaurs, but argon–argon dating of cores drilled in 1991 and 1992 put it about 10 million years too early for the Cretaceous–Paleogene boundary.
Early ice sheets and the rolling hills along Interstate 80
In the Pleistocene, continental glaciers began to advance from the north. The earliest, known collectively as the Pre-Illinoian and spanning roughly 2.5 million to 200,000 years ago, included about 11 separate glaciations, and they left thick sheets of till: unsorted clay, sand and boulders scraped up and dumped by the ice. Streams have had hundreds of thousands of years to dissect that till.
That landscape is the Southern Iowa Drift Plain, the region most travellers know, because most of Interstate 80 crosses it. It is a surface of rolling hills where younger, wind-blown loess drapes the old till, and it holds some of the most productive farmland in the United States. North-east of it, the Iowan Surface has gentler relief broken by paha, long ridges of loess or loess-capped till aligned north-west to south-east and rising more than 6 m (20 ft) above their surroundings. The word means “hill” in Dakota; the town of Mount Vernon grew up on one, and Casey’s Paha in Tama County is another. These are the soft, rounded hills that Grant Wood painted.
Wind-blown silt piles up in the Loess Hills
During the last two glacial stages, the Illinoian and the Wisconsin, meltwater carried huge loads of finely ground rock, known as glacial flour, down the Missouri River valley. When the floods receded, the mud flats dried out and strong westerly winds lifted the silt, dropping the heaviest share on the valley’s eastern edge. Layer after layer, named the Loveland, Pisgah and Peoria loess after their periods of deposition, built dune fields that grass eventually stabilised.
The result is the Loess Hills, a narrow band about 320 km (200 mi) long, usually 2 to 24 km (1 to 15 mi) east of the Missouri River channel, rising about 60 m (200 ft) above the floodplain. The loess is often more than 27 m (90 ft) thick, and the only deposits of comparable depth and extent are in Shaanxi, China. Because dry loess stands in near-vertical walls, erosion has sculpted “peak and saddle” skylines, razor ridges often less than 3 m (10 ft) wide, and “cat-step” terraces where the silt slumps. Locals call the yellow soil “sugar clay”: hard when dry, it loses all cohesion when wet. The hills run from the Blood Run Site in South Dakota to Mound City, Missouri, passing Sioux City, Council Bluffs, Sidney and Hamburg on the Iowa side.
The Des Moines Lobe, 12,000 years ago: potholes and the Iowa Great Lakes
The youngest landform region is also the flattest. A tongue of the Laurentide ice sheet, the Des Moines Lobe, pushed south from Minnesota and reached as far as the site of the capital, Des Moines, before melting back; the area was under ice until about 12,000 years ago. It had too little time to develop a proper stream network, so the land is level to gently rolling, broken by moraine ridges, knob-and-kettle hummocks and countless shallow depressions. Geographers call it the Prairie Pothole Region. Before settlement, 4 to 6 million acres of Iowa were wetland; about 95% has since been drained with tile and ditches for farming, and remnants such as Barringer Slough in Clay County show what the lobe once looked like.
Iowa’s largest natural lakes lie on the lobe. In Dickinson County the Iowa Great Lakes fill glacial potholes that formed along the lobe’s south-western edge about 13,000 years ago. Big Spirit Lake, at 2,300 ha (5,684 acres), is the largest natural lake in the state and has a northern shore on the Minnesota line, while West Okoboji Lake is the deepest, reaching 41 m (136 ft). Clear Lake, in Cerro Gordo County, lies further east on the same glacial surface.
The Mississippi and Missouri alluvial plains
The newest chapter is still being written by the rivers that frame the state. Iowa is the only state whose eastern and western borders are formed almost entirely by rivers: the Mississippi on one side and the Missouri and Big Sioux on the other. Both have built alluvial plains. The Mississippi Alluvial Plain is a broad valley of stream terraces, abandoned channels, backwater sloughs and oxbow lakes, and it holds the state’s lowest ground where the Des Moines River, 845 km (525 mi) long and the largest river crossing Iowa, joins it below Keokuk. The Missouri Alluvial Plain is probably the only truly flat region of Iowa; the river runs in a narrower channel there, and its eastern edge is the abrupt front of the Loess Hills. Carter Lake, a small city stranded on the Nebraska side of the Missouri, is the only Iowa city west of the river.
For a broader look at the state’s rivers, climate and borders, see our guide to Iowa’s geography. The same Loess Hills carry on south across the state line, which our post on landforms in Missouri follows, and the ice sheets behind the Des Moines Lobe are explained in how glaciers form.