Iowa was generally flat and low for millions of years before dinosaurs evolved and probably ever since. Like most of the Midwest, it was frequently covered with shallow seas during the Paleozoic Era and was a question mark during the Mesozoic Era, the Age of Dinosaurs. Mesozoic rocks and fossils have simply been eroded away.
Ancient Marine Fossils
Iowa may have the most complete record of Paleozoic fossils in the interior of North America, and most of these fossils are marine. In the northeast corner of Iowa are Cambrian (500-570 million years ago) rocks with trilobites and brachiopods. A wide variety of marine animals are found in northeast Iowa in rocks from the Ordovician Period (435-500 million years ago).
One of the world’s most diverse collections of Early Silurian (395-435 million years ago) crinoids is found in eastern Iowa. From eastern and north-central Iowa come Devonian (345-395 million years ago) marine invertebrates and fishes. Also found here are Iowa’s oldest land plants and some of the oldest coals in the world.
In 1993, severe floods exposed fossils in the Coralville Reservoir north of Iowa City. (The city of Coralville is named for the area’s abundant fossil corals.) The site became a major tourist attraction and is now called Devonian Fossil Gorge. Probably few visitors realize how common Devonian fossils are elsewhere in Iowa.
Rocks from the Missippian Period (310-345 million years ago) stretch from southeast to north-central Iowa. They harbor fossils of marine invertebrates and fishes and land plants. Fossils of the oldest known four-legged animals in North America were discovered here. These amphibians grew as long as six to eight feet.
Abundant fossils of marine animals and coal swamp plants are found in rocks from the Pennsylvanian Period, which ended about 280 million years ago, in central and southern Iowa. Cretaceous formations (65-140 million years ago) in western Iowa contain fossils of marine invertebrates, fishes, and long-necked marine reptiles called plesiosaurs. A scrap of dinosaur bone has also been found here.
Geology & Fossils
Crinoids
Iowa’s best known fossils are probably crinoids. Complete crinoids appear to be growing in slabs of rock found in Mississippian limestones at LeGrand and Gilmore City. Exquisite Mississippian crinoids are also well known from the Burlington and Keokuk area. Large numbers of articulated crinoid cups are known from Pennsylvanian, Devonian, and Silurian formations. Iowa’s oldest crinoids include beautiful intact specimens found in Ordovician limestones in the northeast corner of the state.
A crinoid was adopted as state fossil by Missouri’s neighbor to the south, Missouri. Indians have also talked about adopting a crinoid as state fossil.
Manson Meteor Crater
Something lies buried 50 to 300 feet deep under the farmlands of north-central Iowa, near the town of Manson. Geologists once thought the circular structure was an ancient volcano. Many now believe it is an ancient crater created by the impact of a meteor or asteroid.
Scientists estimate that an object measuring two miles across created the crater, which measures twenty miles in diameter. It hurtled toward Earth at 43,000 miles per hour, smashing into the planet with ten times the force of all the world’s nuclear bombs put together. On impact, it pulverized itself and everything in its path in a crater five or six miles deep.
What kind of effect would such an impact have on the planet?
It would throw an immense cloud of dust into the atmosphere, blocking out sunlight. Without sunlight, plants would die, and, without plants to eat, animals would die. Scientists disagree about whether this particular explosion created enough dust to cause the extinction of the dinosaurs.
However, there is something else about the Manson Crater that is remarkable. It was formed about 66 million years ago, near the end of the Cretaceous Period. That’s about the time the dinosaurs disappeared!
Was Earth hit by a swarm of asteroids or meteoroids? More craters could be hidden in tropical forests, under glaciers, or under the sea.
Whatever created the Manson Crater may not have killed all dinosaurs on Earth, but it certainly killed many that might have been living in Iowa!
Ice Age Iowa
Before corn and prairies, there was ice. All of Iowa was probably covered by ice at one time or other during the Ice Age, or Pleistocene Epoch, which lasted from about two million to 10,000 years ago. The enormous ice sheets left glacial till or drift — a jumble of clay, sand, gravel, and boulders.
Ice Age Iowa was inhabited by such mammals as northern bog lemmings, mammoths, caribou, muskox, and giant beaver. Today, Ice Age fossils are found throughout Iowa. Although large mammals are the most spectacular finds, fossil snails and insects also help scientists understand past environments.
North-central Iowa was covered by the last, or Wisconsinan, glaciation not much more than 10,000 years ago. This region is now known as a young glacial plain. Because streams here have had little time to carve good drainage channels, much of this land was swampy when early settlers arrived. People have since drained many swamps for agriculture and settlements, however.
Outside this young glacial plain, most of Iowa has been free of glaciers much longer. Rivers have had much more time to carve these dissected, or eroded, till plains, which include the Wisconsin Driftless Area, a region that was untouched by the last Ice Age glaciation. In northeastern Iowa, where the bedrock is limestone or sandstone — both easily eroded by water — the valleys are narrow with steep sides, and high bluffs rise along the Mississippi River.
Wind blew glacial dust, or loess across much of southern Iowa, helping form soil. In western Iowa, loess was piled into bluffs along the Missouri River.
Also in western Iowa are Miocene (22.5 to 5 million years ago) fossils such as teeth of rhinoceroses and a three-toed horse. Rare Pliocene (5 to 1.8 million years ago) bones include the elephant Stegomastodon.
Iowa’s Ice Age Snail
Iowa boasts a living fossil that is something of a symbol. Although we think of the Ice Age as a harsh time, some animals are adapted to cold conditions. Can you imagine a muskox living in the desert? The end of the Ice Age was bad news for such creatures as muskoxen, mammoths, and the Iowa Pleistocene snail.
When Ice Age glaciers began melting, muskoxen and mammoths could no longer survive in what is now Iowa. But the Iowa Pleistocene snail was lucky. Because it is so small, it was able to find refuge in tiny Ice Age habitats that still survive.
In Iowa’s Driftless Area are cool, moist, shaded cliffs with a special property — many of them act as refrigerators! In winter, water freezes in cracks that are connected to sinkholes in nearby uplands. This underground ice remains in summer, cooling air and water flowing out of the talus — the loose rocks that lie on the slopes. Readings of 33 degrees — just one degree above freezing — have been recorded from some slopes in mid-June! Although the summer sun may warm the surface, the underground ice keeps things cool below.
The Iowa Pleistocene snail was thought to be extinct until it was rediscovered in this area, along with other snails new to science. The snail’s habitat is called an “algific talus slope,” a name that describes the refrigerated slopes described above. Many of these species are glacial relicts — souvenirs of the Ice Age.
Fortunately for the Iowa Pleistocene snail, glaciers of the Wisconsin Glaciation didn’t touch its home in the Driftless Area, part of which is now protected as Driftless Area National Wildlife Refuge.
The Shock Doctrine: The Rise of Disaster Capitalism - 
