Geology & Fossils
Some paleontologists regard cranes as prehistoric symbols. They are known to have lived over 38 million years ago during the Eocene Period and have that “dinosaur look.”
For thousands of years, sandhill cranes have been congregating in the fall along the Platte River, which flows across Nebraska from west to east. Another living fossil is the paddlefish, a bizarre inhabitant of the Missouri River recently adopted as Nebraska’s state fish.
With no mountains to stop them, seas invaded ancient Nebraska and retreated again and again during the ancient Paleozoic Era and the Mesozoic Era (the Age of Dinosaurs). Towards the end of the Cretaceous Period, which ended about 65 million years ago, the Rocky Mountains began to rise west of Nebraska. Sediments from the Rockies were deposited on the Great Plains, which grew higher and higher. Eventually, the sea retreated for good.
Today, Nebraska remains one of the flattest states. But it slopes gently downward and eastward towards the Missouri River. Some of the rock layers beneath the surface tilt slightly downward in the opposite direction.
This state of affairs allows erosion to uncover pages from various chapters in Nebraska’s prehistory. The Nebraska prehistory book begins along the eastern border. Here rocks from the Pennsylvanian Period are commonly exposed, with Permian rocks in the southeast corner of the state.
To the west, a broad strip of rocks from the Cretaceous Period angles from south-central Nebraska northeast across the state. From here, rocks representing the Cenozoic Era (the Age of Mammals) cover the entire state westward to Colorado.
Some of Nebraska’s better fossil collecting localities are located along major rivers, such as the Missouri, Platte, Niobrara, Republican, Blue and Nemaha and their tributaries. Fossils from the Paleozoic and Mesozoic eras are generally marine invertebrates. These include cephalopods (nautiloids, ammonoids, belemnites, squids, and octopi), bivalves (clams, oysters, scallops, and brachiopods), and gastropods, or snails.
Fossil gastropods, or snails, do not always indicate ancient seas, however. Freshwater and land snails were common in Nebraska during the Ice Age, when the climate was cooler and moister. A wildlife refuge was even created for a surviving Ice Age snail in neighboring Iowa!
Fossil snails are scattered across Nebraska, along with the bones of Ice Age mammals. Here and there, erosion has uncovered fossils of mammals that lived before the Ice Age, or Pleistocene Epoch.
Agate Fossil Beds National Monument
More than ten million years ago, during the Miocene Epoch, a downpour flooded a stream that flowed roughly parallel to today’s Niobrara River. The waters swept away the carcasses of pig-like oreodonts that had been smothered in dust storms on the dry plains. Living creatures that couldn’t outrun the surging waters were drowned.
As the waters subsided, the carcasses were deposited along stream banks. Beneath them were the bones of thousands of other mammals that had been buried in earlier floods.
In 1877, Captain James Cook, a pioneer scout and rancher, discovered this fossil site. Near the post office at Agate, it was designated Agate Fossil Beds National Monument. Most of the fossils had been worn away, but two fossil-filled hills remained.
Most of the fossils belonged to Diceratherium, a rhinoceros that stood little more than three feet high at the shoulder. Its kind were probably as abundant on the plains as bison were only two hundred years ago. Far away in Washington State, a cast of a rhinoceros in a lava flow is thought to have been made by a Diceratherium.
More formidable than a tiny rhinoceros was Dinohyus, a pig that stood seven feet tall at the shoulder!
Paleontologists can often guess an animal’s identity from a single bone. But Moropus, whose bones also occur at Agate bone bed, fooled scientists. One of the largest of a group of animals called chalicotheres, it looked like half a dozen different animals put together. Its head and teeth were horse-like, but it had long claws! Did Moropus use its claws to dig up roots and bulbs? Or did it use them to pull tree branches down?
Another famous Nebraska fossil site is Ash Fall State Park. Here are preserved fossils of prehistoric mammals that were buried under a blanket of volcanic ash.
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