North America’s fourth and last major glacial invasion was first studied in detail in Wisconsin. The Wisconsin Glaciation flattened much of the Midwest and gouged great chunks out of the ground. The glaciation ended just 10,000 years ago, leaving much of the northern Great Lakes states covered with glacial debris—boulders and gravel left by retreating glaciers. Fossils of western bison, woodlands muskoxen, mastodons, and other animals that followed the retreating Wisconsin icesheet into extinction lie scattered across the state.
Do you want to see what Wisconsin looked like before it was flattened by the Wisconsin Glaciation? Visit the Wisconsin Driftless Area, which covers most of the Western Upland in southwest Wisconsin. Untouched by the Wisconsin Glaciation, this region boasts some of Wisconsin’s most rugged scenery. The Driftless Area is the only region without lakes in a state where more than 8,500 have been charted.
In the Driftless Area, streams have carved narrow ridges and deep, steep-sided valleys out of the limestone as they dug through the evidence of earlier Ice Age glaciers, back more than 65 million years to the Cretaceous Period. These streams have uncovered poorly preserved leaf impressions and petrified wood from a time when dinosaurs stalked the land.
But far more common than dinosaur fossils are the remains of animals that lived during the Paleozoic Era, which ended about 230 million years ago. If what is now Wisconsin had been dry land back then, it would have few ancient fossils. Why? There was little life on land during the early Paleozoic. Fortunately, Wisconsin was generally underwater, as you could guess from its Paleozoic fossils. These include ancient corals, bivalves (such as clams and oysters), and trilobites.
Trilobites are common in many of the Paleozoic Era rocks of southern Wisconsin. Trilobites from the Cambrian Period, which ended half a billion years ago, are often found as crushed or broken fragments, casts, or molds that are difficult to identify. Rocks formed in the Silurian Period (395-435 million years ago) harbor fossils of Wisconsin’s official state fossil, the trilobite Calymene celebra.
Wisconsin’s Ice Age
But the prehistoric period Wisconsin is best known for is probably the Ice Age, which gripped Wisconsin so recently. Someone suggested that the trilobite be declared Wisconsin’s official invertebrate fossil, allowing the adoption of the mastodon as the state’s official vertebrate fossil. Wisconsin’s most spectacular vertebrate fossil is probably a mastodon skeleton found near Richland Center. It is on display in the Geology Museum at the University of Wisconsin-Madison.
One period of the Ice Age is even named for Wisconsin. It’s called the Wisconsinan Glaciation.
Geology & Fossils
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