The Hoosier's Story – Natural history 1

A half billion years ago, the chunk of earth crust that now underlies the 202,000-acre Hoosier National Forest basked in the equatorial sun. During the Paleozoic Era, geologists say, the tectonic plate on which Southern Indiana rests—called the North American Plate—lolled about five hundred miles south of the equator, roughly where the Amazonian city of Manaus, Brazil, lies today.

Like the planet’s other twelve plates, the North American has since merged, diverged, and drifted for five hundred million years through a geologic process known as plate tectonics, momentarily settling some thirty-five-hundred miles to the north.

During the Paleozoic Era, which geologists say lasted from 570 million to 266 million years ago, the Hoosier’s rugged hill country occupied the floor of a shallow tropical sea that incessantly ebbed and flowed and changed depths. As time passed, sedimentary materials—which included skeletons, shells, bones, and other plant and animal remains—settled to the ocean floor and compressed and cemented into the bedrock that underlies Southern Indiana today.

The Hoosier sits atop limestone, siltstone, sandstone, and shale bedrock that formed during the Paleozoic’s Devonian Period, which lasted from 360 million to 320 million years ago. Two subsequent geologic eras have shaped the natural regions that encompass the national forest. The Mesozoic Era, a.k.a. the Age of Dinosaurs or middle life, lasted from 245 million to 66 million years ago. The Cenozoic Era, the Age of Mammals or recent life, continues today.

Since the North American Plate began its northward journey, the crust that now supports the Hoosier National Forest has risen some nine-hundred feet above the sea in its higher elevations and been transformed by other epic forces of nature, most recently and significantly the Pleistocene Epoch, or the Ice Age. According to the Indiana Geological Survey: “No other event since the extinction of the dinosaurs sixty-five million years ago can compare to the Ice Age in terms of the profound effect it had on our landscape and the natural environment in which we live today. In fact, virtually all of societal affairs are in one way or another affected by some facet of the Ice Age.”

The Ice Age began two million years ago when continental-sized glaciers, propelled by their own weights and lubed by warming global temperatures, began drifting south from Canada and the Arctic, grinding, gouging, and reshaping the North American terrain during untold advances and retreats. No one knows how many of these ice blocks covered Indiana, but geologists say there were at least eleven, perhaps as many as eighteen.

As the Ice Age glaciers transported and deposited rocks, soils, and other sedimentary materials in new locations, they redirected Indiana’s drainage patterns south to the post-glacial Ohio River. In pre-Pleistocene times, Southern Indiana drained northwest to the Mississippi River in Western Illinois, via the Teays River, which stretched from North Carolina and Virginia, through Northern Indiana.

The two most recent ice events—the Illinoisan and Wisconsin Glacials—began almost a quarter million years ago and shaped all of Indiana’s present-day landforms, the Hoosier’s included. Both were prompted by a warming climate that caused glaciers in the Hudson Bay and Labrador sections of Canada to glide south across Michigan and Indiana.

The Illinoisan, which lasted from 220,000 to 70,000 years ago, covered the entire state, save for a broad, U-shaped chunk of Southern Indiana that included the Hoosier. The Wisconsin Glacial, which lasted from 70,000 to 10,000 years ago, stopped along an uneven line from Terre Haute to Brookville, well north of the national forest.

While the entire Hoosier National is unglaciated, the Illinoisan and Wisconsin Glacials nonetheless are responsible for its rugged landforms and diverse varieties of flora and fauna.

As the view south from the Hoosier’s Hickory Ridge Lookout Tower in Monroe County suggests, pre-glacial Southern Indiana was a vast, flat plain. From the state's highest point in East Central Indiana to its lowest where the Wabash meets the  Ohio River, the elevation drops less than a thousand feet.

The Hoosier’s 950-foot highpoint at Bald Knobs in Jackson County is a mere 530 feet higher than its 420-foot low on the Ohio River near Tate’s Hollow.

But as the planet warmed and the Wisconsin glaciers began melting some 13,600 years ago, their meltwaters rushed southward, carving Indiana's Ohio River Watershed through those flats and underlying stone into the hill-and-vale topography that typifies the Hoosier National Forest here in the Cenozoic Era.


Hoosier National Forest Photographs: Top, Hemlock Cliffs; Second, McPike Pond; Third, Hickory Ridge Lookout Tower; Bottom, Buzzard Roost


 

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