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Reflections
Glaciers sculpted and shaped the Kendall County landscape : Reflections : Oswego Ledger-Sentinel : Hometown Newspaper for Oswego and Montgomery, IllinoisGlaciers sculpted and shaped the Kendall County landscape
| 3/8/2012
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No one really knows why, but about 1 million years ago, an age of glaciation began on Earth. Today, geologists call that ancient time the Pleistocene Epoch, and it resulted in the Kendall County landscape we see today.
During the Pleistocene, glaciers advanced and retreated four times here in North America. At their greatest extent, glaciers thousands of feet thick covered more than half of North America, including virtually all of northern Illinois.
For one reason or another, glaciers did manage to miss a few areas. The extreme tip of southern Illinois, in the area of today's Shawnee National Forest, was just outside the area covered by Pleistocene glaciers. And about 10,000 square miles in southern Wisconsin and northern Iowa and Illinois, known as the Driftless Area, was apparently an ice-free island when the last great ice advance-the Wisconsin glacier-covered most of North America.
But what is today Kendall County did get buried by thousands of feet of the ice. As the glacial ice advanced, it caused huge ridges of rock and pulverized soil to bulge up at the bottom of the glacial face. Glaciers not so much dozed up rocks and soil as it pushed out material from beneath them. The huge weight of ice caused the ice at the glacier's bottom to extrude and flow outwards creating huge ridges of rock and soil. As the glaciers melted and receded, these deposits, called moraines, were left behind. Today, moraines not only dominate our landscape, but are also mined for the gravel they contain, making them one of Kendall County's major economic assets.
As the glaciers rhythmically advanced and retreated during the Pleistocene, the weather along the ice sheet's front became as wild as the topography.
Glacial melt rivers as large as the Mississippi gushed out of the ice sheet, their force grinding rock and vegetable matter into fine silt. Eventually the ice retreated far enough to allow areas of land covered with silt from the huge ice melt floods and rock the glaciers had pulverized to fine dust to dry out. Then the wild post-glacial winds began to blow, picking up the dust and silt and depositing the resulting fines-called loess by geologists-in the valleys between moraines, creating today's topography of gently rolling prairies broken by ridges.
The glaciers also brought the first humans to North America. As the Pleistocene Epoch progressed, the level of the world's ocean levels began to drop, as more and more water fell as snow, compacted, and was built into polar ice sheets that slowly advanced southward, driven by their own weight.
As ocean levels dropped, areas of seabed became dry land. In Europe, England became part of the continent as the English Channel dried up. In North America, the Bering Strait became a land bridge.
No one really knows how long the Bering land bridge existed, but it was there long enough for some of the gigantic mammals of the era-giant ground sloths, mammoths, mastodons, cave bears, saber-toothed tigers, and giant bison-to wander across it looking for better grazing and prey. As the animals moved, so did the small groups of human hunters who relied on the herds for their food.
Gradually, these people of Asian descent followed their mobile food supply southeasterly around the edge of the ice sheet, some afoot, and some apparently by skin boat closely following the icy shoreline. Others may have even come west from modern Europe following the newly emerged coastline.
After reaching North America they spread out, colonizing the land. Along the way, they left evidence of their passage. Even today, a few distinctive Paleo-Indian spear points are found in Kendall County, the last reminders of the area's earliest residents.
As the ice sheet melted and retreated to the north for the last time, the climate warmed. The wind-deposited loess between glacial moraines became fertile land. Ever so slowly, the evergreen forests of the glacial era gave way to deciduous oak and maple climax forests. Summer and fall brought wind and lightning storms that set the prairie plants afire, killing off shrubs and inhibiting tree growth, thus forming grasslands punctuated with groves of trees. The Native Americans who arrived in this rich land enhanced the natural burning process, to create areas more attractive to the deer, bison, and other game animals both the Indians and the settlers who replaced them used for food.
The prevailing westerly winds drove the prairie fires before them, so groves of hardwoods grew up primarily around watercourses and wetlands, with fewer trees on the west side, and heavier forests on the east banks of local streams.
The area's Indian inhabitants slowly changed their cultural ways. Wandering Paleo hunters changed to the hunter-gather people who relied on plants as well as game for food. Gradually, hunter-gatherer bands settled, and scattered villages grew. Cultivation of a few wild foods began, and fish became a major food source. Eventually, the magic grain, maize, arrived via trade with Meso-America where it had been bred, allowing the complicated and rich Mississippian culture to thrive.
The Mississippians depended on farming for much of their food, and from their capital at Cahokia on the Mississippi River, satellite towns grew along all the state's major watercourses, including the Fox River. Prominent Mississippian settlements grew throughout Kendall County, including at today's Meramech Hill and at Oswego. For reasons still unclear-they left no written history-Mississippian culture disappeared shortly before Columbus arrived, its place taken by the tribal culture the first explorers found here in the 1600s.
Gradually, the Illinois, Pottowatomi, and other Native American people were replaced by American settlers who, like the Native Americans before them, made use of the area's rich soil.
The glaciers scoured out the Great Lakes and created the landforms that are now so familiar, and also created the rich farmland that's so much a part of our region's heritage. You're invited to find out more about the majestic-and ongoing-geologic processes that formed today's landscape at 7 p.m. next Thursday, March 15, when my friend Ray Wiggers speaks on "Kendall County Geology" at Oswego's Little White School Museum. For more information, call the museum at 630-554-2999.
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