The native bedrock of northern Illinois is sedimentary limestone, dolomite, and chert. These rocks were formed from the deposits of marine animals and plants that lived and died in the warm, shallow sea that covered Illinois during the Ordovician and Silurian Periods 490-420 million years ago. However, we also find many igneous and metamorphic rocks in Illinois that are not native, but instead originated in the Great Lakes region and Canada. These were brought here by glaciers during three glacial episodes that began 1.8 million years ago and ended a mere 13,000 years ago. During that time, huge sheets of ice and snow extended and retreated over Canada and parts of North America over and over again as the climate cooled, then warmed, then cooled again. Each time, the glaciers bore down on the landscape, picking up and crushing the top layers of bedrock, carrying it along, and leaving the debris behind when they melted.
We have a very good record of deposits from the two more recent glacial episodes. The earlier of those is called the Illinois Episode because the glaciers reached their southernmost point across the continent in Illinois. It occurred between 180,000 years ago and 125,000 years ago. All but the farthest tip of southern Illinois was covered by ice, and, oddly, a small corner of northwest Illinois that is now Jo Daviess County. If you have ever seen a large boulder in a field in the Rock River region, it was brought here during the Illinois Glacial Episode. A layer of material (till) was also left behind as the glaciers melted into the Mississippi and Illinois River Valleys. Some amount of glacial till is beneath you right now, though in Ogle county it might be only five feet thick, compared to five hundred feet near Peoria.
The next glacial episode was named for Wisconsin, and lasted from 25,000 years ago to 13,000 years ago. It impacted northeast Illinois from Chicago to DeKalb and south past Champaign. The glaciers, traveling through the Great Lakes, barely entered what is now Ogle County. The closest glacial advance reached about 16 miles east of where you are standing. However, large amounts of water washed over this area when the glacier melted, carrying debris to the Rock River and its tributaries and carving new drainage channels such as the Leaf River. Sand and gravel deposits on the shores of the Rock River are from this glacial episode.
You can find the required information to log this Earthcache from anywhere on the property, though in some areas it’s more obvious than in others. The posted coordinates will bring you to a spot where you can find the information you need.
Your task is to determine whether the deposits that make up Bald Hill were from the Illinois or Wisconsin Glacial Episode. To log this cache, please message the CO with your answer.
If the deposits are from the Illinois Glacial Episode, you would see:
large boulders, shallow deposits of gravel (or none), and native limestone at the surface.
If the deposits are from the Wisconsin Glacial Episode, you would see:
sand, gravel, and clay in very thick deposits; small rocks made of quartz, jasper, flint, and granite; no visible limestone outcroppings at the surface.
