Geological Science Field Trips 2008-2009

Situated in the rugged Illinois Ozarks, the Thebes area presents some of the most interesting geology in Illinois. Bedrock exposed here consists of Paleozoic, Mesozoic, and Cenozoic age strata. Some of the rocks are quite fossiliferous, especially the Paleozoic rocks. The Mesozoic gravel deposits in this area contain spectacular specimens of unique agates that are not found in the gravel deposits common to the other parts of Illinois. Other interesting features include the gorge of the Mississippi River at Thebes and Cache Valley, a large abandoned channel of the Ohio River. Many products are mined in the area, including tripoli, used as a fine abrasive; fuller's earth, an absorbent clay; and novaculite, a source of cherty gravels.

In marked contrast to most of Illinois, this region was not glaciated. Instead, it was covered several times by waters from the ancient Gulf of Mexico during Cretaceous and Tertiary Periods when the rest of the state was above sea level. The area lies at the north end of the Gulf Coastal Plain, the vast lowland that extends to the Gulf and contains the Mississippi River. The late Cretaceous and early Tertiary strata in this region consist of unconsolidated (loose) marine and nonmarine clays, sands, and gravels. North of the Cache Valley, limestones of Silurian and Devonian ages are exposed.

Participants will tour the Mississippi River area near Thebes, visit two quarries, and stop at a clay pit where fuller's earth is mined for kitty litter.

Meet before 8:15 a.m. at the Spillway Shelter and picnic area, located along Promised Land Road, within Horseshoe Lake Conservation Area. The park is located south of Olive Branch, along Illinois Route 3.

Directions to Horseshoe Lake Conservation Area

Take I-57 south to Exit 1 at Cairo. Follow Illinois Route 3 north toward Olive Branch.

Horseshoe Lake Conservation Area is located southeast of Olive Branch along Illinois Route 3.