Northeastern Illinois has a great deal of geological history hidden beneath the surface. If you just take a trip to the Larson Quarry or even Pit 11 you can find some interesting rock formations as well as fossils in them. But, if you cannot get to these areas, this paper can tell you what you can find in 5 different locations in Northeastern Illinois. We will be looking at Larson Quarry, Thornton Quarry, Mazon River, Pit 11, and the Glacial Drift which is at Palos Hills and throughout the surface of the Chicago area).
The first stop is the Larson Quarry. This is located near DeKalb, Illinois. The specific coordinates are 42? N, 88? 35’ W at 920 feet. This site contains specimens from 450 million years ago which include trilobites like Flexicalymene, brachiopods like platystropia, rafnesquina, leptaena, the horn coral streptelasma, an unnamed bivalve, bryozoans, various crinoid stem parts, the cephalopod endoceras, and the gastropods cyclonema and malcurites.
The rock types and formations that were found around these fossils were Cincinnatian Maquoketa Shale and Galena Dolomite. “The Maquoketa shale, which contains seams of argillaceous dolomite…and ranges from highly to slightly weathered, with the highly weathered portions having the consistency of a stiff clay and are extremely fissile” (Preber). These fossils were preserved by replacement as well as mold and cast. The Mazon River, which is near Morris, Illinois is located at 41? 20’N, 88? 30’W at 525 feet.
This site had fossils from 290 million years ago and included seed ferns like alethopteris and neuropteris, true fern Pecopteris, horsetail rush parts calamites (trunk), and annularia (leaf whorl), lycopod tree (club moss) parts and bark lepidodendron, and leaf lepidophylites. The rock types were shale and sandstone as well as coal and the formation was Francis Creek. The Illinois State Museum writes in an article about how Francis Creek formation is formed: “Much of the area that we now call Illinois was a mixture of swampy lowlands and shallow marine bays. From the northeast flowed at least one major river system.
The river(s) built large deltas through the low swamps and into the shallow bays. The mud that the river(s) carried was deposited in these deltas and bays. This mud turned into a rock called the Francis Creek Shale”. The preservation type was impressions in concretions. Next is Pit 11 which is the Mazonia/Briadwaood State Fish and Wildlife area at 41? 15’N, 88? 15’W at 590 feet. These specimens were found from 290 million years ago. The fossils include the same basic plants as the Mazon River.
They also include jellyfish like the essexella, the shrimp belotelson, clams like edmontia and aviculopectin, an unnamed marine annelid worm, a possible fish fossil coprolite, the infamous Tully Monster: tullymonstrum, and the echinoderm sea cucumber holothurian. The rock types that held these fossils were shale and sandstone, as well as coal. These were in a Francis Creek formation. These fossils were preserved by impressions in concretions. It becomes quite apparent that there are certain biases in how and what fossils are preserved.
Some species, notably Pecopteris unita, are found with fertile structures nearly half of the time while others like the more common true fern, Pecopteris acadica, are rarely seen with fertile structures being preserved at all” (Mazon Creek…). This tells us how these fossils were formed. Thornton Quarry, at coordinates 41? 35’ N, 87? 35’ W at 590 to 250 feet, is located on the south side of Chicago. This Quarry was formed 420 million years ago and includes fossils such as the trilobite calymene, brachiopod pentamerus, corals like favosite and halysites, an unnamed crinoid stem and the cephalopod moorecoceras.
The rock types that were here were limestone-dolomite in Niagaran Racine Dolomite with fossils in mold and cast. “The most famous reef unearthed in the Chicago area is the coral reef…Debris from the growing reef began to be deposited and cemented. The reefs contained sea lilies, animals resembling horseshoe crabs, and ancient ancestors of squids and octopuses up to ten feet long. The ashy, gray-white limestone mined from the Thornton Reef contains the fossils of these animals” (Herweck). The reason that there are multiple types of coral is because this area was at the bottom of the Silurian Sea and molded well into the limestone.
Lastly, we visit Palos Hills which includes the Glacial Drift. This area was from ~15,000 years ago and showed deposits of unconsolidated pebbles, sand, silt ad clay which contain mixed rocks of 400 million to 1 billion years old at 41? 41’N, 87? 50’ W at 772 feet. The fossils found here are mammoth or mastodon remains that are preserved as unaltered hard parts. There are also mixed marine fossils including crinoid stems, brachiopods, corals like hexagonaria (Petosky Stone) and trilobite parts, where were preserved as mold and cast r replacement.
The glacial till include striated rocks composed of a mixture of many types including sedimentary (limestone/dolomite), igneous (granite, basalt) and metamorphic (gneiss). The formation was Valparaiso Moraine. “At this time, glacial ice covered the entire Chicago region…. the active ice sheet retreated into the Lake Michigan basin, although stagnant, melting ice remained behind. The ice then rapidly re-advanced to the…Valparaiso moraine in the western Chicago region” (Grimm).
The glaciers that were in Chicago made a big impact on picking up rocks and sand and dirt from other regions and bringing it into the area, which is a reason why you may find interesting things in the area and that rocks age from 400 million to 1 billion years ago. The areas that we can find an assortment of fossils and rock formations are abundant in Illinois and have a wide variety of ages. From this summary, you can see what type of rock are around Northeastern Illinois and what type of animals, plants, and rocks were around from different periods and what type of environment they must have lived in.
Courtney from Study Moose
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