Geological History Of Illinois

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Geological History of Illinois

Geological History of Illinois


Chicago is the third populated city in the United State. It is located on the 'north eastern Illinois' on the south western coast of Lake Michigan. It's laid by the thick succession of sedimentary rocks consisting primarily of shale, sandstone, siltstone, limestone, mostly of which is overlain by cross-bedding deposits of silt, clay, gravel and sand. The distribution, thickness, composition and fossil content of these sediments and rocks, including the interregional and regional unconformities which form the boundaries, revealing the important clues about the geological history of Illinois during the past 500 million years.


Chicago's Geology- Cambrian Period

Distributed widely in the US, this is lower grouping of sediments the Potsdam sandstone this was of the Cambrian time. That's the earliest form of Paleozoic formation within the northern territory which gives the evidence that the larger portion of continent was land that was open to erosion whilst the previous western and the eastern seas were being formed by Cambrian formations.

Silurian Period Lower Magnesian group

This Potsdam group is formation of limestone between 160 and 450 feet thick overlying over the beds. Even though a different and new fauna was formed, the clearer waters existed. And possibly the lower magnesian form of limestone were resultant from calcareous remains of sea life, where well potted fossils have been very exceptional.

Early Silurian dolomite's succeeding sand-St Peter group

The St. Peter sandstone formed because of the coming back to flattering condition to deposition of sand and the regions reformed in the fauna, which over lays the Lower form of Magnesian set. This sandstone is extremely white and porous differing greatly in thickness ranging from sixty feet thickness reaching wells, at Goose Island in N.Branch of Chicago River, which is 2.5 miles NW of Chicago Harbor inlet, about 200 ft at Chicago Heights and 450 ft at S.Evanston.

Mid Silurian Time of Magnesian limestone Trenton group

During next period which followed, limestone of the Trenton group was being deposited to this region. And as judge by fossil residue at the places where the rock isn't opened to the outside of the Trenton Sea lived a varied and abundant fauna which consists of crinoids, crustaceans, mollusk, corals and other kind of invertebrates. These limestone are usually magnesian.

Shale or Mud rocks succeeding the Trenton limestone- Hudson or Cincinnati group

Trenston limestones are formed or composed of mudstones or thin bedded shales. These are mostly consists of very fine mud sediments that were carried into ocean or sea by drainage. There is a possibility that those fine sediments could pollute water which affected the organisms living it. As the changes in fauna, the new class of species arrived in and if they weren't able to become accustomed the habitat they got extinct or were forced to migrate.

The two vast islands were came into existence in interior continental at the end of the Hudson epoch in area of Ohio, Tennessee, Indiana and also Kentucky which affect the conditions of rock ...
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