Illinois by Time Period



To find out what was happening in Illinois during specific time periods of Earth history, scroll down the time period of choice. A nice, printable geologic time table from the Illinois State Geologic Survey can be be viewed. This site gives more detail about the specific time period divisions and general character of the sediments deposited at that time.


PRECAMBRIAN: The Precambrian time period is divided into the Phanerozoic and the Proterozoic Eras. During the Precambrian not a lot happened in Illinois. Cascade style volcanics and granite intrusions occurred 1.5 billion years ago. Later, at 1.15 billion year ago the rifting (separating) of continental plates created a weak zone along which the Mississippi River later formed. This is the zone of the New Madrid Earthquake. top

CAMBRIAN: Illinois was emergent for most of the Cambrian period. Toward the end of the Cambrian the sea came in and deposited the sands and muds that created the dolomites, sandstones and shales now found in Illinois. top

ORDOVICIAN: Illinois was covered by shallow seas during the Ordovician. Marine limestones and dolomites were deposited. In the late Ordovician sands were deposited. The St. Peter Sandstone, which formed from these sands, creates the backbone for Starved Rock, Buffalo Rock, and Mattheissen State Parks. top

SILURIAN: Once again shallow seas covered Illinois. The seas at this time were teeming with life. Near Chicago was an extensive reef, with corals, hard-shelled organisms (like snails and clams), trilobites, crinoids, and more. These reef limestones are now quarried for construction materials. top

DEVONIAN: Deep stagnant basins occurred in Illinois during the Devonian. The sediments deposited in these basins would later form thick black shale deposits, which are also found in Indiana and Kentucky. Important changes in life occured during the Devonian, such as the development of fish and amphibians. top

MISSISSIPPIAN: Shallow seas once again invaded Illinois. Limestone were deposited which form the caves and karst topography of Western Illinois. The lead and zinc deposits of Northwestern Illinois, Iowa, and Wisconsin were formed at this time. top

PENNSYLVANIAN: Coastal swamps dominated the Illinois landscape. These swamps later formed the vast coal deposits that underlie 2.3 of the state. Land plants were abundant strange swamp creatures exsisted, including the state fossil, the Tulley

CRETACEOUS: The Age of Dinosaurs! But not in Illinois. Illinois was emergent during this time, except for some near shore clays and sands that were deposited. However, three exciting things were happening in Illinois. A meteorite hit outside of Chicago, the beautiful state mineral, fluorite, was formed, and strange igneous rocks, called cryptovolcanic structures, were intruded below the

TERTIARY/QUATERNARY: During the Tertiary marine sands and clays were deposited in Southern Illinois. These absorbent clays, called Fuller's Earth, are used as kitty litter! The Ice Age occurred during the latest part of the Quaternary, the Pleistocene epoch. Illinois was covered by four different continental glaciers, filling in the river valleys and carving the Great Lakes. When Illinois was not covered by ice, huge animals roamed the area. top