The geologic time scale is the “calendar” for events in Earth history. It subdivides all time into named units of abstract time. The enumeration of those geologic time units is based on stratigraphy, which is the correlation and classification of rock strata. The fossil forms that occur in the rocks provide the chief means of establishing a geologic time scale. Because living things have undergone evolutionary changes over geologic time, particular kinds of organisms are characteristic of particular parts of the geologic record. By correlating the strata in which certain types of fossils are found, the geologic history of various regions—and of Earth as a whole—can be reconstructed.

Major events and evolutionary changes in Illinois can be categorized using the geologic timescale as a framework. For example, human history in Illinois begins in late Pleistocene or early Holocene geologic period.  The Bicentenniel of Illinois Statehood spans the last two hundred years of the Holocene and are higlighted in blue in our Time Periods menu.


  • Cambrian (543 to 485 million years ago)
  • Ordovician (485 to 443 million years ago)
  • Silurian (443 to 420 million years ago)
  • Devonian (420 to 359 million years ago)
  • Mississippian (359 to 323 million years ago)
  • Pennsylvanian (323 to 299 million years ago)
  • Permian (299 to 250 million years ago)
  • Triassic (250 to 200 million years ago)
  • Jurassic (200 to 145 million years ago)
  • Cretaceous (145 to 66 million years ago)
  • Paleogene (66 to 25 million years ago)
  • Neogene (25 to 2.6 million years ago)
  • Pleistocene (2.6 million to 11,700 years ago)
  • Holocene (11,700 years ago to present)

Lincoln College Mammoth

Image of Lincoln College Mammoth bone
In what might have been the most successful college field trip of all time, Lincoln College freshman Judd McCullum discovered the tusk of a Wooly Mammoth (Mammuthus primigenius) during a biology lab at the college’s Creekside Field Station in 2005. McCullum discovered the 12-foot-long tusk while conducting a freshwater mussel survey. The mammoth may have been one of the last of its kind in the region. Carbon-14 dating determined the animal died about 11,500 years ago, well after the glaciers had retreated from Illinois.

Field Guide to the Past

Image of Fossil Scorpion
Most fossil plants and animals from hundreds of millions of years ago were preserved in swampy lowland areas where they had the good fortune to die and be buried quickly by sediments in water. The water preserved the specimens long enough for the living tissues to be replaced by minerals or for the specimen to create an impression in the soft mud that hardened into shale or sandstone under heat and pressure.

All Our Own: The Tully Monster

Image of Tully Monster.

Tully Monster.

Their fossils are locally common but have been found nowhere else on Earth. That’s why the Tully Monster (Tullimonstrum gregarium), a slender soft-bodied creature with a long, narrow snout and sensory organs (primitive eyes) set away from the body on stalks, has come to represent Illinois as the “State Fossil.” Francis Tully found the first one in 1958. He took it to experts at the Field Museum of Natural History in Chicago for identification, but the scientists were stumped. Museum staff with a sense of humor nicknamed it the “Tully Monster.”

Fluorite Crystals

Image of Fluorite crystals.
Fluorite (fluorspar) mining got its start in Illinois in 1842. The mineral is usually referred to as fluorite, while the product that is mined is called fluorspar. It was used as a flux to help remove impurities while smelting metals like iron and aluminum. It also is used in products ranging from optical lenses to fluoride (derived from fluorite) in toothpaste. Fluorspar production peaked in the 1960s when an average of 118,820 tons was mined annually. Fluorspar mining eventually became unprofitable due to competition from overseas producers, coupled with the high costs of underground mining. The last mine in Illinois closed in 1995. Fluorspar is no longer mined in the United States.


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