Page snapshot: Illinois State Geologic Map; Fossil; 3D models; Rock; Mineral; Gem; Highest and Lowest Elevations; Places to Visit; and Additional Resources.
Image above: Foliage of the Carboniferous seed fern Neuropteris flexuosa from the Mazon Creek biota, Carboniferous of Illinois. Photograph by James St. John (Flickr; Creative Commons Attribution 2.0 Generic license; image resized and cropped).
Geologic Map of Illinois
Geologic map of Illinois showing maximum ages of mappable units. Image by Jonathan R. Hendricks for the Earth@Home project developed using QGIS and USGS data (public domain) from Fenneman and Johnson (1946) and Horton et al. (2017).
Illinois State Fossil: Tullimonstrum gregarium (Tully Monster)
The Tully Monster or Tullimonstrum is an unusual animal from the Carboniferous Mazon Creek that is thought to have swum using its three-finned tail and hunted using its eight-toothed proboscis to bring prey to the mouth. Its place on the tree of life is uncertain, though some scientists suspect it is a vertebrate. Without the exceptionally preserved specimens at Mazon Creek, this soft-bodied predator would be unknown to science. Tully Monsters reached lengths of up to 30 cm (1 foot), but most specimens are less than half that length.
Fossil of Tullimonstrum on display at the Museo di Storia Naturale di Milano. Photograph by Ghedoghedo (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license).
Artist's reconstruction of a living Tully monster. Illustration by "Entelognathus" (Wikimedia Commons; Creative Commons Attribution-Share Alike 4.0 International license).
"Tully: Monster vs Method." Video by the Field Museum (via Vimeo).
3D Models of fossils from Illinois
Fossil foliage (part) of the fern Crenulopteris acadica sp. from the Pennsylvanian of Kanakee County, Illinois (PRI 42681). Pinna (portion of a compound leaf). Marattioid fern (Order Marattiales, Family Psaroniaceae). Specimen is on display at the Museum of the Earth, Ithaca, New York. Length of specimen is approximately 16.5 cm.
Fossil foliage of the horsetail (Equisetophyte) Annularia stellata preserved in a concretion from the Pennsylvanian of the Mazon Creek area of northeastern Illinois (PRI T-1762). Note the whorls of leaves around the branch. Annularia is one of the foliage taxa for extinct, tree-size horsetails (Calamitaceae). Specimen is from the teaching collection of the Paleontological Research Institution, Ithaca, New York. Length of surrounding rock is approximately 15 cm.
Portion of a fossil stem of the horsetail (Equisetophyte) Calamites sp. from the Carboniferous of Illinois. Note the nodes (horizontal bands) and internodal ribs (vertical lines). Specimen is from the Cornell University Paleobotanical Collection (CUPC), Ithaca, New York. Length of specimen is approximately 14 cm.
Fossil specimen of the cephalopod Actinoceras beloitense from the Ordovician Platteville Limestone of Ogle County, Illinois (PRI 70326). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Length of specimen is approximately 9 cm.
Fossil specimen of the bryozoan Archimedes wortheni from the Carboniferous (Mississippian) Warsaw Limestone of Hancock County, Illinois (PRI 70774). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Length of specimen is approximately 11 cm.
Fossil specimen of the blastoid Pentremites godani from the Mississippian Paint Creek Formation of St. Clair County, Illinois (PRI 70771). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York.
Illinois State Rock: Dolostone
Dolostone became Illinois's official state rock on January 1, 2023.
Illinois State Mineral: Fluorite
Southeastern Illinois once produced the majority of fluorite in the U.S., and it is the site of the most recently active fluorite mine in the country.
Sample of fluorite from Illinois. Photograph by James St. John (Flickr; Creative Commons Attribution 2.0 Generic license).
Illinois State Gem: None
Illinois does not yet have an official state gem.
Illinois's Highest and Lowest Elevations
Topographic map of Illinois with physiographic regions and point of highest elevation identified. Topographic data are derived from the Shuttle Radar Topography Mission (SRTM GL3) Global 90m (SRTM_GL3) (Farr, T. G., and M. Kobrick, 2000, Shuttle Radar Topography Mission produces a wealth of data. Eos Trans. AGU, 81:583-583).
Highest Elevation: Charles Mound
Charles Mound, Illinois’ highest point at 376 meters (1235 feet), is located in the state’s extreme northwest and is part of the Driftless Area.
Photograph of Charles Mound, the point of highest elevation in Illinois. Photograph by Skye Marthaler (Wikimedia Commons; Creative Commons Attribution-Share Alike 4.0 International license).
Lowest Elevation: Ohio and Mississippi Rivers
Illinois’s lowest point, at 85 meters (280 feet), is the confluence of the Ohio and Mississippi Rivers at the state’s southern tip.
Places to Visit
"Sue" the T. rex on display at the Field Museum. Photograph by Elizabeth J. Hermsen.
Specimen of Dunkleosteus on display at the Burpee Museum. Photograph by Amy Meredith (Flickr; Creative Commons Attribution-NoDervis 2.0 Generic license).
Trilobites on display at the Fryxell Geology Museum. Photograph by Kara Brugman (Flickr; Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic license).
Additional resources
Earth@Home resources about Illinois and nearby states:
- Rocks: Superior Upland, Central Lowland, and Inland Basin
- Fossils: Superior Upland, Central Lowland, and Inland Basin
- Topography: Superior Upland, Central Lowland, and Inland Basin
- Mineral Resources: Superior Upland, Central Lowland, and Inland Basin
Earth@Home resources about the Midwest region of the United States: