The geology of Illinois includes extensive deposits of marine sedimentary rocks from the Palaeozoic, as well as relatively minor contributions from the Mesozoic and Cenozoic. Ice age glaciation left a wealth of glacial topographic features throughout the state.

Precambrian Geology

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

rocks of Illinois are deeply buried by 2000–13000 feet (610–3960 m) of later sediments. Drilling has revealed these rocks to be primarily intrusive felsic igneous rocks, though some extrusive felsic rocks have also been recovered from boreholes. Precambrian rocks of Illinois are highly faulted; tectonic extension and related thermal subsidence have led to the formation of two major sedimentary basins. These basins, termed the

Illinois Basin The Illinois Basin is a Paleozoic depositional and structural basin in the United States, centered in and underlying most of the state of Illinois, and extending into southwestern Indiana and western Kentucky. The basin is elongate, extending app ...

and

Michigan Basin The Michigan Basin is a geologic basin centered on the Lower Peninsula of the U.S. state of Michigan. The feature is represented by a nearly circular pattern of geologic sedimentary strata in the area with a nearly uniform structural dip toward ...

, allowed for extensive deposition of sedimentary rock during the

Palaeozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and '' ...

Era. The

is a northwest-southeast asymmetrical

structural basin A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat-lying strata. They are geological depressions, the inverse of domes. Elongated structural basins are also known as synclines. ...

that is filled with more than 4000 meters of

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s. The basin covers most of Illinois, and extends into western Indiana and western Kentucky. The basin is bounded to the north by the

Mississippi River The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it f ...

and the Kankakee Arch, to the east by the

Cincinnati Arch The Cincinnati Arch is a broad structural uplift between the Illinois Basin to the west, the Michigan Basin to the northwest and the Appalachian Basin and Black Warrior Basin to the east and southeast. It existed as a positive topographic area du ...

, and to the south by the

Ozark The Ozarks, also known as the Ozark Mountains, Ozark Highlands or Ozark Plateau, is a physiographic region in the U.S. states of Missouri, Arkansas, Oklahoma and the extreme southeastern corner of Kansas. The Ozarks cover a significant portio ...

uplift Uplift may refer to: Science * Geologic uplift, a geological process ** Tectonic uplift, a geological process * Stellar uplift, the theoretical prospect of moving a stellar mass * Uplift mountains * Llano Uplift * Nemaha Uplift Business * Uplif ...

and Pascola Arch.

Palaeozoic Geology

For much of the Palaeozoic, Illinois was located much further south than today, instead being near the equator; it was also underwater for much of this time, forming a shallow continental sea.

Cambrian

The oldest Palaeozoic rocks in Illinois are those of the upper

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

Mt. Simon Sandstone, the most basal of which are interpreted as being braided river deposits, while the remainder of the formation seems to represent marine tidal environments. These tidal environments included both tidal channels and tidal flats; desiccation cracks and

ripple marks In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (current or waves) or wind. Defining ripple cross-laminae and asymmetric ripples * ''Current ripple marks'', ''unidi ...

preserve surface features of the time. Overlying the Mt. Simon Sandstone are the Eau Claire Formation, Ironton- Galesville Sandstone, Franconia Formation, Potosi and Eminence dolomites, and Jordan Sandstone; of these, only the Potosi is exposed at the surface, in

Ogle Ogle may refer to: Places * Ogle County, Illinois, United States * Original name of Ashton, Illinois, a village * Ogle, Kentucky, United States, an unincorporated community * Ogle Township, Somerset County, Pennsylvania, United States * Ogle, N ...

and Lee Counties . These rocks suggest a gradual increase in local sea level over the time of the Cambrian; they also suggest that most sediment was being transported to the area from the North. Fossils are uncommon in the Cambrian of Illinois, but

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...

, and

trace fossil A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...

s of worms have been discovered.

Ordovician

Rocks of

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

age are best exposed in the Northwestern part of the state, largely in the Driftless Area (see below). Ordovician rocks in the state are separated from Cambrian and Silurian rocks by

unconformities An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

. Most of the Ordovician saw continued offshore marine deposition throughout the entirety of the state; however, Southern parts of the state saw some deposition of shallow-water carbonates and evaporites, indicating that some areas of the state were significantly shallower than others, and even exposed above water at times. Ordovician rocks in Illinois are divided into three series, each separated by an unconformity; from oldest to youngest, these are the Canadian, Champlainian, and Cincinnatian series. Ordovician features in Illinois include the now-buried Glasford Structure in

Peoria County Peoria County is located in the U.S. state of Illinois. The 2020 United States Census listed its population at 181,830. Its county seat is Peoria. Peoria County is part of the Peoria, IL Metropolitan Statistical Area. History Peoria County was ...

, a crater caused by a meteorite impact roughly 455 million years ago. It and a similar buried crater in Cook County have been associated with the

Ordovician Meteor Event The Ordovician meteor event was a dramatic increase in the rate at which L chondrite meteorites fell to Earth during the Middle Ordovician period, about 467.5±0.28 million years ago. This is indicated by abundant fossil L chondrite meteorites in ...

Silurian

Almost all

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

rocks in Illinois are deep-water

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

deposits; reef habitats were common, and fossils of reef organisms are locally highly abundant, including

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

s, stromatoporoids, and

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a l ...

ns.

Devonian

An unconformity separates the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

rocks of Illinois from those of the Silurian; the oldest Devonian rocks in the state are therefore from the middle part of the period. These rocks are also primarily marine limestones and shales, with the upper Devonian rocks of the state being carbon-rich black shales; some evaporite deposits are also present. There is evidence of significant intervals of hypersaline water in the middle Devonian in Illinois. Fossils include brachiopods,

s, corals, bryozoans, algaes, and

conodont Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

Carboniferous

The earliest

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

rocks sit conformably on top of the youngest Devonian in Illinois; Carboniferous rocks in the state are areally extensive, regionally very well-exposed, and form a large percentage of the state's bedrock. Illinois remained marine for much of the Carboniferous, with limestones making up most of the rock deposited; however, sandstones, shales, cherts, siltstones, and coals are also present; these indicate marine conditions, but also terrestrial swamp conditions. Carboniferous fossils include the world-famous

Mazon Creek fauna Mazon may refer to: Places in the United States * Mazon, Illinois, a village * Mazon Creek fossils * Mazon River * Mazon station, Mazon, Illinois * Mazon Township, Grundy County, Illinois Organizations * MAZON: A Jewish Response to Hunger, a no ...

, home to the Illinois's State Fossil, ''Tullimonstrum gregarium''. A significant unconformity separates Mississippian from Pennsylvanian strata.

Mesozoic Geology

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

rocks are overall poorly exposed in Illinois; those present are

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

in age and only seen in extreme southern parts of the state. They are largely terrestrial sands and gravels, though one marine unit, the

Owl Creek Formation The Owl Creek Formation is a geologic formation ranging from Tennessee to Mississippi. It preserves fossils dating back to the Cretaceous period. Ornithomimids and ceratopsid remains are known from the formation. The ceratopsid tooth could repr ...

, indicates that the

Western Interior Seaway The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that split the continent of North America into two landmasses. The ancient sea, ...

covered parts of the state at one point in time.

Cenozoic Geology

Paleogene

Paleocene rocks are present only in the extreme south of Illinois, in

Alexander Alexander is a male given name. The most prominent bearer of the name is Alexander the Great, the king of the Ancient Greek kingdom of Macedonia who created one of the largest empires in ancient history. Variants listed here are Aleksandar, Al ...

and Pulaski counties. These rocks make up the

Clayton Formation The Clayton Formation is a geologic formation in Illinois. It preserves fossils dating back to the Paleogene period. See also * List of fossiliferous stratigraphic units in Illinois This article contains a list of fossil-bearing stratigraphic ...

and Porters Creek Formation; both units are marine. The Porters Creek Formation preserves fossils of

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...

shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimo ...

s, and

bony fishes Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...

. The only

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

rocks in the state, exposed only in Pulaski County, are those of the

Wilcox Formation The Wilcox Formation is a geologic formation in Tennessee. It preserves fossils dating back to the Paleogene period. The first hydrocarbon discover in the formation occurred in 1928, onshore Texas. Subsequent field discoveries included the S ...

. They were deposited in an ancient

river delta A river delta is a landform shaped like a triangle, created by deposition (geology), deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, res ...

Neogene

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Mounds Gravel lies in the southern part of the state, and the Grover Gravel is found as a scattering of deposits throughout the northern part of the state.

Quaternary

During the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

period, Illinois was subject to multiple intervals of

glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...

; over 90% of Illinois was formerly covered by glaciers, leaving a variety of glacial landscape features. The Mississippi River, fed by ice-sheet melt and water from glacial lakes, cut a deep valley as it flowed through the region. The formation of this valley has been constrained as having occurred between 2.5 and 0.8 million years ago. Among the oldest glacial features is the Buffalo Hart Moraine, located in

Logan County Logan County is the name of ten current counties and one former county in the United States: * Logan County, Arkansas * Logan County, Colorado * Logan County, Idaho (1889–1895) * Logan County, Illinois * Logan County, Kansas * Logan County, ...

. This is a terminal glacial moraine; however, unlike most other moraines in the state, it is not

Wisconsinan The Wisconsin Glacial Episode, also called the Wisconsin glaciation, was the most recent glacial period of the North American ice sheet complex. This advance included the Cordilleran Ice Sheet, which nucleated in the northern North American Cor ...

in age but rather

Illinoisan Illinois ( ) is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolitan areas include, Peoria and Rockfor ...

, and as such is roughly 125,000 years old. French Canyon - Starved Rock - panoramio

Kankakee River State Park, located in

Kankakee County Kankakee County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 113,449. Its county seat is Kankakee. Kankakee County comprises the Kankakee, IL Metropolitan Statistical Area. History St ...

, contains evidence of the catastrophic

Kankakee Torrent The Kankakee Torrent was a catastrophic flood that occurred about 19,000 calibrated years agoCurry, B.B., Hajic, E.R., Clark, J.A., Befus, K.M., Carrell, J.E. and Brown, S.E., 2014. "The Kankakee Torrent and other large meltwater flooding ev ...

that occurred roughly 19,000 years ago. This event occurred when the dam of a glacial lake located in what is now the Lower Peninsula of

Michigan Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...

failed catastrophically, leading to a massive influx of water down the channel of the modern

Kankakee River The Kankakee River is a tributary of the Illinois River, approximately long, in the Central Corn Belt Plains of northwestern Indiana and northeastern Illinois in the United States. At one time, the river drained one of the largest wetlands in N ...

. Evidence of this flood can be seen in the high rubble bars that run parallel to the modern river. The torrent also cut through the bedrock of the Joliet Dolomite; this caused the formation of waterfalls in tributaries of the Kankakee as their waters flowed over the hard bedrock and fell down into the canyon cut by the torrent. The Rock Creek Canyon is home to one such waterfall, which is eroding upstream at a rate of 3 inches (7.5 cm) per year The most obvious glacial feature in Illinois is

Lake Michigan Lake Michigan is one of the five Great Lakes of North America. It is the second-largest of the Great Lakes by volume () and the third-largest by surface area (), after Lake Superior and Lake Huron. To the east, its basin is conjoined with that o ...

, the basin of which was carved out by glaciers. As the glaciers receded they left a large number of recessional moraines; among the more visible moraines in the state is the

Bloomington Moraine Bloomington may refer to: Places U.S.A. (most commonly) * Bloomington, Illinois *Bloomington, Indiana * Bloomington, Minnesota U.S.A. (less commonly) * Bloomington, California * Bloomington, Idaho * Bloomington, Kansas *Bloomington, Maryland * ...

, a

terminal moraine that can be seen in Bureau County. This moraine is also associated with a large number of

esker An esker, eskar, eschar, or os, sometimes called an ''asar'', ''osar'', or ''serpent kame'', is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North Amer ...

s and a substantial glacial

outwash plain An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and ca ...

; also associated with this terminal moraine is a series of

sand dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, fl ...

created from sand deriving from glacial outwash. These are primarily parabolic and longitudinal dunes. Located to the Southwest the Bloomington Moraine is a large, very flat plain; this area is in fact the lakebed of

Glacial Lake Pontiac A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...

, which drained about 17,000 years ago. Glacial Park, in McHenry County, preserves a wealth of glacial landscape features, including a delta

kame A kame, or ''knob'', is a glacial landform, an irregularly shaped hill or mound composed of sand, gravel and till that accumulates in a depression on a retreating glacier, and is then deposited on the land surface with further melting of the g ...

named Camelback Kame, as well as wetlands in three glacial kettles—one

marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found at ...

, one bog, and one

fen A fen is a type of peat-accumulating wetland fed by mineral-rich ground or surface water. It is one of the main types of wetlands along with marshes, swamps, and bogs. Bogs and fens, both peat-forming ecosystems, are also known as mires. T ...

. The very northwesternmost corner of the state lies in the

Driftless Area The Driftless Area, a topographical and cultural region in the American Midwest, comprises southwestern Wisconsin, southeastern Minnesota, northeastern Iowa, and the extreme northwestern corner of Illinois. Never covered by ice during the last ...

, so named because it was never covered by glaciers. This area is marked by much more dramatic topography than in the rest of the state, due to the incision of the Mississippi River and subsequent reworking of river systems during the Quaternary Glaciation. In this area, vertical cliffs have been cut into the resistant dolostone of the Ordovician Dunleith formation by rivers.

Apple River Canyon State Park Apple River Canyon State Park is an Illinois state park on in Jo Daviess County, Illinois, United States. The park was begun with land purchased by the state in 1932, as a result of a prolonged campaign led by botanist Herman Silas Pepoon; wi ...

shows some of this dramatic topography in the eponymous canyon; the tributaries of the Apple River enter the main channel pointing upstream, indicating a reversal in flow direction due to the advance of the Illinoian glacier.

References

{{Geology of the United States by political division

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolita ...

Natural history of Illinois