State of Illinois
Otto Kerner, Governor

DEPARTMENT of REGISTRATION and EDUCATION
William Sylvester White, Director

1961

ILLINOIS STATE GEOLOGICAL SURVEY
John C. Frye, Chief
URBANA, ILLINOIS

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS
42517-20M 2 (78783)

Although the Earth's age is measured in billions of years, the landscape of Illinois is young—a mere 15,000 years old.

During the Ice Age, most of Illinois was repeatedly invaded by massive glaciers, sometimes rising a mile or more high, that carried with them ground-up rock materials they had scraped from the bedrock to the north as they slowly moved south.

When the last glaciers melted in Illinois about 15,000 years ago, the landscape that emerged was very different from the land before the glaciers. Old hills and valleys had disappeared, new ones had formed, and a layer of loose rock material, which was carried by the ice and released as it melted, covered most of the area.

Most of this material, known as glacial drift, was brought in by the ice during the last two of the four main glacial periods—the Illinoian period 100,000 to 150,000 years ago and the Wisconsinan 5,000 to 50,000 years ago. The older drift from the Kansan and Nebraskan glacial periods is mostly buried under the more recent drifts.

The glaciers covered all of Illinois except for the northwestern corner, the southwestern edge along the Mississippi River, and extreme southern Illinois, as shown in figure 1. In those areas, the land is mostly like it was before the glaciers arrived. However, in the glaciated part of the state, the bedrock is usually covered by the rock debris that the ice carried from as far away as Canada. 4 As the edges of the ice melted, these loads of rock material were, in some places, deposited as ridges (moraines), which are the hills and mounds on today's flat prairies. This material also filled ancient river valleys, but new valleys were created by torrents of water released by the melting ice.

The glacial drift is part of the youngest (topmost layer) of the main divisions of our rocks, which geologists call the Pleistocene (the scientific term for Ice Age deposits).

Most of the drift is a mixed bag of clay, pebbles, and boulders known as "till," but some glacial deposits are made up of water-sorted sand and gravel that was carried and dropped off by meltwater from the glaciers. Other materials were deposited by the wind—sand formed into shifting dunes and fine silts were spread like a blanket over the land. This layer of silt is called loess.

The glacial deposits have a diverse mix of rocks, some brought in from northern areas, and others eroded from the native rock layers in Illinois.

The boundaries of the Illinoian, Wisconsinan, and Kansan glaciations are shown in figure 1. Some of the more noticeable moraines are outlined with dark gray 5 lines on plate 1. In this area, glacial drift covers the bedrock except in valleys where streams have eroded and removed it.

Beneath the glacial drift of Illinois, many layers of rocks sit on top of a foundation of ancient crystalline rocks, which can be found at depths ranging from 2,000 to as much as 15,000 feet. The geological map (plate 1, in pocket) is designed to look like the layer of glacial drift has been removed to reveal the bedrock layers, primarily made up of limestone, shale, and sandstone.

The key on the map shows the age order of the rocks, with the youngest on top, and provides the names that geologists have given to the different rock systems. Each system includes rocks that were formed over a long period. The entire sequence of rocks can be compared to a book of Earth's history, with each system serving as a chapter. Systems are divided into formations, which can be viewed as the pages in the book.

As shown on the generalized rock column in figure 2, the rocks that are just older than the glacial drift (Pleistocene) are the Tertiary and Cretaceous sands, gravels, and clays, which are mostly loose. These layers are found only at the very southern tip of Illinois and were formed when that region was covered by a northern extension of the Gulf of Mexico. The Tertiary rocks are indicated on the map in light pink (T) and the Cretaceous in red-violet (K).

Next in age after the Cretaceous are the Pennsylvanian rocks, named for the state of Pennsylvania where they are prominently visible and were first studied.

The Pennsylvanian System is split into two sections on the map (P¹ and P²). The rocks indicated in light gray (P²) sit above the No. 6 Coal in the sequence, while those in medium gray (P¹) are found below it. The No. 6 Coal is among the thickest and most valuable coals in the state. Pennsylvanian rocks are located beneath the glacial drift, and their thickness varies from just a few feet to as much as 3,000 feet.

The Pennsylvanian System includes a variety of rock types, featuring all of our extractable coal. It also has significant deposits of limestone, shale, and clay, along with some areas containing oil and gas.

Next below the Pennsylvanian, we have the rocks of the Mississippian System, which are shown in blue on the map (M¹ and M²). The lower and middle Mississippian rocks (M¹) are mostly limestone in the areas shown, but in the central and eastern parts of the state, where they are covered by Pennsylvanian rocks, they contain a lot of siltstone and cherty limestone. The upper Mississippian rocks (M²) include a series of sandstone, shale, and limestone formations.

This group of rocks is named after the Mississippi River because you can see these layers well along the Mississippi Valley in western Illinois, southeastern Iowa, and eastern Missouri.

The Mississippian rocks provide limestone, fluorspar, zinc, and ganister, and are most economically significant in southeastern Illinois, where they're the key rocks for oil production.

The Devonian (D, dark gray on the map), Silurian (S, violet), Ordovician (O, light pink), and Cambrian (C, deep pink) rocks, listed in that order, are older than the Mississippian layers. Typically, they consist of dolomite, limestone, shale, and sandstone. With the exception of small areas along the Mississippi and Illinois River Valleys, these older rocks only appear at the surface in the northern quarter of the state and in local spots in Alexander, Hardin, Jackson, Monroe, Pike, and Union Counties. They are still economically significant because they provide limestone, dolomite, silica sand, oil, zinc, lead, tripoli, novaculite, and novaculite gravel.

The rocks from the Cambrian to the Pennsylvanian periods are part of the Paleozoic Era. These Paleozoic rocks sit on top of crystalline rocks, like granite, that go down to unknown depths in the Earth's crust. The crystalline rocks aren't found on the surface in Illinois, but they can be reached through some deep well drilling and can be seen in the nearby Missouri Ozarks and central Wisconsin.

The rock formations seem to lie flat in most of Illinois, but they are slightly sloped in many areas. In some places, they are depressed into basins and troughs (synclines), uplifted into domes and arches (anticlines), or fractured by faults.

The largest structural feature in Illinois is a huge spoon-shaped basin—the Illinois Basin—that stretches southeast into Indiana and Kentucky. The deepest part of the basin is in southeastern Illinois.

Because the entire region, including the basin, has been worn down by rain, ice, wind, and many rivers and streams, the youngest Paleozoic rocks (the top layers) are only found in the middle of the basin, as shown in figures 3, 6, and 7. This is why the map shows the coal-bearing rocks from the Pennsylvanian age in the center of the state, with older rocks appearing in successive bands around the edges of the basin in southern, western, and northern Illinois.

Distribution of bedrock in northern Illinois is shaped by a broad upfold or arch (see figures 4, 6, and 7). The map (plate 1) shows the older rocks (Cambrian and Ordovician) visible at the center of the arch, surrounded by younger Silurian and Devonian rocks. Another elongated upfold (anticlinal belt) stretches from the area around Dixon southeastward into Indiana. The top of the upfold is marked on the map by patches of Silurian and Devonian rocks in Champaign and Douglas Counties.

In the far southern part of Illinois and in the north-central area, the rock layers are disrupted by large faults (see figures 5 and 6) that shift the rock layers by distances ranging from just a few inches to as much as 3,000 feet.

The structural map of Illinois (figure 6) shows the position of the main geological structures. The large basin is highlighted with shading, where the darkest pattern indicates the part of the basin that is the deepest. The same layers that are at sea level on the outer edge of the basin are pushed down to 6,000 feet below sea level at its deepest point. The axes of some small anticlines and the locations of major fault zones are also marked.

Reading the history of the earth’s crust is like reading a mystery novel. The geologist has to examine, or read, each rock layer, from the youngest at the top to the oldest at the bottom, looking for clues about its origin, age, and development that will help reconstruct the past.

One of the clues to the past is the fossil remains of plants and animals found in the rocks. For example, if a specific type of coral is known to have existed only during a certain period, then all rocks that contain that coral fossil must have formed during that time.

Records of wells (well logs) and rock cores or samples collected when test holes are drilled into the bedrock also reveal a lot about the sequence of rock layers beneath the surface. The types of rocks found also provide insights into ancient geography, as some were formed on land while others were deposited in long-gone seas. 12

Such knowledge is really important for locating and developing mineral resources like coal and oil.

Minerals commercially produced in Illinois include crude oil, coal, limestone, dolomite, clay, sand, gravel, fluorspar, tripoli, ganister, novaculite gravel, silica sand, and the metals zinc and lead. The distribution of mineral industries, as shown in the various maps from figures 8 to 12, is obviously related to the distribution of the rocks (plate 1). For instance, the coal mines are spread along the edge of the area where Pennsylvanian rocks are found at relatively shallow depths.

The mineral fuels, coal and petroleum, are the top mineral products of Illinois, accounting for about 70 percent of the annual value of all minerals produced in the state. For many years, coal was number one, but petroleum now takes the lead.

In recent years, oil production has averaged around 78 million barrels annually, making Illinois the eighth largest oil-producing state. The main oil region is the deep section of the Illinois Basin in the south-central and southeastern parts of the state, but notable discoveries have recently been made in central and western Illinois. There are approximately 490 oil fields in Illinois, varying in size from a few to several thousand acres.

Oil has been found in rocks from the Pennsylvanian, Mississippian, Devonian, Silurian, and Ordovician systems, but the Mississippian rocks are the most productive and make up about 76 percent of our total oil production.

Illinois is the fourth-largest producer of bituminous coal in the U.S., which is the most commonly mined type of coal in the country. Many manufacturing industries use coal, as do public utility companies that burn it to generate electricity, and individuals use it for heating their homes.

All coal deposits in Illinois are located in the Pennsylvanian System. Among the various types of coal, only a small number are thick enough to be mined profitably. The coal is classified as bituminous, but its quality varies. Illinois' coal reserves in layers thicker than 28 inches are estimated to be 137 billion tons, which is more than any other state.

Around the edge of the coal basin where the coal is found close to the surface, it's usually extracted in open pits (strip mines). However, in the past, most coal was extracted underground, and more than half of Illinois coal is still produced from these types of mines.

There are two types of carbonate rocks in Illinois: limestone and dolomite. They look almost the same, but they have different chemical compositions. Limestone is primarily made up of calcium carbonate (the mineral calcite), while dolomite consists of calcium magnesium carbonate (the mineral dolomite).

Limestone deposits are located in various areas in the western, southwestern, and southern parts of the state. These deposits mainly date back to the Mississippian and Ordovician periods. Smaller deposits of Pennsylvanian limestone can be found in the central part of the state. In the northern fifth of the state, you can see dolomites from the Silurian and Ordovician periods exposed in many areas.

Limestone and dolomite are extracted for various uses, including aggregate for concrete, agricultural limestone, railroad ballast, chips for asphalt roads, and for use in metal production and chemical processes. They are also sourced for building stone in certain areas, and marble is produced in southern Illinois.

Limestone and shale or clay are heated together in kilns to make portland cement. Portland cement is produced from a Pennsylvanian limestone in LaSalle County and from an Ordovician limestone in Lee County.

Limestone or dolomite is also heated in kilns to produce lime. Mississippian limestone is utilized in Adams County, and Silurian dolomite is used in Cook County for lime production.

Illinois has commercial clays from glacial, Cretaceous, Tertiary, and Pennsylvanian deposits. Pennsylvanian shales are also sources of clays. Glacial clays, found as loess (wind-blown materials), alluvial (water-deposited) sediments, and glacial till, are common throughout the state. Cretaceous and Tertiary clays, located at the southernmost tip of the state, are important for making heat-resistant bricks and coatings, as well as for floor-sweeping compounds.

Pennsylvania clays and shales are the primary source of clay for making products like pottery, stoneware, drain tile, sewer pipe, flue tile, building tile, brick, and special heat-resistant fire bricks.

Deposits of sand and gravel, found in many areas of Illinois, provide significant amounts of material for concrete, railroad ballast, road gravel, building sand, molding sand, and other uses. Glacial deposits are the main sources of sand and gravel, but in the far southern part of Illinois, where there is no glacial drift, the primary sources are Tertiary gravel and Devonian chert gravel.

Almost every county has some sand and gravel, but the main production areas are in northeastern Illinois and along some of the major rivers. A significant amount of sand and gravel is dredged from bars in the channels of the Mississippi, Ohio, and Wabash Rivers.

Silica sand is used to make glass, for sandblasting, as grinding sand, as molding sand for casting steel, and in the fracturing treatment of oil wells to boost oil production. Some of the sand is finely ground into a powder that serves as a filler, a fine abrasive, as a ceramic material, and for many other uses.

This unique sand, made up of grains of the mineral quartz (silica), comes from a sandstone that dates back to the Ordovician period, which is mined in LaSalle and Ogle Counties.

Tripoli, or “amorphous” silica, is extracted from Devonian rocks in Alexander County. The silica is finely ground for use as a polishing agent, a filler, in buffing compounds, and for other applications.

Ganister, a granular material with a high silica content, is also mined in extreme southern Illinois. It is used to make products that can handle high temperatures.

For many years, Illinois has been at the forefront of fluorspar production, consistently making up about half of the total output in the United States each year. The industry is located mainly in Hardin and Pope Counties, where the deposits are found in veins and layers within Mississippian System rocks. The fluorspar reserves are sufficient to support the industry for many years to come.

Fluorspar is used to make hydrofluoric acid, as a flux in steel production, in aluminum refining, to produce fluorine compounds used in various products, in ceramic glazes, and for other specialized applications.

Lead and zinc are the only metallic minerals extracted in Illinois. Deposits are located in Jo Daviess County in the northwestern part of the state and in the fluorspar region of Hardin and Pope Counties in the far southern part of Illinois. In northwestern Illinois, the ore is found in rocks from the Ordovician period, while in southeastern Illinois, it is linked to fluorspar in rocks from the Mississippian period.

Between 1820 and 1865, the mining area that includes northwestern Illinois was the country's main source of lead ore. In recent years, zinc production has gone up while lead production has gone down.

Water is also a mineral resource, and it's the one we need the most. Almost all water comes from rain and snow, which gather in lakes and streams or soak into the ground to create our two types of water supplies—surface water and groundwater.

Groundwater in Illinois gathers underground in deposits of sand and gravel or in the porous layers of sandstone or limestone bedrock. These rock materials are known as “aquifers” due to their capacity to store water and let it flow into wells that are drilled into them.

The aquifers aren't evenly spread out across Illinois, so groundwater supplies are plentiful in some areas of the state but limited in others. Some cities in Illinois rely entirely or partially on surface water from lakes and rivers.

The sand and gravel aquifers provide more groundwater than any others in Illinois. The main sand and gravel aquifers were formed by streams and are situated along river valleys. Some of these river valleys aren't visible anymore because they were covered by glacial debris. However, they can be identified from well records or drilling records.

The northern third of Illinois has the most plentiful ground water supplies. Bedrock formations from the Devonian to Cambrian periods are good aquifers and provide drinkable water at depths ranging from 200 to 2,000 feet. However, these formations are much deeper in the Illinois Basin, where the water they provide is too salty for most uses.

West of the Illinois River and at the southern end of the state, the Mississippian, Devonian, and Silurian limestones are the main aquifers. However, their output is low.

In the Illinois Basin, all these formations are covered by Pennsylvanian rocks that provide only small, scattered amounts of groundwater.

A city or farmer needs a new water supply. Where can they find suitable water-bearing formations? They're considering a new limestone quarry. Where are the best limestone deposits? A coal mine is being planned. How deep does a shaft need to be sunk to reach the nearest minable coal, and what will the mining conditions be like? A landowner has a clay deposit on their land. What can be done with it? A road is going to be built. Are there sand and gravel or limestone deposits nearby for construction materials? A farmer wants to know if there might be oil on their property. Are there oil-bearing structures present, and if so, at what depth?

These are common questions asked by the industries and residents of Illinois to the Illinois State Geological Survey. With its extensive collection of information and experience, the Survey provides answers. This wealth of knowledge comes from the Survey’s ongoing efforts in geologic exploration, mapping, research, and the organization and interpretation of data.

The Survey investigates and charts the geology and mineral resources of the state, conducts research to discover new and better uses for our mineral resources, and organizes and analyzes fundamental geologic information collected from well logs, rock samples, and field studies, making it all accessible and useful.

Such knowledge is shared with the public through publications, maps, lectures, and personal consultations with businesses, drillers, engineers, landowners, teachers, and others who need geological information.

The discovery of information about Illinois's rich mineral resources has really just started, and it's the Geological Survey's job to keep gathering and using new data so we can make the most of our resources.

The Educational Extension Section of the Geological Survey organizes six field trips every year in different areas of the state for teachers, students, and the general public. It also puts together and distributes rock and mineral collections for educational groups in Illinois, offers lectures, sets up exhibits, and identifies rocks and minerals for the public.

Educational Extension publications, like this book, are straightforward discussions about geological topics. Priced at 25 cents each, these include:

Many technical discussions about the different phases of Illinois geology are also published by the Geological Survey. Regional reports on the geology and mineral resources of certain areas can be found in many school and public libraries, or they can be purchased from the Survey. Some of the regional reports include:

“Mineral Production in Illinois in 1959,” Circular 300 (free of charge), is part of a series of annual economic summaries. “Caves of Illinois,” Report of Investigations 215, priced at 50 cents, will be available by September 1961.

Illinois State Geological Survey Educational Series 7
24 pages, 1 plate, 12 figures, 1961