Educational Series 5
Guide to
ROCKS AND MINERALS
OF ILLINOIS
Illinois State Geological Survey
STATE of ILLINOIS
William G. Stratton, Governor
DEPARTMENT of
REGISTRATION and EDUCATION
Vera M. Binks, Director
First printing 1959
Second printing 1960
ILLINOIS STATE GEOLOGICAL SURVEY
John C. Frye, Chief
URBANA, ILLINOIS
Printed by Authority of the State of Illinois
Guide to ROCKS AND MINERALS OF ILLINOIS
Illinois has so long been known as the Prairie State that at first glance it seems a most unlikely place in which to collect rock and mineral specimens.
But Illinois has a surprising wealth of rock and mineral resources, not only to be collected as interesting specimens but to be put to practical and profitable use.
The rich prairies that gave the state its nickname are themselves derived from ancient rocks, worn and changed by millions of years of action by weather, water, wind, plants, and animals. Unmeasured depths of rock underlie the prairies, hills, and valleys, and in some parts of the state are exposed in outcrops, canyons, and river valleys. Boulders and gravel brought in by the glaciers thousands of years ago are strewn over many parts of the state.
These resources are of great value. Besides the rich agriculture based on the rock-derived soil, much of our industry, manufacturing, and transportation is dependent on rock and mineral materials. Every county in Illinois possesses some rocks and minerals that either are being used or have potential future value.
The Illinois State Geological Survey several years ago began to prepare sets of typical rocks and minerals of Illinois for use by the schools and other educational groups in Illinois. This booklet is designed to furnish a brief geological background and explanation of these common Illinois rocks and minerals. It also should be useful to the student or amateur interested in making his own collection.
Even though Illinois has no mountain ranges or deep canyons, the geology of the state has many complexities. In fact, the very flatness of our topography is a complicating factor because in order to study the geology at many places in the state it is necessary to use information from mines and descriptions (logs) and samples (cores) of the rock penetrated during drilling of deep wells. There are also geophysical methods of learning something about the rocks beneath the surface.
Fig. 1.—Geologic map of Illinois.
CAMBRIAN ORDOVICIAN ORDOVICIAN-SILURIAN ORDOVICIAN-SILURIAN UNDER DRIFT DEVONIAN-MISSISSIPPIAN PENNSYLVANIAN UNDER DRIFT CRETACEOUS-TERTIARY KEY Wisconsin glacial drift Illinoian glacial drift Pennsylvanian boundary
The complexity of Illinois geology is not produced by the upturning and sharp folding of rock layers such as can be seen in the Rocky Mountains, but rather by the changes in composition, thickness, and character of the rock layers that are only gently warped or relatively flat. At several places in the state, especially in the southern part, faults, or breaks, in the rock layers do occur, but over much of our area this is not common.
The presence of usable minerals at considerable depth is known at many places; coal is mined from depths greater than 800 feet, and oil is produced from saturated rock layers, called pay zones, several thousand feet below the surface. Lead and zinc ores, fluorspar, silica sand, limestone, sand, gravel, clay, and shale are all produced at shallower depths. However, the student can see only those rocks and minerals that are to be found at or near the surface. For that reason the following paragraphs describing their geologic occurrence deal only with surface geology.
The youngest of the major geologic divisions of our rocks is called the Pleistocene, which is the scientific name for the “Ice Age” deposits. During this relatively recent period of geologic time, which began about a million years ago, glaciers flowed southward from Canada and spread a layer of “glacial drift” over all of the state except the northwest corner, the southwest edge of the state along the Mississippi River, and extreme southern Illinois ([fig. 1]).
Most of the glacial deposits that we see were formed by the last two of the four major periods of glacial advance, the Illinoian and the Wisconsin. The Illinoian was the most extensive, reaching as far south as Carbondale and Harrisburg. The Wisconsin, so called because its deposits are so widely spread in that state, reached only to Mattoon and Peoria.
The glacial drift is the youngest and uppermost of the divisions of the rock column ([fig. 2]). Within the drift can be found the widest diversity of rock and mineral types—quartzite, schist, and other metamorphic rocks; granite, gabbro, and other igneous rocks; and of course the sedimentary rocks, limestone, dolomite, sandstone, shale, and even pieces of coal, which occur in bedded layers of the older rocks in Illinois.
Sand and gravel were carried and deposited by flowing streams before, during, and after glaciation, but the major deposits were made while the glaciers were melting. They contain a wide variety of rock and mineral types.
Figure 2—Diagram of layers of rocks in Illinois.
| Era | General Types of Rocks | |||
|---|---|---|---|---|
| Period or System and Thickness | ||||
| Epoch | ||||
| CENOZOIC “Recent Life” | ||||
| Age of Mammals | ||||
| Quaternary 0-500′ | ||||
| Pleistocene or Glacial Age | ||||
| Recent—alluvium in river valleys | ||||
| Glacial till, glacial outwash, gravel, sand, silt lake deposits of clay and silt, loess and sand dunes; covers nearly all of state except northwest corner and southern tip | ||||
| Tertiary 0-500′ | ||||
| Pliocene | Chert gravel; present in northern, southern, and western Illinois | |||
| Eocene | Mostly micaceous sand with some silt and clay; present only in southern Illinois | |||
| Paleocene | Mostly clay, little sand; present only in southern Illinois | |||
| MESOZOIC “Middle Life” | ||||
| Age of Reptiles | ||||
| Cretaceous 0-300′ | Mostly sand, some thin beds of clay and, locally, gravel; present only in southern Illinois | |||
| PALEOZOIC “Ancient Life” | ||||
| Age of Amphibians and Early Plants | ||||
| Pennsylvanian 0-3,000′ (“Coal Measures”) | Largely shale and sandstone with beds of coal, limestone, and clay | |||
| Mississippian 0-3,500′ | Black and gray shale at base; middle zone of thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone | |||
| Age of Fishes | ||||
| Devonian 0-1,500′ | Thick limestone, minor sandstones and shales; largely chert and cherty limestone in southern Illinois | |||
| Age of Invertebrates | ||||
| Silurian 0-1,000′ | Principally dolomite and limestone | |||
| Ordovician 500-2,000′ | Largely dolomite and limestone but contains sandstone, shale, and siltstone formations | |||
| Cambrian 1,500-3,000′ | Chiefly sandstones with some dolomite and shale; exposed only in small areas in north-central Illinois | |||
| ARCHEOZOIC and PROTEROZOIC | ||||
| Igneous and metamorphic rocks; known in Illinois only from deep wells | ||||
As shown by the diagrammatic rock column ([fig. 2]), rocks placed in the divisions called early Cenozoic and Mesozoic are next in age to the Pleistocene. The map ([fig. 1]) shows that the Cenozoic and Mesozoic rocks occur only in the extreme southern tip of Illinois because only that part of Illinois was covered by a northward extension of the forerunner of the Gulf of Mexico in which the deposits of sand, gravel, and clay were laid down.
The next older division of Illinois rocks is called Pennsylvanian—or “Coal Measures”—because during the last century they were first extensively described by geologists working in Pennsylvania.
The Pennsylvanian is one of our most important groups of rock strata because it contains all of our minable coal beds, as well as important deposits of limestone, shale, clay, sandstone, and some oil and gas. The Pennsylvanian rocks are very widespread in Illinois, occurring under the glacial drift from depths of a few feet to several hundred feet throughout about two-thirds of the glaciated area ([fig. 1]).
Next below the Pennsylvanian are the Mississippian rocks. We in Illinois are particularly interested in this division of rocks because they take their name from the excellent exposures along the Mississippi River valley in western Illinois, southeastern Iowa, and eastern Missouri. They are composed of extensive beds of limestone and cherty limestone, sandstone, and shale.
Mississippian rocks are of great economic importance in the structural area known as the Illinois Basin, where they are the most important oil producing rocks. They also contain our fluorspar deposits and along the valley bluffs are an excellent source of limestone for quarrying.
Rocks older than the Mississippian—except for small areas along the Mississippi and Illinois River valleys—are found at the surface only in the northern quarter of the state and locally in Hardin County near the southern tip of the state. They are nonetheless economically important because from these older rocks are produced lead and zinc, some oil and gas, silica sand, limestone, dolomite, and shale.
On the generalized rock column ([fig. 2]) these older rocks are grouped into two units. The uppermost contains the Devonian and Silurian and the lower contains the Ordovician and Cambrian. In general they include dolomite, limestone, and shale, with sandstone at several places, especially in the lower unit.