Guide to Rocks and Minerals of Illinois

Adapted from ISGS Geoscience Education Series 16, 2004
by Wayne Frankie

Old Stoneface, Shawnee National Forest, Saline CountyIntroduction

Most of us know little about the Earth on which we live and thrive. Some of nature's most remarkable wonders, because they are so familiar, go unnoticed. Yet beneath our feet and before our eyes is an intriguing world, if we will just pause to examine it.

At first glance, Illinois, "The Prairie State," may appear to be an unlikely place in which to collect rock and mineral specimens. But Illinois is surprisingly rich in rock and mineral resources, thanks to the great diversity and wealth of materials beneath its land surface. The state contains some of the most interesting mineral and rock specimens in the world.

The fertile prairies that gave the state its nickname are themselves derived from ancient rocks, which have been changed by millions of years of weathering and erosion by ice, water, wind, plants, and animals. Beneath the cityscapes, open fields, hills, and valleys lie layered sedimentary rocks that, in some parts of the state, are exposed in outcrops, canyons, and river valleys. Boulders and gravel, brought in from the north by the glaciers thousands of years ago, are strewn across most of the state.

Why Are Rocks and Minerals Important?

The geologic materials—from soils, to sands and gravels, to rocks and minerals, to petroleum resources—form the basis of our modern society. These resources are important to all of the state's industries, including agriculture, construction, manufacturing, energy production, information delivery, or transportation. Every county in Illinois possesses rocks and minerals that are being used or have potential future value.

The Complexity of Illinois Geology

The majority of the Illinois landscape appears relatively flat and featureless. Hidden beneath this unassuming surface are numerous layers of soils, clay, silt, and sand and gravel that bury the bedrock that lies below them. In Illinois, economic minerals are present deep underground in many places; for example, coal is mined to a depth of 1,000 feet, and oil is produced from porous rock layers, called pay zones, at depths of several thousand feet.

Lead and zinc ores, fluorspar, silica sand, limestone, sand, gravel, clay, and shale are all found at shallower depths. Most people, though, are aware of only those rocks and minerals found at or near the surface.

Geologists gather some information about the state's subsurface directly by looking at and taking samples from outcrops, quarries, and mines. But, for most of the state, geology is "seen" and understood mainly from descriptions (logs) and samples (cores) of the sediments and rocks penetrated during shallow and deep drilling. Other instruments that provide information are geophysical logs, seismic reflection, and electrical resistivity surveys. All of these help geologists interpret subsurface materials.

Understanding a bit of Illinois' geologic history helps us understand how the state's rock and mineral deposits came into being.

Purchase the Book

The printed version of Guide to Rocks and Minerals of Illinois can be purchased from the Shop ISGS web site. for $7.00 plus S&H.

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