Classicly

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.