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. [Illustration: Figure 2—Diagram of layers of rocks in Illinois.] Era General Types of Rocks Period or System and Thickness Epoch