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.