Most of the drift is an unsorted mixture of clay, pebbles, and boulders called “till,” but some glacial deposits consist of water-sorted sand and gravel carried and deposited by meltwater from the glaciers. Other materials were deposited by the wind—sand was piled into shifting dunes and fine silts were spread like a blanket over the land. This mantle of silt is called loess.
The glacial deposits contain a wide variety of rocks, some brought from regions to the north, others scoured from the layers of native rock in Illinois.
The limits of the Illinoian, the Wisconsinan, and the Kansan glaciations are shown in [figure 1]. Some of the more prominent moraines are sketched with dark gray lines on [plate 1]. Within this area, glacial drift covers the bedrock except along valleys where streams have cut through and removed it.
BEDROCK GEOLOGY
Beneath the glacial drift of Illinois many layers of rocks overlie a base of ancient crystalline rocks that in Illinois occur at depths of 2,000 to as much as 15,000 feet. The geologic map ([plate 1], in pocket) is drawn as if the mantle of glacial drift had been removed to expose the layers of bedrock, which are largely limestone, shale, and sandstone.
The key on the map shows the age sequence of the rocks, arranged with the youngest at the top, and gives the names that geologists have assigned to the various systems of rocks. Each system consists of rocks that were deposited during a long period of time. The complete sequence of rocks might be likened to a book of earth history, and each system likened to a chapter. Systems are divided into formations, which might be regarded as pages in the book.
As shown on the generalized rock column in [figure 2], the rocks next older than the glacial drift (Pleistocene) are the Tertiary and Cretaceous sands, gravels, and clays, mostly unconsolidated. They occur only at the extreme southern tip of Illinois and were deposited when that area was covered by a northward extension of the Gulf of Mexico. The Tertiary rocks are shown on the map in grayed pink (T) and the Cretaceous in red-violet (K).
Next older than the Cretaceous are the Pennsylvanian rocks, named for the state of Pennsylvania where they are well exposed and were first studied.
The Pennsylvanian System is divided into two areas on the map (P¹ and P²). The rocks shown in lightest gray (P²) lie above the No. 6 Coal in the sequence and those in medium gray (P¹) lie below it. The No. 6 Coal is one of the thickest, most valuable coals in the state. Pennsylvanian rocks occur under the glacial drift, and their thickness ranges from a few feet to as much as 3,000 feet.
