Figure 5—When rocks are faulted by earth stresses, the layers of rock are displaced or offset.
Figure 6—Major structural features of Illinois—the Illinois Basin, anticlines, and faults. The increasing depth of the basin is shown by progressively darker patterns.
KANKAKEE ARCH MISSISSIPPI R. ARCH LA SALLE ANTICLINAL BELT OZARK UPLIFT KEY Position of major anticline Major faults
Figure 7—The cross section from north to south through Illinois shows the strata down-warped into the basin.
HISTORICAL GEOLOGY
Reading the history of the earth’s crust is like reading a mystery story. The geologist must examine, or read, each rock layer, from the youngest at the top to the oldest at the bottom, searching for clues to its origin, age, and development from which he can reconstruct the past.
One of the clues to the past is the fossil remains of plants and animals found in the rocks. For instance, if a certain coral is known to have lived only during one span of time, all rocks containing that coral fossil must have been formed within that span.
Records of wells (well logs) and rock cores or samples collected when test holes are drilled into the bedrock also reveal much about the sequence of rock strata beneath the surface. The kinds of rocks encountered tell, in addition, something about ancient geography, for some were formed on land and others were deposited in long-departed seas.