Because the entire region, including the basin, has been eroded by rain, ice, wind, and many rivers and streams, the youngest Paleozoic rocks (top-most layers) are preserved only in the middle of the basin, as shown in figures 3, 6, and 7. This is why the map shows the coal-bearing rocks of Pennsylvanian age in the center of the state and the older rocks cropping out in successive bands around the margins of the basin in southern, western, and northern Illinois. Distribution of bedrock in northern Illinois is influenced by a broad upfold or arch (see figures 4, 6, and 7). The map (plate 1) shows the older rocks (Cambrian and Ordovician) exposed at the center of the arch and surrounded by younger Silurian and Devonian rocks. Another elongated upfold (anticlinal belt) extends from the vicinity of Dixon southeastward into Indiana. The crest of the upfold is indicated on the map by patches of Silurian and Devonian rocks in Champaign and Douglas Counties. [Illustration: Figure 4—When rocks are upfolded and the surface is eroded, the older rocks are exposed at the crest of the dome or anticline.] In extreme southern Illinois and in north-central Illinois, the rock layers are broken by great faults (see figures 5 and 6) which displace the layers of rocks by as little as a few inches to as much as 3,000 feet. The structural map of Illinois (figure 6) shows the position of the major geologic structures. The big basin is indicated by shading, with the darkest pattern showing where the basin is deepest. The same strata that lie at sea level at the basin’s outer edge are downwarped to 6,000 feet below sea level in its deepest part. Axes of some small anticlines and locations of major fault zones also are shown. [Illustration: Figure 5—When rocks are faulted by earth stresses, the layers of rock are displaced or offset.] [Illustration: 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 [Illustration: 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.