The rock formations appear to lie flat in most of Illinois, but they are
slightly inclined in most places. In some areas they are down-warped
into basins and troughs (synclines), upfolded into domes and arches
(anticlines), or broken by faults.
The largest structural feature in Illinois is a great spoon-shaped
basin—the Illinois Basin—that extends southeastward into Indiana and
Kentucky. The deepest part of the basin is in southeastern Illinois.
[Illustration: Figure 3—When rocks are down-warped and the surface
is leveled by erosion, the youngest rocks are preserved in the
center and the older rocks are exposed at the margins.]
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.]