were not deeply entrenched in bedrock, but Pleistocene glaciation diverted the Mississippi River to
its present position and scoured the bedrock surface (Horberg, 1950). Below Peoria the present Illi-
nois River occupies the valley of the Ancient Mississippi River, and above Peoria the Illinois River
became established in its present position during Woodfordian glaciation (Willman, 1973).
The uppermost bedrock is mostly carbonate rocks of Mississippian and Pennsylvanian age
(fig. 10).
Mississippian
and Pennsylvanian bedrock are present throughout the LIRB, but in most
areas it is concealed by Quaternary deposits up to 500-ft thick. Many of the Mississippian- and
Pennsylvanian-aged formations are made of cyclic beds of sandstone, siltstone, shale, limestone,
coal, and clay. These rocks contain 1–2 percent of coal by volume. There are 75 identified coal beds
in Illinois (Willman and others, 1975). The Herrin (Number 6) Coal Member of the Pennsylvanian
Carbondale Formation (fig. 11), mined at two
active underground mines in Illinois, ranges from
200 ft below land surface in the northern and western part of the basin to 800 ft in Shelby County
(southeastern part of the basin) and is mostly from 28- to 42-in. thick. Coal beds range from 0- to
150-ft thick. The Mississippian bedrock is mostly shale and limestone.
The Mississippian and Silurian rocks near the confluence of the Illinois and Mississippi Riv-
ers contain fracture and dissolution features, including karst. Many caves are present in this area
(Panno and Weibel, 1993). The uppermost bedrock surface in parts of Bureau, La Salle, and Lee
Counties at the northernmost part of the LIRB includes Silurian, Cambrian, and Ordovician car-
bonates (fig. 10). The
uppermost
bedrock surface along the lower reach, south of Valley City, of the
Illinois River includes Devonian, Silurian, and Ordovician carbonates. The altitude of the bedrock
surface changes as much as 600 ft across the basin
(fig.
12).
Erosion, prior to and during glaciation, from large rivers and tributaries cut two major bed-
rock valleys that dominate the bedrock topography—the Mahomet system from the east and the
Ancient Mississippi system from the north (Wilson
and others, 1994) (fig. 13). The bedrock
valleys
are filled by overlying glacial drift. The time of formation of the bedrock valleys is not well defined
(Melhorn and Kempton, 1991). The glacial materials in these valleys are some of the most produc-
tive aquifers in the LIRB; so an understanding of the configuration and distribution of the valleys
is important. The Illinois and Mackinaw Buried Bedrock Valleys underlie the present Illinois River.
The preglacial Illinois Valley was part of the ancient Mississippi River (Horberg, 1950). The Maho-
met Buried Bedrock Valley is a large, bedrock valley that connects to the Illinois Buried Bedrock
Valley from the east through Champaign, De Witt, Logan, Macon, Mason, McLean, Piatt, and
Tazewell Counties (fig. 13).
Several small, buried bedrock valleys connect to the present Illinois
River Valley from the west: lower Spoon, Crooked Creek, and Wyoming. The Princeton Buried
Bedrock Valley is a large, bedrock valley that connects to the Illinois River Valley in the northern
part of the LIRB in and near Bureau County. The Mahomet Buried Bedrock Valley is part of the
ancient Teays drainage system, which was thought to originate in southeastern West Virginia
(Kempton and others, 1991), but new evidence indicates that the Teays drainage system may not
be one connected valley and that the Mahomet Buried Bedrock Valley had headwaters in western
Indiana (John Kempton,Illinois State Geological Survey, oral commun., 1996).
The LIRB is on the northwest edge of thesubsurface structural Illinois Basin. Five major
tectonic structures (Glasford Structure, Peru Monocline, Downs Anticline, Pittsfield Anticline, and
Cap au Gres Faulted Flexure) and many minor
tectonic structures affect local geology (fig.
14).
Several structures, such as the Osman Monocline and Colfax Syncline, which intersect the western
