Fazio, U.S. Geological
Survey, in designing and creating digital illustrations is greatly appreciated.
The flat topography of the LIRB in centralIllinois is dissected by the Illinois River. Glacial
features are the major landforms. The LIRB is in the Till Plains Section of the Central Lowland
physiographic province (Willman and others, 1975). The Galesburg Plain, Springfield Plain, and
Bloomington Ridged Plain are subsections within
the Till Plain(fig. 2).
The Galesburg Plain and
Springfield Plain are largely in the Illinoian drift, but prominent glacial topography are only local
features. The Bloomington Ridge Plain includes the Wisconsinan glacial moraines and associated
glacial topography. A small area near the confluence of the Illinois and Mississippi Rivers is in the
Lincoln Hills Section of the Central Lowland. The Lincoln Hills Section is part of the Ozark Pla-
teau Province including deeply dissected flat-lying rocks.
The altitude of land surface in the LIRB isgenerally from 600 to 800 ft above sea level. The
area of greatest topographic relief is along the river valley, where topographic relief can range from
200 to 400 ft. Relief reaches 400 ft locally in Calhoun and Jersey Counties near the confluence of
the Illinois andMississippi Rivers at Grafton (fig. 1).
The majority of the basin is extremely flat with less than 20 ft of relief. The distinction in
topography between the older Illinoian drift (Galesburg and Springfield Plains) and the Wiscon-
sinan drift(Bloomington Ridged Plain) is the morainic ridges in the Wisconsinan drift areas. The
morainic ridges are generally from 50 to 100 ft high, 1 to 2 mi wide, and 100 to 500 mi long. The
moraines are separated by areas with more subdued, undulating topography or “rolling” topogra-
phy.
The preglacial physiographic provinces are divided by the type of
bedrock (fig. 3). Subse-
quentglacial deposits also are controlled by bedrock lithology and structure (Horberg and Ander-
son, 1956: Kempton and others, 1991). The LIRB is part of the Pennsylvanian Upland,
Pennsylvanian Lowland, and Havana Lowland. The Pennsylvanian Upland is structurally situated
along the northwest flank of the Illinois Basin just west of the axis of the uplift that separates the
Illinois structural basin from the structural basin in Iowa. The Havana Lowland borders the Illinois
Riverin the center of the basin, and the PennsylvanianLowland lies in the western part of the
LIRB. In the areas where the Pennsylvanian Upland and Lowland are covered by the Illinoian drift,
the surface topography reflects bedrock surface: The area covered by Wisconsinan drift reflects gla-
cial depositional features (Horberg, 1950). The various glacial advances were responsible for
numerous drainage changes, which can be related in part to features of the bedrock topography
(Horberg, 1950). The lowlands are areas where bedrock surface has been eroded below the sur-
rounding area. The broad Havana Lowland was developed at the junction of three important ancient
drainage areas and just above the point where the massive Mississippian limestone crosses the val-
ley (Horberg, 1950). The Havana Lowland area is hydrologically unique in the LIRB because of
the low elevation of bedrock and thick sequence of overlying sand.
