Three-dimensional Geologic Mapping for Transportation Planning in Central-northern Illinois: Data Selection, Map Construction, and Model Development Berg, R.C., E.D. McKay, D.A. Keefer, R.A. Bauer, P.D. Johnstone, B.J. Stiff, A. Pugin, C. P. Weibel, A.J. Stumpf, T.H. Larson, W.-J. Su and G.T. Homrighous Illinois State Geological Survey, 615 East Peabody Dr., Champaign, IL 61820; E-mail: R.C. Berg at berg@isgs.uiuc.edu Introduction. To prepare for a highway improvement project, the Illinois Department of Transportation (IDOT) contracted with the Illinois State Geological Survey (ISGS) to conduct a 15-month, three-dimensional (3-D) geological mapping program along a 24-mile (39 km) segment of Illinois Route 29 in central-northern Illinois from just north of Chillicothe (Peoria County) northward into southern Bureau County (Figure 1). Route 29 is located on the west side of the Illinois River, mostly at the base of a steep and highly dissected bluff (~200 ft (61m) high) composed of Pennsylvanian bedrock (mainly shale, limestone, and coal) overlain by thin (mostly <50ft/15m) glacial deposits along the southern 8.6 miles (13.8 km) of the high-way, and thick (>200 ft/60m) glacial deposits along the northern 5.0 miles (8.1 km) of the highway. The middle and extreme southern portions of the highway traverse Illinois River floodplain and Wisconsin Episode outwash terrace deposits for 10.4 miles Figure 1. Location Map. (16.7 km). The mapping has supported the development of derivative map products specifically tailored to address various construction conditions, predict areas of possible geologic hazards (e.g., landslides and mine subsidence), and locate groundwater supplies for rest areas and other development that may follow highway construction. The mapping also supports the preparation of IDOT=s Environmental Impact Statement for the construction project by characterizing hydrological conditions, slope stability, erosion and sedimentation, and showing how geology potentially impacts ecological systems. The succession of Quaternary deposits in the area reflects the interaction of multiple glacial advances with a major river system, the Mississippi, that has drained much of the upper Midwest United States. Glaciers from the Lake Michigan Lobe have advanced westward across the study area numerous (at least four) times during the Quaternary. The course of the southward-flowing present-day Illinois River in the study area overlies and generally parallels the bedrock valley that last carried the ancestral Mississippi River about 20,500 years ago, when the maximum glacial advance of the Wisconsin Episode diverted the ancestral Mississippi River westward into its present course. In the study area, where these multiple glacial events impinged on a major river in an area of significant bedrock relief, the result was a succession of glacial, fluvial, eolian, and alluvial, deposits and interglacial paleosols, reflecting numerous complexly superimposed episodes of erosion, deposition, and stability. 13 Because of the geological complexities that were encountered, it was necessary to develop two 3- D geological models. (1) A regional 3-D model, which covers about a 200 mi 2 (518 km 2 ) area centering on the highway, delineates unlithified materials from land surface to the bedrock surface. Regional mapping and modeling were necessary to better understand the geology and increase our ability to predict the lithostratigraphic units near IL29. (2) A more detailed model, covering the area extending about 200 feet (61m) on either side of the highway centerline, also was compiled to differentiate lithologic units to a