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The buried Mahomet Valley in east-central Illinois is a complex lowland carved into the surface of Pennsylvanian and older rocks. It consists of a deep channel throughout most of its length and contains numerous benches below erosional remnant hills, suggesting several cycles of early to middle Quaternary erosion. Recent local and regional studies utilizing existing borehole data, including down-hole geophysical logs and seismic profiles, have provided new insights into the valley’s configuration. The usual techniques for interpreting shallow seismic refraction and reflection data are complicated by a seismic velocity inversion in the Quaternary sediments filling the deeper parts of the valley.

Based on the 500-ft (152 m) elevation contour to define the upper limit of the Mahomet Valley Lowland, the valley is about 8 mi (13 km) wide at the Illinois/Indiana border. Westward, the lowland widens to as much as 18 mi (29 km) in Ford County, where a major tributary enters from the north. Here, several ridges rise above a broad bench ranging from 400 to 450 ft (121 to 137 m) in elevation. Sparse well data and some seismic profiling suggest a deep channel at or slightly below an elevation of 350 ft (106 m) in northeastern Ford and northwestern Vermilion Counties. This deep channel extends southwestward where it passes under the villages of Mahomet (Champaign County) and Monticello (Piatt County) and then westward to just east of Clinton (De Witt County). Between Mahomet and Clinton, where there are also several isolated bedrock hills that rise to elevations as much as 500 ft (152 m), the intermediate bench lowers to 350 to 400 ft (106 to 121 m). The valley narrows to an average of 14 mi (22 km) wide between Monticello and its confluence with the Mackinaw Valley segment of the Ancient Mississippi Bedrock Valley in southwestern Tazewell County. Southeast of Clinton, a narrow bedrock “diversion” channel (Kenney Valley) provides a nearly straight connection between the Mahomet Valley and the Ancient Mississippi Valley below the confluence with the Mahomet.

A complex Quaternary history has been established for the Mahomet Bedrock Valley, but as yet no evidence has been found for late Tertiary or preglacial alluvial deposits. Deposits filling the valley include the widespread Mahomet Sand Member, as much as 200 ft (60 m) thick, locally overlying or interbedded with tills of the Banner Formation (pre-Illinoian). Above this succession are Glasford Formation (Ulinoian) and Wedron Formation (Wisconsinan) tills and associated deposits. The varied nature of the bedrock valley topography, the scattered presence of till-like material on bedrock hills underlying the Mahomet Sand, and the presence of lower Banner Formation till interbedded with the Mahomet Sand suggest several episodes of valley erosion and glacial deposition during a long pre-Illinoian history.

The deepest bedrock channel probably originated before deposition of the Mahomet Sand but postdates at least one early Quaternary glaciation. Near the Indiana border, the uppermost surface of the Mahomet Sand, at elevation 560 ft (170 m), appears locally eroded, forming broad terraces that continue down valley at progressively lower elevations. The surface of the Banner Formation forms a broad sag over much of the valley and rises slightly over the uplands. Deposits of the Glasford Formation form the upper fill in the valley and include a significant outwash related to the Vandalia Till Member. The Sangamon Soil, developed in the Glasford, together with the overlying Roxana Silt and Robein Silt, locally forms an important subsurface marker. The topographic expression of this pre-Woodfordian surface shows no evidence of the Mahomet Valley; it was completely buried by the end of the Illinoian.

Aquifers associated with the Mahomet Bedrock Valley and the Ancient Mississippi Bedrock Valley to the west are the only highly productive, nonalluvial sand and gravel aquifers in the southern three-fourths of Illinois. The aquifers associated with the buried Mahomet Valley provide the only large source of irrigation, industrial, and municipal supplies of groundwater in east-central Illinois; 40 municipalities and water districts are currently obtaining groundwater from these aquifers.

The largest groundwater withdrawals occur in the Champaign-Urbana area, averaging 17 × 106 gal (64 × 106 l/day. Total groundwater withdrawals from the valley are estimated to be at least 42 × 106 gal (16 × 107 l)/day. The coefficients of storage for the Mahomet Sand range from 2 × 10−5 to 2 × 10−3, with hydraulic conductivities and transmissivities up to 4,237 gpd/ft2 (2 × 10−3 m/s), and 510,000 gpd/ft (5 × 10−2 m2/s), respectively; for the Glasford sand the coefficients range from 1 × 10−5 to 8 × 10−2, with hydraulic conductivities and transmissivities up to 4,660 gpd/ft2 (2 × 10−3 m/s), and 233,000 gpd/ft (2 × 10−2 m2/s), respectively. Coefficients of vertical hydraulic conductivity of the confining beds range from 2.12 × 10−3 to 0.4 gpd/ft2 (1 × 10−9 to 2 × 10−7 m/s).

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