SURFICIAL GEOLOGIC MAP OF THE HOLDER 7.5 MINUTE QUADRANGLE, MCLEAN COUNTY, ILLINOIS

The Holder Quadrangle is located in McLean County and spans from 40° to 40° 22’ 30” north latitude and 88° 52’ 30” to 88° 45’ west longitude. The majority of the quadrangle consists of the Woodfordian moraine front and the adjacent ground moraine The Quaternary units overlie Pennsylvanian strata of a variety of lithologies. Quaternary units exceed 100 m in thickness where they overlie buried bedrock valleys. Major Quaternary sediment units present in the area are the Wedron Group till units, the Mason Group meltwater deposits, the Peoria Silt, and the Cahokia Alluvium. Carmi Formation lacustrine deposits are also locally present. The Wedron Group consists of the Tiskilwa till (south of the Bloomington Moraine) and the Lemont Formation (north of the Bloomington Moraine). These till units are clay-rich diamicts that are pink and grey in color, respectively. The Peoria Silt is fine-grained yellow-orange silts and clays which covers most of the area in a thin veneer (as much as 2-3 m). The Mason Group is an outwash sand/gravel unit most prominent south of the Bloomington moraine. It is less than 8-10 meters in thickness. The Cahokia Alluvium is less than 5 meters in thickness and is associated with Holocene drainage systems. This map was prepared using soil maps from the McLean County Soil Survey. Parent materials were identified and grouped creating formation boundaries. Units had to be at least 2 m in thickness to be mapped. Water well data was evaluated to indicate thickness of major surficial deposits to indicate their viability as mappable units. After boundaries were delineated, a field check was conducted to adjust contact placement. Cross sections were prepared and subsurface correlations were determined using water well records. More than 100 well records on repository at the Illinois State Geological Survey for the Holder Quadrangle were analyzed as part of this study.