SURFICAL GEOLOGICAL MAP OF THE PECATONICA 7.5” MINUTE QUADRANGLE, ILLINOIS

The Pecatonica 7.5 Quadrangle is located in Winnebago County, and spans from 89°15' and 89°22'30" west longitude and 42°15' and 42°22'30" north latitude. Using soil maps from the Soil Survey of Winnebago County published by the University of Illinois Agricultural Experiment Station, a surficial geologic map was created. Once the soils were identified and located, they were placed into different groups based on their relations to different formations. The area was then field checked, and the positions of contacts were refined. Water well records were used to constrain the geologic soil data. The formations dealt with were: The Cahokia formation (QHca), The Roxana Silt member (Qrs), the Ogle Till member of the Glasford formation (Qgo), the Argyle Till member of the Winnebago formation (Qwa), and Galena-Plateville undivided (Ogp).

The Cahokia alluvium (QHca) is found in the riverbed of the Pecatonica River valley as well majority of valleys found within the quadrangle. The Cahokia Alluvium consists of silt, clay, and silty sand. Modern alluvium can also be found within the Pecatonica River valley, riverbed, and other smaller valleys. The Ogle Till member (Qgo) is a tan to brown, sandy and silty tills with discontinuous beds of sand and gravel. The Ogle Till member (Qgo) is found south of the Pecatonica River at higher elevation, on hilltops and ranges in thickness from 0 to 3 meters. The Argyle Till member (Qwa) is sandy, pink to tan in color and very calcareous. The Argyle is locally found in the northwestern corner of the quadrangle at higher elevation and ranges in thickness from 0 to 3 meters. The Plano Silt member (Qps) is also found locally in the northwestern corner of the quadrangle. Along valley slopes few glacial deposits are present and bedrock underlies the soil. The Galena Plateville undivided (Ogp) consists of dolostone that ranges from coarsely fossiliferous to micritic, and is locally cherty and/or heavily bioturbated. Bedrock outcrops are restricted to quarries and major stream valleys and quarries in the vicinity of the Pecatonica Anticline.