Illinois Quaternary Deposits. Note the fingers of Cahokia (yellow) and Henry (orange) Formations
emanating from the southern and western limits of the Wisconsin stage glaciers (defined by the Wedron
Formation in green).
|These powerful alluvial processes created the expansive floodplain that we know now as the American Bottom. The importance of glacio-fluvial deposits in Illinois and the American Bottom in particular, is illustrated on the map of Illinois Quaternary deposits. Most surficial deposits in the American Bottom fall into two types, the Henry Formation - a gravelly glacio-fluvial deposit of Late Wisconsinan age (~15,000ybp), and the Cahokia Formation - a suite of Late Wisconsin and early Holocene deposits including the Cahokia alluvium (a silty clay floodplain deposit), the Peyton Colluvium (a coarse alluvial and colluvial deposits at the base of valley walls), and the Grayslake Peat (highly organic lake, pond, and swamp deposits).|
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