SURFICIAL GEOLOGIC MAP OF THE EUREKA 7.5 MINUTE QUADRANGLE IN WOODFORD COUNTY, IL

This study was conducted in order to map the surficial geologic units of the Eureka, IL Quadranglet located at 89°15’ to 89°22’30” latitude to 40°45’ to 40°37’30” longitude at the scale of 1:24,000. This Quadrangle includes sediment units deposited during and after the Wisconsin Glacial Episode. The Quadrangle also contains the Eureka Moraine. The methodology used to map this quadrant involved the conversion of soils data, LiDAR data, well water log data and field work. There are five geologic units which are Quaternary age that make up the surficial geology in the Eureka Quadrangle. These units are the Cahokia Alluvium, Peoria Silt, Henry Formation the Tiskilwa Formation, and the Lemont Formation. The Cahokia Alluvium layer is a brown, silt and clay layer which is interbedded with fine sand and rediposited bedrock clasts interpreted to be a post glacial loess and till that has been redeposited on stream beds and valley floors. The Peoria Silt is a light yellow to grey sandy silt which was deposited in an eolian environment and is interpreted to be a pro-glacial loess derived from glacial meltwater channels. The Henry Formation contains stratified layers composed of sand and gravel with cobbles and boulders interpreted to be upland outwash and alluvial fans. The Tiskilwa Formation is a red-gray diamicton associated with gravel, sand, silt and clay interpreted to be derived directly from glacial ice. Lastly, the Lemont Formation is composed of unstratified diamictons, cobbles, boulders and discontinuous layers of sand, gravel, silt and clay which are interpreted to be derived directly from glacial ice and overlain by a thin covering of loess. The distribution of the Cahokia Alluvium is alongside river beds and valleys. The Peoria Silt is just west of the Eureka Moraine and is only mappable if the silt is greater than sixty inches thick. The Henry Formation’s distribution is the smallest amount and is present as outwash alongside river beds. The Tiskilwa Formation is distributed entirely west of the Eureka Moraine. Lastly the Lemont formation is contained in the north-east corner of the map within the Eureka Moraine.