GLACIAL GEOLOGY OF THE JIM FALLS 7.5' QUADRANGLE, CHIPPEWA COUNTY, WISCONSIN

The surficial geology of the Jim Falls 7.5' Quadrangle was mapped during summer 2000. Well logs, airphotos, topographic maps, hand borings, power boreholes, the Chippewa County soil survey, and seven weeks of field work were used to map the Jim Falls Quadrangle at a 1:24000 scale.

Reddish-brown sandy loam till and gravelly weathered outwash of the River Falls Formation are up to 4 m thick in the southern part of the Jim Falls Quad and discontinuously overlie Eau Claire Formation bedrock highlands. Reddish-brown gravelly sandy loam diamicton and silt loam lake sediment of the Copper Falls Formation underlie the northwest-southeast-trending Chippewa Moraine. Hummocky supraglacial sediment, fifteen ice-walled-lake plains, and poor drainage are located within the moraine. Moraine surficial relief is 3-20m. Outwash and pitted outwash plains extend south of the Jim Falls Quad to the most northerly extent of Lake Wissota.

Ice flowed from the Superior Lowland during the Illinoian Glaciation (300-130ka) and deposited till of the River Falls Formation. Following the ice retreat, the Chippewa Lobe briefly readvanced from the north to a position just north of Lake Wissota during the Early Chippewa Phase of the Late Wisconsinan Glaciation (approximately 25 ka). After a retreat of the ice margin, the Chippewa Lobe readvanced from the NE during the Late Chippewa Phase (15 ka). During this event, the ice margin reached the lower elevations of the bedrock highlands in the southwest part of the Jim Falls Quad. The ice margin retreated approximately 1 km and stabilized south of Jim Falls. This event constructed the high-relief Chippewa Moraine. Supraglacial sediment accumulated near the ice margin. As the ice melted, low areas on the glacier surface filled with sediment and formed hummocks. High areas on the glacier surface melted to frm kettles. Water from the melting ice deposited outwash over stagnant ice from the Early Chippewa Phase. When the stagnant ice melted, the pitted outwash plain formed north of Lake Wissota.