Grayish-orange to olive-gray medium-grained dolostone with light gray chert nodules, thin to wavy bedded. The summit is an erosional outlier, once of several mounds in the area, part of a once continuous layer of Edgewood overlying the Ordovician Maquoketa shale. Silurian dolostone overlying Ordovician shale forms a prominent escarpment in many places in the Midwest, for example at Strawberry Point, Iowa, High Cliff State Park, Wisconsin, Door County, Wisconsin, and Niagara Falls, New York.
The view from the northwest end of the summit ridge overlooks the heart of the Wisconsin lead-zinc district centered on Platteville. The town of Galena, named for lead sulfide ore and situated to the west on the Mississippi River, was a major port for export of lead. The Glanville lead-zinc prospect lies near the high point. (When we visited Charles Mound in 1994, road gravel in the roadway to the summit contained bits of pyrite and sphalerite.)
Surficial Geology: Charles Mound lies in the so-called driftless area that includes northwest Illinois, southwest Wisconsin and northeast Iowa, an area which apparently escaped the Wisconsinan ice advance. Traces of older glacial deposits have been found beneath a thin mantle of loess, but overall the topography has much more relief than elsewhere in the Midwest.
Soil Series: Elizabeth silt loam at the summit: dark gray to dark brown calcareous silt loam, shallow, somewhat excessively drained. Weathered from dolostone bedrock at a depth of 10 to 20 inches. Lacrescent silty clay loam is found on the slopes away from the summit: deep, well drained, formed on a thin mantle of loess over bedrock.
- Mullens, T.E., 1964, Geology of the Cuba City, New Diggings and Shullsburg quadrangles, Wisconsin and Illinois: U.S. Geological Survey Bulletin 1123-H.
- Tegeler, R.A., 1996, Soil Survey of Jo Daviess County, Illinois: U. S. Natural Resource Conservation Service.
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