It is within the Driftless Area, a region of Illinois, Iowa, Minnesota, and Wisconsin that was not covered or ground down by the last continental glaciers.

The hill itself is an erosional remnant, similar to that on which the original village site of Scales Mound was located.

[4] As mapped in the early 1960's by Mullens,[5] Charles Mound is an outlier and erosional remnant of Silurian Edgewood Dolomite overlying strata of the Ordovician Maquoketa Formation.

The Maquoketa Formation consists of dark-gray clayey siltstone and silty dolomite that occurs in beds that are 2 in (5.1 cm) to 5 ft (1.5 m) thick.

A prominent grayish-orange and grayish-yellow granular dolomite bed that ranges from 3 to 13 in (7.6 to 33.0 cm) in thickness, which contains abundant fossil bryozoa fragments, locally outcrops near Charles Mound.