A widespread marine environment during the Paleozoic flooded the region, depositing sedimentary rocks which cover most of the center and south of the state.
The rock record contains an uncertain age gap with younger 1.8 billion years old Proterozoic quartzofeldspathic and migmatite gneiss, with amphibolite and biotite schist.
Most of the state, in a loop from Waupaca County in the east, south in a narrow belt to the Iowa line and west to Eau Claire, Dunn, Barron and parts of Polk and Burnett counties, the underlying rocks are sandstone with dolomite and shale belonging to the Trempeauleau, Elk Mound and Tunnel City groups.
A narrow band along the Lake Michigan shore from Milwaukee to south of Sheboygan, including Port Washington displays Devonian shale and limestone.
[2] Very few rocks from the Mesozoic are preserved in Wisconsin, other than occasional areas of non-marine Cretaceous gravel and clay rich in iron.
Select rocks in the Driftless Area have yielded ammonite fossils from the middle Turonian period, when Wisconsin was covered by the Western Interior Seaway.
[3] No rocks from the Paleogene or Neogene period are known from Wisconsin; however, abundant Quaternary deposits can be found as a result of the last Ice Age.