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Belmont Mound State Park

Belmont Mound State Park Geology

The mound's history contains many ancient geologic periods:

  • Niagara dolomite (limestone) of the Silurian Period (about 430 million years ago).
  • Thick blankets of shale, limestone and sandstone of the Ordovician Period (about 470 million years ago).
  • A very thick layer of sandstone and some limestone of the Cambrian Period (more than 500 million years ago.
  • Various pre-Cambrian rocks (more than 520 million years ago).
  • Granitic bedrock.

Geologists say the deep granites are "roots" of mountains that stood here more than two billion years ago, before the mountains were eroded down to a rolling, granite plain.

The sandstones and limestones were originally deposited on top of the granite plain as a sea bottom sands and soft limy sediments. Extensive inland oceans alternately flowed into and retreated from Wisconsin as the region warped up and down repeatedly. The first sea covered this area more than a billion years ago and the last sea (the Silurian Sea) retreated about 400 million years ago. Due to eons of cementation and pressure, the sands and limy sediments changed into sandstones and limestones, respectively.

For the last 400 million years, most of the state has remained above sea level and erosion has carved southwestern Wisconsin into its present much-branched, treelike drainage pattern of rivers, streams, hills and valleys. The streams have removed hundreds of miles of sandstone and limestone from southern Wisconsin during the 400-million-year erosion cycle.

Southwestern Wisconsin has had nearly all of its Niagara dolomite removed. It remains in this region only as tiny remnants atop Blue, Platte, Belmont and Sinsinawa mounds. If it were not for the hard Niagara dolomite "capping," these four mounds would have been cut down to the level of the lowlands surrounding them.

Large areas of Niagara dolomite still exist in eastern Wisconsin, where it forms the steep-walled ridge called the Niagara Escarpment just east of big Lake Winnebago and it is the uppermost layer of rock in Door County, Wisconsin's large "thumb," between the waters of Green Bay and Lake Michigan. Niagara dolomite dips under Lake Michigan, lies beneath Michigan's lower peninsula and surfaces again at Buffalo, New York, where Niagara Falls plunges over its steep edge. Thus, Belmont Mound's "cap rock" is directly related to the rock layer supporting Niagara Falls, 1,000 miles away.

Within the park

Look for these interesting rock formations on the north slope of the mound:

  • The devil's dining table, about 100 yards downhill from the park drive.
  • The table top, about 40 feet across, sits on a narrow pedestal of rock.
  • The devil's chair, a smaller formation of similar shape, a few feet from the table.
  • The cave, a few yards north of the devil's dining table. This is a dark passage through an enormous rock.

There is an abandoned limestone quarry on the south side of the mound. Near the picnic area are the remains of what probably were kilns used to heat the limestone to form lime. Lime has many uses in chemistry, building materials, manufacturing and agriculture.

Last revised: Friday October 17 2014