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Blue Mound State ParkThe Mound's Geology

Over millions of years, the forces of nature changed this place from mountains to a sea bottom to the hill it is today.

An undersea sediment sandwich

Deep down under the Blue Mounds is granite bedrock, the "roots" of mountains that stood here more than 2 billion years ago. The mountains were eroded down to a rolling granite plain.

As Wisconsin warped up and down repeatedly, extensive inland oceans alternately flowed into and retreated from the area. In the process, sand and soft limy sediments were deposited on top of the granite. The first sea covered the area more than 1 billion years ago and the last (the Silurian Sea) about 400 million years ago.

Eons of cementation and pressure changed the sands and limy sediments into sandstones and limestones.

Erosion carves the landscape

For the last 400 million years, most of Wisconsin has remained above sea level and erosion has carved southwestern Wisconsin into its present much-branched, tree-like drainage pattern of rivers, hills and valleys. Hundreds of miles of sandstone and limestone have been remove from southern Wisconsin by the streams during the 400 million year erosion cycle.

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

Bypassed by the Glaciers

Most of Wisconsin, Illinois, Iowa and Minnesota once looked much the same until a series of four glaciers inched their way across the North America thousands of years ago. Massive sheets of ice peeled off hilltops and filled in valleys from Canada to Kansas, leaving a vast, flat expanse in their wake.

Due to certain geological quirks of fate, southwestern Wisconsin was bypassed and encircled by the four glaciers. The area, therefore, stands as an island of hills and valleys amid surrounding plains.

The term "Driftless Area" is given to this region because it is devoid of drift or the accumulated rock and soil left by retreating glaciers.

Last revised: Friday October 17 2014