Winter Sunset over Blue Mound State Park

Winter Sunset over Blue Mound State Park, originally uploaded by jw_creations.

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.

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.