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Mill Bluff State Park Geology

Large bluffs are called mesas. Smaller, more abrupt bluffs are called buttes and other more slender, abrupt bluffs are called pinnacles. You can see examples of all three at Mill Bluff State Park.

Rock formation with snow on one side
The rocky bluffs glow in the sunlight.
Photo by RJ and Linda Miller

Even though this park is in the driftless area [exit DNR], the area the glaciers missed, the geologic features are partially the result of the last (or Wisconsin) stage of glaciation. During this glacial advance, the Wisconsin River was plugged near Wisconsin Dells. The river spread out to form glacial Lake Wisconsin, covering most of what today are Adams, Juneau and other adjacent counties, including the Mill Bluff area. During this time, some of the mesa and buttes stood as islands in the glacial lake, while others were submerged. Wave action hastened erosion of the sides of the rock forms.

The unique flat-topped, cliff-sided rock structures are capped by layers of somewhat more resistant sandstone; and weathering tends to break the rock off in vertical fragments. There are remnants of the Dresbach Group, Upper Cambrian sandstone. The heights of the bluffs range from 80 feet to over 200 feet. The mesa and buttes are isolated "outliers" of the continuous limestone-capped escarpments south of the park.

Rocks and boulders of different compositions than local bedrock are erractics. They have been found on the sides of some of the buttes and mesas. These erractics are boulders that were imbedded in icebergs which floated and lodged against the mesa and buttes. When the ice melted, the boulders remained lodged on the sandy flanks of the bluffs.

Erosion by wind and water eventually wear away the underlying soft sandstone until the weight of the over hanging cap rock causes it to shatter and fall. The mounds will decrease in size until all the capping is removed. Then the mound becomes a conical hill, gradually blending into the plain.

Bluffs
The steep bluffs fascinate travelers today as they did in pioneer days.
DNR Photo

These unique glacial features are protected in Mill Bluff State Park, one of nine units of the Ice Age National Scientific Reserves. Because of the uniqueness of this park and of the delicate sandstone structures of the buttes, climbing on or defacing the bluffs is strictly prohibited. Anyone who disobeys this rule will have to pay a fine.

Pioneer landmarks

The tall buttes of Mill Bluff were landmarks for settlers traveling west through Wisconsin and are often mentioned in their journals or diaries. Today, the beautiful scenery and buttes arouse traveler's curiosity to stop in for a closer look.

Ice Age National Scientific Reserve

In October 1964, Congress authorized establishment of the Ice Age National Scientific Reserve. With later joint planning between the U.S. Department of the Interior and the State of Wisconsin, Mill Bluff was officially established in the Federal Register in May 1971, as a unit of the Ice Age National Scientific Reserve.

Last revised: Thursday July 30 2015