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Mirror Lake State Park Geology

The local names Dell, Dells and Delton all come from the French dalles, meaning gorge.

Rock overhang on lake shore
The waters of Dell Creek sculpted sandstone into these cliffs.
DNR Photo

Dell Creek winds its way for 25 miles through sand and hill country west of Wisconsin Dells. It begins in southern Juneau County, heads southeast into Sauk County, turns a right angle six miles northeast of Reedsburg and runs northeast through Mirror Lake and Lake Delton to the Wisconsin River south of Wisconsin Dells.

Why the right angle turn? Originally, it was likely that the Wisconsin River flowed through the Baraboo Hills to the south and that Dell Creek flowed southeast through a gap in the Baraboo Hills and joined the Wisconsin River near Baraboo.

Some 10,000 to 20,000 years ago the gigantic Wisconsin Glacier covered much of our state; in Sauk County U.S. Highway 12 is very near its westernmost boundary. Dell Creek is entirely within non-glaciated country, but geologists believe that glacial outwash blocked the creek, diverting it to the northeast where it cut a new channel in which Mirror Lake and Lake Delton now lie.

The rock forming Mirror Lake's gorges is sandstone. Geologists call it Late Cambrian, formed about 500 million years ago. Its sand grains are thought to have been deposited by rivers draining into shallow inland seas. The seas retreated and the sand compacted into sandstone. Within the last 10,000 years or so, Dell Creek cut its gorges into this thick, soft sandstone.

Last revised: Friday October 17 2014