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Why are the Wisconsin Dells so beautiful? The surprising answer begins more than a billion years ago, when ancient oceans created the sedimentary rock that makes the Dells' stunning cliffs and canyons.
Mountain ranges eroded to form a sandy ocean bottom known as Precambrian and Cambrian sandstone. Hundreds of millions of years passed and as life filled the oceans, the remains of sea creatures littered the ocean floor. The fossil record shows that microscopic creatures came first, followed by algae, sponges, coral, scuttling trilobites, ammonites, delicate crinoids, underwater plant life, swimming tusks called endoceri (ancient relatives of octopi), and various types of bivalve mollusks. Nearly a mile of sedimentary sandstone, limestone, and dolomite layers built up before the seas dried out nearly 400 million years ago.
The Dells region might have resembled the high, stream-cut plateau of southwestern Wisconsin, but a million years of glaciation changed its appearance dramatically. Massive sheets of ice extended down over northern part of the continent. As these glaciers cycled through freezing and melting, their repeated advance and retreat ground down Wisconsin's limestone and sandstone layers to create gravel and sand.
An enormous ice dam formed near Portage, Wisconsin, during one period of glacial melting. Water trapped behind the dam forced its way through the ancient quartzite mountain range (the Baraboo Hills) and created Devil's Lake. When the ice dam at Portage broke free, hundreds of trillions of gallons of glacial meltwater spilled through what is now the Wisconsin River Valley. The pressure of this water over thousands of years cut through the rock layers at the Wisconsin Dells, exposing cliffs formed by harder, upper-level limestone, and softer, lower-level sandstone.
The flowing Wisconsin River continues to carve the whimsical shapes of the Dells, first seen by Native Americans who entered the region 12,000 years ago. When H.H. Bennett discovered photography in the late 1860s, the rock formations of the Wisconsin Dells became his most famous subject, shared with millions of people around the world. Bennett gave many of the rocky sculptures lasting nicknames, and his stereoscopic camera enabled people to enjoy three-dimensional views of the remarkable landscape along the Wisconsin River in the place called Wisconsin Dells.
-Tracy Will
For more information on Wisconsin Geology, visit the the Milwaukee Public Museum's Web site.