More than 14,000 years ago, at the end of the last ice age, a melting glacier flooded the Baraboo Hills region of Wisconsin. The sheer force of the rushing rapids carved into the surrounding rocks, creating a 430-mile-long chasm that would become the Wisconsin River. The flood deposited ancient sandstone, limestone and dolomite into the river and chiseled out steep sandstone cliffs and canyons along the river known as "The Dells."
Cambrian sandstone rock found in the Wisconsin River dates back approximately 520 million years to when Wisconsin was still under the sea. During this time, according to Stewards of the Dells, "the sandstones in the cliffs were deposited as windblown dune sands." These sands still line the bed of the river today. Sandstone also forms one of the major tourist attractions along the river: the 100-foot cliffs known as The Dells.
Lower magnesian limestone dates back 510 million years to the Ordovician Period. Much like sandstone, this limestone is a product of sand deposits, and it once capped the bluffs and hills of the Wisconsin River area. Today, limestone creates a solid and durable presence along the banks of the river. Its mineral composition also enriches the river's soil. According to Sketches of Minnesota, "the soil derived from the decomposition of the lower magnesian limestone is usually of excellent quality; rich, as well in organic matter as in those mineral salts which give rapidity to the growth of plants, and that durability which enables it to sustain a long succession of crops."
According to Wisconsin Natural Resources Magazine, "dolomite is a rock similar to limestone; it holds groundwater in interconnected cracks and pores." Wisconsin dolomite also dates back approximately 510 million years to the Ordovician Period, and served as a sedimentary shield for the area's bluffs and hills. At the end of the last ice age, dolomite helped form the border of the Wisconsin River. Due to its dense composition, dolomite is currently used in aquifers throughout the state.
