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| Cave of the Mounds, Wisconsin. |
Of the eight types of caves, as categorized by the National Caves Association, Wisconsin boasts three in addition to those of the man-made variety. The state has solutional, littoral and corrasional caves.
A solutional cave occurs when natural acid in water seeps through rock that is soluble, such as limestone, dolomite, chalk, salt and gypsum. Cracks in the rock gradually expand, creating huge openings and tunnels. These caves typically contain stalactites and stalagmites. This is the most common type of cave, and almost all of Wisconsin’s caves are solutional.
Primary caves form at the same time that the surrounding rock is created. These usually are lava tubes, such as those found in Hawaii and eastern Idaho. A lava tube forms when the surface of a lava flow cools and hardens, as the hot lava inside drains out, leaving a hollow tube.
Littoral caves are found along coastlines where wave action has eroded weak rock and formed cavities. In colder climates, the freezing and thawing of water in the caves increases the erosion and can result in beautiful ice caves. These sea caves can be found in Wisconsin’s Bayfield and Door peninsulas.
Caves also can form when blowing wind or streams carrying hard stones cut through soft rock, forming tunnels and caverns. These are called corrasional caves. Often solutional caves after widening enough undergo a corrasional process. Southeast Wisconsin has a number of corrasional caves created when wind, rain and the freezing-thawing cycle carved out openings in sandstone cliffs.
Glacier caves form when melting ice inside a glacier form streams that hollow out tunnels. If the tunnels reach a glacier’s end, they come out as waterfalls or rivers. During the last ice age, Wisconsin’s glaciers included several such caves, evidence of which can be seen today in tunnel channels that break up long moraines.
A fracture cave occurs when soluble rock between layers of harder rocks, dissolves out, causing the ceiling to collapse in large blocks.
Talus caves form when boulders and rocks fall off cliffsides into large piles. If the boulders are large enough and depending on how they land, “tunnels” can form in the open spaces between them.
An anchialine cave is a subterranean passage that connects a pond or lake on land to a sea or ocean. The cave floor usually is covered in water.
In addition to these recognized categories, we might include a ninth type of cave – man-made. In the past, cavities would be burrowed out of cliffsides as homes or so goods could be stored in a cooler environment. Tunnels also were dug deep underground for mining (for example, the iron mine that is now the Neda Mine Bat Sanctuary in Dodge County). Such caves exist all across Wisconsin.
