MYSTERIOUS WORLD: Autumn 1999: Cave of the Mounds

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A view of the south cavern. The cavern was originally filled with acidic water, which carved out the soft limestone to form a the cavern. Look carefully, and you can see the "lifeline" of the cavern snaking along the center of the ceiling. The lifeline is the crack in the ceiling of the cave through which the acidic water still drips, continues creating new rock formations. Image copyright © 1995 Cave of the Mounds. All Rights Reserved.

he Cave of the Mounds is one of the most spectacular natural wonders the Upper Midwest has to offer. Located in southern Wisconsin just west of Mount Horeb, the Cave has been the host to millions of visitors since 1940. It takes its name from the Blue Mounds, two large hills which from a distance appear to have a bluish hue. The Cave actually underlies the southern slope of the eastern mound.

The cave was first discovered by accident in 1939 by workers who were quarrying for limestone at the foot of the eastern mound. Using dynamite, they blasted out huge chunks of limestone, only to find a great underground tunnel where their quarry had once been. The underground tunnel turned out to be a limestone cavern more than 20 feet high, opening into numerous other rooms and galleries.

The discovery attracted so many visitors so quickly that the cave had to be sealed off in order to preserve it. Unfortunately, during the early days, souvenir hunters actually broke off numerous stalagtites and stalagmites, the stumps of which can still be seen. Fortunately, most of the cave's natural features were preserved by the temporary closing, and the cave was reopened to the public in 1940.

The story of the formation of the cave begins over 400 million years ago, during the Ordovician Period of Earth's geologic history, with the formation of the limestone rock from which the cave was carved. During

One of many stalagtites to be be found in the cave. The bright coloring in the stalagtite is caused by the presence of brightly colored minerals such as iron and manganese oxide.

this period, warm, shallow seas covered much of North America. These seas teemed with shellfish, whose decaying shells over millions of years accumulated to create massive quantities of limestone rock.

The Cave itself began to form around a million years ago. The shallow inland seas had receded, but the water table remained high, and the layers of limestone created by the shells of uncounted shellfish remained saturated with water. The top layer of this water table became acidic due to rainwater and melting snow combining with carbon dioxide as they seeped through the surface soils, forming carbonic acid. This acid, though weak, is capable of dissolving limestone, and a large crack in the surface rock allowed a large amount of carbonic acid to seep through and dissolve out large cavities in the rock. This crack, called a "lifeline", was responsible both for the initial formation of the cave, and for the spectacular rock formations that would eventually be formed therein.

Not so much a stalagmite as a vast pile of calcite, or "cave onyx", this stalagmite will eventually grow to the ceiling to form a mighty column.

Over time, the water table subsided, allowing the long, undulating cavity formed by the acidic water to fill with air. Without the water to dilute it, water droplets falling through the lifeline, filled with calcium carbonate after filtering through the limestone, fell directly on the rock floor, each drop leaving a small amount of calcium carbonate behind it. Over time, each of these small amounts of calcium carbonate formed "speleothems", the most common of which are known as stalagtites, columns that grow from the ceiling of the cave down, and stalagmites, columns that grow from the floor of the cave up. The amount of time it takes to build a speleothem varies, but it typically takes anywhere from 50 to 150 years per cubic inch of material.

Stalagtites start off as small rings of calcium carbonate, which gradually form long, hollow straws. Over time these straws become plugged, and the stalagtite begins to grow in length and thickness. Stalagmites form in a similar fashion, except they are never hollow, simply floorbound versions of stalagtites. Eventually, the stalagtite and stalagmite will join to form a column.

There are many types of speleothems other than stalagtites and stalagmites. One variant type are "ribbon stalagtites", which are formed when water flows along the ceiling of a cave, instead of falling vertically. When calcite-laden water drips into shallow pools of standing water, speleothems called "lily pads" or "cave rafts" can be formed. A light plate of calcite crystals lays suspended on the surface of the water, on top of which a small stalagmite may form. When the plate becomes too heavy, it sinks, allowing another lily pad to form. Several pads may form one atop another to form a complete stalagmite. Another speleothem variant is called a "helictite", stalagtites that grow both downwards and sideways. Oolites, or "cave pearls", are small, pearl-like concretions, where calcite forms around grains of sand, much like pearls.

Left: A helictite, an unusual form of stalagtite that grows sideways.
Center: A "lily pad", a type of stalagmite that forms in pools of standing water.
Right: Two oolites, or "cave pearls", that form in much the same way as standard pearls do. Images copyright © 1995 Cave of the Mounds. All Rights Reserved.

The brilliant colors to be found in various speleothems are due to the mixture of minerals present in the water. Red and brown colors are caused by iron oxide, and blue and grey colors are caused by the presence of manganese oxide. Some speleothems are partially luminescent, giving off light for a brief period after

This calcite structure is briefly luminous after having been exposed to bright light. Roll over it to see! Image copyright © 1995 Cave of the Mounds. All Rights Reserved.

exposure. The effect of these brilliant colors creates brilliant vistas of color, inspiring some to coin colorful names for the various areas to be found throughout the cave, including "The Dream River Room" "The Gem Room", "The Painted Waterfall", and "The Cathedral Room".

Some of these rooms wind on for hundreds of feet leading, some believe, to yet more passages in the deep unexplored recesses of the cave. Though there have not yet been found any additional passages or connecting caves, some believe that there are yet more passageways and caves connecting the Cave of the Mounds with a network of other caves to be found throughout the area.

The Cave of the Mounds is open daily spring, summer, and fall, but during the winter it is open weekends only, with scheduled guided tours year round. The Cave is located twenty miles west of Madison, Wisconsin, just off of U.S. Highways 18 and 151 between Mount Horeb and Blue Mounds. For more information, contact: Cave of the Mounds, Brigham Farm, P.O. Box 148, Blue Mounds, WI 53517; phone: (608)437-3038; email: info@caveofthemounds.com.

Official Cave of the Mounds site

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