CAVE OF THE MOUNDS
Erosion has scoured away most of the Silurian dolomite that once
lay over all of Wisconsin. Its remnants exist in the Niagara Escarpment on the eastern side of the state and on the tops of several
mounds in the southwest corner, which are outliers of the more
widespread Silurian strata now eroding to the south in Illinois and
Iowa. Two of these mounds, called the Blue Mounds, are located
about 20 miles west of Madison on US 151. The western mound
is the higher of the two, standing 415 feet above the surrounding
plain, the highest point in southern Wisconsin and the site of Blue
Mound State Park. The eastern mound is 230 feet shorter.
Cave of the Mounds, a few miles east of the state park on the
south face of the eastern mound, was discovered in 1939 when a
routine dynamite blast at a quarry uncovered a 20-foot-high room
with passages to other chambers. The landowner immediately
halted the quarrying and, within a year, opened a commercial cave.
Lying in Galena Dolomite deposited about 460 million years ago, the
cave is 70 feet deep and about a half mile long. Groundwater, made
weakly acidic by carbon dioxide absorbed from the environment,
dissolved calcium carbonate along fissures and bedding planes in
the dolomite, creating crevices and caverns. Percolating water combined with the sulfur in lead and other minerals deep underground
to form sulfuric acid, which also dissolves calcium carbonate.
At Cave of the Mounds, this dissolution process began between
1 and 2 million years ago when the dolomite was below the water
table. As the local streams carved their valleys deeper, the water
table dropped and underground erosion continued at deeper
levels. Eventually, groundwater rivulets and then streams flowed
through the cave, enlarging the passageways, and water dripped
into the spacious caverns. Upon interacting with the air, the water’s
carbon dioxide escaped, triggering the deposition of calcium car-
bonate (the mineral calcite). Cave of the Mounds is famous for its
spectacular dripstone formations, collectively called speleothems,
that grew slowly as the calcite was deposited, crystal by crystal. One
cubic inch of growth can take one hundred years or more. It is
important to never touch any of the formations to avoid acciden-
tally damaging them. Also, oil from our skin can interfere with the
deposition of calcite and stop the growth of the speleothems.
Cave of the Mounds, designated a National Natural Landmark
in 1988, is open for touring throughout the year. Many caves close
during winter to allow resident bats to hibernate undisturbed.
However, this cave was a closed underground system until it was
discovered and quickly sealed in 1939, and there is no evidence
that bats have ever lived in it. Thus, the cave remains open in winter when its temperature is an even 50 degrees, just as in summer.
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South Cavern shows a variety of speleothems, including flowstone, where water, flowing in sheets, deposits layer upon layer of calcite on the walls or inclined floor of a cave.
—Courtesy Cave of the Mounds
The Gem Room in Cave of the Mounds displays three aspects of speleothem formation: prominent stalactites and stalagmites (center), flowstone (lower center to lower
left), and color—red showing the presence of iron oxides in the calcite and blues and
grays showing the presence of manganese oxides. —Courtesy Cave of the Mounds