story of the geologic formation of the Cave of the Mounds
begins with the creation of the rock in which the Cave is
found. The Cave was formed within sedimentary rock, a limestone
made of compacted seashells. This rock dates back over 400
million years to the Ordovician Period of the earth’s
During this Ordovician Period,
warm shallow seas covered the continent where we find Wisconsin
today. Abundant shell life could thrive in these seas. Layers
and layers of calcium carbonate shell debris accumulated
and slowly hardened into limestone. Thousands of feet of
limestone and other sedimentary rocks were laid down during
this Ordovician Period.
Millions of years ago, the
seas receded leaving these layers of rock behind. Erosion
began to wear them down. Today the exposed rock in Blue
Mounds is a limestone called Galena dolomite, which is a
specific kind of limestone containing some magnesium. Cave of the Mounds itself began to form about a million
to a million and a half years ago when the Galena dolomite
was still beneath the water table. The water table is defined
as that level below which
all of the rock is saturated with water. Rock above the
water table contains air spaces, while rock below the water
table has all its spaces filled with water.
Often the top layer of the
water table becomes acidic because rain water and melting
snow absorb carbon dioxide as they seep through surface
soils. The water combines with the carbon dioxide to form
a weak carbonic acid, which can dissolve limestone and create
cavities in the rock. When a major crack lets large amounts
of acidic water into the limestone below the water table,
large amounts of rock dissolve along this crack. This is
what happened at Cave of the Mounds. The cave was formed
along a major crack which can be seen today. This crack
is called the "lifeline" of the cave.
The story of Cave of the
Mounds does not end with the dissolution of limestone to
form the hollow cavern. Even as the dolomite beneath the
ground was being dissolved to form the cave, surface streams
were eroding deeper and deeper valleys in the landscape.
the stream levels lowered, so did the water table. Eventually,
the water table dropped below the level where the cave had
been formed. Now the large natural cavity far below the
earth’s surface was filled with air. Now a new stage
in the life of the cave could begin.
When surface water seeps
through soil and then through porous limestone rock, it
dissolves small amounts of limestone. Every droplet of water
entering the cave below carries dissolved calcium carbonate.
As the water drops enter the air-filled cave, this calcium
carbonate is precipitated into the form of speleothems.
Each drop leaves calcite crystals on the cave ceiling or
walls or floor. The crystals adhere to each other and grow
into different kinds of formations. Eventually, stalactites
reach down from the ceiling, stalagmites tower upward from
the floor and sheets of flowstone cover the walls.
Speleothems grow slowly.
The rate of growth depends on how fast the water flows and
on how much dissolved calcium carbonate it contains. It
can take from 50 to 150 years to deposit a cubic inch of
are speleothems which form on the ceiling of a cave. All
stalactites begin as hollow circles when crystals of calcite
form around the outer surface of a droplet of water hanging
form the ceiling. As each new drop of water appears, it
leaves another crystal ring. Soon long crystal soda straws
hand from the ceiling. These tubular stalactites grow into
cones of icicles if the initial tube becomes plugged and
the crystals form around its outer surface.
Sometimes water trickles
along the cave’s ceiling and walls leaving thin trails
of crystals. Each new trail builds upon prior ones to shape
ribbon stalactites. Various minerals in the water create
the many colors in the speleothems. Reddish brown colors
are oxides of iron; blue and grey colors are manganese oxides.
sometimes forms lily pads or cave rafts on standing pools
of water. These occur when a light plate of calcite crystals
is held in suspension on the water’s surface. When
the crystal plate becomes too heavy, it sinks; other plates
form and fall upon one another successively until they make
a pad or a raft. As water drips onto the exposed pad, it
may form a stalagmite. Then the whole structure looks like
a lily pad.
Speleothems made of pure
calcite with few other added minerals are light colored
and translucent when exposed to light. Here crystals of
calcite have built on one another to form a single large
beautiful parrot-shaped crystal.
are strangely shaped stalactites which grow sideways as
well as downward. They seem to defy the laws of gravity.
Tiny and rare are the oolites or cave pearls which form
as the bottom of small pools of water. Grains of sand act
as catalysts for the deposit of calcite crystals.
Chambers of curving rock
disappear into the distance underground. The shapes and
colors of the walls and ceiling are mirrored in a large
reflecting pool, as if in a dream river.
Anyone looking down this
long tunnel begins to wonder where else the Cave leads,
whether the rooms we visit are part of a much larger cave
system with unseen possibilities of beauty and color.
To date, efforts to find the link between the known passageways
of Cave of the Mounds and other possible caves have been
unsuccessful. However, the hope of uncovering other vistas
like the dream river room remains real.
we dream of additional caves, we strive to cherish the beauty
which has already been revealed. The Cave is a treasure
to be shared. But, if it is to be shared with future generations,
it must be carefully preserved. The beauty of the Cave makes
it imperative the we remember the motto used by cavers everywhere:
but pictures, leave nothing but footprints, kill nothing
To learn more about cave science and conservation, visit the National Speleological Society.
To learn more about America's Show Caves, visit the National Caves Association.