Geology of Cave of the Mounds
The 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 geologic history.
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. As 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 centimeter of “cave onyx.”
Stalactites 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.
Calcite 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.
Helectites 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.
While 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:
“Take nothing but pictures, leave nothing but footprints, kill nothing but time.”
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.