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  • The story of the geologic formation of the Cave of the Mounds begins with the creation of the rock in which the Cave formed. The Cave was formed within limestone, a sedimentary rock formed from compacted seashells and other marine sediments. This rock dates back over 400 million years to the Ordovician Period of the Earth’s geologic history. During the 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 the limestone we see today. 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 and 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 at least 20% magnesium.

  • Cave of the Mounds itself began to form 1 or 2 million 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. Often, the top layer of the water table becomes acidic because rainwater and melting snow absorb carbon dioxide as they seep through surface soils. The water combines with the carbon dioxide to form weak carbonic acid, which can dissolve limestone and create cavities within 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 that can still be seen today.

  • 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. This allowed a new stage in the life of the Cave to begin.

  • When surface water seeps through the soil and then through the porous rock, it dissolves small amounts of the limestone (also called calcium carbonate). 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 in the form of calcite. Each drop leaves calcite crystals on the cave ceiling, walls or floor. The crystals adhere to each other and grow into different kinds of formations, called speleothems. Eventually, stalactites reach down from the ceiling, stalagmites tower upward from the floor, and sheets of flowstone cover the walls.

  • Speleothems grow very 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 one cubic inch of “cave onyx”. This process continues today – a design forever in process and never complete.

Learn more about the science of the Cave on our Cave geology page!

Fun Fact – It takes approximately 100 years for cave onyx to grow 1 inch.
Learn more fun facts.