Cave of the Mounds

Cave of the Mounds
Inside Cave of the Mounds
Map of Wisconsin
Location	Blue Mounds, Wisconsin
Coordinates	43°01′05″N 89°48′58″W / 43.018°N 89.816°W / 43.018; -89.816Coordinates: 43°01′05″N 89°48′58″W / 43.018°N 89.816°W / 43.018; -89.816
Designated	1987
Official website

Cave of the Mounds, a natural limestone cave located near Blue Mounds, Wisconsin, United States, is named for two nearby hills called the Blue Mounds. It is located in the southern slope of the east hill. The cave's beauty comes from its many varieties of mineral formations called speleothems. The Chicago Academy of Sciences considers the Cave of the Mounds to be "the significant cave of the upper Midwest" because of its beauty, and it is promoted as the "jewel box" of major American caves. In 1987, the United States Department of the Interior and the National Park Service designated the cave as a National Natural Landmark.

The limestone from which the cave was carved began forming approximately 488 million years ago, during the Ordovician Period. During this time, much of North America was covered with warm, shallow seas. Over millions of years, calcium carbonate shells from tiny marine organisms accumulated on the sea floor, forming enormous quantities of limestone. The type of limestone in which the cave was formed is called galena dolomite because of its high concentration of the lead ore galena.

The cave began forming about a million to a million and a half years ago, in a manner similar to many caves. A large crack in the surface of the rock, called the cave's "lifeline", allowed rain water to seep into the stone. In the air, rain combines with carbon dioxide to form a weak carbonic acid. Though this acid is not very strong, it is strong enough to dissolve away the limestone after it seeps in through the "lifeline". Over time, large cavities were dissolved in the stone and as the water table dropped, the water drained out and the cavities filled with air. Now that the cavities are filled with air instead of water, the water droplets coming from the "lifeline", which are saturated with dissolved calcium carbonate, deposit solid calcium carbonate, which builds up over time to create speleothems. This is a very gradual process, usually taking 50 to 150 years to form one cubic inch of material, and continues to this day.