Types of Wisconsin Caves

There are two types of caves in Wisconsin – limestone and sandstone. Limestone caves are the ones you might typically think of – stalactites dripping from the ceiling, dark, wet places with steady cool temperatures, and bats lurking in the corners. These caves are found in limestone rock formations and are the most prominent type found here. Sandstone caves formed primarily in the southwestern part of the state and are sometimes merely rock outcroppings or shelters. These tend to be much smaller and drier than their limestone cousins. Both types of caves offer opportunities for fun and adventure. The easiest to access are a few of the larger limestone caves that are open to the public.
Once inside a cave, curiosity soon takes over. How were the caves formed? Who found them? What are the different rock and mineral formations on the walls and ceiling? Why it is cold and damp?
As luck would have it, no two caves are alike, just as you never step into the same river twice. Each cave is ever-changing and water constantly shapes the landscape above ground and underground.
Limestone caves are also known as 'solution' cave that formed by the dissolving limestone rock as the surface came in contact with water that is slightly acidic. It is sort of like seeing an Alka-Seltzer tablet dissolving in water.
Looking into vast caverns and rooms with high ceilings, it is hard to imagine Alka-Seltzer™ getting the job done. A key component is time – millions and millions of years of constant dissolving. Fiske explains, "As rain water soaks into the ground, it filters through leaf and other organic matter, drops off larger particles and picks up dissolved carbon compounds. These carbon compounds combine with water to form a mild solution of carbonic acid. If there are some remnant petroleum products in the limestone, sometimes the sulfur compounds in the petroleum products attach themselves to the water and dissolve forming a mild sulfuric acid. Both of these acids will dissolve the limestone rock.

Water seeping through limestone slowly forms hollow tubes called soda straws. ©AS
Water seeping through limestone slowly forms hollow mineral tubes called soda straws.
Soda straws are also known as Tubular stalactites.

Water containing the mild acid solutions flows through the limestone rock formations following cracks in the earth's crust. As the mild acid dissolves the limestone the cracks get bigger and form passages. Some of the dissolved limestone and water comes in contact with air and re-solidifies. That is what makes formations inside the caves.

Most of Wisconsin's limestone caves were formed in this manner over millions of years, and the results inside can be spectacular. The different features that decorate a cave are referred to as cave formations or speleothems. The most common speleothems are familiar to people, even if they can't remember which is which. They are stalactites and stalagmites. Stalactites are the ones that "stick tight" to the ceiling, while stalagmites build up from the cave floor. These deposits of calcite are concentrated along cracks where the water drips through the rock and dries out. In fact these formations are often referred to as drip stones or flow stones. They form much as an icicle does in the winter, constantly dripping and leaving a little more calcite to crystallize.

Another type of speleothem is known as onyx, Fiske says, "Onyx is a highly crystallized form of rehardened limestone that covers the ceilings and formations in a cave and can sometimes be several inches thick. It begins as a liquid solution and attaches itself to rock as it dries out. It is a lot harder and more glass-like than normal limestone, so the formations often appear very different from usual, as if they have been coated with milky glass."

There are also "popcorn speleothems," small formations made from drippings or splashing liquid on the floors. Where lots of these little bubble growths are clustered next to each other, it does look like popcorn.

Some caves also have box-work formations. These are thin blades of calcite that hang from the ceilings or stick out from the walls. They go every which way making a honeycomb pattern. The blades can intersect at various angles forming boxes, hence the name. Wind Cave in South Dakota is one of the best examples of this, but box-work formations also are found in some Wisconsin caves.

Only about 250 sandstone caves have been recorded and mapped in the state. Sandstone caves are also usually cool in temperature, but they are dry and are generally much smaller than their limestone cousins. The longest one in the state is 100 meters in length, but most are tiny by comparison. Sandstone caves are usually barren and lack the more typical cave formations because they don't have mineral-rich water dripping from the ceiling. Some of these caves were formed through dissolution, but most were formed by running water and wind erosion centuries ago.

Sandstone caves were used by Native Americans as places to live and visit. We can tell from the excavations of sandstone caves and rock shelters, that Native Americans would spend their winters in and around these caves that must have provided protection from the elements. Deer bones found at these sites have definitely been identified as animals that were killed in the winter. Additional evidence comes from etchings made in the sides of the soft sandstone walls of these caves. Pictures of buffalo illustrate a life of nomadic people, tribes that would head west into Iowa and southern Minnesota to hunt buffalo and then return to the fertile Wisconsin river valleys for the winter.

The presence of abstract art on some cave walls suggests that these spaces may have been used in summer for ceremonial purposes such as a youth's "vision quest" or part of a rite of passage.

Archaeologists have also found a cave with a drawing of a human figure with zigzag lines coming from his head. It is thought this is a picture of a storyteller, perhaps recounting the buffalo hunt depicted on the other side of the cave.