Jumping into Sinkholes
Written by Stalac-Tate
There are some misconceptions of sinkholes in different places of the world. Most have heard of sinkholes prominently in the news. However, the answer to why sinkholes sink slower in Wisconsin than in Florida is limestone. The difference lies in our geology. In Wisconsin, the karst bedrock forms in dolomite, which is much less easily dissolved than the limestone that forms the karst bedrock in Florida. As a result, we have fewer and smaller voids and cavities in Wisconsin’s karst. The sinkholes are proportionally smaller as well.
Before I get too far ahead, let’s talk about
What is a Sinkhole?
When the ceiling of a cave or cavity gets too close to the surface and it cannot support itself anymore, it sinks in. This creates a bowl-shaped, funnel-shaped, or elongated depression on the surface called a sinkhole, and underground it is called a collapse. Sinkholes may be open, covered, buried, or partially filled with debris. Sinkhole dimensions vary by region. Wisconsin sinkholes generally range between 20 to 30 feet in diameter and about 4 to 10 feet deep, however, some can be wider and/or deeper.
How do Sinkholes Form?
Sinkholes are the evidence of a near-surface cave below. These holes or depressions form when water washes sediment down into cracks and voids in karst bedrock, which for us is limestone. Geologists describe a landscape that includes sinkholes and caves as a Karst topography. When limestone rock dissolves underground, which is an important process in the origin of limestone caves, the land surface above ground is also affected. These geologic processes lead to this distinctive type of landscape.
Therefore, water flowing in from the surface goes underground first to create the cavity or cave below. Then, further erosion causes the surface, or ceiling of the cave, to slowly sink into this space, especially if the cavity is near the surface.
Not all sinkholes are the result of karst. Some manmade sinkholes occur when a water main break washes sediment out of the area, creating a large cavity. These types of sinkholes are often on the news.
Where Do Sinkholes Occur?
Sinkholes occur worldwide. However, in different regions, there are different factors. In Wisconsin, sinkholes are most likely to occur in the Driftless Area. However, Wisconsin’s Karst topography ranges across the state in a v-shaped pattern. Some of the largest caves in the state are found in Wisconsin’s Karst topography.
Here at Cave of the Mounds, we have 5 sinkholes. Two are active sinkholes with steep sides, rocks/dirt visible from shifting ground, and no large trees or plants in/near because the ground is still shifting too much for a good root system to develop. One sinkhole is semi-active, meaning it was once active but is now on its way to becoming inactive. And two inactive sinkholes which are easy to identify because it looks like normal land.
Learn more about sinkholes by traveling along our Karst Trail using our Karst Trail Guide.