Sinkholes, caves and springs: Karst envelopes the Driftless region

erial view of sinkholes outside Fountain in Fillmore County. (Image from Google Earth 2023)

By Zach Jensen,

erial view of sinkholes outside Fountain in Fillmore County. (Image from Google Earth 2023)

Minnesota may be the land of 10,000 lakes, but a Harmony Township resident said Fillmore County, itself, is home to more than 10,000 sinkholes — all because of a geographic phenomenon called “karst”.

Aaron Bishop, 33, has been studying karst topography practically his entire life, since his parents first took ownership of Niagara Cave, close to the Iowa/Minnesota border, in 1995.

“Having grown up with a cave as a basement has given me a particular perspective regarding the relationship between surface-use and the underworld,” said Bishop, a 2009 graduate of Fillmore Central High School. “We train guides to interpret karst features for guests who visit the cave. There are over 400 known caves in Fillmore County alone, but most are not accessible to the public. Mystery Cave (near Preston) and Niagara Cave are the only two caves open to the public in the state of Minnesota, so this is a rare opportunity for visitors of the region to be exposed to the underground.”

Bishop, who earned a bachelor’s degree in geology from the University of Hawaii at Hilo, said the word “karst” is the German form for the word “kras”, which is Slavic in origin, meaning “bleak waterless place”, and it’s difficult to define because it often looks very different from one location to the next.

“I usually describe karst as a topography characterized by features such as joint systems, sinkholes, blind valleys, disappearing streams, caves and springs,” he explained. “Karst topography results from soluble rock, usually carbonate rock such as limestone or dolostone, that is dissolved away from carbonic acid. Carbonic acid is the combination of carbon dioxide and water; CO2 + H2O = H2CO3. Just like how carbonic acid in soda, or pop, can dissolve away your calcium-based teeth, so too can it dissolve away the calcium-based limestone bedrock in our region, usually along natural vertical fractures (joints) and horizontal bedding planes.

“If we look back in time to the Early Paleozoic Era (541 million years ago), this region was once in the southern hemisphere near the equator and occasionally covered by shallow inland seas,” Bishop continued. “The proximity to shore would determine which sediment would be deposited at any given time. Nearer to shore, the primary sediment deposits would be coarse sand grains, slightly farther out would be finer grains of muds and silts, and beyond that would be carbonates.” 

The rural Harmony native said that under the right conditions, calcium carbonate (CaCO3) precipitated out of the water accumulating on the shallow seafloor and would be covered by more and more carbonate sediment forming layers. 

“Over time and pressure, this carbonate sediment would lithify (turn to stone) from compaction,” he added. “As the engine of plate tectonics moved the landmasses about the face of the planet for the next four and a half or so hundred million years, the area we now reside ended up far north of the equator where we find ourselves today.”

However, as Bishop explained, something else happened during this time: The Appalachian Mountains. “What do they have to do with us here in the Midwest?” he asked. “During the episodes of mountain building (orogenies), folding and faulting (diastrophism) occurred throughout the North American continent. Compression and shearing of rock resulted in two main joint sets; one oriented primarily east-west, and the other northeast-southwest. Joints are not faults but rather fractures without the major movement associated with faults. These joints are key to water movement, and subsequently the development of sinkholes and caves within the limestone rock.”

Bishop said the reason this area, in the upper Midwest United States, has more carbonate limestone rock near the ground’s surface is because the entire area lacks glacial drift.

“Drift is not a verb. It’s not about movement. Rather, glacial drift is a noun,” said Bishop. “Drift is all of the rocks, muds, clays, grit and gravel that was scraped along as glaciers moved southward during the Pleistocene Epoch 2.5 million years ago to 11.7 thousand years ago. Because southeast Minnesota, southwest Wisconsin {…}, northeast Iowa, and northwest Illinois were not covered by these recent glaciers, we don’t have as much glacial drift, and our shallow soils are just above the shallow bedrock. Due to the limestone’s exposure to acidic water and its proximity to the surface, we can experience the karst topography. 

Bishop explained that even just one county to the west also has limestone, but it’s buried beneath 50 to 100 feet of glacial drift — making any potential karst features inaccessible. However, that doesn’t make karst rare. 

“Karst is not uncommon,” he said. “Covering more than one-eighth of the surface of Earth, the development and evolution of karst is not the same everywhere and exists as a continuum. There are no two karst regions exactly alike. What karst does is it blurs the line between surface water and groundwater. Remember the word kras? It meant ‘bleak waterless place’, because water finds it very difficult to stay on the surface in karst areas. Minnesota is known as the land of 10,000 lakes, but Fillmore County has zero lakes and over 10,000 sinkholes. We also boast the largest blind valley in the state as well as the largest discharge spring in the state. All in one county. All because of karst.”

“Karst is incredibly exciting,” he continued. “It’s predictably unpredictable nature ensures a quality experience when research or exploring. Any time there is an opportunity to go on a vacation, we look for where the caves are. No two caves are alike.”

However, because karst is so permeable, Bishop said that topography not only allows water to move relatively freely — it also allows potential toxins to also more-freely enter our underground water supply.

“Limestone’s texture is very porous, like a sponge,” he said. “It holds water that we can drill a well and draw water from (aquifer). The shallow soils of the driftless, coupled with the fractured and porous nature of our bedrock, allows for the swift movement of water (and contaminants) from the surface to the groundwater supply. What happens on the surface plays a huge role in the quality of water from the aquifers below.

“We know the source for high nitrate contamination,” Bishop added. “According to the Minnesota Pollution Control Agency, 70 percent of nitrate pollution comes from cropland for most of Minnesota. In the Karst Region, it’s closer to 89 to 95 percent. Now, if we look at it another way, we know the source of the pollution, that means we know the source of the solution. Corn and soybeans are nitrate intense crops. Throwing in cover crops to draw the excess nitrates from the ground and/or adding other small grains into the rotation may help reduce the nitrates leaching into the groundwater.”

In summary, Bishop explained that more karst equals more groundwater pollution, which means any toxins or other contaminants in the surface water will, eventually, flow into our drinking water.

“After the rains fall and the ground dries up, folks may forget about it,” Bishop said. “Not farmers. Not cavers. The story of water after it soaks into the ground is just the beginning. We’ve learned so much in the past several decades, including the harmful health effects of contaminated water. More and more research is showing the long-term health effects (that) nitrate contaminated water leads to thyroid disease, colorectal cancers and neural tube defects. But, we also know there are opportunities to clean up the water from the top down with cover crops and other farming practices. It is my hope that more people begin to think about, put effort into, and take pride in protecting Earth’s most valuable resources.”

For more information on groundwater pollution in Minnesota, visit the Minnesota Pollution Control Agency’s website at 

Niagara Cave is open seven days each week, from May 1 through Oct. 29 and is open for tours Nov. 3-5. For information on Niagara Cave, visit 

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Kim-Ann Johns
5 months ago

Absolutely a beautiful cave, and is maintained by a great family. They have given to the cave by educating people and sharing her with the public and the cave has given back to them. Fillmore county is trult a mystical place in Minnesota.