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Karst Topography in Fillmore County

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Harmony Area Historical Society
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Aerial view of sinkholes in Fillmore County

Aerial view of sinkholes outside Fountain in Fillmore County, July 3, 2011.

Southeast Minnesota boasts a unique geological landscape of rolling hills, fertile fields, dramatic bluffs, and freshwater streams. Beneath its lush surface is a hidden world of underground caves and rivers created after thousands of years of rain and snow seeped through fractured bedrock, slowly softening and dissolving sedimentary rock.

Between 14,000 and 12,000 years ago, the last glacier moved across most of the upper Midwest. It buried much of the area under deposits of silt, clay, gravel, and rocks known as “drift.” Because the distinctive bluffs and narrow valleys of Southeast Minnesota, Northeast Iowa, Southwest Wisconsin, and Northwest Illinois stopped the glacier’s advancement, these areas are known collectively as the Driftless Area. The unique terrain of the Driftless Area, created by dissolving sedimentary rock, is known as karst topography.

Karst is formed by a chemical reaction between water and carbon dioxide in the soil and atmosphere. It makes the water slightly acidic, which causes soluble rock to gradually dissolve. Soluble rock includes limestone, one of the most common types of bedrock in Fillmore County.

Limestone was formed from an accumulation of mud, sand, and the remains of animals that made a prehistoric sea in the area their home during the Paleozoic era. This shallow sea came and went over thousands of years, depositing layers of mud, sand and fossils. As these deposits slowly broke down in Fillmore County, they left behind bluffs, narrow river valleys, and, most notably, sinkholes and caves.

Fillmore County is home to more than 10,000 sinkholes. They were formed when soluble rock beneath the surface soil collapsed following centuries of exposure to rain, snow, and seasonal freezing and thawing. Most Fillmore County fields have outcrops of trees where only green leaves and branches can be seen. Tree branches jut upwards from these sinkholes; trunks and roots are hidden from view. Sinkholes can measure anywhere from a few feet to a hundred feet in depth, and occasionally become entrances to the underground caves that pepper Southeast Minnesota.

Because so many visitors to Harmony ask about those unusual tree clusters, the Harmony Area Historical Society (HAHS) purchased a sinkhole on the northwest side of town along the Harmony-Preston Bike Trail. They created the Karst Exhibit at the Harmony Visitor Center and the Karst Interpretive Site along the trail to explain how sinkholes are created, as well the area’s geology. The Minnesota Department of Natural Resources now manages the Karst Interpretive Site.

Visitors to Fillmore County can experience the unique karst geology of the Driftless Area up close by visiting two caves within a few miles of each other. Public tours are offered to see Niagara Cave near Harmony and Mystery Cave near Preston. Both caves were formed nearly 400,000 years ago and provide excellent examples of intricate underground passageways carved by underground water flow.

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© Minnesota Historical Society
  • Bibliography

Bertalan, Dan, and Rob Nelson. “Mysteries of the Driftless–‒The Documentary.” YouTube. Posted by Untamed Science, September 17, 2013.
https://www.youtube.com/watch?v=xn_DOPumFkU

“Finding Minnesota: The Cave That Pigs Discovered.” WCCO TV, July 6, 2014.
http://minnesota.cbslocal.com/2014/07/06/finding-minnesota-the-cave-that-pigs-discovered/

Karst Interpretive Site. Harmony Area Historical Society.
http://www.harmonymnhistory.org/Karst_Interpretive_Site.html

Haynes, Cyd. “3 ½ Hours to Natural Wonder.” Maple Grove magazine, July 2014.
http://maplegrovemag.com/3%C2%BD-hours-natural-wonder

Karst topography exhibit. Harmony Visitor Center (Harmony, Minnesota), May 2018.

“Niagara Cave Celebrates 80 Years.” KTTC, June 11, 1014.
http://www.kttc.com/story/25698348/2014/06/Thursday/niagara-cave-celebrates-80-years

Weiss, John. “Niagara Cave Celebrates 90 Years, With Lantern Light.” Rochester Post-Bulletin, June 7, 2014.
http://www.postbulletin.com/news/local/niagara-cave-celebrates-years-with-lantern-light/article_fb193f37-706c-57a9-99a0-0d857e2ea4c1.html

Related Images

Aerial view of sinkholes in Fillmore County
Aerial view of sinkholes in Fillmore County
Karst sinkholes in Fillmore County, Minnesota
Karst sinkholes in Fillmore County, Minnesota
Karst sinkholes in Fillmore County, Minnesota
Karst sinkholes in Fillmore County, Minnesota
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Photograph of Lynch Creek, Chatfield, MN
Photograph of Root River Valley
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Limestone Bluff, Olmsted County Park, Simpson MN
Limestone Bluff, Olmsted County Park, Simpson MN

Turning Point

The last glacier moves across the upper Midwest between 14,000 and 12,000 years ago, but does not cover Southeast Minnesota, Southwest Wisconsin, Northeast Iowa and Norwest Illinois, collectively known as the Driftless Area, due to the region’s karst topography.

Chronology

470,000,000 BCE

Intermittent prehistoric shallow seas deposit mud, sand and animal remains which, after millions of years, form layers of soluble bedrock that create Southeast Minnesota’s karst topography.

400,000 BCE

Niagara Cave near Harmony and Mystery Cave near Preston are created as water from rain and snow soak through the topsoil and into the limestone bedrock, dissolving the rock and gradually creating large caverns and underground waterways over thousands of y

14,000 - 12,000 BCE

The last glacier moves across the upper Midwest. Southeast Minnesota, southwest Wisconsin, northeast Iowa, and norwest Illinois remain uncovered due to the region’s karst topography.

1924

Niagara Cave is discovered five miles south of Harmony.

1934

Niagara Cave opens for public tours.

1937

Mystery Cave is discovered between Preston and Spring Valley.

1947

Public tours of Mystery Cave begin.

1989

Mystery Cave becomes part of Forestville State Park.