GEOLOGY OF SINKHOLES
Located off US highway in Fountain City park. Park in the parking lot. Walk east on the Root River bike trail one-quarter mile to a grove of trees that are collapsing into the sinkhole.
Text On Markers:
The surrounding area and much of southeastern Minnesota are karst landscapes. Minnesota's karst landscapes consist of limestone and dolostone bedrock that lies very close to the surface. This carbonate bedrock is often riddled with features eroded by slightly acidic water: sinkholes, passageways, extensive underground water systems, and caves.
Rainwater becomes slightly acidic by absorbing carbon dioxide in the atmosphere, and if it seeps through the soil, by absorbing the carbon dioxide given off by plant roots, bacteria, and other organisms. Over time, this water following bedrock joints, or fractures, dissolves the carbonate rock and gradually enlarges the cracks. Eventually, a system of underground drainage will develop that bypasses the surface drainage pattern. Sinkholes are inlets to that system. A sinkhole may begin to develop where joints in the bedrock intersect and the downward flow of water is more rapid. Over time, a funnel-shaped cavity often forms in the rock. Infiltrating surface water erodes the soil and moves it down the hole, thus forming a pronounced depression in the ground. When erosion into the subsurface is slow, sinkhole formation is also a slow, gradual process. When erosion is rapid, a sudden collapse of overlying sediment can occur. Sinkholes sometimes collapse suddenly after heavy rains. A sinkhole may become temporarily closed as newly collapsed sediment clogs the passageway. In a karst landscape, water flowing into sinkholes bypasses the natural filtering action of a lengthy percolation through thick soil and sediment layers. Once in the bedrock, water can move rapidly through a complex system of passageways at rates as high as several kilometers per day. Using dye to color the water, scientists have shown that water entering this sinkhole emerges in about a day at the headwater springs of Trout Creek, about two kilometers northwest of here.
In karst terrains, bedrock aquifers, a common source for drinking water, are susceptible to rapid contamination from activities on the surface of the land. Likewise, water quality in spring-fed streams, which mark the end point of underground drainage in a karst landscape, may also be affected.
Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation and the Minnesota Geological Survey. 2003
Marker Current Status:
Site visited. Locals state that no GSM marker in town.
- Updated Coordinate Needed
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