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This earthcache is located on the Root River State Trail in Fountain, Minnesota. Please park at the trailhead. Fountain, Minnesota is located in Bluff Country, known for excellent biking trail system, scenic highways and byways. You will have a nice stroll on the paved multiuse trail. Please be aware of bicyclist, cross country skiers and snowmobiles. Handicap assessable earthcache in the summer only.
Parking coordinates: N 43 44.328 W 092 08.046
A sinkhole is not a hole in the rock.
A common misunderstanding is that a sinkhole is a hole in a rock. Actually, the sinkhole is what we see on the ground surface because the hole is in the rock below. This earthacache will provide insight to the geological basis and unique features of sinkholes.
Karst Processes
Carbonate rocks are the basis of sinkholes, caves, sinking streams and subsurface drainage. Dolostone and limestone are the common carbonate rocks. Dolostone is the mineral dolomite plus some calcite. Limestone is calcite plus some dolomite. These are not the only soluble rocks in which karst forms, but they are the only important soluble rocks in Fillmore County. Karst is a type of topography that is formed by dissolution. Natural karst processes occur over hundred to thousand of years. The formation of karst includes interactions between carbonate rocks and slightly acidic water. Carbonic acid is a mild acid formed when rainwater and carbon dioxide react. As the rainwater passes through the soil, the water absorbs more carbon dioxide and becomes more acidic. These chemical reactions are natural processes.
Since calcite dissolves faster then dolomite, rocks with high calcite to dolomite ratios dissolve faster and develop more distinctive karst landforms. The carbonate chemistry provides the basis for karst processes. Other factors such as topography, depth of bedrock, type of sediment cover, rock structure, and climate affect karst development.
In Fillmore County, the best-developed karst landscapes are controlled by topography. The highest sinkhole density is on flat hilltops between river valleys. Flat hilltops are result of erosion processes that cut across the pre-existing starata.
The depth of bedrock and nature of the ground cover also control the location of visible karst features. Fillmore County karst features are covered with more than 50 feet of ground cover.
Climate plays another role in karst development. Limestone and dolostone dissolve more rapidly in warm temperate, humid climates than in arid glacial climates. The limestone and dolostone of Fillmore County have been subjected to both slow and rapid processes since Paleozoic time, when invertebrates, reptiles, amphibians, and seed bearing plants appeared. A long period of warm, moist weathering during the Cretaceous and Tertiary periods produced a large number of “paleokarst” features that were buried by glacial deposits in Pleistocene time period. The alternating cold glacial and warm interglacial periods during the Pleistocene influenced the development of karst in Fillmore County.
Carbonic rock contains openings between beds of rock and fractures or joints. The slightly acidic water drains in the rocks through these openings. The acid water enlarges the openings. The entire process is self-accelerating: openings that are enlarged will drain more water, thus increasing the rate that acid water is brought into contact with the rock, increasing the dissolution of the rock. The cycle continues, increasingly more acid water drains into the openings, creating larger fractures, until a sinkhole develops.
Fillmore County sinkholes range from less than 3 feet to more than 100 feet in diameter and from 1 foot to about 60 feet in depth. The majority are 10 to 40 feet in diameter and 5 to 40 feet deep. Sinkhole shapes are circular or elliptical. The walls range from nearly vertical like cones to shallow bowl-like shapes.
Sinkholes occur worldwide. Sinkholes are depressions formed in karst by either slow, downward solution or rapid collapse of the land surface by activities of man. Man made sinkholes are those caused by human activates as a result of ground-water depletion, construction activities or a combination of both.
Environmental impacts of Sinkholes
Ground-water contamination is a major concern for all karst features worldwide. Sinkholes serve as direct connections between surface water and underlying water aquifers. Rainwater rapidly travels from the surface to underlying aquifers. Agricultural chemicals sprayed on fields may dissolve in water or are carried in sediment washed into sinkholes, which can then move into ground water. Chemicals or bacteria leached from garbage placed in sinkholes can contaminate ground water. In Fillmore county, nitrates, bacteria, drainfields, municipal waste treatments, salt from roads, and storm runoff, chemical leaching from landfills, petroleum storage tanks, and transportation accidents are some of the causes of ground water contamination.
Dye trace studies are useful in understanding the relationships between surface water and ground water in karst. Fluorescent dye is released into a sinkhole. Springs in the area are monitored to detect the dye. Less than 5 gallons of dye is released. Special monitoring “bugs” composed of charcoal are placed in the springs. The “bugs” are analyzed at a lab to determine the amount of dye going through the spring in a certain time frame.
Dye trace studies have been conducted in Fillmore County.
At ground zero, there are additional signs explaining sinkholes. Read the signs. Your task is to apply the geological content to the signs.
Tasks: Answer the following questions.
1-How many sinkholes are in Fillmore County?
2-Name the main types of Best Management Practices listed on the sign.
3-A dye trace study was conducted from this site and nearby. What are the locations the dye trace “bugs” detected the dye? What directions did the dyes travel from the dye trace study locations?
Tour the area, reflecting upon the diameter and depth of this sinkhole. There are many sinkholes in the area, enjoy the variety of shapes and sizes of each sinkhole.
Another publicly accessible sinkhole is located at this waypoint.
N 43 44.326 W 092 08.326
In your answers, please include the earthcache name Root River Sinkhole, number in your party, and cacher’s names. One email per caching group.
Additional Hints
(No hints available.)