Minnesota Geology Monday - Karst

Carbonate rock, like limestone and dolostone, are the primary bedrock components of most of the southeastern corner of Minnesota.  These carbonate rocks in southeastern Minnesota are Ordovician in age (488 - 443 million years old).  Because the glacial drift is relatively thin, or nonexistent in this part of the state, these limestones and dolostones are at or near the surface.  Karst easily develop in ten counties in the area as the carbonates are dissolved by water.

The name karst comes from the German word Kras, a region in Yugoslavia.  Landscape features common to karst include caves, sinkholes, and disappearing streams, among others.  Fillmore County in southeastern Minnesota has more of these karst features (including over 10,000 sinkholes) than all of the other counties in the state combined.

There are numerous caves in the area that have formed as water has dissolved carbonate minerals over time.  Cave formations (commonly called speleotherms), like stalagmites, stalactities, and flowstone, radiometric dating has determined the ages of these structures to  between recent to 350,000 years old.  Two caves are open to the public via guided tours, Mystery Cave (http://www.dnr.state.mn.us/state_parks/forestville_mystery_cave/index.html) and Niagara Cave (http://www.niagaracave.com/default.asp).

Disappearing streams is another feature common to karst landscapes.  About a half mile downstream from the visitor center at Mystery cave is example of a dissappearing stream.  The South Branch of the Root River is flowing over fractured bedrock and dissappearing into Mystery Cave.  The picture below demonstrates the view upstream from a county bridge.

Looking downstream from the same bridge and the river is gone.  The river cuts approximately four miles off of its surface path by escaping through fractured carbonate rock and flowing through Mystery Cave.

The most common karst feature in Minnesota are sinkholes.  Sinkholes develop when overlying sediments collapes into cavities in carbonate rock.  Many sinkholes form above joints in the bedrock.  As demonstrated by the picture below, the city of Fountain claims to be 'The Sinkhole Capital of the U.S.A.'.

The easiest way to see sinkholes in this area is to drive the county roads past the local farm fields.  As sinkholes develop, farmers can no longer farm that location, so trees and other vegetation begin to grow out of the sinkhole.  The farmer continues to plant crops around the sinkhole.

A Google Earth image below demonstrates how many sinkholes have developed in the area over time.  The image is one local field and shows numerous sinkholes, the farmer continues to farm the area around the sinkholes.

Karst can be linked with pollution of ground and surface waters.  Because water quickly drains from the surface via fractures and joints in the bedrock, any chemicals or wastes are quickly transported to other locations.  The city of Lewiston discovered just how quickly water can drain away from the surface via a sinkhole.

The grown over pictures below show Lewiston's three sewage lagoons that are no longer in service.  On February 20, 1991, public works employees found a sinkhole had developed in lagoon number 2.  Over a period of a few hours to a day, and estimated 7.7 million gallons of partially-treated water drained into the groundwater system.

101 American Geo-Sites

The new geo-meme suggested by Callan Bentley has individuals highlighting places listed in the new book 101 American Geo-Sites You've Gotta See, written by Albert Dickas.

I scored a 21 out of 101, not particularly good.  It's apparent that I need to spend some more time traveling.  Never visiting the American Southwest (including California) or the Northeast really limited my scores.  After the list, I do have a suggestion for another location to be added.

1. Wetumpka Crater, Alabama
2. Exit Glacier, Alaska
3. Antelope Canyon, Arizona
4. Meteor Crater, Arizona
5. Monument Valley, Arizona
6. Prairie Creek Pipe, Arkansas
7. Wallace Creek, California
8. Racetrack Playa, California
9. Devils Postpile, California
10. Rancho La Brea, California
11. El Capitan, California
12. Boulder Flatirons, Colorado
13. Interstate 70 Roadcut, Colorado
14. Florissant Fossil Beds, Colorado
15. Dinosaur Trackway, Connecticut
16. Wilmington Blue Rocks, Delaware
17. Devil’s Millhopper, Florida
18. Stone Mountain, Georgia
19. Kilauea Volcano, Hawaii
20. Borah Peak, Idaho
21. Menan Buttes, Idaho
22. Great Rift, Idaho
23. Valmeyer Anticline, Illinois
24. Hanging Rock Klint, Indiana
25. Fort Dodge Gypsum, Iowa
26. Monument Rocks, Kansas
27. Ohio Black Shale, Kentucky
28. Mammoth Cave, Kentucky
29. Four Corners Roadcut, Kentucky
30. Avery Island, Louisiana
31. Schoodic Point, Maine
32. Calvert Cliffs, Maryland
33. Purgatory Chasm, Massachusetts
34. Nonesuch Potholes, Michigan
35. Quincy Mine, Michigan
36. Grand River Ledges, Michigan
37. Sioux Quartzite, Minnesota
38. Thomson Dikes, Minnesota
39. Soudan Mine, Minnesota
40. Petrified Forest, Mississippi
41. Elephant Rocks, Missouri
42. Grassy Mountain Nonconformity, Missouri
43. Chief Mountain, Montana
44. Madison Slide, Montana
45. Butte Pluton, Montana
46. Quad Creek Quartzite, Montana
47. Ashfall Fossil
Beds, Nebraska
48. Scotts Bluff, Nebraska
49. Crow Creek Marlstone, Nebraska
50. Sand Mountain, Nevada
51. Great Unconformity, Nevada
52. Flume Gorge, New Hampshire
53. Palisades Sill, New Jersey
54. White Sands, New Mexico
55. Carlsbad Caverns, New Mexico
56. Shiprock Peak, New Mexico
57. State Line Outcrop, New Mexico
58. American Falls, New York
59. Taconic Unconformity, New York
60. Gilboa Forest, New York
61. Pilot Mountain, North Carolina
62. South Killdeer Mountain, North Dakota
63. Hueston Woods, Ohio
64. Big Rock, Ohio
65. Kelleys Island, Ohio
66. Interstate 35 Roadcut, Oklahoma
67. Mount Mazama, Oregon
68. Lava River Cave, Oregon
69. Drake’s Folly, Pennsylvania
70. Hickory Run, Pennsylvania
71. Delaware Water Gap, Pennsylvania
72. Beavertail Point, Rhode Island
73. Crowburg Basin, South Carolina
74. Mount Rushmore, South Dakota
75. Mammoth Site, South Dakota
76. Pinnacles Overlook, South Dakota
77. Reelfoot Scarp, Tennessee
78. Enchanted Rock, Texas
79. Capitan Reef, Texas
80. Paluxy River Tracks, Texas
81. Upheaval Dome, Utah
82. Checkerboard Mesa, Utah
83. San Juan Goosenecks, Utah
84. Salina Canyon Unconformity, Utah
85. Bingham Stock, Utah
86. Whipstock Hill, Vermont
87. Great Falls, Virginia
88. Natural Bridge, Virginia
89. Millbrig Ashfall, Virginia
90. Catoctin Greenstone, Virginia
91. Mount St. Helens, Washington
92. Dry Falls, Washington
93. Seneca Rocks, West Virginia
94. Roche-A-Cri Mound, Wisconsin
95. Van Hise Rock, Wisconsin
96. Amnicon Falls, Wisconsin
97. Green River, Wyoming
98. Devils Tower, Wyoming
99. Fossil Butte, Wyoming
100. Steamboat Geyser, Wyoming
101. Specimen Ridge, Wyoming
Site I would add to the list:
102.  Sudbury Impact Breccia, Minnesota

Minnesota Geology Monday - Sudbury Impact Breccia

Approximately 1,850 million years ago, an asteroid between 6-12 miles in diameter struck Earth at what is now Sudbury, Ontario.  This impact is the second-largest known impact, as well as one of the oldest.  The effects of this impact is left behind in the rock record that have recently been discovered near Thunder Bay, the Gunflint Iron Range, Michigan's Upper Penisula, among others.

There are several locations near Gunflint Lake, along the Gunflint Trail (County Road 12), that bear witness to the impacting event even though the impact was nearly 450 miles to the east.  One location is found approximately one mile down National Forest Road 1347, off of County Road 12.

Approximately a quarter mile after leaving the Gunflint Trail (County Road 12), you drive past a gravel pit in late-Wisconsin glacial deposits of the Rainy lobe.

After a little over a mile drive on National Forest Road 1347, a hiking trail intersects the road.  One exposure of the breccia is several hundred feet to the north of this intersection.

After following the trail north of the road for a few hundred feet, you must leave the trail and hike through the brush for another 100-200 feet to reach the exposure.  The breccia is found on the far side of the middle ridge in the picture below.

At the time of the impact, this area of Minnesota was located along the shore of an ancient sea.  The impact breccia lies directly upon the Gunflint Iron Formation (1,878 million years old) that was being deposited prior to impact.  The breccia at this location, has been interpreted as a submarine debris flow caused by earthquakes resulting from the impact.  The breccia layer is almost 7 meters thick, with larger pieces of iron formation (some almost 3 meters long) found near the bottom and smaller pieces found near the top.

After depostion of the impact breccia, the ancient sea again began to deposit fine-grained material, though no more iron.  Mudstones and shales of the Rove Formation (1,850 million years old) were deposited directly above the impact breccia.  These mudstones and shales were later metamorphosed into slate by the 1,115 million year old diabase sills of the Logan Intrusion found above the Rove formation.

Beyond the intersection of Cut Across Trail and National Forest Road 1347 approximately a quarter mile is another interesting exposure of impact breccia that shows ejecta.  As part of the impact, most material would have fallen back into the newly formed crater, though some of it was thrown nearly 450 miles to Minnesota.  The ejecta is found at the uppermost portion of the impact breccia and includes accretionary lapilli.  These lapilli are comprised of multiple layers of ash and melted fragments that were removed from the impact site.

The Sudbury Impact Breccia located in Minnesota demonstrates a catastrophic moment in Earth's history.  A much more detailed read on the exposures, prepared by Mark Jirsa of the Minnesota Geological Survey, found along the Gunflint Trail can be found here,

Minnesota Geology Monday - Gooseberry Falls

Many of the rivers and streams along highway MN 61 on Minnesota's North Shore have high gradients as they flow to the lowest location in Minnesota, Lake Superior.  Because of the high gradient, most of the rivers and streams have waterfalls and the Gooseberry River is no different.  Gooseberry Falls State Park (http://www.dnr.state.mn.us/state_parks/gooseberry_falls/narrative.html) is the second-most visited state park, only Fort Snelling is visited more often.

Bedrock of the area is comprised of 1,100 million year old North Shore Volcanic Group of the Midcontinent Rift System.  The vast majority of rock types found within the Midcontinent Rift System along the North Shore basalts, though there are smaller amounts of rhyolites and intrusive gabbros and granites.  There are several hundred lava flows exposed along the North Shore as part of the Midcontinent Rift System.

The Gooseberry River flows over several distinct basaltic lava flows forming waterfalls at more weathering-resistant areas.  The three main waterfalls are called the Upper, Middle, and Lower Falls, with the Lower Falls being located closer to Lake Superior.  All three of the waterfalls are easily reached via paved, handicap-accessible trails from the Joseph N. Alexander visitor center that was built in 1996.

The Upper Falls, pictured above, are located just north of the MN 61 bridge.  The trail takes you underneath the highway.

Walking towards the lake leads to a footbridge over the Gooseberry River that allows views of the Lower and Middle Falls further upstream.

The Middle Falls are located closest to the visitor center and are more easily visited.  These falls are the most impressive as well, especially during periods of high flow on the river.

When the Gooseberry River has low water levels, it is quite easy to walk out onto large basalt boulders that have come downstream from the waterfalls.

Minnesota Geology Monday - Minnemishinona Falls

Access to Minnemishinona Falls has been given to the general public in recent years.  In the mid-2000s, Nicollet County obtained the site from private ownership.  The larger falls are a 42-foot waterfall, relatively recently a much smaller waterfall has developed slightly upstream from the main waterfall.