Twin CitiesMississippi River

GorgeProfessor Mark Lawler

Geology 113Field Assignment

Jennifer Benker

My field observations were taken from the Mississippi River Gorge in

Minnesota.After an overview of the area, the

following are included:

3 Features that demonstrate change through time:

➢Minnehaha Falls➢Mesic Oak Savanna

➢Bald Eagle

5 Rock Samples:

➢ St. Peter Sandstone➢Platteville Limestone➢Decorah Shale➢Gneiss➢Granite

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The Mississippi River Gorge

➣ The Mississippi river spans over 2,350 miles, with its only true gorge in Minnesota; the gorge begins in downtown Minneapolis and ends at Fort Snelling.

➣ It was created by the recession of the St. Anthony falls, which formed around 12,000 years ago in downtown St. Paul.

➣ The force of the water from the falls eroded the layer of Saint Peter Sandstone, undercutting the shale and limestone above.

➣ From this erosion and carving action the falls receded upstream, on average 4 feet per year, to their current location in Minneapolis and created the Mississippi River Gorge.

(Friends, 2013)3South view from the Marshall Avenue Bridge

Geologic History

4North view from the Marshall Avenue Bridge

➢During the Ordovician Period (500-430 million years ago), sediment was deposited from an ancient inland sea, creating the rock bluffs visible along the gorge today.

➢Lake Agassiz was a giant glacial lake situated over North Dakota, Minnesota, and Canada. About 11,700 years ago, as the River Warren began to drain the lake and converge with other rivers near the Twin Cities, it caused massive erosion and initiated the Mississippi River Gorge

➢The Gorge began from a 60ft waterfall near downtown St. Paul, River Warren falls. As these falls retreated past Fort Snelling the same erosion created the St. Anthony Falls. Then River Warren Falls flattened due to a change in bedrock geography.

➢About 10,000 years ago the Mississippi joined the River Warren and the St. Anthony falls continued to retreat up the Mississippi until it was stabilized in the 1870’s to utilize its power.

(Brewer, 1998)

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Minnehaha Regional

ParkHome of

Minnehaha Falls

36th StreetRestoration

Siteof

Mesic Oak Savannah

Marshall Avenue Bridge

Minnehaha Falls➣Minnehaha Falls is a geologically significant feature of Minnehaha creek and the Mississippi river.

➣The falls are located about a mile upstream at the confluence of the Mississippi river and Minnehaha creek.

➣The falls have changed dramatically over the course of time with help from the Mississippi River and St. Anthony falls. Originally non-existent, Minnehaha falls appeared about 5,000 years ago when St. Anthony falls gradually eroded its way upstream.

6(Plaque)

(States, 2014)

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➢Minnehaha falls is an excellent mini version showcasing the same process that shaped the Mississippi River Gorge.

➢Minnehaha Creek and the Gorge share the same bedrock. As the St. Anthony falls moved upstream and passed over Minnehaha Creek, the Minnehaha falls formed.

➢As the water from the falls tumbles forcefully over the harder Platteville Limestone, it begins to erode the softer St. Peter Sandstone below. With no bottom layer to uphold the Platteville Limestone, it breaks away. This reveals a new section of bedrock and the cycle would begin again.

(States, 2014)

Mesic Oak Savanna

➢Originally found in the transitional zone between the Mississippi River’s edge and the oak savanna on the bluff tops.

➢Once part of a 2-million-acre landscape before European settlement. Now less than .01% of oak savannas remain.

➢Is currently 1 of 3 native plant communities found along the Mississippi River Gorge.

(Shaw, 2002)

This pictured mesic savanna, is under current restoration and is one of the rarest plant communities in the region.

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➢Mesic Oak Savanna is a prairie between wet and dry, with medium moisture in its soil and is scattered with bur, red, or white oaks. Also known by early settlers as “Oak Openings” (Prairies).

➢Fires, animal activity, and/or soil moisture conditions have prevented a closed canopy forest from developing.

➢The oak savanna became a transitional community from the fire swept prairies and burned forested areas.

➢Without fire or other disturbances, oak savannas can and have evolved into denser oak woodland-brushlands.

9(Shaw, 2004)

Bald Eagle (Haliaeetus Leucocephalus)

(Pingstone, 2004; States, 2014)

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➢Minnesota has the 3rd largest eagle breeding population after Alaska and Florida. 872 active nests were located in 2005.

➢Bald eagles who survive until adulthood will live between 20-25 years.

➢They can distinguish more colors than a human, and can see in the UV light range allowing them to see the urine trail of their prey.

➢Have been known to migrate and nest in and around Mississippi River Gorge area(Eagle, 2014; Zumeta).

➢ During the Mesozoic Era, around 144 to 66 million years ago, the first birds began to evolve from reptiles.

➢ The order of Falconiformes, appeared during the Eocene epoch, around 50 million years ago.

➢ The Eocene epoch was also a time when the Kites evolved, creating the family Acciptiridae.

➢ Incorporated into that family is the genus of Haliaeetus, from which the bald eagle belongs to.

➢ The exact age of the bald eagle species (Haliaeetus leucocephalus) had not been determined, although there have been fossils found which date back to over 1 million years ago.

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CLASSIFICATION

Kingdom: AnimaliaPhylum: ChordataClass: AvesOrder: FalconiformesFamily: AcciptiridaeGenus: HaliaeetusSpecies: Haliaeetusleucocephalus

(Rochelle, 2006; American)

St. Peter Sandstone (Sedimentary)

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➢St. Peter Sandstone was formed during the Paleozoic Era in the Middle Ordovician Period, 450 Million years ago. Sediment was deposited in shallow seas covering most of the United States.

Texture Very fine grained and well sorted sandstone.

MineralsConsists almost entirely of mineral quartz. It is mined in many parts of the Midwest as raw material for glass manufacturing, silica sand for abrasive uses, and for foundries to make sand castings (Geology, 2014).

Platteville Limestone (Sedimentary)

➢ Platteville limestone began as limestone and transformed to dolomite after deposition.

➢ Ordovician sea organisms produced shells of calcium and carbonate. The chemical reaction of the sea water and accumulated minerals creates limestone. As time passes the limestone can be subjected to seepage and infiltrated by seawater; some of the calcium in the limestone is replaced with magnesium which produces the mineral dolomite (Geology, 2014).

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Decorah Shale (Sedimentary)

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➢ COOL FACTORContains the most diverse and abundant fauna

of the Ordovician System in Minnesota (Mossler, 2006).

➢Decorah Shale formed from much of the same process’ as the previous limestone, from shallow sea deposition over North America during the Ordovician Period.

➢ Soft clay sediments once buried and compacted become shale.

➢Decorah Shale sits above Platteville Limestone, which sits above St. Peter Sandstone(Thomas; Plante, 2001).

Gniess (Metamorphic)

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TextureFoliated - Layered in sheets with light and dark bands.

MineralsQuartz, feldspars, dark micas, hornblende, pyroxene, garnets, and others (Thomas).

➢Gneiss is formed under great pressure from the moving plates of Earth’s crust. Can be formed from metamorphic rock called schist, and also some igneous rock, especially granite (Plante, 2001).

➢ COOL FACTORGneiss rocks are some of the oldest rocks

from Earth.

Granite (Igneous)

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➢ Granite forms deep in the Earth’s crust from cooling magma which contains silica. The slow cooling process is what creates the large crystals in granite (Plante, 2001).

TexturePhaneritic - Grains large enough to be seen.MineralsQuartz composes more than 10% of the rockGreater ratio of Potassium feldspar to plagioclase (Thomas).

References

American Eagle Bald Eagles' Evolutionary Ancestors. (n.d.). PBS. Retrieved June 10, 2014, from http://www.pbs.org/wnet/nature/episodes/american-eagle/bald-eagles-evolutionary-ancestors/4274/

Brewer, A. (1998, January 1). Center for Urban and Regional Affairs. Ecology and Geology of the Mississippi River Gorge.. Retrieved June 8, 2014, from http://www.cura.umn.edu/publications/catalog/npcr-1097

Eagle Biology. (2014). National Eagle Center. Retrieved June 7, 2014, from http://www.nationaleaglecenter.org/eagle-info/biology/

Friends of the Mississippi River. (2013). FMR. Retrieved June 1, 2014, from http://www.fmr.org/participate/ongoing/gorge_stewards/history

Geology Page: Cenozoic Era. (2014). Retrieved June 11, 2014, from http://janeaddamstrail.com/trail-history/geology

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References

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Mossler, J., & S. Benson. (2006) Minnesota at a glance: fossil collecting in the Twin Cities area. Minnesota Geological Survey, St. Paul, Minnesota. Retrieved June 10, 2014, from http://equatorialminnesota.blogspot.com/2013/12/cornulites.html

Pingstone, A. (2004, July 1). Haliaeetus leucocephalus. - Wikimedia Commons. Retrieved June 1, 2014, from http://commons.wikimedia.org/wiki/Bald_Eagle#mediaviewer/File:Bald.eagle.closeup.arp-sh.750pix.jpg

Plante, A., & Peck, D. (2001). Rock Key. Rock Key. Retrieved June 11, 2014, from http://www.rockhounds.com/rockshop/rockkey/#granite

Plaque Inscription. (n.d) 1-Main entrance minnehaha lower glen. Minneapolis, MN, Minnehaha Regional Park.

Prairies & Oak Savannas. (2014). The Southeast Fox River Partnership. Retrieved June 6, 2014, from http://fyi.uwex.edu/southeastfox/files/2013/01/prairies.pdf

References

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Shaw, D. B., & Carr, C. E. (2002). Mississippi River Gorge Ecological Inventory and Restoration Management Plan. Great River Greening. Retrieved June 9, 2014, from http://www.greatrivergreening.org/wp-content/uploads/2013/07/Gorge_Management_Plan.pdf

States. National Park Service. (2014, May 24). Twin Cities Geology. National Parks Service. Retrieved June 02, 2014, from http://www.nps.gov/miss/naturescience/twingeol.htm

Shaw, D. B., & Carr, C. E. (2002). Mississippi River Gorge Ecological Inventory and Restoration Management Plan. Great River Greening. Retrieved June 9, 2014, from http://www.greatrivergreening.org/wp-content/uploads/2013/07/Gorge_Management_Plan.pdf

Thomas, J. J. (n.d.). Fairly Simple Exercises. Fairly Simple Exercises. Retrieved June 12, 2014, from http://www.skidmore.edu/~jthomas/fairlysimpleexercises/rockid.html

Zumeta, D. (n.d.). Birds of the Mississippi River Gorge Area. . Retrieved June 20, 2014, from http://www.minneapolismn.gov/www/groups/public/@regservices/documents/webcontent/convert_281839.pdf