A field assignment on the Mississippi River Gorge which Identifies three different aspects that represent change through time and five different types of rock, all throughout the area.

1. Twin Cities
Mississippi River
Gorge
Professor Mark Lawler
Geology 113
Field Assignment
Jennifer Benker

2. 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
2

3. 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

4. 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)

5. 5
Minnehaha
Regional
Park
Home of
Minnehaha Falls
36th Street
Restoration
Site
of
Mesic Oak
Savannah
Marshall Avenue
Bridge

6. 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)

7. (States, 2014)
7
➢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)

8. 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.
8

9. ➢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)

10. Bald Eagle (Haliaeetus Leucocephalus)
(Pingstone, 2004; States, 2014)
10
➢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).

11. ➢ 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.
11
CLASSIFICATION
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Falconiformes
Family: Acciptiridae
Genus: Haliaeetus
Species: Haliaeetus
leucocephalus
(Rochelle, 2006; American)

12. St. Peter Sandstone (Sedimentary)
12
➢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.
Minerals
Consists 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).

13. 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). 13

14. Decorah Shale (Sedimentary)
14
➢ COOL FACTOR
Contains 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).

15. Gniess (Metamorphic)
15
Texture
Foliated - Layered in sheets
with light and dark bands.
Minerals
Quartz, 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 FACTOR
Gneiss rocks are some of the oldest rocks
from Earth.

16. Granite (Igneous)
16
➢ 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).
Texture
Phaneritic - Grains large enough to
be seen.
Minerals
Quartz composes more than 10%
of the rock
Greater ratio of Potassium feldspar
to plagioclase
(Thomas).

17. 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
17

18. References
18
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

19. References
19
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/webcont
ent/convert_281839.pdf