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University of Minnesota DEPARTMENT OF EARTH SCIENCES

Historic Campus Geology Exploration

(Photograph courtesy of Minnesota Historical Society)

Name:

Lab Instructor:

Historic Campus Geology Exploration –

Background Information for the Historic Campus Geology Exploration. Although the self-guided exploration of the geology of the historic knoll and mall campus areas was originally

intended to follow Labs 5 and 6 of the ESCI 1001 program, the only prior knowledge you really need to complete

the exploration is a basic understanding of the three major rock groups.

As igneous rocks form from cooling magma, they are typically composed of crystals. If the magma cooled beneath

the Earth’s surface, its crystals are often large enough to be seen without magnification, forming a distinctive pattern

of different colored minerals. On the other hand, if magma is ejected at the Earth’s surface as part of a volcanic

eruption, any crystals formed will be much too small to see, so the resulting rocks appear to be uniform masses.

Sedimentary rocks form from either the deposition of sediment on the Earth’s surface or from the precipitation of

crystals in its surface waters. So sedimentary rocks include detrital sandstones and mudstones, as well as crystalline

limestone and evaporate.

Metamorphic rocks form from the alteration of other rocks as they are compressed, squeezed and heated.

Consequently most metamorphic rocks display a distinct textural alignment that ranges from the flat surfaces of slate

to the mineral bands of gneiss. Metamorphic rocks that consist of non-banded crystal masses, like marble and

quartzite, typically have a uniform composition distinct from the heterogeneity of most crystalline igneous rocks.

Armed with that information, you should be able to complete the exploration on your own. Hope that you enjoy

exploring the geology of our campus.

Copyright © 2013 - all rights reserved KCK


Historic Campus Geology Exploration

Logistics

This is a self-guided exploration of the geology, both natural and anthropogenic, of the knoll and mall campus areas.

As the exploration may take up to two hours, be sure to dress appropriately for the weather. While completing the

exploration do not try to identify rocks by any method other than sight. In particular, do NOT scratch any of the

building stones to test their hardness. Other people, including university police, may not appreciate the often subtle

nuances that distinguish geological curiosity from wanton vandalism. Also note that a few of the interior locations

(like Burton Hall and the Education Sciences Building) are quiet areas with few outside visitors. Please try not to

disturb building residents as you complete the exploration.


Introduction

While on the tour, think about the origin of the rocks you encounter and the geologic history behind each outcrop or

building stone. Countless people pass these rocks every year without realizing how much they reflect of our Earth’s

past. As a group, the rocks on campus attest to the collision of tectonic plates that formed North America, the slow

creation and destruction of now-vanished Minnesota mountain ranges, and times when ancient volcanoes, vast ice

sheets or tropical seas dominated our state’s landscape. Some achieved their present form within the past 20,000

years while mammoths and giant sloths roamed the campus area, others formed at a time when microbes were the

only life on earth. The tour even includes some of the oldest Earth material that you are likely to ever encounter.

To keep things simple, the tour will begin close to the Ford Hall lab rooms.

Stop 1 – West Entrance of Amundson Hall (across from the east entrance of Ford Hall)

Next to Amundson Hall’s Church Street entrance are five short pedestals of igneous rock (Figure 1). Despite their

varying hues, all five of these pedestals are varieties of granite, which illustrates why it pays to be cautious when

using color to identify minerals and rocks. Even the darkest rock, which looks like gabbro, contains clear quartz

crystals characteristic of granite. Try to find a few of these quartz crystals (doing so is easier in the reddish pedestals

than the dark ones).

Question 1 - What characteristic of their textures1 tells you that these rocks formed from magma that cooled

miles below the Earth’s surface, rather than from lava erupted in a volcanic eruption at the Earth’s surface?

From Amundson Hall, head west towards the Washington Avenue Bridge. Keep on the bridge’s down current

(south) side and stop a third of the way across near the first door into the bridge’s covered central corridor.

Stop 2 – East end of Washington Avenue Bridge (by Weisman Art Museum)

Figure 2. Panorama of river valley from south side of Washington Avenue Bridge

From the Earth’s perspective, the valley below you is relatively young. Less than twenty thousand years ago this

whole area was covered by an immense ice sheet. After the ice melted, rivers meandered back and forth across the

newly exposed landscape and began to carve down through glacial sediment to flow across the underlying bedrock.

As one of these rivers worked its way south of what is now downtown St. Paul, it encountered an older river valley

filled by loose glacial sediments. The river quickly cut down through those sediments to create an immense

waterfall, which then retreated upstream to leave a deep river valley in its wake. Within the past thousand years, this

retreating waterfall passed through this area, carving the river valley below you (Figure 2).

1 ‘Texture’ here refers to the rock’s internal fabric - the size, shape and pattern of its components, not its surface. Whether a rock surface was left rough or polished only reflects the architect’s desires, not the rock’s origin.

Figure 1. Granite pillars in front of Amundson

Hall’s Church Street entrance with a close up

of one pillar’s texture in image at right.


Look at the bluffs beneath the Weisman Art Museum - one of the campus’ more distinctive structures (Figure 3). In

the past, ESCI 1001 lab groups used to walk along the path beneath the bluffs, but when Coffman Union was

renovated a number of years ago, shock waves from dynamite blasts used to expand its underground parking

structure traveled through the bedrock to destabilize blocks along the bluff. Oddly enough, our University

administration questioned the wisdom of putting students beneath tons of unstable, overhanging rocks so this tour

will be limited to gazing at the bluffs and river valley from a distance.

Layers of gray resistant rock, called the Platteville Formation, form the vertical

bluff walls while the sloping surface from the cliff base to the river is underlain by

more easily eroded St. Peter Sandstone (the boundary between the two units falls

halfway between the river and street levels and is most visible close to the bridge

where vegetation is limited). While the river valley is geologically young, the

rocks it cuts through are far older. The Platteville Formation and St. Peter

Sandstone are over 450 million years old and formed along the margin of an

inland sea at a time when North America straddled the equator and Minnesota had

a tropical climate. Intense tropical weathering broke rocks and sediment down to

leave remarkably pure quartz sands that formed beaches and dunes along the

seashore. These beach and dune sands became the St. Peter Sandstone while

offshore carbonate mud accumulated and compacted to eventually form the

overlying Platteville Formation.2 The top of the Platteville Formation (just below

the street level) is an erosion surface that represents all of the missing time

between the formation of the underlying Platteville Formation rocks (over 450

million years ago) and the overlying glacial deposits that left by a melting ice

sheet a mere 18,000 years ago.

Question 2 – How can you tell from the relative position of the two rock units whether this ancient sea was

becoming deeper or shallower at the time the Platteville Formation and St. Peter Sandstone formed? What

reasoning supports your answer?

The bluff cliffs owe their existence to a set of vertical fractures that extend through the resistant layers of Platteville

to the underlying less-resistant St. Peter Sandstone. These fractures break the Platteville Formation up into large

blocks. As the waterfall moved through this area, water flowed down fractures to erode the underlying sandstone

and undercut the Platteville blocks. Once undercut, the blocks broke along fracture surfaces to tumble into the

downstream channel. This process caused the waterfall to retreat and created a bluff-lined valley in its wake.

Note the asymmetry of the river valley. On the east side, the river hugs the bluff edge while there is a broad flat area

between the river and west bluffs. This low area formed from sediment deposited along the river bank and during

times when flood waters covered the valley floor. Downstream, this flat area disappears as the river curves and the

water flow swings over to hug the valley’s other wall.

Return to the East Bank campus and cross over to the north side of the bridge by the Science Teaching and Student

Services Building to look across the river valley upstream of the bridge.

2 Locally the Platteville Formation is used as building stone or aggregate for road construction. The pure quartz sand of the St. Peter Sandstone is

quarried for glass manufacturing and as frac sand used to prop fractures open in oil and gas reservoirs. Similar sands have been in the news lately as Minnesota struggles with the environmental and economic issues associated with increased demand for frac sand quarries.

Figure 3. Bluff below Weisman Art Museum


Stop 3 – Overlook of West Bank river flat (north of the east end of the Washington Avenue Bridge by the Science

Teaching and Student Services building)

As you gaze across the river at downtown Minneapolis, realize that only 18,000 years ago, ice covered this area up

to 200 meters thick – a thickness nearly the height of the tallest building you can see from the bridge. This was only

the southern margin of an immense ice sheet that covered Canada with up to 3 kilometers of ice (30 times the height

of the tallest building in downtown Minneapolis), enough ice to depress the Earth’s lithosphere. When this ice sheet

began to melt, tremendous amounts of melt water flowed across the newly exposed land to carve a new river pattern

through an irregular, slowly rising landscape. Part of this drainage pattern is the river in front of you, while the land

beneath you is still slowly rising (about a millimeter per year) as it rebounds from the weight of now-vanished ice.

Again note that the river currently only occupies part of its valley. Across the river is a continuation of the low-lying

flat area you saw south of the bridge. In the mid-1880’s the area beneath the Washington Avenue Bridge hosted a

relatively poor, frequently flooded, residential area known as Bohemian Flats. Eventually the residents were evicted

when construction of the downstream Lock and Dam #1 (close to Minnehaha Falls Park) raised the river level here.

Despite occasional flooding, part of the flats continued to be used for industrial purposes until 1983. In light of the

flats’ flood-prone history, you might take a moment to consider the location of the University’s library archive (the

archway in the bluff just north of the bridge). As the archive lies just above the floodplain and groundwater table,

and down flow from soils that hold toxic waste from a long gone coal gasification plant3, the archive’s location was

a matter of some controversy.

Question 3 – Using the map at left, what relationship

exists between the location of the river flat areas and

the shape of the river channel? What does this

suggest about the relative speed of the water flows

along the inner and outer sides of a river bend?

Go back to Pleasant Street and turn left (north) to reach Fraser Hall. Enter Fraser Hall, walk towards the back hall

and turn left to take central staircase up a half flight of steps to the first level.

3 This coal gasification plant once generated the gas used to light street lamps and homes in downtown Minneapolis, before gas lamps were replaced by electric lights that were powered by a hydroelectric plant at Saint Anthony Falls.

Stop 2

Stop 3


Stop 4 – Fraser Hall (central interior stairway)

The central staircase at the back of Fraser Hall is primarily composed of a light-colored limestone with a distinctive

pattern of small pores aligned in parallel rows (Figure 4). Geologists call this pattern ‘birdseye’ or ‘fenestral’

fabric4, which is formed by gas bubbles trapped in microbe-bound carbonate mud. Rocks like this typically form in

the carbonate mud of intertidal and supratidal settings. Thin layers of algae grow across carbonate mud binding it

together. As new algae and mud cover older layers, the older algae begin to die and decay. As part of this decay, gas

bubbles form that prop up the overlying layers. Decaying organic matter also changes the pH (acidity) of the mud’s

water causing it to precipitate crystals that solidify the mud and preserve the pores in the newly formed rock texture.

Although this distinctive porosity pattern increases the rock’s worth as decorative stone it also has some negative

practical considerations, such as trapping dirt.

Question 4 – Compare the stairway sections between the entry and first levels with the one between the

first and M levels. What role might the rock’s porosity play in the choice of adding red tiles to stair edges

in the more heavily traveled lower staircase?

Exit Fraser Hall and cross Pleasant Street to enter Walter Library.

Stop 5 – Walter Library (Pleasant Street entrance and main lobby)

4 These names arose as people thought the pores looked like birds’ eyes or windows (fenestrae) into the rock.

Figure 4. Fraser Hall staircase and close up of fenestral

fabric in stone of steps.

Figure 5. Mottled burrow fabric seen in

stone covering wall of Walter Library’s

lobby entrance.


As you enter Walter Library from Pleasant Street, take a moment to examine the stone covering the lobby walls

(Figure 5). Commercially sold as ‘Winona Travertine’, geologists call this stone the Oneota Dolomite. Similar in

texture and composition to the Platteville Formation of the river bluffs, these Oneota Dolomite rocks are slightly

older and come from quarries at Winona, Minnesota. Pores in the rock formed as calcite fossils dissolved out of the

more chemically-stable dolomite matrix. The rock’s distinctive mottled pattern formed as soft-bodied organisms

burrowed through shallow marine mud on the bottom of a tropical sea that once covered much of Minnesota. Some

of these organisms swallowed the mud to digest its associated organic material and then excreted it, leaving trails of

fossil excrement, which oddly enough were used much later by tool-using bipedal primates to decorate the lobbies

of their public buildings.

Question 5a – As you go through Walter Library look for and identify at least one other area where Oneota

Dolomite was used as decorative building stone.

Walk through Walter Library to the large open lobby close to the Northrop Mall entrance.

Like many campus rocks, the lobby’s marble-covered floor and staircases share a common theme of having

originated as marine rock. Although all the lobby marbles originated as fine-grained carbonate mud deposited in

shallow marine areas, they underwent different degrees of metamorphic alteration after their deposition and initial

lithification. Some experienced relatively little alteration and should really be considered as limestone rather than

true marble. In those rocks, you can still see small irregular white grains that are fossils of marine organisms. Most

of these fossils are fragments of crinoids, which were particularly abundant during Ordovician to Permian time (254

to 485 million years ago). Other marbles were buried more deeply and underwent extensive metamorphism that

largely obliterated their original depositional fabric, including fossils, recrystallizing it into a more uniform

crystalline texture. During burial, dark irregular lines (called stylolites) also formed. Calcite, the mineral that makes

up most of the rock, is more easily dissolved as pressure increases during burial, leaving behind seams of insoluble

residue.

Most of the lobby floor is covered by light-colored marble, but red marble is used as borders and decorative patterns

between large open areas. Its reddish hue is due to slight amounts of iron impurities.

Question 5b – Based on their fabrics, which of these two rocks (the light-colored marble or the red marble)

underwent the least metamorphism? Briefly explain which features support your answer.

While the rocks of the lobby walls, staircases and floor originally formed from carbonate mud

accumulating on the bottoms of inland seas and ocean seafloors, the oceanic seafloor itself was the

source material for the lobby’s dark green columns. Serpentinite, named for the mineral serpentine, is

a dark green metamorphic rock that forms from the alteration of mantle rock and oceanic crust (mostly

peridotite, along with some gabbro and basalt) at divergent and convergent plate tectonic boundaries.

Although serpentinite underlies most of the Earth’s ocean areas, it is usually only exposed at the

Earth’s surface in subduction zones, where slivers of oceanic rock become caught and uplifted in the

collision of converging mountain belts. Consequently, while the marble of the lobby walls and floor

tell a tale of past oceans, these green pillars reflect the tectonic creation and destruction of past oceanic

crust, the ongoing generation and recycling of the Earth’s surface (Figure 6).

Compare the texture of the serpentinite to that of the more highly metamorphosed marbles. At Folwell

Hall you will be asked to distinguish serpentinite from a green variety of marble, so focus on

differences in the rocks’ fabrics here rather than their color.

Exit through the lobby doors out to the Northrop mall area.

Figure 6. Serpentine pillar in

Walter Library’s lobby entrance.


Stop 6 – Northrop Mall (outside Walter Library)

From the 1930’s on, it became a running joke to ask newcomers to the University of Minnesota to meet on the mall,

at the brick building with columns.5 While this tour highlights the natural building stones used on campus, a glance

around the mall should drive home the realization that the most common building stones on campus are of human

origin.

Bricks are essentially manufactured rocks. Clay is mined from the Earth, compressed in a mold and baked in a kiln.

Individual clay particles recrystallize into an interlocking mosaic of crystals that greatly increases the brick’s

strength. This alteration takes place in a solid state; the clay never actually melts, it only recrystallizes. Most of the

campus bricks, including those on Northrop Mall, are red - even though their original clay was gray to greenish gray

in color. During heating, ferrous iron in the clay (which gives the clay a green color) is oxidized to ferric iron (which

gives it a red color). Until a few decades ago, brick factories lined the base of those bluffs across from downtown St.

Paul, mining clay from a rock unit called the Decorah Shale to produce most of the brick for older St. Paul and

Minneapolis buildings.

Concrete is another manufactured rock. It is produced by mixing sand and gravel with water and carbonate cement

(crushed, dried limestone). Any water exposed to the atmosphere contains dissolved carbon dioxide and is

essentially a weak carbonic acid. Mixing powdered limestone and weakly acidic water generates an exothermic

(heat-producing) reaction that bonds the cement to the sand and gravel grains to cement them together into a more

cohesive solid.

Even the glass of the mall area windows and doors is manufactured rock. Quartz sand, like that of the St. Peter

Sandstone beneath the campus river bluffs, is melted down and quickly cooled to form clear transparent sheets of

rock. Unlike brick or concrete, glass does not contain any minerals as it cooled too quickly for crystals to form. If

melted quartz did cool slowly, it could form a crystal fabric similar to many of the campus building stones.

However, those crystals would destroy its clarity and render it useless as window panes.

Question 6 – Each of these artificial rocks (brick, concrete, and glass) is analogous to which type of

natural rock (igneous, sedimentary, or metamorphic)? Briefly explain the reasoning behind your answers.

Brick is an artificial form of ____________ rock, because:

Concrete is an artificial form of ____________ rock, because:

Glass is an artificial form of ____________ rock, because:

Walk towards Northrop Auditorium to look at the low stone wall along the plaza’s south side facing the mall and

Coffman Union.

Stop 7 – Northrop Plaza wall

Benches and sidewalk borders throughout the mall area are made of a gray ‘salt and pepper’ colored rock quarried

near Rockville, Minnesota. Distinctive for its large feldspar crystals, this rock is probably best displayed in the

plaza’s low southern wall.

Question 7 - What type of rock (igneous, sedimentary or metamorphic) do you think this rock is? What features

of the rock support your choice?

5 Northrop Mall’s uniform design was created by Cass Gilbert, who also designed Minnesota’ State Capital building in Saint Paul.


In addition to the gray ‘salt and pepper’ rock, a red rock

forms diamond patterns at sidewalk intersections

throughout the mall area (Figure 7). This rock also came

from the St. Cloud area. Thousands of students walk over

these rocks every day without realizing the story their

presence on campus represents. Both rocks formed in the

root of a now-vanished mountain range that billions of

years ago stretched across Minnesota as micro-continents collided to form the core of what would become North

America. As smaller plate fragments collided together, compression pushed part of the crust down to form mountain

roots. The plaza rocks formed miles below the Earth’s surface as part of this process and then slowly rose towards

the Earth’s surface as the overlying mountain range eroded away. Now exposed in the St. Cloud area and used as

campus building stones, these rocks reflect a billion years of our state’s past, as well as the birth of our continent.

From here, walk over to Church Street and then turn right on Pillsbury Drive to reach the Civil Engineering

Building. Go down the stairs through the outdoor amphitheater (Figure 8) to enter the building on the first floor,

then walk west (right) to reach the large sculpture composed of Morton Gneiss in the lobby area one floor below.

Stop 8 –Civil Engineering building (Morton Gneiss sculpture on second floor)

Take the staircase down to the second floor to the column-like sculpture composed of Morton Gneiss. Although

some older rocks have been found within the past few decades, the dark components of the Morton Gneiss are still

the oldest Earth materials that you are likely to encounter. Granted, the meteorites you see in museums are even

older, but those meteorites did not originate on Earth.

Question 8a – How old are these components? (See the placard at the base of the sculpture for this answer.)

Besides its age, the Morton Gneiss is significant as it reflects multiple stages of tectonic activity even before North

America formed. An intimate mixture of red continental and dark oceanic components, the rock’s distinctive pattern

originated miles below the Earth’s surface as slivers of micro-continents and seafloor collided and welded together.

Question 8b – From the rock’s texture, how can you tell that the Morton Rock originated from mixing and

metamorphism of pre-existing rocks, rather than from cooling of magma below the Earth’s surface? In other

words, how does the gneiss’ crystal pattern differ from that of the granites you saw earlier on the tour? (You can refer back to Figure 1 for a typical granite pattern.)

Return to the street level and follow Pillsbury Drive west to Pillsbury Hall.

Figure 8. Entrance to Civil Engineering building and column of Morton Gneiss in lobby

Figure 7. One of the diamond patterns present in mall sidewalks.


Stop 9 – Pillsbury Hall (exterior)

Pillsbury Hall is composed of Hinckley Sandstone and

Fond du Lac Formation rock, which most likely came

from small quarries between Duluth and the Twin Cities.

Hinckley Sandstone is the light colored rock while iron

staining gives the Fond du Lac Formation a reddish hue.

Some of the Fond du Lac blocks display irregular buff-

colored areas where iron impurities were reduced rather

than oxidized. Both the Hinckley and Fond du Lac stones

formed from current-deposited sands and often display

cross-bedding, sets of parallel thin lines that lie at an

angle to the blocks’ edges (Figure 9).

Beyond recording the movement of ancient water currents, these rocks reflect a period nearly 1.1 billion years ago

when continental rift valleys began to split North America apart to create new oceanic crust. A line of volcanoes

formed across Minnesota, running from Iowa to the Lake Superior region, spilling lava flows that piled atop one

another. Fortunately for North America’s future as a continent, rifting and volcanism stopped and rivers eventually

brought sediments to fill the rift valleys and bury the lava flows. Those sediments became the sandstones of the

Fond du Lac and Hinckley rock units. The older Fond du Lac sands contain significant amounts of both feldspar and

quartz grains, but the younger Hinckley sands are primarily quartz.

Question 9 – While the choice of stone for the ground level and first floor of Pillsbury Hall was strictly an

architectural decision, does it parallel or reverse the natural sequence of the two source rocks? In other

words, is the building’s oldest rock placed beneath the youngest or did the builders put younger rock

beneath older? (Obviously both are used to form a decorative pattern on the upper floors, so this question

just concerns the ground and first levels.)

If you have time, the first floor of Pillsbury Hall contains many displays of rocks, minerals, and fossils. In the

basement, by the west stairway, is a large block of red 'pipestone' from western Minnesota. Occurring as a thin layer

at Pipestone Monument, this is a slightly metamorphosed, easily carved, clay layer. Pipes carved from this rock

played a prominent role in many Indian cultures and calumets made from Minnesota pipestone were traded across

North America.

Like many older restrooms on campus, the stall walls in the Pillsbury restrooms are composed of a gray, thin

bedded, stylolite-rich limestone that displays many fossils and burrows. Crinoids are the most abundant fossils, but

brachiopods, rugose corals and bryozoans are also present. You are welcome to examine these features on your own,

but the restrooms are not a part of the formal campus tour.

Stop 10 – Pillsbury Hall Rock Garden (on the northwest corner of Pillsbury Hall)

On the northwest corner of Pillsbury Hall, a small outdoor rock garden holds a large cylindrical greenstone core

taken from a mine ventilation shaft at Ely, MN. This rock is nearly 2.7 billion years old. Greenstone, which is

composed of serpentine and chlorite, forms from the metamorphism of igneous basalt. Large ellipsoid outlines on

the surface of the greenstone column are characteristic of pillow lava, which suggest the original basalt formed from

underwater volcanic eruptions. Consequently, this rock shows the Ely area formed as an ancient sea floor was

trapped between two colliding continental fragments as the North American plate began to form.

Question 10 – Of the rocks that you have seen on the tour so far, which one would most likely form from the

continued metamorphism of greenstone (granite, carbonate, sandstone, marble, serpentinite, or the rocks

composing the wall of Northrop Plaza)?(You can refer to Figures 1, 4, 5, 6, and 9 to jog your memory).

Close to the Ely core is a buff-colored block of Oneota Dolomite (the real geologic name for Winona Travertine)

that contains two potholes. As turbulent water swirls in eddies over bedrock, mud and silt in the water can scour out

circular potholes. These pothole examples are relatively small, but huge potholes at Interstate Park near Taylor Falls

(meters in diameter) formed as a flood of glacial melt water plunged over a low basalt cliff.

Figure 9. Sandstone with cross-bedding near Pillsbury Hall entrance.


Before leaving Pillsbury Hall, look at the decorative pattern of building stones in its upper level. For decades a

heating plant south of Pillsbury Hall (where the Church Street Garage is now located) pumped out smoke that

discolored Pillsbury Hall. By the time Pillsbury Hall was finally cleaned in 1985, most people had forgotten that it

was composed of different rocks and the rediscovery of its beauty was a remarkably pleasant surprise.

Walk north from Pillsbury Hall past Williamson Hall to the pillar-bounded south entrance of Folwell Hall (Figure 10).

Stop 11 – Folwell Hall exterior (south entrance)

Stop 11 – Folwell Hall exterior (south entrance)

‘Xenolith’ is Greek for ‘foreign rock’ or ‘stranger rock’ and a number of xenoliths are encased in the granite walls

and pillars of Folwell Hall’s south entrance. Once you start looking, it is easy to find a number of small irregular

shapes distinct from the course matrix of the surrounding granite (Figure 10). These xenoliths formed as pieces of

surrounding rock became caught up in magma that later cooled to form the granite. So Folwell’s exterior entrance

not only hints at ancient plate collision and mountain building like other granites on the tour, but even gives you a

glimpse of the country rock present before those processes began.

Question 11 – Do most of the xenoliths you see here share a roughly similar appearance or do they appear

to have come from a wide variety of different rock types?

Enter Folwell Hall through the south entrance and walk up the marble-lined staircase.

Stop 12 –Folwell Hall interior (first floor hallway)

Folwell’s first floor hosts one of our campus’ more beautiful hallways. The central staircase and much of the floors

and walls are covered by white marble. Rectangles of gray, stylolite-rich, crinoidal limestone and white marble

cover the floor while the walls are composed of fractured white marble surrounded by panels of yellow marble

(Figure 11a).

Figure 10. South entrance of Folwell Hall and

xenoliths in stone covering entrance wall.

Figure 11. First floor (a) of Folwell Hall

with a close up of one of the archways’

narrow limestone panels that contains algal

encrusted fragments (b).

Xenolith

Granite

a. b.


This yellow marble apparently came from a rather heterogeneous quarry. Originally, it was all carbonate mud, but

the mud’s setting varied from restricted marine environments, characterized by layered algal stromatolites, to open

marine environments where the mud contained crinoids and other fossil material. Many of the wall panels show a

rather complex post-depositional history. Besides the original depositional textures, there are open fractures (now

filled with grout), brown areas that represent alteration of the limestone, and fractures filled by coarse calcite

cements.

Side panels in the archways by the east and west stairways show another, quite distinctive, variety of yellow marble

containing fragments of limestone ripped up by storms and coated by layers of algae and marine cements. Multiple

generations of green sediment partially filled open areas beneath these algal bound limestone fragments and a final

stage of calcite cement filled any remaining pore space (Figure 11b).

Both the floor and walls have rectangular borders of green stones, but one of these borders is composed of

serpentinite, the same metamorphosed ocean crust and mantle rock you saw in Walter Library’s pillars while the

other is simply a green variety of marble.

Question 12 – Based on the textures of the two rocks, which of these borders is composed of serpentinite

and which is composed of marble?

Leave Folwell Hall and walk past the circular traffic intersection of Pleasant Street and Pillsbury Drive SE to reach

Burton Hall.

Stop 13 – Burton Hall (interior first floor lobby)

The entrance stairway and first floor lobby of Burton Hall is another of the campus’ hidden architectural gems.

Compared to other stops you have visited, relatively few people visit Burton Hall, so please try not to disturb the

folks who work here.

.

A variety of rocks cover the lobby’s floor and lower walls, most of which are similar to other limestone and marble

seen on the tour. An exception is the dark, nearly black rock6 that forms a rectangular border between sections of red

limestone and light-colored marble (Figure 12). At first glance, this dark rock appears to be distinct from the other

tour rocks, but it actually is just a dark-colored variety of rocks you examined at previous stops.

6 This dark border is not the same material as the staircase steps, so do not confuse the two.

Figure 12. Burton Hall stairway and lobby, with a close up view of the dark border for Question 13


Question 13 – Based on this dark rock’s texture and internal components, is it most likely a variety of black

granite or a variety of black limestone? What features of the rock support your answer? (The best clues to

its origin can be seen in the border at the top of the left staircase shown in Figure 12.)

Leave Burton Hall and continue west on Pillsbury Drive SE to the pedestrian suspension bridge

spanning the railroad valley just beyond the intersection of Pillsbury Drive and East River Road

(Figure 13).

Suspension Bridge (near intersection of Pillsbury Drive and East River Road)

Although now widened and extended to make room for railroad tracks, the valley below the

bridge began as a natural ravine cut by a stream flowing into the Mississippi River. Hundreds of

students cross this bridge daily, few of which are aware that the valley below them played a

historic role in the area’s commercial development.

During the ninetieth century, the Metis came to dominate the plains of western Minnesota.

Descendants of mixed First Nations and European ancestry, Metis built a unique culture centered

about agriculture, fur trade, and bison hunting. Every year large trains of their Red River ox carts

wound their way across the state’s western prairies toward Saint Paul to trade furs. A number of

separate trails existed, but most of the Saint Paul trade eventually merged together into a

combined trail as they were constrained by the Mississippi River and this ravine. That combined

ox cart trail was the forerunner of University Avenue.

No question here – However, if you have never crossed the bridge, this

is an excellent opportunity to experience its distinctive bounce!

Walk south on East River Road curving around the children’s playground to reach the Educational Sciences

Building. Enter through the north door which usually has a coffee shop sign in front of it (and always has an ‘M’

carved above it).

Stop 14 – Educational Sciences Building interior (gallery overlooking the Mississippi River)

In the open lobby, just to the left of the elevator, take the metal staircase up to a window lined gallery that overlooks

the Mississippi River valley. Although the Educational Sciences Building is not as quiet as Burton Hall, it is still off

the beaten trail for most University students. So please try to be considerate of the building residents who see

relatively few outside visitors.

The view south gives a different perspective on the river valley and river flats seen earlier on the tour (Stops 2 & 3)

while the northern view includes downtown Minneapolis and a complex of locks, dams, and bridges beyond which

lies Saint Anthony Falls. Although you cannot see the falls from here because of intervening structures, almost

everything you see before you exists because of Saint Anthony Falls. Water cascading over the falls was the primary

reason for Minneapolis’ existence and without falling water to drive machinery, turbines, and industry, the view

below you would most likely consist solely of suburbs sprawling out from a much smaller version of Saint Paul.

Milling at the falls began with lumber but quickly moved to flour milling and associated industries. Eventually the

United States’ first central hydroelectric plant was built on an island next to Saint Anthony Falls in 1882.

Saint Anthony Falls, and consequently the city, arose as a reflection of the local geology. Fractures through the

resistant Platteville Formation allowed water to reach and erode the St. Peter Sandstone. As the sandstone eroded

away, blocks of the overlying carbonate rock broke free along fractures to create a vertical face, the waterfall itself.

Waterfalls never stay in one place though. Erosion continued to undercut more blocks, forcing the waterfall to

retreat. St. Anthony Falls originally formed close to Fort Snelling, but retreated eight miles north over the last

10,000 years. To prevent further retreat, the falls are now held at their present position by a sloping concrete apron

and underground wall beneath the river channel that prevent further erosion.

Figure 13. Suspension

bridge above railroad tracks


Note that the river valley, which extends from the top of one bluff to an equivalent height on the other side of the

river, is larger than the river channel itself.

Question 14 – Upstream, the river is constrained by the lock and dam downstream of Saint Anthony Falls,

but looking downstream, roughly how much of the total river valley is occupied by the river channel?

O – over 9/10 O – over 3/4 O – roughly 1/2 O – less than 1/4 O – Less than 1/10

Before you leave the gallery, look down to see the old Northern Pacific Bridge No. 9 which is now a pedestrian

bridge across the Mississippi River. Your campus exploration will end on the bridge after an intervening stop at the

outdoor patio below you.

Take the elevator or stairs down to the Ground Level and exit out the river side of the building to the garden patio

by the coffee shop.

Stop 15 – Educational Sciences Building exterior (rocks by outdoor Garden Patio)

Three large dark colored rocks by the garden patio (two of which have had mirrors attached to their side) came from

the Iron Range of northern Minnesota. They not only reflect the Educational Sciences Building’s earlier role as

home to the Mineral Resources Research Center, but a time when the chemical balance of the Earth’s oceans and

atmosphere was fundamentally different. Although photosynthetic organisms began to produce free oxygen very

early in Earth’s history, almost all of this oxygen quickly reacted with immense amounts of dissolved silica and iron

present in the Earth’s early oceans. Consequently, there was little to no free oxygen in the atmosphere or water and

no multicellular life. It took nearly 2.5 billion years before most of the dissolved silica and iron was oxidized and

oxygen began to accumulate in the ocean and atmosphere and multicellular life began. Prior to that time, marine

deposition was dominated by silica and iron oxides, whose scattered remnants are now valuable iron ore deposits.

The Iron Range of Minnesota not only played a crucial role in the economic life of northern Minnesota but in world

history as well. Much of the steel for the Allied war effort during World War II came from the Iron Range. If these

deposits had not existed, the war may have had a very different, potentially tragic, ending.

Question 15 – Concentrating on the large rock (and ignoring the two with mirrors), do the rock’s silica

and iron components appear to form a homogenous crystal pattern (similar to the igneous rocks you have

seen on the tour) or are they separated into layers? (The different textures would reflect whether water

chemistry was uniform throughout the year, or varied seasonally.)

Walk toward the pedestrian bridge, stopping for a moment to look

at the two light colored rocks just to the left of the bridge entrance

(Figure 14).

Stop 16 – Pedestrian Bridge (glacial erratics and overlook of river valley from bridge)

Although people undoubtedly shifted these two boulders to their present off-road position, their presence on campus is

due to natural processes. Large boulders like this are common across the Upper Midwest. Early First Nations people

recognized their often out-of-place nature and considered them to be of spiritual importance. Early Euro-American

scientists instead erroneously attributed their existence to past immense floods. Not until the middle of the 19th

century

did another, even more staggering idea arise. Boulders like this were not deposited by immense floods but by immense

ice sheets that had covered much of the northern hemisphere many times within the recent geologic past.

Ice is viscous enough that it does not sort sediment by size, but deposits large and small grains together. While ice

brought these large rocks here, it also brought the soil beneath them and most of the campus area is covered by a

veneer of glacial sediment.

Question 16 – Looking at the texture of the rock, do these appear to be igneous rocks (which would have

had to be transported from northern Minnesota or Canada) or local sedimentary rocks (sandstones or

carbonates)?

Figure 14. Erratics by

east end of pedestrian bridge.


Continue walking out on the bridge to the third street light on the downstream (south) side of the bridge.

To the north, in the distance below the other bridges, you should be able to see the arches of the Stone Arch Bridge,

which hides Saint Anthony Falls from view. South, you can see the bluffs, river flats and river valley beneath the

Washington Avenue Bridge. Although the campus building stones are beautiful and tell stories of ancient seas,

ancient climates and the slow collision and rifting of plates that gave rise to continents and mountain chains, as a

geology tour, it is only fitting to end with natural exposures.

Wrap Up Exercise: Looking south from the bridge, briefly summarize the geologic history behind what

you can see from this location. Your summary should include at least three significant steps in the evolution

of the present river valley and campus area.

When you have completed the tour, the simplest route back to the east bank campus is to backtrack through the

Educational Sciences Building to reach East River Road. You can also follow the road in front of the building up to

the right (south) to reach the campus area. However, taking the road to the left puts you on a bike path to 17th

Ave.

and 5th St. on the north side of campus, so the other options are more direct. To reach the west bank campus, you

can continue across the bridge and take side streets back to campus, but if you are unfamiliar with that area, it may

be simpler to return to the west bank campus via the Washington Avenue Bridge.

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