Download Boulder Walk brochure

Guide to the BoulderWalk
at Cincinnati Nature Center’s Rowe Woods
In just 15 minutes and 1,500 feet, the Boulder Walk takes you on a journey
back through earth’s history to a time when glaciers covered most of North
America. The Boulder Walk is located along the berm on the northeast side
of the Visitor Center Parking lot. All specimens described in this brochure are
found on the right side of the trail as you walk away from the Visitor Center.
The Illinoian glacier, one of the last
of the giant ice sheets to invade
southwestern Ohio (about 400,000
years ago), would have covered Rowe
Woods with a sheet of ice 2,000 feet
thick. Glaciers form like pancake batter
being poured on a griddle: heavy snows
in the far north pile up causing ice to
press out on all sides. This immense
“pancake” of slowmoving ice picked
up huge rocks and
carried them south,
tumbling and grinding
them along the way.
The force of the glacier shaped the
landscape we see today. For example,
the current course of the Ohio River
was carved by massive amounts of
water coming off the end of the glacier.
As temperatures gradually increased
and the glacier melted, the rocks it had
pushed along its snout were deposited
thousands of miles from where they
were picked up. These boulders are
called “glacial erratics.”
This Boulder Walk is a miniature
model of a terminal moraine showing
how the landscape might look at the
farthest extent of a glacier.
Snout
Ice Lobe
Terminal
Moraine
ICE
BEDROCK
Thrusting
Ground Moraine
Erratic Till
Glacial Terminal Moraine
Many of these boulders were brought
to CNC through the joint efforts of
Richard and Lucille Durrell, members
and trustees of CNC, and Gale Gordon
of Earl Park, Indiana. Other rocks
were found and added when the dam
for Crosley Lake was excavated. The
moraine was constructed in the early
1970s at the same time the Visitor
Center was being built.
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Obsidian – extrusive igneous rock
Group I – This group represents typical igneous rocks, those that
have been formed from molten rock (magma). Igneous rocks form
in two ways. Intrusive igneous rocks form under the surface of the
earth, cooling slowly and forming large crystals. Extrusive igneous
rocks form when magma is forced to the earth’s surface through
a volcanic eruption. These rocks cool more quickly and thus have
smaller crystals.
Glaciers transported the rocks from Group I from the huge
Canadian Shield, a region of approximately 3.1 million square
miles encompassing much of Canada and parts of the upper
midwestern United States. This region represents an area greatly
affected by glaciers, with shallow soils and bedrock as old as 1.2 2.7 billion years.
The Canadian Shield, outlined
in this image, is a large area
where the bedrock is composed
mainly of igneous rocks.
Cincinnati
Granite – intrusive igneous rock
All the rocks in this group were formed in or around volcanoes that
existed in northern North America billions of years ago. They were
then transported south by glaciers to the midwestern United States.
Mineral composition and the speed of cooling determine each rock’s
specific texture and color.
1. GABBRO. Using your fingers can
you tell which way this rock was
dragged by a glacier over the
underlying bedrock?
4. GRANITE. The small lens shaped darker
area near the top of this large boulder is
an inclusion - a portion of an older rock
that became attached to this granite.
2. GRANITE. Hard and resistant to
weathering, granites “travel well”.
Granites are made mainly of the
mineral orthoclase feldspar; it can be
white, cream, pink or red depending on
how much iron is present.
5. DIORITE. This boulder is typical of
a family of rocks whose composition
is intermediate between granite and
gabbro. The shape of this erratic is a
text-book example of a rock having
been transported a long distance by ice.
3. PINK GRANITE. This smaller boulder
contains two areas of much coarser
crystals of pink orthoclase and quartz.
These areas may represent a condition
of slow cooling producing larger
crystals followed by a more rapid
cooling producing the finer texture of
the rest of the rock. Orthoclase is used
in porcelain,
china, and
glazes for
bathtubs.
6. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
7. GRANITE. Look carefully and you can
see that this rock has been intruded
by magma which cooled slowly so that
very large crystals can be seen in the
three joints running from top to bottom
in the rock.
8. GABBRO. This boulder has an almost
perfect shape for an erratic. The rust
covering it is due to minerals on the
surface breaking down to expose iron
to air and water. This is the same rust
that forms on cars and old cans.
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Group II – This group shows examples of igneous rocks that have
been forced (intruded) into other rocks (igneous, sedimentary and/
or metamorphic). This group also shows what happens to all rocks
when they are exposed to the atmosphere and become weathered
(broken down).
1. PEGMATITE. This rock was found at
CNC when the dam was being built for
Powel Crosley Lake. This intrusion goes
into a deep metamorphic rock called
a schist (metamorphic rocks will be
considered in detail in groups IV and V
of this trail)
5. GNEISS INTRUDED BY GRANITE. The
granite intrusions in this specimen are
more resistant to weathering than the
surrounding gneiss.
6. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
2. DIORITE INTRUDED BY GRANITE.
Which part of this rock is weathering
faster, the granite or the diorite? Hint:
if you need to remember what granite
looks like check out Group I #7.
Gneiss
Layers in sedimentary rock
Cincinnati’s bedrock formed during the Ordovician Period, approximately 490
– 443 million years ago. At that time a warm shallow sea covered the area and
layers of shells of sea creatures covered the sea floor. Those shell fragments
eventually formed limestones while even smaller particles of mud and shells
formed shales. Limestone and shale are the two most common rocks at the
Cincinnati Nature Center. To see examples of them head down the limestone
steps to Avey’s Run. You may even find some fossils!
1. DOLOMITE. This rock is a type of
limestone that contains fossils. By
comparing the fossils in a particular
rock with those of bedrock it is possible
to trace a rock back to where it was
formed. This rock is from an outcrop in
Columbus, OH.
3. MYSTERY ROCK. Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
4. DIORITE GNEISS. This large dark,
faceted boulder is typical of many
metamorphic rocks. Gneiss rocks
generally have bands where similar
minerals have grouped together. The
intrusions show up as small pink areas.
Group III – This group contains examples of
sedimentary rocks. These rocks differ from igneous
and metamorphic rocks by being composed
of layers and layers of pieces of rocks or shells.
These rocks are particularly important to us here
because the bedrock of the Cincinnati region is
sedimentary.
2. CONGLOMERATE. This large rock
came from one of the sand and gravel
pits in the Little Miami valley near CNC.
If you look closely you can see that it
is made up of rounded, not faceted,
pebbles of limestone and some erratics.
The rounding is characteristic of
stream travel and this rock was formed
when water from the melting glacier
deposited pebbles in valleys. When the
water evaporated the pebbles became
loosely glued together.
3. SANDSTONE. Many sedimentary
rocks show features such as ripples,
mudcracks, raindrops, footprints, trails,
etc. This rock shows ripple marks
formed by wind-blown waves. These
marks are called wave or oscillation
ripples and indicate that the rock was
formed in shallow water.
Sandstone
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2. AUGEN GNEISS. This rock once
contained well formed crystals which
later were deformed by pressure and
movement to create pink “eye-shaped”
or circular areas which are called augen
in German.
3. GRANITE GNEISS. This rock was
scraped and broken by rocks embedded
in a glacier, giving it a large flat
surface.
4 and 5. BANDED GNEISS. These rocks
show excellent examples of banding
that occurs when sedimentary rocks
are transformed into metamorphic
rocks. When exposed to heat and
pressure, the minerals will tend to
form into distinct bands. In these rocks
we can observe bands of clear quartz.
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Group IV – This group shows examples of metamorphic rocks; those
that have been transformed by heat and pressure. Metamorphic rocks
occur throughout earth’s crust and are formed when rocks of any type
are exposed to high heat and pressure either deep under the earth’s
surface, at the junction of continental plates, or where magma from
the earth’s interior seeps into bedrock.
1. GRANITE GNEISS. This rock looks
similar to granite because it was
granite before being metamorphosed
into gneiss. One way to distinguish the
two is that gneiss shows more distinct
bands of minerals than does granite.
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These bands differ from those seen in
sedimentary rocks because each band is
composed of an individual mineral.
6. JASPER CONGLOMERATE
QUARTZITE. This brightly colored
rock is metamorphosed sandstone.
Pieces often occur in glacial till (rock
fragments ground up by a glacier.)
7. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
8. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
Pink Granite Gneiss
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Group V - Additional metamorphic rocks.
1. PINK BANDED GRANITE GNEISS.
This metamorphic rock shows examples
of sheeting or exfoliation. This process
occurs when changes in temperature
and pressure result in surface layers
peeling off.
Half-Dome in Yosemite National Park is
a prime example of exfoliation.
2. BANDED GNEISS. This was a
sediment before metamorphism.
3. GRANITE GNEISS. Note how
weathering has etched out bands in
this small specimen of gneiss.
4. GRANITE GNEISS. This rock has a
large vein of quartz. Quartz is highly
resistant to weathering and so has
become exposed as the surrounding
minerals wash away.
5. RED QUARTZITE. This small rock’s
surface has been smoothed by its
time under a glacier. Even though this
is a metamorphic rock, if you look
closely you can still see some of the
sedimentary layers.
6. BLACK SLATE. (metamorphosed
shale). Slate tends to split into long flat
sections and is often used on roofs and
floor tiles.
7. GREEN ARGILLITE.
This rock was formed
from mud deposited
in a lake 1.2 billion
years ago! The lines
in the rock tell the
story of the seasons
at this lake. The thick layers were
deposited in summer and the thin in
winter. Like the rings in a tree trunk,
one pair equals one year.
8. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
9. MYSTERY ROCK - Based on what
you’ve learned so far, can you name
this rock? Answers on last page.
10. METATILLITE. This rock is
metamorphosed tillite. Tillite is formed
when rock fragments become cemented
together by fine rock dust mixed with
water. This boulder was once part of a
rock formation in Ontario representing
a 1.2 billion year old glacial deposit.
Try out the Self-Guided Geology Hike. This hike will lead you along the
Geology trail and past some of the features that help shed light on CNC’s
long geologic history. Brochures are located inside the Visitor Center.
ANSWERS TO MYSTERY ROCKS
Group I - number 5 - DIORITE
Group II - number 3 - PINK PEGMATITE INTRUDED INTO PINK GRANITE
Group II - number 6 - BANDED GNEISS INTRUDED BY PINK GRANITE
Group IV - number 7 - AUGEN GNEISS
Group IV - number 8 - PINK GRANITE GNEISS
Group V - number 8 - BANDED GRANITE GNEISS
Group V - number 9 - GNEISS
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Celebration
Garden
Rowe Woods
4949 Tealtown Road
Milford, Ohio 45150
513.831.1711
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Picnic
Tables
Main
Parking
En
Boul
Map to the
Boulder
Walk
at Cincinnati Nature
Center’s Rowe Woods
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Trailhead
Visitor
Center