Wissahickon Follow Up
Name _________________________
Date _________
Directions: Read the information and answer the questions
Part 1. General Background Information: The
Wissahickon rock formation is quite different from
that of the rest of Pennsylvania. The Wissahickon
Creek is sort of a backward creek. We usually
think of a creek as starting in a mountainous gorge
and dropping quickly as it reaches flatter land. The
Wissahickon starts in gently rolling farmland and
ends in a mountainous gorge. How can you explain
that? The explanation has to do with plate
tectonics.
There are some places where there are rifts,
where new oceans form, and some places where the
continents collide, where new mountains form.
This is an ongoing process, but each cycle of
mountain building and rifting/ocean formation is
called an orogeny. One orogeny includes a period
of mountain building and a period of rifting of the
continent to form an ocean. The last Orogeny that
formed Pangaea, the Appalachian Mountains and
the Atlantic Ocean was called the Allegheny
Orogeny. There were three orogenies before that.
Between each orogeny there is a period of time
where erosion and weathering wear down the
mountains.
1. 1. What is an Orogeny?
2. 2. What two things form in one Orogeny?
3. 3. What geological feature is the
Wissahickon and where did it come from?
4. 4. Why wasn’t the Wissahickon rock
formation created at the same time as the
Appalachian Mountains were forming in the
last orogeny?
5. 5. Why are the rocks of the Wissahickon so
different from the rest of the rocks in
Pennsylvania?
During an earlier Orogeny, the ocean located
where the Atlantic Ocean is now, was called the
Lapetus Ocean . As the rifting occurred and the
Lapetus Ocean opened up, small fragments of one
plate sometimes were torn away from their
continents and traveled separately until they got
attached to a different continent. These little pieces
are called terranes. When the Lapetus Ocean
closed up and continents collided to form Pangaea,
a terrane called Avalonia collided with the east
coast of North America. Since it is a piece of
another plate, it’s rock formations are different from
those of the North American Plate. It is also made
of much older rocks. As the Avalonia terrane
collided with the North American Plate parts of it
were shoved deep underground. Eventually the new
ocean-the Atlantic-opened up but the Terrane is still
attached here. Over the last two hundred million
years, erosion has leveled most of the land of the
region and rivers have cut down through the layers
exposing them so they are no longer ten miles
underground. The Wissahickon formation is part
of this terrane.
Stop 1: From Mud to Morphism
A long time ago when the rocks of the Wissahickon
were still part of another plate that was far away,
the layers of sediments that formed these rocks were
deposited. The Quartzite was once sandstone, a
sedimentary rock made of sand. The Schist was
once shale, a sedimentary rock made of clay mud.
Sand particles are larger and heavier sediments than
mud particles so they tend settle out quickly when
water slows down its motion. However, clay mud
will only settle out in very still water. Since layers
of sand and mud form under different conditions the
problem is how they could be interlayered with each
other in the same place. There must have been
some special environment with changing conditions
that would cause the clay to mix with the sand
1. Which type of rocks were the layers made
of before they were changed to quartzite
and schist?
2. Which sediments were deposited to form
the layers before they turned into
sedimentary rock?
3. Which type of rock-quartzite or schist was
harder? In other words, which type of rock
was more resistant to weathering and how
could you tell?
4. Which type of rock-quartzite or schist-was
foliated and therefore looked like the pages
of a book?
5. Notice that the layers appeared to be
vertical rather than horizontal to the surface
of the ground. What could have happened
to cause that?
repeatedly. It is believe that the Wissahickon was
on the edge of the continent-the part that is
underwater called the continental shelf. Whenever
there was an earthquake (occurring often during
episodes of plate tectonics) , there would be an
underwater landslide down the continental slope
that would cause a layer of sand to slide over the
clay and this is how the alternating layers of
sandstone and shale formed. So, then how did they
change into quartzite and schist? Metamorphism
must have occurred from plate tectonic collisions.
As the sedimentary rock layers were put under
intense heat and pressure, they deformed and
changed into what we now see-the layers of
quartzite and schist.
Stop 2: Gems of Pennsylvania
If you brushed away the fallen leaves and
looked a little closer, you might have found
beautiful gems and minerals in Pennsylvania. Some
of the best places to look are road cuts, stream cuts,
or old mines and quarries. The Wissahickon
formation is part of the Piedmont Province which
has some of the best gem minerals of Pennsylvania.
Some of the minerals that can be found here are
muscovite micas, garnets, and tourmaline. Intense
heat, intense pressure and super-heated mineralized
water cause the molecules in the rocks to rearrange
and form new minerals. In shales this caused
silvery mica (muscovite) crystals to grow in a
foliated pattern like the page of a book.
Geologists use the minerals found in rocks to
estimate the temperatures and pressures that
affected the rock during metamorphism. The
Wissahickon schist is a type of schist known as
Garnet Schist because it formed under high
temperatures of at least 550 degrees Fahrenheit and
with pressures equal to 20-25 kilometers beneath
the earth’s surface. At this particular temperature
and pressure garnet crystals formed in the schist.
Garnets are deep red crystals that look like soccer
1. What are two ways gemstones are formed?
2. What helpful information do certain minerals
give geologists?
3. Were you able to find any minerals on the
hike? Explain.
balls due to their dodecahedral geometrical shapes.
During heavy rains hundreds of garnet crystals are
washed out of the schist and these range in size
from ½ inch to microscopic in diameter. Since
they weather out of the schist good places to look
for them in the sand of the path on top of the bluff,
and in the gravel bars of small streams that feed the
Wissahickon Creek. Another mineral to keep your
eye out for is tourmaline embedded in the igneous
rock called pegmatite. Mineral-laden water was
heated by the very high temperatures within magma
still in the process of crystallizing. The superheated
fluids then worked through the surrounding rock,
changing its structure and mineral composition by
adding or removing minerals. Tourmaline looks like
black, shiny needle shaped crystals.
Gemstones are minerals prized for their
beauty, durability, and rarity. Their worth depends
on their scarcity, color, purity, brilliance, hardness
and demand. Most stones are cut and polished so
they glow and sparkle. Diamonds, rubies, emeralds,
sapphires, and opal are all transparent gemstones
that are called precious stones. Semiprecious stones
include agate, amethyst, garnet and peridot.
Stop 3: A Semi-Recumbent Fold
Notice the layer of quartzite on the outcrop above.
It bends up and then back on itself. This type of
deformation is called a recumbent fold and
provides more clues about history. To visualize
this, imagine holding two ends of a sheet of paper
and move them together to make a fold and then tip
it over on one side. Rock layers are being pushed
together on either side (compression), but not with
the same exact amount of force on each side.
The reason the fold is a Semi-recumbent fold is
that it is not completely turned over. Did you notice
that the layers of schist do not appear to be folded in
the same way as the quartzite. This may seem
puzzling as the rocks were all together when they
were folded. The reason for the difference is that
schist with its foliated mica grains tends to dissolve
and recrystalize under this kind of stress. This
causes them to re-position and to lose the folded
shape. Whereas quartzite is not foliated so tends to
bend or break when it is under uneven compression.
1. What is a recumbent fold?
2. How do quartz-rich layers and micarich layers respond differently to
uneven compression?
Stop 4 Tributary to the Wissahickon Creek
At the bridge you are standing at a small tributary
(feeder stream) into the Wissahickon Creek. If you
look uphill, you will see the creek forms a Vshaped valley with the sides of the valley sloping to
the edges of the stream. Since this part of the
stream is sloping, it runs quickly and erodes away
sediments especially during times of high flow after
a rain, it picks up larger rocks and logs. Now look
the other way, toward the Wissahickon Creek.
Notice how the V-shaped valley disappears, and
instead you can see rocks, dirt, leaves, and tree
trunks spread over a broad fan-shaped area called an
alluvial fan. Here the slope has changed from a
steep slope to a flat area and so the stream is now
depositing the materials it was carrying.
When you Took a look around the Wissahickon
watershed how clean did the land area look? How
would you classify this watershed: highly
developed and paved, somewhat developed with
trails, or undeveloped. Did you notice any erosion
along the trails? What was the quality of the river
water? What is your assessment of the health of
this watershed and Creek?
1.
Name one erosional feature and one
depositional feature of the tributary feeding
into the creak? Which one was erosional and
which was depositional?
2. How would you classify this watershed in
terms of its development?
3. Did you notice any erosion along the trails?
4. When you looked into the water, did it look
clear or was brown and filled with sediment
(turbid)?
5. What was your assessment of the health of this
watershed and Creek?
Stop 5 Four Different Rocks in a Fold
Did you notice the alternating layers of Wissahickon schist and quartzite at this stop. Because they don’t fold
exactly the same , when they fold, they tend to separate and form cracks. In these cracks, pressure is released
and this melts some of the rock into magma, that flows between the layers of the schist and quartzite. Did you
notice the light-colored rock between the layers. That rock is a type of granite called pegmatite that is an
intrusive igneous rock. Did you notice that the minerals in that rock are not aligned in any one direction, but
are randomly oriented. The Pegmatite must have formed from magma oozing (intruding) into the weak areas
between the other layers after they formed. There is evidence that the pegmatite is younger than the schist. Did
you also see the dark rock at the very edges of the pegmatite. This rock formed when the schist was baked by
the high temperatures of the magma. This rock is called gneiss. It is harder and more highly metamorphosed
than the schist. Remember the very small micro-folds called crenulations. These occur when folding of the
same rock takes place more than once from multiple orogenies.
1. How does pegmatite form?
2.
What observable evidence was there that the
pegamatite is felsic and intrusive?
3. List the four rocks in the order that they
formed.
Stop 6. A Sill and a dike
Did you look up at the outcrop above see the sill and a dike. Remember that a sill is made of igneous rock that
has oozed into separations between rock layers. A dike would form when a crack opens up either across rock
layers or within one particular layer.
1.
What type of rock is in the sill and dike?
2.
Explain how you figured out the location of
the sill and dike?
Analysis: What geological event or events occur to form the following geological features at the Wissahickon?
a. The alternating layers of schist and quartzite.
b. The garnets in the Wissahickon Schist.
c. The recumbent fold
d.
The V-shaped valley and the alluvial fan.
e.
The pegmatite.