CENTRAL PARK FIELD TRIP GUIDE.docx
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CUNY College of Staten Island *
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Course
100
Subject
Geology
Date
Apr 3, 2024
Type
Pages
17
Uploaded by ChancellorBoar4201
CENTRAL PARK GEOLOGY FIELD TRIP
This is a self-guided field trip. There will be three stops on this field trip.
Please follow social distancing guidelines if you go to Central Park in person. You may also take a virtual
tour using
Google Earth
. To take a Google Earth tour of this field trip, type in ‘Central Park New York’ in
the search button and then zoom into the coordinates given below for each stop. Use the different
features available including street view and familiarize yourself with the surroundings for each stop.
Figure 1. Central Park Map 1
Enter CENTRAL PARK at 67
th
street and 5
th
Ave:
INSTRUCTIONS: As you enter Central Park you will see a small play area for children on your right.
Continue walking straight (west) towards the statue of
Balto-
the heroic sled dog. Before you reach the
statue you will see a large exposure of rock on your left. This is
rock exposure A .
Field Trip stop #1:
Google Earth coordinates: 40
0
46’ 10.29” N, 73
0
58’ 15.51” W
Figure 2. The path to Rock Exposure A
Go to
the rock exposure A.
(Locations are shown on the Map above). When you arrive at the exposure,
make the required observations, and answer the questions.
Figure 3. Exposure A
1.
Examine the rock that you are standing on. Note that the
rock is layered
. Sketch a rough map of
Stop#1 and Exposure A. In the area marked "exposure A", draw a series of parallel lines to
indicate the direction of layering.
Parallel lines in the image above. ^
Sketch:
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2.
In the space below, sketch any feature that suggests the layering in exposure A has been
deformed.
The elongated parts of the exposure are deformations.
3.
Name one
mineral
present in the rock in abundance.
A rock mineral abundant in the rock is Mica Muscovite. (There are also Milky Quartz.)
4.
Classify this rock. Is it an Igneous, sedimentary, or metamorphic rock? Explain your reasoning.
What is the name of the
major rock type (igneous, sedimentary, or metamorphic)
seen here?
This is a metamorphic rock as seen by the foliated layering. This rock has undergone regional
metamorphism related to plate tectonics, which is supported by its state after differential stress
and by deformations such as the elongated parts.
5.
Standing
on Exposure A,
if you look around in all directions, you will see that there are many
similar exposures of rock cropping out of the grass. In loose boulders, features such as layering
generally do not show any consistency from one boulder to the next. What is there that suggests
that, underneath the soil and grass that separates them, rock exposures A and the ones around
are joined, and are not just the tops of large, buried, loose boulders?
The folds and faults of these metamorphic rocks seem to be consistent in this general area.
Based on this formation and the presence of similar minerals and deformations, it is most
reasonable to assume the exposure was part of a larger mountain chain, where weathering and
erosion caused those specific sections to be exposed.
These rock exposures are actually ‘
outcrops’
, places where bedrock (the continuous crust of the
earth) appears at the earth’s surface. The folding that you see in this area may be understood in terms of the
geologic history of the
New York City region:
About 500 million years ago, this region was
shallow sea floor, off the coast of the American
Continent
, and was the site of deposition of great thicknesses of
sediment
derived from the
erosion of the nearby land
(Fig. 1).
Sediments were deposited near the start of the Cambrian
Period (542 million years ago) on the shores of the
Iapetus Ocean
.
During that time, the region was in the central part of the American plate. Later, a new
,
convergent plate boundary
developed here, along which the
ocean lithosphere was pushed
under the continental lithosphere
(forming a
subduction zone).
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Subduction of the
Iapetus Ocean
led to its destruction and
the collision
of different
continental
blocks
and
island arcs
onto the American continent. As a result, the region became subject to
compression,
and a
mountain range
formed
(Fig. 2).
From the subduction zone,
heat, magma,
and chemically active fluids
penetrated the core of the mountain range, deforming and
metamorphosing the sedimentary layers.
Those collisions gave rise to
the Appalachian Mountain
belt that sutured together the
continental blocks into a
supercontinent
that we call
‘Pangea’
. The
three mountain-building
events called
‘orogenies’
that eventually built the lofty Appalachian range are:
1.
the Taconic Orogeny
in the Middle Ordovician (about 472 million years ago);
2.
the Acadian Orogeny
in the Middle to Late Devonian (at 390 million to 370 million
years); and
3.
the AlleghenianOrogeny
in the Late Carboniferous to Permian (300 million to 250
million years ago).
About
200 million years ago,
the region
ceased being a convergent plate boundary,
and active
mountain-building processes
came to a halt.
Gradually the mountains were eroded away until
the rocks that composed their
igneous
and
metamorphic roots
were exposed at the surface
(Fig.
3).
The deformed rocks at which you are now looking are the
roots
of that ancient mountain
range.
In the roots of the mountain range where these rocks formed,
pockets of melt
developed which
were then squeezed and forced (intruded) into the adjacent solid rock. When the melt cooled, it
formed bodies of rock called
"intrusives
". Such intrusives may also be seen in this area.
Figure 5. Outcrop with an intrusion (compass for scale).
6.
Find the
intrusion
and outline it in red. Note its sharp contact with the surrounding rock. Outlined above. ^
Next, leave
Rock Exposure A
and walk toward and past the statue of
Balto
the sled dog and
under the small bridge and onto the other side.
Continue walking past
Shakespeare’s statue
toward ‘
Sheep’s Meadow’
.
Enter
Sheep’s Meadow
through the small gate and walk toward the large outcrop of rock on
your left with many boulders perched on top. This is stop #2.
Figure 6. Rock Exposure B
Field Trip stop #2
Google Earth coordinates: 40
0
46’ 14.21” N, 73
0
58’ 28.54” W
1.
Stand on the wide mound of rock. This is
rock exposure B
. Observe that the rock is layered, and
the layers appear
folded
. Is the rock exposure here an outcrop of
bedrock
like rock exposure A?
Evidence?
The rock exposure is an outcrop of bedrock like rock exposure A because they share similar
characteristics: it has grooves perpendicular to its foliations, it has undergone differential stress
(in a pattern like exposure A), there are larger Mica Muscovite minerals, and the surface is
exposed because of weathering and erosion.
2.
What is the major rock type?
This exposure is a metamorphic rock; specifically, a Schist.
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3.
Look again at the layers in this area. Note the numerous
"grooves"
parallel to the layers that
exist where some of the layers have been
worn (eroded) more deeply
than others. Why have
they been worn more deeply?
Some of the layers could have been worn more deeply depending on the mineral composition in
that area and the amount of water that has run along these layers. The erosive power of water is
often influenced by the type of minerals and the hardness of the rock; similarly, minerals within
the rocks may dissolve or undergo chemical reactions when exposed to water.
Figure 8. Outcrop with complex folding.
Note the
complex folding
of the layers in part of this area.
4.
Trace the course of an individual folded, contorted layer for as great a distance as possible. Note
where you begin to follow the layer and where you finish following it. Now measure the length
of that layer in terms of the length of your foot; that is, follow along the layer walking
heel-to-toe, heel-to-toe. Write your answer below.
The length of the layer is 29 (size 9) foot lengths.
Next, walk
heel-to-toe
in a straight line from where you began your traverse along the layer to
where you finished the traverse.
The straight-line distance is 5-foot lengths.
5.
If we assume that the layer you followed was originally straight, then the difference between the
two measurements you made represents the amount of shortening that the deformation
(folding) accomplished.
By approximately what percent of its original length was the layer
shortened?
𝐶????𝑎𝑙 𝑆ℎ?????𝑖?𝑔 ???????
= ??𝑖𝑔𝑖?𝑎𝑙 𝐿??𝑔?ℎ−𝐹?𝑙??? 𝑙??𝑔?ℎ
??𝑖𝑔𝑖?𝑎𝑙 𝐿??𝑔?ℎ
× 100
Shortening percent = 82.76%
6.
Step off
rock exposure B
on the west side, stand on the soil or grass, and look at the rock face
that slopes gently down toward you. Locate the foot-or-so wide,
parallel grooves
that extend for
ten yards or so up the outcrop.
Figure 9. Outcrop with parallel grooves.
a.
Are the grooves parallel to the layering in the rock?
The grooves are perpendicular to the foliations.
b.
Could they be due to
differential erosion
of the layering in the rock?
If the grooves were caused by differential erosion of the layering in the rock, then there
would be more uneven erosion patterns and they would align with the bedding or
layering of the rock. Thus, it could be partially caused by this considering the glacial
deposits, massive boulders, and general tectonic movement.
c.
Did the grooves form
before or after
the folding of the rock layering?
The grooves formed after the folding of the rock layering.
The origin of the grooves may be explained in terms of
the Glacial Theory
. About
15 thousand years
ago, a giant body of flowing ice (a glacier) covered this area.
Embedded in the ice
at the bottom of
the glacier
were large boulders
. As the ice pressed down on these boulders and dragged them over
the underlying bedrock,
grooves were carved in the bedrock
. Smaller particles of rock that were
dragged along created smaller grooves called
"striations".
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Figure 10. Outcrop with striations and grooves.
d.
Find some striations. What is their orientation with respect to the grooves?
The striations are parallel to the grooves and perpendicular to the folding.
e.
What are the possible
directions
from which the glaciers may have come to this area? (See
map for true north.)
Direction 1: Northwest Direction 2: Southeast
At a later outcrop we shall determine which of these two possible directions is most likely.
Notice the many
boulders
that lie scattered on the surface of the
rock exposure B
. Walk to the
large
boulder perched on the rock
. There is a similar large boulder in the distance to the right of
the carousel. These boulders are called
‘erratics’.
Figure 11. Examples of glacial erratics
7.
Examine the boulders.
a.
What is the general grain size?
The general grain size ranges from phaneritic to mostly pegmatitic and they are
subangular.
b.
Name three minerals present in the largest boulder.
Muscovite Mica, Potassium Feldspar, and Quartz (some staining from Hematite.)
c.
What
major rock type
is the boulder?
This is an igneous rock; specifically, a type of Granite (Pegmatite).
d.
What is the major rock type upon which the boulder rests?
The boulder rests on the metamorphic rock present underneath the grass and soil.
e.
Explain how the boulder got to its present position. (It was not placed there by people)
The boulder got to its present position by being moved by either a glacier or a landslide.
Leave Sheep’s meadow through the small gate near the boulders. Turn left and walk downhill
towards the Carousel.
FIELD TRIP STOP #3
: Google Earth coordinates: 40
0
46’ 9.01” N, 73
0
58’ 28.08” W
Go through the
underpass to the left of the carousel
. Upon emerging from the underpass,
follow the path a short distance
to outcrop C
on the right. The outcrop is just below the red brick
octagonal building. Stand so that beyond the outcrop, on the horizon, you can see the sign for
the Essex Hotel.
Figure 12. Outcrop C. Note the glacial grooves that run from right to left across the outcrop.
8.
Look at the right end of the outcrop where it slopes down to the soil and grass. Now look at the
left end of the outcrop. In profile, which end, the right or the left, looks steep and
abruptly "cut
off"?
Which ends looks more
"streamlined"?
Steep Right (looks smooth) Streamlined Left (looks streamlined and steep)
The asymmetry of this outcrop provides an ambiguous answer to the question "from which
direction did the glacial ice advance?" As glacial ice moves over bedrock that is hilly, it tends to
carve the "upstream" end of the hill into a smooth, streamlined shape. At the same time, as the
ice flows over and then leaves the hill, it tends to "grab" at any loose, fractured parts of the
bedrock and remove them, causing that end of the hill to have a steep, cliff-like profile (see Fig.
4). The resulting asymmetric hill is called
a ‘roche moutonnee’.
9.
From your examination of the
roche moutonnee
before you, from which direction did the glacial
ice advance?
The glacial ice advanced from the northwest to the southeast.
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10. In Fig. 4, note the piece of detached bedrock embedded in the ice. What other erosional glacial
feature that you have seen might be caused by such fragments?
This detached bedrock embedded in the ice may cause glacial striations as the bedrock would
scratch and scrape the bedrock surface as the glacier would advance.
11. What effect does this have on the angularity of the fragment?
As a glacier continues to drag along the bedrock, it could lead to the smoothing and rounding of
rocks; therefore, the angularity of the fragments would be reduced: abrasion and plucking.
12. If the fragment is not destroyed, what feature that you have observed might it become when the
ice ultimately melts?
If the fragment is not destroyed, it could form a glacial erratic or a glacial deposit and specifically
contribute to the formation of a moraine, a glacial landform created by the deposition of
sediment.
13. To verify the direction of glacial movement indicated by the
roche mountonee
, where would you
go and what would you look for? (Hint: Consider your hypothesis concerning the origin of the
erratic boulders.)
To verify the direction of glacial movement indicated by the roche mountonee, one would look
for striations on the bedrock (scratches or grooves) as striations are parallel to the direction of
glacial movement and can provide a clear indication of the glacier’s path. Additionally, one could
look for the type of minerals left on the bedrock as well as assess the shape of valleys in the
region (U-shaped vs V-shaped).