Read the following statements
describing how sedimentary layers are deposited.
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STENO'S LAWS
Principle of Superposition, states
that in an undisturbed sedimentary sequence, the older beds
are on the bottom, and the younger beds are on the top.
Principle of Original Horizontality,
states that sediments are deposited in flat, horizontal layers.
Sedimentary particles settling under the influence of gravity
in a body of water will deposit horizontal layer of sediment.
Principle of Original Lateral Continuity.
If we assume that sediment is being deposited on the seafloor,
the sediment will not only be deposited in a flat layer, it
will be a layer that extends for a considerable distance in all
directions. The layer is said to be laterally continuous.
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These three laws apply to
the sediments forming the bedrock units within the Lehigh Gorge.
Many times rock units contain "bedding"
which is seen as a layered look to the rocks. A bedding
plane is a surface separating layers of sedimentary rock.
Examine the bedding shown in the pictures below.
Click on the images below to enlarge.
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Use the images on this page
to answer the following questions in complete sentences.
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1. Look
at the picture above, which rock layer is the oldest
and which is the youngest?
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Many times layers of rock are
not observed as horizontal sheets. Sometimes they are compressed,
extended or sheered in opposite directions. These forces distort the
horizontal layers and produce different folds or
cracks in rock layers. |
Compression
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Tension
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Shear
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When rock is compressed, sometimes
the layers bend and form folds. An anticline is a
fold in which the layers (limbs) dip away from the axis or center
of the fold. The picture on the left (A) is an anticline.
A syncline is a fold in which the layers (limbs)
dip toward the axis or center of the fold. The picture on the right
(B) is a syncline |
Anticline
Syncline
A B
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Examine pictures A
and B. Using your knowledge of Steno's laws, answer
the following questions below in complete sentences. Note the symbols
for an anticline and syncline. An anticline is shown by a line representing
the axis or center of the fold and arrows pointing away
from each other, the arrows are pointing toward each
other in the syncline symbol. |
2. In picture
A, which layer is the oldest and which layer is the
youngest? |
3. In picture
B, which layer is the oldest and which layer is the
youngest? |
Use the picture of the anticline (A)
to answer the following question.
4. If you stand at the center of an anticline
and walk perpendicular to the fold axis, are the
rocks you cross over younger or older than the rock exposed in the
center of the anticline? (Imagine walking from the green rock
layer in a straight line to the aqua colored rock layer)
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Use the picture of the syncline
(B) to answer the following question.
5. If you stand at the center of a syncline and
walk perpendicular to the fold axis, are the rocks
you cross over younger or older than the rock exposed in the center
of the syncline? (Imagine walking from the yellow rock layer in
a straight line toward the pink rock layer) |
Look at the bedrock
map of the Lehigh Gorge
6. Are the rock layers in the Lehigh Gorge horizontal?
Explain your answer.
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7. What must
have happened to the bedrock to form the pattern
you see in the Lehigh Gorge? |