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Activity 3 - How are sediments deposited?

 

Read the following statements describing how sedimentary layers are deposited.

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.

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.
Use the images on this page to answer the following questions in complete sentences.
1.   Look at the picture above, which rock layer is the oldest and which is the youngest?
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
Tension
Shear
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

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)

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.

7.   What must have happened to the bedrock to form the pattern you see in the Lehigh Gorge?

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