Cross-Section Histories

In this assignment you will be asked to take a look at some cross sections through the Earth’s surface and reconstruct the history of this area. You will use the principles of geologic time and what you’ve learned about rock formation to puzzle through the events of the past using these rocks.

Name:

Cross Section 1

This area has a number of sedimentary layers and a fault line from an earthquake, plus some deep igneous activity.

1. List the order that things happened in from oldest to youngest, including any erosion or deformation that occurs (8 points)

Mudstone (A)              Sandstone (B)              Siltstone (C)                 Shale (D)
Limestone (E)              Shale (F)                      Granite (H)                  Fault

2. Granite often includes heavy, radioactive elements like Uranium-235 which has a half-life of 710 million years. How can this information help you with more precisely dating the rock layers in this cross section? (2 points)

Cross Section 2

This cross section shows a mix of sedimentary and metamorphic rocks, with several different shapes depicted. Where there are two rocks of the same type but different adjectives (e.g. “oil shale” and “shale”) these are just to make them different in your description.

3. List the order that things happened in from oldest to youngest, including any erosion or deformation that occurs. (10 points)

Limestone (A)              Sandstone (B)              Shale (C)                      Mica Schist (D)                        Erosion
Garnet Schist (E)                      Conglomerate (F)                     Oil Shale (G)                Tilting
Regional Metamorphism

4. If the limestone layer (A) contains a marine fossil that lived only a short window of time, how could you use that fossil to help with reconstructing the history of these rocks? (2 points)

Cross Section 3

This cross section shows beds of rock that have been exposed by a river valley carved down into them. Just like last time, the rocks of the same type with different adjectives (e.g. “river sandstone” and “marine sandstone”) these are just to make sure they’re different in your description.

5. List the order that things happened in from oldest to youngest, including any erosion or deformation that occurs. (11 points)

Siltstone (A)                 Mudstone (B)               Shale (C)                      Conglomerate (D)
Coquina (E)                 River Sandstone (F)                 Marine Sandstone (H)              Greywacke (I)
Valley Formation                     Tilting              Erosion

6. What other layers would you expect to see represented in the cobbles of the conglomerate (D)? (2 points)

Big Picture Stuff

The following questions refer to all three cross sections together. When answering them, look back through your answers to the three sections above before answering.

7. Find at least one example of each of the following principles in the cross sections. (8 points)
   1. Principle of Superposition:
   2. Principle of Original Horizontality:
   3. Principle of Lateral Continuity:
   4. Principle of Cross-Cutting Relationships:
   5. Principles of Inclusions:
   6. A Nonconformity:
   7. A Disconformity:
   8. An Angular Unconformity:

8. In which of the layers in the cross sections above would you not expect to find fossils preserved? (2 points)

9. Imagine that further study matches up some of these rock layers as forming in the same regions, though separated by great distances at their formation. In Cross Section 1, the limestone (E) is the same as the limestone (A) in Cross Section 2. At the same time, the river sandstone (F) in Cross Section 3 is the same as the sandstone (B) in Cross Section 2. How does this new information help you? (3 points)

10. Using the information from the question above, give the order of the cross sections themselves from youngest to oldest. (2 points)