Lab 3: Ocean Processes

Geology 115/History 150Name(s):

Lab 3: Ocean processes

Objective: To observe the types of sediment that make up coastal lands, and then to use map information to identify different types of coastal processes.

Needed:

• Dissection microscope sediment samples A, B, C, D

• Aerial Stereo Photographs book

• Kingston, Rhode Island topographic map (1957)

• Point Reyes National Seashore and Vicinity (1973)

• Geology of the Point Reyes Peninsula, Marin County, CA (1976)

The dissection microscope sediment samples

To use the dissection scopes, turn the knob at the base of the body to turn on the illumination lamp; turn the large knob until the sample comes into focus in the eyepiece. The squares on the tray are 0.5 cm on a side.

1. a. Sample A is sand from Cape Cod NS. Estimate the size of an average sand grain in this sample (1/10 of a square = 0.05 cm; 1/20 of a square = 0.025 cm, etc.).

b. Did the sand derive from a single source, or from many sources? How can you tell from your observations?

c. Are the sand grains in general more rounded or more angular? What does this observation tell you about the distance that these sand grains have been transported?

2. a. Sample B is loess from the Palouse in eastern Washington. Estimate the size of an average loess grain in this sample (1/10 of a square = 0.05 cm; 1/20 of a square = 0.025 cm, etc.). You may also state that it is less than a particular size, if the actual size is hard to estimate.

b. Are these grains more uniform or less uniform than the Sample A sand grains, in composition? Or is it hard to tell? If you can answer this question, suggest a reason for your observation. Hint: where did the loess come from?

c. Would you judge these grains to have traveled a greater or lesser distance than the Sample A grains? Justify your answer.

3. a. Sample C are skeletal coated grains from the Florida Keys. Estimate the size of an average grain in this sample (1/10 of a square = 0.05 cm; 1/20 of a square = 0.025 cm, etc.). Is it larger, smaller or the same size as Sample A’s sand grains?

b. Justify the word “skeletal” in this sample’s name; in other words, what do some of the larger grains appear to be?

c. The “coated” means that the grains have the mineral calcite precipitate (= undissolve) around them as they sit in the water. Would this cause the grains to grow smaller or larger? Is this what happens to grains in Sample A or B?

4. Sample D is grapestone (yes, a real word) from the Florida Keys; these grains are made from coated fecal pellets that were produced by marine invertebrate organisms (yes, say “ew”). Again, the word “coated” means having calcite precipitated on them. Would this type of sediment be associated with a reef? Why or why not?

5. Finally, which of these sediments (A, B, C, D) will be found on a carbonate platform? Which of these sediments will be found on a siliciclastic platform?

Aerial Stereo Photographs book

You can use the stereographs while viewing the photographs in this book to get a 3-D effect, but it is not necessary.

Plate 31: St. Joseph Point, Florida

6. If north is to the top of the photograph, which compass direction does the spit seem to be growing? How can you tell this? What are the dark areas in the photograph, anyway?

Plate 33: Coatue Point, Nantucket Island, Massachusetts

7. This is a bit south of Cape Cod, but the same processes take place in both areas. The photograph shows a barrier island (the long linear feature). Was the barrier island always this cuspate shape? What evidence from the photograph suggests that erosion is at work?

Kingston, Rhode Island topographic map (1957)

8. Locate this map on the larger Cosmogenic Isotope Ages of the New England Coastal End Moraines map on the north wall of the classroom. “Point Judith” mentioned on the larger map is the lower right corner of this topographic map. What larger feature is the hilly area parallel to the coast and contains “Bull Head Pond” and “Broad Hill” called?

9. Is there evidence of a barrier island on this map? If so, write some local place names that are part of that island. What happened to the island geologically? What will eventually happen to Green Hill Pond?

10. What do the main roads and towns in this area try to avoid? Would this affect the development/settlement/use of this area? In fact, what is the principal use for the roadless and townless area of this map?

Point Reyes National Seashore and Vicinity (1973)

11. a. What is the highest elevation at the tip of Point Reyes? Don’t forget units.

b. In contrast, what is the highest elevation along Limantour Spit?

c. Which area is therefore more resistant to erosion by the ocean?

12. Drake’s Bay has a striking arcuate shape on the map. Use the transparency with the graph of a logarithmic spiral to overlay the mathematical function onto the map. Can the shape of Drake’s Bay be considered a log spiral? Are there any other bays on the map that can be considered to have a log spiral shape? If so, name them.

13. The conditions to create a bay in the shape of a log spiral are known: there needs to be a erosion-resistant headland and erodible land downcurrent from the headland. Given these conditions, in what compass direction does the current flow in this part of the Pacific Ocean?

Geology of the Point Reyes Peninsula, Marin County, CA (1976)

14. Even without the label, how can you tell that there’s a fault running down the length of Tomales Bay? Hint: look at the rock descriptions on either side of the bay.

15. a. In the previous section, you determined whether the tip of Point Reyes or Limantour Spit was more resistant to erosion. What formation(s) make(s) up the tip of Point Reyes? Write the rock name(s), not just the formation abbreviation or name.

b. What formation(s) make(s) up Limantour Spit? Again, write the rock name(s), not just the formation abbreviation or name.

c. From what you’ve seen in lab already, which rock is more resistant to water erosion? Is this consistent with your answer to question 11c?