SELF-GUIDED GEOLOGIC TOUR OF TOURMALINE BEACH
*****Print this page out and bring it with you when you go!*****
DUE DATE: Final Exam.
TO BRING: Hat, digital or disposable camera, water, clip board, pencil/pen.
BEFORE YOU GO: Check the tide chart at http://www.tidelinesonline.com/. For region, select California. For location, select La Jolla. You’ll want to do your tour close to low tide. At high tide, you may get wet, and the beach may be underwater.
NOTE: Part of your assignment will involve taking photos of various geologic features, so make sure you have a camera!
Driving Directions:
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Follow I-5 south to Pacific Beach. |
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Take the Grand/Garnet/Balboa Ave exit. |
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Turn right (west) on Garnet; stay on Garnet all the way to Mission Blvd. |
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Turn right (north) on Mission Blvd. Then turn left onto La Jolla Blvd. Almost immediately, turn left onto Tourmaline Street. |
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Follow Tourmaline Street down the hill and park anywhere you can along the roadside. If you're feeling lucky, try the beach parking lot. |
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Once you've parked, walk down the hill, to Tourmaline Beach. |
OBJECTIVE: To observe, describe, photograph, and interpret various geologic features in the sea cliff at Tourmaline Surf Park.
ASSIGNMENT: As you walk along the beach, you’ll be asked to answer various questions and also TAKE PICTURES of various geologic features. For each picture, include the following information in your field notes: (1) a brief description of what you’re photographing, (2) the view direction of the photograph, (3) some sense of the scale of the photograph (e.g., “field of view, 20 ft”; or, you can include a small object (keys, etc.) for scale in the photo. Upon your return, please have your photos developed (or printed out). Then attach them to sheets of paper (1-2 photos per page). Below each photo, include the following information: (1) photo title, (2) photo description, (3) view direction.
Following your trip, please write a 3-page paper (typed, double-spaced, 1-inch margins) summarizing your geologic observations and discussing the questions presented below. Your paper should also include several labeled photographs, as described below. In addition, it should include the following information: date/time of your trip, weather conditions, brief discussion of photographs, and overall impressions/thoughts about your trip. Please note that photograph pages don’t count as part of your write-up.
Okay…let’s get started!!!
From the parking lot, walk south along the beach about 25 to 50 yards. The rock unit exposed in the sea cliff is the San Diego Formation (an approximately 3 million-year-old deposit consisting mostly of poorly cemented sandstone.
If you look closely, you’ll notice that the sandstone exposed in the sea cliff contains numerous fossils…
TAKE PICTURES of all the different fossils you observe. Note: at a minimum, you should be able to find at least two different types of fossils, including (1) thin-shelled scallops, and (2) spiral snail shells called gastropods. Look carefully! You’ll see ‘em!
Based on the types of fossils you found, was this sedimentary rock unit deposited in a marine- or a non-marine environment?
Now, walk north along the beach (i.e., back toward the parking lot)...
What evidence do you see that coastal erosion is actively occurring here?Hint: look at the homes near the top of the cliff. List at least 4 observations and TAKE A PICTURE illustrating at least one of your observations.
Once you're back at the parking lot, continue walking north. Notice that the sedimentary rocks exposed in the sea cliff to your right are not horizontal but are slightly tilted (dipping). This tilting is due to the growth of Mt. Soledad a few miles to the northeast. Mt. Soledad is being pushed up along a compressional bend in the Rose Canyon Fault Zone.
The well-cemented, well-layered strata toward the bottom of the cliff are siltstones, mudstones, and sandstones of the Eocene Scripps Formation (approx. 46 million years old).
Estimate the dip angle (tilt angle with respect to the horizontal) of the Scripps Formation. Are these rocks dipping (tilted downward) toward the north or south?
Higher up on the Sea Cliff is a layer of conglomerate—a coarse-grained, gravelly sedimentary rock, which is capped by brownish, finer-grained sandstone. Both the conglomerate and the sandstone are part of the Pliocene San Diego Formation (less than 3 million years old).
Highest up, toward the top of the sea cliff, you’ll see chocolate brown, poorly cemented sandstone and conglomerate of the Pleistocene Bay Point Formation (approx. 120 thousand years old). Notice how unstable the Bay Point Formation is!
TAKE A PICTURE of the Scripps, San Diego, and Bay Point Formations exposed in the sea cliff. Label each Formation on your photograph.
Based on your observations of the upper and lower parts of the sea cliff in this area, develop a simple summary statement about how the degree of cementation in these rocks changes with age. That is, do the rocks in this area become better or more poorly cemented with increasing age?
Now, walk up to the sea cliff. Keep walking north until you notice a series of criss-crossing fractures filled with a very soft, clear mineral. This mineral was precipitated inside cracks in the rock by groundwater.
What mineral is this?
Hint: it’s so soft you can scratch it with your fingernail, it doesn’t fizz in hydrochloric acid, and it’s listed as having a hardness of 2 on the Mohs’ hardness scale. If you’re not familiar with the Mohs’ hardness scale, look it up in the index of your textbook, or consult p. 53 of your lab manual.
Is the age of these mineral veins younger or older than the age of the Scripps Formation? How can you tell?
Hint: if you have trouble answering this question, try looking up "cross cutting relationships" in your textbook, or consult p. 152 in your lab manual.
As you continue walking north along the base of the sea cliff, notice how “messed up” (folded, swirled, etc.) the Scripps Formation is in some spots. These irregular swirls are the result of underwater mass movement events (slides, slumps, etc.) that occurred in this rock unit as it was being formed.
TAKE A PICTURE of these ancient underwater mass movement deposits.
Now, walk a fair distance further north, about ¼ mile past the parking lot, to where the cliff makes a sharp bend to the left. Walk up to this south-facing cliff, to a spot about 25 yards to the left of the bend and observe the “chunky” rock unit closely. This rock unit belongs to the Cabrillo Formation, and consists of rounded pebbles and cobbles in a matrix of sandstone. The rock in this area is approximately 72 million years old and was deposited on an ancient, underwater submarine fan.
Name this rock type (hint: it’s a coarse-grained detrital sedimentary rock containing rounded rock fragments):____________________________________.
Estimate the average fragment size within the Cabrillo Formation (consider the pebbles and cobbles only, not the sandstone):
Is this sedimentary deposit poorly sorted (rock fragments exhibit a wide range of sizes) or well sorted (rock fragments are all about the same size)?
The rock fragments that comprise the Cabrillo Formation are generally igneous rocks, whereas the Cabrillo Formation itself is a sedimentary rock unit. Given this relationship, are the rock fragments themselves younger or older than the Cabrillo Formation? Explain.
The crystals that make up the rock fragments within the Cabrillo Formation are generally visible / invisible (indicate best choice), which means that they formed as a result of rapid / slow cooling (indicate best choice).
TAKE A PICTURE of the Cabrillo Formation.
What evidence indicates the clasts within the Cabrillo Formation have been transported some distance before being deposited?
Would current velocities have been generally fast or slow on the surface of this ancient submarine fan? How can you tell?
Okay, that’s it! Make sure you've answered all the questions on this page (shown in bold), and double-check that you've taken photos of everything. Now, turn around and head back to the parking lot (or stay awhile and enjoy the beach!).