Self-Guided Geologic Tour -- Torrey Pines State Beach Print this page out and bring it with you when you go!
DUE DATE: Final Exam
TO BRING: Light jacket, 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: From I-5 North, go past the I-5/805 split. Exit at Carmel Valley Road in Del Mar. Turn left onto Carmel Valley Road. Follow Carmel Valley Road all the way to the coast (about 2 miles), until you come to a “T” intersection. Turn left at the “T”, onto North Torrey Pines Road (although the sign says Camino Del Mar). Park on the right side of the road, as close as possible to the Torrey Pines Sign and Toll Booth (you can park in the Torrey Pines parking lot, but it’ll cost you 5 bucks).
Assignment: From the Torrey Pines parking lot, proceed south along the beach. As you read through this guide, you’ll be asked to 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 tour, please prepare a 3 page paper (double-spaced, 1-inch margins) of your experience that you’ll submit along with your photos. Your write-up should answer all of the questions posed below. In addition, it should include the following information: Date/time of your tour, weather conditions, brief discussion of photographs, overall impressions/thoughts about your trip.
GEOLOGY TOUR
Okay, let’s get started! From the Torrey Pines parking lot, proceed south along the beach.
1. The two rock units most visible in the sea cliff include the Torrey Sandstone (light tan--brownish sandstone) and the Del Mar Formation (greenish shale, siltstone, and sandstone). Based on direct observation, which rock unit is older, and which is younger?
Both of these rock units are Eocene in age, based on their distinct mix of fossils. At other locations around the world, rock units containing these same fossils have been dated at approximately 45.5-49 million years old, so we can be relatively sure that the rocks we’re seeing today are about the same age.
2. Based on the above paragraph, how do geologists distinguish the Torrey Sandstone from several other brownish sandstones exposed throughout the San Diego area?
3. Notice that the sedimentary rocks exposed in the sea cliff are not horizontal, but are dipping (tilted) slightly. Are the rocks dipping (tilted downward) toward the north or south?
4. Notice the wide, well-developed beach along this stretch of coastline. Name two possbiel sources for the sand on this beach (if you're not sure, think about various places where beach sand might originate):
Now, see if you can find evidence of ancient life (fossils) preserved in the Del Mar Formation…keep walking south along the base of the sea cliff until you start seeing abundant fossils in the greenish Del Mar Formation. Look for flat benches developed at the base of the cliff, and you’ll start seeing the fossils.
Please TAKE PICTURES of the various fossils you observe
Look for the following fossils:
Oysters
Snails
Many other clues in addition to fossils provide information about ancient depositional environments. Sedimentary structures are useful for this purpose, because we can observe such structures in modern depositional environments. A sedimentary structure is any feature in a sediment or sedimentary rock that develops during or after deposition.
Please TAKE PICTURES of the following features as you notice them:
Bedding (i.e., sedimentary layering—how we recognize most sedimentary rocks)
Cross beds (inclined sandstone layers indicative of ancient, underwater dunes that formed in fast-moving tidal currents…they look like this):
The downward slope of the cross beds tells us which way the ancient current was flowing. In the above picture, the current was flowing from left to right.
Liesegang Bands (convoluted layers stained reddish brown by iron-rich groundwater)
Concretions in this area have "grown" inside the Torrey Sandstone via precipitation of quartz- and/or calcite-rich groundwater...they're really cool if you can find one).
Mass movement refers to the downslope movement of soil and rock due to gravity. As you continue to walk south along the cliffs, look for evidence of recent mass movement events (mostly rock falls). TAKE A PICTURE.
Keep walking south along the cliffs until you come to Bathtub Rock. Bathtub Rock is a resistant slab of the Del Mar Formation that sticks out into the ocean. Climb onto Bathtub Rock and examine the bathtub. Legend has it that this partially sand-filled shaft was dug long ago in hopes of locating coal deposits. Do you see any marine organisms clinging to the inside of the bathtub? TAKE A PICTURE.
That’s it! Before you head back to your car, jot down some notes about what you saw and what you learned today so you can prepare your write-up.