Los Trancos

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Los Trancos Open Space                            

San Andreas fault walk:

 The purpose of this field trip is to introduce the student to landforms created by the movement of the San Andreas and associated strike-slip faults. The one to two hour walk takes the student through the rolling hills scarred with landslides and covered with oaks and grasslands typical of the Coast Ranges.

  1. Complete the preliminary questions before attending the field trip.
  2. Answer the appropriate questions
  3. Answer one of the three questions in essay form. Attach the typed response to completed packet.

 HOW TO GET THERE:

     The preserve's entrance is on Page Mill Road, 7 miles west of Highway 280 and 1 mile east of Skyline Boulevard. Parking is available for 20 vehicles. Additional parking and restroom facilities are available at the Monte Bello Open Space Preserve parking area, directly across Page Mill Road.

          From San Jose State University, take 4th St. to 280 north. Travel north on 280 until Page Mill Road in Palo Alto. Take Page Mill Road exit, stop at the stop sign, turn left or west. Follow this road to Los Trancos open space. Allow 30-40 minutes for travel time from 280 to the open space due to road conditions.

 Answer these questions before going on the trip.

 The San Andreas fault runs roughly parallel and east of the park. This fault represents a transform boundary. We will examine surface features formed in association of a strike-slip fault.  The website that contains a description of the walk can be found at: http://www.openspace.org/TRANCOS.html or http://pubs.usgs.gov/gip/earthq3/ , a USGS website.

 1.       The San Andreas fault is a __________________ plate boundary. It  separates the ______________ plate and the ______________ plate. This fault is classified as a ________________ lateral strike slip fault which means that the relative movement is to the ______________. The fault is ________ miles or __________ kilometers long.

2.     Draw a diagram of California. Include the San Andreas fault.

 3.     Santa Cruz, Monterey, and Half Moon Bay are on the ____________ plate.

 4.     San Jose, San Francisco, and Berkeley are on the ____________plate.

 5.     There are variations in the straightness of the San Andreas fault. This produces different landforms. Terms are defined in the glossary: http://earthquake.usgs.gov/image_glossary/ .

  1. Define the following terms (use a diagram to help with the explanation):

Offset:

 

 Offset streams near Watsonville, California.

Offset of streams:

 

 

Linear valleys:

 

 Crystal Springs Reservoir formed by the San Andreas fault.

 

Shutter ridges:

 

 

 

Springs:

 

 

 

  1. If the fault has a right bend tensional stress forms depressions (explanation found in book):

Draw this configuration

 

 

 

 

 

 

Sag ponds are often found in this setting:

  1. If the fault has a left bend, compressional stress forms uplift (information found in book).

How were the Santa Cruz Mountains formed?

 

6.     Explain the significance of the rocks we viewed on the first stop.

 

  

7.     What major topographical features do we see that show the trace of the San Andreas fault?

 

 

to the road conditions.

         

Answer the following questions on the field trip:

Stop 1:

          What is the name of the tectonic plate you are standing on?

 

Stop 1 continued:

If you are looking at Loma Prieta Mountain, you are facing what compass direction?

 

The sedimentary rock that contains pebbles from Loma Prieta is called a conglomerate. Define this term:

If the rocks are 2 million years old and traveled 23 miles, what is the calculated rate of movement?

  

What did you learn from this stop?

 

 

Stop 2:

The topography of the Bay Area is determined by rock types, fault movement and weathering processes. Look toward the north (toward San Francisco) and east. Describe two landforms you see.

 

 

 

 

Fault zones produced crushed rock or fault gouge. This material is weathered more easily than the unaltered rock. Crystal Springs reservoir is the body of water in the depression formed by this weathered rock.

 

Sketch a map of the Bay Area from where you are standing. Where are the highlands and the lowlands? Now look at the map of the Bay Area, guess where the faults are and mark them on your sketch.

 

 Major faults of the Bay Area.

 

 

 

 

Stop 3:

          Surface rupture is considered an earthquake hazard. Why?

 

 

Stop 4:

The displacement or offset of the fence is due to the movement on the fault. If one stands by one section of the fence and looks across the offset, the relative movement is in which direction? Right  Left

 Stop 5:

 Describe in your own words the definition of a spring.

 

  

 

If one comes across a spring in the Coast Ranges, what could one assume?

 

Stop 6:

Take a look around. Here the vegetation is somewhat different than what is found in the surrounding area. Notice the depression.

Stop 7:

          What do you think of this information? Do you believe it? Why or why not?

 

 

 

Stop 8:

          Do you understand this explanation?

 

          What is a contour?

 

 

Stop 9:

 The field trip is complete. Have a safe walk back to your car and a safe ride home.

 

 

 

This page was last modified by P. Jefferis on 08/22/05 . She is a lecturer in the Department of Geology at San Jose State University