06 de junio de 2018

MROSD Earthquake Hike

The Mid-Peninsula Region Open Space District holds an earthquake hike on the first Sunday of every month at the Los Trancos Open Space Preserve. Attending one of these and reviewing it is mandatory for all future MROSD Docents so on June 3, 2018, all but one person on this hike was a docent, future docent or spouse. The leader was John Seyfarth and Strether Smith was the sweep.


Because almost all of the audience was future docents, a lot of attention was placed on how to lead a nature hike.

  1. Introduce the docents and their history with MROSD and geology. Neither of the docents was trained as a geologist.
  2. Talk about MROSD, its organization and the history of Los Trancos and Montebello Open Space Preserve. (Get the flyer from MROSD office.) Started in 1972 with 26 preserves to save open space in the SF Peninsula. Funded by taxpayer bonds except the Coastal Annexation which has representation without taxation.

Monte Bello has an interesting history because "The Land" commune was setup near the fault along Canyon Trail and MROSD kicked them off at 6am one morning without warning and removed all their structures. Since we were exploring the fault, we also looked for remnant of their structures. For more on this commune and their 2008 reunion, http://theland.wikispaces.org . Lone Oak Hill was used for the commune's celebrations and parties and also the Mid-Peninsula Open Space Logo.

  1. Discuss hazards on this hike.
    a. Not flat and about 2 miles plus it was in the 90's on a sunny day. Participants need basic conditioning and proper clothes.

    b. Lots of ticks and we found a blade of grass with two deer ticks to show everyone.

    c. Snakes. In particular, a large rattlesnake living in the culvert under the trail to "rattlesnake viewpoint." It wasn't out but we went looking for it.
    d. Poison Oak and lots of it.

  2. Expectations:
    a. This is a nature, education hike and won't be going very fast or far.

    b. It's hot and dry.
    c. Survey the participants by making them raise 1-5 fingers based on expertise. One professional geologist and one whose undergraduate degree in geophysics.

  3. Basic introduction to Plate Tectonics. As the participants introduced themselves, each had to pick the plate on which they were born. Almost all came from the N. American plate. Af ter discussion about plates, Strether showed the main faults in the SF Bay Area and described the fault zones, not lines, above each one.

  4. Additional Resources. Strether showed three books (some of which are out of print and now expensive.) "Field Guide to the San Andreas Fault," "Geology of the SF Bay Area," and "The South Skyline Story."

  5. We walked up to "rattlesnake point" after crossing the culvert and admired the many wildflowers still blooming. At rattlesnake point, Strether pointed out the conglomerate rocks which were from Loma Prieta Peak, 23 miles to the South on the North American Plate. We were on the North moving Pacific plate. https://www.inaturalist.org/observations/13104738

Also from this point we could see Black Mountain which is either the largest or the third largest piece of Limestone known. Cinnabar Winery built huge caves in the limestone to store it's wine. Ridge Winery is also just past Black Mountain.

Black Mountain itself is still growing because of an S Curve in the San Andreas Fault. Strether took out a Raisin Bran Cereal box back with an S Curve and showed how the north moving Pacific Plate was compressing the area south of Black Mountain and forcing it up.

  1. We stopped to look at Coyote Scat at Rattlesnake, specifically the little bones and hair. John asked the audience "why do coyotes poop on the trail?" a. First, as the apex predator, coyotes use the trails because it's easier to walk on, while their prey stay to the grass and trees. b. To mark the territory.
  2. From Rattlesnake Point we crossed Page Mill Road to the entrance of the Monte Bello Preserve where several boulders are placed near the map kiosk. John told us to be sure to warn participants about these boulders because while standing around them many have tripped on the rocks. He asked us to identify the three types of rocks found here, metamorphic, igneous and sedimentary. We picked out the Loma Prieta Conglomerate but missed on the green rock which we took to be Greenstone. But it was a large piece of Serpentinite, the California semi-precious State Rock. We completely missed the crushed granite covering the parking lot, the igneous example.
  3. We stopped at the bench and junction for the Steven's Creek Nature Trail to talk about the view South toward Mt. Umunhum and crowd control. Strether pointed out that stops need to be planned where there's enough space for everyone to hear and participate and, possibly, get off the trail when bikers, hikers and horses pass.

I've taken this view so many times that I didn't take it this day. However, you can see both the San Andreas and the Pilarcitos Fault zones. The Pilarcitos Fault runs along the Stevens Creek headwaters to Half Moon Bay and it used to mark the boundary of the Pacific and North American Plates but for some reason that boundary moved inland to the San Andreas Fault which follows Canyon Trail through Portola Valley and Woodside to Crystal Springs Reservoir now. https://pubs.usgs.gov/of/2005/1127/chapter8.pdf

  1. We stopped along the trail toward Canyon Trail to look for fossils in the pebbles didn't find any and we examined the remnants of past human settlements. The black walnut grove was planted by a homesteader who gave up after the 1906 earthquake.

  2. On the Canyon Trail, Strether used the same Raisin Bran Box to show how the S Curve creates Sag ponds as the plates shift past each other. We looked at the one on the north side of Black Mountain and talked about the pond succession we witnessed in our lifetimes. Frances, a retired Ranger, described how when the City of Palo Alto flushes their water tank, the debris flows into this Sag pond and is filling it up.

While walking along the the Canyon Trail, we saw tar weed, salsify and gold back ferns and lots of grasses.

  1. As we walked up the Canyon Trail toward Page Mill Road, much of the discussion turned to the location of "The Land" commune buildings. There two main buildings with running water were quite near Page Mill on a flat section on the other side of an old Sag pond. After MROSD kicked the commune off Monte Bello, some of them moved to Long Ridge Preserve near the now Zen Retreat. Apparently, the Zen Retreat was originally a school but wasn't successful.
  2. After crossing Page Mill Rd back to the Los Trancos Earthquake Trail, we found some wild strawberries.
    And we talked about why one particular trail was now closed due to infestation of Sudden Oak Death in the trees around it. Much of the discussion came from Frances, the retired Ranger, who knew about early spraying efforts to stop the spread of Sudden Oak Death and the futility of trying to clean off our shoes to prevent its spread. Sudden Oak Death is caused by an old world phthiraptera water mold and is spread by the wind during foggy days. It affects more than just oaks and it's likely it can't be stopped.

15, At a wide trail junction, Strether used a slinky attached to a board with a rattle to demonstrate P- and S-waves and their speed and damage they cause. By dragging the board on the trail, he demonstrated the stickiness and unpredictability of sliding motion.

Finally we talked about dating the 1906 quake from new growth from fallen trees. A tree topples over but doesn't die and new branches grow vertically out of the fallen trunk.

Ingresado el 06 de junio de 2018 por wooac wooac | 15 observaciones | 0 comentarios | Deja un comentario