Moss Beach Field Trip Guide

By Dylan Duverge, Jake Kardon and Alissia Austell



Figure 1: Moss Beach Syncline (ES 150 – Winter 2004 Website)


1. Introduction


Moss Beach is a stretch of coastline north of Half Moon Bay Airport that belongs to the Fitzgerald Marine Reserve.  The outcrop in firgure1 shows an example of one of the interesting geologic features we will encounter on this field trip (can anyone guess?).  Be prepared for all possible conditions, including high tides, wind, rain or blazing sun.  You should know what to bring toprepare.  If not, you deserve your sunburn or your cold.


According to Christie, our objectives are four fold:


1.     To observe the interaction of faults and folds, apparent structures developed when deposition happens concurrent with fault activity,

  1. To describe the sedimentary and igneous rocks affected by the fault and remark on the effects of deformation and resultant fault rocks. We will use the depositional structures in the sedimentary rocks to assist us in tracking deformation.
  2. To create a thorough and meaningful field map with sufficient, accurately placed strike & dip data and qualitative information to reconstruct the structures in the lab.
  3. To relate fold and rotational structures observed in the field to long-term evolution of the San Gregorio Fault Zone.


Be sure to bring all your geology things, including your brunton, notebook, handlens, rulers, pencils and measuring tape.  Again if you don’t bring food and water, you deserve to starve.  And don’t go mooching on everyone either.  That’s my job.


Figure 2: Location of Moss Beach (Friends of Fitzgerald Marine Reserve Website)

2. Geologic History


From the late Mesozoic to early Cenozoic California lay along a convergent plate boundary in which the Farallon Plate was subducting below the North American plate. At about 28 Ma the ocean ridge began to subduct and the North American Plate began a more northerly trajectory resulting in the formation of the Right Lateral Strike Slip Fault we know of today as the San Andreas Fault (SAF) (USGS FTG 2001). This slip along the SAF then transferred the Salinian Block up the coastline from Southern California.


Above the Salinian basement rock is the Miocene Montara Granodiorite, which is in turn overlain by the Pliocene Purisima Formation (Stanford Field Guide). These marine deposits also show the effect sea level had in the area (as well as leaving behind some cool fossils) since most of the San Francisco Bay area was to some degree covered in water at one time or another (Fitzgerald Marine Preserve Paper). At the last glacial maximum (20ka) sea level was an estimated 130m lower, accompanied by a coastline as much as 35km west of today’s (USGS FTG 2001). Starting about 18ka the ocean slowly began to rebound, and world sea level rose, exposing the area to erosion and other weathering processes (USGS FTG 2001).


3. Lithologies


Moss Beach contains three main formations.  The upper part of the Pliocene Purisima Fm. is the dominant lithology on the beach.  It is underlain by Cretaceous Montara grandiorite, which crops out to the north, and is overlain by Quaternary terrace deposits.  The grandiorite outcrop belongs to the Salinian block and is interpreted as an ancient granitic highland that was sub-aerially exposed. 


In general the Purisima fm. contains fine to very coarse-grained clastic rocks that were deposited in a shallow-marine environment.  There are three facies exposed that reflect various depositional environments:


Facies 1: This is the shallowest of facies and consists of alternating shell/pebble conglomerate beds with massive, hummoky and cross-stratified sandstone beds.  Unlike the others, this facies does not have very large clasts.


Facies 2: This facies is interpreted as sediment gravity flows that were deposited in a deeper shelf environment.  It doesn’t contain the massive sandstone of facies 1 but tends to contain more sedimentary and granitic clasts.


Facies 3:  This facies contains granule conglomerate as well as massive sandstone with interbeds of granule conglomerate.  Ganitic clasts are pervasive in some sections.  In some places, clasts are jimongous.


Source: Wiley and Moore, 1983, Pliocene shallow-water sediment gravity flows at Moss Beach, San Mateo County, California in SEPM Pacific Section, Cenozoic Marine Sedimentation, Pacific Margin, USA, p. 29-43


The Pleistocene terrace fluvial and reworked fluvial deposits range from crudely bedded clast supported deposits with sandy matrixes to brown dense and gravely to clayey sand; with a range of sorting quality (thanks Pescadero group).


4. Active Tectonics


Over the last 28 million years, the transform motion between the Pacific Plate and the North American Plate has made the San Andreas Fault System and many other right lateral strike slip faults, including the San Gregorio Fault.  These faults have moved the Salinian Block north along with the Pacific Plate, relative to the North American Plate.


The San Gregorio Fault zone is the western boundary of The Salinian Block in this area.  This fault system has experienced an estimated 115-km of displacement, and has had motion in the Holocene with an estimated slip rate between 0 and 12mm/yr (Pescadero FTG).  The strand of the SGF that cuts through Moss Beach is the Seal Cove Fault, which puts the Cretaceous Granodiorite of the Salinian Block next to the sedimentary Perisima formation (Stanford FTG 2001).


The damage zone of the faults at Moss Beach can be observed at the coast where they dive into the ocean trending northwest. The pattern of the damage zone, from gouge, to brecca, to damaged rock is visible and discernable.  See Figure 3 on Casey’s website (


Faults are not the only point of interest at moss beach, there is also a Northwest trending syncline in the Perisima Formation just to the north of the Fault damage zone.


Figure 3: Photograph of the folded clay-rich fault gouge in the core of the Seal Cove Strand of the San Gregorio Fault, Moss Beach, California. (Geology website at University of New Mexico)


8. Vocabulary


Facies (n.): The sum total of primary features such as sedimentary rock type, mineral content, sedimentary structures, bedding characteristics, fossil content, etc. which characterise a rock as having been formed in a given environment, esp. distinguishing the rock from adjacent or associated units.


Syncline (n.): A concave up fold which generally has the youngest rock unit at its core.


Jimongous (adj.): A mixture of the words “humungous” and “giant” meaning big ass ____(fill in blank here). Ex: “Dude! That is a jimongous sandwich!” “Why yes, dude. Yes, it is a big ass sandwich.”


Internet Sources - Last year’s  ES 150 website - For the location map. - Fold picture


Print Sources


Anderson et. Al. (2001) USGS, Geology and Natural History of the San Francisco Bay Area: A 2001 NAGT Field-Trip Guidebook.


Leech, M. (2001) Stanford Univerity, Geology of the Marin Headlands and Half Moon Bay Coast, GEO 116 Field Trip Guide


Wiley and Moore, 1983, Pliocene shallow-water sediment gravity flows at Moss Beach, San Mateo County, California in SEPM Pacific Section, Cenozoic Marine Sedimentation, Pacific Margin, USA, p. 29-43  **** BEST SOURCE EVER *****


Wright, T.L, Greene, H.G, Hicks, K.R, Weber, G.E (1990) AAPG June 1990 Field Trip Road Log: Coastal Geology—San Francisco to Monterey.  Used for depositional history in intro.