Marine Terraces, Trinidad, California

[Relief map of Trinidad area] [Photo of strata exposed along bluff, Trinidad State Beach]

Most of the Trinidad coast is well above the ocean on a marine terrace bordered by rugged sea cliffs.  A flight of older, higher terraces extend a few miles inland. The relief map (above left) shows the terraces' extent, plus some other prominent features.  A few outcrops of harder rock, former sea stacks, stand out here and there.  Streams dissect the terrace surfaces.

The labelled photo (above right) shows strata exposed by cliff erosion along the bluff between College Cove and Trinidad Beach.  The dark layer at the very top is wind-blown silt and fine sand that has developed into topsoil.  The yellowish brown layers immediately below are unconsolidated, weathered beach sand and pebbles of Pleistocene age, about 80,000 years old(1, 4, 5). The elevation of the terrace near the Memorial Lighthouse is 160 feet. Thus the mean rate of uplift has been 2 feet per 1,000 years over the time period.

The dark gray rocks at the very bottom belong to the Franciscan complex.  The Franciscan complex consists of weakly metamorphosed igneous and sedimentary rocks of Mesozoic age, former ocean-floor crust, conveyed to the Pacific coast by ocean-floor spreading (3).

The jump in elevation going from present-day sea level to a marine terrace does not always take the form of an abrupt sea cliff.  The slope form varies, depending upon the makeup and strength of the underlying Franciscan bedrock.  Where the rock is weak and argillaceous, slopes are less steep but more irregular.  Weak argillites become saturated with water during the rainy season and disintegrate into a viscous slurry that oozes slowly downhill.  The terrain reflects earthflow and debris slide activity.  This terrain is most common along Scenic Drive, but it appears off-and-on all along the Trinidad coast.

Further Reading

Aalto, K. R.  1989.  Geology of Patrick's Point State Park, Humboldt County, California.  Geology 42: 125-133.  (The link is to a former College of the Redwoods class web page that referenced Aalto's scientific paper.)

Aalto, K. R.  2009.  Geology of Trinidad, California, Trinidad Museum Society, Trinidad, California.  18 p. (PDF - 22MB)

Martin, K.  2007.  Geologic and seismic characteristics of Trinidad, CA.  Background Report for the City of Trinidad Planning Commission.  Streamline Planning Consultants, Arcata, CA.  27 p. (PDF)

Padgett, J. S., Kelsey, H. M., and Lamphear, D.  2019.  Upper-plate deformation of Late Pleistocene marine terraces in the Trinidad, California, coastal area, southern Cascadia subduction zone.  Geosphere (2019) 15 (4): 1323-1341.

Speer, R. R.  1998.  Geological time machine.  University of California, Berkeley, Department of Geology.  (Speer not only defines terms, such as "Holocene, Pleistocene and Miocene".  He provides an overview of the setting and major events on earth in each geological time period.)

Lechner, H.  1998.  West of Westhaven: A walk along Trinidad Scenic Drive. Lechner describes in detail his hiking adventure and the complex of dynamic natural features.

Images by Jim Popenoe