A "toilet bowl" along the coast near Swift Street. Photo © Mobile Ranger. — A “toilet bowl” along West Cliff Drive just east of Swift Street. Photo © Mobile Ranger.

Along Santa cruz’s popular West Cliff Drive are some really unique features known by geo-geeks as “toilet bowls”. They are a clue to what the modern-day surf paradise was like 5-10 million years ago.

To set the stage in your mind, think about ancient or paleo-oceans. Imagine yourself lying down on soft sediments, under several hundred feet of calm water. Fine clay particles and many billions of tiny single-celled, silica-rich, organisms (diatoms) slowly drift downward and gently settle on top of you. Occasionally you are tickled by cold methane gas bubbles that rise out of the sea floor. This was the calm, deep water environment along West Cliff Drive when these gray elongated blobs formed from calcium carbonate that precipitated when the methane gas vented from the sea floor into the ocean. The same type of calcium carbonate deposits are being formed right now at the bottom of Monterey Bay.

Santa Cruz Mudstone in front of Long Marine lab. A great example of "pocket beaches". Photo © Mobile Ranger. — Santa Cruz Mudstone in front of Long Marine lab. A great example of “pocket beaches”. Photo © Mobile Ranger.

During the same time, the Santa Cruz Mudstone formed due to the settling of the fine grained silts, clays and billions of diatoms. Santa Cruz Mudstone is the rock type that shapes the landscape characteristics of the northern Santa Cruz coast from the north county line through to Mitchell Cove (the stinky beach with the big cement staircase). It is a fairly hard rock type, though its many layers vary in their hardness and thus erodibility. If it were a much more erodible rock, or did not have layers, the landscape we see today would be very different. It might be without the dramatic sea cliffs, famous arches and our beautiful pocket beaches.

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Sources Used
- Anatomy and origin of carbonate structures in a Miocene cold-seep field. Ivano W. Aiello, Robert E. Garrison, J. Casey Moore, Miriam Kastner, Debra S. Stakes. Geology. December 1, 2001;29(12). geology.gsapubs.org/cgi/content/abstract/29/12/1111.
- Natural Bridges Field Trip Guide. Christie Rowe. University of California Earth Sciences Webpage. www.es.ucsc.edu/~crowe/structure/natbridges.html.
- Our Ocean Backyard: Ancient mud, diatoms and whales. Gary Griggs. Santa Cruz Sentinel. February 27, 2010. www.santacruzsentinel.com/localstories/ci_14482797.
- Paleosurf: The Ancient Beaches of Santa Cruz. Christie Rowe, Catherine Riihimaki. Hellatite Gallery Website. http://es.ucsc.edu/~geoclub/paleosurfpics.html.
- Principles of Sedimentology and Stratigraphy. Sam Boggs Jr. Columbus, OH: Merrill Publishing Company; 1987.