Recession of Marine Terraces – With Special Reference to the Coastal Area North of Santa Cruz, California

The concept "wave base" (or "surf base"), i.e. the maximum depth below mean sea level at which shoaling waves will effectively erode the ocean bottom leading to the recession of a shoreline, is discussed. Also, past and present opinions as to the magnitude of wave base in general and specifically in the area near Santa Cruz, California, and the variables controlling this phenomenon are presented. Then, an account of the author's successful and unsuccessful attempts to determine average rates of cliff retreat in the study area is presented along with the specific cliff recession rates obtained. These compare favorably with the recession rates measured by the U. S. Army Corps of Engineers for nearby areas of similar geology and topography and with rates determined for similar coastal areas in various parts of the world. A brief discussion of the spectrum of cliff recession rates found in areas of varying geology and wave climate is also presented.

The accepted history of sea level since the last glacial maximum,particularly during the last 7,000 years, is reviewed as well as pertinent information on the geology, topography and wave climate of the study area.

It is then shown that average recession rates estimated by relating extrapolated bedrock profiles of the lowest marine terraces with the accepted history of the latest sea level rise compare favorably with the recently measured recession rates. However, a conflict exists between the present wave-cut terrace profiles, the accepted history of sea level and the accepted value of wave base.