A pre–late Pliocene mega-landslide was discovered in southwest San Diego County during grading of several residential subdivisions. Outcrop evidence indicates that a single lateral spread occurring on a single continuous, bentonitic clay bed and encompassing approximately 125 km2 occurred between 1.5 and 28 Ma. Steeply dipping shears separating lateral spread blocks mimic the structure in the nearby La Nacion fault zone. Future studies of faults in areas suspected to be within the lateral spread will need to differentiate tectonic faulting from lateral spread deformation. Investigations should include deep borings to determine if bentonite clay beds exhibit evidence of vertical offsets as a result of normal faulting. In addition to a well-developed basal rupture surface, the lateral spread debris contains discontinuous and unpredictably-oriented internal shears and fractures within large blocks of debris that will affect slope stability. Because much of the structure cannot be evaluated completely by subsurface investigations, in-grading mapping and engineering analysis are necessary to provide appropriate recommendations for stabilization of site-specific slopes.

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