Clearly defined and persistent coastal terraces characterize the Pacific slope topography of northern Baja California. The post-Miocene stratal record, generally expressed as thin terrace cappings of shallow marine deposits, occurs on the lowest 2 or 3 terraces at elevations of approximately 200 m. or less. Thicker stratigraphic sections, including slightly deeper marine deposits, occur just north of the international boundary in the San Diego embayment and, presumably, offshore. Terrace deposits, as well as Late Cretaceous and younger strata into which the terraces were cut, appear to be undisturbed or exhibit only minor and local deformation.

Numerous littoral and inner sub-littoral communities can be recognized in post-Miocene faunules of the terrace deposits. Those on the oldest (highest) terraces are repeated, with modifications, on younger (lower) terraces. Discrepant associations of species and genera, with latitude and bathymetric ranges which are not known to overlap in the modern eastern Pacific, characterize the older post-Miocene faunules. Occurrences of species and genera associated in the younger faunules are more nearly compatible with extant associations along the adjacent coast.

Critical evaluations of field and taxonomic relationships suggest that depositional contamination and misleading comparison are responsible for seemingly discrepant fossil associations. Other anomalies may reflect post-Miocene distributional shifts imposed by factors other than those of the physical environment.

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