Recent excavations in the downtown area of San Diego have exposed fossiliferous marine sands of middle and late Pleistocene age. Molluscan assemblages recovered from these sands can be grouped into two distinct faunas referred to here as the Broadway fauna (middle Pleistocene) and the "E" Street fauna (late Pleistocene).

Amino-acid racemization age estimates by K. R. Lajoie on shells of Chione from these faunas are 560,000 ± 75,000 years B.P. and 250,000 years B.P., respectively. Both faunas possess a decidedly warm-water aspect and reflect protected littoral to sublittoral environments. The Broadway fauna contains several local biostratigraphic index genera including Turritella gonostoma Valenciennes, Argopecten abietis abbotti (Hertlein and Grant), and Pecten vogdesi (Arnold), that are not present in the younger "E" Street fauna.

Historically, all marine Pleistocene deposits in the San Diego area have been referred to as the Bay Point Formation. New evidence suggests that temporally, faunistically, and geologically distant units can be recognized within the local Pleistocene section.

The deposits containing the Broadway fauna and the "E" Street fauna occur in low-lying areas at or near sea level and appear to have been deposited in an earlier formed topographic depression. This is in marked contrast to other, younger Pleistocene marine deposits in the San Diego area which occur as thin veneers on elevated marine abrasion platforms (i.e., the Nestor Terrace and the Bird Rock Terrace).

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