The San Diego Formation is a mostly sand, with rock layer from a former bay of San Diego of Late Pliocene age.
Besides those of clams and other mollusks, quite a few bird fossils (which are generally rare) have been found in this geological formation. Among them is a possible ancestor of Cassin's Auklet (Ptychoramphus aleuticus) and the loons Gavia concinna and G. howardae. The former was quite likely a close relative or even ancestor of the large black-headed[1] loons; the latter was perhaps a small relative of the large grey-headed[2] loons[3].