Up to 30 m of muds deposited in a Sangamon(?) estuary are found in boreholes drilled for the proposed Southern Crossing of central San Francisco Bay. Microfossils and plant fragments in the core samples represent deposits of shallow estuarine and deeper marine environments.

The most abundant species is Elphidium excavatum (including its variants E. selseyense, E. lidoense, and E. clavatum). It occurs with Elphidiella hannai to comprise over 70% of the total population in all samples. Bucella frigida, Elphidium gunteri, and Ammonia beccari comprise at least 15-25% of the taxa in many samples. All other species occur with frequencies of less than 2%.

Two associations are defined: (a) one in which E. excavatum comprises over 50% of the total population, and A. beccari and E. gunteri are also abundant; and (b) one in which E. excavatum is common but less abundant, and Elphidiella hannai and B. frigida are also common. The Elphidium excavatum association is found in the lower samples in each core, the Elphidiella hannai association in the upper samples.

In the present bay, E. hannai is most abundant at depths greater than about 12 m, whereas Elphidium excavatum and associated species are the most common species at shallower depths, which suggests that the Sangamon(?) estuary was of moderate depth and gradually became deeper. The presence of B. frigida, a species not found in the present bay but common in inner netritic environments of the West Coast, suggests that the later Sangamon(?) bay may have had a more open-marine aspect. Foraminifera common to shallow-bay environments are apparently not preserved in the cores.

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