The formation preserves
fossils dating back to the Late/Upper Miocene epoch of the
Neogene period. Common fossils in the area include
Spisula,
Calyptraeidae,
Clinocardium, and
Simomactra falcata. The presence of shell beds and
mollusk fossils suggests the area was once under a shallow bay, with relatively uniform levels of salinity and sand porosity. The shell beds form a conglomerate in a calcerous sandstone matrix; today, this conglomerate is a major component of notable geological features in the Bay Area, including
Mission Peak. The base of the formation includes
sandstone and
siltstone in distinct, parallel layers. The presence of rocks of the Briones formation near the surface creates conditions conducive for the development of
chaparral and
grasslands. ==See also==