The Barstow Formation, a deposit of alluvial-fan, fluvial, and lacustrine sediments 3,280 to 3,940 ft (1,000 to 1,200 m) thick, was deposited in the Barstow basin during Neogene time. The formation is well exposed in the Barstow syncline about 9 mi (14 km) north of Barstow. The palynomorphs of the Barstow basin were probably deposited in a wide, but relatively small, body or bodies of fresh to slightly saline water with fluctuating shorelines. The basin formed on the northern extremity of the Barstow-Bristol trough.
The most abundant palynomorphs found in the Barstow Formation belong to Asteraceae, Chenopodiaceae, Ephedra, Eriogonum, Pinus, and Quercus. Also present in lesser abundances are specimens belonging to Onagraceae, Ericaceae, Juglans, Alnus, Carya, Arbutus, Typha, and Platanus. The palynomorph assemblage is dominated by a lowland shrub community in association with an upland community of oak and pine. This association is indicative of a dry summer climate similar to, but probably wetter than, the present climate.
A middle to late Miocene age has been assigned to a vertebrate mammalian fauna near the sample locality, and radiometric dates of 13.3 to 15.9 m.y.B.P. have been obtained from local tuff beds.
The large relative abundances of Ambrosia, as well as the presence of Artemisia, indicate an age of early Pliocene for the Barstow Formation, based on present palynologic information.