Black Hawk Ranch Fossil Site
Map SatelliteThis is the type section of fauna of the Montediablan (Black Hawk Ranch) Provincial Mammalian Age which dates from the Clarendonian of the Lower Pliocene. Plant and marine invertebrate fossils from the same epoch have been recovered from the upturned strata here.
Among the smaller mammals represented are a rabbit, Hypolagus sp., a muskrat-size beaver, Eucastor lecontei, and a primitive ground squirrel of the extant genus Citellus. Carnivores include representatives of three extant genera: two foxes, Vulpes vafer and Urocyon sp., and a relative of the ringtail cat, Bassariscus parvus. Several premolars of a raccoon-like animal, probably related to the Central American coatimundi, Nasua, whose ancestry is unknown, have been found here. Extinct carnivores include a large cougar-like cat, Pseudaelurus thinabatse, and probably sabre-toothed cats, Machairodontidae.
Fossils of the mastodon Gomphotherium simpsoni are plentiful. Three horses are known from this site, Hipparion sp., Hipparion forcei, a mule-deer size horse with three toes, and Pliohippus cf. leardi, a larger animal without the side toes. Other remains include a peccary, Prosthennops, an oreodont, Ustatochoerus, and three camels, Procamelus, Pliauchenia and Paracamelus, the latter being much larger and taller than those surviving today in the Old World. An ancestor of the pronghorn, the small 0.6-meter (2-foot) high antilocaprid Merycodus, was quite common. Other vertebrate fossils include those of a crane, Grus conferta, and a lizard.
The flora includes abundant impressions of leaves of poplars, Populus, willow, Salix, elm, Ulmus, and sycamore, Platanus, all trees common to the borders of streams. Leaves of oak, Quercus, sumac, Rhus, and mountain mahogany, Cercocarpus, are, not unexpectedly, less common as these plants grow further from the streams where the chance of the leaves being embedded in the sediments is considerably lessened. The stream-side trees were common in all of Western America flora during the Miocene, and two, the willow and the sycamore, have descendants living in the area today. The elm is not found in California naturally now, but grows well under cultivation. Comparisons of the flora with others give evidence of a gradual trend towards less rainfall in the area during the early Pliocene.
The site itself was on the edge of the saltwater basin that extended inland to the Sierra. In the vicinity were marshes and higher grasslands as well as forested areas.
Integrity: Development includes orchards, grazing lands, jeep roads, ranch and residential buildings and a bulldozed fossil quarry. Further development of the area has been proposed but portions, including the scientifically valuable areas, may be incorporated in the adjacent Mount Diablo State Park.
Use: Private
Ref: Stirton, R. A. 1951. Prehistoric Land Animals of the San Francisco Bay Region, in Geologic Guidebook to the San Francisco Bay Counties. Calif. Div. Mines Bull. 154, pp. 177-186.
Axelrod, D. I. 1944. Pliocene Floras of California and Oregon: The Black Hawk Ranch Flora. Carnegie Inst. Pub. 553, pp. 91-101.
October 1975
Inventory of California Natural Areas
Revision © 2009 Steven Louis Hartman