Anza-Borrego Desert State Park

Blair Valley

Blair Valley is a valley in the State Park.[2] It consist of main Blair Valley and Little Blair Valley separated by a small mountain range over which Foot and Walker Pass leads. To the west of the valley lies Granite Mountain, to the east the range of Vallecito Mountains.

The valley can be crossed by dirt roads, e.g. to reach a look-out point over Smuggler Canyon or sites of Indian pre-hispanic art.

Some areas are habitats for the Peninsular bighorn sheep, often called desert bighorn sheep. Few park visitors see them, and the sheep avoid human contact. Observers count this endangered species to study the population, and monitor its current decline from human encroachment.[8]

Anza-Borrego rock outcrop and flora

Agua Caliente Springs and valley towards south

ABDSP lies in a unique geologic setting along the western margin of the Salton Trough. This major topographic depression with the Salton Sink having elevations of 200 ft (61 m) below sea level, forms the northernmost end of an active rift valley and a geological continental plate boundary. The trough extends north from the Gulf of California to San Gorgonio Pass, and from the eastern rim of the Peninsular Ranges eastward to the San Andreas Fault zone along the far side of the Coachella Valley.

Over the past seven million years, a relatively complete geologic record of over 20,000 ft (6,100 m) of fossil-bearing sediment has been deposited within the park along the rift valley's western margin. Paleontological remains are widespread and diverse, and are found scattered over hundreds of square miles of eroded badlands terrain extending south from the Santa Rosa Mountains into northern Baja California in Mexico.

Both marine and terrestrial environments are represented by this long and rich fossil record. Six million years ago, the ancestral Gulf of California filled the Salton Trough, extending northward past what would become the city of Palm Springs. These tropical waters supported a profusion of both large and small marine organisms. Through time, the sea gave way as an immense volume of sediment eroded during the formation of the Grand Canyon spilled into the Salton Trough. Little by little, the ancestral Colorado River built a massive river delta across the sea way. Fossil hardwoods from the deltaic sands and associated coastal plain deposits suggest the region received three times as much rainfall as now.

The Anza-Borrego region gradually changed from a predominately marine environment into a system of interrelated terrestrial habitats. North of the Colorado River Delta and intermittently fed by the river, a sequence of lakes and dry lakes has persisted for over three million years. At the same time, sediments eroded from the growing Santa Rosa Mountains and other Peninsular Ranges to spread east into the trough. These sediments provide an almost unbroken terrestrial fossil record, ending only a half million years ago. Here, the deposits of ancient streams and rivers trapped the remains of wildlife that inhabited a vast brushland savannah laced with riparian woodlands.

This record of changing environments and habitats includes over 550 types of fossil plants and animals, ranging from preserved microscopic plant pollen and algal spores to baleen whale bones and mammoth skeletons. Many of the species are extinct and some are known only from fossil remains recovered from this park. Combined with a long and complete sedimentary depositional sequence, these diverse fossil assemblages are an unparalleled paleontologic resource of international importance.

Both the Pliocene-Pleistocene epoch boundary and the Blancan-Irvingtonian North American land mammal ages boundary fall within the long geological record from the Anza-Borrego region. Environmental changes associated with these geological time divisions are probably better tracked by fossils from the Anza-Borrego region than in any other North American continental platform stratum. These changes herald the beginning of the Ice Ages, and the strata contain fossil clues to the origin and development of modern southwestern desert landscapes.

The first fossils, marine shells from the ancient Gulf of California and freshwater shells from a prehistoric era Lake Cahuilla, precursor of present-day Salton Sea, were collected and described by William Blake in 1853. Blake was the geologist and mineralogist for the Pacific Railroad Surveys commissioned by Congress and President Franklin Pierce to find a railway route to the Pacific. Blake first named this region the Colorado Desert.

Pre-Columbian era rock art in the Indian Hill archeological area.

Since the late 19th century, numerous scientific studies and published papers have centered on the marine organisms that inhabited the ancient Gulf of California. Fossil assemblages from the classic 'Imperial Formation' include calcareous nanoplankton and dinoflagellates, foraminifera, corals, polychaetes, clams, gastropods, sea urchins, sand dollars, and crabs and shrimp. The deposits also yield the remains of marine vertebrates, such as sharks and rays, bony fish, baleen whales, walruses and dugongs.

Marine environments such as outer and inner shelf, platform reef, and near shore beach and lagoon, are all represented within the "Imperial Formation". As the sea became more shallow, estuarine and brackish marine conditions prevailed, typified by thick channel deposits of oyster and pecten shell coquina that now form the "Elephant Knees" along Fish Creek. Many of the marine fossils are closely related to forms from the Caribbean Sea. They document a time before the Isthmus of Panama formed, when the warm Gulf Stream of the western Atlantic Ocean invaded eastern Pacific Ocean waters.

As North and South America connected about three million years ago, terrestrial faunal north-south migrations began on a continental scale called the Great American Interchange, and are present in Anza-Borrego's fossils. Animals such as giant ground sloths and porcupines made their first appearance in North America at this time.

The oldest terrestrial vertebrate fossils from the Colorado Desert predate the late Miocene invasion of the Gulf of California. These very rare fossils, which include a gomphothere (elephant-like mammal), rodent, felid and small camelid, and were collected from 10– to 12-million-year-old riverine and near-shore lake deposits. However, the most significant and abundant vertebrate fossils have been recovered from the latest Miocene through late-Pleistocene riverine and flood plane deposits of the Palm Spring Formation in the Vallecito and Fish Creek Badlands and Ocotillo Conglomerate exposed in the Borrego Badlands. These fossil assemblages occur in a 3.5-million-year-long, uninterrupted stratigraphic sequence that has been dated using horizons of volcanic ash and paleomagnetic methods.

The bestiary for this savannah landscape includes some of the most unusual creatures to inhabit North America – animals such as:
Geochelone, a giant bathtub-sized tortoise; Aiolornis incredibilis, the largest flying bird of the Northern Hemisphere, with 17-ft (5.2-m) wing span; Paramylodon, Megalonyx and Nothrotheriops, giant ground sloths, some with bony armor within their skin; Pewelagus, a very small rabbit; Borophagus, a hyena-like dog; Acrtodus, a giant short-faced bear; Smilodon, a saber-toothed cat; Miracinonyx, the North American cheetah; Mammuthus imperator, the largest known mammoth; Tapirus, an extinct tapir; Equus enormis and Equus scotti, two species of extinct Pleistocene horses; Gigantocamelus a giant camel; and Capromeryx minor, the dwarf pronghorn.

Morteros, bedrock mortar grinding holes, in the Indian Hill area