The expanses of Anza-Borrego Desert State Park's eroded
badlands also provide a different view into the region's long-vanished tropical past. The inland of southeastern California was not always a
desert.
Paleontology, the study of the
fossilized remains of ancient life, is the key to understanding this prehistoric world. The park has an exceptional fossil record which includes preserved plants, a variety of
invertebrate shells, animal tracks, and an array of bones and teeth. Most fossils found in the park date from six million to under a half million years in age (the
Pliocene and
Pleistocene epochs), or about 60 million years after the last
dinosaur age ended.
Geology Anza-Borrego Desert State Park 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 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 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 seaway. 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 predominantly 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 the 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.
Fossils This record of changing environments and habitats includes over 550 types of fossil plants and animals, ranging from the 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, the 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.
Marine period 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 an outer and inner shelf, platform reef, nearshore 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.
Terrestrial period 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 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 plain 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;
Arctodus, 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. Within the state park, Mammuthus remains, including two complete skulls and a partial skeleton, have been uncovered at 46 different archeological sites. Most of these sites are located in the Borrego Badlands.
Gompotherium, Stegomastodon, Mammuthus meridionalis, and
Mammuthus columbi are among the various mammoth fossils to be found in Anza-Borrego Desert State Park. A
Gompotherium jaw is on display at the University of California Museum of Paleontology, Berkeley. grinding holes, in the Indian Hill area ==Native Americans==