page_title: Anza-Borrego Desert State Park
section_title: Terrestrial period
hierarchical_section_title: Anza-Borrego Desert State Park / Geology and paleontology / Terrestrial period
caption_attribution_description: English: Photo of morteros or grinding holes in the Indian Hill area of Anza-Borrego State Park in southern California.
context_page_description: Anza-Borrego Desert State Park is a California State Park located within the Colorado Desert of southern California, United States. The park takes its name from 18th century Spanish explorer Juan Bautista de Anza and borrego, a Spanish word for sheep. With 600,000 acres that includes one-fifth of San Diego County, it is the largest state park in California.
  The park occupies eastern San Diego County and reaches into Imperial and Riverside counties, enveloping two communities: Borrego Springs, which is home to the park's headquarters, and Shelter Valley.
context_section_description: 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.
caption: Morteros, bedrock mortar grinding holes, in the Indian Hill area
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