The San Diego Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in southwestern

San Diego County San Diego County (), officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the f ...

southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban a ...

(United States), and northwestern

Baja California Baja California (; 'Lower California'), officially the Free and Sovereign State of Baja California ( es, Estado Libre y Soberano de Baja California), is a state in Mexico. It is the northernmost and westernmost of the 32 federal entities of Mex ...

(México).

Geology

It is a coastal transitional marine and non-marine pebble and cobble conglomerate deposit and marine

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

rock with marine fossils, from a former bay, deposited during the

Middle Pliocene Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ( ...

Late Pliocene Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

ages (2–3 million years ago), of the Pliocene period during the

Cenozoic Era The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

. This formation is found from the south side of

Mount Soledad Mount Soledad, also known as Soledad Mountain, is a prominent landmark in the city of San Diego, California, United States. The mountaintop is the site of the Mount Soledad Cross, the subject of a 25-year controversy over the involvement of reli ...

Rosarito Beach Rosarito is a coastal city in Playas de Rosarito Municipality, Baja California, situated on the Pacific Coast of Mexico. As of 2010, the city had a population of 65,278. Located south of the US-Mexico border, Rosarito is a part of the greater S ...

in northern

, including

Tijuana Tijuana ( ,"Tijuana"
(US) and < ...

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

, and the southwestern corner of San Diego County from San Ysidro to Pacific Beach. San Diego Formation deposits were formed in a large, open, crescent-shaped bay similar in size to

Monterey Bay Monterey Bay is a bay of the Pacific Ocean located on the coast of the U.S. state of California, south of the San Francisco Bay Area and its major city at the south of the bay, San Jose. San Francisco itself is further north along the coast, by ...

that existed on the coast in Pliocene times.

Aquifer

The formation contains the San Diego Formation Basin, a large aquifer under Imperial Beach, Chula Vista, National City, and southern portions of the city of San Diego. The San Diego Formation Basin is a confined shallow aquifer. It has a basin ground surface area of and an estimated groundwater storage capacity of 960,000 AF. The depth to groundwater is about .Metropolitan Water District of Southern California (2007). Groundwater Assessment Study. Retrieved from http://edmsidm.mwdh2o.com/idmweb/cache/MWD%20EDMS/003697466-1.pdf The groundwater in the San Diego Formation is brackish, and its quality is considered to be fair to poor. Due to its proximity to the ocean, the risk of seawater intrusion is a primary concern regarding water quality. To avoid intrusion, the Sweetwater Authority constantly monitors the basin water levels, which have remained stable since the 1980s. A factor that contributes to limit the groundwater pumping is the importation of Colorado River water. However, to reduce demands for imported water, there have been installed desalination plants. The Richard A. Reynolds Groundwater Desalination Facility, in Chula Vista, was completed in 1999 and holds a production capacity of 4 million gallons of drinking water per day. The Claude “Bud” Lewis Carlsbad Desalination Plant, in San Diego, was completed in 2015 and produces 50 million gallons of water per day. In this way, these plants contribute to reducing vulnerability and secure water supply, especially during drought periods. The use of groundwater contributes to a regional effort to reduce demands for imported water.

Fossil content

Besides those of clams and other

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s, quite a few

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

fossils (which are generally rare) have been found in this

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

. Among them is a possible ancestor of

Cassin's auklet Cassin's auklet (''Ptychoramphus aleuticus'') is a small, chunky seabird that ranges widely in the North Pacific. It is the only species placed in the genus ''Ptychoramphus''. It nests in small burrows and because of its presence on well studied ...

(''Ptychoramphus aleuticus'') and the loons ''Gavia concinna'' and ''G. howardae''. The former was quite likely a close relative or even ancestor of the large black-headed loons; the latter was perhaps a small relative of the large grey-headed

Pacific loon The Pacific loon or Pacific diver (''Gavia pacifica''), is a medium-sized member of the loon, or diver, family. Taxonomy and etymology The Pacific loon, previously considered conspecific with the similar black-throated loon, was classified as ...

Geology

Aquifer

Fossil content

Fauna

Cetaceans

Mysticetes

Odontocetes

Pinnipeds

Sirenians

See also

References

Further reading