San Diego Formation facts for kids

The San Diego Formation is a special layer of rock found in the ground. It's located in southwestern San Diego County in southern California, USA, and also in northwestern Baja California, Mexico. This rock layer tells us a lot about what the area was like millions of years ago.

What is the San Diego Formation?

This rock formation is made up of two main types of rock: conglomerate and sandstone. Conglomerate is like natural concrete, made of pebbles and larger rounded stones cemented together. Sandstone is made from sand grains. These rocks formed in a big bay that was once on the coast.

The San Diego Formation was created about 2 to 3 million years ago. This time is known as the Pliocene period, which was part of the much longer Cenozoic Era. The Cenozoic Era is often called the "Age of Mammals."

You can find parts of the San Diego Formation from the south side of Mount Soledad in San Diego County all the way to Rosarito Beach in northern Baja California. This includes cities like Tijuana, Mexico, and parts of San Diego like San Ysidro and Pacific Beach.

Scientists believe these rocks formed in a large, curved bay. This ancient bay was similar in size to today's Monterey Bay in California.

Water Underground: The Aquifer

The San Diego Formation holds a huge natural underground water storage area called an aquifer. This aquifer, known as the San Diego Formation Basin, is located under cities like Imperial Beach, Chula Vista, National City, and parts of San Diego.

An aquifer is like a giant sponge made of rock and soil that holds water underground. This particular aquifer covers a large area and can store a lot of water. The water here is about 100 feet (30 meters) below the surface.

The water in this aquifer is a bit salty, which is called "brackish." Because it's close to the ocean, there's a risk of seawater getting into the aquifer. To prevent this, local water authorities carefully watch the water levels.

To help provide fresh water and reduce the need for water brought in from other places, special plants have been built. These are called desalination plants. They take salty water and remove the salt to make it safe to drink.

• The Richard A. Reynolds Groundwater Desalination Facility in Chula Vista started in 1999. It can produce 4 million gallons of drinking water every day.
• The Claude “Bud” Lewis Carlsbad Desalination Plant in San Diego opened in 2015. It's even bigger, making 50 million gallons of water daily.

These plants are very important. They help make sure there's enough water, especially when there isn't much rain.

Ancient Life: Fossil Content

The San Diego Formation is famous for its fossils. Fossils are the preserved remains or traces of ancient life. Many fossils of clams and other mollusks (like snails and oysters) have been found here.

What's really cool is that many bird fossils have also been discovered. Bird fossils are quite rare to find! Some of the bird fossils found include:

• A possible ancestor of today's Cassin's auklet, a small seabird.
• Two types of ancient loons, Gavia concinna and G. howardae. One was likely related to modern large black-headed loons, and the other might have been a smaller relative of the large grey-headed loons.

Ancient Animals: Fauna

The San Diego Formation has also given us clues about the larger animals that lived in this ancient bay. Many fossils of marine mammals have been found, showing us what the ocean was like millions of years ago.

Whales and Dolphins (Cetaceans)

Fossils of different types of whales and dolphins have been discovered.

• Baleen Whales (Mysticetes): These are whales that have baleen plates instead of teeth to filter food from the water. Fossils include an extinct type of rorqual (a group that includes modern humpback and blue whales) and a relative of the humpback whale.
• Toothed Whales (Odontocetes): These whales and dolphins have teeth. Fossils found here include a close relative of the Chinese river dolphin (called Parapontoporia sternbergi). Another interesting find is Semirostrum ceruttii, a type of porpoise that had a lower jaw extending a few inches beyond its upper jaw, similar to a halfbeak fish!

Seals and Walruses (Pinnipeds)

Fossils of Pinnipeds, which include seals, sea lions, and walruses, have also been found.

• One fossil is a relative of the northern fur seal (called Callorhinus gilmorei).
• Two types of ancient walruses have been discovered: Dusignathus seftoni and Valenictus chulavistensis.

Sea Cows (Sirenians)

Fossils of sirenians, also known as sea cows, have been found. These gentle, plant-eating marine mammals include manatees and dugongs today. An ancient relative of the giant, extinct Steller's sea cow (named Hydrodamalis cuestae) was found in the San Diego Formation.