The Imperial Valley is an area rich in its geology. From its hard volcanic rock, to its rare earth minerals and even gold, it’s no wonder that the Imperial Valley is known by geologists as one of the most geological diverse areas of the American Southwest. For millennia, these resources have been mined by humans to help them adapt, survive, and thrive in an otherwise harsh landscape. From the area’s earliest indigenous peoples, to early pioneers and prospectors, to modern industry, the geology of the Imperial Valley continues to fuel its people, history, and future.


Prehistoric mining

Imperial County was once home to multiple tribes, including the Kumeyaay, Quechan, Cahuilla, Cocopah and Pai Pai peoples. Today, evidence of that early activity is visible across the region, especially in their worked stone tools. While some of the raw materials for these tools are abundant right off the ground, others could only be taken from within it through early mining of natural quarries.

Two such resources local to Imperial Valley are obsidian and wonderstone (rhyolite). Both stones find their origins with the volcanic activity of the region’s own Salton Buttes volcano, which last erupted nearly 3,000 years ago! Though that eruption was then under the waters of the ancient Lake Cahuilla, the eventual loss of the lake brought these treasures to light.

Today, it’s still possible to see the evidence of the early excavation of these stones. Otherwise known as volcanic glass, obsidian is very fragile and breaks with a razor’s edge, making it useful for knives and projectile points. The slightly harder and less fragile rhyolite was often also sharpened to a cutting edge and used in the likes of hand axes. Today, obsidian cores from the Salton Buttes volcano are found as far south as Baja California, demonstrating the value and importance of these local stones!


Historic mining

Mining the Imperial Valley did not change drastically until the 1800s, when it become one of the last heavily mined areas during California’s Gold Rush. Successful mines were eventually purchased by large mining companies and continue today, while the rest were eventually abandoned.

One the last surviving “ghost” mining towns in California is Tumco, in eastern Imperial County in the Cargo Muchacho Mountains. Gold production at the company town boomed quickly, with a rapid swell in the town’s population. Miners there would use hydraulic stamp presses to crush the gold-rich ore and cyanide baths to separate it from the pulverized material. At one point, the town mined nearly $1,000 in gold a day! Sadly, Tumco soon began to face diminishing returns and went bankrupt after a series of bad investments. Today, the remains of Tumco town are preserved by the Bureau of Land Management and are open to the public for hikes and visits.


Current mining practices

Today, Imperial Valley is still served by a number of active mines. Mesquite Mine, also in the Cargo Muchacho Mountains, is actually one of the largest gold mines in the United States. Since 1985, this mine has produced more than 4 million ounces of gold (over $6 trillion in today’s market)! In the eastern part of the county, the Fish Creek Mountain Mine, operated by US Gypsum, has been in operation since 1922. Working as a part of the Plaster City plant, over 8 million tons of gypsum has been mined (~$24 million) and mining continues today with no signs of stopping.

The amount of resources in Imperial Valley have locally is astounding! Want to feel obsidian and wonder stone with your own hands or try panning for gold? Today come celebrate and “rock” out with that unique geology with the Imperial Valley Desert Museum at its free, community event: Ocotillo Rocks!


The Imperial Valley Desert Museum is located in Ocotillo. It is open Wednesdays through Sundays, 10 a.m.  4 p.m.


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