California’s “Nine Sisters” have existed throughout today’s San Luis Obispo County for 20-25 million years, but most of us drive right past them, unaware of their prehistoric connection.
This chain of extinct volcanic plugs comprises nine characteristic rocky landforms, ranging from Morro Rock, sometimes known as the “Gibraltar of the Pacific,” to the towering Bishop Peak, a popular hiking destination.
Here’s a closer look at these ancient California landmarks.
Geology of the Nine Sisters
The Nine Sisters are all the solidified cores of extinct volcanoes, formed by magma condensing in the volcanic vents between 20 and 25 million years ago. Over geologic time, the softer surrounding stone eroded, leaving behind the dense volcanic plugs that we can still see today.
Many geologists include the Nine Sisters in a larger set of 23 similar volcanic plugs and domes throughout SLO.
Get to know the Sisters
Some of the Nine Sisters are protected areas supporting local wildlife, and others are open to the public for popular hikes. Here’s a closer look at each of these ancient volcanic cores.
- Morro Rock (576 ft): One of the most unmistakable landmarks on the California coast and a Chumash sacred site, Morro Rock rises directly out of the ocean at the mouth of Morro Bay, earning it comparisons to Gibraltar. It’s a bird sanctuary for species including the endangered peregrine falcon, meaning climbing the rock is prohibited, but you can drive up to its base via a causeway.
- Black Hill (655 ft): This low peak in Morro Bay State Park has easy trails and views over Morro Bay.
- Cerro Cabrillo (911 ft): Another rugged hill in Morro Bay State Park, Cerro Cabrillo offers hiking and climbing trails as well as bay views.
- Hollister Peak (1,404 ft): This striking volcanic plug is located on private land, although you can see it from Highway 1.
- Cerro Romauldo (1,306 ft): Partly located on military land at Camp San Luis Obispo, Cerro Romauldo is mostly reserved for military training.
- Chumash Peak (1,257 ft): This privately-owned summit is named to honor the Indigenous Chumash people and has no public trails.
- Bishop Peak (1,559 ft): This is the tallest and most popular of the Nine Sisters, famous for its rock crown and challenging trails.
- Cerro San Luis Obispo (1,292 ft): Famous for the big hillside “M” above downtown SLO, Cerro San Luis Obispo has multiple hiking and biking trails.
- Islay Hill (775 ft): Located on the southeastern edge of SLO near the airport, this smaller hill has easy trails and views of the Nine Sisters.