The Rocks of Santa Cruz’s Central Coast: The Purisima Formation

An example of the Purisima Formation along West Cliff Drive. The top part of the cliffs where most of the ice plant is growing are composed of Quaternary deposits. The bottom section is the Purisima Formation. The contact between them can be seen clearly in the middle of the photo.
An example of the Purisima Formation at Mitchell Cove along West Cliff Drive. The top part of the cliffs where most of the ice plant is growing are composed of much younger Quaternary deposits. The bottom section is the Purisima Formation. The contact between them can be seen clearly in the middle of the photo.

The Purisima Formation is the rock type that shapes the landscape characteristics of the central and southern Santa Cruz coast. It is present from the intersection of Swift Street and West Cliff Drive all the way down the coast to Seacliff Beach in Aptos. Relative to the Santa Cruz Mudstone it is not very resistant to erosion. In fact, the change of rock type from the more resistant Santa Cruz Mudstone to the less resistant Purisima Formation near Lighthouse Point, allowed the landward erosion that caused the Monterey Bay to form where it did.

Creation of the Purisima

This formation consists of inter-layered yellow-gray siltstones and blue-gray sandstones and gets its name from Purisima Creek in San Mateo County. It was created during the Pliocene and the last gasp of the Miocene about 3-7 million years ago. The deposits were laid down in a shallow sea that was gradually shrinking. If you were to walk from West Cliff Drive and Merced Avenue all the way to Seacliff Beach, you would be getting younger in geologic time.

Over this 4 million year period of deposition, the rocks record a shift from deeper off-shore conditions (primarily yellow-gray silts with a lot of silica-rich organisms, known as diatoms, deposited in fairly quiet water) to shallower near-shore conditions (primarily blue-gray sandstone deposited from rivers dumping into estuaries and bays). There are a few mollusk fossils and whalebones in the cliffs of the Purisima Formation near West Cliff Drive but you will find the best exposures of them along the cliffs below Depot Hill in Capitola.

Capitola Overlook from Depot Hill, May 2011. Image adapted from photo by fogcat5 via Flickr.
Capitola Overlook from Depot Hill, May 2011. Image adapted from photo by fogcat5 via Flickr.
Depot Hill Bluffs in Capitola, June 2012. Image adapted from photo by fogcat5 via Flickr.
Depot Hill Bluffs in Capitola, June 2012. Image adapted from photo by fogcat5 via Flickr.

The Purisima and Lighthouse Point

Lighthouse Point is composed of a particularly resistant part of the Purisima Formation. In general the Purisima erodes fairly easily. However, Lighthouse Point, and a few other resistant headlands like San Lorenzo Point and Pleasure Point, have withstood the ocean’s erosive powers. Lighthouse Point, in fact, protects Monterey Bay’s northern beaches from much of the wave energy they would otherwise get from the dominantly northwest waves. Even so, the Purisima at Lighthouse Point does erode. In fact it tends to form caves …

Modern day Lighthouse Point showing the Purisima Formation and  the Mark Abbott Memorial Lighthouse which was built in 1967.
Modern day Lighthouse Point showing the Purisima Formation and the Mark Abbott Memorial Lighthouse which was built in 1967.
A cave in the Purisima Formation which is undercutting modern day Lighthouse Point.
Under cutting of Lighthouse Point by caves continues. This picture was taken on the westside of the point in 2014.

Caves Endanger Original Lighthouse

The original location of the Santa Cruz Lighthouse, built in 1869, was close to the cliffs. In 1878, less than ten years after it was built, a letter was sent to the Lighthouse Board noting that three large caves (all over 50 feet long and 50 feet wide) penetrated the point, and the largest extended to within 12 feet of the lighthouse.

To preemptively save the lighthouse from potential catastrophic cliff erosion, the Lighthouse Board ordered it to be picked up and moved 300 feet landward. Thus from 1879 to 1948 the lighthouse was located on the north side of what is now West Cliff Drive.

Aerial view of Lighthouse point circa 1941. You can see the beacon and the old lighthouse on the north side of West Cliff Drive. Image is used courtesy of Frank Perry. Ed Webber was the photographer.
Aerial view of Lighthouse point circa 1941. Image is used courtesy of Frank Perry. Ed Webber was the photographer.

While the caves did not collapse catastrophically, they did erode about a foot per year and eventually created the cove that exists today in front of the modern day lighthouse. Check out our piece on Lighthouse Point for a more complete history and the interesting story of the lighthouses that lit the way for ships before the one that stands today.

Take the Self-Guided Mobile Tour

This piece is part of the West Cliff Drive Tour. Download the free app with many tours of the Santa Cruz area and beyond.

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Go to Mobile Ranger Guides in the Apple App Store
Go to Mobile Ranger Guides in the Google Play Store
  1. Sources Used

    • Lighthouse Point: Illuminating Santa Cruz. Frank A. Perry. Santa Cruz, California: Otter B Books; 2002.
    • Living With the Changing California Coast. Gary B. Griggs, Kiki Patsch, Lauret E. Savoy. University of California Press; 2005.
    • Personal Communication with Frank Perry, Historian, Santa Cruz County, February 2012.
    • Personal Communication with Gary Griggs, Distinguished Professor of Marine Sciences, University of California Santa Cruz, February 2012.
    • Solid beam shines from lighthouse once again. Shanna McCord. Santa Cruz Sentinel. December 19, 2013.



About The Author

I really enjoy field trips. I love being in a cool place and having someone tell me about it. The problem is, you can’t always find a professor or park ranger-type to tell you all they know about the local rocks, plants, and history. So I decided to combine my love of things natural with mobile technology.

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