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