Geological Outings Around the Bay: Point Año Nuevo

^{Photo courtesy Doc Searls of Flickr under Creative Commons license.}

Point Año Nuevo gets a crush of visitors during the mating season of elephant seals, a spectacle well worth the trip down the San Mateo coast. The rest of the year, especially at low tide, is good for enjoying the Point's geology. For instance, nowhere else on the California coast is a major fault zone so well exposed.

But first let's notice something about the Point: it's all a big flat space. In fact, looking at it from the edge, down on the beach, you'll see that it's a classic marine terrace, just like ones I've described previously at Pebble Beach a few miles to the north and at Shell Beach in Sonoma County.

The top layer of this geologic cake is a thin frosting of brown soil, some orangish stream deposits just beneath it and lighter-colored beach sands. The bottom layer is solid rock all the way down to the beach. The contact between the two layers is a strip of vegetation, where groundwater collects. That contact represents an ancient wave-cut platform, from a time about 105,000 years ago (105 ka) when the sea level was higher than today (because the glaciers had melted even more than they have today).

Here's a closer look at that wave-cut surface. It appears just like the modern seafloor, complete with the borings of pholad clams.

The reason I could photograph this without having to climb the cliffs is that the ground in places has been faulted since 105 ka.

Time to look at the geologic map, which shows several faults crossing Point Año Nuevo. The photo above is from the beach cliff above the word "Bay."

All of these faults are part of the San Gregorio fault zone, an obscure part of the San Andreas family that runs mostly offshore. It is thought capable of a very large magnitude 7.5 earthquake. The two most prominent strands at Point Año Nuevo are on the right-hand side of the map; they're informally named the Coastways fault (on the east) and the Frijoles fault. The Frijoles is at the center of this view:

This closeup shows where Año Nuevo Creek, long ago, took advantage of the downdrop on the fault and cut into the underlying Purisima Formation (which is about 5 million years old).

Point Año Nuevo is a very active place today, even though no large earthquakes have been documented here for about a thousand years. When first logged by European explorers in 1603, there was no island here. By the end of the 1700s there was one. Researcher Gerald Weber, who has tramped the Point since 1973, has evidence that this change created a huge washout of sand that protected the southern seacliffs of the Point until just recently. In a field trip recently, he paced off the amount he has seen the cliffs retreat since about 1980. The hat sits where renewed erosion uncovered an old pier post dating from the 1850s, when the pier served lumbering operations in the nearby hills.

Weber thinks that this sand has moved down the coast and is now affecting Santa Cruz Harbor. Eventually it will spill into Monterey Canyon and wash out to the deep sea.

The San Gregorio fault zone is an important mystery. This image from an old map shows its extent in this part of the San Mateo coast.

Recent mapping has improved this picture, but the work is very difficult. Lidar technology promises big advances, though. This light-based form of radar mapping allows us to strip buildings and vegetation off the underlying ground surface, making even subtle fault features stand out. Lidar imagery now covers this region:

. . . and the Frijoles fault trace on Point Año Nuevo stands out beautifully in it.

Learn more about Point Año Nuevo's geology from the park's 2009 docent training materials.

Lidar data from many Northern California fault traces is freely viewable in Google Earth.