Beachside geology: the Kehoe cliffs

Contorted and faulted white dikes cutting through Granite of Inverness Ridge

Most people visit a beach to play and recreate. Kehoe Beach invites visitors to additionally experience cliffs-on geoscience and to explore the natural history of the San Andreas Fault Zone. Kehoe Beach is an excellent, easily accessible field-trip destination in western Marin County north of San Francisco. Doris Sloan featured the geology of the Kehoe region in a recent Bay Nature article [1].

The generalized geologic map of Point Reyes National Seashore after Clark and Brabb (Figure 9-2 in [2]) shows that the area south of the Kehoe Beach Trail consists of late-Pleistocene-to-Recent features: alluvial depositions, beaches and sand dunes; obvious to today's visitor. The units north of Kehoe Trail include Laird Sandstone from the middle Miocene and Salinian granitic rocks from the Cretaceous period; the latter forming the Inverness Ridge.

The Laird Sandstone you can see at the Kehoe trailhead is the oldest of the sedimentary rocks in the area [3]. It originated by deposition when the sea rose or the land sank between 16 to 11 million years ago. On top of it, Monterey Formation sandstone was deposited eight to six million years ago. This formation is distributed from south to north over the Point Reyes Peninsula and exhibits a strong similarity with deposits found in Monterey further south. Therefore, it is assumed that Point Reyes, riding north on the eastern edge of the Pacific plate, was located in the Monterey area up until Miocene times [4]. Layers of uplifted Monterey Formation are exposed north of where the Kehoe Beach Trail traverses a sand-dune ridge to meet the shoreline.

Monterey Formation exposure next to Kehoe Beach Trail

Walking north on Kehoe Beach, you soon will find yourself in front of Laird Sandstone cliffs. Continue north and you continue walking back in time, geologically speaking. The gray granitic rocks to the north have been uplifted relative to those of the sandstone cliffs on the south. A small landslide marks the contact zone, a vertical fault between the sandstone cliffs and the Salinian granitic formation. The latter is also called the Granite of Inverness Ridge, which is plutonic igneous rock that formed about 85 million years ago (within the Cretaceous period between 145 to 66 million years ago) deep in the crust from magma produced during long-ago plate collisions [1].

A striking feature of the lower granitics are the bold white dikes cutting through them in puzzling, strangely twisted patterns (top picture). Doris Sloan helps us to solve the puzzle, or, at least, to make sense of it in plate-tectonic terms [1]:

If Yosemite comes to mind when you think of granite, this outcrop of granitics north of the landslide may puzzle you. No high cliffs of light-colored rock; no straight white dikes—bands of lighter, younger rock—cutting through the granitic rocks for hundreds of feet. Here you see a jumble of multicolored rocks, cut by contorted and faulted white dikes. The rocks are so mixed up and fractured, so different, that you would think there is no connection to Yosemite. However, these granitic rocks once formed the southernmost end of the Sierra Nevada, about 350 miles south of their present location; they ended up on the other side of the San Andreas Fault system and got hijacked by the Pacific plate on its way north.

Vertical fault between formations

Point Reyes is a merger of different points in time and space. You can be sure that this peacefully appearing location has gone through lots of sinking, lifting and shifting—and still does. Rising sea levels and predicted earthquakes will have their future impact on this preserve.

Keywords: geology of Point Reyes, shifting ground, deposition, sedimentation, earth science, plate tectonics.

References and more to explore
[1] Doris Sloan: A Trip Through Time On the Pacific Plate. Bay Nature July-September 2015, pp. 12-16.
[2] Geology at Point Reyes National Seashore and Vicinity, California [pubs.usgs.gov/of/2005/1127/chapter9.pdf].
[3] The Laird Sandstone [www.marin.edu/~jim/ring/ptreyes/pr_tl.html].
[4] Monterey Formation [www.marin.edu/~jim/ring/ptreyes/pr_tm.html].