|Contorted and faulted white dikes cutting through Granite of Inverness Ridge|
The generalized geologic map of Point Reyes National Seashore after Clark and Brabb (Figure 9-2 in ) 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 . 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 . 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|
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 :
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
 Doris Sloan: A Trip Through Time On the Pacific Plate. Bay Nature July-September 2015, pp. 12-16.
 Geology at Point Reyes National Seashore and Vicinity, California [pubs.usgs.gov/of/2005/1127/chapter9.pdf].
 The Laird Sandstone [www.marin.edu/~jim/ring/ptreyes/pr_tl.html].
 Monterey Formation [www.marin.edu/~jim/ring/ptreyes/pr_tm.html].