The rocks are part of the Franciscan Formation and are about 100 million years old. This area was once covered by the ocean and evidence of submarine volcanism can be found to the north and south of this area. The primary rock seen in most locations is sandstone. Often called graywacke, it ranges from light tan to dark gray. It is easy to identify from a freshly broken surface because it looks and feels like coarse sandpaper.
Shale is present in some places.
It tends to break easily into many small, flaky pieces which do not have the gritty feel of sandstone. The most difficult of all the rocks to identify is basalt. Basalt is the type of lava extruded during eruptions of Hawaiian volcanoes. Here it is very old and decomposed. It usually looks more like a hardened mudstone.