The California coast has been subject to movement because of transform
motion of two major plates. Mudflats that were once home to millions of
organisms slowly were uplifted exposing their remains along the cliffs.
Layers of fossils represent millions of years of accumulation. You can
find mollusk including bivalves and gastropods, bones from
marine mammals, sand dollars, and even crab claws.
Many times the shells are in distinct layers, with
few still having two shells. Paleontologists have deduced that this
represents storm layers. A storm would come in with violent waves and
rip up the mollusk community. After the storm the shells would create a
"burial" layer. In between the storm layers, you can find bivalves whose
shells are still intact, so they lived to a ripe old age.
Mollusk fossils in mudstone