Portions of this coastline are underlain by
what geologists call the Franciscan melange.
The melange is composed of immense blocks
of highly resistant rocks (chert, greenstone,
and sandstone) within a matrix of softer, more
easily eroded rock (mudstone). Here, the more
resistant blocks survive the erosive power of
the ocean to form the seastacks that you see
just offshore. Seastacks are most commonly
associated with the Franciscan melange.
The rocky North Coast is especially
rich in tidepool habitats. Pools
found among the seastacks offer
relatively quiet, nutrient-rich
waters needed by tidepool animals.
How Seastacks are formed
The constant assault from the
ocean erodes the softer rock
matrix of the melange causing
the shoreline to retreat inland.
More resistant blocks are
detaches from the headland
by wave erosion and are left
standing offshore as seastacks.