During the ice ages sea level fluctuated considerably,
alternately exposing and submerging the lower lands around the islands
and causing the islands size and shape to change frequently.
Because of continuing uplift of the islands during the last few million years
and the changing sea level caused by glacial advances and retreats
there's evidence on the islands of ancient shorelines at different elevations.
Erosion around these ancient shorelines carved flat areas of land called
marine terraces which are found at many different elevations on the islands.
From twenty feet to a thousand feet above sea level.
During the last ice age which ended about 10,000 years ago. Sea level was about 400 feet lower than it is now.
At that time before northern Channel Islands were connected into one large island called Santarosae.
At its nearest point it was about five miles from the mainland.
Today the rise in sea level since the end of the last ice age has caused many
of the ancient shorelines, cut during glacial periods to be submerged below
the ocean surface because the rate of sea level rise is greater than the rate of tectonic uplift.
As waves crash onto the shoreline at the base of sea cliffs erosion from wave action cuts a terrace in the surf zone.
Ice Age glaciation causes sea level to drop exposing the terrace.
A second terrace is created at the new sea level about 400 feet below the original terrace.
Slight tectonic uplift raises the elevation of the original terrace
because uplift is going on all the time.
During the next interglacial period sea level rises and a third terrace is cut
below the original terraces which now has been uplifted above the new surf zone.
The second terrace, cut during the glacial period is now submerged.
Repeated glacial and interglacial cycles cut and exposed additional levels of terraces
creating the stair step like landscape shown.