Travel Reference
In-Depth Information
once-flat-lying sedimentary rocks have been involved in a regional compres-
sional episode of the sort that can produce mountains. Millions of yeats ago,
the flat layer cake of strata making up Sucia was slowly pushed against an un-
yielding wall of mountains to the east. The layers first bent, then buckled
and broke. All such effects are manifest on Sucia as tilted strata and faults.
The originally flat layers dip at an angle of nearly 60 degrees, making the
complete book of strata that is now Sucia Island easy for us to read. Simply
by strolling along a beach of the island, we can walk through time—through
the entire pile of sedimentary beds.
The oldest sedimentary rocks that make up Sucia Island are located on
its southern margin. Let us start out there on a sparkling May morning, warm
sun prickling our still wintry skin, and walk the intertidal, one eye watching
our step on the beach strewn with boulders and driftwood, the other watch-
ing the ever-changing cliffside rocks. We will need a low tide, because thete
are several landslides that would otherwise ban our way along the path we
must follow on Sucia's southern coast.
The most ancient strata on the island are conglomerates, the type of
rock made up of larger rock particles, such as gravel and cobbles. These con-
glomerates on Sucia are a chaotic and jumbled pile ot rounded rocks, a few
as large as a bowling ball. In some areas the conglomerates contain beautiful
white quartz; in other places a more variegated rock assemblage is visible.
The various rocks in this basal conglomerate are sure clues to the nature of
the land area that was nearby when these rocks were deposited, for they are
so large that they could not have been transported far from their original
mountain sources. We can surmise that these cobbles were brought to the
edge of a sea by rivers tumbling down from nearby coastal mountains; in this
conglomerate we see bits and pieces of these ancient mountains. They give
us our first glimpse of ancient Sucia Island—it was near mountains.
The aggregate thickness of Sucia's basal conglomerate is perhaps 100
feet, but because the beds are tilted on their side at a 60-degree angle, we
rapidly pass through this thickness of rock as we walk along. The rocks are
exposed both as a low cliff lining the beach and as a bench along the beach
