Travel Reference
In-Depth Information
In places, these conglomerates (which are called the Extension Forma-
tion), are also as much as a thousand feet in aggregate thickness. Where this
formation is exposed, as on the south face of Waldron Island, we see only a
huge wall of rounded rocks all jumbled togethet, with only the faintest hint
of sedimentary bedding. Near their top we see them thin and change to
smallet clast sizes, eventually to be succeeded by siltstone with fossils.
If we continue our tout through the Nanaimo Group, we find at least
five such transitions from fossil-bearing shale to more coatsely grained de-
posits without fossils. This alternation between marine deposits and mar-
ginal marine or terrestrial deposits that makes up the Nanaimo Group con-
tinued until the end of the Age of Dinosaurs. The section on Sucia is roughly
in the middle of this long history.
Why do such dramatic changes in lithology take place? Normally only
two explanations are offered. Either the level of the land is rising or falling
(which happens when mountains form nearby), or the sea level itself is chang-
ing. It is the latter explanation that is now favored in most cases. If this expla-
nation is correct, the level of the sea rose dramatically over a 20-million-year
period and then receded at least five times. Sucia Island was deposited during
the third of these cycles. Are these cycles the result of a change in sea level?
Every day the sea rises and falls, its tide a clock of nature. Although we
think of tides as coming in and going out (we have even invented two words
for the phenomenon,
flood
and
ebb),
the change is more a vertical than a
horizontal effect. The tides are familiar to all of us as a normal consequence
of gravity, a large moon so near, and a sun tugging on everything. Tidal
change is usually imperceptible—unless you live in the Bay of Fundy, where
it takes a steady trot to outrun the tide. The tides ate the most familiar
change in what we may call
sea level.
Yet daily tidal change is not the only routine change in sea level; it is
only the fastest. There are longer-term changes that are far less intuitive and
familiar. Off the east coast of North America, for instance, fishing boats rou-
tinely trawl up cobbles and rocks indicative of the seashote from depths as
deep as 150 metets. The teeth of mastodons and mammoths are found in this
