Travel Reference
In-Depth Information
The fossils change as well. The large number of clam fossils, made up—
until now—of many types quite familiar to us, become dominated by larger,
flat clams that resemble oysters. These are the inoceramid clams, perhaps the
most ubiquitous species of the Cretaceous Period and a form that went com-
pletely extinct at the end of the Cretaceous. They are found singly in the
Sucia sediment and are often encased in concretions, the hard mud balls so
characteristic of deeper-water deposits. Ammonites now appear for the first
time—mostly the straight
Baculites,
but other species are found as well.
Larger coiled varieties are found, some smooth-shelled, others ornamented
with an astonishing array of ribs and knobs. How did these shells get here?
Ammonites were swimmers like fish, and they used their astonishing, cham-
bered shells to maintain buoyancy much as a submarine does. They must
have competed with fish as active predators. We can imagine the ammonites
swimming above fine muddy bottoms—bottoms covered with worms and
clams, crustaceans and urchins, bottoms perhaps 100 feet deep or more—and
sinking down onto these muddy bottom after their death.
We are now in a very fine sediment, a silty matrix with many concre-
tions. We are also near the end of our walk, for although more rock is pre-
sent, we would have to dive to see it. The strata on Sucia can be seen to con-
tinue underwater from the edge of the shore, but no observations can be
made because of the profusion of marine life growing over these underwater
deposits. We have crossed a quarter-mile of beach and have traversed more
than 600 aggregate feet of sedimentary beds. One message comes through
loud and clear: The ancient environment of Sucia Island saw an ever-rising
level of the sea, a sea that first lapped on a seashore and ended at a level far
above a deep muddy bottom. Did the sea rise, or did the land subside? And
did this change happen only here or all over the world at the same time? To
answer these questions, we need to step back and examine a larger area than
Sucia Island, and we need to look at a longer slice of time as well. The ma-
rine rocks on Sucia are but a small bit of the much thicker pile of strata that
make up the Nanaimo Group. Let's us look at the lithologies of this great pile
of strata in the hope of extracting more information about the ancient envi-
ronment of this place.
