Travel Reference
In-Depth Information
Sea level change and ancient environments
Just across the international border from Sucia Island is an assemblage of is-
lands big and small named the Gulf Islands. They are almost entirely made
up of the same sedimentary rock that constitutes the northern tier of the San
Juan Islands. (The name change is purely a matter of nationalism, not geo-
logical history.) The seaway in which these islands now float was carved by
the glaciers of the last 2.5 million years, which repeatedly slid down through
the regional bedrock to carve out this spectacular fjord-like real estate. But the
actual creation of the
rock
that makes up these islands took place much longer
ago, near the end of the Age of Dinosaurs, as we have seen.
If we take a boat tour through these islands, we can see the entire pack-
age of ancient sedimentary rock that makes up this assemblage, which is
named the Nanaimo Group. It is really a group of formations, each formation
being defined by a distinctive lithology of rock type. About eleven or twelve
formations have been recognized, and their aggregate thickness would be more
than 15,000 feet—3 miles of strata—if they were piled up in one place. The
name Nanaimo Group comes from a large town on Vancouver Island where
these rocks were first identified. The lowest strata of the Nanaimo Group are
composed of gravel and sandstone quite coarse in nature, and our best guess is
that they are about 85 million years old. There is abundant fossil and litholog-
ical evidence that these basal Nanaimo Group strata were deposited in rivers
or on land. Perhaps it was a river valley or perhaps a flood plain; whichever,
these oldest deposits were certainly not laid down in the sea. But overlying
these continental deposits, which themselves sit upon far older volcanic rock,
is a dark shale that is indeed the remains of an ancient ocean bottom.
These lowest marine rocks are called the Haslam Formation. They are
widely exposed over much of Vancouver Island, as well as being found on
members of both the Gulf and San Juan Islands. The Haslam Formation is
usually more than 1000 feet thick and rich in fossils. The most abundant of
these fossils are small forms called inoceramid clams, a type now extinct. But
also found in substantial numbers are ammonites. At least twelve species
occur, sporting the disc-like shells as well as curious, uncoiled ones. Some
