Travel Reference
In-Depth Information
dissimilar sedimentary rock types. Gabb, however, made this correlation on
the basis not of
rock
type but of
fossil
content. Both the European and the
Californian rocks had one aspect in common: the presence of fossils particu-
lar to and diagnostic of the Cretaceous System as it was originally defined.
These fossils were the ammonites, the group William Smith had spent so
much time studying in the late 1700s and that d'Orbingy used so successfully
in the mid-1800s.
Ammonites were among the fastest-evolving creatures ever to have
graced the earth, and individual species are thus diagnostic of relatively short
intervals of earth history. Cretaceous ammonites differ from those that came
earlier by their highly ornamented shells, often uncoiled shapes, and very
complicated septal sutures (the shell junctions uniting their shell walls with
the chamber walls, or septa).
By 1862 the small survey crew had moved into the Great Valley of Cal-
ifornia and began surveying and excavating along the eastern flank of the
mountains known as the Sierra Nevada. Deep in creek bottoms already pil-
laged by the gold miners of a decade earlier, they found sandstone and mud-
stone rich in fossils. Gabb found ammonites in abundance, the most com-
mon being a straight ammonite called
Baculites.
On two tributaries called
Chico Creek and Butte Creek, these fossils were present in untold numbers,
preserved in a fashion superior to any from Europe, for the dusky mudstones
of California had protected the fossils from ground water and erosion. Their
iridescent shells gleamed red and green as Gabb and his crew exposed them
to California sunshine for the first time since the Mesozoic Era.
Gabb and his crew discovered that the Cretaceous System was wide-
spread not only in California but along the entire Pacific Coast as far north
as Canada. In 1863 Gabb took time away from his California work and trav-
eled through Oregon and Washington to Vancouver Island, where he col-
lected fossils in many localities. His conclusion was that many of the same
species he had already encountered on Chico Creek could be found in the
Vancouver Island region as well.
By the time the first Geological Survey of California had finished its
initial four-year charter, it had collected thousands of fossils; and in 1864 it
