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monites belonging to the same genera, if not the same species, as European
forms convinced him that he was sampling not just Cretaceous-aged rocks,
but
upper
Cretaceous rocks, equivalent in age to the chalk of the English and
French coastlines. His collections were hurriedly made; he had, after all, an
entire state to investigate, and no one portion, however physically beautiful,
could monopolize his time. 1 le hoped that subsequent expeditions would re-
veal some ammonite species occurring in both France and California, which
would allow those two distant relics to be tied together by a filament of time.
It was not to be. In the decades following Gabb's pioneering studies, the
countless scientists and amateur fossil collectors who had been drawn to the
deep valley of Chico Creek by its rich fossil content searched in vain. For all
the great collections subsequently amassed, no investigator stumbled on any
fossil diagnostic ot the European Cretaceous. The great rampart of North
and South America apparently created an impassable barrier for European
species: They could not colonize the new world of the Mesozoic Era. No
species in Europe exchanged DNA during the Mesozoic Era with any species
in western North America, just as no mollusk species in these two areas do
so today. If you take a paleontological specialist to see fossil collections made
from deposits in the Pacific region, he or she will look in vain for old friends
and will confront only strangers. Yet while the ammonites and other diag-
nostic European fossils migrated westward with the currents over their myr-
iad generations (ultimately to find more westward expansion blocked by ge-
ography), the earth's magnetic field created an indelible time marker in the
Chico sedimentary rocks.
Through a great portion of the Cretaceous Period, which has been ra-
diometrically dated as beginning about 145 million years ago and ending 65
million years ago, the earth's magnetic field was stuck in one direction.
There were no reversals for tens of millions of years. And then, about 80 mil-
lion years ago, this long quiescent period came to an end with a reversal. Al-
though any teversal is a worldwide event, there have been so many through
earth history that a reversal is essentially useless without other, time-bound
data such as those obtained through fossils or radiometric decay. But the first
reversal of the Cretaceous "long normal" episode was a diagnostic fingerprint
