Travel Reference
In-Depth Information
that could be used with only a minimum of other age control. All you needed
to know was that you were sampling somewhere in the upper Cretaceous
Period—a time interval itself more than 30 million years long. If you then
encountered a zone of reversed magnetic polarity sitting atop a long interval
of normal polarity, you could be assured of having found the single most age-
diagnostic magnetic event in the history of the earth. This magnetic rever-
sal was given the unromantic name of 33R. It was known from the deep sea
and from the Italian Apennines Mountain work. In 1980, analysis of my drill
cores from Chico Creek showed it there too. Like an elusive fish finally
hooked, the Cretaceous of the west coast of North America now had a
line—in this case a time line—securely attached to the global time scale.
According to Ken Verosub, who analyzed the cores drilled from Chico
Creek sediment in his paleomagnetics lab at the University of California at
Davis, the magnetic signal captured in our cores was strong. But was it accu-
rate? Did the signal emanating from these rocks record the directions of the
earth's magnetic field from a time when dinosaurs roamed the earth in pro-
fusion, or was it from a more recent time? In the lab, I learned that paleo-
magnetics is fraught with uncertainty and that the black and white reversal
patterns shown on all summary charts of the earth's magnetic field history—
apparently so clean and unambiguous in their binary glory—are really any-
thing but.
Like so much else, paleomagnetics sounds simple in theory. The sedi-
ments we had drilled from Chico Creek were sandstone and siltstone de-
posited on shallow sea bottoms long (but exactly how long ago?) ago.
Among the minute sand and clay particles drifting down onto that ancient
sea bottom were trillions and more of magnetite grains; all were eventually
squished together into the bottom sediment. The last thing these tiny mag-
netite grains did before being cemented into place was orient themselves
parallel to the earth's magnetic field. Each core so laboriously drilled from
the rocks held some number of magnetite grains, and each core thus exhib-
ited a tiny but measurable magnetic field of its own, created by its enclosed
magnetite. Fed the orientation of a core, the magnetometer could tell us the
orientation of the magnetic field emanating from it. One by one, the inch-
