Travel Reference
In-Depth Information
whitecaps, diving and sometimes fighting for surface-dwelling fish. Am-
monites lived and died in countless numbers in the sunlit portions of the sea,
their empty shells littering the shallow sea bottoms and sandy shorelines. To
the west of this sea the Rocky Mountains were rising, urged skyward by huge
volumes of magma welling up among their roots. In places this magma
pooled far underground, slowly solidifying to become giant bastions of gran-
ite, the speckled foundation of any continent. Elsewhere in the rising arc of
mountains, the magma successfully fought to the surface, blasting outward
from volcanic cones, creating smokestack pillars of blackness to the west of
the inland sea. Falling ash from the volcanoes periodically covered the
shorelines of the seaway, creating a rich, fertile soil.
Riotous jungles grew and died in the swampy lowland areas along the
margins of the great sea. Rivers large and small poured into the seaway, car-
rying to the central sea unnumbered tons of sediment derived from the new
mountains as they began to erode. Carried within or buried beneath this set-
tling sediment were the remains of many creatures of that time: the skeletons
of fish, the shells of long-extinct mollusks. Yet the most valuable product for
geological dating in this region came not from the fossils but from the vol-
canic ash that fell on this land. The intersection of this fine volcanic dust
with sedimentary rock gives us a means of dating the fossiliferous sediments
in terms of absolute years from the present, not merely relative position. The
ash can be dated
radiometrically.
This methodology is among the most scien-
tifically useful ot time machines.
The age of the earth
There has always been a need to put absolute age dates—the time, in thou-
sands or millions of years ago, that given rock formed—on the units of the
geological time scale. Fossils alone cannot do this. They can help us sort out
relative positions of strata but cannot gives us their age in numbers of years
ago. The solution to that problem awaited the discovery of radioactivity and
the invention of machines that can measure the tiny quantities of chemicals
called isotopes, that are used to date a rock. Those discoveries took place in
