Travel Reference
In-Depth Information
The microscopic ash particles erupted out of the ancient Rocky Moun-
tains during the Cretaceous Period are the source of our hest age dates for the
Age of Dinosaurs. When these microscopic crystals first hit the air, they
cooled, which allowed tiny feldspar crystals rich in potassium to form. Most
of these ash particles hit the surface of a wide inland sea east of the volca-
noes, the seaway that bisected the North American continent between 100
million and about 70 million years ago.
The ash that fell on the land was quickly washed away by rain and de-
stroyed. But, each monumental volcanic hurst also sent a shower of ash onto
the seaway's hroad surface, where it slowly sank, finally to come to rest on
deep, murky sea bottoms where fish and shells—and above all ammonite
cephalopods—lived and died in numbers beyond counting. Inside each of
the tiny crystals of feldspar forming this sunken ash, isotopes decayed from
one to another like, well, clockwork.
The thick shale, sandstone, and ash deposits of the Western Interior
Seaway can be seen throughout the central part of North America. In Col-
orado, Wyoming, the Dakotas, Arizona, Alberta, Manitoba, Montana, Ne-
braska, and Kansas, the sedimentation of more than 50 million years is visible.
Better than any other place on earth, this stack of sediment contains markers,
both fossil and igneous, that can be used to tell time, Cretaceous time.
The fossils alone make the region extraordinary. The best-preserved
ammonites in the world come from hete—beautiful pearly shells from the
Pierre Shale and its Canadian equivalents. So gorgeous are these fossils that
a new mineral industry has sprung up, producing a reddish jewel called
am-
molite.
Yet it is not the extraordinary completeness and pristine nature of the
fossils that makes them so useful to the geologist (though all of that helps);
rather it is their very abundance, and the rapid dance of
evolut
ion
they
seem
to have engaged in.
The Western Interior Seaway was a cauldron of evolutionary change.
All manner of creatures evolved rapidly in this nutrient-rich inland sea.
Ammonites seem have been especially bitten by this itch to evolve, which
accelerated the creation of new species. Most speciation is aided and abetted
by geographic separation; some small band is split off from the larger popula-
