Geology Reference
In-Depth Information
commencement, have perhaps moved and overturned to a great depth the entire outer crust
of the globe.”
5
We now know of at least five mass extinctions in the geological past, and biologists say
another one is under way as we wipe species off the planet 100 to 1,000 times faster than
nature did before we started helping out. Since the evolution of life on land, several events
have killed off over half of all animal species. Every school kid learns that dinosaurs died
off and mammals began rising 65 million years ago during the great Cretaceous-Tertiary
extinction event. The less well-known, but far deadlier, Permian-Triassic extinction event
251 million years ago killed off almost all of the animal species on Earth, ending the age
of trilobites and setting up the rise of dinosaurs. More recently, the last glaciation of the
Quaternary Period (the so-called ice age of the past several million years) saw the demise
of megafauna, like mammoths, and ushered in a modern world increasingly dominated by
people. When viewed through the geologic record millions of years from now, the modern
extinction event we are living through may well look similar to past grand catastrophes that
ended ancient worlds.
After Cuvier, the drive to find evidence for Noah's Flood in the rocks was well and truly
dead, although modern creationists would later resurrect the idea. While natural philosoph-
ers were long wedded to the idea that fossils confirmed the biblical account of a great flood,
once they established the reality of extinctions in the geologic record, it showed that Noah's
Flood could not have deposited all the world's fossils. They then shifted to looking for the
signature of the Flood in the overlying unconsolidated deposits of gravel and boulders. This
new view helped natural philosophers and theologians alike accept a pivotal reinterpreta-
tion of the Bible, one that made room for a new concept of time—time enough that fossils
need not have all died, or lived, at the same time. Thanks to a Scottish farmer, today we
know this idea as geologic time.
