Geology Reference
In-Depth Information
and shale, like the Bright Angel formation I kicked apart so easily, form under com-
pletely different conditions than limestones like the Muav, Redwall, Toroweap, and Kaibab.
Marine limestone forms when organisms whose bodies are made of calcium carbonate
(CaCO
3
)—like coral, clams, or microscopic foraminifera—die. Their shells and skeletons
pile up on the seafloor and, if subjected to enough pressure, temperature, and time, even-
tually form carbonate rock. Because the organisms that become carbonate rocks take time
to grow and don't live in turbid waters, the alternating layers of biologically precipitated
limestone and mechanically deposited sandstone and shale that settled out from turbid wa-
ter could not have formed during the same event. The alternating sequence of different rock
types stacked one atop the other in the canyon walls records a long series of events and
environments.
A single enormous flood simply can't explain the geology of the Grand Canyon. As we'll
see, geologists discredited the idea that Noah's Flood created the world's topography and
deposited its sedimentary rocks in the early nineteenth century, decades before the first ex-
pedition down the Grand Canyon. When creationists argue that they want to “teach both
sides” of the argument about earth history in science classes—their view and those of the
“evolutionists” they vilify—they neglect to mention that geologists had disproved the cre-
ationist view of a young Earth shaped by Noah's Flood before Darwin ever began thinking
about evolution.
Today, the real debate about the formation of the Grand Canyon is between geologists
who agree about its geologic history but argue about its topographic history—exactly how
and when the canyon itself formed. The conventional view is that by six million years ago
the Colorado River had established its modern course to the Gulf of California. Radiomet-
ric dates on cave deposits record that the water table draining into the river dropped steadily
downward at about a hundredth of an inch a year for at least the past three million years.
Younger lava flows that spilled into the canyon also tell of gradual incision over the past
half-million years. But a recently published study suggests that the modern canyon was
preceded by an older canyon first carved by a river that drained in the opposite direction
between eighty and seventy million years ago. Arguments about the topographic evolution
of the canyon center on whether the modern canyon formed as a river eroded headward
back into the Colorado Plateau, a plateau-bound lake spilled over a drainage divide to cut a
