Page 18 - 2009 DT 7 issues

It’s Our Fault benchmarks are used to determine submerged western continental edge
which strata were laid first, second about 240 million years ago, Nevada
eologic time is agonizingly and so on, helping us to draw a picture began to lift itself—on and off—out
slow. Its passage is measured of the past. (It is often said, however, of the water. The Sevier Orogeny
Gin grand dimensions: one that two geologists in one room will (mountain building) process took
million, one hundred million, one bil- generate at least eight opinions.) But place between 180 million and 70
lion years. We devise elegant ways to the picture becomes more compli- million years ago. Slabs of ocean
put this “deep time” into context, as cated when these sequences go awry. floor scraped off the subducting
if we could truly grasp the abyss be- So it is with Red Rock Canyon’s plates stitched themselves piece-
hind us: “Stretch your meal to Nevada and the
arms out horizontally eastern Great Basin,
on either side of your squeezing and deform-
body. The skin at your ing the rocks, creating
furthest fingertip rep- volcanoes and mountain
resents all of human @2008 Andrew Alden chains. The resulting
history.” Feel better? compression forced
Not really. whole ranges to ride
While we cannot over one another, creat-
truly internalize the ing an Overthrust Belt
concept of so much stretching from Alaska
time, we can at least to Central America in
get our hands around what is known as the
some of it. That pebble Sevier Thrust System.
at your feet is a small The oceanic conveyor
time capsule. Its history belt eventually delivered
is ancient beyond your a great slab of Precam-
imagination. Strangely brian ocean floor (older
enough, deep time Keystone Thrust behind the Wilson Cliffs than 543 million years,
begins to speed up as give or take) to South-
you sleuth the rocks, measuring Keystone Thrust Fault, one of the ern Nevada.
their angles, dissecting their layers, two most famous highly exposed and About 65 million years ago with
analyzing the fossils . . . immersing accessible thrust faults in the world. nowhere to go but up and over, the
yourself in the details. Held in your Unraveling exactly when and how massive limestone slab ramped itself
hands, immense chunks of time are the 600 million-year-old limestone on top of the younger Wilson Cliffs
cut down to size. Their secrets are of the Bonanza King Formation came of Aztec sandstone through what is
accessible . . . or are they? to rest atop the 180 million-year-old called “high angle thrust faulting.”
Before the advent of radiometric Aztec Sandstone of the Wilson Cliffs Known as the Keystone Thrust (the
dating—by which absolute age can was no easy task. The fault represents Red Rock Canyon segment of the
be measured—geologists determined a whopping disconformity of over Sevier Thrust System), this 13-mile-
the relative age of a landform by 400 million years! long knife-edged thrust fault ,which
placing rocks and events according to For much of its history, Southern created the Spring Mountains, is one
fundamental principles: older rocks Nevada was located at the western of the most studied in the world for
underlie younger ones; most sedi- edge of the North American conti- its complexities and accessible expo-
ments are laid down horizontally; nent, mostly under water. As ocean sures. Only the Himalayas provide
etc. Even now, such simple physical plates began subducting beneath the a more famous example of exposed
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