The Earth is very old—4.5 billion years or more according to recent
estimates. Most of the evidence for an ancient Earth is contained in
the rocks that form the Earth’s crust. The rock layers themselves—like
pages in a long and complicated history—record the surface-shaping
events of the past, and buried within them are traces of life—the
plants and animals that evolved from organic structures that existed
perhaps 3 billion years ago.
Also contained in rocks once molten are radioactive elements whose
isotopes provide Earth scientists with an atomic clock. Within these
rocks, “parent” isotopes decay at a predictable rate to form
“daughter” isotopes. By determining the relative amounts of parent and
daughter isotopes, the age of these rocks can be calculated.
Thus, the results of studies of rock layers (stratigraphy), and of
fossils (paleontology), coupled with the ages of certain rocks as
measured by atomic clocks (geochronology), attest to a very old Earth!
But this is not the whole story. As indicated earlier, younger Mesozoic
and Tertiary rocks more than 1 mile thick that once covered the area
have been carried away by erosion, and, if we include these, the span is
increased by another 50 million years or so.
If we consider the geologic formations that make up the national parks
(N.P.), national monuments (N.M.) (excluding small historical or
archaeological ones), Monument Valley, San Rafael Swell, and Glen Canyon
National Recreation Area, all in the Colorado Plateau, it becomes
apparent that certain formations or groups of formations play starring
roles in some parks or monuments, some play supporting roles, and in a
few places the entire cast of rocks gets about equal billing. Let us
compare them and see how and where they fit into the “Geologic Time
Spiral” (fig. 61).
Dinosaur N.M. and Colorado N.M., with exposed rocks ranging in age from
Proterozoic to Cretaceous, cover the greatest time spans (nearly 2
billion years), but most of the rocks are missing at Colorado N.M., as
noted below. Dinosaur N.M. has one unit—the Jurassic Morrison
Formation—in the starring role, for this unit contains the many dinosaur
fossils that give the monument its name and fame, although there are
several older units in supporting roles. Grand Canyon N.P. is next, with
rocks ranging in age from Proterozoic through Permian (excluding the
Quaternary lava flows), but here is truly a team effort, for the entire
cast gets about equal billing. Canyonlands N.P. stands third in this
category, with rocks ranging from Pennsylvanian to Jurassic, but we
would have to give top billing to the Permian Cedar Mesa Sandstone
Member of the Cutler Formation, from which The Needles, The Grabens, and
most of the arches were sculptured. The Triassic Wingate Sandstone and
the Triassic(?) Kayenta Formation get second billing for their roles in
forming and preserving Island in the Sky and other high mesas. Now let
us consider other areas with only one or few players in the cast,
beginning at the bottom of the time spiral. Black Canyon of the Gunnison
N.M., cut entirely in rocks of early Proterozoic age with only a veneer
of much younger rocks, obviously has but one star in its cast. Colorado
N.M. contains rocks ranging from Proterozoic to Cretaceous (equal to
Dinosaur in this respect), but Colorado is unique in that all rocks of
the long Paleozoic Era and some others are missing from the cast. Of
those that remain, the Triassic Wingate and the Triassic(?) Kayenta are
the stars, with strong support from the Jurassic Entrada Sandstone and
from the Proterozoic rocks, which floor the U-shaped canyons.
All the bridges in Natural Bridges N.M. were carved from the Permian
Cedar Mesa Sandstone Member of the Cutler Formation, also one of the
stars in Canyonlands N.P. In Canyon de Chelly (pronounced dee shay) N.M.
and Monument Valley (neither a national park nor a national monument, as
it is owned and administered by the Navajo Tribe), the De Chelly
Sandstone Member of the Cutler Formation—a Permian member younger than
the Cedar Mesa—plays the starring role.
Wupatki N.M. near Flagstaff, Ariz., stars the Triassic Moenkopi
Formation. Petrified Forest N.P. (which now includes part of the Painted
Desert) has but one star—the Triassic Chinle Formation, in which are
found many petrified logs and stumps of ancient trees. The
Triassic-Jurassic Glen Canyon Group, which includes the Triassic Wingate
Sandstone, the Triassic(?) Kayenta Formation, and the
Triassic(?)-Jurassic Navajo Sandstone, receives top billing in recently
enlarged Capitol Reef N.P., but the Triassic Moenkopi and Chinle
Formations enjoy supporting roles.
The Triassic(?)-Jurassic Navajo Sandstone, which has a supporting role
in Arches N.P., is the undisputed star of Zion N.P., Rainbow Bridge
N.M., and Glen Canyon National Recreation Area. The Navajo also forms
the impressive reef at the east edge of the beautiful San Rafael Swell,
a dome, or closed anticline, now crossed by Highway I-70 between Green
River and Fremont Junction, Utah.
As we journey upward in the time spiral (fig. 61), we come to the
Jurassic Entrada Sandstone, which stars in Arches N.P., with help from
the underlying Navajo Sandstone, and a supporting cast of both older and
younger rocks. The Entrada also forms the grotesque erosion forms called
“hoodoos and goblins” in Goblin Valley State Park, north of Hanksville,
Utah.
Moving ever upward in the spiral, we come to the Cretaceous—the age of
the starring Mesaverde Group, in which the caves of Mesaverde N.P. were
formed, and which now house beautifully preserved ruins once occupied by
the Anasazi (the ancient people who once dwelt in many parts of the
Plateau).
This brings us up to the Tertiary Period, during the early part of which
the pink limestones and shales of the Paleocene and Eocene Wasatch
Formation were laid down in inland basins. Beautifully sculptured
cliffs, pinnacles, and caves of the Wasatch star in Bryce Canyon N.P.
and in nearby Cedar Breaks N.M. This concludes our climb up the time
spiral, except for Quaternary volcanos and some older volcanic features
at Sunset Crater N.M., near Flagstaff, Ariz.
Thus, one way or another, many rock units formed during the last couple
of billion years have performed on the stage of the Colorado Plateau
and, hamlike, still lurk in the wings eagerly awaiting your applause to
recall them to the footlights. Do not let them down—visit and enjoy the
national parks and monuments of the Plateau, for they probably are the
greatest collection of scenic wonderlands in the world.