Having finished our geologic ramble through Canyonlands National Park,
let us see how this pile of eroded rocks fit into the bigger scheme of
things—the geologic age and events of the earth as a whole, as depicted
in figure 80. As shown in figure 9, the rock strata still preserved in
the park range in age from Pennsylvanian to Jurassic, or from about 300
to 175 million years ago, a span of about 125 million years. This seems
an incredibly long time, until you note that the earth is some 4.5
billion years old and that our rock pile is but one twenty-fifth, or 4
percent, of the age of the earth as a whole. Thus, in figure 80 the
rocks exposed in the park occupy only about the left-hand third of the
top whorl of the spiral.
But this is not the whole story. As indicated earlier, about 10,000 feet
of younger Mesozoic and Tertiary rocks that once covered the area have
been carried away by erosion, and if we include these, the span is
increased to about 250 million years, or nearly a full whorl of the
spiral.
Deep tests for oil and gas tell us that much older rocks underlie the
area, and we have seen that some of these rocks played a part in shaping
the park we see today—note the breaks in the deep-seated Precambrian
rocks and the salt in the Paradox Member. In addition to the Precambrian
igneous and metamorphic rocks, there are about 2,000 feet of Paleozoic
sedimentary rocks older than the Pennsylvanian Paradox Member. Most of
these sedimentary rocks were laid down in ancient seas during Cambrian,
Ordovician, Devonian, Mississippian, and Pennsylvanian times (fig. 80).
There are some gaps in the rock record caused by temporary emergence of
the land above sea level and erosion of the land surface before the land
again subsided below sea level so that deposition could resume. Silurian
rocks are absent altogether, presumably because here the Silurian Period
was dominated by erosion rather than deposition.
While Pennsylvanian and Permian sediments were being deposited in and
southwest of the park, a large area to the northeast—called by
geologists the Uncompahgre highland, because it occupied the same
general area as the present Uncompahgre Plateau—rose slowly above sea
level. Whatever Paleozoic rocks there were on this rising land, plus
part of the underlying Precambrian rocks, were eroded and carried by
streams into deep basins to the northeast and southwest. Thus, while
mostly marine or nearshore deposits were being laid down in and near the
park, thousands of feet of red beds were being laid down by streams in
an area between the park and the Uncompahgre Plateau. During part of
Middle Pennsylvanian time a large area including the park and known as
the Paradox Basin was alternately connected to or cut off from the sea,
so the water evaporated during cutoff periods and was replenished during
periods when connection with the sea resumed. In this huge evaporation
basin were deposited the layers of salt and gypsum plus some potash
salts and shale that now make up the Paradox Member.
The old Uncompahgre highland continued to shed debris into the bordering
basins until Triassic time, when it began to acquire a veneer of red
sandstone and siltstone of the Chinle Formation (Lohman, 1965). The area
remained above sea level during the Triassic Period and most if not all
the Jurassic Period, although the Jurassic Carmel Formation was laid
down in a sea that lay just to the west.
Late in the Cretaceous Period a large part of central and southeastern
United States, including the eastern half of Utah, sank beneath the sea,
as shown in figure 81, and received thousands of feet of mud, silt, and
some sand that later compacted into the Mancos Shale. This formation and
all the younger and some older strata have long since been eroded from
the park area but are present in adjacent areas, such as the lower
slopes of the Book Cliffs north of Green River, Crescent Junction, and
Cisco (fig. 7).
The land rose above the sea at about the close of the Cretaceous and has
remained above ever since, although inland basins and lakes received
sediment during parts of the Tertiary Period. Compressive forces in the
earth’s crust produced some gentle folding of the strata at the close of
the Cretaceous, but more pronounced folding and some faulting occurred
during the Eocene Epoch, when most of the Rocky Mountains took form.
During the Miocene Epoch molten igneous rock welled up into the strata
to form the cores of the nearby La Sal, Abajo, and Henry Mountains (fig.
7). Additional uplift and some folding occurred in the Pliocene and
Pleistocene Epochs.
[Illustration: LATE CRETACEOUS SEA, which covered parts of central
and southeastern United States. (Fig. 81)]
Much of the course of the Colorado River was established in the Miocene
Epoch, with some additional adjustments in the late Pliocene and early
Pleistocene Epochs (Hunt, 1969, p. 67). Erosion during much of the
Tertiary Period and all of the Quaternary Period, combined with some
sagging and breaking of the crust brought on by solution and lateral
squeezing of salt beds beneath The Needles, The Grabens, and the Meander
anticline, produced the landscape as we now see it.
The Precambrian rocks beneath the area are about 1.5 billion years old,
so an enormous span of time is represented by the rocks and events in
and beneath Canyonlands National Park.
If we consider the geologic formations that make up the Colorado
Plateau—including national parks (N.P.), national monuments (N.M.)
(excluding small historical or archeological ones), Monument Valley, San
Rafael Swell, and Glen Canyon National Recreation Area—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. 80).
Dinosaur N. M., with exposed rocks ranging in age from Precambrian to
Cretaceous, represents the greatest time span (nearly 2 billion years)
but 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; several older units have supporting roles. Grand
Canyon N. P. and N. M. are next, with rocks from Precambrian through
Permian (excluding the Quaternary lava flows in the N. M.), but here
there is truly a team effort, for the entire cast gets about equal
billing. Canyonlands N. P. stands third in size of cast, 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 Kayenta Formation
get second billing for their roles in forming and preserving Island in
the Sky and other high mesas.
Now let us consider those 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 Precambrian age (except for only a
veneer of much younger rocks), obviously has but one star in its cast.
Colorado N. M. contains rocks ranging from Precambrian to
Cretaceous—equal to Dinosaur in this respect—but it is unique in that
all the rocks of the long Paleozoic Era and some others are missing from
the cast; of those that remain, the Triassic Wingate and Kayenta are the
stars, with strong support from the Jurassic Entrada Sandstone.
All the bridges in Natural Bridges N. M. were carved from the Permian
Cedar Mesa Sandstone, also the star in Canyonlands N. P. In Canyon de
Chelly (pronounced “dee shay”) N. M. and Monument Valley (neither N. P.
nor N. M., 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) also has but one star—the Triassic Chinle Formation,
with its many petrified logs and stumps of ancient trees. The
Triassic-Jurassic Glen Canyon Group (fig. 9), which includes the
Triassic Wingate Sandstone and 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, erosional remnants of which are
found on the high mesas of Canyonlands N. P., is the undisputed star of
Zion N. P., Rainbow Bridge N. M., and Glen Canyon National Recreation
Area, despite the fact that the latter is the type locality of the
entire Glen Canyon Group (fig. 9). The Navajo also forms the impressive
reef at the eastern edge of the beautiful San Rafael Swell (a dome, or
closed anticline, fig. 7), now crossed by Highway I-70 between Green
River and Fremont Junction, Utah.
As we journey upward in the time spiral (fig. 80), 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 erosional 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, whose caves in Mesa Verde N. P. now house
beautifully preserved ruins once occupied by the Anasazi, the same
ancient people who once dwelt in Canyonlands N. P.
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 nearby Cedar Breaks N. M. This concludes our climb up the time
spiral, except for Quaternary volcanoes and some older volcanic features
at Sunset Crater N. M., near Flagstaff, Ariz.
Thus, one way or another, many geologic units that 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. Don’t 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.
[Illustration: Dinosaur]