The Geologic Story of Arches National Park 11

The Geologic Story of Arches National Park 11

[Illustration: DOUBLE O ARCH, viewed about north from northwest end

of Devils Garden trail. Large opening is 71 feet wide and 45 feet

high; small one at lower left is 21 feet wide and 11 feet high. Span

of large opening is 11 feet wide and 6 feet thick. Arch frames a

part of the Book Cliffs about 14 miles to the north. Photograph by

Hildegard Hamilton, Flagstaff, Ariz. (Fig. 56)]

 

[Illustration: DARK ANGEL, a shaft of the Slick Rock Member that is

an erosional remnant of a once high, narrow fin. About one-half mile

northwest of Double O Arch. Photograph by National Park Service.

(Fig. 57)]

 

[Illustration: “INDIAN-HEAD ARCH,” in upper Devils Garden. Arch and

most of head are in Slick Rock Member, top of head is basal part of

Moab Member. Opening is 4 feet wide and 4½ feet high. Photograph by

Professor Dale J. Stevens, Brigham Young University. (Fig. 58)]

 

[Illustration: GEOLOGIC TIME SPIRAL, showing the sequence, names,

and ages of the geologic eras, periods, and epochs, and the

evolution of plant and animal life on land and in the sea. The

primitive animals that evolved in the sea during the vast

Precambrian Era left few traces in the rocks because they had not

developed hard parts, such as shells, but hard shell or skeletal

parts became abundant during and after the Paleozoic Era. (Fig. 59)]

 

 

 

 

GEOLOGIC TIME

The Age of the Earth

 

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!

 

Professor Stevens found 14 arches in what he called upper Devils Garden,

northwest of Double O Arch, and two arches in the northwesternmost

extension of the park known as Eagle Park (fig. 1). One of the unnamed

arches in upper Devils Garden is shown in figure 58. I am tentatively

calling it “Indian-Head Arch,” because of the rather obvious

resemblance.

 

This ends our journey through Arches National Park, but there remains

for consideration a summary of the principal geologic events leading to

the formation of this beautiful part of the Colorado Plateau and a brief

comparison with the geology of other national parks and monuments on the

Plateau.

 

 

 

 

Summary of Geologic History

 

 

Having finished our geologic trip through Arches National Park, let us

see how the arches and other features fit into the bigger scheme of

things—the geologic age and events of the Earth as a whole, as depicted

in figure 59. As shown in figure 4, the rock strata still preserved in

the park range in age from Pennsylvanian to Cretaceous, or from about

300 million to 100 million years old—a span of about 200 million years.

This seems an incredibly long time, until one notes that the earth is

some 4.5 billion years old, and that our rock pile is but 1/23 or 4½

percent of the age of the Earth as a whole. Thus, in figure 59, the

rocks exposed in the park occupy only about the left half of the top

whorl of the spiral.

 

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 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 played a part in shaping the

park we see today. In addition to the Precambrian igneous and

metamorphic rocks, there is about 2,000 feet of Paleozoic sedimentary

rocks older than the Pennsylvanian Paradox Member of the Hermosa

Formation, most of which was laid down in ancient seas. This includes

strata of Cambrian, Ordovician, Devonian, Mississippian, and

Pennsylvanian ages (fig. 59). 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, presumably because,

here, the Silurian Period was dominated by erosion rather than

deposition.

 

While Pennsylvanian and Permian rocks were being laid down 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 part of the present Uncompahgre Plateau), rose slowly

above sea level. Whatever Paleozoic rocks 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

some marine or near-shore deposits were being laid down in and south of

the park, thousands of feet of red beds were being laid down by streams

between the park and what is now the Uncompahgre Plateau. During part of

Middle Pennsylvanian time, a large area, including the park, known as

the Paradox basin, was alternately connected to or cut off from the sea,

so that the water was evaporated during cutoff periods and replenished

during periods when connection with the sea resumed. In these huge

evaporation basins were deposited the salt and gypsum plus some potash

salts and shale that now make up the Paradox Member of the Hermosa

Formation.

 

Arches National Park contains four northwesterly trending major

folds—the Salt Valley and Cache Valley salt anticlines, the Courthouse

syncline, and the faulted Moab-Seven Mile anticline, which forms the

southwestern border. How these folds were formed was explained on pages

27-32. The history of their growth, however, was a long one that began

about 300 million years ago in the Pennsylvanian and ended about 50

million years ago in the early Tertiary. The growth of these folds

occurred in two stages. The first stage, which involved the development

of the salt cores of the anticlines, ended in the Jurassic with the

beginning of Morrison time; the second stage, which involved additional

folding that intensified the magnitude and shape of existing folds,

occurred in the early Tertiary and was followed later by collapse of the

salt anticlines. The formation and collapse of the Salt Valley and Cache

Valley anticlines was accompanied by pronounced jointing (fig. 12),

which allowed differential erosion to produce the tall fins in which the

arches were formed.

 

The old Uncompahgre Highland continued to shed debris into the bordering

basins until Triassic time, when it began to be covered by 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, of 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

and received thousands of feet of mud, silt, and some sand that later

compacted into the Mancos Shale. This formation, as well as all younger

and some older strata, has long since been eroded from most of the park

area, but a little of the Mancos is preserved in the Cache Valley graben

(fig. 11), and the entire Mancos Shale and younger rocks are present in

adjacent areas, such as the Book Cliffs north of Green River, Crescent

Junction, and Cisco (figs. 7, 50, 56).

 

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 igneous rock welled up into older rocks to form

the cores of the nearby La Sal, Abajo, and Henry Mountains. Additional

uplift and some folding occurred in the Pliocene and Pleistocene Epochs.

 

Much of the course of the Colorado River was established during the

Miocene Epoch, with some additional adjustments in the late Pliocene and

early Pleistocene Epochs (Hunt, C. B., 1969, p. 67). Erosion during much

of the Tertiary Period and all of the Quaternary Period plus some

sagging and breaking of the crest of the anticlines, brought on by

solution and lateral squeezing of salt beds beneath the Moab-Seven Mile,

Salt Valley, and Cache Valley anticlines, combined to produce 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 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. 59).

 

Dinosaur N.M., with exposed rocks ranging in age from Precambrian to

Cretaceous, covers 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, although there are several older units in supporting

roles. Grand Canyon N.P. and N.M. are next, with rocks ranging in age

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 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.