The Geologic Story of Arches National Park 6

The Geologic Story of Arches National Park 6

Uplift And Erosion of The Plateau

 

 

Next among the main events leading to the formation of landforms in the

park was the raising and additional buckling and breaking of the Plateau

by Earth forces partly during the Late Cretaceous but mainly during the

early Tertiary. After uplift and deformation, the Plateau was vigorously

attacked by various forces of erosion, and the rock materials pried

loose or dissolved were eventually carted away to the Gulf of California

by the ancestral Colorado River. Some idea of the enormous volume of

rock thus removed is apparent when one looks down some 2,000 feet to the

river from any of the high overlooks farther south, such as Dead Horse

Point (Lohman, 1974, fig. 15). Not so apparent, however, is the fact

that younger Mesozoic and Tertiary rocks more than 1 mile thick once

overlaid this high plateau but have been swept away by erosion. In all,

the river has carried thousands of cubic miles of sediment to the sea

and is still actively at work on this gigantic earth-moving project. In

an earlier report (Lohman, 1965, p. 42) I estimated that the rate of

removal may have been as great as about 3 cubic miles each century. For

a few years the bulk of the sediment was dumped into Lake Mead, but now

Lake Powell is getting much of it. When these and other reservoirs

ultimately become filled with sediment—for reservoirs and lakes are but

temporary things—the Gulf of California will again become the burial

ground.

 

According to Cater (1970, p. 65-67), who made an intensive study of the

salt anticlines, collapse of their crests seemingly occurred in two

stages—the first stage following Late Cretaceous folding; the second

following uplift of the Plateau later in the Tertiary. Solution and

removal of salt by ground water played the leading role in the ultimate

collapse.

 

[Illustration: TILTED BLOCK OF ROCKS IN CACHE VALLEY GRABEN, viewed

to the east toward Cache Valley from point on gravelled side road to

Wolfe’s cabin, about half a mile east of paved road. Steep slope on

left composed of Jurassic Morrison Formation, hogback on top formed

by Dakota Sandstone of Late Cretaceous age, and gentle slopes to

right composed of the Mancos Shale of Late Cretaceous age. (Fig.

11)]

 

As shown by Dane (1935, pl. 1, p. 121-126), collapse of the Salt Valley

and Cache Valley anticlines was accompanied by considerable faulting and

jointing, particularly along their northeast sides; by the upward

intrusion of two large areas of the Paradox Member of the Hermosa

Formation, one just northwest of the park and one in the middle of Salt

Valley south of the campground; and by two downdropped masses of rock

known to geologists as grabens (pronounced gräbǝns)—one just northwest

of the park and one called the Cache Valley graben, which extends both

east and west from Salt Wash. The Cache Valley graben has preserved from

erosion the youngest rock formations in the park, as shown in figure 11.

 

The remarkable jointing of the rocks on the northeast limb of the Salt

Valley anticline is shown in figure 12. All the arches in this section

of the park were eroded through thin fins of the Slick Rock Member of

the Entrada Sandstone, and some, like Broken Arch, figure 16, are capped

by the Moab Member.

 

Differences in the composition, hardness, arrangement, and thickness of

the rock layers determine their ability to withstand the forces of

fracturing and erosion and, hence, whether they tend to form cliffs,

ledges, fins, or slopes. Most of the cliff- or ledge-forming rocks are

sandstones consisting of sand deposited by wind or water and later

cemented together by silica (SiO₂), calcium carbonate (CaCO₃), or one of

the iron oxides (such as Fe₂O₃), but some hard, resistant ledges are

made of limestone (calcium carbonate). The rock column (fig. 4) shows in

general how these rock formations are sculptured by erosion and how they

protect underlying layers from more rapid erosion. The nearly vertical

cliffs along the lower reaches of Salt and Courthouse Washes and the

Colorado River canyon upstream from Moab consist of the well-cemented

Wingate Sandstone protected above by the even harder sandstones of the

Kayenta Formation. (See figs. 21, 22.) To borrow from an earlier report

of mine (Lohman, 1965, p. 17), “Vertical cliffs and shafts of the

Wingate Sandstone endure only where the top of the formation is capped

by beds of the next younger rock unit—the Kayenta Formation. The Kayenta

is much more resistant than the Wingate, so even a few feet of the

Kayenta * * * protect the rock beneath.” In some places, as shown in

figures 19 and 20, the overlying Navajo Sandstone makes up the topmost

unit of the cliff.

 

[Illustration: JOINTED NORTHEAST FLANK OF SALT VALLEY ANTICLINE,

viewed westward from an airplane. Light-colored wedge in middle

background is Salt Valley bordered on extreme left by Klondike

Bluffs. Dark-colored fins and pinnacles on left, of Slick Rock

Member of the Entrada Sandstone, form Devils Garden. Sharp pinnacle

above valley is the Dark Angel. (See fig. 57.) White bands of

sandstone extending to foreground are composed of Moab Member of the

Entrada. Note vegetation in the joints. Photograph by National Park

Service. (Fig. 12)]

 

Last but far from least among the factors responsible for the grandeur

of Arches National Park and the Plateau in general is the desert

climate, which allows one to see virtually every foot of the vividly

colored naked rocks, and which has made possible the creation and

preservation of such a wide variety of fantastic sculptures. A wetter

climate would have produced a far different, smoother landscape in which

most of the rocks and land forms would have been hidden by vegetation.

On the Plateau the vegetation grows mainly on the high mesas and the

narrow flood plains bordering the rivers, but scanty vegetation also

occurs on the gentle slopes or flats.

 

The combination of layers of sediments of different composition,

hardness and thickness, the bending and breaking of the rocks, and the

desert climate, has produced steep slopes having many cliffs, ledges,

and fins with generally sharp to angular edges, rather than the subdued

rounded forms of more humid regions.

 

 

 

 

Origin And Development of The Arches

 

 

Among the questions commonly asked by visitors are, “How do arches

form?”, “Why are some openings called windows, others arches?”, “What is

the difference, if any, between arches or windows and natural bridges,

such as those at Natural Bridges National Monument?”, and “How many

arches are there in Arches National Park?” Before taking up the origin

and development of arches, I shall attempt to explain the differences

between the three types of natural rock openings named above and comment

upon the number of arches.

 

[Illustration: INDEX MAP, showing localities where most of the

photographs were taken. Arrows point to distant views. Numbers refer

to figure numbers. (Fig. 13)]

 

I believe most geologists and geographers are in general agreement with

Cleland (1910, p. 314) that “a ‘natural bridge’ is a natural stone arch

that spans a valley of erosion. A ‘natural arch’ is a similar structure

which, however, does not span an erosion valley.” According to this

definition, Natural Bridges National Monument includes three true

bridges, whereas all the larger rock openings in Arches National Park

with which I am familiar are properly termed “arches,” but some are

called windows. If we were to distinguish between arches and windows, we

might say that arches occur at or near the base of a rock wall, as do

the doors of a house or building, whereas windows are found well above

ground level. This distinction was not followed in naming the rock

openings in the park, however; for example, Tunnel Arch (fig. 14) is

considerably higher above the ground than North Window (figs. 37, 38) or

South Window (fig. 39).

 

As to the number of arches in the park, I might begin by saying that

there is no universal agreement as to how large a rock opening must be

to qualify as an arch. The pamphlet formerly handed to visitors entering

the park proclaimed that “Nearly 90 arches have been discovered, and

others are probably hidden away in remote and rugged parts of the area,”

but the average visitor probably sees less than a third of this number.

 

David May, Assistant Chief of Interpretation and Resource Management,

Moab office of National Park Service (oral commun., Oct. 1973), believes

that if only those in the park having a minimum dimension of 10 feet in

any one direction were considered to be arches, the number would boil

down to about 56 or 57. The most complete count of arches and other

openings in all of southeastern Utah was made by Dale J. Stevens,

Professor of Geography at Brigham Young University, during the period

February through April 1973. He considered those with openings of 3 feet

or larger and found more than 300 in southeastern Utah, of which 124 are

in Arches National Park, although he stated that several areas of the

park were not intensively searched because of time limitations (written

commun., July and Sept. 1973). The 124 arches and openings are

distributed among the several named areas of the park, as follows:

Courthouse Towers, 13; Herdina Park, 11; The Windows section, 25;

Delicate Arch area, 3; Fiery Furnace, 19; Devils Garden, 25; upper

Devils Garden (northwest of Devils Garden), 14; Eagle Park, 2; and

Klondike Bluffs, 12.

 

Professor Stevens generally used a range finder or a steel tape to

measure the width and height of the openings and the width and thickness

of the spans, but estimated a few of the dimensions. In the text

descriptions of arches or captions of figures that follow, I am

including all or part of these measurements, without further

acknowledgment.

 

All the arches in the park were formed in the Entrada Sandstone, mainly

in the Slick Rock Member but partly in the Slick Rock and Dewey Bridge

Members, and a few in the Slick Rock Member occur not far beneath the

base of the overlying Moab Member. The sandstone of the three members is

composed mainly of quartz sand cemented together by calcium carbonate

(CaCO₃), which also forms the mineral calcite and the rock known as

limestone, but the Dewey Bridge Member also contains beds of sandy

mudstone. Limestone and calcite are soluble in acid, even in weak acid

such as carbonic acid, HHCO₃, also written H₂CO₃, formed by the solution

of carbon dioxide (CO₂) in water. Ground water, found everywhere in rock

openings at different depths beneath the land surface, contains

dissolved carbon dioxide derived from decaying organic matter in soil,

from the atmosphere, and from other sources. Even rainwater and snow

contain a little carbon dioxide absorbed from the atmosphere—enough to

dissolve small amounts of limestone or of calcite cement from sandstone.

The calcite cement in the Entrada and in many other sandstones is

unevenly distributed, however, so that all the cement is removed first

from places that contain the least amounts, and, once the cement is

dissolved away, the loose sand is carried away by gravity, wind, or water.