The Geologic Story of Arches National Park 7

The Geologic Story of Arches National Park 7

Both nearly flat but slightly irregular beds of sandstone and relatively

thin walls or fins of sandstone are prime targets for this differential

erosion. Potholes, as shown in figure 18_A_, may be formed in relatively

flat beds by the dissolving action of repeated accumulations of

rainwater or snowmelt, even in arid regions like the Plateau.

 

Relatively thin walls, or fins as they are called in parts of the

Plateau including Arches, are targets for the formation of alcoves and

caves by solution of cement and removal of sand by gravity, wind, and

water, aided by the prying action of frost in joints, bedding planes, or

other openings. Once a breakthrough of a wall or fin occurs, weakened

chunks from the ceiling tend to fall, and natural arches of various

shapes and sizes are produced. Arches form the strongest shapes for

supporting overlying rock loads, as the rock in the arch is compressed

toward each abutment by the heavy loads. Blocks of compressed rock

beneath a relatively flat ceiling tend to be dislodged also by expansion

due to release of pent-up pressure, until a strong self-supporting arch

is formed. Release of pent-up pressure in rock walls may help also in

initiating the formation of alcoves or caves in cliff faces. Man,

including the ancient Greeks, Romans, Egyptians, and others, has long

made use of arches in building bridges, aqueducts, temples, cathedrals,

and other enduring edifices.

 

As vividly shown in figure 12, the Entrada Sandstone on the northeast

flank of the Salt Valley anticline has been broken by Earth forces into

thin slabs mostly 10 to 20 feet thick between nearly parallel joints,

but, as will be noted in the descriptions of individual arches, some

rock walls are only 1 or 2 feet thick, whereas others are 50 feet thick

or more. Some weak or thin slabs have weathered away, leaving the

stronger or thicker ones as towering fins, particularly in the Fiery

Furnace and Devils Garden areas. Jointing on a less spectacular scale

also has broken the Entrada in areas south of Salt Valley, leaving walls

or fins of rock.

 

[Illustration: TUNNEL ARCH, reached by short trail north of main

trail through Devils Garden. Opening is 26½ feet wide and 22 feet

high; span is about 14 feet thick. (Fig. 14)]

 

Although all the arches in the park were carved from the Entrada

Sandstone, slight differences in their mode of origin or placement

within the Entrada allow them to be grouped into three classes: (1)

vertical arches formed in the Slick Rock Member alone or in the Slick

Rock and Moab Members, (2) vertical arches formed mainly in the Slick

Rock Member but partly in, and with the aid of, the incompetent

underlying Dewey Bridge Member, and (3) horizontal arches, or so-called

pothole arches, formed from the union of a vertical pothole and a

horizontal cave. Hereinafter, the three members will be referred to

alone, without reference to the Entrada.

 

[Illustration: “BABY ARCH,” just southwest of Sheep Rock in

Courthouse Towers area. For details, see text. (Fig. 15)]

 

Before giving examples of arches in each of the three classes, it is

appropriate to remark that the arches and other erosion forms in the

park represent but a fleeting instant in geologic time. Many of the

pinnacles or piles of rock may be the broken remains of former arches,

and many of the arches we see may be gone tomorrow, next year, or a few

hundreds of years and, certainly, before many thousands of years. On the

other hand, many new arches will form by the processes described above

as the geologic clock ticks on.

 

[Illustration: BROKEN ARCH, reached by a ½-mile trail leading

northward across field that separates Fiery Furnace from Devils

Garden. White thin-bedded unit at top is the Moab Member, which

rests upon the massive salmon-colored Slick Rock Member. Opening is

59 feet wide and 43 feet high. (Fig. 16)]

 

 

Examples of Arches

 

Tunnel Arch (fig. 14) is a good example of an arch eroded entirely

within the massive Slick Rock Member. Just southwest of Sheep Rock (fig.

31) is an unnamed opening in the lower part of the Slick Rock Member

which I call “Baby Arch,” because it is one of the newest ones visible

from the park road (fig. 15). It is only 25½ feet wide and 14 feet high

and penetrates a wall 14 feet thick. Note that the breakthrough probably

began along the prominent recessed bedding plane at the base of the

arch. Its youthfulness is also indicated by the sharp, angular breaks in

the ceiling and by the pile of freshly fallen rocks. Some visitors have

asked park personnel why they have not cleared away such debris! Despite

its youthfulness, the ceiling has already taken on the shape of an arch.

 

Broken Arch (fig. 16) was formed near the top of the Slick Rock Member

and is strengthened and protected by the more resistant overlying Moab

Member, which forms the upper half of the span. The crest is only 6 feet

thick at the thinnest point and is not broken as the name seems to

imply.

 

Double Arch (fig. 17), “one” of the most beautiful in the park, is in

The Windows section near the east end of the road. The southeast arch,

which is 160 feet wide and 105 feet high, is the second largest in the

park, but the west arch measures only 60 feet wide and 61 feet high. In

common with most arches in The Windows section, these two arches of the

Slick Rock Member rest upon bases of the weak, easily eroded Dewey

Bridge Member. More rapid erosion of the Dewey Bridge undercut the

arches and hastened their development.

 

[Illustration: DOUBLE ARCH, in The Windows section. (Fig. 17)]

 

[Illustration: PROBABLE STEPS IN FORMATION OF POTHOLE ARCH. _A_,

Original pothole probably formed in relatively level bed of

sandstone, such as this one, which is in an older rock unit—the

White Rim Sandstone Member of the Cutler Formation, a unit not

present in Arches. This pothole, which contains 4 feet of water, is

in nearby Canyonlands National Park (Lohman, 1974, fig. 17), just

north of the edge of the White Rim, about 4½ miles north of the

confluence of the Green and Colorado Rivers. Photograph by E. N.

Hinrichs. _B_, Pothole is being deepened by solution while cliff is

receding toward pothole by weathering. _C_, As erosion continues,

pothole and cave in cliff face are growing deeper. _D_, Pothole Arch

formed by union of vertical pothole and horizontal cave. _E_,

Telephoto view of Pothole Arch from park road near stop 14. Visible

span is 90 feet across and 30 feet high. (Fig. 18)]

 

[Illustration: Fig. 18 B]

 

[Illustration: Fig. 18 C]

 

[Illustration: Fig. 18 D]

 

[Illustration: Fig. 18 E]

 

The cause of the wavy bedding in the Dewey Bridge Member, as shown in

figure 17 but as better shown in the frontispiece, is not known for sure

but generally is regarded to be the result of irregular slumping during

or just after deposition of the sediments in a body of water, caused by

the weight of overlying sediments.

 

The last example I shall take up is Pothole Arch (fig. 18), which

differs from all the other examples in that this arch is roughly

horizontal rather than vertical. Most park visitors, including me, were

not aware of this interesting feature until after publication of the

pamphlet “The Guide to an Auto Tour of Arches National Park,” which, as

previously noted, may be purchased at the Visitor Center. Pothole Arch

caps a ridge high above the road half a mile northwest of Garden of

Eden, so only those who happened to look up at the right place were

aware of its existence.

 

A different mode of origin than that given in the caption for figure 18

is depicted on a poster in the Visitor Center, which shows the pothole

being formed by a waterfall having an apparent flow rate of several

cubic feet per second. Potholes can be formed in this manner in places

where sufficient streamflow is available, either continuously or

following rainstorms, but I believe the process depicted in figure 18 is

a more likely mode of origin for Pothole Arch.

 

 

 

 

How to See the Park

 

 

As aptly stated on a poster in the Visitor Center, how to see the park

depends in part upon the question “How long can you stay?” Inasmuch as

the park entrance and Visitor Center are beside a through U.S. Highway

(163), many motorists first become aware of the park’s existence from

the entrance sign, and some take time for at least a quick visit, such

as a round trip to The Windows section, which can be made in an hour or

so.

 

For those who have or take more time and are able to walk at least short

distances, a visit of 1 or 2 days is a very rewarding experience.

Others, particularly avid shutterbugs and those with camping gear,

profitably spend from several days to a week or more and hike all or

most of the trails.

 

Regardless of how long you plan to spend, I urge at least a brief stop

at the Visitor Center, where excellent displays and a narrated slide

show help materially in conveying just what the park has to offer. At

the counter you can purchase a copy of “The Guide to an Auto Tour of

Arches National Park,” which explains the views from each of 25 numbered

stops along the park road, as well as other reports describing arches or

other parks and monuments.

 

The park is open the year round, but, like most high deserts, it gets

rather hot in the summer and cold enough in the winter for occasional

snows and is sometimes closed temporarily because of heavy snowfall. The

weather generally is ideal during the spring and fall. Even though

summer daytime temperatures may exceed 100°F (37.8°C) and slow down

hikers, the nights are cool enough for comfortable sleeping beneath

ample covers. Before beginning our trip through the park proper, let us consider a

beautiful part many people fail to realize actually belongs to the

park—the Colorado River canyon forming the southeastern boundary.