The Geologic Story of Arches National Park 8

The Geologic Story of Arches National Park 8

A Trip Through The Park

 

 

Colorado River Canyon

 

The southeastern boundary of the park for about 11 miles is the Colorado

River, from the bridge on which U.S. Highway 163 crosses the river to a

point upstream about half a mile below the mouth of Salt Wash.

Illuminated night float trips down part of this reach are run during the

summer, as noted on p. 16. Partly paved State Highway 128 follows the

southeast side of the river for about 30 miles to Dewey Bridge, then

goes northward about 15 miles to Cisco, where it connects with Highway

I-70.

 

The rocks of the Glen Canyon Group form the southernmost corner of the

park, as shown in figure 19. About 2 miles northeast of the bridge, we

cross the axis of the Courthouse syncline (fig. 9), which brings the

Navajo Sandstone down nearly to river level, as shown in figure 20. The

underlying Kayenta Formation is largely hidden by vegetation and

alluvial deposits in this view.

 

[Illustration: GLEN CANYON GROUP, forming southernmost point of

park, as viewed across the Colorado River from State Highway 128

half a mile above Moab bridge carrying U.S. Highway 163. Massive

sandstone forming about the lower third of cliff is the Wingate

Sandstone, darker thin-bedded sandstones and mudstones forming

middle section of cliff comprise the Kayenta Formation, upper cliff

is the lower part of the Navajo Sandstone. Note that the saltcedar

(tamarisk), which lines both banks of the river, is in full bloom.

(Fig. 19)]

 

[Illustration: NAVAJO SANDSTONE CLIFFS, bordering west bank of

Colorado River in Courthouse syncline, from State Highway 128 about

2 miles above the Moab bridge. Note rounded domes at top of cliff.

(Fig. 20)]

 

[Illustration: MOUTH OF SALT WASH, viewed across Colorado River from

point on State Highway 128, 11 miles above Moab bridge. Dark cliffs

on upper right and left are of Wingate Sandstone capped by thin

protective cover of resistant sandstone beds of the Kayenta

Formation. In background Wingate is overlain by entire Kayenta

Formation and lower part of the Navajo Sandstone. Wingate is

underlain to river level by weathered slope of the Chinle Formation.

Water in Salt Wash is largely backwater from the bankfull river;

actual flow in wash generally is much less but at times reaches

flood proportions. (Fig. 21)]

 

About 11 miles above the Moab bridge is the mouth of Salt Wash (fig. 1),

as viewed from State Highway 128. (See fig. 21.) Seventeen miles above

the bridge (east of area shown in fig. 1), we get an excellent view of

the southeast end of the highly faulted Cache Valley anticline, as shown

in figure 22. The background shown in the photograph formerly was the

easternmost part of the former monument, but when the monument graduated

to a park on November 16, 1971, this part of Cache Valley along with

most of Dry Mesa was withdrawn from the park and put under the

supervision of the Bureau of Land Management, also a part of the

Department of the Interior.

 

[Illustration: SOUTHEAST END OF FAULTED CACHE VALLEY ANTICLINE,

viewed northwestward across Colorado River from a point on State

Highway 128, 17 miles above Moab bridge. High cliff of Wingate

Sandstone on left is capped by thin protective layer of the Kayenta

Formation. About upper third of slope below base of cliff is the

Chinle Formation, below which is the Moenkopi Formation extending to

high-water level. Note bent and broken beds on right. (Fig. 22)]

 

As noted on page 16, part of “Run, Cougar, Run” was filmed just upstream

from the irrigated field in the foreground of figure 22, in a wide part

of the valley called Professor Valley (fig. 7). This valley and the

Richardson Amphitheater on the southeast side of the river were named

after a Professor Richardson who settled in the area in the 1880’s. The

long abandoned townsite of Richardson was 1¼ miles due east from the

point from which figure 22 was taken.

 

 

Headquarters Area

 

The junction of the park road with U.S. Highway 163 is shown at the

lower left of figure 23, and the entrance station, Visitor Center,

parking lot, and several buildings are seen at the lower right. Several

residences for park personnel and other buildings are shown in figure

25. As shown in the lower part of figure 23, the geology at the park

entrance is rather complex, as the park boundary here is partly along

the Moab fault and partly along a branch fault—both in the Seven

Mile-Moab Valley anticline (fig. 7). The Moab fault extends

northwestward from Moab for more than 30 miles (McKnight, 1940, p. 120,

121, pl. 1).

 

As shown in figure 23, soon after leaving the checking station the park

road begins to ascend the first of several switchbacks, and cuts first

into the Slick Rock Member, then the Dewey Bridge Member, and finally

the Navajo Sandstone the rest of the way to and beyond the top of the

hill.

 

From points a mile or so up the hill may be seen interesting features in

several directions.[5] The view to the southwest is shown in figure 23,

to the west are the Three Penguins (fig. 24). A good view of the Moab

Valley is had by looking southeastward (fig. 25). A well in the Navajo

Sandstone at the base of the hill supplies water to all the residences

and to the Visitor Center, where a drinking fountain and modern

restrooms are available to the public. Storage is provided by a steel

tank hidden in a ravine above the buildings shown in figure 25.

 

To the north the wall of Entrada Sandstone is cut by a normal fault

(fig. 6), as shown in figure 26.

 

[Illustration: FAULTED SEVEN MILE-MOAB VALLEY ANTICLINE. Top, View

toward the southwest from park road about 1 mile above entrance

station. Bottom, Geologic interpretation of photograph in part after

McKnight (1940, pl. 1). Moab fault and branch fault (both normal

faults, fig. 6) unite just beyond ridge of Slick Rock Member. Total

vertical displacement along both faults is about 2,500 feet. H.F.,

unnamed upper member of Hermosa Formation; M.F., Moenkopi Formation;

D, downthrown side of faults; U, upthrown side. Valley fill and

slope wash of recent (Holocene) age obscure faults and underlying

rocks. The original sequence of the rocks may be visualized by

placing the Navajo Sandstone, the upper part of which is exposed at

the lower right, on top of the Kayenta Formation, the lower few feet

of which cap and protect the cliffs of Wingate Sandstone in the

background. The Pacific Northwest (gas) Pipeline mentioned on page

15 is buried beneath the slice of the Moenkopi Formation between the

two faults, which accounts for the disturbed appearance of the rock.

(Fig. 23)]

 

[Illustration: Geologic interpretation of photograph]

 

[Illustration: THREE PENGUINS, viewed westward from park road about

1 mile above entrance station. Penguins are carved in massive Slick

Rock Member seen resting upon thin-bedded Dewey Bridge Member. (Fig.

24)]

 

[Illustration: MOAB VALLEY, viewed southeastward from park road

about 1 mile above entrance station. Moab fault in about middle of

valley, hidden beneath recent (Holocene) valley fill and slope wash,

separates unnamed upper member of Hermosa Formation just above U.S.

Highway 163 on right from Navajo Sandstone forming hills on left and

ledges in foreground. Park Service residences at base of hill. White

patch bordering Colorado River on northwest is tailings pile of

Atlas Corporation’s uranium mill. Moab and Spanish Valley are beyond

river, and south end of La Sal Mountains forms distant skyline.

(Fig. 25)]

 

[Illustration: FAULTED WALL OF ENTRADA SANDSTONE, north of park road

about 1 mile above entrance station. Fault is nearly vertical and

normal (fig. 6), but fault trace slopes steeply downward to right,

separating upthrown Slick Rock and Dewey Bridge Members on left from

downthrown Slick Rock Member on right. Light-colored rock in

foreground is Navajo Sandstone. Displacement probably does not

exceed 50 feet. (Fig. 26)]

 

[Illustration: PARK AVENUE, viewed to the north along trail. (Fig.

27)]

 

 

Courthouse Towers Area

 

About 2.3 miles from the entrance station is a turnoff and parking area

at the south end of the Park Avenue trail (stop 2), which is about 1

mile long and ends at another parking area 1.7 miles farther north. An

interesting hike is best made from south to north in a downhill

direction, and hikers generally meet the cars of relatives or friends

awaiting them at the northern parking area. The trail begins in a canyon

cut in the soft Dewey Bridge Member and walled by high fins of the Slick

Rock Member (fig. 27), but farther north the canyon is floored by the

bare Navajo Sandstone. The avenue was named from the resemblance of the

east wall to a row of tall buildings. Atop the west wall, just to the

left of the view in figure 27, are two balanced rocks (fig. 28). The one

on the left, which resembles somewhat the head of an Egyptian queen, is

offset to the right along a bedding plane, and this offset may have been

caused by an earthquake.

 

As we progress toward Courthouse Towers proper, lofty fins and monoliths

lie mostly on our left, and to the right are fine distant views of the

La Sal Mountains (stop 4). A general view of the Courthouse Towers is

shown in figure 29, and closeups of two of the named rock sculptures—the

Three Gossips and Sheep Rock—are shown in figures 30 and 31. Just beyond

Sheep Rock, which some think resembles the Sphinx, we see “Baby Arch,”

shown in figure 15.

 

Five miles from the entrance station, the road crosses Courthouse Wash

on a modern bridge (stop 6)—a distinct improvement over the two tracks

in the sand we used in 1946. The Courthouse syncline, named after the

wash, extends northwestward through here. (See figs. 8, 9, 20.) About a

mile west of the bridge, Professor Stevens found another pothole arch. A

mile and a half north of the bridge is stop 7, where attention is called

in the booklet to the vast area of “petrified dunes” east of the road,

which are simply dunelike exposures of the crossbedded Navajo Sandstone

formed originally by the cementation of a vast area of sand dunes. My

view of these was taken about 1 mile beyond the stop (fig. 32).

 

[Illustration: BALANCED ROCKS ON SOUTH WALL OF PARK AVENUE, at south

end of trail. (Fig. 28)]

 

[Illustration: COURTHOUSE TOWERS, viewed to the northwest from point

on park road about three-fourths of a mile northeast of the south

end of Park Avenue trail. Sandstone towers are Slick Rock Member

resting on Dewey Bridge Member, which also forms foreground. Three

Gossips at upper left, Sheep Rock just beyond. The Organ and Tower

of Babel are on right. (Fig. 29)]

 

[Illustration: THE THREE GOSSIPS, shown at upper left of figure 29.

(Fig. 30)]

 

[Illustration: SHEEP ROCK, shown on center-left skyline in figure29. (Fig. 31)]