The vivid and varied colors of the bare rocks and the fantastic buttes,
spires, columns, alcoves, caves, arches, and other erosional forms of
Arches National Park result from a fortuitous combination of geologic
and climatic circumstances and events unequalled in most other parts of
the world.
First among these events was the piling up, layer upon layer, of
thousands of feet of sedimentary rocks under a wide variety of
environments. Sedimentary rocks of the region are composed of clay,
silt, sand, and gravel carried and deposited by moving water; silt and
sand transported by wind; and some materials precipitated from water
solutions, such as limestone (calcium carbonate), dolomite (calcium and
magnesium carbonate), gypsum (calcium sulfate with some water),
anhydrite (calcium sulfate alone), common salt (sodium chloride), potash
minerals, such as potassium chloride, and a few other less common types.
Some of the beds were laid down in shallow seas that once covered the
area or in lagoons and estuaries near the sea. Other beds were deposited
by streams in inland basins or plains, a few were deposited in lakes,
and the constituents of deposits like the Navajo Sandstone, were carried
in by the wind. The character and thickness of the exposed sedimentary
rocks and the names and ages assigned to them by geologists are shown in
the rock column (fig. 4) and in the cross section (fig. 8). The history
of their deposition is summarized on pages 98-102. Figure 4 was compiled
mainly from generalized sections given by A. A. Baker (1933), Dane
(1935), McKnight (1940), and Wright, Shawe, and Lohman (1962), and, in
part, from Hite and Lohman (1973).
[Illustration: ROCK COLUMN OF ARCHES NATIONAL PARK. Average
thickness of units 250-1,000 feet is exaggerated two times; those
less than 250 feet, four times. 1 foot = 0.305 meter. (Fig. 4)]
AGE (millions of yrs ago)
GEOLOGIC AGE
NAME OF ROCK UNIT
KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION
THICKNESS (feet)
NAMED FOR OCCURRENCE AT OR NEAR
100
Late Cretaceous
Mancos Shale
Lead-gray fossiliferous marine shale. Forms slopes.
?
Mancos, Colo.
Dakota Sandstone
Conglomeratic sandstone, gray shale, carbonaceous shale, and
coal. Forms ledge.
100
Dakota, Nebr.
Unconformity
Late Jurassic
Morrison Fm.
700
Morrison, Colo.
Brushy Basin Member
Variegated shale, some sandstone and conglomerate, petrified
wood, chert, and dinosaur bones. May contain some beds
of Burro Canyon (Early Cretaceous) age.
Salt Wash Member
Crossbedded white and gray conglomeratic sandstone beds and
lenses, locally carnotite bearing, and red and gray
sandy mudstone. Forms slopes.
Unconformity
160
San Rafael Group
(San Rafael Swell, Utah)
Summerville Fm.
Thin bedded red sandstone and shale. Some cherty limestone
concretions. Forms slopes.
0-40
Summerville Point, Utah
Entrada Ss.
(Entrada Point, Utah)
Moab Member
White, crossbedded fine-grained sandstone. Caps Slick Rock
Member north of Devils Garden and Fiery Furnace and on
Klondike Bluffs.
0-100
Moab, Utah
Slick Rock Member
Salmon-colored to pink and white fine-grained generally
crossbedded sandstone, containing some medium- to
coarse-grained sand. Generally forms cliffs or narrow
fins many of which contain arches or windows.
0-240
Slick Rock, Colo.
Dewey Bridge Member
Red muddy sandstone and sandy mudstone, with contorted
bedding. Forms easily eroded bases to arches in
Windows Section, hence aided in their development.
0-175
Dewey Bridge, Utah
Unconformity
190
Jurassic and Triassic(?);
Glen Canyon Group
Navajo Sandstone
Massive crossbedded buff, gray, and white fine-grained
sandstone, and local beds of gray limestone. Forms
cliffs along Colorado River, floors Windows Section.
0-350
Navajo Country, Four Corners (Glen Canyon, U.)
Late Triassic(?)
Kayenta Formation
Lavender, gray, and white lenses of sandstone, red sandy
shale, and conglomerate. Contains some freshwater
shells. Caps and protects cliffs of Wingate Sandstone.
0-250
Kayenta, Ariz.
Late Triassic
Wingate Sandstone
Massive, horizontally bedded and crossbedded reddish buff
fine-grained sandstone. Forms vertical cliffs along
Colorado River, Cache Valley, Salt Wash, and
Courthouse Wash.
0-350
Fort Wingate, N. Mex.
200
Chinle Formation
Irregularly bedded buff to red sandstone, red mudstone,
limestone, and conglomerate. Lenticular sandstone and
conglomerate (Moss Back Member) locally at base.
Freshwater shells, petrified wood, reptile bones.
Forms slopes.
0-700
Chinle Valley, Ariz.
Moss Back Ridge, Utah Unconformity
Middle(?) and Early Triassic
Moenkopi Formation
Thin-bedded brown shale, gray and brown sandstone, arkosic
grit, and conglomerate. Crops out on southwest side of
Moab Valley and in several places in Salt and Cache
Valleys. Forms slopes.
0-1,300
Moenkopi Wash, Ariz.
Unconformity
250
Permian
Cutler Formation
Chocolate brown and red sandy shale, maroon and pinkish-gray
arkose and conglomerate. Lower part probably
equivalent in age to Rico Formation in areas to south
and east. Crops out in Moab Canyon west of Moab fault.
Forms slopes.
0-2,500
Cutler Creek, Colo.
Pennsylvanian
Hermosa Formation
Unnamed upper member
Gray marine fossiliferous sandy limestone, gray and
greenish-gray sandstone and sandy shale, and red sandy
shale. Exposed in ledges southwest of Moab fault in
highway cut west of park entrance.
0-1,500
Hermosa Creek, Animas River Valley, Colo.
300
Paradox Member
Salt, gypsum, and anhydrite, with black and gray shale and
limestone. Few exposures in Salt and Cache Valleys.
Forms slopes.
0-11,000
Paradox Valley, Colo.
Unconformity
Pennsylvanian(?)
Unnamed conglomerate
Yellow sandstone with boulders of limestone and chert
containing Mississippian fossils. Exposed at two
places in Salt Valley.
?
Not exposed in the area but present far beneath the sedimentary cover
and exposed in several places a few miles to the northeast are examples
of the other two principal types of rocks—(1) igneous rocks, solidified
from molten rock forced into or above preexisting rocks along cracks,
joints, and faults, and (2) much older metamorphic rocks, formed from
other preexisting rock types by great heat and pressure at extreme
depths. Igneous rocks of Tertiary age (fig. 59) form the nearby La Sal
Mountains. The particles comprising the sedimentary rocks in the area
were derived by weathering and erosion of all three types of rocks in
various source areas.
Arches National Park and nearby Canyonlands National Park are both in
the heart of the Canyon Lands Section of the Plateau; therefore, it is
only reasonable to wonder why the differences in their general character
seemingly outweigh their similarities. First, let us consider the
similarities. Both parks are underlain by dominantly red sedimentary
rocks, both parks feature unusual erosional forms of sandstone, and both
contain beautiful natural arches, although the arches in Canyonlands are
restricted almost entirely to the southeastern part of The Needles
section and are in much older rocks than those in Arches.
To be sure, differences in the rocks themselves play a part in the
dissimilarity of the two parks, and these differences are of two types.
First, there are lateral changes in the character of the strata, known
to geologists as facies changes, brought about by differences in the
environment, in the type of materials, and in the mode of deposition
even within relatively short distances. Thus, during parts of the
Permian Period while sand, later to be known as the Cedar Mesa and White
Rim Sandstone Members of the Cutler Formation, was being deposited in
the southern part of Canyonlands, red mud, silt, and sand of the Cutler
were laid down farther north in Canyonlands (Lohman, 1974, fig. 9), and
similar, though somewhat coarser, beds of the Cutler were laid down at
Arches (fig. 4). Further comparisons of the rock columns in the two
parks show that while limestones of the Rico Formation were being
deposited in a shallow sea in the southern part of Canyonlands,
additional red mud, silt, and sand of the Cutler were being laid down
above sea level in areas to the northeast. The source of the coarser
materials was the ancient Uncompahgre Highland, which stood above sea
level from Late Pennsylvanian time to Late Triassic time (figs. 7, 59).
Although wider and longer, it occupied about the same position as the
present Uncompahgre Plateau between Grand Junction and Gateway, Colo.
Streams eroded the hard igneous and metamorphic rocks from this ancient
landmass and dumped the material into basins to the northeast and
southwest. The basin to the southwest, now called the Paradox basin
(after Paradox Valley, Colo.), at intervals contained shallow seas and
lagoons, which I will discuss later.
Comparison of the rock columns for the two parks also reveals other
differences. Both parks contain exposures of rocks as old as the
Pennsylvanian Paradox Member of the Hermosa Formation. However, only in
the Horseshoe Canyon Detached Unit of Canyonlands are rocks as young as
the Jurassic Entrada Sandstone, whereas all the spectacular natural
arches that make Arches famous were formed in the Entrada Sandstone, and
Arches also contains several younger formations of Jurassic and
Cretaceous age (fig. 4).
A commonly asked question is “Why are most of the rocks so red,
particularly those in which the arches were formed?” This can be
answered with one word—iron, the same pigment used in rouge and in paint
for barns and boxcars. Various oxides of iron, some including water,
produce not only brick red but also pink, salmon, brown, buff, yellow,
and even green or bluish green. This does not imply that the rocks could
be considered as sources of iron ore, for the merest trace, generally
only 1 to 3 percent, is enough to produce even the darkest shades of
red. The white or nearly white Navajo Sandstone and the Moab Member of
the Entrada Sandstone contain little or no iron.
As pointed out by Stokes (1970, p. 3), microscopic examination of the
colored grains of quartz or other minerals shows the pigment to be
merely a thin coating on and between white or colorless particles. Sand
or silt weathered from such rocks soon loses its color by the scouring
action of wind or water, so that most of the sand dunes and sand bars
are white or nearly so.