The vivid and varied colors of the bare rocks and the fantastic canyons,
buttes, spires, columns, alcoves, caves, arches, and other erosional
forms of the canyon country result from a fortuitous combination of
geologic and climatic circumstances and events unequaled 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 particles
ranging in size from clay and silt through sand and gravel carried to
their resting places by moving water, silt and sand particles
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 materials were laid down in shallow seas that once covered
the area (fig. 8) or in lagoons and estuaries near the sea. Some beds
were deposited by streams in inland basins or plains, a few were
deposited in lakes, and some, like the Navajo Sandstone, were carried in
by the wind. The character and thickness of the sedimentary rocks, and
the names and ages assigned to them by geologists, are shown in the rock
column in figure 9 and in the cross sections in figures 10 and 15, and
the history of their deposition is discussed in the chapter “Summary of
Geologic History.” The rock column was compiled mainly from generalized
stratigraphic sections given by Baker (1933, 1946), McKnight (1940),
Hinrichs and others (1967, 1971b), and F. A. McKeown and P. P. Orkild
(U.S. Geol. Survey, unpub. data, Mar. 16, 1973).
Not exposed in the area but present far beneath the sedimentary cover,
and exposed in a few surrounding places, are examples of the other two
principal types of rocks: (1) igneous rocks, solidified from molten rock
forced into or above younger rocks along cracks, joints, and faults and
(2) much older metamorphic rocks, formed from other pre-existing rock
types by great heat and pressure at extreme depths. The particles
comprising the sedimentary rocks were derived by weathering and erosion
of rocks of all three types in the headwater regions of the ancestral
Colorado River basin. Igneous rocks of Tertiary Age (fig. 80) form the
nearby La Sal, Abajo, and Henry Mountains (fig. 7).
[Illustration: SHALLOW INLAND SEA which covered Canyonlands and
vicinity during Middle Pennsylvanian time. (Fig. 8)]
Second among the main events leading to the formation of the canyon
country was the raising and buckling of the Plateau by earth forces so
that it could be vigorously attacked by various forces of erosion and so
that the rock materials thus pried loose or dissolved could eventually
be 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 you look down some 2,000 feet to the river from any of the high
overlooks, such as Dead Horse Point (fig. 15) or Green River Overlook
(fig. 23), or when you lay a straightedge across the three high mesas in
figure 10 and note the large volume of missing rocks below. Not so
apparent, however, is the fact that some 10,000 feet of younger Mesozoic
and Tertiary rocks that once overlay this high plateau also has been
swept away. In all, the river has carried thousands of cubic miles of
sediment to the sea and is still actively at work on this gigantic
earthmoving 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 it 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.
[Illustration: ROCK COLUMN OF CANYONLANDS NATIONAL PARK. One foot
equals 0.305 meter. (Fig. 9)]
AGE (millions of yrs ago)
GEOLOGIC AGE
NAME OF ROCK FORMATION
KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION
THICKNESS (feet)
NAMED FOR OCCURRENCE AT OR NEAR
150
Jurassic
Entrada Sandstone
Crossbedded white fine-grained sandstone at top (Moab
Member); salmon colored to pink, fine-grained,
generally crossbedded sandstone in middle (Slick Rock
Member); red earthy sandstone and siltstone at base
(Dewey Bridge Member, grades into Carmel Formation
west of Green River). Forms steep-sided buttes north,
east, and west of park.
400-500
Entrada Point, San Rafael Swell, Utah
175
Jurassic & Triassic(?)
Glen Can Group
Navajo Sandstone
Crossbedded buff to gray sandstone, some red sandstone, and
thin beds of limestone. Residual rounded patches on
highest mesas.
325-550
Navajo Country, Ariz., New Mexico, Utah
Late Triassic(?)
Kayenta Formation
Irregularly bedded stream-laid gray, buff, lavender and red
fine-to-coarse-grained sandstone and siltstone. Caps
most high mesas and forms tops of highest cliffs.
Contains fresh-water fossils.
160-300
Glen Canyon, S. Utah, Kayenta, Arizona
200
Late Triassic
Wingate Sandstone
Buff and light red generally crossbedded medium-grained
sandstone. Forms highest cliffs, many of which are
coated with black desert varnish.
210-340
Ft. Wingate, New Mexico
Chinle Fm.
Unnamed upper member
Reddish siltstone, mudstone, and sandstone locally bleached
to bluish or greenish gray, and few thin beds of
limestone. Forms steep slopes at base of highest
cliffs. Contains some fossil wood and reptile bones.
205-740
Chinle Valley, Ariz.
Moss Back Mbr
Gray, brown, and gray-green sandstone and conglomerate.
0-80
Moss Back Ridge, Utah
215
Middle(?) and Early Triassic
Moenkopi Fm.
Unnamed upper member
Brick red, reddish-brown, and brown mudstone and sandstone,
and some conglomerate and gypsum. Forms slopes broken
by thin ledges.
250-940
Moenkopi Wash, Ariz.
Triassic(?)
Hoskinnini Tongue
Pale-brown fine-to-coarse-grained sandstone; forms ledges.
0-120
Hoskinnini Mesa, Ariz.
250
Permian
Cutler Fm.
Undivided Cutler Formation in northeastern part of area is
composed of buff, red, and purple arkosic sandstone
and conglomerate. South of Indian Creek is the thick
Cedar Mesa Sandstone Member, composed of massive
mainly crossbedded white to pale red sandstone with
thin beds of cherty limestone. Forms needles, arches
and other erosional features. Thickening southwestward
is the White Rim Sandstone Member of white crossbedded
sandstone.
800-1,000
Cutler Creek, Colo.
White Rim Sandstone Member
White Rim, Wayne Co., Utah
Organ Rock Tongue
Organ Rock, Utah
Cedar Mesa Sandstone Member
Cedar Mesa, S. E. Utah
Rico Formation
Buff, red, and purple arkosic sandstone and conglomerate
containing several thin beds of marine fossiliferous
limestone. Forms moderately steep slopes.
250-585
Rico, Colo.
300
Pennsylvanian
Hermosa Formation
Hermosa Creek, Animas River Valley, Colo.
Unnamed upper member
Blue, greenish, and gray fossiliferous limestone interbedded
with white, gray, and greenish sandstone and gray to
green shale. Lower part known only from deep wells.
Forms steep canyon walls.
900-1,800
Paradox Member
Salt, gypsum, and anhydrite with interbedded black and brown
shale; some limestone.
?
Paradox Valley, Colo.
Last but far from least among the factors responsible for the grandeur
of the canyon country is the desert climate, which allows us to see
virtually every foot of the vividly colored naked rocks and has made
possible the creation and preservation of such a wide variety of
fantastic sculptures. A wetter climate would have produced a far
different and smoother landscape in which most of the rocks and land
forms would have been hidden by vegetation. In the canyon lands the
vegetation is mainly on the high mesas and on the narrow flood plains
bordering the rivers, but scanty vegetation does grow on the gentle
slopes or flats.
The desert climate has combined with the nearly flat lying layers of
sediments of different character, hardness, and thickness to produce
steep slopes having many cliffs and ledges and generally sharp to
angular edges rather than the subdued rounded forms of more humid
regions. This has led geologists to refer to such terrain as having
“layer-cake geology,” and this is brought out by the profile in the rock
column (fig. 9), by the cross section (fig. 10), by figure 15, and by
many of the other photographs. But the baker of this cake was rather
careless—not only do the layers range widely in thickness and character,
but some are wedge shaped, thick on one side of the cake but thin or
absent on the other. Then too, when he ran out of icing in the midst of
a layer, he was apt to finish with a different kind or color, for no
inspector was on the job to insure orderly construction.
If all the rock strata in the park were present at one locality, their
sequence and thickness would be those shown on the right-hand side of
the graphic section in figure 9. However, because of the lateral changes
in thickness and character and the wedging out of certain beds, such as
the White Rim Sandstone Member of the Cutler Formation, no two sections
of the strata are exactly alike. This will be brought out in photographs
of different exposures of rocks in various parts of the park.
An often-asked question is, why are most of the rocks so red? This can
be answered by 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
of iron, generally only 1 to 3 percent, is enough to produce even the
darkest shades of red. The only rocks in the park that contain virtually
no iron are white sandstones of the White Rim Sandstone Member of the
Cutler Formation (figs. 21-24) and the Navajo Sandstone.
[Illustration: Fig. 10, first section]
[Illustration: SECTION ACROSS CANYONLANDS NATIONAL PARK from North
Point at west (left), via Grand View Point in middle, to Needles
Overlook at east (right), showing the three principal topographic
levels and character of the rock strata. Line of section bends at
Grand View Point, which is northernmost part. (Fig. 10)]
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 most of the sand dunes and sand bars are
white or nearly so.
The map (fig. 1) and cross section (fig. 10) of the park show that in
general the major features of the landscape lie at three different and
distinctive levels. A recently erected plaque on Grand View Point
appropriately refers to these levels as the “Three Worlds.” The high
plateaus, or mesas, in and adjoining the park dominate the skyline—in
fact, the central one, between the Green and Colorado Rivers, is
appropriately named Island in the Sky. If you stand on either the east
or the west shore of this towering cliff-bordered island, you can look
across a sea of fantastic erosional forms to a similar cliff-bordered
shore at about the same level. Closer inspection of the sea of rocks on
either side shows relatively flat benches or platforms about halfway to
the bottom; below these are the generally steep sided or cliff-bordered
canyons of the two rivers and their larger tributaries. From some
vantage points along the shore, such as Dead Horse Point (fig. 15) or
Green River Overlook (fig. 23), you can see the deepest level of all—the
channels and flood plains of the Green and Colorado Rivers.
What caused the “Three Worlds” and the formidable cliffs supporting the
high mesas or forming towering monoliths like Angel Arch or Druid Arch
(figs. 43, 54)? 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 their tendency to form
cliffs, ledges, or slopes. Most of the cliff- or ledge-forming rocks are
sandstones consisting of sand grains 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. 9)
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 supporting the highest mesas consist of the
well-cemented Wingate Sandstone protected above by the even harder
sandstone of the Kayenta Formation. 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 remnants of the overlying Navajo Sandstone make up the
topmost unit of the cliff.
[Illustration: Petroglyph]