Colorado National Monument is a land of brightly colored cliff-walled
canyons and towering monoliths—a majestic sample of mysterious
canyonlands that stretch hundreds of miles to the west and south. Now a
desert region more than a mile above the sea, it was not always so. More
than a billion years ago the site of the Monument was deep beneath the
sea. Later, lofty mountains were pushed up only to be obliterated
eventually by the slow but relentless forces of erosion. Millions of
years later the earth shook to the stride of 10-ton dinosaurs—then the
sea returned again and sharks swam over the region looking for food.
These are but a few samples of the interesting—even exciting—events in
the long geologic history of the Monument. Many pages, indeed several
whole chapters, of its history are missing and must be inferred from
nearby regions where the story is more complete. Thus, the cliffs and
canyons you are looking at did not get that way overnight. An
understanding of the geologic processes and events that led to the
scenic features of today should help you toward a clearer picture and
greater appreciation of nature’s handiworks (fig. 6).
Geologists recognize rocks of three distinctly different modes of
origin—sedimentary, igneous, and metamorphic, and there are many
variations of each type. The sedimentary rocks of the Monument are
composed of clay, silt, sand, and gravel carried and deposited by moving
water; silt and fine sand transported by wind; and some limestone,
composed mainly of the mineral calcium carbonate, which was precipitated
from water solutions in freshwater lakes. In areas not far to the
northeast and southwest are many sedimentary rocks of marine origin,
that is, materials that were deposited in the ocean or shallow inland
seas, but in the Monument marine sedimentary rocks occur only in parts
of the Dakota Sandstone; however, the overlying marine Mancos Shale
underlies the adjacent Grand Valley and forms the lower slopes of the
Book Cliffs across the valley (fig. 25).
Igneous rocks were solidified from liquid molten rock intruded upward
into any preexisting rocks along cracks, joints, and faults. Molten rock
that reaches the land surface and forms volcanos or lava flows is called
extrusive igneous rock. Joints are cracks or breaks in rocks along which
no movement has taken place. Faults are cracks or joints along which one
side has moved relative to the other. Different types of faults are
shown in figure 28. Metamorphic rocks were formed from either of the
other types by great heat and pressure at extreme depths in the Earth’s
crust. Metamorphic rocks and some intrusive igneous rocks make up the
hard, dark rock that floors all the deep canyons in and near the
Monument. The nearest extrusive igneous rocks are the thick, dark lava
flows that cap towering Grand Mesa to the east and Battlement Mesa to
the northeast.
[Illustration: INDEPENDENCE MONUMENT, separating the two entrances
of Monument Canyon. Looking north from Grand View; Colorado River,
Grand Valley, and Book Cliffs in distance. Roan Cliffs are white
cliffs at extreme distance on right skyline. Dark rock flooring
canyon is Proterozoic metamorphic rock, red material in slope at
base of cliffs is the Chinle Formation, vertical cliffs are Wingate
Sandstone, thin protective caprock on top of cliffs is lower
sandstone of the resistant Kayenta Formation. The top of
Independence Monument is nearly 450 feet above the floor of the
canyon. (Fig. 6)]
[Illustration: ROCK COLUMN OF COLORADO NATIONAL MONUMENT. 1 foot =
0.305 meter. (Fig. 7)]
AGE (millions of years)
GEOLOGIC AGE
NAME OF ROCK FORMATION
KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION
THICKNESS (feet)
NAMED FOR OCCURRENCE AT OR NEAR
80
Late Cretaceous
Mancos Shale
Gray and black shale, and thin beds of sandstone and limestone.
Contains sea shells. Eroded from Monument, but underlies
Grand Valley and forms lower part of Book Cliffs.
3,800
Mancos, Colo.
Dakota Sandstone
Coaly shale, sandstone, conglomerate, and lignite coal. Contains
plant remains. Forms benches and slopes. Caps highest hill
in Monument.
150
Dakota, Nebr.
115
Early Cretaceous
Burro Canyon Formation
Green siltstone and shale, and sandstone and conglomerate. Forms
benches and slopes. Crops out on highest hill in Monument.
60
Burro Canyon San Miguel Co., Colo.
EROSIONAL UNCONFORMITY
150
Late Jurassic
Morrison Formation
Brightly colored siltstone and mudstone, and sandstone and
limestone. Contains dinosaur bones and fresh-water shells.
Forms slopes and badlands. Lower third with sandstone
lenses is Salt Wash Member, upper two thirds is Brushy
Basin Member.
600
Morrison, Colo.
170
Middle Jurassic
Summerville Formation
Brightly colored siltstone and mudstone, and thin sandstones.
Forms slopes.
54
Summerville Point San Rafael Swell, Utah
Entrada Sandstone
White and salmon-red sandstone. Upper level-bedded Moab Member
forms stair steps, lower mostly cross-bedded Slick Rock
Member forms cliffs.
150
Entrada Point Moab, Utah Slick Rock, Colo.
195
Jurassic and Triassic(?) (missing)
EROSIONAL UNCONFORMITY
210
Late Triassic(?)
Kayenta Formation
Red and purple siltstone and shale, and sandstone and
conglomerate. Forms bench between two cliffs and mesas
between canyons.
45-80
Kayenta, Ariz.
Late Triassic
Wingate Sandstone
Buff and light red sandstone. Cross-bedded and level-bedded.
Forms highest cliffs and most of named rock features in
Monument.
350
Fort Wingate, New Mex.
Chinle Formation
Red siltstone and shale, and some limestone conglomerate. Forms
steep slopes at foot of cliffs.
80-100
Chinle Valley N.E. Ariz.
GREAT UNCONFORMITY
240-1000
Triassic, Paleozoic, Younger Proterozoic (missing)
Unnamed
Schist, gneiss, granite, and pegmatite dikes. Floors main
canyons and forms high bluff above The Redlands.
Unknown
1500
Older Proterozoic
After the materials of the sedimentary rocks were deposited and covered
by younger layers, they generally became saturated or partly saturated
with ground water containing small amounts of dissolved minerals. Some
of these minerals precipitated from solution and cemented the loose
particles into rocks of varying hardness. Thus, most of the sandstones
are partly cemented with the mineral calcite, composed of calcium
carbonate (CaCO₃), but some are cemented also with silica (SiO₂) or
hematite (Fe₂O₃).
Look almost anywhere in the Monument and you will see that the rocks are
piled up in layers of different color, thickness, and hardness—much like
a vast layer cake. In most of the Monument, these layers are flat or
slope gently down to the northeast, but along the northeastern boundary
they are sharply bent or broken as though the cake had been carelessly
placed over the edge of a table and had sagged.
Let us consider these layers one by one, beginning with the oldest at
the bottom, for each is a partial record of events long past. Layers of
rock that can be easily recognized and distinguished from other layers
are called formations and are named after a place where they are well
exposed. For the name to be accepted for general usage it must be the
first published description in a technical report of a particular
sequence of rock layers. The places after which the formations of the
Monument were named are given in the rock column (fig. 7), and the
outcrops of the formations are shown on the geologic map (fig. 8). In
the pages that follow, the geologic events that shaped the Monument we
see today are discussed in chronological order, beginning with the
oldest rocks that floor the deep canyons.
[Illustration: Petroglyph]
[Illustration: GEOLOGIC MAP of Colorado National Monument and
vicinity, simplified and greatly reduced from part of maps at scale
1:31,680 by Lohman (1963, 1965a). For additional surficial deposits
in the Grand Valley and Orchard Mesa see Cashion (1973). (Fig. 8)]
EXPLANATION
QUATERNARY
Qal—ALLUVIUM
Qls—LANDSLIDE DEPOSITS
CRETACEOUS
Km—MANCOS SHALE
Kdb—DAKOTA SANDSTONE AND BURRO CANYON FORMATION, UNDIVIDED
JURASSIC
Jms—MORRISON AND SUMMERVILLE FORMATIONS, UNDIVIDED
Je—ENTRADA SANDSTONE
TRIASSIC
TRk—KAYENTA FORMATION
TRwc—WINGATE SANDSTONE AND CHINLE FORMATION, UNDIVIDED
PROTEROZOIC
PL—SCHIST, GNEISS, GRANITE, AND PEGMATITE
CONTACT FAULT—Dashed where approximately located; dotted where
concealed. U, upthrown side; D, downthrown side
ANTICLINE
SYNCLINE
CENTRAL AXIS OF SYMMETRICAL MONOCLINE—Showing direction of plunge
UPPER BEND OF MONOCLINE—Showing direction of plunge
STRIKE AND DIP OF BEDS
ABANDONED MINE
Geology simplified from Lohman, 1965a
_(Showing location of—)_
DEVILS CANYON MONOCLINE
KODELS CANYON FAULT
LIZARD CANYON MONOCLINE
FRUITA CANYON MONOCLINE
LADDER CREEK FAULT