The Mesozoic Era, popularly known as the Age of Reptiles or Age of
Dinosaurs, is divided into three periods. The climate of the entire
earth appears to have been warmer then than it is at present, perhaps
because of a different distribution of land and sea areas, or because
continental areas were not as high and mountainous as they are just now.
Colorado was a rather low land area for most of the first two Mesozoic
periods; then a vast sea covered the entire state for the remainder of
the era.
[Illustration: The pink cliffs of Colorado National Monument are made of
Wingate and Entrada Sandstones. Underlying them, in the valley bottom,
Chinle shales form steep red slopes. (William C. Bradley photo)]
Triassic Period
(180-225 million years ago)
Saharan conditions continued to prevail in western North America during
the early part of the Mesozoic Era. In central Colorado, the lowest
Mesozoic deposits are the Triassic Lykins Formation, a series of soft,
bright red sandstones and shales. Where the Lykins is exposed along the
Front Range, its bright red color identifies it. Because of its
softness, it is often less prominent than adjacent rock layers in the
mountain foothills. The Lykins Formation includes some evaporites,
apparently derived from Permian evaporites washed into the Triassic
ponds and lakes which existed occasionally in this region.
Over almost the entire state, the rocks deposited at this time are very
similar. Formation names may differ—Lykins, Moenkopi, Chinle, Ankareh,
Wingate—but the rocks are almost universally fine-grained sandstones and
shales with a red or pink color. They represent ancient coastal plain,
dune, or delta deposits. Toward the western edge of the state they
coarsen, and contain layers of conglomerates similar to the Triassic
conglomerates of northern Arizona and Utah. These suggest that
mountain-building was taking place west of here at that time.
There are virtually no fossils known from Triassic rocks in Colorado,
although some fossil palm fronds have been found west of the San Juan
Mountains, in the southwestern corner of the state.
Jurassic Period
(135-180 million years ago)
During the Jurassic Period, Colorado was still a low, flat desert area
with intermittent streams flowing eastward over the surface of older
sediments. The Navajo Sandstone, formed from dune sands, was deposited
in the western part of the state. Streams flowing eastward from Utah
brought fine sediments—silts and muds—to western Colorado, forming what
is now the Carmel Formation. Near Canon City, coarse gravels bear
witness to local uplift in Jurassic time. Both these gravels and the
Carmel Formation were overlain by more dune sands, now hardened into the
Entrada Sandstone.
In Late Jurassic time the Colorado area, which had been predominantly
desert since Permian time, appears finally to have been submerged once
more. Fine calcareous muds of the Curtis Formation, containing
ammonites, belemnites, and other marine shellfish, show us that a
shallow sea transgressed from the west over the wind-blown sands. This
sea was, geologically speaking, of short duration—only a few million
years. Bounded on almost all sides by desert, it seems to have dried up,
depositing the gypsum that is now present in a thin layer along the
Front Range between Denver and Canon City in the Ralston Formation.
At about this time, however, the climate underwent a major change.
Deposits above the Ralston indicate an increasingly moist environment,
the environment in which the Morrison Formation was deposited over most
of Colorado and parts of the adjacent states of Kansas, Arizona, Utah,
and Wyoming. The Morrison Formation is exposed in many places, and is
characteristically composed of layers of fine, limy mud, brightly
colored in streaks of red, brown, green, and blue. In most areas it is
so soft that it becomes soil-covered; it is well exposed only in
roadcuts or where it is protected from erosion by a “caprock” of harder
sediments or lava. Spectacular outcrops can be seen in new roadcuts
along U. S. Interstate highway 70 just west of Denver.
[Illustration: In this roadcut along U. S. Interstate 70 west of Denver,
Jurassic and Cretaceous rocks are unusually well exposed in the Dakota
hogback. Green and purple shales represent the dinosaur-bearing Morrison
Formation. The Cretaceous Dakota Group forms the eastern, higher half of
the cut. Black layers are carbon-rich clays of the South Platte
Formation, frequently quarried locally for ceramic uses. (John Chronic
photo)]
Fossil dinosaur bones occur in great numbers in the Morrison Formation
near the towns of Morrison and Canon City and at several other places in
Colorado. Those at Canon City have been quarried extensively, and are
now mounted in a number of museums in the United States. At Dinosaur
National Monument, in eastern Utah and northwestern Colorado, many
excellent remains have been found; those in Utah can be seen in place in
the rock in a striking exhibit at the National Monument.
[Illustration: In an old painting, a paleontologist contemplates fossil
bones found near Morrison. The date is 1877. The bones are those of the
70-foot dinosaur _Apatosaurus_, more commonly known as _Brontosaurus_,
shown below in reconstruction.]
[Illustration: Apatosaurus]
Some of the dinosaurs known from the Morrison Formation reached 80 feet
in length. Both plant-eating and meat-eating types are known. In
addition to the bones themselves, gastroliths or gizzard stones can
frequently be found; these highly polished stones were as essential to
dinosaur digestion as gravel is to a chicken or a caged canary.
Along with the dinosaur fossils are found abundant remains of water
plants called charophytes. These plants formed tiny spiralled balls of
calcite as part of their reproductive activities; both the little balls
and the stalks of the plants themselves occur in many parts of the
state. In western Colorado, near Grand Junction, silicified shells of
freshwater snails can also be found in the Morrison.
Early in the 1900s vanadium, radium, and uranium were discovered in
Jurassic sandstones and mudstones of western Colorado. Extensive mining
in this area has revealed that these elements often become concentrated
by groundwater in organic material such as fossil plant stems or
dinosaur bones. The search for radioactive minerals has thus brought to
light many ancient fossil accumulations.
Cretaceous Period
(70-135 million years ago)
Early in Cretaceous time, marine conditions once more prevailed in
Colorado. This is indicated by a marked change in rock types from beach
and near-shore deposits to true marine sediments.
[Illustration: Between the Front Range and the Plains the Cretaceous
Dakota Formation forms a hogback ridge which can be traced for 200 miles
or more. The well-cemented sandstone resists erosion, and so remains as
a ridge when softer layers are stripped away. (Jack Rathbone photo)]
The sandstones derived from beach sands sometimes include coarse pebbles
of chert which can be traced to sources in Permian rocks of Utah and
Nevada. Occasionally the beach and near-shore deposits include marine
shells like oysters, indicating that there were brackish and salt water
lagoons and marshes along the shore. The Dakota Formation represents the
beach of the transgressive or advancing sea. This formation contains oil
in eastern Colorado, Nebraska, and Wyoming; the oil itself may have been
derived from decay of organic materials in swamps behind the beaches and
bars.
As the sea deepened in eastern Colorado, finer sediments were deposited.
These included the black muds of the Benton Shale, and the Niobrara
Limestone, a shallow-water deposit containing abundant shells of clams
(_Inoceramus_ and _Ostrea_) and ammonites and tiny one-celled animals
called Foraminiferida. Above the Benton and Niobrara Formations lie the
fine gray muds of the Pierre Shale. Several thousand feet thick, the
Pierre contains occasional beautifully preserved ammonite shells as well
as bones from fossil fish and swimming reptiles.
[Illustration: Cretaceous rocks in Colorado are rich in fossil
pelecypods. Each of the fossils illustrated above may grow to a much
larger size than shown.]
[Illustration: Shales of the Laramie Formation contain many recognizable
plant fossils.]
The rocks deposited in western Colorado at this time are markedly
different from those deposited in eastern Colorado. In the east,
deposits are fine and very limy, containing abundant shells and little
in the way of coarse debris. In the west, sandstones of the Mesa Verde
Formation dominate, and coal beds suggest marshy or swampy conditions
inshore from the ancient ocean. This is just the pattern we would expect
from a low-lying region bordering a shallow sea, a region similar
perhaps to the southeastern Atlantic and Gulf coasts of the United
States today.
Toward the end of the Cretaceous Period, the sea receded from Colorado.
Beaches and bars of the retreating sea left a sandstone layer which now
outcrops prominently east of the Front Range as the Fox Hills Sandstone.
Above lie interbedded sands and coals, the Laramie Formation. The
presence of coal above beach sands shows that the coal swamps moved
eastward as the sea retreated.
The exact age of the shoreline deposits and coal beds varies from place
to place in such a way as to indicate that the sea withdrew slowly and
irregularly. In general the shore moved eastward, but there are
localities such as North Park where deposition lasted much longer than
elsewhere. In some places no real beach was formed at the ancient strand
line.
In western Colorado, the end of Cretaceous time is marked by coarser
beds, indicating an increased rate of uplift in Utah. Conglomerates were
deposited in the beds of the McDermott Formation, now visible along the
Animas River south of Durango.