Geologists have divided the second great era of geologic time into units
called Periods. The rocks deposited during a Period are called Systems,
but more often than not it is convenient to discuss them in terms of
easily recognized units of rock, called Formations. Formations are named
after areas in which they are well exposed.
The stratigraphic column given in Chapter I shows the Periods and
Systems in their correct order, and gives the age in years for each, as
determined by radioactivity methods. As you read, refer as often as
necessary to this column.
The geologic map on page 35 will help you locate areas where the rocks
discussed in the text are exposed, and will greatly facilitate your
understanding of the geology of the state.
[Illustration: The Cambrian Sawatch Sandstone lies almost horizontally
on Precambrian granite in Glenwood Canyon. In the foreground is the
Colorado River. (Jack Rathbone photo)]
Cambrian Period
(500-570 million years ago)
The first fossiliferous rocks in Colorado were deposited during the
Cambrian Period, at a time when over much of the world the seas were
creeping in across wide, level plains formed during the Lipalian
Interval. Colorado was not covered by these seas until quite late in the
Cambrian Period. Beach deposits progressively younger in age suggest
that the sea invaded from the west, and spread slowly eastward,
inundating most of the central part of the state but not the extreme
north or south.
The beach deposits, now called the Sawatch Sandstone because they are
well exposed in the Sawatch Range, are composed mostly of fine quartz
sand. They are colored with glauconite, a green mineral, and hematite, a
dark red mineral, so that the rock has a variegated appearance. The post
office at Manitou is built of this red and green rock, and good
exposures of it exist in Williams Canyon near Manitou, along U. S.
Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in
Glenwood Canyon.
The sea which crept over Colorado at this time contained small
conical-shelled mollusks, brachiopods, and trilobites. Their shells can
occasionally be found in Cambrian rocks in Williams Canyon and in the
Sawatch and Mosquito Ranges. At two localities unusual fossils called
graptolites have been found in thin Upper Cambrian shales overlying the
Sawatch Sandstone.
[Illustration: These fossils can occasionally be found in Cambrian rocks
in central Colorado.]
Ordovician Period
(440-500 million years ago)
The sea deepened and widened as the Ordovician Period began, and a
series of limestones and dolomites was deposited, either on top of the
Sawatch Sandstone or, where the Sawatch had not been deposited, directly
on the Precambrian. These rocks are now called the Manitou Formation.
The fossils in these rocks are much more varied than those in the
Sawatch Sandstone: snails, echinoderms, sponges, cephalopods,
brachiopods, and trilobites are common. The Ordovician sea must have
teemed with life, as many rocks deposited at this time are more than
half composed of animal remains. In addition to hard-shelled animals
which formed fossils, there were probably abundant soft-bodied animals
such as jellyfish and worms, which left no record of their presence.
After deposition of the Manitou Formation, the seas receded slightly. A
new series of sands was deposited above the Manitou in central Colorado.
These now form the Harding Sandstone, a formation of unusual interest
because it contains remains of the earth’s earliest known vertebrates,
primitive jawless fish called Agnathids. In places in the Harding
Sandstone there are dense accumulations of the tiny polygonal armor
plates from these fish. Although no whole fish have been found, we can
reconstruct their appearance by comparing individual plates or groups of
plates with later, better known relatives.
Also present in great quantities in the Harding Sandstone are conodonts,
peculiar tiny brown tooth-like fossils. Relationships of the conodonts
are unknown; they may be parts of the Agnathids, or perhaps they
represent some entirely different group of animals, with no living
relatives.
After deposition of the sands of the Harding Sandstone, the sea deepened
locally and the Fremont Limestone, a massive gray crystalline limestone
containing many marine fossils, was deposited. Mollusks (some quite
large), brachiopods, and corals contributed their shells to the Fremont
Limestone. The chain coral _Catenipora_ and the horn coral
_Streptelasma_ may often be used to identify the formation.
The Fremont Limestone was deposited very late in the Ordovician Period.
Probably the seas were much more extensive then than present deposits
indicate; subsequent erosion has at several times erased the evidence in
uplifted areas.
[Illustration: These Ordovician fossils can be found in the Manitou
Formation in the Colorado Springs area.]
[Illustration: The earliest known fish remains come from the Ordovician
Harding Sandstone of central Colorado. These fragments of the protective
plates have been magnified about five times.]
[Illustration: Corals and coral-like organisms occur in the Ordovician
Fremont Limestone.]
Silurian Period
(400-440 million years ago)
Until very recently, no Silurian rocks or fossils were known in
Colorado, and it was thought that seas did not extend into the state
during this period. However, a few years ago good Silurian corals and
brachiopods were discovered near the northern edge of the state. They
occur in broken blocks and patches of Silurian limestone, mingled with
blocks of other sedimentary rocks and, oddly enough, with volcanic
material.
What seems to have happened here is that sedimentary layers of Silurian
age _were_ present over northern Colorado at one time. During some
subsequent period of volcanism, volcanic lavas penetrated these
sediments from below. Near the volcanic tubes, broken, angular fragments
of the surrounding sedimentary rocks were sometimes carried upward or
downward by the motion of the lava.
Much later, both the volcanic outpourings (if the lavas ever reached the
surface) and the sediments were stripped away by erosion, probably at a
time when mountains were rising in the area. Only the deep portions of
the tubes that fed the volcanoes were preserved. These tubes are called
diatremes, and thanks to the blocks of sedimentary rock in them we know
that there were indeed seas in Colorado during Silurian time, seas
containing the abundant life of a shallow marine environment very much
like that existing at the same time in Illinois, Iowa, and Indiana.
Devonian Period
(350-400 million years ago)
As far as we know now, Colorado was just a little above sea level during
most of Devonian time. Early and Middle Devonian deposits are lacking.
Late in the period, however, Colorado was widely inundated once more.
Embayments of a western sea covered most of the central part of the
state and an area in southwestern Colorado around Ouray.
Deposits formed in these embayments have been given several names.
Chaffee Formation is the name most commonly used in central Colorado;
Ouray Formation identifies rocks of the same age in southwest Colorado.
The Chaffee Formation has been subdivided into two well defined units,
the Parting Sandstone or Quartzite, and the Dyer Dolomite or Limestone.
Many ore deposits are associated with these rock units—notably deposits
of lead and zinc. The Parting Sandstone is frequently so well cemented
with silica that it is actually a quartzite; thin shale beds or
“partings” make it easy to recognize. It frequently contains remains of
fossil fish and distinctive beds of algae.
The Dyer Dolomite contains brachiopods and bryozoans, mollusks and
corals. Some of the best fossil hunting in Colorado is in Dyer beds
around the White River Plateau, where the fossils frequently weather out
of the rock as almost perfect specimens.
[Illustration: These Devonian brachiopods come from the White River
Plateau in western Colorado.]
Mississippian Period
(310-350 million years ago)
The sea continued to cover most of Colorado after the end of the
Devonian Period, well into Mississippian time. Mississippian rocks are
characteristically thick, massive gray limestones collectively called
the Leadville Limestone. This unit is well known as the host rock for
many Colorado ore deposits, notably those around the town of Leadville.
During Mississippian time the western sea, warm and rich in organisms,
covered much of North America. Brachiopods and corals flourished, as did
many other forms of life. The seas during part of this time extended
completely across Colorado to merge with seas that covered the
midwestern part of the United States.
Over all this vast area, as well as southwest into Arizona, the gray,
massive, fossiliferous Mississippian limestone is remarkably uniform and
easily recognized, although it is called by different names in different
areas.
Late in Mississippian time, the Colorado area rose slightly and the sea
in which the Leadville Limestone was deposited receded. An interval of
erosion followed. The surface of the limestone was dissolved and pitted,
tunnels and caves formed where running water etched deep into the rock,
and a reddish soil formed on the surface and in the hollows. This
portion of the limestone, which in some places also contains pebbles of
chert, is named the Molas Formation. Part of the Molas may be
Pennsylvanian in age.
[Illustration: Mississippian fossils from western Colorado show that
seas covered much of the state about 330 million years ago.]
Pennsylvanian Period
(270-310 million years ago)
As the Pennsylvanian Period began, the Colorado area continued to rise.
Earliest deposits of this age are fine-grained black shales and
sands—the Glen Eyrie Formation along the southern Front Range and the
Belden Formation in west central Colorado. Then, through millions of
years, mountain-building took place. Some areas rose more than others,
so that formerly flat-lying marine sediments were bent and broken, and a
series of high mountain ridges and deep basins were formed. Geologists
sometimes call these the Ancestral Rocky Mountains.
Although the pattern of the mountains changed repeatedly, the Ancestral
Rockies consisted principally of two large ranges. One range roughly
paralleled the present Front Range, but lay thirty to fifty miles
further west. The other extended from the San Luis Valley northwest
toward Colorado National Monument, including the area around the Black
Canyon of the Gunnison and the present Uncompahgre Plateau. Coarse
sediments washed off both sides of both ranges, and accumulated as
alluvial fans and valley fill along the mountain margins. These exist
today as the Fountain Formation of the eastern Front Range, the Minturn
Formation between the ancient uplifts, and the Hermosa Formation west of
the western uplift.
[Illustration: This paleogeographic map reflects the distribution of
land and sea during the early part of the Pennsylvanian Period and shows
where coarse sediments derived from the Ancestral Rockies were
deposited.]
FOUNTAIN FORMATION
MINTURN FORMATION
HERMOSA FORMATION
[Illustration: West of Denver, the main line of the Denver & Rio Grande
Railroad tunnels beneath steeply dipping sandstones and conglomerates of
the Fountain Formation. (Jack Rathbone photo)]
[Illustration: Corals, brachiopods, and fusulinid Foraminifurida can be
found in the Pennsylvanian Minturn Formation at many places in the
Mountain and Plateau Provinces.]
[Illustration: In western Colorado, where vegetation is sparse, rock
structures are clearly defined. This photograph shows beds of the
Pennsylvanian Minturn Formation sharply folded, probably as a result of
the deformation of gypsum in underlying layers. (Jack Rathbone photo)]
In the Flatirons near Boulder, Red Rocks Park near Denver, and the
Garden of the Gods near Colorado Springs we see well exposed examples of
the Fountain Formation. The Minturn Formation is visible along the Eagle
River west of Wolcott, and along Gore Creek near Vail. The Hermosa
Formation forms striking red cliffs north of Durango. In the Sangre de
Cristo Mountains area, exceptionally great and rapid deposition took
place, and the Minturn Formation is very thick.
In west central Colorado, near the towns of Eagle and Gypsum, a large
basin formed. In it, gypsum and other salts were deposited as arms of
the sea were cut off from the main marine area. The unusual appearance
of the hills along the Eagle River, especially north of U. S. Highway
24, is caused by the presence of gypsum in the bedrock.
In a similar manner, the Paradox Basin was formed in southwestern
Colorado. Thousands of feet of gypsum, salt, and potash were deposited
here, probably also precipitated in restricted arms of the sea. These
minerals, the so-called evaporites, have since significantly controlled
development of the landscape in Gypsum Valley and other parts of this
region. (See The Plateaus in Chapter I and the section on Gypsum in
Chapter III).
Between the mountain masses and their surrounding alluvial deposits,
shallow seas repeatedly invaded the lowland areas of the state. Marine
fossils in some parts of the Minturn Formation bear witness to as many
as twenty marine cycles. Strangely, the Pennsylvanian Period appears to
have been cyclical in other parts of the United States as well, for
marine sediments are found alternating with nonmarine sediments in
Pennsylvania, Illinois, Kansas, Nebraska, and New Mexico. In middle
Pennsylvanian time, general uplift occurred in Colorado, and almost the
entire state was above sea level for the rest of the period.
Permian Period
(223-270 million years ago)
By the end of the Pennsylvanian Period, the mountains of the Ancestral
Rockies had been almost entirely removed by erosion, and the deep basins
were filled with sediments. Colorado was once more a great plain,
sloping gently to the northeast. In eastern Colorado, a shallow sea
gradually dried up, leaving some thin limestone and gypsum beds along
its margin. The western shore of this sea was edged with beaches and
sand dunes, preserved as the Lyons Sandstone. The buildings of the
University of Colorado, as well as many homes and other structures in
the Boulder-Denver area, are faced with this beautiful salmon-colored
sandstone.
[Illustration: Balanced Rock, in the Garden of the Gods northwest of
Colorado Springs, is an erosional remnant of iron-rich conglomerate and
sandstone. It remains while the rest of the surrounding layers are gone
because it is harder and more completely cemented together by silica.
The rock is part of the Late Paleozoic Fountain Formation. (John Chronic
photo)]
In the western part of the state, Permian deposits consist mostly of
shales and sandstones. The red color of these rocks, and the complete
absence of fossils in them, suggest that the environment in which they
were deposited was not marine, but was a vast, level mudflat subject to
alternating wet and dry periods. The shales and sandstones collectively
are called the Maroon Formation, named for Maroon Bells, near Aspen,
where they are dramatically exposed in the mountain cliffs.
[Illustration: Tracks of Permian reptiles called _Laoporus
coloradoensis_ occur in the Lyons Sandstone near Lyons. These are about
life size.]
During part of Permian time, a shallow sea extended from Idaho, Utah,
and Wyoming into the northwest corner of Colorado. In this sea was
deposited the Phosphoria Formation, a highly phosphatic limestone
containing only rare, poorly preserved molluscan fossils.
As the Paleozoic Era ended, Colorado was still flat and low-lying. By
this time land plants and animals had evolved, but if vegetation grew in
the Colorado area, or animals roamed it, they left few fossil remains.
Tracks of early reptiles have been found in the Lyons Sandstone. Dune
sandstones here and in adjacent areas suggest that desert conditions may
have prevailed, in which case Colorado would have been very similar,
scenically and climatically, to Sahara regions today.
[Illustration: Dark red Pennsylvanian and Permian conglomerates form the
Flatirons that overlook the University of Colorado campus at Boulder.
University buildings are faced with Permian Lyons Sandstone quarried
along the foothills of the northern Front Range. (University of Colorado
photo)]