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By:
Sarah
Age: 17
Grade: 12
Colorado |
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Pikes Peak, as seen from its northwest shoulder |
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BRITTLE, AMBER GRASSES CAST TRANSPARENT SHADOWS OVER THE PEBBLY TERRAIN, AND A CARPET OF PINE NEEDLES
CRACKLES BENEATH MY FEET, AS I SCALE A STEEP HILL ON THE FLORISSANT
FOSSIL BEDS NATIONAL MONUMENT IN CENTRAL COLORADO. Four-mile Creek
shimmers down in the valley below, slicing its path through the
fossil-rich shales of prehistoric Lake Florissant and the ancient
volcanic deposits surrounding them. The surrounding forest of
ponderosa pine shrouds the lower portion of a massive granite
form. This is Pikes Peak, one of the many mountain summits that
comprise Colorado's Front Range in the southern Rocky Mountains.
I sit down on the crest of the hill, absorbing the mountain shapes,
muted by the crisp winter light, that ring the horizon around
my home: Pikes Peak in the east, Crystal Peak to the north, the
long fringe of the Sangre de Cristos in the distant south, and,
bisecting the state as part of the Continental Divide, the Sawatch
Range in the west.
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Living in the shadow of these mountains, I've gradually become
acquainted with their habitats and seasons, but the granite soil
eroding from Pikes Peak and rounded boulder outcroppings that
pattern the landscape remind me that the land's present character
is only the most recent facet of an epic saga. Looking now at
the rugged, ice-sculpted northern shoulder of Pikes Peak, I sense
the vast scale on which events took place during the mountain's
existence. For Pikes Peak, my human life represents but a single
instant in the eternally changing pageant of geologic time.
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Geologists divide Earth's 4.6-billion-year history into four major
chronological eras: the Precambrian, which encompasses the first
three-fourths of the planet's existence from its formation to
the appearance of multicellular life; the Paleozoic, beginning
approximately 600 million years ago as complex life began to evolve;
the Mesozoic or "Age of Reptiles," lasting from 230 million to
70 million years ago; and the Cenozoic era, which saw the rise
of mammalian life and the passing of the ice ages. Each era is
further subdivided into periods, which in turn are dissected into
epochs.
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The opening chapters of the Pikes Peak region's geologic story
may be read in its Precambrian foundation, or basement rock. The
oldest rocks in the Front Range (1.8 billion-year-old schist and
gneiss) reveal evidence of extensive volcanism, metamorphism,
and igneous intrusions, indicating that the Precambrian witnessed
more tectonic activity than any other time in Earth's history.
The foundation rocks of North America are divided into provinces
depending on their age and content. About 1.8-1.6 billion years
ago, a wide strip of crust containing the Proterozoic Province
on which the Front Range stands today accreted (or welded onto)
the western edge of the original North American landmass, or craton.
The intense tectonic activity also broke up the Precambrian foundation
into fault blocks and fissures, ultimately setting the stage for
the uplift of the Rocky Mountains at the close of the Mesozoic.
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I scoop up a handful of knobby pebbles lying at my feet. Salmon-pink
feldspar, black biotite, and opaque gray quartz combine to produce
Pikes Peak granite. This hard rock comprises Pikes Peak itself,
as well as the coarse gravel eroding from the exposed boulders
around me. The first steps in the formation of Pikes Peak occurred
1,050,000,000 years ago, when a tremendous bubble of magma forced
its way up into the ancient rocks. This granite injection halted
its upward course a few miles below Earth's surface, cooling very
slowly to become the Pikes Peak Batholith. Partially uncovered
by erosion after the uplift of the Rocky Mountains, the batholith
stretches 40 square miles.
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I shift my gaze from the mountains to the course of a shallow
stream trickling through the valley below. As the water rounds
the curves of my hill, it creates a miniature lagoon against the
face of a granite boulder. This still sanctuary hosts a variety
of microscopic life, such as diatoms and bacteria. Prior to the
Paleozoic, single-celled organisms such as these were the only
form of life existent. As the Cambrian period opened, more complex
creatures began to appear rapidly in the fossil record. This (relatively)
sudden influx of multicelled and hard-shelled animals is known
as the Cambrian Explosion.
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Aquathid frog scales from the Harding Sandstone Formation |
The Paleozoic was a time of restless seas. Although the area in
which I live now possesses a dry, almost arid climate, at times
in the past it has been underwater. Shallow oceans invaded large
portions of North America repeatedly throughout the era. Thick
formations of sandstone and limestone are enduring evidence of
the tons of sand, silt, and clay deposited by the fluctuating
waters. In outcrops of the Harding Formation, an Ordovician sandstone
south of Pikes Peak, I have collected fossil scales of primitive
agnathid fish, some of the earliest known vertebrates.
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| The first land-dwelling organisms materialized late in the Silurian.
Like early oceanic life, the first terrestrial pioneers were microscopic
colonies of cyanobacteria. Gradually these gave way to green algae
and, eventually, to primitive vascular plants. The first animal
colonists on land were arthropods, whose hard external skeletons
made the task of supporting body weight out of water easier.
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Slow regional uplift began in the Mississippian period. As the
land rose, eroded debris filtered into valleys and lowlands and
produced wide, alluvial plains just above sea level. It was on
this boundary between earth and water that the first amphibians
evolved. The uplift continued into the Pennsylvanian and eventually
resulted in the formation of a primordial mountain chain, the
Ancestral Rockies. One range, Frontrangia, emerged from Colorado's
shallow ocean within 30-50 miles of the present day Front Range.
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By the end of the Permian period at the close of the Paleozoic,
erosion had leveled the Ancestral Rockies. Colorado became a gently
sloping plain, flanked by seas and vast ranks of sand dunes. Reptiles
and protomammals (a diverse animal group from which mammals evolved)
entered the scene about this time.
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I descend through the forest into the valley. Willows line the
stream and numerous feathery grasses ripple around my feet. This
world is the result of millions of years of evolution, and yet
I wonder what other paths it might have taken were it not for
the Permian Extinction. The Paleozoic ended with the greatest
mass extinction in Earth history, annihilating 96 percent of all
marine life, 75 percent of amphibians, many reptile families,
and the protomammals. The cause of this global disaster is unknown.
It could have been the effects of violent volcanic eruptions in
Asia or the sudden habitat reduction resulting from the withdrawal
of the shallow seas, or a combination of many factors. Despite
this, reptiles endured to diversify and eventually dominate the
planet.
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The Mesozoic era dawned upon a dry, warm world. Paleozoic seas
withdrew for the duration of the Triassic period, and an arid,
desert-like climate prevailed. Mesozoic climatic conditions were
predominately warm, and 200 million years ago Colorado was a region
of tropical temperatures. Triassic sediments accumulated as terrestrial
deposits on mudflats and dune fields, today forming fine-grained
sandstones and shales exposed along the east side of the Front
Range. Popularly known as the "Age of Reptiles," the Mesozoic
encompassed the rise of the dinosaurs, their diversification,
and eventual extinction.
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Thirty miles south of Pikes Peak, I thread my way through the
pinon/juniper forest lining the multicolored rock walls of Garden
Park, just north of Canon City. Though the air I breathe is dry
and cool, 180 million years ago this area was covered by vast
tropical floodplains. The bones of the dinosaurs that inhabited
these moist lowlands were engulfed by fine-textured mudstones
and shales, preserved in what would eventually be called the Morrison
Formation. I crouch down at the base of a crumbling, gray-green
exposure and sift the shards of shale through my fingers. Pressed
within the layers of the hillside is the legacy of Jurassic life.
Fossil bones embody the story of a reptile-dominated world, and
plant imprints provide trace glimpses into their habitat.
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Over the course of the Jurassic, the Ancestral Arctic Ocean or
Sundance Sea transgressed over the northern portions of North
America four times, bringing with it a gradual change to moist,
maritime climatic conditions. At the advent of the Cretaceous
period, the northern seas crept down the middle of North America
to join another encroaching ocean from the south. These two fused
together to create the immense Western Interior Seaway, which
bisected the entire continent. The sea laid down deposits of sand
and clay thousands of feet thick. For approximately 30 million
years, the area in which I stand and, indeed, all of the southern
Rocky Mountains were part of an ocean floor patrolled by ammonites
and other marine creatures. As the sea withdrew, it uncovered
wide flatlands on which the last of the dinosaurs, such as Triceratops,
died out.
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My eyes glide over Pikes Peak's sharply shadowed ridges. Though
the land underwent previous uplifts, the mountain I know did not
materialize until the close of the Cretaceous. Like a grand finale
to the Mesozoic era, the Laramide Orogeny (mountain-building episode)
created not only the Rockies but also the rest of the great chain
of mountain ranges stretching from Alaska to South America. Because
the land was close to sea level, the young mountains were much
lower than they are today, but their identity was secure.
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The cause of the upheaval remains elusive, though a wide range
of theories attempt to explain it. One of the most likely suggests
that plate tectonics was responsible; a long period of accelerated
convergence between the Pacific seafloor plate and the North American
plate could have caused deformation as far inland as Colorado.
This idea is supported by geologic evidence of an increase of
Atlantic seafloor spreading around the same time. This would have
quickened the rate of collision between the Pacific and North
American plates, possibly precipitating an episode of orogeny.
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Widespread uplift continued into the early Cenozoic era, and erosion
stripped away the sediments overlying the Pikes Peak Batholith.
Intensive volcanic activity broke out along the Front Range as
another result of the uplift. Volcanoes played an important role
in the Tertiary chapter of Colorado's geology, affecting both
the character of the Tertiary landscape and our present understanding
of it.
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Strung out through a shallow valley within walking distance of
my home, Four-mile Creek threads its way northward. As I follow
its route, I envision the landscape as it must have looked 35
million years ago: a long lake lapped at the edges of the hills,
fed by a stream that ran almost parallel to Four-mile Creek. Humid
air filtered through the foliage of giant sequoia trees fringing
what geologists now call Lake Florissant. The crumbling shale
outcrop at the valley's edge that I explore is the result of volcanic
ash settling to the floor of the lake. These shales contain an
astonishingly detailed record of life in the lake's Eocene existence.
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Forty miles southwest of Florissant in the Sawatch Range is the
remnant of one of many Tertiary volcanoes, Mt. Aetna. Active during
the Eocene, Mt. Aetna produced a hot, glassy material that welded
together as it cooled to create a hard tuffstone over a massive
area of central Colorado. Known today as the Wall Mountain Tuff,
this fine-grained, dove-gray rock provides a datable reference
with which to measure the age of Lake Florissant's sedimentary
deposits.
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Winged Ant
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Just 15 miles southwest of the Florissant valley is the Thirty-Nine-Mile
volcanic field, which extends from Four-mile Creek west to the
edge of the Mosquito Range. At its center is a ring of hills encircling
the present-day town of Guffey, believed by many geologists to
be all that remains of the tremendous Thirty-nine Mile Mountain,
or Guffey Volcano. The height of this giant fell within the range
of 25-30 thousand feet—some estimates place it higher than Mt.
Everest. At the close of the Eocene, the Guffey Volcano disgorged
great flows of mud and ash called lahars. Running east through
paleovalleys, the lahars sometimes crossed streams, including
the one that once ran where Fourmile Creek flows today. The lahar
created a natural dam, and the stream backed up some 8 to 10 miles,
then turned west for another 2 miles to form an L-shaped lake.
Thus Lake Florissant came into being. Just how long the lake existed
is unknown, but its habitat, inhabitants, and even clues to the
climate of its epoch are beautifully preserved in its surviving
shales.
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During the Eocene, Colorado's climate would have been "warm temperate,"
bordering on subtropical, resembling the temperature and seasonality
of today's Californian redwood forests. Immense, water-loving
Sequoia affinis trees were common all over western North America,
and their petrified remains are exposed at Florissant. At the
time of the formation of the lake, lahars also surrounded the
sequoia trees for up to 15 feet of their height, suffocating the
trees but preserving their bases. Five stumps have been excavated
at the Fossil Beds, but dozens more have been located by sonar,
still buried.
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I hold a fragment of shale bearing the imprint of a tiny hymenopteran.
From the insect's segmented body to its frail wing membranes,
it is perfectly preserved. Such fossils of soft-bodied organisms
and plant tissues are rare in most parts of the world, where only
hard materials like bones and shells are preserved. The majority
of Florissant's fossils, however, are insects and leaves and other
plant parts (1,200 insect species and 450 plant species have been
recorded). The secret of this extraordinary reversal lies in the
ash-cloud eruptions of the Guffey Volcano that followed the lahars
into the Florissant valley.
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Dinosaur tail vertebra from the Morrison Formation in Garden Park
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I trace my finger along the paper-thin shale layers, imagining
the storms of flour-fine ash that sifted down from the Guffey
Volcano. Organisms knocked into the lake by the ash shower were
buried quickly in very fine sediments—the two essential components
of successful fossilization of fragile organisms. Coarse-grained
material deposited over a longer period of time would preserve
a skeleton or shell, but any soft parts would be lost. By contrast,
the fine ash could preserve the frail membrane of an insect wing
in all its delicacy.
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| The Dewey Limestone Formation overlies the Nellie Bly Formation.
Two samples of this formation were collected. The first sample
is of a brachiopod shell wackestone. It was collected from an
outcrop in W/2, NE/4, SE/4 Section 10, Township 26 North, Range
13 East, on Bison Road, just east of Bartlesville. It is grayish
brown, hard, and argillaceous.
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Although insects are abundant in the Florissant shales, very few
vertebrates have been found here. A few fossil fish, four birds,
and only one mammal (a mouse opossum) comprise the list. One explanation
for this proportional scarcity suggests that mammals and birds
could have avoided the ash showers by running or flying away,
while insects, smaller and more localized, were more susceptible
victims.
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The volcanic rhythm of eruptions and interludes continued at Lake
Florissant until the end of the Eocene, when the Guffey Volcano
self-destructed in a spectacular final explosion. This covered
the Florissant shales with a concrete-like conglomerate, which
sealed off the fossils and protected them from erosion and weathering,
until the stream-cutting of today's Four-mile Creek exposed the
fossils. Standing before the excavated wall of rock in a private
quarry, I can view the entire sequence of alternating paper-thin
shales and thicker mudstones, sandwiched between the Wall Mountain
Tuff below and the Thirty-Nine-Mile capstone above. These two
volcanic layers have been radiometrically dated at 36.6 million
and 34.4 million years old, respectively. Thus, Lake Florissant
and its fossils are about 35 million years old.
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Even after the Guffey Volcano's demise, dramatic volcanic activity
persisted in the Front Range. During the Miocene, an uplift of
the Pikes Peak region shifted Four-mile Creek's southerly direction
to its current northward course.
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Above and below: Dinosaur bone fragments from the Morrison Formations
in the Garden Park
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Until the boundary of the Tertiary and Quaternary periods, Pikes
Peak's appearance would have been vastly different from the mountain
of today. As the Tertiary ended, widespread uplift elevated both
the vast regions of the Great Plains and the Rocky Mountains to
their present elevations. Pikes Peak had finally attained its
current elevation of 14,110 feet and would soon undergo even greater
alterations after the onset of the ice ages in the form of glacial
scouring.
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| Spanning most of the Quaternary period, the ice ages caused glaciers
and ice fields to form all across Canada and the northern United
States, extending south along the Rocky Mountains. These tremendous
masses of ice dramatically altered the land on which they rested.
As I look at Pikes Peak's northern face, my eyes are drawn to
the wide basin gouged from the granite by the grinding pressure
of an ancient glacier. This giant depression, called a cirque,
is a typical result of glacial movement. In addition to the cirques
that formed at their heads, Pleistocene glaciers carved valleys
and furrows out of the mountains as gravity pulled them downward,
and left great deposits of debris called moraines as they fluctuated
in alternating episodes of ice encroachment and recession. Climatic
conditions favorable to full-scale glaciation waned as the Pleistocene
faded into the Holocene, although the ice did advance and regress
several more times. Holocene glaciers generally formed in pre-existing
cirques and basins, deepening and refining the landforms of their
predecessors but leaving few of their own. The Holocene began
only eleven thousand years ago, and its last two episodes of glaciation
provided the finishing touches to the Rocky Mountain's present
facade. Pikes Peak's evolution into the mountain I know so well
was complete.
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I watch a wisp of snow drifting off the summit of Pikes Peak,
carried by the wind. The Peak's silent presence inundates the
landscape. I imagine the young mountain, newly formed, witness
to a world light-years different from the one in which I live.
Throughout its existence, Pikes Peak beheld a world of kaleidoscopic
change, and the landforms and the underlying sequences of sediments
and fossils are a record of that immeasurably rich history. Although
the Front Range is at present seismically stable, erosion continues
to reshape the mountains, and the forces of orogeny may resurface
in the future. Facing the rugged, azure contours of Pikes Peak,
I ponder a future that I will never see but one in which the mountains
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