Natural History of the Rocky Mountains
The Rocky Mountains
The natural history of
the Rocky Mountains began over 170 million years ago and
has followed a repeating cycle of land upheaval followed
by thousands of years of erosion. The western United States
and the Rocky Mountains took shape during three major mountain
building episodes between 170-40 million years ago (MYA).
The Laramide Orogeny (70-40 MYA) was the last of these and
formed the fundamental
structures of the modern Rocky Mountains.
Today, the Rocky Mountains extend
two thousand miles through two countries, from British Columbia
and Alberta, Canada, to New Mexico. The Rockies also pass
through the states of Idaho, Montana, Wyoming, Utah and
Colorado and comprise over 40 distinct mountain ranges.
information on individual ranges.
The Colorado Rocky Mountains
Long before today's
Rockies began building, the Ancestral Rockies formed about
fifty miles to the west around 320 million years ago (MYA).
Considered at least as high as the current-day Himalayas,
Colorado's Ancestral Rockies consisted of two ranges, Frontrangia
and Uncompahgria. The mountains pushed upwards for 70 million
years and then began eroding until the landscape was relatively
flat again. Remnants of these ranges still can be seen in
the Devil's Backbone west of Loveland, the Red Rocks in
Morrison, the Maroon Bells near Aspen, and the Garden of
the Gods in Colorado Springs.
Around 85 MYA, seas spread across most
of Colorado, forming white sandbars and beaches known today
as the Dakota Sandstone layer. By 70 MYA, tectonic plates
had begun to converge and clash under the Western U.S.,
causing the continental crust to buckle and fold like an
accordion. As the land rose, so did molten magma which formed
theColorado Mineral Belt that runs from the Front Range
down through the San Juan Mountains and contains almost
allthe gold, silver, lead and zinc deposits that fed the
voracious Colorado mining industry. This period, knownas
the Laramide Orogeny, lasted until about 40 MYA and was
followed by another period of erosion which lowered the
mountains to hills once again.
Between 35 and 26 MYA,
volcanoes erupted in the San Juans throwing hundreds of
cubic miles of volcanic ash into the air. When it settled,
the hot ash hardened to form a light colored glassy layer
known as the San Juan Tuff. The Never Summer and West Elk
ranges also saw volcanic activity between 27 and 21 MYA
Around 26 MYA, great faults creased
the land, forming particularly the Rio Grande rift between
the Sangre De Cristos and San Juan mountains and the upper
Arkansas valley between the Sawatch and Mosquito ranges.
The hills were thrust upwards over six thousand feet. Wind
and water continued shaping the landscape, eroding away
less resistant rock to form valleys and gorges. The final
major mountain-shaping forces occurred during glacial episodes
around 130,000 and 14,000 years ago. The glaciers scoured
mountain valleys, carved out new ones, and left behind lakes
and glacial formations like moraines and hanging valleys.
Today, Colorado is topographically
divided into three major geological zones: the Eastern Plains,
the Rocky Mountains, and the Colorado Plateau. About 40%
of the state isplains, 30% is mountains and 30% is plateau.
The eastern plains and western plateau are primarily made
up of sedimentary rock, while the rocky mountains are comprised
of igneous, metamorphic, and sedimentary rock.
The Eastern Plains rise from 3,500
feet above sea level at the eastern border to
6,000 feet at the eastern foothills of the Rockies. The
plains are distinguished by two
shallow river valleys, the Arkansas and the South Platte,
and by the rolling grasslands in
The Rocky Mountain zone lies in
the center of the state and consists of six distinct mountain
ranges (the Front Range, Wet Mountains, Sangre de Cristo,
Park Range, Sawatch, San Juan) that vary from 6,000 to over
14,000 feet above sea level. Mount Elbert in the Sawatch
Range is the highest mountain in the state at 14,431 feet.
The RockyMountains are also distinguished by the Continental
Divide, which winds its way through the mountains and separates
rivers that flow down to the Pacific and Atlantic Oceans.
All drainage west of the Divide flows into the Colorado
River and out to the Gulf of California, with major tributaries
including the Yampa, White, Gunnison, Dolores, and San Juan
Rivers. East of the divide, water flows either via the South
Platte and Arkansas rivers into the Gulf of Mexico or from
the San Juan mountains into the Gulf of Mexico via the Rio
The Colorado Plateau marks the
final major zone in the state and is located west
of the Rocky Mountains. These plateaus and mesas decline
away from the mountains
with elevation variations between 11,000 feet down to 5,000
feet above sea level. The
major features of the region include the White River Basin,
Grand Mesa, Uncompahgre
Plateau, Paradox Basin, and San Juan Basin.
of the major geologic regions of Colorado:
As you stand in Telluride's
box canyon and look up at the towering mountains where miners
toiled for gold and other minerals, you might first want
to know about all that gold – and whether or not any
of it is still up there. But if you stare up at those mountains
long enough, you might begin to ask some different questions
about them thar hills.You may wonder, for instance, why
are the peaks so jagged and why do they shoot up 4,500 feet
from the valley floor like that? What's up with all that
red rock, and how the heck did gold get up there, anyway?
The Telluride region, located in the San
Juan Mountain range, has been shaped over millions of years
by both changes in the climate and the formation of various
rock layers. Originally this region was flooded by a vast
inland sea until a mountain building episode called the
Laramide Orogeny began pushing up the land 70 million years
ago. A period of volcanic activity followed about 5 million
years later, which substantially added to the mass of these
mountains."There were eruptions of volcanic activity
in the area which shaped the mountains near us," explains
local geologist Marcie Ryan.
"These episodes capped
the tops of the mountains with what is called the San Juan
Tuff, a mixture of volcanic ash and glass shards welded
together. The resulting deposit is identified by the colorful
purple and green fragments cemented together." This
mountain range, which is the youngest in the Rockies, looks
jagged not because of these volcanic deposits but because
it hasn't had as much time to erode as the others. The range
went through another climactic change around 1.6 million
years ago when a series of glaciers moved in, causing fundamental
changes to the landscape. Ryan has documented evidence of
at least 5 episodes of glaciation, while Rob Blair in The
Western San Juan Mountains estimates that there could have
been as many as 15 glacial advances in the last 2 million
The Telluride valley shows distinct
evidence of the effects of glaciation. The valley itself
is a classic U-shape, indicating a glacier carved out its
walls. Above the valley, Bridal Veil and Ingram basins are
textbook examples of "hanging valleys" or valleys
carved out by smaller glaciers that couldn't keep up with
the main one. Other visible clues to glaciation are called
"moraines." Society Turn is the site of a large
terminal moraine."A terminal moraine is where the front
of the glacier pushes debris in front of it," explains
Ryan. "When the ice stops movingand starts melting,
it leaves a ridge that extends across the valley. After
the glacierstarted melting, the valley filled up with water.
The valley floor is filled with 500 feet of lake sediment."
Down valley from Society Turn,
the profile changes from a U- to a V-shape, and according
to Ryan, the land here was carved into a narrow valley by
water rather than ice.On highway 145 between Placerville
and Telluride, several layers of rock are visible on the
hillside above. According to a chart provided by Ryan, there
are 19 distinct rock layers or formations around Telluride,
varying in thickness from 80 to 2,000 feet. Between mile
markers 77 and 75, excellent examples of this stratification
can be seen.The most visible layer in the lower canyon is
the 1,150-foot thick Cutler formation, which was formed
around 220 million years ago. Consisting of sandstone and
shale deposited by streams, its rust-red color comes from
the iron-oxide rich cement that binds the grains of sand
together. Above this layer is a very distinctive and unusual
black layer made of petroliferous limestone called "Pony
Express." This layer is 155 million years old.
Additional rock formations are
visible around Telluride. By Society Turn, people oftenpractice
climbing on an outcropping of rock that is part of the Dakota
formation. Formed around 100 million years ago from sands
deposited by streams, this 150-foot thick hard and tan-looking
sandstone layer is the top rock surface holding up the nearby
From the top of Lawson Hill, another
outcropping is visible on the opposite side of the valley.
Mancos Shale, a 2,000-foot layer of mudstone, was formed
90 million years ago of black and gray clays. This layer
generally weathers easily, forms rounded slopes and, as
its contents suggest, can shrink, expand, and shift horizontally
or vertically depending on its exposure to moisture. The
result is an unstable surface prone to movement and mudslides
like the one that occurred in 1987 near the Telluride airport.
The aftermath of this mudslide still is visible from the
entrance to Mountain Village.
Much higher up and more difficult to
see is the 250-foot thick prominent cliff layer known asthe
Telluride Conglomerate."Rock formations are named after
the locality near which they were found," explains
Ryan. " The Telluride Conglomerate was an old river
deposit made up of older, rounded pebbles and cobbles that
were lifted up and cemented together. It crops out just
below the volcanic layer, and it is exposed well because
the glacier eroded it."Climactic history and rock formations
aside, the question still remains: how did all that gold
get up there in the first place?
"During the episode of mountain
building, there was faulting and fracturing in the rock,"
said Ryan, who also leads mineral collecting and geology
trips in the area. "Hydrothermal solutions filled the
cracks and fissures and then started precipitating out minerals
along with other precious metals that are soluble in hot
water."Ryan goes on to describe how the miners found
gold:"Sometimes it was really obvious. Other times
it wasn't, and they looked for clues, such as if there was
a vegetation anomaly where the vegetation looked stunted
or different if it was near acidic rock. They also used
geometry. If they saw a vein on one side, they would ask
where it would come through on the other side. The richest
mineral concentrations were where two veins intersected.
A lot of times, of course, it was pure luck."
Much easier than mining for gold
ore in rock, which then had to be crushed, melted and separated,
was panning for placer gold in the river. Ryan suggests
that not only is there still plenty of gold in the San Miguel
River, but that more gold still remains in the mountains
thanwas ever taken out. Nowadays, however, it is either
too dangerous or too costly to get to.
George Cappis, a miner in the Telluride
region for over 50 years, agrees with Ryan, recounting an
intriguing example of how miners left gold behind."During
World War II, the government gave out money to mine for
lead and zinc because they wanted it for the war effort,"
Cappis recalls. "We were mining mostly for gold back
then, and one night we were told to get allour tools and
equipment out of this one tunnel. We never went back. There
was still plenty of gold down there though."
Gold wasn't the only mineral mined out
of these mountains. Ryan lists the other major economic
mineral deposits as copper, silver, lead, and zinc. And
what about Telluridium, the ore that supposedly gave the
town its name?Tellurium combines with other metals to form
Telluride ore (Telluridium)," Ryan explains. "To
be honest, there's not a lot of it here. Maybe they just
liked the name."
(reprinted from an article by Allison Johnson in the 1998
Telluride Times Journal
Summer Examiner). Information for this article was taken
from an interview with Marcie
Ryan, columns Ryan published in The Norwood Post, and from
The Western San Juan
Mountains, edited by Rob Blair. For information on geology
or mineral collecting trips,
call Ryan at (970) 728-3391.
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