A Range of Many Colors
Last modified 10/17/04
A Range of Many Colors
The incomparable Elk Mountains of Colorado are home to the Maroon Bells
Cathedral Lake, Crater Lake,
a rich late 1800's mining history, high-end ski resorts like Aspen and Snow
Mass, billion-dollar estates, an airport chock-full of private jets, and
strongly related to all of the above, a fetching brew of
Paleozoic sedimentary and
mid-Tertiary igneous geology.
The Elks are the westernmost range in the
Rockies proper. To their west, the west-dipping Grand Hogback
marks the transition to the Colorado Plateau, just as the east-dipping Dakota
Hogback marks the transition to the High Plains on the east.
The Elks are unique among Laramide uplifts in that they lack a core of
Precambrian crystalline basement, but they more than make up for it with their
spectacular interplay of sedimentary rocks and igneous processes.
Elk Range Evolution
Over the last 510 Ma, many earth processes
have combined to create the Elk Mountains
we know and admire today, and they couldn't have done a nicer job. Here's what
From Middle Pennsylvanian through Triassic
times, sediments washed to the west off the Frontrangia uplift of the Ancestral
Rocky Mountain Orogeny accumulated in the Maroon Basin (also known as the Central
Colorado Trough) between the Uncompahgria and Frontrangia island ranges. Prominent among these were great thicknesses of Late
Pennsylvanian through Permian dark red clays, sands and gravels now known as
the Maroon Formation, but Pennsylvanian evaporites and Triassic redbeds also
Laramide Uplift, Deformation and Thrusting
During the Latest Cretaceous
through Early Tertiary Laramide
Orogeny, the massive
Precambrian-cored Sawatch Range block moved upward and
to the west over Paleozoic and Mesozoic Maroon
Basin sediments along the Castle Creek fault zone (CCFZ), a complex
east-dipping high-angle Laramide thrust system exposed in Castle
Creek valley, among other places along the western Sawatch front.
West-dipping and overturned strata exposed in the Roaring Fork Valley just north
of Aspen are part of the CCFZ's footwall syncline. (A syncline is
a downward " rug fold" wherein the tops of the folded strata bend
toward each other; a syncline folded up just ahead of an overriding thrust block
is a footwall syncline, a structure common in the Laramide orogen). West
of the syncline, continued advance of the Sawatch block buckled Maroon sediments into
a large, thick anticline (an upward "rug fold" wherein the
tops of the folded strata bend away from each other). This anticline would
become the Elk Range.
This much seems clear, but how the Elk anticline ended up at least 5-6 km
west of its initial position relative to the Sawatch block remains
controversial. Additional facts to be explained include the following:
The Elk Range is the only major Rocky Mountain Laramide
structure lacking a core of crystalline Precambrian basement.
The sedimentary strata of the Elk Mountains now
steeply not to the west but to the southwest now, as seen in the photo above at Cathedral Lake.
(Initial Elk folding due to westward thrusting of the Sawatch block along
the CCFZ would have produced strata dipping to the east and west, not to the
A popular gravitational collapse model holds that further uplift of both the
Sawatch block and the Elk anticline eventually caused the latter to detach above
its basement and slide off the Sawatch uplift to the southwest, in part along the Elk Mountain
Thrust (EMT), a low-angle east-dipping thrust fault of Late Cretaceous to
Early Paleocene age exposed along the west side of the Elk Range for more than
50 km. This model implies the presence of one or more large high-angle
west-dipping normal faults of similar age east of the EMT. None of my geologic
maps show such faults, but more detailed mapping presumably supports the model.
How the Elk block came to fall off the Sawatch uplift to the southwest is
unexplained in this model, but Elk strata might have ended up with a southwest
dip if the block happened to spin counterclockwise in map view as it slid. The
northern east side of the Elk Range contains a number of normal faults that
might permit such a rotation, but they seem too short to do the job, even in
Just Another Laramide Thrust?
Another school of thought considers the EMT a large, oblique western splinter
of the CCFZ. In this view, the low-angle EMT continues down to the east below
the Elk Range to merge with (root into) the higher-angle CCFZ somewhere beneath
the Sawatch uplift, presumably near the brittle-ductile transition at the base
of the upper crust. Riding the relatively
small wedge between the EMT and the CCFZ, the Elk anticline simply shot out
ahead of the massive Sawatch block in response to continuing Laramide
contraction. No normal faults are required.
These models differ substantially regarding the eastern extent of the EMT,
but both view its western segment as a southwest-directed low-angle thrust
similar to many other Laramide thrusts. Fault-related folding along this western
segment could impose a net southwest dip on overlying Elk strata in either
model. Asymmetric folds commonly develop in rocks both above and below angled
thrusts, with anticlines forming in the hanging wall above the thrust,
and synclines in the footwall below the thrust. When an angled thrust
fault cut its way up through initially flat-lying strata, the steepest fold dips
develop in the strata closest to the fault. To simulate hanging wall folding,
thrust your hand fingers-first across a table top, allowing your fingertips to
curl under as they go. Elk strata moving up the thrust ramp to the southwest
above the thrust segment of the EMT would curl the same way, acquiring a
southwest dip in the process.
Ma, Oligocene magmas intruding the Colorado
Mineral Belt along lines of weakness left over from Late
Proterozoic continent building hit the all-sedimentary Elk Mountain anticline
and the adjacent Sawatch uplift particularly hard. In the Elk Range, magmas—mostly granodiorites
(right) and quartz
horizontally and unevenly along the Elk Mountain Thrust to pool in two large intrusive bodies
known as the White Rock and Snowmass plutons. In doing so, the intrusions
further uplifted the overlying Maroon Basin sediments in a process known as magmatic
Mid-Tertiary magmas reached the surface in great volume in the nearby West
Elks and in the San Juan Mountains, but volcanism may or may not have
occurred in the Elks. Any Elk Range volcanics that did erupt are long lost to
Continued high heat flows and a substantial gravity low along the Colorado
Mineral Belt together s0uggest the ongoing presence of large, hot magma
bodies at depth. These are presumably the deep reservoirs that fed the
mid-Tertiary Elk Range intrusions, but the story may go deeper yet. Tomographic
imaging reveals a broad region of unusually low seismic velocity known as the Aspen
anomaly in the upper mantle beneath the Sawatch and Elk Ranges. Whether this
hot mantle resides in the asthenosphere
or in the lithospheric mantle remains
unclear, but it may be generating melts either way.
Directly or indirectly, the
Oligocene intrusions deserve much of the
credit for the great heights, the sheer topography and the uncommon majesty that
together set the Elks apart from other Colorado ranges. The tough, medium-
to fine-grained, light-colored Elk intrusives tend to form prominent peaks like Mt.
Sopris (12,660', right) in the north and Castle Peak (14,265') in the
south. The intense heat and mineral-rich fluids the molten intrusions injected
into the surrounding sediments altered the latter in several ways. The Mississippian
Leadville Limestone acquired rich deposits of zinc, silver and gold, mostly in
combination with the lead sulfide ore known as galena. In the late 1800s,
this legacy turned Aspen and Leadville into world-famous mining centers. Near
the town of Marble, the Treasure Mountain intrusion recrystallized gray
Leadville Limestone into the exquisite gleaming white Yule Quarry Marble of
Lincoln Memorial and Tomb of the Unknown Soldier fame. A large sill
(slab-like horizontal magma body) intruding the Maroon Formation along the Elk
Mountain Thrust baked and basted the normally dark red beds of the Late
Pennsylvanian through Permian Maroon Formation into the hard, handsome
maroon strata of the Maroon Bells and the durable
gray-green hornfels of the Cathedral Lake basin.
Uplift, Dissection and Glaciation
A final and ongoing pulse of regional uplift over the last 10 Ma reinvigorated stream
erosion throughout the Rockies and the Colorado Plateau. Uplift has been
greatest around the intersection of two of the most profound structural trends
in Colorado—the Colorado Mineral Belt
and the Rio Grand Rift. In fact, most of Colorado's 55 Fourteeners cluster around that
point (near Leadville),
and many of the largest rivers in the West—in clockwise order from north, the North
Platte, Laramie, South Platte, Arkansas, Rio Grande, San Juan, Gunnison,
Colorado, White and Yampa—drain the resulting dome in a radial pattern. The Elk Mountains
sit astride the Colorado Mineral Belt just
35-40 miles WSW of Leadville.
were already deeply dissected when
glaciers arrived to sculpt them into final form in Pliocene and Pleistocene
time. The strong Elk intrusives
and surrounding indurated metasediments proved to be ideal media to hold
the sharp-edged cirques, sheer mountain faces and deep U-shaped valleys (right) that
glaciers are wont to carve. This combination makes Elk Range alpine scenery
second to none.
Elk Range Gallery
Pass, western gateway to the Elk Range:
A stunning early morning drive up CO133 from Somerset
took us up over McClure Pass (8,755') and down into the Crystal River
basin toward Carbondale.
Looking ahead to the east from the pass (2nd frame), we got our first glimpse of the
incomparable Elk Mountains. Dramatic canyon walls and road cuts
exposing the Maroon Formation
and Early Pennsylvanian
sediments and evaporites graced the drive beyond.
Looking back to the west (3rd frame), we
glimpse the west-dipping Grand
Hogback marking the western edge of the Colorado Rockies as a
geomorphic province. The Grand Hogback is a bookend
to the Dakota Hogback along the east flank of the Front Range and
contains equivalent Mesozoic and Paleozoic strata. Beyond it to the west
lie the dissected tablelands of the Colorado Plateau.
Mt. Sopris and vicinity, north of Aspen
Sopris: The erosional remnant of a large Tertiary stock, Mt. Sopris
(12,660') dominates the northern tip of the Elk Mountains and much of
the scenery between Carbondale and Aspen as well. It's not a volcano per
se, but the frozen cylindrical magma reservoir now exposed on Mt. Sopris
may have fed a
vent or two in its day. The uniform light gray color of the quartz
monzonite stock immediately differentiates Mt. Sopris, Castle Peak and
similar Elk intrusions from the pink granites, the dark banded gneisses
and schists, and the red, orange and buff sedimentary strata of the Gore, Ten Mile and Front Ranges to the north and east. If you're coming
from either direction, the color alone tells you that something
different has gone on here.
The top frame in this series shows a group of Carbondale grade-schoolers taking in
an outdoor Colorado history lesson.
The remaining frames came from a spectacular drive along West and
East Sopris Creek Roads, wide and well-maintained dirt thoroughfares skirting the north and
east flanks of the mountain south of Carbondale, respectively. The route
eventually joined CO82 north of Aspen and south of Basalt. We hit these fall colors in
mid-September just before their peak, but we weren't complaining.
Glacial outwash stream terraces are clearly visible at the foot of
Mt. Sopris in the 5th frame. In the 6th frame, a pair of large high cirques loom
over wooded slopes built on moraine.
The quaint little
cabin behind me sits on moraine on the east flank of Mt. Sopris. It's
badly out of focus here, in favor of yours truly, but it looked like
rough duty from the road.
Basalt-draped Triassic redbed ridges rise to the east from this turnout
on East Sopris Creek Road. These Late Tertiary basalts probably postdate
mid-Tertiary Elk intrusions like Mt. Sopris by 20
My or so. The town of Basalt, on CO82, is probably below them to the
Castle Creek Valley, south of Aspen
These ^near infrared shots
capture a few of the buildings preserved at Ashcroft, an old mining town
near the top of the paved portion of Castle Creek Road.
Castle Creek Valley, Summer: 
Upper Castle Creek Valley, Fall: Looking south from
vantages along the bumpy dirt road to the Cathedral Lake
trailhead, we see Castle Peak
looming beyond golden Aspen glades reaching
down into the upper reaches of Castle Creek Valley. Around 20 Ka, a large Pleistocene
valley glacier filled Castle Creek Valley from the high cirques in the distance
all the way down to Aspen, the town—a distance of 14 miles. Halfway down, it
merged with another large glacier filling Conundrum Creek Valley, the next big
drainage to the north.
Aspen trees are eager fire-scar pioneers. Here, their
sinuous golden stands reveal the traces of ancient wildfires on the alluvial
fans and moraines below tree line. This strategy allows them to compete with the
much taller conifers for sunlight, water and nutrients—and ultimately for real
Cathedral Lake Trail: The turnoff to the Cathedral Lake
trailhead is ~1.4 mi up from Ashcroft and the Toklat Lodge on the west
side of Castle Creek Road. From there, the steep but rewarding route
climbs 1,986' over 3.2 mi to the glacial Cathedral Lake (11,866') and then on to
The trail climbs first through an aspen forest to reach a
thinly-soiled pediment of light gray, fine-grained granodiorite
intruded here ~34 Ma. The middle part of the trail follows Pine Creek
along a narrow staircase defile (1st frame) cut into the hard
granodiorite. Pine Creek drains the Cathedral Lake cirque to Castle
Creek and takes a nice fall here. Near the top of the first set of
switchbacks is the somewhat jumbled contact between the intrusion and
overlying Maroon sediments. Along the contact, heat and fluids from the
intrusion baked and basted the overlying normally dark red Maroon
sediments into a hard gray-green metamorphic rock called hornfels
that nevertheless retains the original SW-dipping Maroon bedding planes
(2nd and 3rd frames). The trail then climbs relentlessly over moraines,
talus slopes, steep rock walls and tundra along the south face of Leahy
Peak (13,322') before crossing Pine Creek at the lip of the glacial
bench and dipping into the hummocky terrain of the Cathedral Lake cirque
top: One of the many false summits on the trail afforded this
SW view of the Elk Mountain crest (center) and a large rock glacier
(lower left). Cathedral Lake sits behind the glacial bench beneath the
sharp triangular peak (?? Malamute Peak, 13,348') at right center.
Lake: At 11,866', this classic glacial lake occupies a large,
deep cirque drained below the southern Elk Mountain crest. The moraine
damming the lake is behind us in the first frame.
Among the many tilted, angular gray-green
hornfels spires surrounding the lake is Cathedral Peak (13,943', 2nd
frame, behind the rock in the middle ground). The trail from the lake up
to Electric Pass (~13,465', shown above the rock
glacier below) looked intriguing, but we were running behind
schedule and had no taste for more climbing at this juncture.
A late lunch on the lake shore consisted of weary leftovers from our
previous 4 day-hike lunches, but that didn't seem to dull its appeal.
The big can of peaches I'd lugged all the way up the trail was worth
every pound—especially the gulp of light syrup at the end.
It was mighty tempting to linger, but we had to be on our way. It
was already 3:30 pm, and we needed to make Twin
Lakes on the other side of Independence Pass before we could bed
down. Due to the steepness of the trail and the variable footing, the
way down didn't seem much easier than the way up, but we managed to make
||Gee, wish we'd thought of that:
This frequent Aspen visitor from Texas "hired" her eager companion Fred from
a kennel near the airport. Fred's unbridled enthusiasm for
anyone who came along had us missing our own dogs in no time. My
Belgian sheepdog and border collie would have given a paw to come on
||Rock glacier: A rock
glacier snakes down toward the trail from the high ragged ridge
connecting Cathedral Peak and Electric Pass (top center). Ice bound into
the base of the rock mass serves as a lubricant, but the bulk of the
glacier is rock debris, not ice. Nevertheless, rock and ice glaciers
share many morphologic similarities.
Coming down from Cathedral
Lake, we spotted this near-vertical gradient in the alteration of Maroon
Formation rocks (1st and 2nd frames). Under the influence of heat and
fluids emanating from the magma intruded below, the gray-green rocks at the base of the hill
reached hornfels-grade metamorphism, while the more normal red rocks at the
higher in the column suffered less alteration.
intrusive heat and fluids responsible for the alteration came from
below, the vertical variation in metamorphic grade seen here makes
sense. But horizontal metamorphic gradients also become evident as scans
the rock colors around the Cathedral Lake cirque. Notice the marked
short-range variation in the wall behind John. In the final frame, the
trail crosses a talus slope exposing a jumble of variably-altered Maroon
The horizontal metamorphic variations suggest two end-member
scenarios: The contact between the intrusion and the overly Maroon
sediments has quite a bit of relief, or the heat and fluids responsible
for the alteration found some directions easier to move in than others.
The reality is probably a mix of the two.
Range from the Elks: To the east and south, distant ridges of
the Sawatch Range from the Cathedral Lake Trail. The Sawatch is on
highest range in Colorado, but it doesn't look all that exceptional from
Maroon Bells and Crater Lake
Bells: No Colorado geology site would be complete without a
homage to the nearby Maroon Bells, the crown jewels of the Elk
Mountains and deservedly the most photographed
mountains in Colorado. Looking west across Maroon Lake in the first two
frames, we see Maroon Peak (14,156') on the left (south) and North Maroon
Peak (14,014') on the right. Maroon Lake looks for all the world like a
glacial lake dammed by a moraine, but a rockslide from the north wall of
the canyon is the real culprit. The bottom frame was taken from a meadow just above the
The thinly bedded red Maroon Formation clays, sands, and gravels of the
Maroon Bells accumulated in coalescing alluvial fans draping the
Uncompahgre Uplift during Colorado's Late
Paleozoic Orogeny. (They're equivalent to the Fountain Formation of
the eastern Front Range.) Heat and silica-rich fluids from an underlying
Oligocene intrusive sill hardened and added a gray cast to the normally
deep red Maroon sediments. In places, the alteration advanced to a hard,
dark green to gray-green metamorphic rock known as
hornfels. The sill also lifted
the Maroon Bells, but it isn't exposed there.
The visible grains in this Oligocene granodiorite boulder
encountered on the trail below Crater Lake
tells of an injected magma freezing slowly well below the surface. The
light gray color indicates a high silica content and viscosity and is typical of
Elk stocks and sills. If this magma had erupted, it would probably
have done so explosively.
Contorted Triassic redbeds form the north wall of Maroon Creek Canyon
between Crater and Maroon Lakes. If these peaks had popped up anywhere
other than next to the Maroon Bells, they'd
be attractions in and of themselves.
Seen here to the SE from the lower end of the trail to Crater
Pyramid Peak (14,018') holds up the south side of Maroon Creek's upper
canyon. Alluvial fans draping the slopes here support forest growth
better than the intervening shallow alpine soils and hard rock surfaces.
Lake: In the 1st and 2nd frames, the Maroon Bells tower over misnamed Crater Lake,
which was dammed by a rockslide across Maroon Creek's upper canyon. The U-shaped valley above the lake clearly
harbored a glacier, but the dam at the opposite end isn't a glacial
moraine. Nor does the lake occupy a crater, volcanic or otherwise.
In the top left corner of the 2nd frame, a rock
glacier spills down a hanging canyon left behind in the Pleistocene
as the large Maroon Creek valley glacier outpaced the downcutting of its
In the 3rd frame, magenta flowers (I'm guessing fireweed) line the
trail on the way down from Crater Lake. Alluvial fans of the north flank of
Pyramid Peak rise to the left.
The 4th frame looks down Maroon Creek to the east
over Maroon Lake from the rockslide impounding Crater
Lake. The far end of Maroon Lake is dammed by alluvial fans, not by
In addition to the references cited on the home
page and in supporting articles, this
article relies on the following sources, in alphabetical order by first author: