Colorado Geology Photojournals

A Tribute to Colorado's Physical Past and Present

Right: Trees and snow mark major Laramide uplifts in green and white while salmon pink marks the Colorado Plateau in this true-color satellite image of Colorado and surrounding states, courtesy NASA, ^Visible Earth

Colorado in first snow, courtesy NASA, Visible Earth,



Groundwork articles



The Gore Range, the Lower Blue River Valley and the Williams Fork Mountains

More Complicated Than They Look

Southern Gore Range from above Silverthorne, CO
Southern Gore Range, looking west

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Faults and Glaciers



Last modified 11/22/03
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Faults and Glaciers

Buffalo Mountain

The first day hike of our September, 2002 Colorado geology road trip aimed for the summit of Buffalo Mountain, a distinctive 12,777' granite dome at the southern end of the Gore Range. Buffalo is a widely visible and easily recognized central Colorado landmark that looms over Dillon Basin, the hub of human presence in Summit County. We were hoping for panoramic views of the central Colorado Rockies from Buffalo's summit, but an early fall snowstorm forced us off the mountain just short of our goal.

A month before, I'd visited Lower Cataract Lake, a postcard hike at the foot of the east side of the northern Gore Range a few miles off CO9 south of Kremmling. Between Upper and Lower Cataract Lakes, both of glacial origin, Cataract Creek takes an impressive 900' plunge over a steep wall of Precambrian rock.

Across the lower Blue River Valley from both Buffalo Mountain and Lower Cataract Lake are good views of the Williams Fork Thrust, the fault that bounds the Laramide Front Range uplift on the west. This nearly horizontal thrust carried the Precambrian core of the Front Range at least 10 km west over Late Cretaceous Pierre shale

The Gore Range

East side of the northern Gore Range

Geologically speaking, the Park, Gore, Tenmile and northern Mosquito Ranges are all cut from a single south-trending Laramide basement uplift running down the center of the Rockies from the Wyoming border roughly to Leadville. Topographic features of fairly recent origin demarcate the individual ranges. Fault-controlled Late Tertiary Tenmile Canyon separates the Gore Range from the Tenmile Range and northern Mosquito Ranges on the south, with the Continental Divide arbitrarily separating the Tenmile and Mosquito. At the north end west of Kremmling, Gore Canyon, a deep and narrow antecedent canyon of the Colorado River, separates the Gore and Park ranges. 

Structurally speaking, major faults define the east and west sides of the Gore Range.

The Blue River Frontal Fault

The steep east flank of the Gore Range is largely the scarp of the Blue River Frontal fault, a major Late Tertiary normal fault with several thousand meters of down-to-the-east movement last active in the Pliocene. As the Gore Range rose along this fault, the adjacent west edge of the lower Blue River Valley block dropped down to form a half-graben (an elongated fault-bounded block dropped down on one side only).

Southern Gore Range from above Silverthorne, CO
Glacial South Willow Creek Canyon, east side of the southern Gore Range

Thanks to a steep slope, a high crest and prevailing easterly winds, the rugged east side saw unusually heavy glaciation during Pliocene and Pleistocene times. Lofty knife-edge arętes separating deep, steep-walled cirques, large U-shaped hanging valleys like South Willow Creek Canyon at right, and massive moraines are glacial legacies easily seen from CO9 as one drives the lower Blue River Valley between Silverthorne and Kremmling.

The Gore Fault

The less abrupt west slope of the Gore uplift is defined by the Gore fault, which separates the Precambrian core of the Gore Range from Late Paleozoic Maroon Basin sediments to the west. The Gore fault was probably last active in Laramide time, when the Gore Range block was likely thrust up and to the west. It was clearly functioned as a high-angle reverse fault during the Pennsylvanian uplift of the Ancestral Rocky Mountains and may participated in Precambrian deformations as well. Faults with many lives are not uncommon in the Rocky Mountain region.

North Tenmile Canyon, southern Gore Range

Gore Range Highlights

The Gore Range hosts many spectacular alpine summits over 12,000' but no Fourteeners. Mt. Powell (13,575') is its highest peak. The bold and widely-visible granite dome of Buffalo Mountain west of Silverthorne sits near its southern tip on the east side. Much of the Gore Range falls under the protection of the Eagle's Nest Wilderness (named after Eagle's Nest, 13,901'), but its spectacular hiking remains easily accessible from population centers like Silverthorne, Frisco and Vail. Worthwhile Gore destinations include Buffalo Mountain, South Willow Creek Canyon, Lower Cataract Lake and North Tenmile Canyon (right). The southern end of the west slope is home to Vail, the town and vaunted ski resort.

The Lower Blue River Valley

The lower section of the Blue River Valley (LBRV), between the Gore and Front Ranges, is a structural rather than an erosional feature borne of Late Tertiary normal displacement along the Blue River Frontal fault. Technically, the LBRV is a half-graben, an elongated fault-bounded block dropped down on one side only relative to surrounding blocks. In the case of the LBRV, the west side went down relative to the flanking Gore and Williams Fork Ranges while the east side stayed put.

Even though it's now well over 8,000' in elevation, the LBRV never hosted a significant valley glacier. Some of the larger valley glaciers flowing east out of the Gore Range nosed into it from time to time, but they neither widened nor deepened it. The Blue River uses the 5-9 km wide LBRV half-graben as a convenient route north from the Continental Divide to the Colorado River; it's done little to remodel the valley filling during its residence.

Apron of glacial deposits ringing the base of Buffalo Mountain's granite dome

The Gore Range forms the valley's rugged west wall; a thick apron of confluent glacial deposits (right) shed east from the Gore crest blankets its base. The Williams Fork Mountains of the Front Range form the east wall. Late Cretaceous Pierre shale floors the LBRV and runs a variable distance up its unstable east wall as well. Landslides in the Pierre shale and in the highly-fractured Precambrian rock overlying it above the Williams Fork Thrust present ongoing hazards. Landslide deposits and gullied cliffs of soft black Pierre shale alternate along the base of the east wall.

The impermeable Pierre shale inhibits deep-rooted plants and trees on the flat valley floor, but scrub oak and lush grass flourish there with the help of abundant Blue River water. The LBRV has long been one of the premier cattle ranching localities in the Rockies. With its many working ranches and its imposing walls, the picturesque lower Blue River Valley still delivers a heady dose of Western romance, but in recent years, the valley's come under increasing attack by developers. If you haven't driven the valley along CO9, I suggest you do it soon.

Rio Grande Rift Involvement

The Blue River Frontal fault and the lower Blue River Valley half-graben are Late Tertiary extensional structures superimposed on older Laramide structures. The tell of ongoing regional uplift and extension along a north-trending broad welt centered just west of the Park-Gore-Tenmile-Mosquito Laramide uplift. In fact, they are currently the northernmost geologic (as opposed to geophysical) manifestations of the Rio Grande Rift, the main trend of which runs west of the Gore Range into southern Wyoming.

The Williams Fork Mountains and Thrust

The Laramide Williams Fork thrust (WRT) marks the western structural boundary of the massive Front Range uplift. Just east of Dillon, the WRT runs near the base of the Williams Fork Mountains, the last major Front Range ridge to the west. From there north, it gradually ascends the east wall of the lower Blue River Valley toward the Williams Fork crest, but its trace is largely hidden by trees and colluvium. The WRT is well exposed in a side canyon along I-70 just east of Dillon, but access may be tricky. 

Rounded crest of the Williams Fork Mountains
Craggy crest of the northern Gore Range

East-west shortening of the Rockies during the Laramide Orogeny forced the Precambrian crystalline core of the Front Range block at least 9 km to the west over Late Cretaceous Pierre shale along the WRT. The east-dipping WRT is nearly horizontal over the Pierre shale, but further under the Front Range to the east, it presumably bends downward into a steeper attitude once it's traversed the Mesozoic sedimentary cover and has stiff Precambrian basement both above and below. The ~1.7 Ga gneisses, schists and granites above the horizontal WRT segment are pervasively fractured, presumably by stresses sustained as they rode up and over the bend in the fault and sagged onto the Pierre shale. 

The intense fracturing left the normally strong crystalline rocks of the Williams Fork crest weak and unstable. The unusually rounded topography of the steep-sided crest reflects ongoing gravitational collapse of the weakened rock above the WRT. (You can make a sugar pile only so pointy.) In contrast, unfractured but otherwise similar crystalline rock along the Gore crest is strong enough to support a very sharp glacial topography, as you can see at right. Yes, the Williams Fork Mountains were probably less glaciated than the Gores, but that alone doesn't account for the striking difference in topography.

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Gore Range Gallery

Gore Range

Southern Gore Range from above Silverthorne, CO Gore Range:  Glaciers deeply dissected the rugged Gore Range, an imposing Laramide uplift rising just west of the Front Range and just north of I-70. These west-looking aerials from above Silverthorne (1st frame) and Kremmling (2nd frame) respectively show the southern and northern ends of the east side of the range, which was steepened by thousands of meters of normal displacement along the Blue River Frontal fault during Late Tertiary extension associated with the Rio Grande Rift

Buffalo Mountain stands on the left in the 1st frame. The large, broad U-shaped South Willow Creek Canyon to Buffalo's north (on its right in the photo) and the sharp "ice cream scoop" topography of the northern Gores are classic glacial landforms.

The 3rd frame show the steep glacier-carved north wall of South Willow Creek Canyon from Buffalo Mountain on the south wall.


Northern Gore Range from above Kremmling, CO

Buffalo Mountain

Buffalo Mountain:  Buffalo Mountain (12,777') is a handsome and striking granite dome (not a volcano) at the southeast corner of the Gore Range. Buffalo's pink, coarse-grained 1.7 Ga granites and pegmatites intruded darker 1.7 Ga banded gneiss and schist during the Early Proterozoic Colorado Orogeny. The summit's reddish brown tinge comes from the granite's pink feldspar. A gray remnant of the metamorphic host rock forms the rubbly shoulder on the right limb of the granite summit in the 3rd frame. Subtle but distinct color variations like these can be very helpful, but be careful to correlate them with a geologic map before relying on them—granites are often neutral grays, and metamorphics can contain pink feldspars, too. 

The deep glacial cirque stair-stepping down Buffalo's south face and the broad gently-sloped morainal deposits ringing its base are reminders of the heavy glacial erosion the Gore Range sustained in Pleistocene and Pliocene times. The domal shape, however, relates to exfoliation, not to glaciation. Domes are common granitic landforms shaped by a sheet-like delamination spurred by the pressure release attending overburden removal. Many such domes dot the eastern parts of ^Rocky Mountain National Park, but they're less common in Summit County.

In the 3rd frame, a lenticular cloud and a gibbous moon grace Buffalo's summit, here seen from Timber Ridge above Silverthorne one cold January morning in 2001. 

Snow highlights the cirque and both avalanche scars in the 4th frame, taken a month earlier from Dillon Dam. We would climb the smaller avalanche scar on the right in September, 2002. Buffalo hosted another large avalanche during the heavy snowfalls of 2003.

The 5th frame shows Red Peak (13,189') over Buffalo's shoulder in a view to the NW from Timber Ridge at Wildernest in Silverthorne, CO. Red Peak's sharp, craggy ridgeline is typical of glaciated Gore Range topography further the north. Buffalo's smooth dome is an outlier. The aerial photo in the 6th frame shows the deep glacial valley of South Willow Creek separating Buffalo and Red Peak.

The 7th and last frame shows part of a much larger boulder field, one of many along the Buffalo's east flank near South Willow Creek Canyon. I'm betting that these represent rockfalls spalling off Buffalo's granite dome during exfoliation, but I haven't entirely ruled out a glacial origin.

Southern Gore Range from above Silverthorne, CO

Buffalo Assault, September, 2002

Jeremy at the base of Buffalo Before the cold front hit on this bright, clear mid-September morning, a T-shirt was plenty warm enough at 10,000' and 0645 hours. From the Forest Service trailhead at the top of Ryan Gulch Road in Silverthorne, the Buffalo summit trail climbs an ever-steeper moraine to reach the base of Buffalo's smaller avalanche scar east of the cirque. Many pegmatite and quartz cobbles and boulders litter the trail below the scar.
Once above tree line, we could see small glades of already-yellow aspen dotting the moraines that drape the base of Buffalo Mountain, but this little shrub was the best early fall color we found near the trail.
John next to downed trees in the avalanche scar. Behind John, a jumble of downed timber marks the base of the smaller of Buffalo's south face avalanche scars. The log mass, the teeth of the avalanche, came to rest near the 10,500' level. From here, the ill-defined trail to the summit braids its way up the scar to tree line near 12,000. That's about where we got snowed off the mountain this time around, but according to The Summit County Hiker, the path to the summit is an unmarked boulder hop from there.
Here I'm looking SE toward the Continental Divide and Dillon Reservoir from halfway up the avalanche scar. Colorado's worst drought in over 100 years left the reservoir 60% below capacity. Lumpy glacial moraines stretch from the foot of Buffalo to the lake. 

Aspens and conifers use very different strategies to survive the severe mountain environment and to compete for sun and nutrients on the moraines. Aspens are common post-fire pioneers—hence the sinuous shapes typical of aspen glades.

A snap cold front bringing the first snow of the 2002-2003 season noses through Ten Mile Canyon south of Buffalo Mountain. The temperature and barometric pressure had already dropped quite a bit by this time, but the snow was yet to come.
Frisco: To the south, Frisco nestles against the thoroughly fractured Precambrian hornblende gneiss of Royal Mountain, a steep 1500' climb from Frisco with rewarding views of Middle Park, the Continental Divide, the Gore Range, and Tenmile Canyon
Metamorphic fin:  A fin-shaped remnant of rubbly gray country rock (1.7 Ga banded gneiss and schist) clinging to Buffalo's 1.7 Ga granite dome creates a false summit behind me, but I'm standing on granite here. Glowering clouds and building fits of snow and wind turned us back at this point, about 800' vertical feet below the true summit.

Metamorphic fins like this one are common decorations on the granite domes of the Front Range. The 3rd frame shows granite dome with several fins in Horseshoe Park on the east side of ^Rocky Mountain National Park

John Photo ops galore:  When the going gets this steep (3,000' in 1.8 miles), lots of must-have shots keep popping up magically along the trail, even on the way down. The weather was still very unsettled on the mountain, but this welcome snatch of sun felt warm and reassuring. We even talked about turning around for another whack at the summit, but things looked a lot less inviting up there.
Wondering:  Second-guessing mountain weather is always a risky, high-stakes business. Looking back at our unattained goal from Timber Ridge at the end of the hike, we couldn't help wondering if we should have pushed on to the top in spite of the weather. The storm clouds had lifted and thinned by this time, but heavy rains would come at 10,000' shortly thereafter as we packed for our next stop, Colorado National Monument. Judging from the snow pack we saw at 12,000' and up over the next few days, we weren't prepared for amount of snow Buffalo's summit likely drew that day. I'll settle for alive and wondering.

We turned back at the top of the shrub line (about 12,000') between the cirque and the gray fin of 1.7 Ga gneiss and schist clinging to the dome on the right in the 2nd frame. The 3rd frame show a granite dome with several such fins on the east side of ^Rocky Mountain National Park.

Williams Fork Mountains

Williams Fork Mountains:  Taken from the  the 11,000' level on Buffalo Mountain, the 1st frame looks to the NE across the lower Blue River Valley to the Williams Fork Mountains at the western edge of the Front Range uplift. Severely weakened Precambrian crystalline rock runs along their lofty but rounded crest, while gullied black cliffs of soft Late Cretaceous Pierre shale crop out along their base. Somewhere in between is the largely obscured Williams Fork thrust, along which the Precambrian core of the Front Range pushed up and over the Cretaceous shale during Laramide contraction of the Rockies. Between Silverthorne and Kremmling, impermeable shale floors the lower Blue River Valley half-graben.

The 2nd frame is a closer look at the Williams Fork Crest from a point lower Buffalo's east slope. Note the rounded topography at the crest.

The 3rd and 4th frames view the Williams Fork Mountains from Lower Cataract Lake on the east slope of the Gore Range ~20 miles north of Buffalo. At center in the 3rd frame are rumpled Pierre Shale cliffs somewhere below the Williams Fork thrust. The 4th image is a wider-angle SE-facing ^near infrared view of the Williams Fork Mountains.

Lower Cataract Lake

(Sorry, pictures coming) [Cataract Lake photos]
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Gallery Note: Unless noted otherwise below, all the images on this page are from the summer and fall of 2002.
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