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Last modified 11/22/03Top Page IndexFaults and Glaciers
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
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 FaultThe 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).
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 FaultThe 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.
Gore Range HighlightsThe 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 ValleyThe 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.
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 InvolvementThe 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 ThrustThe 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.
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. Top Page IndexBottom of Gallery
Top of GalleryGallery Note: Unless noted otherwise below, all the images on this page are from the summer and fall of 2002.Top Page IndexReferencesIn 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:
Top Page IndexURL: http://www.cliffshade.com/colorado/gore/index.htm
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