Black Canyon of the Gunnison

A spectacular day hike on the north rim section of the Black Canyon of the Gunnison National Park (BCGNP) was to my mind the high point of our September, 2002 Colorado geology road trip. In geology, as elsewhere, extraordinary results point to extraordinary events or, perhaps more often, to extraordinary combinations of less exceptional happenings. The extraordinary Black Canyon of the Gunnison (BCG) is a prime example of the latter pathway, and "extraordinary" hardly does it justice.

Nothing—neither the Black Canyon pictures we'd seen, nor anything we encountered along the approach to the north rim—prepared us for our first disbelieving look into the chasm. South of Crawford, the 15-mile dirt road to the North Rim Visitor Center runs on a flat, monotonous high-desert plateau of hard Precambrian rock flanked by unimposing, gently dipping cuestas of Morrison shale capped by Dakota sandstone. Nothing out of the ordinary struck the eye as we entered the park entrance and headed east along North Rim Road to Kneeling Camel View, our first stop. We could hear the distant rush of a hidden river as we parked the car, but even here, within a hundred yards of the brink, we could see nothing but plateau.

Exclamation Point

Suddenly before us was a guard rail and an unimaginable vertical gash in the earth some 1,700 to 2,772 feet deep and in places only 1,300 feet wide, cut smack through the most resistant of Colorado's basement rocks. Half a mile straight down tumbled the mighty Gunnison River, looking innocent enough at the moment—a glinting emerald and white thread on the canyon floor.

We tried to imagine the utterances (no doubt, some prehistoric version of "Holy shit!") that must have passed the lips of anyone stumbling onto this chasm unprepared. The mind reels at its improbability as the gut recoils from the terrifying drop. How on earth could such a thing have happened? Every overlook here could have been called Exclamation Point.

Top Page Index

How to Make a Black Canyon

Local and regional events of the last 70 Ma generously endowed the upper Gunnison with all the tools a river needs to cut deeply into hard rock—a steep gradient, an abundance of water, and a heavy and abrasive sediment load. Laramide basement uplifts, voluminous mid to late Tertiary volcanism, and late Tertiary regional uplift together progressively elevated the headwaters of the Gunnison. The volcanics provided ample sediments, and the late Tertiary climate added great volumes of water.

The Gunnison River

Above and along the Black Canyon, the waters of the Gunnison River gain momentum like a runaway train on a steep grade. Over its 48-mile run through the Black Canyon, the Gunnison now drops over 4,600'—farther than the Mississippi falls over its entire 1,500 mile course. Along the 12-mile stretch within the National Park, the river's gradient averages a steep 96' per mile (about 2%) but reaches 240' per mile at The Narrows. A wet, stormy late Tertiary climate, an already high and areally extensive drainage basin, and nearly 2 Ma of Pleistocene glaciations and interglacials pushed huge volumes of water and sediment into the river, often in floods of unimaginable fury. Glaciers never reached downstream as far as the Black Canyon, but their outwash and melt waters helped to shape it nonetheless. Coarse debris plucked from of the upper Gunnison highlands by glaciers and brought down by high-energy streams gave the river big, sharp teeth needed to cut through the Precambrian basement rocks that now form the sheer canyon walls. As at Red Canyon in the Colorado National Monument and the Inner Gorge of the Grand Canyon, the great strength of the Precambrian rocks forced a very narrow cut.

The stage for something extraordinary was set, and the Gunnison played its part with a ferocity hard to imagine from the dam-tamed river we see today. As with Glenwood Canyon along the Colorado River to the north, most of the downcutting in the BCG took place in the Pleistocene.

Step-By-Step Recipe

1. Starting at ~72 Ma in the Laramide Orogeny, elevate the future upper Gunnison basin in several discrete basement uplifts cored with resistant 1.7-1.4 Ga metamorphic and granitic rocks. The resulting faulted anticlines now underpin some of Colorado's highest ranges—the Sawatch, the Elks and the San Juans.

2. To the southwest, raise a lesser Gunnison Uplift and begin to erode away its Mesozoic sedimentary cover to expose its tough Precambrian core. Specifically, remove the Lower Cretaceous Dakota coastal sands marking the arrival of the Western Interior Seaway, the Late Jurassic Morrison floodplain clays and fluvial sands, and the Early Jurassic Entrada desert dune sands resting unconformably on the Precambrian core. Complete the stripping in Step No. 5.

3. Starting ~36 Ma, begin covering the upper Gunnison basin and the Gunnison Uplift with a volcanic pile more than 4000' feet thick accumulated in 3 long and prolific episodes of explosive and basaltic volcanism centered in the San Juan Mountains and the West Elk and Sawatch Ranges. Continuously sprinkle the pile with easily mobilized volcanic debris—cinder and ash falls, ash flows and mudflows. Heap up West Elk volcanic flows to the north to push the course of the Gunnison River progressively to the south across the top of the ever-changing volcanic pile. Stop at ~6 Ma when the river is finally and irreversibly entrenched directly over the long-buried Precambrian core of the Laramide Gunnison Uplift.

5. Starting in the late Tertiary around 28 Ma, add a wet, stormy climate and 5,000' of regional uplift affecting all of the Southern Rockies. This will further energize the streams feeding copious volumes of water and abrasive sediment to the Gunnison and further steepen the Gunnison River's own gradient. The river will now cut downward with vigor, even after reaching the hard Precambrian rock coring the Gunnison Uplift.

6. At elevations above 8,000', ice down the West Elk and San Juan volcanic fields with a series of voraciously erosive glaciations starting at around 1.8 Ma and lasting throughout the Pleistocene.

7. Rinse and scour the narrow Gunnison channel copiously, first with repeated torrents of glacial outwash heavily laden with coarse glacial debris and then with frequent and furious Quaternary (post-glacial) floods. Stop around 10 Ka.

Top Page Index

Bottom of Gallery

Black Canyon of the Gunnison and West Elk Mountains
Hotchkiss and Crawford
	Hotchkiss: An imposing Mt. Lamborn (11,401') of the West Elk Range looms in the east as we pass through rural Hotchkiss on our way to the north rim portion of the Black Canyon of the Gunnison National Park (BCGNP). From Hotchkiss to Crawford, CO92 passes though lush rolling green farmlands far removed from the urban sprawl of the I-25 corridor just east of the Front Range.

	Crawford: Looking east from CO92 just west of Crawford, the West Elks rise behind a low dark ridge of rocks (2nd frame) likely to be Early Phase volcanic breccias. The West Elks are resistant mid-Tertiary intrusions now supporting erosional remnants of the volcanic pile that the intrusions once fed. Flows from the West Elk volcanic field set the Gunnison's course across the Gunnison Uplift, and volcanic debris shed from the field gave the river teeth.

Black Canyon of the Gunnison National Park, North Rim
	Kneeling Camel View: This west-looking series, panned from the rim down to the canyon floor, records our first look into the canyon. Above the rim in the distance are late Jurassic Morrison Formation claystones capped by resistant Early Cretaceous Dakota Sandstone in a north-dipping cuesta aptly named Mesa Inclinada, here with Green Mountain (8,563') at its crest. Beneath the Morrison are desert dunes of the early Jurassic Entrada Sandstone (not visible here). Between the Entrada and the Morrison, Colorado's climate got wet. Between the Entrada and the planed off Precambrian basement exposed in the canyon walls are ~1.5 Ga of missing earth history—the local version of the Great Unconformity. Black Canyon Gneiss: The dark wall rocks seen here are the Black Canyon Gneiss, a suite of 1.78-1.65 Ga quartzitic gneisses, mica schists, intruding granites and migmatites, the last with the swirling banded pattern of light and dark taffies pulled together. The gneisses and schists are the remains of one or more oceanic volcanic arcs that docked against the nascent continent along the southern margin of the Wyoming Craton ca. 1.7 Ga. The 300 Ma-long slow-motion collision heated and compressed the arc rocks to their present metamorphic state. Everywhere, the canyon walls are laced with white and pink veins of granite and pegmatite related to 1.4 Ga and later intrusions particularly well displayed in the Painted Wall. Occasional dark Cambrian dikes mark the last known igneous intrusions at 510 Ma.



	The Gunnison River: Before the Gunnison River was dammed upstream for irrigation water and flood control, its water and sediment load gained tremendous momentum both above and within the Black Canyon. Today, the Gunnison drops over 4,600' along its 48-mile run through the Black Canyon — farther than the Mississippi falls over its entire 1,500 mile course. Along the 12-mile stretch within the National Park, the river's gradient averages a steep 96 ft/mile (about 2%) and tops out at 240 ft/mile at The Narrows. With that kind of head, historical Gunnison flows reached a thundering 12,000 cubic feet per second at flood stage in the Black Canyon prior to damming. According to the BCGNP brochure, that represents 2.75 million horsepower of concentrated cutting power. By all accounts, Pleistocene floods were much greater. The violence of such events are beyond imagination. Canyon Comparisons: A hundred miles or so below the Black Canyon, the mighty Gunnison joins the Colorado River at Grand Junction. (Above the confluence of the Green River at Dead Horse Point in western Utah, the Colorado used to be rather aptly known as the Grand River—hence place names Grand Junction, Grand Valley, etc.) Gunnison waters and sediments helped the Colorado cut a spectacular canyon of its own through another Laramide uplift—the Kaibab Uplift of northern Arizona, at the western margin of the Colorado Plateau. I refer, of course, to the Grand Canyon of the Colorado, the longest and largest canyon on the planet. Most Grand Canyon vistas feature the 2-mile-wide outer canyon the Colorado and its local tributaries cut through the thick, relatively soft sedimentary cover of the Kaibab Uplift. To anyone standing on the brink of the Grand Canyon, these sedimentary walls look sheer enough, but the Black Canyon's walls are much steeper on average. Since little sedimentary cover remains over the higher and generally wetter Gunnison Uplift, the Black Canyon has no outer canyon to speak of. When the meandering Colorado finally cut down to the hard Precambrian rock at the core of the Kaibab Uplift, it settled into a much narrower Inner Gorge, the true analog of the Black Canyon. The Inner Gorge penetrates an impressive ~1,800 ft into the Precambrian basement, while the Black Canyon cuts through 2,772 ft of Precambrian rock just as hard. Worldwide, few rivers can claim great canyons deeper than they are wide. The Gunnison is a standout among them; the Colorado never made the cut.









	North Rim Ranger Station: When we pulled up to the remote and usually quiet North Rim Ranger Station around noon, the place was hopping with National Park Service (NPS) personnel, sheriff's crews and a swarm of official vehicles, including this support helicopter. A hiker reported missing several days earlier had been found dead on the floor of the canyon that morning. Whether the deceased had fallen or jumped had yet to be determined, but an NPS anthropologist pulled into the search from his dig 50 miles to the south told us that many people come to national parks to end their lives each year. The sequelae had become an unwelcome and all too frequent part of life in the NPS. Later, on our North Vista Trail hike, we ran across a contemplative park ranger sitting elbows on knees, staring across the canyon. His young muscular face, military bearing, short hair, fatigues and side arm made him look more like a soldier, as I suspect he had been not so long ago. Some of the overlaps between soldiering and rangering were beginning to dawn on me. John wondered aloud if his 22 year old outdoor-minded daughter Heather might end up doing something like the NPS. She holds a passion for climbing and a degree in environmental chemistry. When these photos were taken, she was a Peace Corps volunteer stationed in Nicaragua. As we chatted with NPS personnel returning from the search, John decided that she could certainly do worse. For that matter, both of us wondered how the life of a ranger might have compared to our own lives in medicine. Carrying guns and cleaning up after suicides doesn't sound much better than doing radiology, we concluded, even if it is outside.





	Chasm View Nature Trail: The day's hiking began with the Chasm View Nature Trail, a self-guided 0.5 mile loop that served as an excellent introduction to the area. We enjoyed former ranger Paul Zaenger's lovingly written guide book, Beyond the Brink of Time, nearly as much as the hike itself. Together with the pinyon pine, the shaggy-barked Utah junipers shown here dominate the pinyon-juniper woodlands rimming the canyon. After surviving nearly 8 centuries, the pinyon pines at the brink of the canyon are some of the oldest and largest in the US. Not far from here, we came across a pocket of rounded cobbles worn smooth and left behind by the ancestral Gunnison as it entered the Precambrian core of the Gunnison Uplift. How these conspicuously out-of-place river rocks managed to escape removal, by the elements or by visitors, is unclear.


	The Painted Wall: At a sheer 2,300 feet, the Painted Wall, a prominent segment of the Black Canyon's north rim, is Colorado's highest cliff. The darker rock is Black Canyon Gneiss. The bold white and pink bands are granites and pegmatites injected during Middle Proterozoic (1.4 Ga) through Cambrian (~510 Ga) intrusions. Large books of white mica (muscovite) and crystals of pink potassium feldspar and translucent quartz give the pegmatites their lustrous pink look. Just as Precambrian joints and faults controlled the placement of these igneous intrusions, the current regional jointing system now controls the locations of the side canyons cut by smaller streams left behind as the mighty Gunnison cut through the Gunnison Uplift. These views look west from the Chasm View Nature Trail overlook, which is across the canyon from Chasm View on the South Rim.





	North Vista Trail: The deservedly popular North Vista Trail starts at the North Rim Ranger Station and ends atop the Cretaceous and Jurassic strata of Mesa Inclinado at Green Peak (8,563'). It's an easy but dramatic walk with a sky-high gain/pain ratio. A common and worthwhile halfway destination is the aptly named Exclamation Point, just upstream from the Painted Wall.







	Beyond the rim: A short scramble up the face of the cuesta above the North Vista Trail near Exclamation Point opened up the view to the south beyond the canyon rim. Looking SW (top left), the north flank of the Uncompahgre Plateau (bottom left) looms as a long dark ridge on the horizon. To the SE (bottom left), we caught our first and only glimpse of the San Juans. The peak is Sheep Mountain (13,168'), a volcanic remnant on the north flank of the range.

	John (top left) and I take a breather on the way back to the ranger station, still on the North Vista Trail. Over his long career as a boy scout leader, John had perfected a slow and steady approach to hiking. My long legs seem to have a certain minimum speed that gobbles up flat ground but makes for frequent breathers on the uphills. To a hiker-photographer, the rabbit style can be very useful, but we all know that the turtle usually wins.

	Our parting shot of the Black Canyon caught late afternoon sun on the north rim near the ranger station. We were sorry to be leaving, but it had been a very good day indeed.
West Elk Mountains
	West Elk Mountains: The lure of a much-anticipated hike to Cathedral Lake the next day finally pulled us away from the Black Canyon around 5 PM. We had to push now to make Castle Creek Canyon south of Aspen by mid morning, but this magical view of the West Elks to the NW from the dirt road back to Crawford was reason enough to stop one last time. Landsend Peak (10,807') on the far left towers over Needle Rock, a volcanic neck in the center distance. The hard, light-colored Oligocene intrusions forming the skeleton of the West Elk Range are more apparent in the late afternoon light than they had been in that morning. The West Elks are erosional remnants of an immense volcanic field erupted in two waves of explosive volcanism that inundated much of central and southwest Colorado in mid- to Late Tertiary time. Early Phase (36-31 Ma) eruptions blanketed large areas with andesitic lava flows, volcanic mudflows and ash falls, particularly in the West Elk and San Juan Mountains. The far-flung West Elk Breccia dates from this phase. Erosion has since carved it into many fantastic shapes, including the Castles near the town of Gunnison. Middle Phase (30-26.5 Ma) eruptions seared the by this time eroded Early Phase landscape with fast-moving incandescent ash flows (AKA pyroclastic flows, glowing cloud eruptions) depositing ignimbrites (welded tuffs) and unwelded tuffs over nearly a third of the state. Middle Phase tuffs apron the south flank of the West Elks to form the Palisades on the Gunnison. The Middle Phase coincides in timing and style with the Ignimbrite Flare-up that turned the entire Basin and Range into a lifeless moonscape around 30 Ma. Non-explosive Late Phase volcanism (25-5 Ma) blanketed the state with less violent basaltic lava flows. This final volcanic episode isn't preserved in the West Elks but left basalts capping Grand Mesa and ridge tops north of Aspen.





Somerset—Coal Country
	As we drove north from Hotchkiss on beautiful CO133 in the failing light, we couldn't bring ourselves to press on and lose the scenery to the dark. Just past Somerset, we took a tiny cabin (top left) on Coal Creek at Crystal Meadows Resort, a rustic West Elk hunting and fishing lodge at the turnoff for County Road 12 (a well-graded dirt road leading to Crested Butte and Gunnison — a must-drive for anyone interested in seeing the best of Colorado). Owner Cheryl Dix (2nd frame) took care of us with an ever-present warm smile that could brighten anybody's day. That evening, we plopped down to a hearty hunter's dinner. When Cheryl learned that we'd be shoving off for Cathedral Lake at dawn, she insisted on getting up early to make us a breakfast that kept us going into early afternoon. The Somerset area is prime Colorado coal country. Most of the coal mined from the richly organic late Cretaceous Mesaverde sandstones (3rd frame, here above Coal Creek and CO133) is valuable low-sulfur bituminous, but heat from the nearby West Elk volcanic field cooked up an especially prized anthracite here. The Somerset mining operation founded by Cheryl's father fills over a hundred 100,000-ton coal cars every other day. After this stop, we noticed long coal trains everywhere.

Gallery Note: All images on this page are from September, 2002 unless noted otherwise below.

Top of Gallery

Top Page Index

References

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:

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

Photojournal