THE LANDSCAPE AND ITS ATTRIBUTES
The Eastern Uinta Mountains
Although the main divide is very well defined in the Western Uinta Mountains it is very poorly defined in the Eastern Uintas. Because the Eastern Uinta Mountains are more complex structurally than the Western Uintas, and because they have had a more complex Tertiary history, their physiography is more complex, and they lack the grand simplicity of the west half of the range. The main divide has shifted with time from a position that once must have coincided with the crest of the anticline to its present position on the south flank. The crest of the anticline, moreover, passes beneath the valley of Browns Park, which extends deep into the heart of the range and separates the northeast flank from the rest of the range.
The physiography of the Eastern Uinta Mountains is further complicated by the great canyon systems of the Green and Yampa Rivers. These canyonssources of awe and wonder since the days of the mountain menstimulated early-day thinking on many geologic problems, most particularly on folding and faulting, which are grandly portrayed, and on the relations of drainage development to the formation and growth of mountains. Speculation as to how the Green River established its course across the Uinta Mountains led Powell to introduce such terms as "superposition"" and "antecedence" to identify processes by which streams are able to establish and maintain courses across mountain barriers. Although Powell coined the terms, he alone did not devise the concepts.
The canyons of the Green and Yampa are the most impressive features of the Eastern Uinta Mountains, but even without them, the mountains would possess much to fire the interest of the traveler and the imagination of the geologist. Physiographically, the Eastern Uinta Mountains are almost totally unlike their western counterpart. The lofty summits are lacking; no part has been glaciated; and there is no climatic timberline, although many of the summits are bare of trees. The highest point, Diamond Peakaltitude 9,710 feet, and site of the infamous diamond hoax of 1872is an outlier completely separated from the rest of the range. Mount Lena, south of Flaming Gorge Dam, at an altitude of 9,600± feet, is the highest point on the main divide. Most of the country lies below 8,000 feet.
Lacking high peaks to intercept moisture-laden air masses and standing in the rain shadow of the higher mountains to the west, the Eastern Uintas are relatively arid. Whereas parts of the Western Uintas probably receive 50 inches or more of moisture per year, only the Mount Lena area in the Eastern Uintas is likely to get half that much, and such places as Lucerne and Ashley Valleys get only 8-10 inches. Consequently, the pine, fir, and spruce forests of the Western Uintas give way largely to pigmy forests of juniper and pinyon, although stands of lodgepole pine are dense in the Mount Lena area, and, locally, elsewhere there are groves of ponderosa pine or stands of Douglas-fir. The lush high-altitude meadows of the Western Uintas have no eastern counterpart. If any one characteristic thus identifies the Eastern Uinta Mountains, thanks to the aridity of the climate, it is well-exposed bedrock. Bedrock is everywherein the bottoms and sides of gullies, along the ridges, in tremendous cliffs along the canyon walls, and on the faces and barren tops of the mesas. The plant cover is light, and the soil is thin.
Topographically, the Eastern Uinta Mountains are divisible into four large highland blocks separated by the canyons of the Green, the valley of Browns Park, and the canyon of the Yampa. These blocks, for convenience, can be referred to as (1) the Dutch John-Cold Spring highland, (2) the Diamond Mountain highland, (3) the Douglas Mountain highland, and (4) the Blue Mountain highland (fig. 14). The intervening canyons are formidable barriers that make vehicular access from one highland block to another difficult, if not impossible. Thus, Diamond Mountain, Douglas Mountain, and Blue Mountain are mutually exclusive; none can be reached directly from either of the others. The Dutch John-Cold Spring highland can be reached from the south by a scenic mountain highway (Utah State Highway 44) that connects Dutch John to Vernal, by way of Flaming Gorge Dam. But before the dam was built, access from the south involved many circuitous miles of difficult driving.
Dutch John-Cold Spring highland
The Dutch John-Cold Spring highland is not really a topographic entity; rather, it is a heterogeneous belt of bare-topped hills, hogbacks, and mesas bounded vaguely by the Green River Basin on the north and the Flaming GorgeRed CanyonBrowns Park valley complex on the south. On the east it terminates at Vermilion Creek, in a narrow slot 600 feet deep. This highland contains some of the most diverse topography, some of the most complex geology, and the oldest rocks in the entire Uinta Mountains; it also contains Utah's oldest commercial gas field, Clay Basin.
Large flat-topped mesas, such as Bare Top (Bear Mountain), Goslin, and Cold Spring Mountain, are remnants of a broad erosional plain that once extended along the flank of the mountains for the length of the range, between the high peaks on the south and the basin on the north. Named the Gilbert Peak erosion surface and described by W. H. Bradley (1936), this plain has been faulted and tilted and has been dissected by deep canyons since it was formed.
Most of the Dutch John-Cold Spring highland is on the north east limb of the Uinta anticline, where a great thickness of upturned strata is beveled by erosion and exposed to view. More than 20,000 feet of strata is exposed in one continuous section on Cold Spring Mountain, even though much of the total section is omitted from surface exposure by faulting. Altogether, the area contains more than 40,000 feet of strata, not counting the ancient metamorphosed Red Creek Quartzite, which forms the crystalline core of the range and which itself is probably 20,000 feet thick. Thanks to large-scale faulting and deep erosion, the Red Creek Quartzite is well exposed along the north side of Browns Park. The age of the Red Creek, as determined by radiometric dating, is 2.3 billion years.2
Diamond Mountain highland
The Diamond Mountain highland, south of Red Canyon and Browns Park, is a dissected upland along the main divide of the Uinta Mountains. As seen from the south near Vernal, it presents an even skyline that belies a more rugged interior. It is surmounted by many craggy peaks and ledges of somber red quartzite and is flanked on the south by a thick sequence of colorful, well-exposed rock formations made up mostly of sandstone and limestone. It is almost completely enclosed by deep canyons and steep escarpments. On the west it is separated from the Western Uinta Mountains by a low pass which also provides the route for Utah State Highway 44. The pass drains north into Cart Creek, which meanders through a verdant meadow and plunges into a gorge a thousand feet deep before joining the Green River near Flaming Gorge Dam. The south side of the pass drains into Little Brush Creek through another gorge nearly as deep and even more precipitous.
On the north the Diamond Mountain highland is bounded by rugged terrain along Red Canyon and Browns Park. Several small spring-fed streams flow north in open valleys with floors of the soft Browns Park Formation. None of these streams has a mean flow of more than a few cubic feet per second. They then descend into narrow quartzite canyons, some of which rival Red Canyon itself, before joining the Green River. Crouse Creek is typical: its canyon affords access into Browns Park by way of a rough but generally passable truck road. The creek bottom is chocked with brush and treescottonwood, boxelder, alder, choke cherry, red-osier dogwood, willow, wild rose, and poison ivya good habitat for beaver, bobcat, and cougar.
The relation of these north-flowing streams to Pot Creekthe "Summit Valley" of Powell's dayis very strange. Pot Creek flows southeast the full length of the highland to Lodore Canyon. Yet the several divides between Pot Creek and the north-flowing drainages are low saddles only a few feet high. The whole drainage system of the Diamond Mountain highland is on the Browns Park Formation, which, in turn, fills an older pre-Browns Park valley system cut into the hard quartzite of the Uinta Mountain Group. In many places the present drainage is directly opposite the original drainage, and in some places, creeks flowing in opposite directions share the same pre-Browns Park valley. Many minor tributaries are "barbed"; they flow south into creeks that flow north. The distribution of the Browns Park Formation is very significant because the discordant relation of the Green and Yampa Rivers to the Uinta Mountains is tied to the deposition and removal of the Browns Park Formation.
On the east the Diamond Mountain highland is bounded by the deep chasms of Lodore and Whirlpool Canyons. From the rim of Wild Mountain at the southeast corner of the highland to the river below is a drop of more than 3,000 feet. Though a delight to white-water boatmen, these canyons are insurmountable barriers to cross-country travel; west of the canyons, a corner of Colorado is completely cut off from the rest of the State and can be reached only through Utah.
On the south the highland is bounded by the rim of the Diamond Mountain Plateau, a broad nearly flat upland and a likely correlative of the Gilbert Peak erosion surface of the north flank of the range. Diamond Mountain is capped by gravel and volcanic ash of the Browns Park Formation. Diamond Gulch drains southeastward across this area in a course nearly parallel to that of Pot Creek. It joins the Green River in Whirlpool Canyon via Jones Hole, a hidden spring-fed valley of rare beauty.
Douglas Mountain highland
East of the Diamond Mountain highland, across Lodore Canyon, is Douglas Mountain. This dissected highland, nearly triangular in outline, is a jumble of cliffs, ledges, ravines, and parklike clearings. It is bounded on the north by an arid extension of Browns Park, on the east by the Little Snake River, and on the south by the twisty canyon of the Yampa. Access is only from the north over rough mountain roads and trails. Zenobia Peak, at an altitude of 9,006 feet, the highest summit, commands impressive views in all directions. It also is the highest point in Dinosaur National Monument.
Douglas Mountain is an area of brightly colored rocks. The brilliant white exposures of the Browns Park Formation, dazzling in the sun, contrast with the deep-red hues of the adjacent Uinta Mountain Group. Blue, gray, and pale-pink limestones cap Zenobia Peak and reach south in long sloping ridges toward the Yampa River. The canyons themselves are variegated in reds, gray, and buff, depending partly on the particular rock formation exposed in the walls and partly on the time of day. Douglas Mountain has a few small springs but no perennial drainage.
Blue Mountain highland
South of Douglas Mountain, across Yampa Canyon, is Blue Mountain, the southeasternmost highland block of the Uinta Mountains. Steep, even precipitous, escarpments on the west, south, and east of this highland block are surmounted by an open, rolling interior, which rises gradually northward to the bald summits of Round Mountain, Marthas Peak, and Tanks Peak. Rewarding views are had from these peaks, also. To the north, the ground drops away more than 2,000 feet in a giant step down to the flats that rim Yampa Canyon. The river itself is another 1,000 feet below.
Abrupt monoclinal folds characterize the flanks of the Blue Mountain block, and they are grandly displayed. Monoclines are steplike bends in otherwise flat or gently dipping strata. The monoclines were formed by the warping of strata across concealed faults, or by the sharp flexing of strata on the flanks of flat-topped anticlines. On Blue Mountain these structures owe their dramatic appearance to the erosive resistance of the Park City Formation, to the massive character of the underlying Weber Sandstone, and to the easy erosion of the overlying Moenkopi Formation. As the soft Moenkopi is stripped away, the Park City and Weber are left standing in enormous dip slopes and "flatirons."
One of the more spectacular folds of this type is just north of U.S. Highway 40, where Stuntz Ridge, a westward extension of Bitic Mountain, forms a jutting promontory nearly 3,000 feet high (fig. 15). This remarkable flexure passes downward and laterally into a large fault.
Other flexures nearly as dramatic form the north face of the Blue Mountain, where the strata have been folded into sharp monoclinal bends along the Yampa fault and subordinate fractures in the stairstep noted before. These structures are seen to advantage from observation points along the high road to Harpers Corner, from Harpers Corner itself, or from the access road to Echo Park far below (fig. 60). Harpers Corner, incidentally, is the best canyon overlook in Dinosaur National Monument. It projects north as a sharp ridge from the main mass of Blue Mountain, where it looks down on Whirlpool, Lodore, and Yampa Canyons, and on Steamboat Rock at the confluence of the Green and Yampa Rivers.
To the west, on the same structural trend as the Yampa fault, is Split Mountain, a nearly symmetrical anticline breached to its core by the Green River (fig. 30). The outer shell of Split Mountain consists mostly of Weber Sandstone, which is deeply eroded into parapets, buttresses, and other fanciful architectural forms; even so, the outer shell reflects the overall shape of the fold. Younger rocks at the flanks form encircling hogback ridges and racetrack valleys; hard layers are etched into relief. The famous dinosaur quarry is on the south limb of the fold, where the dinosaur-bearing Morrison Formation is laid bare by erosion.
Split Mountain has been a source of wonderment since the days of Powell, particularly because a seemingly easier course for the Green River is available in the soft beds of the racetracks that encircle the anticline, as opposed to the hard rocks in the core. The river, however, had no other options at the time that downcutting began. Split Mountain did not exist as a topographic feature, and once entrenched, the river had no alternative but to pursue its established course. The "easy" course in the racetrack was eroded out later by tributaries. Meanwhile, the shape of the fold was brought into relief as soft beds were removed and bard beds were uncovered.
The great canyons of the Green and Yampa Rivers
The Green and Yampa Rivers arise in distant watersheds, fed by snowmelt from the Wind River and Park Ranges, as well as from the Uintas. Both streams flow in broad valleys, but they enter the Uinta Mountains in narrow gorges and flow across the mountains with a startling disregard for the geologic implications of their crossing. The sequence, altitudes, and gradients of canyons through the range are shown graphically in figure 16.
One hundred years ago Powell and his aides floated leisurely through the twisty canyons of southern Wyoming on the first leg of their historic boat trip down the Green and Colorado Rivers. In Powell's time the Green was regarded as the source of the Colorado. Just below the 41st parallelnow the Utah State linethe current quickened, and Powell's party abruptly entered the Uinta Mountains. "The Green River enters the Uinta Mountains by a flaring, brilliant, vermilion gorge, a conspicuous and well-known locality, to which, several years ago, I gave the name Flaming Gorge." (Powell, 1876, p. 146). The same scene, viewed by E. C. LaRue in 1914, is shown in figure 17.
Flaming Gorge to Browns Park
Flaming Gorge Dam, completed in 1964, has greatly changed the appearance of the country upstream. Today, a manmade lake extends 90-odd miles to Green River, Wyoming; it forms a broad embayment above Flaming Gorge but constricts tightly at the gorge entry. The rapids are quiet, replaced by a fjordlike body of still, blue water, hardly more than twice the width of the former river in some places but elsewhere widened into bays and inlets. Whole mountains are now islands.
Flaming Gorge is cut into Jurassic and Triassic rocks. Its sweep and grandeur are due largely to its high cap of Glen Canyon Sandstone (the Nugget or Navajo Sandstone of earlier reports), but it takes its name from the riot of reds, ochers, and oranges in the shaly Triassic beds below. It is more a water gap than a gorge in the usual sense. At the portal the rocks are standing on edge, but, arching over, they flatten rapidly a short distance downstream. Just below Flaming Gorge, the shoreline of the reservoir swings left into the Weber Sandstone, which forms the smooth walls of Horseshoe Canyon. Curving right, through Horseshoe Canyon, the reservoir then turns back into Triassic rocks in a broad curve to the left, then right again into the Weber in Kingfisher Canyon. Powell met his first real rapid at the mouth of Kingfisher Canyon, where he crossed the great Uinta fault and entered Red Canyon (fig. 18). Red Canyon, about 30 miles long, is cut entirely in Precambrian rocks, except at Little Hole, a parklike opening underlain by the Browns Park Formation and a pleasant contrast to the rugged canyons upstream and down.
The roar of the river overprints the scenery of Red Canyon. Red Creek Rapid below Little Hole is particularly unrestrained. Just about all the rapids in Red Canyon are at the mouths of tributaries, where from time to time, coarse rock debris is flushed into the main channel by flash floods. Most rapids in Lodore, Yampa, and Split Mountain Canyons were formed in the same way.
At the mouth of Red Canyon the river emerges into Browns Park, the legendary hideout of Butch Cassidy and his notorious "Wild Bunch," who terrorized southern Wyoming, eastern Utah, and western Colorado near the turn of the century.
Browns Park is a picturesque intermontane valley, which, in Powell's words, is "really an expansion of the ca&ntidle;on." It is broad, averaging about 4 miles across, but it is much longer than wide. Its actual length is arguable, but most people believe that it extends from Red Creek on the west to Vermilion Creek on the east, a length of about 25 miles.
Steep mountain fronts bound the valley on both sides, the mountaintops rising 3,000 feet or more above the valley floor. The north front is more abrupt than the south, partly because it is bounded for most of its length by faults, which drop the valley side relative to the mountains. Small creeks, after descending from the heights, enter the valley from both sides through deep brushy canyons.
Browns Park gains most of its distinctive appearance from its thick fill of Tertiary gravel, sand, clay, and volcanic ashthe Browns Park Formation. This fill forms the broad flat bottom of the valley, and its brilliant white exposure contrasts with the somber appearance of the nearby mountains (fig. 19). Flat-topped Quaternary terraces are characteristic, also.
Browns Park is doubly curious because of its structure. The Browns Park Formation accumulated on the floor of a preexisting canyonlike valley. The valley was formed partly by ordinary erosion and partly by downfaulting along the crest of the great Uinta anticline. Faulting probably helped to establish drainage there in the first place. As the fill accumulated, the contour of the old valley gradually softened, until it took the shape of a shallow elongate syncline (something like an extra-long celery dish). Thus, we have a syncline of deposition, modified by later faulting, superimposed on the crest of a folded anticline. The axis of the syncline is well north of the present valley bottom, which crowds the south margin of the valley. Clearly, the deepest, thickest part of the old fill is well to the north of the present river (fig. 20).
As the old fill spread over the floor of Browns Park, it slowly rose up the valley sides until it eventually buried tributary canyons and intervening ridges alike. Ultimately, it probably filled the valley to the rims. Then, when downcutting was resumed, the soft fill was removed preferentially, so that buried promontories, such as Kings Point, were exhumedan ancient Tertiary landscape faithfully, if incompletely, restored.
Tertiary drainage through Browns Park was obstructed from time to time by one thing or another: alluvial fans of sand and gravel spread across the valley from its margins, chiefly its north margin, and formed lakes. Earth movements along faults may also have caused impoundments. Tiny organisms, such as ostracodes and diatoms, thrived in standing water, and their remains were preserved in the lake-bottom muds. The environment may have been too hostile for more complex forms of life, for no other remains have been found.
Frequent falls of volcanic ash blanketed the whole countryside. Ash that fell in the valley is preserved just as it accumulated it gives the Browns Park Formation its white brilliance. Much ash that fell on the adjacent hills was washed into the valley by rains and freshets. Thus, the resulting Browns Park Formation is a varied accumulation of stream-laid sand and gravel, lacustrine clay, and volcanic ash. Its known thickness is more than 1,500 feet.
After entering Browns Park, the Green River flows swiftly but quietly for several miles across the Browns Park Formation. Just after crossing the axis of the Uinta anticline on the south side of the valley, the Green enters Swallow Canyon (a short but steep walled gorge) cut into the Uinta Mountain Group in a classic example of drainage superposition. The course of the river through Swallow Canyon was fixed at a higher altitude than now in soft Browns Park beds that have since been removed. As the river eroded downward, it reached a buried spur of hard red quartzite through which it was constrained to cut its gorge. The spur, since exhumed, is now King's Point, high above the canyon walls. Although Swallow Canyon is perhaps the most obvious example of superimposed drainage in the entire Uinta Mountains, many other canyons in the regionand elsewhere in the Rockies for that matterwere carved in the same way. Below Swallow Canyon the river swings back again onto the Browns Park Formation, and as its grade flattens and its current slackens, it meanders lazily the 20 miles or so to the Gates of Lodore.
As noted before, the river crosses the axis of the Uinta anticline in Browns Park above Swallow Canyon. Lodore Canyon, therefore, is on the south limb of the fold. The river enters Lodore Canyon deep in the red Precambrian quartzite of the Uinta Mountain Group. And, inasmuch as the dip of the strata, though gentle, is steeper than the gradient of the river, the river crosses successively younger rocks en route. About half way through the canyon, above Triplet Falls, the Lodore Formation (Cambrian Period), the Lodgepole Limestone, and the Deseret Limestone (both Mississippian) first come into view. The Mississippian limestones form a sheer gray cliff 400 feet high that surmounts the somber red beds below. At each bend of the river the cliff descends lower, finally reaching river level 2 miles above the mouth of the canyon. At the mouth of Lodore Canyon, at Echo Park, the river flows on Weber Sandstone, having crossed strata, about 1-1/2 miles thick, of Precambrian, Cambrian, Mississippian, and Pennsylvanian age. Then, around the next bend, a sharp fault (Mitten Park fault) has raised the Precambrian back again above river level.
Lodore is the deepest and most precipitous of the great river canyons in the Uinta Mountains (fig. 21), and the Gates of Lodore could hardly be more imposing. Looking into the gates, one senses some of the mixed emotionsexhiliration, wonder, and apprehensionthat must have bestirred Powell and his boatmen.
The river, after meandering parallel to the mountain front through Browns Park, swings to the right in a broad arc and drives headlong into the canyon. The current quickens, but the river flows smoothly and in a nearly straight line for about 3-1/2 miles. Then, a thundering roar ahead signals approaching turbulence, and after a sharp bend to the right, the river plunges into its first rapid. It drops from one rapid into another for the next 8 miles, with short respites of quiet water in between. Powell's names for some of these rapidsUpper and Lower Disaster Falls, Hells Half Milegive some idea of the fury of the river.
At the Gates of Lodore the canyon walls rise abruptly from the water's edge. A mile downstream the canyon rim is more than 2,000 feet above the river. The walls are most precipitous near the head of the canyon, although they are higher downstream at Wild Mountain, where the drop from rim to river is more than 3,000 feet. Lodore Canyon may have been cut in two stages. As shown in figure 22, a steep inner gorge rises 600-800 feet above the river, then a series of buttresslike forms slope up several hundred feet to the base of precipitous outer walls a thousand feet higher. Canyon profiles (fig. 23) suggest that the postulated outer canyon was about 1,500 feet deep when the inner gorge was cut, probably in Quaternary time.
At Echo Park, or Pats Hole, as it is also known, the Yampa River flows into the Green. Echo Park (fig. 24) is a longtime favorite of canyon visitors and is accessible to automobiles by way of a rough but scenic dirt road. Yampa Canyon extends upstream about 45 river miles from Echo Park to a point near the east end of the Uinta Mountains called Lily Park. Just below Lily Park the Yampa River cuts abruptly across Cretaceous, Jurassic, Triassic, and Permian strata, then across the Weber Sandstone, and into older Pennsylvanian rocks generally called the Morgan Formation. The river flows across the Morgan at about the same stratigraphic level for 18 miles. It then cuts back into the Weber Sandstone and stays mainly in the Weber to its confluence with the Green.
Because of marked differences in color, bedding habit, and lithology between the Morgan and the Weber, the appearance of Yampa Canyon changes drastically across the formation boundary. Through the Morgan, the canyon is mostly wide and asymmetrical; it is steep, or precipitous, on the south side, and ledgy and sloping on the north. Layers of pink sandstone and siltstone alternate with gray, tan, and pink limestone. The course of the river is curvy but not tortuous. Through the Weber Sandstone the canyon is narrow, precipitous, and spectacular. Smooth canyon walls overhang the river in places, rising sheer for hundreds of feet above the water, particularly on the outside bends of meanders. The course of the river is extremely twistyin less than 10 miles, straight line, the river meanders 22 miles, and each turn opens anew vista (fig. 25).
Whirlpool Canyon and Island Park
Just below Echo Park the Green River makes a long hairpin bend around Steamboat Rock, crosses the Mitten Park fault, and enters Whirlpool Canyon, a spacious gorge about 2,500 feet deep and 1-1/2 miles across. Shown in profile in figure 26, the walls of Whirlpool Canyon are less precipitous than those of Lodore Canyon and are less picturesque than those of Yampa, but they are very impressive, nevertheless. The dip of the strata, again, is downstream, so that the river flows across successively younger rocksthe same formations exposed in Lodore Canyon. The Uinta Mountain Group forms a vertically walled inner gorge 300 feet deep in the headward part of the canyon, and the river fills the bottom, wall to wall. Then at the first bend, the Uinta Mountain Group passes below drainage. Although the heights of the canyon are eroded mainly from Mississippian and Pennsylvanian strata, Cambrian rocks (Lodore Formation) are well exposed in the canyon bottom from the Mitten Park fault downstream to about midcanyon. In the upper canyon walls, massive limestone beds of Mississippian and Pennsylvanian age form giant stairsteps of sheer cliffs and sloping ledges, capping off the rims at Wild Mountain and Harpers Corner.
Halfway through Whirlpool Canyon, entering from the right (north) is spring-fed Jones Hole Creek, a sparkly stream that issues at full volume from openings in the Pennsylvanian Round Valley Limestone. Jones Hole is dominated by towering cliffs of Weber Sandstone in a picturesque and dramatic setting. It is one of the foremost attractions of Dinosaur National Monument and a remembered highlight of any float trip down the Green. It can also be reached overland, on foot.
Large faults bound Whirlpool Canyon upstream and downthe Mitten Park and Island Park faults, respectively. Between these faults, the canyon area has been raised about 3,000 feet relative to the areas upstream and down. One must realize, however, that the faulting took place long before the canyon was eroded. The Mitten Park fault can be seen very well from the river and from Harpers Corner high above (fig. 27). The Island Park fault can be seen well from the mouth of Whirlpool Canyon (fig. 28) and from Jones Hole.
Whirlpool Canyon opens downstream through a flaring gateway into Island Park, which together with Rainbow Park is an attractive alluviated valley of a few square miles between Whirlpool and Split Mountain Canyons. Island Park is on the axis of a faulted asymmetrical syncline, first cited by Powell; it trends west between the main Uinta anticline to the north and the Split Mountain anticline to the south (fig. 29). The syncline axis is crowded against the flank of Split Mountain. Cretaceous rocks are preserved in the trough of the syncline, their first occurrence downstream from Flaming Gorge.
Split Mountain Canyon
After meandering placidly through Island and Rainbow Parks, the Green River heads into Split Mountain Canyon, an eroded anticline (fig. 30). Flowing swiftly across upturned beds, the river passes from Jurassic rocks at Rainbow Park onto rocks of Mississippian age in the core of the Split Mountain anticline. After crossing the fold axis, the river then recrosses the same beds, but in reverse order, on the south limb of the fold. Varicolored Mississippian, Pennsylvanian, and Permian rocks form the sloping inner canyon walls. Weber Sandstone forms the heights, eroded into a craggy terrain of fanciful turrents, domes, and ramparts. On the south flank of the Split Mountain anticline, the Weber is eroded into an array of buttresslike formsgreat monoliths separated from one another by deep, extremely narrow tributary ravines, some of which expand upstream into wide but nearly hidden alcoves.
The Green River runs fast through Split Mountain Canyon, having there its greatest average rate of fall20.7 feet per milein the Eastern Uinta Mountains. At the mouth of the canyon, the gradient flattens, the current slackens, and the river emerges at last from the Uinta Mountains into the Uinta Basin, 118 miles below its point of entry at Flaming Gorge.
How the canyons were formed
Catastrophism as a rationale to explain the origin of canyons has gone out of style. In the past, many people viewing the great river gorges of the west would have guessed that the earth just split open, sundered by some frightful cataclysm, the waters then simply plunging headlong into the void. Cataclysmic events have indeed been recorded in geology. The overflow of extinct Lake Bonneville was such an event; so was the Alaska earthquake of 1964. But these events are rare and, anyway, are not on the order of magnitude needed to cut canyons across mountain ranges. The energy requirements of canyon cutting are infinitely greater, although the energy is expended much more slowly. So, the concept that the great canyons were carved slowly and inexorably by their own drainage is a truism that most people now accept without further explanation.
In fact, the events leading up to the cutting of a canyon by a river can be quite complex and can involve the interaction of many separate processes. To be sure, the river is the chief agent of erosion and the prime mover of eroded rock, but it alone could not do the job. Atmospheric weatheringchemical decomposition, as well as mechanical disintegration of rockfrost action, gullying, rill action during showers, and downslope movement of rock debris under the influence of gravity all help prepare the ground, widen the canyon walls, and transport material to the canyon bottom. Only then does the river take over, wearing away and carrying off its suspended load and, at the same time, attacking and eroding its bed.
The river is most effective during high water, in early summer, when runoff is swollen by snowmelt from the high mountains or by thunderstorms over the watershed. As the volume of water increases, so does the velocity. Increase the velocity, and the carrying power increases geometrically. If the velocity is doubled, the capacity is tripleda principle spelled out many years ago by G. K. Gilbert, whose namesake Gilbert Peak dominates the Western Uinta Mountains on the north. The roily water of flood stage is an obvious expression of the erosive power of the river. And no one who has seen a large river in flood can doubt its ability to do its work.
But the most intriguing question related to the origin of the great canyons of the Uinta Mountains is not so much the mechanics of erosion as it is how the rivers were able to cross the mountains in the first place. How could rivers establish and maintain their courses across a great mountain range in utter disregard for the structural complexities within the range? Such canyons as Swallow and Split Mountain provide clues, but not answers, and the evidence for the whole canyon system is even less obvious. Differences of opinion through the years have marked the development of thought.
Three concepts, at one time or another, have gained some acceptance: antecedence, superposition, and stream capture. Antecedence, the concept favored by Powell, assumes that the drainage pattern predates uplift and that, as uplift begins and folding progresses, vigorous rivers are able to lower their beds as fast as the land is elevated. Antecedence has been authenticated in some drainage basins, but cogent evidence rules it out for the Eastern Uinta Mountains. For example, early in the history of the range, drainage clearly was away from the rising mountain mass, just as one would expect, rather than toward or across it, as it is now. Rock materials eroded from the heights were redeposited at the flanks, where they now comprise several rock formations younger than the mountains but older than the canyons. As the mountains rose, closed basins formed on both flanks. These basins, the Uinta and Green River, contained great lakes whose deposits now make up the Green River Formation, famous for its fossil fishes, oil shale, and enormously large deposits of sodium salts. Finally, broad erosion surfaces or pediments (for example, the Gilbert Peak erosion surface) were formed on both flanks of the range, sloping outward to the basins. As long as these surfaces were intact, drainage across the range was impossible.
Emmons in 1877 propounded the concept of superposition. He believed, and I think correctly, that the Green River established its course on an old fill of Tertiary sediments (the Browns Park Formation) that spread across the eastern part of the range after the range had been uplifted and deeply eroded. Much of this old fill is still preserved. The Green River, then, according to Emmons' concept, cut down through the fill and incised itself into the buried, older rocks in the core of the range. The Green was thus able to cut across anticlines, synclines, and broad fault zones without undue deflections in its course. It was, in fact, compelled to do so because it was held in by its own banks, and the deeper it cut, the more firmly committed it was to its course. The present discordance between drainage and topography is therefore due more to selective erosion of hard and soft rocks after the drainage pattern was fixed than to differential uplift.
W. H. Bradley (1936) studied the north flank of the Uinta Mountains in the 1920's-30's. Bradley surmised that the ancestral drainage of the Green River Basin was east, toward the North Platte River, and possibly into the Gulf of Mexico. He also found evidence that the ancestral drainage in the Browns Park area was eastward, possibly into the ancestral Yampa River, or possibly across the present Continental Divide and into the Mississippi drainage system as well.
As a topographic eminence, the present Continental Divide is indeed very subdued all the way from the southern Wind River Range to the northern Sierra Madre in southern Wyoming. In that area, broad closed depressions actually exist along the divide, and the true position of the divide is very obscure. The Sweetwater River arises on the west side of the Wind River Range in what structurally is part of the Green River Basin; it turns south, then east, and flows to the North Platte. Thus, even now, part of the drainage from the Green River Basin flows to the Gulf of Mexico.
Bradley's suggestion of eastward drainage is also supported by remnants of an old valley filled with the Browns Park Formation. The old valley trended east from Browns Park toward Craig, Colorado. Beyond Craig, the valley may have extended north into Wyoming toward Baggs, but the evidence is obscure. Northeast of Baggs, remnants of the Browns Park Formation or its equivalent extend well into the present North Platte drainage, though there is no real assurance that any of those deposits originated in the Uinta Mountains region.
Bradley agreed that the course of the Green was superimposed upstream and down from Lodore Canyon, but he argued that if the course were superimposed through Lodore, an inordinate amount of fill had to have been removed from Browns Park. Bradley also called attention to the relatively straight course of Lodore Canyon, which he contrasted with the sweeping meanders farther upstream and down. Following a suggestion by J. D. Sears (1924), Bradley postulated that an east-flowing stream in Browns Park (the ancestral Green) was diverted south by a small but vigorous stream eroding headward in what is now Lodore Canyon. This stream, nibbling away at the intervening divide, undercut the channel of the ancestral Green River and turned it southward. Bradley thus introduced the concept of stream capture to explain Lodore Canyon and the southward flow of a river that very clearly once flowed east.
In more recent years, widespread Tertiary fills have been found in the highlands south of Browns Park (fig. 31). These fills are identical with the Browns Park Formation and no doubt correlative with it. Along Pot Creek, Crouse Creek, Cart Creek, and other streams on Diamond Mountain are deposits of white volcanic ash and standstone at altitudes comparable to the rims of Lodore Canyon. In fact, most of the valleys are blanketed by such deposits; only the ridges and peaks protrude through them. At widely scattered localities on Douglas Mountain, Harpers Corner, and Blue Mountain, deposits of conglomerate and volcanic ash probably are eroded remnants of the Browns Park Formation also. Most of Harpers Corner is capped with gravel or conglomerate. The extent and distribution of all these deposits demand that the old Tertiary fill was very thick indeed and that a tremendous volume was surely removed by erosion. These deposits, therefore, seem to confirm Emmons' view that the course of the Green River across the Uinta Mountains is superimposed.
The establishment of the Green River across the Uintas, as I see it, is thus postulated as follows: An ancient river flowed eastward from Browns Park, as Bradley suggested, hypothetically toward the north end of the Sierra Madre and into the North Platte. If so, the crestline of the Uinta Mountains at that time was the Continental Divide. Steps in the drainage history are shown in figure 32. Meanwhile, crustal movements preceded, perhaps accompanied, and certainly followed deposition of the Browns Park Formation. In brief, the whole eastern part of the Uinta Mountains between the Uinta fault on the north and the Yampa fault on the south was involved in a complicated "graben" movement, or collapse. First noted by Powell, the collapse of the Eastern Uintas has since been affirmed by many geologists, particularly by Sears and by Bradley. The present Continental Divide across central Wyoming is viewed as having begun to rise at about the same time. All these crustal movements, combined with immense outfallings of volcanic ash from some distant source, led to the stagnation and ultimate ponding of all the drainage in the Eastern Uinta Mountains. The old valley of Browns Park finally became filled to overflowing with a great thickness of gravel, sand, clay, and volcanic ash. The fill spread onto and across the main highland mass of the mountains to the south, leaving only the peaks and ridges exposed. Sears expressed much the same view in 1924. The fill also extended east to west at least from Craig to Flaming Gorge and north to south across the Uinta Mountains from Browns Park at least to Diamond Mountain. It enveloped Cold Spring Mountain on the north and possibly even overtopped it. Its southward limit in the Uinta Basin is unknown, for it has been erased by erosion.
Conditions were then ripe for superposition. With drainage obstructed to the east by the rising Continental Divide and with Browns Park filling with sediment, the stagnating, meandering Green River spilled across the Uinta crest, along what is now Lodore Canyon, to the Uinta Basin, where it became tributary to the Colorado. Below the spillover point, the course of the river probably was nearly straight, just as it is today.
Drainage presumably had already been established out of the Uinta Basin to the south, across the high Tavaputs Plateau. The Green there has since cut a canyon 4,800 feet deep, with a rim 9,200 feet above sea level. Either this rim was breached before the Eastern Uinta Mountains collapsed or it rose after it was breached, possibly both; otherwise, the basin would have drained north across the Uintas to the Gulf of Mexico. Even now, the Continental Divide in southern Wyoming is little more than 6,800 feet above sea levelmuch lower than the rims of the Tavaputs Plateau (and scarcely as high as the rims of the Lodore).
In any event, the Green River, rejuvenated in its new course, must have quickly incised through the soft Browns Park Formation atop the Uintas down into the harder rocks underneath.
At about this time, as thus visualized, the main drainage of the Green River Basin was still flowing eastward at a hydraulic disadvantage across the rising Continental Divide. But the invigorated Green River, with a new, lower base level, now began to entrench its meanders upstream from Browns Park, carving out the spectacular loops in Red Canyon, Horseshoe Canyon, and Flaming Gorge. Near Flaming Gorge the river captured drainage that once flowed north into the Green River Basin and possibly even reversed the flow direction of the present reach of river between Flaming Gorge and Green River, Wyoming. Presumably, it then captured the main drainage of the Green River Basin. Bradley (1936) noted that the main Uinta Mountain tributaries of the Green flow well north into the basin before turning east and south into the Green, as if in response to a diverted master stream.
Meanwhile, the course of the Yampa River was being established on the south flank of the range. The ancestral history of the Yampa is still in some doubt, even though it has received considerable attention. The pattern of drainage suggests that the Yampa may once have even flowed east from the Uinta Mountains, along with the Green. The Yampa may have joined the ancestral Green somewhere near Cross Mountain. At any rate, the remarkably entrenched meanders of the Yampa in Yampa Canyon point clearly toward superposition of that reach of river. J. D. Sears (1962) discussed in detail the character and origin of these features. Once again, the distribution of the Browns Park Formation leads to the inference that the Yampa entrenched itself down through a Browns Park fill onto the underlying, harder formations. There, down through hundreds of feet of hard rock, it faithfully reproduced the intricate meander pattern that it had originally developed on an old valley fill long since removed.
Last Updated: 18-Jan-2007