Geological Survey Bulletin 1291 The Geologic Story of the Uinta Mountains

TIMBERLINE in the Uinta Mountains is at an altitude of about 11,000 feet. All the high peaks and ridges above that altitude, from Mount Watson on the west to Marsh Peak on the east, are in the west half of the range. The range, in fact, can be divided topographically as well as structurally into west and east counterparts, here referred to as the Western and Eastern Uinta Mountains, respectively, or more simply, as the Western and Eastern Uintas. The often-used name "High Uintas" is synonymous with the Western Uinta Mountains. A broad low pass north of Vernal, drained by Cart Creek on the north and by a tributary of Brush Creek on the south, separates the two parts of the range.

The Western Uinta Mountains

Peaks and ridges

The main divide, or "backbone," of the Western Uinta Mountains is a narrow sinuous ridge more than 60 miles long and rarely as much as a mile across at its base, extending from Hayden Peak on the west to Leidy Peak on the east. Until late Tertiary time this ridge probably was the Continental Divide. The base of the ridge stands near or somewhat above timberline, 11,000-12,000 feet above sea level. The crest is 1,000-2,000 feet higher. From the head of Stillwater Fork, east of Mount Agassiz, to Island Lake, east of the Burro Peaks—a distance of about 43 miles—the base of the ridge is nowhere less than 11,500 feet above sea level; most of it is higher than 12,000 feet. North and south from the main divide are many subsidiary spurs and ridges, most of them equally narrow, equally sinuous, and as high or higher than the main divide. The new topographic maps of the Uinta Mountains show 26 individual summits, including some subordinate peaks, that stand more than 13,000 feet above sea level; however, only nine of these are on the main divide.

Particularly near the crest of the range, the summit ridges are mostly steep sided forms that alpinists would call arêtes. Exposed ledges of bedrock, mantled only here and there by talus, are surmounted by little or no flat space at the ridgeline (figs. 3 and 4). Subordinate ridges between canyons, on the other hand, commonly broaden away from the crest into lofty plateaus 10,000-12,000 feet above sea level. These ridges, and wide places along the crest itself, are covered with loose angular rubble—a vast felsenmeer or "block field," subjected since Tertiary time to the relentless attack of frost and water (fig. 5). Perhaps most of this rubble is a product of the past severe climate of the Pleistocene Epoch, or Great Ice Age. At that time, the valleys below were buried repeatedly under great accumulations of moving ice, which swept away all loose rock and redeposited it downstream. But the exposed ridges, standing high above the ice, were under almost continuous attack by frost.

ARÊTE. Glacially sharpened Red Castle, viewed from the west, dominates skyline above Smith Fork just north of the Uinta crestline. Massive flat bedding characterizes the Uinta Mountain Group (Precambrian) in this area. (Fig. 3)

GLACIAL LANDSCAPE high on the south flank of the Uinta Mountains. Unnamed ice-sculptured peak, altitude 12,385 feet, at head of Rock Creek basin, looms above morainal ridge in foreground. Photograph by Max D. Crittenden, Jr. (Fig. 4)

FELSENMEER. Jumble of frost-riven blocks tops ridges unreached by Ice Age glaciers. Valley of Yellowstone Creek, viewed toward the northeast. Compare with figures 3 and 4. Photograph by Max D. Crittenden, Jr. (Fig. 5)

Frost has a ratcheting effect on rocks. Water percolates into an opening, freezes and expands, widens the opening, thaws, percolates deeper, refreezes, widens the opening further, and so on. Eventually, solid rock is split asunder. The process is effective at high altitudes throughout the World and in the polar regions. It is particularly effective on brittle jointed quartzite, like that in the High Uintas, where moisture has ready access to cracks and bedding planes. Even today the process there is very active, inasmuch as freezing temperatures can occur in any month of the year. The effect of the process is to soften the sharp outlines left by the ice—the arêtes, headwalls, and cirque basins are all being modified by frost, snowmelt, gullying, and wind. If the present process continues long enough, the steep-sided ridgelines will eventually become mere rubble-covered mounds, buried in their own debris.

Drainage basins

The summit ridges of the Western Uintas form the watersheds of several major drainage basins. Water falling on the north flank drains to the Great Basin via the Weber and Bear Rivers, which empty into the Great Salt Lake, and to the Green River Basin via Blacks Fork of the Green, Smith Fork, Henrys Fork, Beaver Creek, Burnt Fork, Sheep Creek, and Carter Creek. The south flank is drained by the Provo, which flows to Utah Lake in the Great Basin, by the Duchesne, which flows to the Green River in the Uinta Basin, and by such tributaries of the Duchesne as Rock Creek, Lake Fork, Yellowstone Creek, the Uinta River, and the Whiterocks River. Ashley Creek and Brush Creek, near Vernal, flow independently to the Green River in the Uinta Basin.

Sculputure by moving ice

All these streams head near the crest of the range in broad amphitheaterlike basins, or compound cirques, some of extraordinary size, scoured out by now-extinct Pleistocene glaciers. Lying near timberline, most of these basins are floored by glacially abraded bedrock mantled only in part by soil and vegetation. In aggregate, the basins contain hundreds of lakes and ponds—some in hollows eroded from solid rock, others dammed by glacial moraines (fig. 6). Many lakes are arranged in stair-step fashion, one above another; the seven Chain Lakes at the foot of Mount Emmons and the Red Castle Lakes of Smith Fork are good examples. In a setting of deep forests and alpine meadows the silent beauty of these lakes enhances the lonely splendor of the mountains. Emmons, in 1877, duly impressed by what he had seen, reported that "The scenery of this elevated region is singularly wild and picturesque, both in form and coloring. In the higher portions of the range, where the forest-growth is extremely scanty, the effect is that of desolate grandeur; but in the lower basinlike valleys, which support a heavy growth of coniferous trees, the view of one of these mountain lakes, with its deep-green water and fringe of meadowland, set in a sombre frame of pine forests, the whole enclosed by high walls of reddish-purple rock, whose horizontal bedding gives almost the appearance of a pile of Cyclopean masonry, forms a picture of rare beauty." A few lakes can be reached by automobile, especially those in the Mirror Lake-Bald Mountain area on Utah State Highway 150. But most of them are accessible only on foot or horseback, just as in Emmons' time.

GLACIAL LAKE near the Duchesne-Bear River divide. Hundreds of such lakes were left in the Uinta Mountains in the wake of the melted Ice Age glaciers. Photograph by Hal Rumel. (Fig. 6)

Extent of the ice

The great amphitheaters along the crest of the range extend basinward into long steep-sided canyons 2,000-3,000 feet deep. During the Great Ice Age these canyons contained large trunk glaciers that moved outward from their catchments in the amphitheaters toward their termini near the mountain flanks (fig. 8). The form of the amphitheaters and canyons, in fact, is due to the scouring action of the ice on their walls and floors. At one time some of the glaciers extended well beyond the mountains onto the adjacent plains. Several were more than 20 miles long; the longest known was the Blacks Fork glacier on the north slope, which, according to Bradley (1936, plate 34), deposited its outermost moraine on the plains of the Green River Basin 38 miles north of the crest. At an early glacial stage several glaciers on the north slope merged into broad piedmont ice sheets. The longest known glacier on the south slope was the Uinta River glacier, which was about 27 miles long. However, the merged Lake Fork and Yellowstone Creek glacier was more massive. Altogether, the ice at its maximum extent covered at least 1,000 square miles, according to W. W. Atwood (1909), who first studied in detail the effects of the glaciers on the Uinta Mountains. No doubt, the ice was much more extensive at one time than Atwood estimated because he based his estimate on the extent of relatively late Pleistocene glaciers, which were confined largely to the valleys. Atwood was unaware of the farreaching older glaciations, discovered later by Bradley, or of extensive piedmont deposits more recently mapped by the Geology Department of the University of Utah (Stokes and Madsen, 1961). Perhaps the total extent of the glaciers was closer to 1,500 square miles.

ICE AGE GLACIERS of the Uinta Mountains. Maximum known and inferred extent of ice. Piedmont glaciers shown on the north flank of the range existed only in earlier glacial stages. (click on image for an enlargement in a new window) (Fig. 8)

Atwood identified two distinct "epochs" of glaciation and suggested a third. Bradley (1936), although he made no attempt to study the glaciations systematically, identified three stages, including one considerably older than Atwood's "earlier glacial epoch." This, he called the "Little Dry Stage" for the morainal deposits near Little Dry Creek south of Mountainview, Wyoming. Richmond (1965) recognized two additional younger "stades" high in the mountains and subdivided Atwood's two "epochs" into five "stades" (fig. 7).

The effects of the glaciers on the Uinta Mountains are still clearly visible today, though many streams have since incised themselves into narrow inner gorges. U-shaped canyon profiles, truncated ridge spurs, hanging tributary canyons, falls and cataracts, and polished rock surfaces all testify to the abrasive action of moving ice. (fig. 9).

STILLWATER FORK of the Bear River, a glaciated valley in the High Uintas. Pyramidal peaks, hanging tributaries, and flat valley bottoms are scenic products of the Ice Age. Ostler Peak is just left of center. Photograph by Hal Rumel. (Fig. 9)

Although the glaciers swept the upper reaches of many canyons nearly clean of loose rock, they deposited hummocky moraines in most of the downstream reaches. Some of these moraines are well shown on the new topographic maps, particularly those of the Taylor Mountain, Whiterocks Lake, Leidy Peak, Hole-In-The-Rock, and Gilbert Peak Northeast quadrangles. The Taylor Mountain quadrangle map shows massive looped moraines crowded with hundreds of little kettle ponds along the South Fork of Ashley Creek. The Whiterocks Lake and Leidy Peak quadrangle maps show several sequences of downstream moraines on the forks of Sheep and Carter Creeks, as well as younger moraines high in the cirques and many cirque lakes. The Hole-In-The-Rock quadrangle map shows massive moraines of Bull Lake and Pinedale age and broad outwash plains along the West and Middle Forks of Beaver Creek. The Gilbert Peak Northeast quadrangle map shows a complex of large pitted moraines along Henrys Fork and an unusual constriction of the valley where the glacier flowed between resistant ramparts of Mississippian limestone.

Many canyons are choked with morainal debris for several miles above their mouths. Conspicuous terracelike lateral moraines extend along the valley sides. Especially on the north flank of the range in such canyons as Smith Fork, Henrys Fork, and the West Fork of Beaver Creek, the drainage through the morainal belts is interrupted by ponds, marshes, and meadows—an ideal habitat for America's (and no doubt the World's) southernmost moose herd.

In the eastern part of the High Uintas, some valleys, such as Sheep Creek, Carter Creek, and Ashley Creek, were glaciated only in their upper reaches. Downstream from the heads of these valleys, broad U-shaped canyons carved by glaciers give way to extremely narrow precipitous gorges cut entirely by running water. Such gorges are greatly influenced in form and character by the particular rock formation into which they are cut. For example, canyons eroded into the Weber Sandstone—one of the prime cliff-forming units in the Uinta Mountains—are wild and picturesque. Sheep Creek, Brush Creek (fig. 10), and Dry Fork of Ashley Creek, as well as the wonderful canyon of the Yampa in the Eastern Uinta Mountains, owe most of their grandeur to the Weber Sandstone. (See also fig. 25.)

BRUSH CREEK GORGE. A precipitous nonglacial gorge carved by running water. Walls of Weber Sandstone are capped by the Park City Formation. Flat-topped Taylor Mountain in distance is a remnant of the Gilbert Peak erosion surface, an ancient high-level plain. (Fig. 10)

A glance at a topographic map shows plainly that the crest-line of the Western Uinta Mountains is closer to the north flank of the range than to the south flank. Streams draining the south flank are therefore longer than those draining the north flank. On the average, they are about twice as long. Their canyons are correspondingly long, and the amphitheaters at their heads are both longer and wider. Because the area available for the catchment of snow was larger on the south flank than on the north, the glaciers there were generally larger, too, except in an early piedmont (Little Dry Glaciation) stage of the north flank, when coalesced sheets of ice spread out beyond the mountain front.

At their maximum the glaciers on the south flank of the Uintas were able to eliminate many of the intervening divides of their headward tributaries and, in so doing, to carve out the great compound cirques, or amphitheaters, which now make the scenery so striking. In the southwestern part of the range, centered near Bald Mountain, the ice succeeded in removing all but a few isolated peaks. These peaks rose as nunataks, or islands of rock, above a massive icecap from which long tongues of ice extended radially down the canyons of the Bear, the Weber, the Provo, the Duchesne, and Rock Creek (fig. 11). Even Bald Mountain and other nearby peaks may have been covered by ice at an early stage of glaciation. Bald Mountain appears to have been reshaped by ice moving across it from the northwest. If the ice ever was thick enough to cover Bald Mountain, the whole western part of the range at one time must have been locked in an immense, unbroken expanse of ice.

DUCHESNE-BEAR RIVER DIVIDE, Western Uinta Mountains. All the dark forested area was covered by a Pleistocene icecap which flowed radially outward from the mountains. Low round summits in middle distance were overridden by the ice, as, perhaps, was Bald Mountain (lower left) at an early glacial stage. Photograph by Hal Rumel. (Fig. 11)

The glaciers of the south flank carved much larger amphitheaters than their northerly counterparts did and, thus, were evidently more effective in attacking their headwalls and broadening their cirques—perhaps because they generally were larger and had larger catchment areas. Why, then, should the northerly landscape be more rugged and alpine? The Uinta Mountains are at their scenic best north of the crestline between the Bear River on the west and Henrys Fork on the east; Red Castle at the head of Smith Fork is the crown jewel of the range.

Undoubtedly, the great amphitheaters date from an early glacial episode, perhaps the Little Dry Glaciation, which may really be a composite of several early glacial advances and a time or times when great thicknesses of ice nearly buried the range. The Little Dry ice may thus have opened the cirques, sapped away the tributary divides, and excavated the amphitheaters. Then, after a long warm interglacial period, when weathering had softened the contours of the land and the rivers had cut new canyons into the valley bottoms, the younger Bull Lake and Pinedale glaciers reoccupied the area. Although large, these glaciers were thinner and less massive than their predecessors. They reexcavated the valleys, eroded new U-shaped inner gorges, and attacked north-facing slopes and hillsides with great vigor, but they had minimal effects on the south-facing slopes of the divides themselves. Almost without exception, the younger glaciers were more effective against north-facing exposures than against south-facing ones.

Rock-rubble deposits at high altitudes

The felsenmeers of the Western Uinta Mountains (p. 14) are residual deposits that accumulated in place through long periods of sustained severe climate. Other deposits of loose rock, also riven by frost but transported some distance largely by gravity, have accumulated near the bottoms of slopes at many places in the higher parts of the range. Three distinct kinds of loose-rock accumulations grade into one another: talus, protalus, and rock glaciers. Although they are relatively minor features of the alpine landscape, in bulk and in topographic relief, they contribute greatly to the overall scenery. All three require a brittle blocky bedrock source.

Talus (fig. 12) is a sheetlike or conelike deposit familiar to all hikers in the high country. It blankets the feet of steep slopes or cliffs in large accumulations of loose angular rubble often only marginally stable and containing poorly balanced blocks of rock weighing many tons—a fine place for the unwary hiker to turn an ankle. Talus, however, is not confined to high altitudes in the Uinta Mountains; it is abundant below cliffs everywhere, as in Lodore and Red Canyons along the Green River in the Eastern Uinta Mountains.

DEBRIS-MANTLED SLOPES at the head of Blacks Fork of the Green River Rocky peaks and ridges are slowly being engulfed in their own talus. Note protalus ramparts, outlined by snowbanks (lower right). Photograph by Max D. Crittenden, Jr. (Fig. 12)

Protalus and rock glaciers, on the other hand, are restricted to alpine settings in the Western Uintas, largely to areas above timberline. They are abundant, for example, in the Kings Peak, Wilson Peak, and Red Castle areas, in the highest part of the range.

Protalus accumulates in rampartlike deposits at the foot of semipermanent snowbanks or ice banks. Rocks are dislodged from above. They roll down the snowbank to its base where, when the snow finally melts, they remain behind as low ridges. And inasmuch as thick accumulations of snow or ice obviously are essential to the process, protalus ramparts formed chiefly in the recent geologic past, the "neoglaciation" of the last few thousand years, when such accumulations of snow and ice were prevalent. Some ice accumulations at that time grew into small true glaciers and, thus, provided a link between protalus ramparts and small glacial moraines. No clear-cut distinction exists, in fact, between moraines and protalus ramparts. They share the same climatic and topographic setting, and they grade transitionally into each other. At the foot of Kings Peak, modern talus cones are forming across inactive neoglacial protalus ramparts.

Rock glaciers are abundant in the Kings Peak, Wilson Peak, and Red Castle areas also. Rock glaciers were first identified by S. R. Capps (1910) in the mountains of Alaska, but they have since been widely recognized in high mountains elsewhere. They have been described in a definitive paper by Wahrhaftig and Cox (1959).

Although rock glaciers consist largely of coarse angular rock fragments, they also commonly have cores of ice mixed with boulders, gravel, sand, and silt. Indeed, fed from above by avalanches and streams of talus, they are thought to move by intergranular flowage between ice and rock. Interstitial ice and a climate in which the ice can form are therefore essential for the nourishment and growth of rock glaciers.

Like true glaciers, active rock glaciers in the Uinta Mountains produce rock flour, a milky suspension of pulverized rock that causes turbidity in nearby lakes and streams. Rock flour is formed by the grinding action of rock particles rubbing against one another during glacier flowage.

Rock glaciers resemble small true glaciers in shape, size, and mode of flowage (fig. 13). They form lobate mounds a few hundred feet high, a few hundred to a few thousand feet across, and generally less than a mile long. They have steep snoutlike fronts, sinuous longitudinal ridges analogous to medial moraines, and transverse ridges and furrows analogous to crevasses. They are, in brief, among the more intriguing details of the alpine scene.

ACTIVE ROCK GLACIER causes turbidity in adjacent glacial lake, east side of Red Castle. Rock glacier is nourished by talus and avalanching. Lake is dammed by a moraine. (Fig. 13)