GILBERT PEAK EROSION SURFACE
The Gilbert Peak erosion surface is the pediment on which the Bishop Conglomerate accumulated, named by Bradley (1936, p. 170) for extensive remnants on the north and west slopes of Gilbert Peak on the north flank of the Western Uinta Mountains. This surface is an excellent datum for the reconstruction of Neogene events, having been modified in many places by warping, faulting, and dissectionclues to neotectonic activity in the area. Bradley described in detail the origin and history of this surface as it exists on the north flank of the range, and he discussed its geomorphic relationship with the Bishop Conglomerate. He noted its presence on the south flank, also, but restricted his observations to the north. Powell (1876, atlas) and King (1878, atlas) in a general way and Schultz (1918, pl. 5) in more detail had earlier depicted the extent of the Bishop Conglomerate (and hence the underlying Gilbert Peak surface) on both flanks, but they did not map the extensive and geomorphically critical exposures in the Blue MountainYampa Plateau area or the deposits along the crest of the range near Pot Creek (fig. 5). The Blue MountainYampa Plateau deposits are critical in that their source was the crestal part of the range across the present canyons of the Green and Yampa Rivers. These rivers, therefore, could not have existed at the time of Bishop deposition, and Powell's concept of antecedence, which called for a Green River older than the mountains, is thus invalidated.
Rich (1910, p. 615, 619) first described the Gilbert Peak surface but did not name it. Rich regarded it as a peneplain, probably formed under humid climatic conditions, and he inferred that the overlying Bishop Conglomerate was deposited in response to renewed uplift of the Uinta Mountains accompanied by a shift from humid to arid conditions. Rich also toyed with the idea of planation by wind erosion under an arid climate. The pediment concept at that time had hardly yet emerged, and Rich attributed the concavity of the Gilbert Peak surface profile to renewed uplift of the mountains. Sears (1924a, p. 302) also identified the surface as a peneplain, but he correctly sensed its basic origin and, in later years, privately referred to it as a pediment. Sears (1924a) suggested that it perhaps merged with a similar plain on the southern flank of the Wind River Range, described by Blackwelder (1915).
At the time of its formation, the Gilbert Peak erosion surface was an extraordinarily broad, monotonously flat plain, better depicted by photographs than by words (figs. 6, 8, 9, and 10). Much of what remains today survives because it is covered and protected by a capping of Bishop Conglomerate. Mountainward parts of the surface on hard rock, however, lack the gravel cap and may have been stripped or may never have been covered. These areas and the higher mountains were the sources of the gravel. The smoothly graded, slightly concave surface sloped gently away from the mountains into the Green River Basin to the north (fig. 7), the Sand Wash Basin to the northeast, the Piceance Creek Basin to the southeast, and the Uinta Basin to the south. In all probability it also extended southeastward some distance across the present Yampa River toward Axial Basin, where it may have merged with a similar surface extending down from The Flat Tops of the White River uplift, although the evidence of such a link has been erased by erosion. According to Bruce Bryant (written commun., 1983) the oldest dated Neogene deposits on The Flat Tops are about 24 m.y. old and thus equate with the older part of the Browns Park Formation rather than the Bishop. The subjacent erosion surface, however, might still be about the same age as the Gilbert Peak surface.
Several even-topped ridgelines far out in the Tertiary basins accord rather closely in altitude with the Gilbert Peak erosion surface, and though they lack a Bishop Conglomerate cap rock, I regard them as likely correlatives of the Gilbert Peak surface. They owe their present prominence to resistant bedrock. Among them are the crestline of White Mountain west of Rock Springs; Kinney Rim and Laney Rim bounding the Washakie Basin on the west and north, respectively; the crest of the Citadel Plateau south of Maybell; Pinyon Ridge south of Elk Springs; Calamity Ridge east of Rangely; and the Cathedral Bluffs rim west of the Piceance Creek Basin. Aspen Mountain, on the Rock Springs uplift, is a monadnock that stood above the old pediment surface. So is Wilkins Peak, southeast of Green River. All these features are shown on figure 1.
The Gilbert Peak surface truncates hard and soft rocks alike, with little regard for lithology or structure, although resistant rocks stand well above the surface locally as hogbacks or monadnocks. The Madison Limestone (Mississippian) in particular has resisted pedimentation more stubbornly than has the Uinta Mountain Group (Middle Proterozoic); its limestone outcrops rise above the truncated Uinta Mountain Group in many places. Limestone Ridge, on the northeast side of Cold Spring Mountain, is just one example. Locally, the Round Valley Limestone (Pennsylvanian) stands above the surface also, especially on the south slope of the range near Vernal. On the higher flanks of the pediment on the south slope of the range, scattered monadnocks or inselbergs (island mounts) surrounded by Bishop Conglomerate merge gradually with the old mature topography of the mountains. In such places the headward parts of the Gilbert Peak surface penetrate deep into the range as a complex of graded or partly graded, flat-bottomed, alluviated, dendritic valleys; basinward, these valleys coalesced with the flanking pediment.
Many such valleys still exist on the south slope of the Uinta Mountains, little changed from their middle-Tertiary form (fig. 8), especially in the area between Ashley Creek on the west and the Canyon of Lodore on the east. (For example, see Kinney, 1955; Rowley and others, 1979; Hansen and others, 1980, 1981, 1983.) Bradley (1936, p. 171) noted places in the western half of the range where ramplike remnants of an old surface (Gilbert Peak?) extend completely across the crest, joining the north flank with the south in a smooth, unbroken profile. The process through which two opposed concave ramps have merged at the crest into a continuously graded convex divide has not been addressed and remains unclear. I suspect that the remmants cited are parts of the Wild Mountain upland surface (p. 14) rather than the Gilbert Peak.
As visualized by Bradley (1936, p. 174), the Gilbert Peak erosion surface was formed in a semiarid environment by laterally corrading streams. Borrowing from Rich (1910, p. 614), Bradley cited as evidence the fresh, unweathered character of the truncated rock and the lack of a buried soil beneath the overlying Bishop Conglomerate, and he noted the thinning of the Bishop toward the mountains and toward the basins from a thicker section in between. In an arid climate, mostly intermittent streams flowing away from relatively well-watered sources in the mountains would have lost volume and competence to infiltration and evaporation in the basins. They would, therefore, have dropped most of their loads en route at intermediate places in the profile. Here, as their channels became blocked by their own debris, they would have been forced to shift laterally, constantly diverting into new courses across the growing plain.
A long period of crustal and climatic stabilityfollowing earlier crustal and climatic unrestmust have been required to develop so broad and uniform a plain. As will be discussed further, the time at the onset probably was early Oligocene. Then, when the climate became drier, the Bishop Conglomerate was deposited across the medial part of the plain, thickest in the deepest concavity of the profile (Bradley, 1936, p. 179). Most of the runoff flowed in ephemeral but occasionally energetic, highly competent streams, and most of it probably failed to reach the centers of the basins. One effect would have been to slightly flatten the depositional profile. At the time of its deposition the Bishop Conglomerate must have formed a nearly continuous bajada around the north, east, and south flanks of the range.
Botanic evidence suggests widespread cool, dry conditions during the latter part of the Paleogene Sub-period, in contrast with warmer, more humid earlier climates (MacGinitie, 1969; Leopold and MacGinitie, 1972). This climatic change has been attributed to widespread explosive volcanism in western and southern North America (Axelrod, 1981), but the tectonic, epeirogenic rise and topographic diversification of the Cordilleran region at that time must have affected the climate also. (See p. 24.)
The bare-rock surface of the Gilbert Peak pediment is preserved in many places, though slightly to moderately eroded. In most places, however, the surface is mantled, and protected from erosion, by the Bishop Conglomerate, which rather faithfully reflects its form and extent. (See figs. 9 and 36.)
On the north flank of the range, north of Red Canyon of the Green River, well-preserved but warped remnants of the gravel-free surface truncate the Uinta Mountains Group from Flaming Gorge to Browns Park and include, among others, Bear Mountain (Bare Top), Dutch John Bench, and Goslin Mountain (Hansen, 1965, p. 115, 163, 171). All these places are identified on figure 20. North of Browns Park extensive remnants include Head of Cottonwood, O-Wi-Yu-Kuts Mountain and Cold Spring Mountain, the latter made classic by Bradley's study (1936). The tops of these mesas are slightly dissected by differential erosion, mostly along shaly zones, but viewed from a distance, most of them appear as almost perfectly flat plains. Cold Spring Mountain is especially noteworthy, but Dutch John Bench alone is almost pristine, virtually unaltered by erosion since middle Tertiary time. Being bare of gravel, except locally, all these remnants must have been near the mountainward limit of the pediment, close to or above the depositional margin of the Bishop Conglomerate, although they are now lower than conglomerate-capped remnants to the north and northeast at Little, Pine, and Middle Mountains and Diamond Peak, having been lowered by subsequent faulting. Local gravel patches were once more extensive than now, having been partly stripped by erosion, but much of the bare-rock surface may never have supported gravel. Bouldery gravel deposits high on the north flank of Cold Spring Mountain above Talamantes Creek (figs. 5 and 40) are remnants of Bishop Conglomerate in my opinion, although some geologists have mapped them as Browns Park. These remnants contain boulders as large as 1.2 m across.
Farther southeast, on Douglas Mountain, bare-rock remnants of the Gilbert Peak surface also truncate the Uinta Mountain Group. The dissection of these remnants has gone further than that above Red Canyon and Browns Park, but seen in profile they, too, present remarkably even skylines. Remnants of the old surface skirt the northeast slope of Zenobia Peak, along a rough dirt road called Douglas Mountain Boulevard, then are lost in the dissection near Lodore Canyon.1 Remnants form the canyon rim farther south in the Zenobia CreekPot Creek area, however, and in places they pass under patches of Bishop Conglomerate and younger colluvium (fig. 10; Hansen and others, 1983). At the Gates of Lodore, the flat tops of both canyon rims are isolated remnants of the Gilbert Peak erosion surface also.
On the south side of Douglas Mountain the Gilbert Peak surface has largely been destroyed by dissection. No flat area of any great extent remains, but the form of the restored surface is vaguely preserved in the accordance of ridgelines truncating the Morgan, Weber, and Park City Formations along the north rim of Yampa Canyon in Dinosaur National Monument. Seen in profile, these ridgelines describe a well-formed surface. Near the confluence of the Yampa River with the Green, the restored surface is at about 2,190 m elevation and slopes gently northward at a rate of about 9.5 m/km. It accords nicely, therefore, with the base of the Bishop Conglomerate at Harpers Corner to the southwest, slightly higher at about 2,285 m, and with the benchland along Pot Creek to the northwest, a bit lower at about 2,130 m. The restored surface also slopes gradually east at a rate of about 5-6 m/km and truncates bedding at an angle of about 6-10 degrees.
Last Updated: 09-Nov-2009