Geological Survey Professional Paper 1356 Neogene Tectonics and Geomorphology of the Eastern Uinta Mountains in Utah, Colorado, and Wyoming

Flattish to rolling uplands at various places in the Eastern Uinta Mountains are regarded as remnants of a truncation surface higher and older than the Gilbert Peak erosion surface (fig. 11). This surface, here called the Wild Mountain upland surface, may have been shaped by altiplanation, as its remnants generally lack the smooth gradients of stream-cut profiles. Abundant coarse gravels in the Wasatch and Green River Formations near the mountains indicate intensive early degradation at the flanks of the range and alluviation of the basins, with the added implication that a degradation surface was forming in the mountains at the same time. Some remnants of the Wild Mountain upland surface stand 180 m or more above the Bishop Conglomerate. In general they consist of resistant bare rock, such as limestone or quartzite, with no surficial cover, and they truncate the subjacent formations without regard for rock structure. Likely candidates include the top of Wild Mountain (figs. 5, 11) in the northwest part of Dinosaur National Monument, for which the surface is named, and Hoy Mountain to the north of Wild Mountain, both at altitudes of 2,620-2,680 m. The top of Offield Mountain east of Hoy Mountain may be a remnant also. Wild Mountain is topped by the Madison Limestone and limestone beds in the Morgan Formation. Hoy and Offield Mountains are composed entirely of the Uinta Mountain Group. West or north from Boy Mountain, in the highlands south of Browns Park, no remnants of an upland surface remain, but summit levels there are in good general accordance despite rather complete dissection. These summits are generally highest to the south and west, diminishing in altitude to the north and east. Bradley (1936, p. 171) speculated about the possible existence of a pre-Gilbert Peak surface in more westerly parts of the range, where accordant ridgelines flank many of the higher peaks and locally form the crestline itself. The rolling upland at the head of Ashley and Brush Creeks is regarded here as a remnant of this surface, as it stands 300 m or more above nearby deposits of the Bishop Conglomerate.

FIGURE 11.—Wild Mountain, on skyline, as seen from "Harpers Plateau," looking north. "Harpers Plateau" is the local name of the Bishop-capped mesa southwest of Harpers Corner. Whirlpool Canyon is out of view in the middle distance in front of Wild Mountain. Rolling upland surface of Wild Mountain here truncates the Morgan Formation (Pennsylvanian).

East of Wild Mountain, across Lodore Canyon, the Madison Limestone forms a continuous outcrop about 40 km long, reaching to the east end of the range at Lone Mountain; most of the summits on this outcrop are nearly accordant. Here, there is no broad platform either, just a general accordance, declining gently from west to east. Some points, such as Zenobia Peak and Allred Peak, rise well above the general level. Limestone Ridge,² 5 miles south-southwest of Greystone, has an even crestline mantled by a felsenmeer of coarse subangular blocks (fig. 12), a product perhaps of long exposure to a harsh, cold climate. Limestone Ridge is about 2,380 m above sea level, though it must have stood much higher before the tectonic subsidence of the Eastern Uinta Mountains. Both it and Wild Mountain are visible in profile from the Harpers Corner road several miles to the south in Dinosaur National Monument.

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²The name "Limestone Ridge" is applied locally to at least three separate ridges in the Eastern Uinta Mountains.

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FIGURE 12.—Felsenmeer on the Madison Limestone at Limestone Ridge, Douglas Mountain, about 8 km south-southwest of Greystone, Colo. The larger rock fragments are about 30 cm across. The crest of Limestone Ridge is a slightly dissected remnant of the Wild Mountain upland surface.

Along the south flank of the Eastern Uinta Mountains, south of Yampa Canyon, is the extensive Blue Mountain highland. Blue Mountain basically consists of two west-to-east summit ridges separated medially by the drainages of Wolf Creek and tributaries of Hells Canyon. These drainages are partly controlled by old downwarps and faultlines and are partly floored with Bishop Conglomerate. Several broad, nearly accordant summits along Blue Mountain may be remnants of the Wild Mountain upland surface. Noteworthy among these, on the northern ridge, are (1) Round Top Mountain, altitude 2,620 m, topped by limestones of the Morgan Formation; (2) the high ridge between Marthas Peak and Serviceberry Gap (fig. 33), altitude about 2,600 m, also topped by the Morgan Formation; (3) Tanks Peak, altitude 2,657 m, topped by Morgan; and (4) the long ridge north of Bear Valley (fig. 33), topped by the Madison Limestone. This latter ridge extends east from Tanks Peak to Thanksgiving Gorge, sloping gradually from an altitude of about 2,380 m on the west to about 2,270 m on the east, a slope of about 13 m/km. All these summits stand hundreds of meters above nearby deposits of Bishop Conglomerate. Tanks Peak stands even higher, well above the general level, and might be regarded, therefore, as an ancient monadnock on the old Wild Mountain surface.

South of Wolf Creek and Hells Canyon the ridgelines are supported by much younger rock but attain comparable heights. Buckwater Ridge, elevation 2,560 m, and Lazy Y Point, north of the town of Dinosaur and of Skull Creek (fig. 13), are underlain chiefly by the Dakota Sandstone and the Glen Canyon Sandstone respectively. Together they form a rather even-topped ridgeline about 24 km long, sloping gently eastward and standing 790 m or so above the valley floor to the south. At the east end is Skull Creek Rim, altitude about 2,330 m.

FIGURE 13.—General distribution of Bishop Conglomerate and Browns Park Formation in southeastern Uinta Mountains and some of the principal named geographic features. Formation boundaries generalized from Rowley and others (1979). (click on image for a PDF version)

It is significant that the profiles of all these ridgelines slope eastward. As will be further discussed later, the south flank of the Eastern Uinta Mountains was tilted northward and eastward after the Bishop Conglomerate was deposited, and the eastward tilt is reflected in the slopes of the profiles. The northerly tilt component on the Wild Mountain surface is too fragmentary to reconstruct, but subsequent northerly tilt is clearly shown by the attitude of the nearby Bishop Conglomerate.

Cross Mountain, a sharp, fault-bounded local uplift, is separated from the Eastern Uinta Mountains by the Lily Park syncline (Dyni, 1968). Cross Mountain has a truncated crestline that may be a remnant of the Wild Mountain surface. If so, it too has been tilted and in a complex fashion. Unlike the Uinta crestline, however, the crestline of Cross Mountain slopes southward, although the Bishop Conglomerate on the west flank of the mountain shows northward tilting of a later time and at a lesser magnitude. Along the limbs of the Lily Park syncline the Bishop Conglomerate has a northerly tilt of about 9 m/km.