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HW-6
Spanish Fork to Green River, Utah



0.0  Junction of U.S. Highway 50-6 with Interstate Highway 15 Access Road to U.S. Highway 89 Northeast of Spanish Fork. The road is across Lake Bonneville prodelta and lake-bottom clay. A short distance to the east and southeast the gentle slope of the foreset beds of the deltas of Spanish Fork River and Hobble Creek limit the lake clays. A series of springs issue at the contact where porous sand and gravel overlie the impervious clay and mud.

0.8  Access road crosses junction wilt h U.S. Highway 91 northeast of Spanish Fork. Southeast of the intersection the highway climbs up the front of the deltaic foreset beds.

1.2  Cross overpass cast of north end of Spanish Fork. Here the foreset beds of the Provo-level delta into Lake Bonneville are expressed as the slope zone both to the southwest and northeast. The highway climbs onto topset beds of the delta southeastward.

3.4  Spanish Fork Peak on Maple Mountain is the high promontory to the east. Faceted spurs with triangular terminations show on the mountain (fig. 1. 1) and suggest recurrent movement along the Wasatch Fault at the base of the mountain. Bonneville and Alpine Terraces of Lake Bonneville are inscribed into the base of the Maple Mountain. The level flatland at the highway was formed at the Provo level of the lake.

The sag pond is particularly well preserved to the west, (fig. 1.2) but is obscured by the water tank and gravel quarries to the east.

Figure 1.2. Scarp of a post Provo-level minor fault, a branch of the Wasatch Fault, as seen west from Mile 4.6 in the first part of the section. The brushy escarpment is the fault scarp and the relatively barren pasture land in the immediate foreground is in the sag pond. This is the same stratigraphic surface as that at the top of the brushy escarpment and indicates the amount of displacement on the small fault. Paleozoic rocks in the background in the Wasatch Mountains are on the upthrown part of the main fault block along the Wasatch Fault, which here has a displacement in excess of 8,000 feet.


4.9  Cross main line of Denver and Rio Grand Railroad.

5.0  Junction of Access Road U.S. Highway 50-6 with U.S. Highway 89. Continue toward the southeast along the next part of Geologic Guide Segment 1.

0.0  Intersection of U.S. Highway 50 and 6 with U.S. Highway 89 near the mouth of Spanish Fork Canyon. The highway junction is on the Provo level of Lake Bonneville an ancient Ice-Age lake that covered much of the desert area of western Utah. North of the junction a small fault has offset the upper surface of the Provo-level deltaic sediments. The flat area north of the fault, beyond one-half mile north of the junction, was deposited as the upper part of a delta of Spanish Fork River. When the delta was being built these flats were at the same level as the flat at the highway junction, but they have been dropped between 25 and 35 feet by subsequent faulting.
< br/> Higher levels of Lake Bonneville are marked by prominent lake terraces to the cast and southwest of the highway junction. The most prominent terrace is at the Alpine level of the lake, at an elevation of approximately 5,200 feet. A less prominent higher terrace at the Bonneville level is inscribed into the flank of Maple Mountain at approximately 5,300 feet and marks the level of spillover of the lake into the Snake River and Columbia River. Following spillover, the rushing outpouring lake water quickly eroded the dam across the rim in southern Idaho and the lake level lowered to the Provo level at approximately 4,800 feet. The lake stayed at this level for some time until climatic changes reduced precipitation in the basin and the lake finally desiccated to its present remnants: Great Salt Lake, Utah Lake, and Sevier Lake. See the discussion of Lake Bonneville in the introductory part of the guide.

0.6  Overpass over spur railroad into Trojan Powder Company dynamite plant. Just beyond the overpass the front of the mountains marks the trace of the Wasatch Fault, one of the largest displacement and longest recently active faults of western United States. The fault has estimated displacement of 10 to 15 thousand feet near here, with the valley filled with 6 to 8 thousand feet of unconsolidated sediments. Only the youngest of these sediments are related to Lake Bonneville.

Bedrock of the mountains visible approximately 300 yards south of the overpass is of the Oquirrh Formation, one of the thickest formations in Utah (fig. 1.3). These rocks record the beginning of deepwater deposition in the subsiding Oquirrh Basin of Utah during Pennsylvanian-Permian time and the end of shallow-water fossiliferous limestone deposition.

U.S.Highway 89 and 50-6 have been constructed along the floor of Spanish Fork Canyon for several miles (fig. 1.4). The canyon is, in part, rejuvenated with the removal of Lake Bonneville-related sediments which partially filled the valley. Terraces along the margins of the canyon mark former levels of fill.

2.9  Strongly-jointed Oquirrh Formation limestone and sandstone is exposed in road cuts on the north. Silty and organic-rich layers which separate the thicker units have been burrowed and contain trails suggestive of a deepwater environment,

4.9  Quarry in tan Diamond Creek Sandstone. A warm sulphurous spring has been developed along the valley floor east of the quarry. The spring rises here at the crest of an anticline expressed in the sandstone. East of the spring area badly broken outcrops of the same sandstone are exposed in road cuts.

5.6  U.S. Highways 50-6 and 89 cross the Permian-Triassic boundary. Reddish easily eroded Triassic shale and siltstone to the north near the telephone line rest on ragged weathering cherry Permian rocks to the west.

6.1  Junction of Diamond Fork road with U.S. Highway 89. The road cut on the north of the junction is in fossiliferous Triassic Thaynes Formation and the reddish slopes east of the junction are in the Ankareh Formation. Both of these formations are dipping to the east and are overlain along the skyline to the south by nearly flatlying Tertiary rocks (fig. 1.5). The erosional surface between these two marks an angular unconformity and helps date the period of folding of the older beds.

The younger Tertiary rocks are soft and have flowed down into Spanish Fork Canyon on the south margin. This produces the hummocky rolling landscape visible above the railroad Cuts in the sweeping bend of the canyon. A long tongue of landslide debris extends down through a gap in Navajo Sandstone on the west of the canyon 1.5 miles ahead, north of Thistle. Diamond Creek is now nearly choked with debris swept down the canyon. The stream course has adjusted to a major increase in water volume produced by diversion from Strawberry Reservoir. As volume increases the formerly small meanders will increase to form large meanders that will be in equilibrium with the greatest stream flow. Such adjustment will result in major modification of canyon bottom meadows.

Figure 1.5. Generalized cast-west structure section between Thistle (1) and Red Narrows (3). The Morrison Formation (2) is exposed at Mile 10.1. The horizontal line through the middle of the diagram is the approximate road level and shows the preunconformity rocks, below, dipping steeply toward the east with the Tertiary rocks, above, relatively flat lying.


8.6  Thistle Junction: Separation of U.S. Highways 50 and 6, to the East, from U.S. Highway 89, to the South. High cliffs of Jurassic Navajo Sandstone form promontories north of the junction (loc. 1, fig. 1.5). Less prominent, in part fossiliferous, Twin Creek Limestone overlies Navajo Sandstone on the east and forms the small valley, ledges, and road cut cliffs to the east. Navajo Sandstone here is part of the same unit which forms the cliffs in Zion National Park, Arches National Park, Capitol Reef National Park, and in other scenic areas of southern and southeastern Utah. Continue East on U.S. Highways 50 and 6. For a continuation of the geologic guide south along U.S. Highway 89, see HW-89 Road Guide..

10.1  Jurassic Morrison Formation exposed in road cuts on the north (loc. 2, fig. 1.5). Fragments of bones and plant have been found here. Morrison Formation has produced most of the Jurassic dinosaurs known from western North America. The exhibit building at Dinosaur National Monument in northeastern Utah is centered on fossils in. the Morrison Formation. High terraces to the south and north were probably related to a high level of Spanish Fork Creek during Lake Bonneville time.

10.8  Prominent very light gray Cretaceous sandstone ledges north of the highway are remnants of an upturned barrier island complex. Flat-lying Tertiary North Horn (red) and Flagstaff Formations (white) occur above tilted Cretaceous sandstone, above an erosional surface and angular unconformity (fig. 1.5). The North Horn Formation is composed of debris weathered from uplifted mountains to west which were created during the Sevier orogeny in Cretaceous time (see introductory section). This wedge of redbeds thickens to the east and is part of the thick conglomerate exposed in Red Narrows (fig. 1. 6).

14.1  West entrance to Red Narrows (loc. 3, fig. 1.5). Conglomerate ledges here are composed of cobbles and pebbles of Precambrian and Paleozoic limestone and quartzite eroded from mountains of the Sevier orogenic belt in western and central Utah and were deposited here as part of alluvial fans or piedmont deposits. Lenses and filled stream channels are typical of the coarse textured units.

Figure 1.6. Conglomeratic exposures of the tongues of conglomerate of the North Horn Formation, like that shown in fig. 1.5., as seen up a small canyon toward the north at the west edge of Red Narrows, at approximately Mile 13.5.


15.8  Spring with Fountain at Roadside Rest Area, east end of Red Narrows. Red conglomerate fills a faulted square-profiled channel across the railroad tracks and stream to the southeast of the fountain. Tufa cones on the north (fig. 1.7) have developed at the main springs and are composed of calcium carbonate. Layers of tufa-coat leaves and twigs of red-barked birch and other plants nearby. These springs are located along minor fractures associated with a major fault system that is present a short distance to the east. These faults allow water to well up from below or have allowed access to water percolating into the openings from adjacent porous beds.

Figure 1.7. Small tufa cones associated with carbonate-charged springs near the eastern end of Red Narrows at Mile 15.8. Red bark birch and other plants grow around the wet margin of the spring deposits.


16.5  Outcrops of Flagstaff Limestone in low ledges at the northern margin of the bend, across the old road. Pisolitic and oncolitic algal balls form onion like masses in some of the limestone ledges. These algal balls probably accumulated along the shore or beach of Lake Flagstaff and enlarged by back-and-forth water motion and precipitation of calcium carbonate.

Carbonaceous shale and siltstone east of the Flagstaff ledges, near the low double road cuts on U.S. Highway 50-6, are marsh deposits associated with late stages of Lake Flagstaff or early stages of Lake Green River. Crocodiles, turtles, and many clams and snails occur with much broken or macerated plant debris in these beds.

17.2  Baer's Bluff. Lower beds of the Green River Formation are well exposed here. Greenish very low-rank oil shale, light gray limestone, tan papery laminated dolomite, and tan sandstone are part of deposits of Eocene Lake Green River. Fossil fish, snails, clams, ostracods, turtle fragments all occur in some of the limestone and silty beds, but not in the greenish rocks. These rocks show a transition from deltaic marshy environments in the lower part to open lake environments in the upper part. Algal coated logs form mounds in the middle of the cut. The canyon through which the highway is constructed for the next several miles is in the lower Green River Formation. Small faults offset some of the white limestone beds in dramatic fashion in railroad and highway cuts to the east (fig. 1.8).

23.8  Railroad Overpass. Excellent exposures of Green River Formation and its cycles show near the overpass. Two miles east of the overpass (Mile 25.7) small faults are well exposed in railroad cuts north of the highway and east of a service station- motel complex.

Figure 1.8. Small-displacement fault in the Green River Formation on the north side of road cuts at Mile 19.4. The prominent white limestone on the right shows fault drag into the fault. The same bed is exposed to the left of the fault as the two light outcrops in the lower left of the photograph. Darker beds are very low grade oil shale and green shale and siltstone that make up much of the lower part of the Green River Formation.


26.7  Roadside Rest Area and Junction to Skyline Drive , a scenic Forest Service summer dirt road along the crest of the Wasatch Plateau to the south. Excellent bird tracks have been collected in platy beds along the west flank of Starvation Creek, south of the rest area.

Erosion of sediments of the western edge of the Colton delta forms the valley through which the highway is constructed beyond Starvation Creek. The Colton Formation is a series of interbedded reddish shale and siltstone in this part of the delta. This part of the section, plus part of the underlying Flagstaff Formation, is susceptible to slumping when steeply tilted. Hummocky landslide and creep topography is particularly well shown on the south side of the canyon for the next four miles.

29.1  Railroad Overpass. Colton Formation is the reddish rocks near the valley bottom, with light greenish gray Green River Formation exposed along the canyon crest to the north. Hummocky topography to the south for a mile east and west of the overPass is Produced by Colton slump masses. Some trees to the south have twisted trunks and have adjusted to downslope movement of their substrate.

31.1  Spanish Fork Canyon is narrowed here by landslide and slump mass from south. Step-type topography is related to dams formed by the toes of slumps, with flats formed by partially filled valley floors above the steep dams. The small stream has been displaced northward as the landslide mass moved out into the canyon.

33.8  Wasatch County-Utah County Line in Community of Soldier Summit. Strike valley is formed by softer Colton beds between more resistant Green River Formation on the north and Flagstaff Limestone on the south. Small dumps east of town are related to abandoned ozokerite mines and processing plants. Ozokerite is a purplish to black waxy solid hydrocarbon which was mined here and at other places between here and Colton, Utah to the southeast. The Soldier Summit Mine worked pockety deposits to a depth of 600 feet. Crushed rock was put into vats of water and ozokerite was skimmed off, then the skimmed fragments were melted to purify the wax,

36.7  Utah County-Wasatch County Line. Meandering White River occupies a subsequent valley in soft Colton deposits, some of which are exposed in gravel-capped terraces to the east. Indian Head (elevation 9,837 feet) is the high peak on the skyline to the east and is held up by Green River rocks,

39.7  Junction Utah State Highway 96 with U.S. Highway 50-6. Utah Highway 96 leads westward to Scofield Reservoir and Clear Creek. Scofield Reservoir is in a major graben, an elongate faulted trough. Clear Creek, Scofield, and Winterquarters were established as coal towns. Scofield was shipping coal as early as 1879. Winterquarters was the site of the worst mine disaster in Utah history when 206 men lost their lives in a coal-dust explosion there on May 1, 1900.

40.6  Minor junction to the south leads to old railroad siding of Colton. Approximately I mile beyond the side road the highway Crosses a major fault (fig. 1.9) and then passes between double road cuts of the lower part of the Colton Formation which expose excellent channel-fill sandstone lenses cut into flat reddish siltstone and mudstone (fig. 1.10). Light-colored thin beds near the base of the cuts are fossiliferous lacustrine deposits over which the Colton delta was deposited.

Figure 1.9. Southeastward across the town site of Colton, a railroad switchyard in earlier days on the railroad. The prominent escarpment behind the buildings to the left is a fault-line feature associated with one of the major grabens in the crest of the Wasatch Plateau. Flagstaff Limestone caps the ridge to the left of the fault scarp and also forms the lower exposures at the extreme right of the photograph on the opposite side of the fault' on the downdropped block


Figure 1.10. Cross section of the fillings of a meandering stream in the lower part of the Colton Formation in double road cuts at Mile 32.5. This is one of the three channel-filling sandstones exposed in the roadcut and has channeled into lacustrine and fluvial deposits.


41.5  Cross Price River and D&RGW Railroad Main Line Track. Flagstaff Limestone is exposed in gorge south of the highway and Colton beds are exposed to the north. The highway continues to the east through these same rocks.

44.6  Roadside Rest Area and Road Junction. The Utah State Highway heads east along the subsequent valley on the Colton Formation. U.S. Highway 50-6 heads southeastward down Price River Canyon. Flagstaff Limestone is exposed near the rest area, and forms the well-bedded cap of the stripped surface east of the canyon and rest area.

45.8  Utah County-Carbon County Line , at the lower end of deep double road cuts in North Horn Formation. North Horn rocks here are a mixture of stream and lake deposits and similar rocks are exposed for the next 3 miles down the canyon. Lower parts of the formation contain some thin beds of shiny black coal, as for example in cuts near Ford Creek at Mile 47.3.

48.1  Road junction to Price Canyon Recreational Area. Base of the North Horn Formation and top of the Price River Formation is placed at the red shale exposed in the road cut approximately one mile downstream from here. Below the red shale the highway continues in relatively massive sandstone and shale of the Price River Formation. On some cuts, particularly at Mile 49.6, joints and interbedded sandstone and shale combine to produce an area of perennial slumping and road damage.

51.6  Scenic Turnout to View Castlegate (fig. 1.11). Castlegate Sandstone forms the 'gate" ahead and also the vertical cliffs in the vicinity of the viewpoint. The sandstone is of deposits of sediment-charged braided streams draining across the piedmont, in front of the Sevier orogenic mountains, into the coastal area and shallow sea that was at that time at approximately the Utah Colorado state line. The highway drops down section into the underlying Blackhawk Formation (fig. 1.12) after passing through the Castlegate.

Figure 1.11. View toward the south of The Castlegate. one of the Castlegate Sandstone is exposed in the shaly carbonaceous breaks that form the dark band in the road cut on the right. The very lenticular bedding in the Castlegate Sandstone is typical of the formation over much of its extent in the Northern Colorado Plateau.



53.4  Scenic Turnout and Geologic Stop at Overlook Near Tipple of North American Coal Corporation at Castlegate. Blackhawk Formation (fig. 1.12) is the major coal producing rock unit in central and eastern Utah. It erodes to low ledges and slopes alternating with more massive light colored sandstone cliffs. The thicker sandstone units are barrier island accumulations and coal accumulated in coastal marshes and swamps behind these islands (fig. 1. 13). The prominent white-capped sandstone behind the tipple, on the east side of the canyon is an example of one such barrier island deposit. The top of this sandstone is exposed in the road cut on the west of the highway (figs. 1. 14, 1.15) and is overlain by a coal scam approximately 3 feet thick that accumulated in the marsh behind the island. The coal was buried by the marsh-filling stream deposits as the barrier coastline was crowded eastward by sediments being dumped in from the mountains to the west. A lower and older barrier island and coal sequence is exposed in the deep double road cuts to the south (fig. 1. 15).

-Figure A. Lagoon developed between two barrier beaches that are formed as a result of sediment being dumped into the Mantes sea and longshore drift of sediments along the coast from the northeast to the southwest.-


  Figure 1.13. Relationships of coal accumulations and barrier beach developments along the Cretaceous shorelines as an example of the sedimentary model for Cretaceous coal accumulation in the Northern Colorado Plateau. (From Young, 1966) Pinkish rocks to the northeast and near the top of the road cut on the west are the result of oxidation of iron-bearing minerals and development of clinker produced by burning of coals in place. Most burning appears to have been prehistoric and perhaps started by forest fires or lightning where the coal was exposed at the surface.

Figure 1.14. High roadside exposures of the lower part of the Blackhawk Formation at the scenic turnout near the now-abandoned town of Castlegate at Mile 53.4. The lower sandstone beyond the cars is the Lop of the Aberdeen Sandstone and is overlain by the Aberdeen Coal, here approximately three feet thick. Additional lenticular lagoonal sandstone, coal, and argillaceous Carbonaceous accumulations that are typical of the late swamp filling occur higher in the roadcuts. The bleached upper part of the Aberdeen Sandstone is typical of some of the major barrier island sandstones that are overlain by coal.


  The Castlegate mine connects with the Kenilworth mine and as such is one of the largest coal mines in the state. Numerous dinosaur tracks are found in the lagoonal deposits of the mines, particularly on top of the coal beds.

The coal-burning electricity generating plant of Utah Power and Light Company is in the canyon to the south (fig. 1.16). It is one of the few mine-mouth fed generating plants in the West.

Figure 1.16. View southeastward over the lower part of Castlegate showing the "white-capped" Aberdeen Sandstone Member beyond the tipple and the power plant in the eastern canyon wall at Mile 53.4. The smoke plume is from the electricity generating plant of Utah Power and Light Company in the canyon to the south. The town of Castlegate formerly extended for approximately one mile up canyon from here but has been recently removed to allow coal development in the canyon bottom.


53.9  Junction of Utah State Highway 33 with U.S. Highway 50-6 west of steam generating plant. Upper beds of the Star Point Formation arc visible in the canyon bottom a short distance downstream opposite the mine waste dump, near the Port of Entry Station. 54.0 A short distance beyond the Port of

Entry the Panther Sandstone Tongue (fig, 1.120, one of the lower barrier-island-based sandstones, forms distinctive outcrops on both the east and west canyon wall. The sandstone on the east portrays well the shingled nature of the sandstone and siltstone barrier island front (fig. 1.17). Wellexposed, numerous, and variable trace fossils are preserved in the Panther Sandstone Tongue here. Outcrops on the west side of the highway are most accessible for study. Panther Sandstone overlies the Mancos Shale, one of the thick gray Cretaceious shales of the Mountain West. U.S. Highway 50 and 6 is constructed in large part on Mancos Shale or alluvial valley fill over Mancos Shale from here to beyond Grand Junction in western Colorado

56.5  Junction and Railroad Crossing in Northern Helper. Beheaded pediments which are cut across gently dipping Mancos Shale show beyond town to the east. They are armoured with gravel derived from the overlying sandstone units. Cretaceous units show in The Helper Face to the north (fig. 1.18).

59.7  Cross Price River. Garley Canyon Sandstone, a lower tongue of sandstone in the Mancos Shale, caps the bluff on both sides of the highway. Exposures of the sandstone here are of the outer seaward part of a barrier island sandstone like the massive beds seen near Castlegate and like the shingled Panther Sandstone in the mouth of Price Canyon. The Garley Canyon Sandstone thins out into silty shale in a short distance to the cast. Gravel-capped pediments are developed along Price River above poorly exposed gray Mancos Shale. The Book Cliffs rise above Price River Valley to the west and northeast, and mark the boundary between the northern and western high plateaus provinces and the canyonlands section of the Colorado Plateau.

Figure 1.17. Exposures of the Panther Sandstone at the type locality on the eastern side of Price Canyon as seen from approximately Mile 46.0. The Panther Sandstone is the prominent ledge and recess Cliffforming unit in the foreground. Within the exposure the gently inclined beds are typical of the shingled appearance of the unit which was deposited as a barrier island migrated from north to south along the coastline in the Cretaceous. Younger Blackhawk Formation and Star Point Formation form the white-capped sandstone cliffs and slope zone in the head of Panther Canyon.


Figure 1.18. View northward from west of Helper, at approximately Mile 48.0, toward The Helper Face. Lower slopes are on the Mancos Shale and the first prominent ledge is the silty remnant of the Panther Sandstone. Massive cliff-forming units above, near the top of the sheer part of the face, are in Blackhawk Formation and show rather characteristic coal and barrier island facies of the formation.


64.5  Junction of Utah State Highway 10 with U.S. Highway 50-6 at the west edge of Price business district. Price City Museum with natural history exhibits is in the Carbon County Courthouse on the north side of the highway 0.2 miles east of the intersection.

East of Price the highway continues on terraces of the Price River that are cut into Mancos Shale. Toward the east an extension of the Book Cliffs forms the southern and southwestern limit of the Tavaputs Plateau which is capped by Green River Formation and younger rocks. Red Plateau and Cedar Mountain, at the northern end of the San Rafael Swell, are on the skyline to the south. Toward the southwest the southern extension of the Book Cliffs defines the boundary between the high Wasatch Plateau and the lower Canyonlands section of the Colorado Plateau.

71.5  Wellington Community Park. Naturally occuring CO, recovered from the Navajo Sandstone in Farnham Dome, 5 miles east of town, is converted here into "dry ice." The plant south of the highway at the east end of town supplies much of the "dry ice" demand in the state. A coal washing plant built along the Denver & Rio Grande right-of-way southeast of town is on the floodplain of Price River, East of town the highway continues across lower Mancos Shale and climbs onto the northwest flank of Farnham Dome.

76.0  Beginning of Cat Canyon on the northwestern margin of Farnham Dome. Thick Ferron Sandstone forms [Ile resistant, unit which caps the walls and forms the prominent cuesta around the northern and western part of the dome. Two small reverse faults cut the Ferron Sandstone and are particularly well exposed on the north side of the road (fig. 1-19). Gray Mancos Shale is exposed below the Ferron cap in the eastern part of the gap. The shale forms a subsequent valley around the dome between the resistant Ferron and Dakota Sandstones.

77.2  Primitive side road to the south is along the crest of the dome and leads to the CO2-producing well near the knob of reddish Cedar Mountain Formation exposed in the core of the fold to the south. Dakota Sandstone is exposed in highway cuts east of the side road junction and beyond the fence south of the road.

Ferron Sandstone nearly pinches out over the crest of the structure so that it forms only a silty, low, rounded discontinuous cuesta on the east flank where the highway crosses its trace.

The largest known deposit of asphaltic sandstone in the United States occurs north of Sunnyside on the high part of the West Tavaputs Plateau. It is in beds 10 to 350 feet thick in upper Colton and lower Green River beds and has been mined since 1892, although major development didn't take place until 1928.

U.S. Highway 50-6 continues southeastward over pediments and pediment gravel and through cuts in gray Mancos Shale. Some of the pediment-armouring gravel is composed of coarse blocks and boulders brought down from the escarpment and canyons to the east by mudflows (fig. 1.20). Several levels of pediments are developed, each adjusted to a former position of Price River and its tributaries.

80.7  Road Junction of Utah State Highway 123 with U.S. Highway 50-6 East of Grassy Trail Creek Crossing. Utah State Highway 123 leads east to Sunnyside, a coal mining and former coke town. Coal is mined from a Cretaceous barrier island-lagoon sequence which is similar to that exposed near Castlegate, although it is slightly younger here.

Figure 1.19. View eastward along U.S. Highway 50 and 6 from approximately Mile 67.0 showing faultrepeated exposures of the Ferron Sandstone in Cat Canyon. These steeply dipping rocks are on the west side of Farnham Dome and show three nearly complete sections of the Fenno Sandstone on fault isolated blocks which have dropped down to the cast. The massive sandstone which caps the exposures in the gap, near the bend in the road, is the same as that which forms the prominent exposures beneath the juniper cover at the left of the photograph.


Figure 1.20. View eastward through juniper woods to the western edge of the West Tavaputs Plateau, across course mud flow debris which is blanketing a pediment surface cut across Mancos Shale. Blackhawk and Castlegate Formations form the low ledges and the prominent ridge in the middle distance. Green River beds cap the plateau on the skyline.


86.7  Carbon County-Emery County Line. The road continues in Mancos Shale or over pediment surfaces on the shale. Upper Mancos Shale is exposed in badlands along the cliffs to the east below coal-bearing Blackhawk Formation (fig. 1.21), massive Castlegate Sandstone, and more ledge-and slope-forming Price River Formation that caps the first series of cliffs. Younger rocks up to Green River Formation are exposed in the second series of cliffs and ledges up to Patmos Head (elevation 9,841 feet) the high promontory on the rim of the West Tavaputs Plateau.

Cedar Mountain, to the west, is at the northern end of the Triassic and Jurassic rim of the San Rafael Swell, a large domal structure that dominates much of the geology of east central Utah.

103.5  Post office and service station at Woodside. A CO2-driven geyser (fig. 1.22) erupts from a water well drilled by the railroad in 1910. The water well spouted as much as 75 feet into the air initially but escape of gas has lowered the driving pressure and height of the geyser eruption, East of Woodside the Castlegate Sandstone forms the uppermost massive cliff. A lower massive sandstone in the Blackhawk Formation exposed along the gorge of Price River is subdivided into three whitecapped sandstone beds as the unit is traced northward. Each of these is capped by a coal bed in the lagoonal part of the section. Mancos Shale is exposed near the base of the Book Cliffs escarpment and the highway continues southward over the lower part of the shale beyond the crossing of Price River.

Figure 1.21. View eastward from the highway to badland topography carved on upper beds of Mancos Shale. Cliffs above are in Blackhawk Formation and Castlegate Sandstones and are part of the nonmarine nearshore and coal swamp accumulation associated with withdrawal of the Mancos Sea.


Figure 1.22. Woodside geyser at Mile 94.5 erupts approximately every hour from a well drilled by the railroad in the early 1900s. The photograph shows the last phase of eruption of the CO2-driven geyser.


111.4  Overpass Over Main Line of Denver & Rio Grande Railroad. San Rafael Swell to the west exposed lower Cretaceous and Jurassic beds in cuestas along the east flank. Ferron Sandstone forms the low cuesta immediately west of the flat and the road.

117.6  Bend in the highway. The Reef of the San Rafael Swell to the south and southwest is held up by Triassic Wingate, Kayenta, and Navajo Sandstones where the beds dip steeply off the eastern flank of the uplift. U.S. Highway 50-6 continues through gray Mancos Shale. Beckwith Plateau, to the east, is capped by Castlegate Sandstone which here begins to develop thin coal beds and a lagoonal facies, equivalent to a barrier island sandstone to the east and to piedmont braided stream deposits to the west.

121.2  Route Separation. The road ahead leads eastward toward Green River on eastbound Interstate Highway 70, and that to the west (right) leads to Interstate Highway 70 westbound toward Salina. Both routes continue in the lower Mancos Shale. See I-70 Road Guide. for a continuation westward along interstate 70. I-70 Road Guide continues ahead on eastbound Interstate 70.

121.5  Bridge over Interstate 70 on interchange. The highway now swings to the east in Mancos Shale, parallel to the southern margin of Beckwith Plateau. Castlegate Sandstone caps the plateau above ledges and slopes of upper Blackhawk Formation and the silvery gray slopes of Mancos Shale.

125.1  Downtown Green River , Main Intersection. The gorge of Green River north of town separates the East and West Tavaputs Plateaus. Powell gave the name Desolation Canyon to this stretch of the river gorge across the southern part of the Uinta Basin where the river cuts through the Roan Cliffs and Book Cliffs.

125.7  Emery County-Grand County line marking a former position of the midchannel for the Green River before it abandoned this meander.

  Center of the Bridge Over the Green River.

126.4  Double road cut in silty beds of the Ferron Tongue in the lower Mancos Shale. To the cast at the crossing of Brown's Wash the fossiliferous dark gray laminated part of the member is exposed to the south. To the north the lower cliff-forming part of the Book Cliffs escarpment is made up of Mancos Shale up to Castlegate Sandstone. These rocks are overlain by the valley- forming Buck Tongue of the Mancos Shale and by deltaic beds equivalent to the Price River and North Horn Formations in Price Canyon.

129.1  Underpass Beneath D&RGW Railroad. A short distance east of the underpass a side road to the south leads to Crystal Geyser, a CO2-driven well and geyser. It puts on a spectacular display approximately every 4 hours.

132.7  Cross a small gully. The highway continues in Mancos Shale with a characteristic cap of rusty brown gravel derived from the plateau to the north. The white coating here and there is caliche of calcium sulphate brought to the surface by ground water and left behind by evaporation.

136.3  The road rises to an upper pediment surface. To the north the prominent high cliff in the immediate vicinity is capped by the Castlegate Sandstone. Lower lenses of sandstone appear beneath the coal-bearing, but now very silty Blackhawk Formation. These lower sandstone beds show the structure within the Mancos Shale. The LaSal Mountains are visible ahead at about two o'clock and the Salt Valley Anticline is developed in the Castlegate Sandstone and Mancos Shale to the north. The telephone relay station ahead is on the Castlegate Sandstone.

145.0  Junction of U.S. Highway 160 with Interstate Highway 70 and U.S. Highway 50-6. For a continuation along U.S. Highway 160 southeastward toward Moab and Canyonlands and Arches National Parks gee Geologic Guide. Segment 6. Both highways continue in Mancos Shale, either to the east at the foot of the Castlegate capped Book Cliffs, or southward around the nose and flank of Salt Wash Anticline.





from Field Guide: Northern Colorado Plateau by J. Keith Rigby - Purchase Information