I-15 Junction to Green River, Utah
0.0 North Cove Fort Interchange. Leave the freeway and head east toward the south end of the Pavant Range and the Richfield valley. To the southwest, the high conical hill is a cinder cone related to some of the recent basaltic volcanism of the area. East of the freeway interchange the road is built across volcanic debris.
3.4 Cove Fort. Cove Fort (fig. 4.2) was built by Brigham Young in 1867 as a stopover point between Salt Lake City and St. George. It is constructed of basalt quarried in the low hills to the west, and has pinkish andesitic rocks for chimneys for the numerous fireplaces of the fort.
Figure 4.2.Cove Fort which was built for Brigham Young as a stop-over point on the road between Salt Lake City and St. George. The fort is built, in large part, of basalt which was quarried from flows to the west and has fireplaces of pinkish andesite which .as quarried from the mountains to the east.
4.4 Pass beneath Interstate Highway 70 interchange and head east on the entrance road to join the highway. Sulfurdale, a short distance to the south of the interchange, was a major source of sulfur in early pioneer days and rocks in the open pits still show considerable elemental sulfur which fills voids in some of the volcanic rocks. The deposit appears to be related to recently active fumaroles.
5.9 Boundary of Fish Lake National Forest in pinion and juniper woods. The canyon along which the route travels separates the Tushar Range on the south from the Pavant Range on the north. Bleached and altered volcanic rocks are exposed in low road cuts in both the east- and west-bound lanes.
9.3 Ashy-appearing tuffaceous rocks exposed in cuts on the eastbound lane. The general southeastward dip of the rocks is evident in the cuts and suggests that the Tushar Range is composed of thousands of feet of volcanic rocks.
10.5 Summit and divide between Cove Creek on the west and Clear Creek on the east. Road cuts show ashy volcanic rocks characteristic of the Tushar Range and the Marysvale district. The view to the east is principally of the northern flank of Mount Belknap and the volcanic pile of the Tushar Range.
13.2 Extensive coarse Sevier River deposits are well exposed in road cuts on the north (fig. 4.3). These exposures are typical of bedrock between the summit and here and show the very coarse nature of the accumulation.
Figure 4.3.Coarse Sevier River Formation exposed in roadcuts at Mile 13.2, on the east side of the summit in the head waters of Clear Creek. The Sevier River deposits in general rest unconformably on some of the older volcanic and elastic units in the mountains.
14.2 Excellent exposures of tuffaceous mudflow or lahar deposits occur in ledges to the north of the road.
17.1 Narrows of Clear Creek. Vertical walls are cut in ignimbrites which here show only weak columnar jointing. The canyon, at present (1976) is just wide enough for a two-lane road and the creek.
18.0 Welded ignimbrite of the lower canyon walls shows strong columnar jointing. Such ignimbrites are common in the lower part of the volcanic section to be seen along Clear Creek Canyon between here and U.S. Highway 89, 5 miles to the east. Side canyons to the south and north provide glimpses to the section and suggest that the rocks near the road are only part of a sequence of rocks 2,000 to 3,000 feet which are exposed here.
22.0 Cliff in spectacular exposures of jointed ignimbrite (fig. 4.4). The exposure may be formed in three flow units. Sharp needles are produced by weathering of the same rocks on the south side of the canyon opposite the ranch house.
Figure 4.4.Prominently jointed cliffs of welded ignimbrite, as seen eastward from approximately Mile 21. Two or three flow units are exposed here in the Oligocene volcanic section.
23.0 Junction Interstate 70 with U.S. Highway 89. For a description of the routes north and south from here see HW-89 Road Guide.. Joseph and Elsinore are to the north and Marysvale and Junction are to the south. Strong east dip of the volcanic sequence shows in the cuestas to the west (fig. 4.5) and north.
Figure 4.5.various volcanic units exposed near the mouth of Clear Creek Canyon, as seen westward from near the connection of Utah State Highway 13-Interstate 70 with U.S. Highway 89. Ashy units generally weather to more rounded stores and welded ignimbrites form the more resistant ledges.
40.0 Interstate 70 through Richfield Valley Section of I-70 which connects the southern and northern limbs of Highway 89. The interstate skirts the edges of Joseph, Elsenore, Richfield and Salina. A geologic description of this area still needs to be written.
64.0 Junction of Utah State Highway 89/50 with Interstate 70 just south of Salina. The Interstate Highway skirts around south of town on Arapien Shale and volcanic debris. For a continuation north or south along U.S. Highway 89 see HW-89 Road Guide..
63.5 Mouth of Salina Canyon. Excellent exposures of lower red Arapien Shale overlain by younger Gray Gulch Formation and volcanic rocks occur in barren badlands on the northern and southern margins of the valley fill at the mouth of the canyon.
63.0 Leaving Fish Lake National Forest. Jur Jurassic Arapien Shale (figs. 5.23, 5.30) is exposed both to the north and south near old calcite mill workings.
Figure 5.29.Angular unconformity exposed in road cuts at Mile 88.2. Older deltaic Flagstaff Limestone above rests on North Horn Formation beds below.
Figure 5.30.Barren exposures of Jurassic Arapien Shale as seen in the mouth of Salina Canyon at approximately Mile 90.5. These rocks are gypsiferous and have a peculiar endemic selenium- and salt tolerant flora.
62.5 Angular unconformity of North Horn Formation on Cretaceous sandstone and shale (figs. 5.28, 5.29) is exposed in deep road cut on the north. Older Cretaceous shale, appearing like Mancos Shale, and rocks as old as Arapien Shale appear be neath the unconformity to the west. Arapien Shale up to Morrison Beds are nearly vertical beneath flat-lying Tertiary beds in the lower walls of the canyon approximately I mile to the west.
Figure 5.28.Generalized geologic cross section through the western part of the Wasatch Plateau showing the angular unconformities which separate the late Cretaceous and Tertiary beds from the older Cretaceous and Jurassic rocks. The conclusions shown here are those of Spieker who did much of the early geologic mapping and stratigraphic work in the Wasatch Plateau (from Spieker, 1949).
61.0 Junction Gooseberry Recreational Area Side Road with Interstate 70. Price River beds dip westward beneath North Horn beds at approximately the junction. Broad open valley here is in part on landslide lobes from the south and north. Green River Formation caps the hills to the southwest and northwest, above reddish Colton, resistant Flagstaff, and resistant North Horn beds.
65.3 Exposures on both the south and north of the road are in flow-folded or slumped shale and thin sandstone in the Price River Formation. Massive Castlegate Sandstone dips below road level one-half mile to the east. Price River deposits are stream-dominated and are usually complexly channeled and sandstone units show much cross-bedding.
69.7 Railroad tunnels on the south of the canyon (fig. 5.27) are in Castlegate Sandstone. Exposures nearby show complex lensing and sedimentary structures characteristic of braided stream deposits. West of the tunnels the highway crosses several small faults which form horsts and graben and are best shown by relative offset of the massive cliff-forming Castlegate Sandstone. One of these graben is well shown by offset near Water Hollow where Castlegate Sandstone, on the east, is dropped down against Blackhawk beds on the west. Castlegate Sandstone west of the fault is on the canyon rim. It dips gently westward and is at road level approximately 3 miles ahead on the highway.
Figure 5.27.Tunnel on abandoned narrow gauge railroad through Castlegate Sandstone, at approximately Mile 80.0, as seen from the west. Lower beds of the Price River Formation form the pinion, and juniper-covered slopes above Castlegate exposures. This belt of Castlegate Sandstone has been downdropped along the Water Hollow graben in the western part of the Wasatch Plateau.
70.6 Excellent exposures of coal-bearing Blackhawk Formation are cut along the freeway. Massive Castlegate Sandstone forms cliffs along the canyon wall. Several coal mines were developed in Salina Canyon in the upper part of the Blackhawk sequence. A narrow-gauge railroad was constructed up the canyon to serve the coal mines and part of the right-of-way is still used for a service road off the highway.
74.7 Cross Salina Creek in exposures of pinkish and gray North Horn Beds along the western side of the Musinia graben. Cliffs to the west are of Blackhawk Formation west of the graben boundary fault. The graben boundary faults here have displacements of 1,200 to 1,500 feet. Shale and sandstone west of the fault contain a prolific fossil plant assemblage.
77.2 Excellent view toward the north of nipple-shaped Mt. Musinia, elevation 10,986, and of the fault along the east side of the Musinia graben (fig. 5.26). A graben is a long, relatively narrow downdropped fault block and here the softer pinkish North Horn Beds have been dropped down in the block. More resistant Price River and As tlegate rocks, above coal-bearing Blackhawk Beds (fig. 5.28), are exposed east of the fault. The trace of the fault is visible ahead where ledges of the older Cretaceous rocks terminate westward against softer rounded hills of North Horn Formation. The highway has been constructed here along the trace of the fault.
Figure 5.25.View northward along the eastern escarpment of the Wasatch Plateau. The prominent cliff in the escarpment is the Star Point Sandstone which separates the coal-bearing Blackhawk Formation, above, front the underlying Masuk Shale of the Mancos Shale, below. The Star Point Sandstone continues northward as a prominent cliff along the Wasatch Plateau escarpment as seen in Figure 5.17.
Figure 5.26.View northward along the east boundary fault of the Musinia graben from approximately Mile 72.0. Rounded slopes on the left are in the North Horn Formation, which have been downdropped against Castlegate Sandstone and Price River beds, the ledge -forming units on the right. The highway in the foreground is approximately along the trace of the boundary fault. Mt. Musinia is the snow-covered peak on the skyline toward the left.
81.8 Summit (Elevation 7,900 feet). Bleached sandstone to the north is in the Price River Formation, here still dipping westward off the San Rafael Swell dome.
85.8 Entering Fish Lake National Forest, at the eastern base of the Wasatch Plateau to the north, and the Fish Lake Plateau to the south. Cretaceous barrier island sandstones form cliffs to the north, with the Star Point Sandstone as the major light cliff (fig. 5.25). Coals above the Star Point Sandstone have burned and baked overlying rocks to produce pinkish stained areas high on the plateau. Star Point Sandstone is exposed as the cliff in the canyon ahead.
87.0 Fremont Junction. Utah State Highway 72 leads south to Loa and connects with Utah State Highway 24 which crosses through Capitol Reef National Monument. Emery Sandstone is exposed north of the road, northwest of the junction (figs. 5.23, 5.24). Three or four pulses of barrier-island regressive sandstone show upward coarsening as shaly beds are overlain by tan sandstone. Emery Sandstone separates upper and middle Mancos Shale.
Figure 5.24.Emery Sandstone, as seen northward from Mile 62.8. The sandstone is in shorefront facies, in front of the barrier island sequence which is developed beneath the Wasatch Plateau to the west. Emery Sandstone is one of the several extensive eastward-spread sheets of sandstone deposited along the western border of the Mancos seaway as a response to rising uplifts in the Sevier orogenic belt to the west.
89.0 Bridge on Price-Emery Interchange at Junction of Utah State Highway 10. Terraces along creeks to the north are cut in Mancos Shale and are andesite-boulder protected. State Highway 10, to the north, follows the "race track" along the easily eroded Mancos Shale around the western margin of the San Rafael Swell and connects to U.S. Highway 50-6 at Price. Interstate Highway 70 continues westward in middle Mancos Shale.
97.5 Sevier County-Emery County Line. Road is in Mancos Shale below a pediment surface armoured with basaltic andesite boulders derived from the Fish Lake Plateau, to the southwest. The road rises onto the pediment surface and then descends westward in Mancos Shale.
98.8 Major channel-fill sandstone near the top of the Ferron Sandstone. Top of the sandstone and base of the fossiliferous middle Mancos Shale are exposed in cuts 0.4 miles to the west. Middle Mancos Shale was deposited over Ferron sediments as the Cretaceous seaway again spread westward over the delta.
99.5 Road cuts in westbound lane of top of lower Mancos Shale, overlain by coalbearing barrier-island and deltaic sandstone and shale of the Ferron Sandstone (fig. 5.22). The sandstone tongue here is considerably thicker and coarser grained than equivalent beds on the east side of the San Rafael Swell or where the sandstone is exposed east of Wellington, north of the Swell. The upward coarsening sequence of marine sparingly fossiliferous siltstone, fine-grained burrowed sandstone and coarse sandstone with channels which are overlain by moderately thick coal beds record a major regression of marine deposits !it front of a lobe of a Ferron Sandstone delta. Upper deltaic plain sandstone, lacustrine siltstone, and coal deposits are exposed in road cuts of the upper part of the sandstone, on to the southwest.
102.7 Cross Muddy Creek Bridge. Dakota Sandstone is exposed capping the westdipping cuesta of Cedar Mountain beds immediately east of the bridge. Mancos Shale is exposed west of the bridge and to the south and north along the strike valley. Terraces are cut across the Mancos Shale and are armoured with bouldery debris from the overlying Ferron Sandstone (fig. 5.21). Uneven heaving of the Mancos Shale makes highway construction over the formation difficult, and produces wavy uneven roads like some stretches ahead.
Figure 5.21.Eastward to gray Mancos Shale exposures on the south side of the freeway at approximately Mile 54.0. Ferron Sandstone forms the prominent cliff along the skyline toward the right. Debris fans have locally protected the soft underlying Mancos Shale and have helped produce a variety of irregular erosional features.
Figure 5.22.Northwestward to lenticular, well- bedded, deltaic and barrier island sandstones of the Ferron Sandstone as seen across Ivie Creek from Mile 55.6. Light-colored sandstones on the skyline are river deposits. Those in the more well-bedded lower part of the cliff are thought to be marginal marine and barrier island associated sandstone accumulations. Coal is common in the upper part of the sandstone unit.
103.9 Coarse conglomerate of the Buckhorn Member of the Cedar Mountain Formation exposed in road cuts. This conglomerate separates the more maroon and varicolored Morrison Formation from the distinctly more gray and ashy-appearing overlying Cedar Mountain Formation. Both these units form peculiar barren badland topography perhaps related to the expansion-contraction weathering of the volcanic clays that the rocks contain. Such weathering makes it almost impossible for most plants to populate the badlands.
105.2 Crest of cuesta. Massive light beds to the north in the top of the Summerville Formation are massive gypsum, overlain unconformably by basal sandstone of the Morrison Formation, here somewhat thinner than on the east flank of the dome. Wellbedded maroon and gray shale of the Brushy Basin Member of the formation is exposed above the basal sandy section on the backslope of the cuesta near the road. Several trails lead to uranium prospects within the Morrison Formation northwest of the cuesta crest.
106.1 Deep road cuts through channelled and rippled reddish Summerville Formation, particularly on the westbound lanes, show excellent tidal flat deposits. Pinkish sandstone shows climbing ripple marks, bimodal current directions, and trace fossils. Softsediment deformed structures show in darker reddish brown mudstone beds. Exhumed channel deposits armour ridges to the south of the road and mark locations of paleochannels in the middle part of the formation. Soft sediment has been eroded away from the channels leaving them as linear protective ridge caps.
Figure 5.19.View southward along exposures of the Curtis Sandstone from the rest area at approximately Mile 49.0. The Curtis Sandstone is a sheet like marine sandstone that marks a minor invasion of the Sundance Sea southward into the Colorado Plateau. The formation occurs as a distinctive greenish marker between the reddish tidal flat deposits of the underlying Entrada Formation and the overlying Summerville Formation.
107.0 Deep double road cut through greenish well-bedded Curtis Sandstone. "Stone baby" beds of upper Entrada weather to form goblins and dragons beneath the Curtis Sandstone on the east face of the escarpment. A side road to a rest area at the west end of the deep cut leads to an overlook area in the upper Curtis Sandstone (fig. 5,19). To the east the valley is lined with columns and pillars in the Entrada Formation. A pavement of a bar-like sandstone in the Entrada Formation forms the strongly jointed shelf below the overlook. West of the overlook "castle walls" of reddish Summerville Formation rise above the Curtis Sandstone cuesta (fig. 5.20).
Figure 5.20.Exposures of upper Summerville Formation, in the foreground, are capped by massive gypsum (G) with Morrison beds (M) along the skyline, as seen westward from approximately Mile 50.0. The gypsum deposits record final drying up of the tidal flat Sequence.
108.5 Double road cuts in complexly channeled and lensing Entrada Formation (fig. 5,18). Lacy networks through even the thickest beds are crystalline gypsum.
Figure 5.18.Flat-bottomed sandstone lense in the lower Entrada Formation in road cuts at Mile 470. This sandstone is one of several similar accumulations that are thought to be small bars or splay sandstones in the dominantly tidal flat-marginal marine Entrada Formation.
111.1 Approximate contact of the gypsiferous red and green Carmel Formation with the overlying more reddish orange Entrada Formation. The Entrada Formation weathers to "stone babies" and a variety of columns and pillars. Entrada Formation is thought to be a supratidal flat sequence between deposits of the Carmel and Curtis Seas.
114.4 Side road east to overlook (fig. 5.16). Turn-around of overlook is on lower Carmel Beds. Thick, cross-bedded, lightcolored Navajo Sandstone forms the cliffs in canyons to the east and southeast. Rocks at the road junction are greenish upper Carmel Formation, now dipping moderately steeply off the west flank of the San Rafael Swell. From near the junction a panorama of the brightly colored Jurassic (fig. 5.17) and more somber Cretaceous and Tertiary rocks spreads virtually from horizon to horizon to the west. The road descends down a dip slope held up by the upper Carmel Beds. On the skyline to the west Tertiary rocks cap the Wasatch and Fish Lake Plateaus to the northwest and southwest, respectively. Draped gypsum has flowed into gullies north and south of the road and appears almost as frosting.
Figure 5.16.View southwestward from the rest area at approximately Mile 41.0 Massive exposures in the bottom of the canyon are upper beds of the Navajo Sandstone and are overlain by the well-bedded, fossiliferous lower beds of the Carmel Formation at the canyon Tim. Fish Lake Plateau is in the far distance to the southwest.
Figure 5.17.View westward of upper Jurassic and Cretaceous rocks on the west side of the San Rafael Swell and east front of the Wasatch Plateau taken from approximately Mile 45.0. Light-colored Curtis Sandstone forms the prominent escarpment in the foreground, above slope zones and low ledges on the Entrada Formation. Summerville beds are exposed as the first prominent dark slope above the light- colored Curtis Formation and occur below the banded Morrison Formation. Prominent slopes and the flat country in the middle distance arc on Mancos Shale, the lower unit of which is capped by Ferron Sandstone which holds up the cuesta in front of the prominent escarpment of the Wasatch Plateau. Light uppermost Mancos Shale is overlain by the prominent cliff of Star Point Sandstone and younger rocks on the skyline (photograph by W.K. Hamblin).
118.4 Beginning of double road cuts through upper, cross-bedded, light yellow gray Navajo Sandstone and overlying wellbedded lower Carmel Formation. Navajo Sandstone is considered to be a broad sheet of eolian sandstone, in part deposited along the southeastern shore of the Carmel Sea. Lower beds of the Carmel Formation are dolomitized but locally contain fair molluscs, particularly near the thin reddish unit in the gray shale, above the dolomite ledges. Upper beds in the road cut, east of the Moore Ranch Road exit, contain thick beds of massive gypsum interbedded with sandy thin bedded dolomite and siltstone. Carmel rocks record a relatively rapid invasion of the Carmel Sea, and then a gradual oscillating withdrawal.
120.4 Cross Bridge Over Eagle Canyon. Vertical walls of gorge are in Wingate Sandstone, with the bridge in Kayenta beds (fig. 5.15). The road beyond is in upper Kayenta and lower Navajo Formations.
Figure 5.15.Eagle Canyon as seen westward from the bridge at Mile 36.6. Wingate Sandstone holds up the cliff in the inner gorge and is capped by flaggy to irregular bedded Kayenta Sandstone in the brushy slope zone. Navajo Sandstone rises on the skyline as the series of white cliffs.
122.9 San Rafael Knobs Rest Area. The knobs arc in lowermost Navajo Sandstone with the woodlands at the road on more slabby-bedded Kayenta Sandstone. Canyons northwest of the rest area expose Chinle Beds in their lower channels, vertical angular cliffs of Wingate Sandstone, and upper slabby-bedded Kayenta Sandstone at the canyon rim (fig. 5.14). Cliffs of rounded Navajo Sandstone of the western flank of the San Rafael Swell rim the juniper-covered Kayenta beds in the background.
Figure 5.14.View northwestward from the San Rafael Knobs rest area toward the northwestern margin of the San Rafael Swell. Chinle Beds (C) are exposed in the lower slopes within the canyon beneath the massive Wingate Sandstone cliff (W). Kayenta Sandstone forms the juniper-covered tablelands between Wingate Sandstone and overlying white cliffs of Navajo Sandstone (N). The Wasatch Plateau rises in the far distance along the skyline.
124.9 Pass between rounded yellow brown outcrops of cross-bedded Wingate Sandstone. Wingate beds form the joint controlled fins in the cliffs north of the road. West of here the road rises up through Wingate Sandstone exposures.
127.2 Prominent sandstone cliff to the north is held up by the Moss Back Sandstone Member of the Chinle Formation (fig. 5.13). Road in the upper reddish Moenkopi beds leads to Uranium "glory holes" dug in the lower part of the Moss Back Cliff. To the west the road climbs slowly onto upper Moenkopi redbeds and then cuts up through the complexly lenticular sandstone of the Moss Back Member. The sandstone is probably a deposit of a sediment-charged braided stream system, Beyond the cut, to the west, poor exposures of soft Chinle Formation can be seen in some road cuts.
Figure 5.13.Uppermost Moenkopi beds form the slope in the foreground beneath the cliff-forming Shinarump Sandstone Member near the base of the Chinle Formation at approximately Mile 30. Locally the Shinarump Sandstone has produced minor amounts of uranium minerals and its outcrops were extensively prospected during the uranium boom in the 1950s. Moss Back Sandstone is thought to be the deposits of braided stream systems deposited on top of tidal deposits of the Moenkopi Formation.
135.0 Basal beds of Sinbad Limestone are "posed in road cuts. Beyond the sweeping curve the road rises onto the stripped upper surface of the Sinbad Limestone and continues ahead on this gentle surface over the crest of the San Rafael Swell.
137.1 Crest of the steep climb up the flexed eastern monoclinal limb of the San Rafael Swell. The grade now flattens westward and has been constructed across lowermost Moenkopi beds. Sinbad Limestone forms the cuesta caps above the road. Kaibab Limestone is still exposed in canyons below the road to the south and southeast.
139.3 Long road cut on the east side of the eastbound lane shows tan Moenkopi beds resting disconformably upon cherry laggravel filled channels cut into the underlying Kaibab Limestone (fig. 5.12). Light tan to gray chert obviously was derived from weathering of Kaibab beds. The westbound lane of 1-70 is on the top of the Kaibab Limestone, at the unconformity.
Figure 5.12.Disconformable contact between the Kaibab Limestone (K) and the Moenkopi Formation (M) along the east side of the freeway at Mile 17.7. Channels filled with lag deposits of sharp chert fragments occur at the unconformity. Much of late Permian and early Triassic time is represented by erosion at the unconformable surface.
141.1 Rest Areas for both east- and westbound lanes. Rest area for westbound lane is built on the upper Kaibab Limestone. White Rim or Coconino Sandstone (fig. 5.10) is visible in the gorge to the north. The rest area for the eastbound lane is built on the Sinbad Limestone of the Moenkopi Formation and looks eastward over the narrow cut through the San Rafael Reef to the gray slopes of Mancos Shale and the East Tavaputs Plateau in the distance (fig. 5.11). Angular cliffs of Wingate Sandstone above reddish brown Chinle and Moenkopi beds form much of the scenery along the eastern flank of the San Rafael Swell and contrast with the tan to light gray or gray green lowlands on the top of the Kaibab and in lower Moenkopi beds.
143.6 West end of the narrow cuts, Top of the stream-deposited Chinle Formation and base of the Wingate Sandstone. Chinle beds are maroon and reddish brown shale and siltstone and also include the massive sandstone ledge on top of the prominent slope-forming reddish mudstone (fig. 5.10). Thin bedded Moenkopi Formation is exposed below the mudstone to the west and lacks the thick sandstone beds of the Chinle. Chinle rocks have produced numerous fossil reptiles and considerable fossil plant material. In some areas the formation has also been a significant producer of uranium minerals, usually deposited as replacements of organic plant debris. From here the highway climbs up the east flank of the San Rafael Swell through Moenkopi and older Kaibab beds (fig. 5.11).
Figure 5.11.View eastward from the Test area of the eastbound land showing the highway route through the narrows along the east side of the San Rafael Swell. Kaibab (K) Limestone is the oldest unit exposed in this area and is overlain by light-colored and dark-colored Moenkopi Formation (M) that forms the slope zone near the base of tile excarpment. Chinle Formation (C) forms the upper banded exposures and slope zones beneath the prominent Wingate Sandstone cliffs that hold up the high peaks of the cockscomb along the monocline. Younger Jurassic rocks are exposed through the deep V-shaped notch and Cretaceous rocks form exposures in the far distance in the Book Cliffs area.
144.2 Rest areas here are built on Carmel Formation, the unit which forms the resistant flat-irons and resistant ridges (fig. 5.7) at the base of the San Rafael Reef north and south of the highway (fig. 5.8). Lower limestones of the unit are fossiliferous but upper beds are not and record gradual increase in salinity above that in normal seas until gypsum was precipitated in what were evaporating pans along the shoreline.
Figure 5.7.View northward along the monocline on the east side of the San Rafael Swell from the rest areas at approximately Mile 12.8. Dark flatirons are held by lower fossiliferous limestones of the Carmel Formation which here rest on light-colored, massive Navajo Sandstone.
The overlying Entrada Formation is relatively soft here and has helped to produce a prominent strike valley parallel to the Reef (fig, 5.8). Entrada beds are exposed in cliffs south of the rest area where they form castellate features like the Summerville Formation above. They are separated in the cliff by the greenish Curtis Formation. Entrada rocks here are thought to be tidal flat deposits, like those of the Summerville Formation.
Figure 5.8.View southward along The Reef on the east flank of the San Rafael Swell from near the rest areas at Mile 12.8. Navajo Sandstone forms the top of the cockscomb along the monocline. Permian and Triassic rocks are exposed in the center of the uplift toward the right, and are nearly flat lying away from the steep monoclinal flexure. Light-colored Morrison Formation caps the tableland on the left of the photograph, above the dark ledges on the Summerville Formation. Light-colored Curtis Sandstone separates the Summerville Formation from the underlying Entrada and the banded Carmel Formations which occur in the lowlands between the monocline and the tablelands (photograph by W.K.
West of the rest areas, Interstate 70 has been blasted through deep narrow cuts in the underlying Navajo, Kayenta, and Wingate Sandstone which stand high on the monoclinal fold at the east of the San Rafael Swell (fig. 5.9). Wingate and Navajo Sandstones are both cross-bedded windblown sandstones. They are separated by the more flaggy-bedded, stream-worked Kayenta Sandstone which forms a shoulder and brushy area midway through the narrow cut.
Figure 5.9.Generalized east-west geologic cross section across the monoclinal fold of tile east edge of the San Rafael Swell approximately along Interstate 70. The cross section is drawn as though looking toward the north flank of the San Rafael Swell through Moenkopi and older Kaibab beds (fig. 5. 1).
145.7 Bridge Across the Son Rafael River. Summerville "castle walls" are visible both south and north of the bridge, capped by a massive thick bed of gypsum (fig. 5.6). Thick lenticular stream-deposited sandstone beds of the lower Morrison Formation cap the ridges. The Summerville Formation was deposited as part of an extensive arid-coast tidal flat. Thin gypsum stringers through the formation and the massive gypsum at the top were deposited by evaporation of sea water along the margin of a sea which extended from near here northward along the Rocky Mountain area into northern Canada. West of the bridge greenish Curtis Sandstone, deposited in a late marine tongue of the Sundance Sea, forms low sandy exposures south of the road.
Figure 5.6.View southward from bridges over the San Rafael River along Interstate Highway 70 to the Summerville Formation (S) and the overlying Morrison Formation (M). Light-colored rocks beneath the Summerville Formation at the extreme right are upper beds of the Curtis Sandstone. A thick gypsum bed at the top of the Summerville Formation caps the castellate cliffs.
147.0 Bridge of the Hanksville Interchange (fig. 5.5.). Utah State Highway 24 leads south to Hanksville and the upper reaches of Lake Powell. For a guide to this state highway see HW-24 Road Guide., which also covers the road through Capitol Reef National Monument and westward to join U.S. Highway 89 near Sigurd. Morrison Formation is exposed both east and west of the interchange. Lake and deltaic deposits of the Brushy Basin Member (fig. 5.1) form purplish rounded exposures to the east. Ancient stream channels which are filled with light gray sandstone weather out into three-dimensional relief in the Salt Wash Member (fig. 5.1) west of the interchange. Softer floodplain deposits have been eroded away from some channel fills to leave them as meandering sinuous ridges that mark the old stream courses.
Figure 5.5.View eastward from the Hanksville interchange along State Highway 70 at Mile 10 toward Cretaceous exposures in the distance. Linear ridges in the foreground are held up by sandstone fillings of channels in the Morrison Formation. The cuesta in the middle distance is on the Ferron Sandstone in the lower part of the Mancos Shale (photograph by W. K. Hamblin).
149.3 Oyster beds composed of Gryphaea newberry (fig. 5.4) form low rounded exposures in road cuts at the base of the Mancos Shale. In other areas equivalent oyster beds have been utilized for gravel because of the great abundance of the fossils. Mancos Shale here rests upon gray green, ashy-appearing Cedar Mountain Formation of Lower Cretaceous age (fig 5. 1). The resistant conglomeratic basal Buckhorn Member of the Cedar Mountain Formation is exposed in cuts a short distance west along the highway and separates the more greenish Cedar Mountain beds above from the maroon and purplish gray beds of the Morrison Formation below.
Figure 5.3.Exposures of Mancos Shale in road cuts along Interstate Highway 70 at Mile 8. Thin-bedded units at the top are in the Ferron Sandstone. Laminated beds between the massive, light-colored lower exposures and the Ferron Sandstone contain abundant flattened ammonoids and bivalves.
Figure 5.4.Weathered exposures of the Gryphaea newberryi beds at Mile 187. These oyster beds are the basal unit of the Mancos Shale and rest on lacustrine and fluvial lower Cretaceous Cedar Mountain Formation.
150.3 Crest of Ferron Sandstone cuesta. View to north and behind is of Book Cliffs and East and West Tavaputs Plateaus (fig. 5.2), separated by Desolation Canyon, the gorge of the Green River. In the double road cuts on both the eastand westbound lanes white volcanic ash beds have been faulted and also mark the base of the thin silty Ferron Sandstone and the top of a fossiliferous laminated part of the lower Mancos Shale. Ammonoids and molluscs of various types can be collected from the platy shaly beds (fig. 5.3). The fossils are very fragile, however, and do not transport well. These fossiliferous shale and siltstone beds and the overlying thin silty sandstone are the distal edge of a thick sandstone and coalbearing deltaic sequence which is well exposed along 1-70 on the west flank of the San Rafael Swell.
156.0 Cross Beneath Bridge of U.S. Highway 191/6 Interchange. U.S. Highway 191/6 leads northwestward toward Price, Provo, and Salt Lake City. For a guide to this section consult Guide HW-191 Road Guide.. I-70 Road Guide. proceeds westward across the San Rafael Swell, visible ahead, and through the Wasatch Plateau to Salina. Beyond the interchange Interstate 70 climbs the gentle cuestas produced in Cretaceous beds along the east margin of the San Rafael Swell, a large domal structure rimmed by hogbacks and cuestas in Permian to Cretaceous rocks in east central Utah.Flat-irons and serrate cockscombs on the east flank of the San Rafael Swell are visible ahead in reddish Wingate Sandstone and white Navajo Sandstone. San Rafael River crosses through the "Reef" through the notch at approximately one-thirty ahead.
158.0 West Edge of Green River. Interstate Highway 70 is constructed across Mancos Shale and valley fill along Saleratus Creek. Erosion of gently dipping Ferron Sandstone (fig. 5.1) produces rounded cuesta to the south which rises above the general erosional level on the Mancos Shale. Fossil ammonoids and various molluscs can be collected out of the laminated beds immediately below the silty remnant of Ferron Sandstone all along the outcrop band.
from Field Guide: Northern Colorado Plateau by J. Keith Rigby - Purchase Information
