Albuquerque, New Mexico to Flagstaff Arizona

0.0  Junction of Interstate Highways 25 and 40 in Albuquerque. Geologic Guide Segment 4 leads westward on Interstate Highway 40. See Geologic Guide Segment 3 for description along Interstate Highway 25 to the north.

5.8  Cross Rio Grande, north of the west-bank community of Atrisco. The river is a shallow braided, sandy stream, (fig. 4.1). On the west bank, terrace gravels are well exposed in road cuts and bluffs both north and south of the road. Road climbs onto a prominent terrace on the west side of the river.

Figure 4.1. View north along the Rio Grande from Interstate Highway 40. Bluffs along the west side of the river are in Santa Fe beds and river gravel.

6.6  To the north, basalt flows have poured down onto an upper terrace from the Albuquerque volcanos (fig. 4.2), three cinder cones visible to the north on top of the basalt.

Figure 4.2. North from approximately Mile 6.6 to the Albuquerque volcanos beyond a terrace armoured by a sheet of basalt.

7.2  Cross beneath a highway bridge where scoriaceous basalt has been used for ballast and a veneer over the sand at the bridge base.

8.9  Interstate 40 continues westward through sandy countryside over a gently rising sequence of terraces to the Llano de Albuquerque, in a rolling short-grass prairie.

The Llano de Albuquerque is capped by a caliche armor, as much as 10 feet thick, across Santa Fe beds.

17.7  Cerro Colorado (fig. 4.3) south of the road is an exhumed small volcanic cone in Santa Fe beds. The highway begins the descent from the Llano de Albuquerque to the valley of the Rio Puerco.

Figure 4.3. View south to Cerro Colorado from Mile 17.7. Cerro Colorado is a small volcano exposed again by erosion of the enclosing Santa Fe beds.

18.6  Basalt flows to the north have flowed to the south from a small center.

18.9  Immediately east of Rio Puerco Trading Post, columnar basalt shows well at the west end of the outcrop and is associated with terrace gravels of the Rio Puerco.

19.5  Cross Rio Puerco. West bluffs of the valley are sandy hills of rusty, yellowish-tan, very well-exposed Santa Fe Group beds. This is the approximate western boundary of the Rio Grande graben. At times the Rio Puerco is reported to carry about 45 percent of the Rio Grande sediment load and to have concentrations of suspended sediment of 42 percent by weight. It ranks as the second-highest sediment-carrying stream in the United States.

21.1  Road cuts in carbonaceous shale and sandstone of Cretaceous Mesa Verde Group (fig. 4.4), here dipping moderately to the east on the north side of the road. These exposures are in fault blocks along the western side of the Rio Grande trough. These relatively easily eroded rocks are capped by terrace gravel, much like that over which we have been riding.

Figure 4.4. View northwestward to carbonaceous shale and sandstone beds of the Cretaceous Mesa Verde Group arr Mile 21.1. These rocks are on the faulted western margin of the Rio Grande graben.

22.3  Mesa Verde rocks are overlain unconformably by the Santa Fe Group and form exposures around the base of Mesa Gigante to the northwest. High peaks to the north, beyond the mesas, are cored by small igneous intrusions.

23.6  Cross Rio Salado. Outcrops along the arroyo on the west are in Santa Fe beds. 24.9 Mesa Verde rocks in Puerco Fault Belt. This fault zone marks the eastern edge of the Colorado Plateau along this route.

28.2  To the northwest the massive east-dipping sandstone is Cretaceous, probably the Dakota Sandstone. The sandstone is above the variegated Morrison like beds in the mesas, but below Mancos Shale, below the road.

28.9  Bridge Over the Freeway at the Canoncito Exchange. Recent dune sands shown in the cuts on both sides of the road above, with Cretaceous beds just beyond the interchange both north and south of the road.

29.6  Gallup Sandstone at east end of a long road cut, with Mancos Shale exposed in the western part of the cut. In the far distance, ahead massive Cretaceous sandstone caps the surface of Mesa Gigante above Todilto and Entrada Formations.

32.9  Toward the north, the dark red Correo Sandstone of the Chinle Formation rises above the valley (fig. 4.5). Pink and white banded Entrada Sandstone overlies the Chinle beds and is capped by Todilto limestone and gypsum. These beds are overlain by reddish brown slope-forming Summerville Formation and ledge- forming Zuni Sandstone.

Figure 4.5. View northward from Mile 33.4 of Chinle beds, in the foreground, with overlying Todilto gypsiferous ledges and grey shales, with the entire sequence overlain by the relatively massive Entrada Sandstone.

33.3  Overpass Las Lunas Exit. Massive Mesa Gigante, to the north, shows a Chinle Entrada-Summerville-Morrison section overlain, in the distance, with Dakota Sandstone.

35.7  The land slides on the north have developed where the underlying Chinle Shales have heaved and the overlying sandstone has slumped (fig. 4.6).

36.4  Excellent view to the north of Dakota-Morrison rocks above the ledgy En

Figure 4.6. View northwestward at Mile 35.3 in upper Chinle beds which here have slumped after becoming wet and plastic, dropping blocks of the overlying sandstone.

  trada Sandstone, which rests on Chinle beds at the base of the bluff. Toward the northwest, on the skyline, is Mount Taylor, a rather large composite volcano in the San Mateo Mountains.

38.8  Cross the Atchison Topeka and Santa Fe Railroad Tracks. Toward the northwest the small village of Correo is in front of the Chinle-Entrada cliffs.

40.4  Looking toward the north the Correo Sandstone Member of the Chinle Formation forms a cliff, with Entrada Sandstone beds forming the gray bluff above, and with Summerville rocks eroding to the brushy slope in the middle distance below the massive Bluff Sandstone. The village is Mesita.

42.8  Bridge Over the Freeway at the Mesita Interchange. The gypsiferous beds show very well east of the gap of Arroyo Concho northeast of Mesita. Massive Correo Sandstone forms gaps on both sides of the road to the north and erodes to form prominent little buttes here and there, as the name implies, where the sandstone is particularly massive (fig. 4.7).

43.8  Peculiar slumps occur in the Summerville and Bluff beds (fig. 4.8) and must be related to flowage in the underlying beds, possibly the gypsum of the Todilto Formation before it was removed, or to almost quicksand like flowage of Entrada or older beds. Such flowage also could have produced the sandstone plugs visible here. To the northeast of the double road cuts the highway is entrenched through the Correo and Entrada sandstone. The tip of the Laguna Basalt flow is visible to the northeast. The Laguna flow is a columnar-jointed black basalt which has poured down the present-day drainage of Rio San Jose.

Figure 4.7. View northward from approximately Mile 43 with the massive Correo Sandstone above the lower part of the Chinle beds, east of Arroyo Concho.

Figure 4.8. View northeastward from Mile 43.8. The massive Bluff Sandstone has been folded by draping flowage. The chimney like pillar, to the right of center, is a massive sandstone plug. Both structures are relatively characteristic of parts of the Entrada and younger beds in this vicinity.

45.0  The highway cuts through the massive Bluff Sandstone sequence where fluvial and eolian cross bedding is quite evident in the sandstone.

46.0  West end of the cut. Steep dips show below the basaltic cover. Exit to Laguna Pueblo. The pueblo is on the canyon wall to the north (fig. 4.9), on massive tan Morrison Sandstone cliffs. Basalt dikes show in the exit area.

Figure 4.9. View northward to Laguna Pueblo from near the rest area along Interstate Highway 40.

46.8  New Laguna is by the water tower west of the old town.

47.4  Basalt flow near the highway on the south side has been buried by windblown sand but, to the west of town, the same basalt crops out along the valley floor and wall. An older basalt caps the ridge north of town and records several stages of flows and canyon development. Morrison beds are exposed behind the town, as well as in the steep bluffs to the west, beneath the basalt cap. The highway continues to the west in the valley of Rio San Jose cut into Morrison Formation. Upper sandstone beds of the Morrison Formation are much more continuous than those in the lower part of the section. Basalt of the Laguna flow is along the valley floor. Toward the north volcanic rocks rest on top of a stripped surface on the Dakota Sandstone and are part of the high, volcanic, Mount Taylor toward the northwest (fig. 4.10).

Figure 4.10. View northward to the Mt. Taylor area from approximately Mile 51.4. The volcanic pile is resting on Morrison Formation and Dakota Sandstone, and perhaps younger beds, beyond the valley margin.

52.7  Bridge at the Acoma, Paraje, and Casa Blanca Interchange. Junction of New Mexico Highway 23. The highway is in Morrison beds.

53.6  Morrison exposed in road cuts. To the west the small Cubero cinder cone sits on top of the basal Dakota Sandstone (fig. 4.11). Morrison beds are exposed in the valley wall.

Canyon Seama interchange is in Morrison beds which underlie the cinder cone to the north. The road swings to the right of the Cubero cinder cone and climbs up through the Morrison Formation north of Seama.

Figure 4.11. Cubero cinder cone is above Dakota Sandstone ledges and a slope zone on the Morrison Formation, as seen northwestward from Mile 53.1.

54.8  Bridge over Atchison, Topeka, and Santa Fe Railroad.

56.6  Road cuts in Morrison Formation with massive Dakota Sandstone at the rim fig.. 4.12) on both the east and west sides of the road, where the canyon cuts a gap through the cuesta. Mount Taylor rises to the north (fig. 4.13) above San Fidel Mesa (fig. 4.14).

57.2  Dakota Sandstone on both sides of double road cut, with basalt flows now both south and north of the road.

57.8  Small spatter cones on top of the flow in the valley to the north. In the distance, 15 miles to the north, is Mount Taylor, a large composite volcano, which has been visible for some distance. Cretaceous Sandstones form cliffs around the base of the volcano and are capped with basalt-armored terraces below the cinder cone.

Figure 4.12. Eastward to upper Morrison beds and the Dakota Sandstone which forms the rim of the canyon along a gap through the cuesta at Mile 56.6.

Figure 4.13 View north to Mt. Taylor across a massive ledge-forming Dakota Sandstone, as exposed north from the highway at Mile 56.7. San Fidel Mesa is on the Cretaceous beds in front of Mt. Taylor.

59.6  Mancos Shale forms the slope to the north, below bluffs held up by Cretaceous Sandstone beneath the volcanic pile of Mount Taylor, the peak of the San Mateo Mountains.

60.8  Pass Beneath the Bridge Near San Fidel, a small community to the north, but no interchange. The road continues to the west on the Dakota Sandstone which forms gentle cuestas to the south. The subsequent valley to the north is in Mancos Shale, with a characteristic regressive Cretaceous Sandstone forming the north cuesta of the valley.

63.2  Dakota Sandstone exposed in road cuts near the McCartys exit. The highway drops through the Dakota Sandstone into the Morrison Formation.

64.3  Cross Beneath Bridge at the McCartys Exit. To the west, massive Dakota Sandstone forms a northward-dipping cuesta, as well as a low mesa to the south. The road now is in Morrison Formation. Morrison Sandstones are characteristically light colored and weathered to a rounded surface, although with some iron-oxide staining, in contrast to the much more angular weathering, better cemented sandstones of the Dakota Formation. The Dakota cuesta, on the north, is occasionally blanketed by debris from overlying volcanic beds which stream down as dark plumes over the yellow sedimentary rocks.

66.1  Cross Beneath Atchison Topeka and Santa Fe Railroad. View up the canyon toward the west shows the tip of the Recent McCartys lava flow which poured down the canyon.

66.8  A thin streamer of lava extends eastward down the valley beyond here, on one side, but the main field begins here.

Figure 4.14. Generalized geologic map of the Mount Taylor volcanic area, of the northern Datil section and the southern part of the San Juan Basin, covering from the margin of the Rio Grande depression westward to Grants, New Mexico. Location of various uranium deposits and volcanic centers are shown around the southern edge of the San Juan Basin (modified from Beaumont, Dane and Sears, 1956).

67.2  Rest Area in westbound lane.

67.6  Opposite the rest area on the eastbound lane. Flow tubes, pressure ridges, collapse features, and the ropy surface (fig. 4.15) are well displayed in the immediate vicinity of the highway where small lakes and marshy areas have formed in the depressions (fig. 4.16). The highway continues on the lava surface of the Malpais for some miles to the west, with Morrison beds, to the south, and Dakota beds, to the north, forming the valley wall. These sedimentary units are largely blanketed by volcanic debris spilling over the upper surface.

Figure 4.15. Ropy pahoehoe-type lava crust along the flank of a pressure ridge in the McCartys flow at Mile 67.4. The shiny surface is typical of much of the fresh, almost glassy, basalt on the upper part of the flow surface.

71.2  Quemado Exit, in broken volcanic debris.

  Cross under bridge and return again onto the lava field. The McCartys flow originated in a volcanic field to the south, then flowed down into the valley of Rio San Jose. Beyond the Quemado exit the highway is on an older flow, also apparently derived from the south. The surface of the older flow is marked by pronounced pressure ridges, although the upper ropy or scoriaceous surface has been eroded (fig. 4.17).

Figure 4.16. View northward across the pressure ridges and the small marshy depressions on the crest of the McCartys basalt flow from Mile 68.4. Morrison beds are exposed at the base of the bluff in the valley wall beyond :he lava flow. Dakota Sandstone caps the cuesta on the skyline.

74.5  Volcanic peaks occur both to the light and the left. To the south the dark somber gray peaks on the skyline are of volcanic origin. To the north white tuffaceous units associated with a small volcano show very well in Tanner's Canyon.

Figure 4.17. View northeastward from north of the Quemado exit at Mile 72.1 across a slightly older flow than the McCartys flow. Pressure ridges and other major features show on the surface of the flow but the angular scoriaceous surface has been eroded.

75.5  East Grants Exit.

75.9  Cross beneath the exchange at the East Grants interchange.

77.1  Cross from an old lava flow onto a relatively recent one, with the scoriaceous surface still preserved. This flow is about the same age as the one at McCartys. The basalt has started to weather somewhat but the ropy scoriaceous surface is still preserved on the pressure ridges. A far older basalt flow caps the ridges north of Grants where slumped material has buried the volcanic rocks and upper Cretaceous beds.

79.2  Cross Beneath Birdge at the West Grants Interchange. The highway still continues over the basalt flow.

80.6  The road now is on alluvium north of the flow. Grants airport is to the northwest.

82.0  Bridge over the San Mateo-MilamAmbrosia Lake interchange. The broad valley here is carved in Chinle beds. Beds to the west are dipping eastward off the Zuni Uplift.

83.6  Far to the east, in the valley, can be seen the tailings pit from ore processed from the uranium mines of the Ambrosia Lake area.

84.4  Extensive road cuts through tan Sonsela Sandstone of the Chinle Formation. Some cross bedding shows in the somewhat wavy bedded surface. This tan sandstone forms the dip slope around the east and the northeast side of the Zuni Uplift and the highway has been constructed at about this stratigraphic level from here to near Gallup.

86.2  A view toward the west into the interior of the Zuni Uplift shows rocks older than the reddish Moenkopi beds which occur below the tan sandstone over which the road is constructed. Moenkopi beds are particularly well exposed to the west beneath the cuesta forming sandstone. Upper Chinle and Morrison beds are exposed in bluffs along the east side of the valley, beyond the Anaconda Mill.

89.1  Cross Beneath the Bridge of the Blue Water Village and Anaconda Exchange. Blue Water Village is toward the west and Anaconda toward the east. To the north, out in the center of the valley, is a small cinder cone from which a young basalt has flowed down to near the railroad tracks just north of the exchange. Road cuts are in a relatively old basalt flow. Toward the west soft Chinle beds are exposed as the maroon and gray green slope beneath the tan Sonsela Sandstone Member. Below the Chinle beds to the west Moenkopi rocks form the dip slope which is brush covered above the pink and brown Moenkopi beds of the canyon walls. Sonsela Sandstone Member of the Chinle Formation caps the cuesta on the west. Interstate Highway 40 is skirting through the limit of the basalt flow associated with the small cone to the east.

92.4  Valencia-McKinley County Line. 93.4 North edge of the basalt flow which here is lapping against the dip slope of Sonsela Sandstone. Entrada, Morrison and Dakota Formations are exposed in bluffs along the east side of the valley.

95.4  The highway leaves the broad alluvial valley and climbs up onto the Sonsela Sandstone cuesta. The broad flat valley here, in part, is fill related to damming by the basalt flows in the Grants area and, in part, to ease of erosion on the upper Chinle and overlying beds. Double road cuts in both the east and the west-bound lanes are in the Sonsela Sandstone.

98.2  Cross Beneath the Bridge at the Prewitt and Blue Water State Park Exchange. Road cuts east and west of the exchange are in pinkish beds of the Chinle Formation. Cuestas on the north are held up by

upper Chinle and Wingate-Entrada beds, capped by the Todilto Limestone below a slope zone on Morrison beds that is capped by Morrison Sandstone which is overlain by Dakota Sandstone (fig. 4.18). The highway continues essentially on the Sonsela Sandstone dip slope. The extensive north flank of the Zuni Uplift rises to the south through the pinyon and juniper forest.

Figure 4.18. View northward from Mile 104.9. The prominent cliffs above the Chinle Valley are Entrada Sandstone, overlain by the Todilto sequence which forms the strike valley beneath the Morrison Cliffs. Dakota Sandstone caps the escarpment along the skyline, here along the northeast flank of the Zuni Uplift at Mile 121.5.

102.3  Road cuts in the Sonsela Sandstone with some interbedded maroon and gray green shales of the Chinle Formation.

108.4  Bridge Over the Thoreau Exchange. Thoreau is the community to the north and is also a pump station for El Paso Natural Gas. The peak far to the north is in Mancos Shale with a protective sandstone cap. Rocks from Dakota down to Chinle Formation are exposed in the lower part along the highway. To the south resistant Sonsela Sandstone forms limited exposures in the wooded area.

112.5  Road cuts are through reddish Chinle beds above the Sonsela Sandstone.

113.8  Continental Divide, elevation 7,268 feet. The highway continues to skirt around the north flank of the Zuni Uplift, staying in lower Chinle beds. The valley to the north is in valley fill over upper Chinle beds Wingate and Entrada sandstones form prominent steep-walled cliffs to the north and northeast. Rocks as young as Dakota Sandstone are exposed around the slopes of Mount Porvell on the north. The brush-covered slope to the south is on north-dipping Triassic and Pennsylvanian rocks.

116.9  Coolidge. Bridge at the Exchange. Exposures of cross-bedded sandstone and green and purple shale of the Chinle Formation.

19.0  Roadside exposures of massive, white, cross-bedded Sonsela Sandstone Member of the Chinle Formation.

120.0  Small high pass with exposures of channel cut-and-fill Sonsela Sandstone Member in road cuts and in scattered gullies both north and south of the road, in the vicinity of the small oil refinery.

121.7  Underpass Bridge at Ciniza Refinery Shell Oil Company. The road now drops off the Chinle Sandstone outcrop belt onto the valley bottom of Rio Puerco of the West.

122.9  To the north Owl Rock Limestone Member of the Chinle Formation forms the lowest ledges beneath an orange red slope-forming Wingate Sandstone (fig. 4.19). Entrada Sandstone forms the cliff, above the valley and is capped by a thin, gray Todilto Limestone.

127.7  Wingate Station.

130.3  Interchange Wingate Ordinance Depot. Wingate Sandstone forms an orange red slope to the north of Interstate 401 below cliff-forming, well-bedded to cross-bedded Entrada Sandstone. Todilto Limestone caps the cliff.

132.5  Exchange to Indian Village and

  Kit Carson Cave. The "cave" is in the Jurassic section to the north. Navajo Church Rock is the prominent peak in Morrison beds to the north (fig. 4.19). Cuts along the highway are in limestone in the Chinle Formation. Massive cliffs of Entrada Sandstone and overlying units form the north wall of the valley cut in Chinle beds (fig. 4.20).

133.5  Pyramid Rock, to the north (fig. 4.20), has a lower cliff in Entrada Sandstone overlain by thin Todilto Limestone. Summerville, Morrison, and Dakota rocks are exposed on the canyon wall and up to the peak. To the south, beyond our view, Precambrian rocks are exposed in the core of Zuni Uplift. These old rocks are overlain unconformably by Permian rocks.

Figure 4.19. View toward the north at Mile 121.5 from the north end of the Zuni Uplift, across the broad valley carved in Chinle beds. Entrada Sandstone forms the first prominent cliff, with Morrison beds forming the interrupted cliffs along the buttes in the background. The high butte toward the right is capped by Dakota Sandstone.

136.4  Nutria Monocline East of Gallop.

  Jurassic and Cretaceous rocks dip steeply toward the west so that we go very quickly through the stratigraphic section. Morrison beds show as massive light gray sandstone and softer gray to maroon shale. Dakota Sandstone holds up the eastern fin of "The Hogback". Mancos Shale forms the shallow strike valley and is overlain by the ledge-forming coal-bearing Mesa Verde Group (fig. 4.21). The monocline here has steep dips but within a few hundred yards east and west of the flexure the beds become flat again.

137.3  Abrupt flattening on the west side of the monocline. Coal-bearing Cretaceous Crevass Canyon and Dilco Formation are exposed to the south and north. The community of Gallup is in a syncline in Mesa Verde rocks. Pinkish clinkered beds to the north are in the coal-bearing Crevasse Canyon Formation, in the Mesa Verde Group, where coal has burned and baked the overlying rocks.

Figure 4.20. View north-northeastward to Navajo Church Rock, from Mile 132.5. Navajo Church Rock is in the Morrison Formation. Entrada Cliffs flank the notch. The valley in the foreground is on Chinle beds.

Figure 4.21. View northwestward to Pyramid Rock from Mile 133.5. Massive Entrada Sandstone, in the foreground, overlies orange Wingate beds at the base of the cliff. Thick Morrison Sandstone forms most of the overlying sequence, with Dakota Sandstone at the peak.

137.7  Coal exposures on the south are beneath the main Gallup Sandstone of the Mesa Verde Group.

139.7  Gallup Sandstone outcrops along the Rio Puerco, north of the railroad station, are approximately on the synclinal axis.

140.0  Junction of U.S. Highway 666 With Interstate 40 and U.S. Highway 66 in Downtown Gallup. U.S. Highway 666 leads north across the western part of the San Juan Basin to Shiprock and Cortez. For a description of the geology along that route see Guide Segment 5. Continue west on U.S. Highway 66.

141.2  In the motel stretch on the west side of Gallup. East-dipping Crevasse Canyon Formation of the Cretaceous Mesa Verde Group shows well to the north, on the east flank of a small anticline.

149.7  Junction of Main Street with Interstate Highway 40 at the west end of Gallup.

149.9  Begin freeway west. To the south is small Twin Buttes intrusion (fig. 4.22). The mass has been quarried for road metal for Interstate Highway 40.

150.2  Cross crest of small anticline. 151.2 Cross through cuestas of Mesa Verde Sandstones into Mancos Shale.

151.8  To the north, coal-bearing Mesa Verde rocks are exposed along the east flank of a small anticline (fig. 4.23), above well-exposed Mancos Shale (fig. 4.24).

Figure 4.22. View southeastward along the Nutria Monocline at the east edge of Gallup. Dakota Sandstone forms the prominent hogback, barely visible toward the left. Mesa Verde Group Sandstone forms the prominent ledges along the skyline to the right.

Figure 4.23. View southward to Twin Butte Intrusion from Mile 150. Bedded Mesa Verde rocks form the base and the country rock for the intrusion.

Figure 4.24. View northward to massive Gallup Sandstone and the gentle cuesta of Mesa Verde rocks at Mile 151.2.

152.6  Approximate axis of Torrinio Anticline (fig. 4.25).

153.2  The west flank of the anticline is expressed in alternating sandstone, shale, and coal within the Mesa Verde Group in double road cuts. Gallup Sandstone is the thick-bedded unit.

153.6  Cross the axis of a small syncline. Rocks now are dipping to the east directly ahead.

155.6  The Cretaceous Mesa Verde Sandstone is exposed north and south of the road and in the road cuts on the south. Carbonaceous streaks and coal can be seen beneath the channeled sandstone.

Figure 4.25. View northward to excellent exposures of Mesa Verde rocks and upper beds of the Mancos Shale at Mile 151.8. These exposures are on the east flank of the Torrinio Anticline.

156.5  Excellent exposures of a Cretaceous coal-bearing lagoonal section occur in the road cut. Exposed are a series of fluvial sandstone-filled channels, coal, and carbonaceous shales.

157.8  Defiance and Manuelita Exit in Mancos Shale below the Mesa Verde coal-bearing sequence. Gallup Sandstone forms the rim to south and north.

158.1  Cross Rio Puerco.

160.2  Road begins to rise over Dakota Sandstone at the base of the Cretaceous section. The cross-bedded sandstone is exposed beneath a red, windblown soil in road cuts.

161.6  Thick, massive, Morrison Sandstone in the upper part of the sequence is exposed on either side of the valley walls, beneath the Dakota Sandstone (fig. 4.26). On the south side, a few carbonaceous streaks are visible on top of the main massive sandstone. Reddish Morrison sediments separate two massive sandstones at the western edge of the outcrop.

Figure 4.26. View eastward at Mile 161.6 to massive white Cow Springs Sandstone overlain by angular-weathering beds of Dakota Sandstone.

162.8  On the south side, massive, cross-bedded white Entrada Sandstone has large amphitheaters and alcoves cut into it (fig. 4.27) near the highway rest area.

164.5  Cross Rio Puerco in valley fill.

165.4  Bridge Over Atchison Topeka and Santa Fe Railroad. Several large alcoves have been cut into the massive white Entrada Sandstone.

166.2  New Mexico—Arizona State Line.

Figure 4.27. View eastward from Mile 162.9 of Morrison and Dakota beds. The prominent alcove is in the Cow Springs Sandstone of the Morrison Formation, which is separated from upper Morrison beds by a shaly slope zone in the middle part of the cliff. Dakota Sandstone caps the sequence here, on the east side of the Rio Puerco Valley.

166.5  Entering Navajo Indian Reservation. Minor cross beds show in the alcove-forming Entrada Sandstone.

167.4  Lupton. Massive Entrada Sandstone is overlain by cliff-forming Cow Springs Sandstone and reddish Morrison beds, as the slope former. Dakota Sandstone caps the bluff (fig. 4.28). From a distance the Entrada Sandstone appears to be horizontally bedded, but in outcrops close at hand it is seen to be complexly cross-bedded. Rocks dip gently to the east and reddish Wingate Sandstone appears at the base of the cliff to the west (fig. 4.29).

Figure 4.28. View southeastward at Mile 168 of Morrison and Dakota rocks along the southeast side of the broad gap near Lupton. Morrison Sandstone forms most of the prominent cliffs, above the massive Entrada and Cow Springs sandstone. Dakota Sandstone forms the cap on the skyline. Chinle beds are exposed toward the left of the photograph.

167.8  Sandy Wingate and Chinle beds in cuts on either side of Interstate Highway 40 and U.S. Highways 66 and 666.

168.3  Lupton—Window Rock Exchange. Typical variegated shales of the Chinle Formation are exposed near the road. The entire sequence from Chinle and Wingate beds up through Dakota Sandstone is visible to the north and south. The highway is built on Chinle Shale, which is a poor roadbase. The freeway has been built only a relatively short time yet is beginning to develop a wavy pattern. Interstate 40 continues for some distance to the west across Chinle Shale.

Figure 4.29. View northeastward to Entrada and Morrison beds in the prominent bluff at Lupton. Well-bedded rocks toward the left foreground are the reddish Wingate Sandstone.

170.0  Shale of Petrified Forest Member of the Chinle Formation is capped by Sonsela Sandstone Member of the Chinle Formation. This sandstone divides the shaly Petrified Forest Member into an upper and lower part. The Sonsela Sandstone pinches out to the west.

174.4  Cross Beneath Bridge at Allentown Road Interchange near Houck and Tegawitha Mission. An extensive series of Indian displays and exhibits has been built for tourists.

177.6  Tourist-oriented Fort Courage on the north. The highway continues on west in Chinle beds through the Navajo Indian Reservation.

180.2  Double road cuts down through resistant Shinarump Conglomerate. The sandstone thickens to the south and overlies Moenkopi beds. Near Querino Wash the massive Shinarump Sandstone is at bridge level, beneath Chinle Shale, particularly in the west-bound lane.

182.1  Bridge over Querino interchange, in Chinle beds.

184.1  Cross Beneath Bridge at the Cedar Point Interchange. Reddish Lower Chinle beds near the road dip gently to the south.

184.9  Leaving the Navajo Indian Reservation.

185.6  Arizona Port-of-Entry Station.

186.4  Cross Beneath Bridge of Sanders Exchange. U.S. Highway 666 separates and heads toward Saint Johns, to the south. Interstate Highway 40 continues to the west in Chinle beds.

187.4  Striking greenish shale in the lower Chinle Formation.

192.4  Chambers Interchange. Arizona Highway 23 leads northward to Chinle and Canyon De Chelly National Monument.

196.0  Bridge of McCarrell Road interchange. The highway continues to the west heaving its way across lower Chinle beds.

200.1  Cross Beneath the Bridge at the Navajo Interchange. The small Kerr-McGee helium plant, here, is supplied by gas from the Permian Coconino Sandstone where that formation has been penetrated on the Pinta Anticline, 4 miles to the south.

203.0  Cross Crazy Creek. Kind of boring country isn't it. The highway continues westward on essentially the same stratigraphic units that we have been traveling over for the past 30 miles. Beds are dipping gently toward the north into the Black Mesa Basin.

205.7  Sonsela Sandstone, still part of the broad sheet like unit that we've been traveling along for some distance.

206.1  Cross Beneath the Bridge in the Pinta Interchange.

209.4  Red Petrified Forest Member of the Chinle is exposed in semibadlands near the road. In general the Chinle beds are only shallowly weathered and buried by drifted sand.

210.0  Cross Dead River. A rest area has just been constructed on the west side.

211.5  Buttes, mesas, and residual masses of Chinle beds are exposed in the gully to the south. The Sonsela Sandstone persists alongside the road and capping the mesas to the south.

213.0  Somber dark gray and maroon beds of the Petrified Forest Member are exposed in bluffs on the north.

214.4  Exit to Petrified Forest National Park. The monument stretches both south and north of the highway. See HW-180 Road Guide. for a description of the geology through the monument and then into Holbrook along U.S. Highway 180.

215.0  ark access road rejoins Interstate 40 heading west toward Holbrook. Minor basalt flows occur to the north in the Painted Desert. Visitors Center for the Painted Desert section of the park is north of the freeway. The Painted Desert parallels the highway on the north almost to Holbrook. The highway continues southwestward over the lower part of the Chinle Formation.

228.6  Small knobs on the skyline to the north are in the Hopi Buttes volcanic field. The volcanic activity there was of Pliocene age, when nearly 200 centers of volcanic action produced pyroclastic cones, lava flows, or diatremes. Bedrock grades from Tertiary on the north to Triassic on the south.

237.6  Enter Holbrook.

239.0  Shinarump Sandstone in lower Chinle beds on both sides of the highway. This unit has produced considerable fossil wood near here. The member is siltier here than at most localities.

239.5  Cross beneath freeway bridges. The highway descends from the terrace on the north down onto the floodplain of the Little Colorado River in the main business district.

240.3  Junction of U.S. Highway 180, Arizona State Highways 77 and 377 With U.S. Highway 66 and Business Route Interstate Highway 40 in downtown Holbrook. U.S. Highway 180 leads eastward to Petrified Forest National Park. For a description of the geology along that route through the park see HW-180 Road Guide.. Turn westward and follow U.S. Highway 66 and Interstate 40 toward Winslow and Flagstaff.

241.5  The west edge of business district of Holbrook. Rocks to the north are of the Moenkopi Formation overlain by the Holbrook Sandstone Member. The highway to the west rises off the floodplain of the Little Colorado River.

244.6  Road cuts through Moenkopi Formation.

245.9  Veined Moenkopi Formation with criss-crossing gypsum veinlets is exposed in road cuts on the north.

248.2  Road cuts of Moenkopi Formation show the channeled lenticular nature of the unit (fig. 4.30). The section here is capped by the Holbrook Sandstone.

Figure 4.30. View northeastward to channeled and veined Moenkopi beds at Mile 248. Holbrook Sandstone Member of the formation caps the road cuts and the escarpment.

249.0  Cross beneath power lines. To the south is the Cholla electric generating plant.

250.0  Side road to the south to the Cholla Plant and Cholla Lake. Road cuts to the west are through Little Colorado River silt.

251.5  Joseph City. The road continues along the floodplain of the Little Colorado River with road cuts in river silt. Road cuts in the bedrock are in the Moenkopi Formation and the bluffs to the north are held up by the Holbrook Member of the formation.

256.0  Cross Manila Wash.

257.6  Bridge over the >strong>Jackrabbit interchange.

261.0  Rest area on the north. Moenkopi Sandstone forms the isolated blocks and ledges in which the picnic areas have been constructed (fig. 4.31).

263.0  Bridge over the Hibbard Road interchange. Green and red Moenkopi beds are exposed in road cuts on the north. These rocks are dipping northward into Black Mesa Basin off the Mogollon Rim, which is in the far distance to the south.

268.2  Cross Cottonwood Wash.

269.2  Temporary end of the freeway. Junction of Arizona State Highway 87 which leads northward to Second Mesa and the Hopi Indian Reservation.

Figure 4.31. Moenkopi Sandstone exposed in isolated blocks and ledges along the north side of the road at Mile 261. The rest area has taken advantage of the irregular topography formed by these moderately resistant units.

270.9  Cross the Little Colorado River. 272.6 Enter Winslow.

273.7  Junction of Arizona State Highway 65 with U.S. Highway 66 and Interstate 40. The Arizona highway leads south to Phoenix over the Mogollon Rim. Continue westward through town on toward Flagstaff.

275.8  Leave Winslow.

276.8  West of town the highway climbs up through double road cuts in Moenkopi beds. To the north the basal part of the Chinle Formation is exposed. The rim of Meteor Crater, or Canyon Diablo, shows to the southwest at about ten o'clock, as a low ragged, flat-topped ridge in front of the ridge on the skyline. To the west the San Francisco Peaks are visible on the skyline. Mount Humphreys is the highest peak, with an elevation of 12,670 feet.

282.8  Bridge Over the Interstate Highway at the Leu Exchange. The prominent sandstone cuesta to the north separates the Moenkopi, below, from the Chinle beds above. Meteor Crater rim rises at about ten o'clock as the light-colored ragged ridge, approximately 12 miles to the southwest (fig. 4.32). From this vantage point several of the smaller isolated volcanic cones, show on the plateau toward the west, east of the main San Francisco composite volcanic peak.

284.0  Low road cuts and borrow pit cuts through Moenkopi beds. Sunset Crater and the northeastern part of the San Francisco volcanic field show toward the north of San Francisco Peaks, directly ahead. The Painted Desert section is to the north in the badlands bluffs visible on the horizon.

287.9  Interchange at Red Gap road in Moenkopi beds.

  The highway continues through Moenkopi beds now forming broad low bluffs both north and south of the road.

Figure 4.32. Meteor Crater, as seen from a short distance south from Interstate Highway 40, looking towards the south. The museum and visitors center is along the rim crest toward the left (photography by W. K. Hamblin).

289.3  Double road cuts through red Moenkopi Formation.

290.5  Sunshine Overpass over the Atchison, Topeka and Santa Fe Railroad. Outliers and buffs to the north are of a massive sandstone near the base of the Moenkopi Formation, that, immediately west of the bridge, is seen resting on Kaibab Limestone, which forms the tan bluffs in the gully to the north and the south.

291.3  Roadside rest area built in a basal sandstone of the Moenkopi Formation. To the west the highway continues through the lower Moenkopi Sandstone.

293.7  Interchange Bridge at Exit 233 to Meteor Crater. For a description of the geology along the access road see I-40 Road Guide.. The highway continues through the basal part of the Moenkopi Formation, with the resistant sandstone forming pedestaled rocks here and there.

296.8  Cross beneath the bridge of the Two Guns exchange. Ruins, to the south, are modern and are in upper beds of the Kaibab Limestone. The contact of the Moenkopi redbeds on Kaibab Limestone is exposed at the bridge interchange.

297.4  Bridge Over Canyon Diablo. Upper rim is of the Kaibab Limestone. From here the highway continues to climb toward the high elevations at Flagstaff. From here to Flagstaff we cross the upper stripped surface of Kaibab Limestone. To the south outliers of Moenkopi Formation form the forested hills in the national forest along Mogollon Rim.

302.3  Bridge over the Buffalo Range Road interchange. From high points either east or west of the small valley, Meteor Crater is visible to the southeast along the skyline. To the north lava flows and volcanic cones of the eastern part of the San Francisco volcanic field show very well. Most of the mounds are tephra cones (fig. 4.33) and many of them are flanked by flat mesas armoured with basalt flows.

Figure 4.33. View northward from Mile 307.5 of the southeasternmost tephra cone in the Sunset Crater volcanic field. Light rocks in the foreground are of the upper part of the Kaibab Limestone.

307.8  Cross beneath the bridge at the Twin Arrows Exchange. The road continues across the stripped top of the Kaibab Limestone which here and there is littered with volcanic debris from the San Francisco volcanic area.

310.2  Small hill to the south is a tephra cone which is being mined for road metal. It is the southeasternmost of the small volcano

of the recent basaltic eruptions of the San Francisco-Sunset Crater area. The road continues westward on the top of the Kaibab Limestone and climbs up into a juniper and scattered pinyon forest.

312.5  The highway is on volcanic ash and cinders. All the low mounds to the north are small volcanic cones or are remnants of lava flows from the main center, which is still to the north of us.

313.6  Kaibab Limestone is exposed beneath the volcanic cover in road cuts and gullies to the north and the south. To the north, beyond the railroad, basalt flows have angular ledges at their southern terminations.

314.6  Permian Kaibab Limestone is exposed in double road cuts. The tip of a basalt lava flow forms the bluffs to the north. To the west a quarry in one of the tephra cones supplies fragmental debris used in construction and maintenance of the railroad and highway.

315.5  Cross beneath power lines providing electricity for central Arizona from the Glen Canyon Dam. Road cuts beyond are through a basalt flow. There is a small quarry in an armoured tephra cone to the south.

316.3  Cross Beneath the Bridge at the Winona Exchange. To the north extensive quarries (figs. 4.34, 4.35) in one of the volcanic cones are utilized by the railroad. Double road cuts beyond are through massive basalt. To the west the highway drops down through Kaibab Limestone exposures.

317.5  Double road cuts through Kaibab Limestone. To the northwest Humphreys Peak rises as the high point of the San Francisco Peaks. Humphreys Peak and the other associated peaks are part of an old composite volcano. Mount Elden is a subsidiary volcano along the south side and is the lower forested hill at the south edge of the major cluster of high peaks. Mount Elden is a younger volcano which poured very thick, pastey, dacite flows that helped make the steep slopes of the southeast part of the mountain.

Figure 4.34. View northward from near the Winona Exchange, at Mile 316.5, to extensive quarries in one of the tephra cones in the southern part of the Sunset Crater volcanic field.

Figure 4.35. View northward from the Winona Ex:hange of the extensive railroad quarries in one of the volcanic cones. The fragmental volcanic debris is itilized as road ballast for the railroad and the highway.

319.3  Rest area. The younger volcanic rocks of the Sunset Peak area are principally basalt. They form the low tephra cones and lava flows to the north of the highway.

320.3  Bridge of the Cosino Exchange, in Kaibab Limestone.

321.3  Forest now is a mixture of Ponderosa pine and pinyon pine. Railroad fills to the north show the extensive use of even coarse tephra and bombs from the nearby volcanos.

322.7  Cross Beneath the Walnut Canyon National Monument Interchange, in Kaibab Limestone. Walnut Canyon Monument, to the south, protects a series of pueblo-type Indian ruins in a canyon which is in Kaibab Limestone. The forest here is principally an open Ponderosa pine-pinyon pine forest.

324.4  Road cuts through badly weathered Kaibab Limestone. Another volcanic cone is being "eaten" into by a series of quarries on the north (fig. 4.36), beyond the railroad. Thick jointed lava flows show very well on Mount Elden, ahead. Paleozoic rocks are exposed on the north flank of the mountain, on the dome margin (fig. 4.37).

325.6  Double road cuts through Kaibab Limestone.

326.1  Exit to U.S. Highways 66, 89 and 180. Leave Interstate 40 and head into the eastern edge of Flagstaff.

326.5  Stop sign, turn north toward U.S. Highway 89.

326.8  Bridge over Interstate highway and railroad tracks.

327.0  Junction With U.S. Highway 89 East of Flagstaff. End of I-40 Road Guide.. For a description of the geology along U.S.

Figure 4.36.. View northward to a small tephra cone from Mile 324.4 near the southwestern margin of the Sunset Crater volcanic field. Utilization of the debris is evident along the railroad in the foreground.

Figure 4.37. View toward the west of Mt. Elden, from approximately Mile 325. Paleozoic rocks form the cliff along the right northern margin of the peak. The steep light slope towards the left is held up by thick pasty lava that formed on the southeast part of the dome.

from J. Keith Rigby, by Field Guide: Northern/Southern Colorado Plateau, Kendall/Hunt Pub. Co., 1976, 207 pages -Purchase Information
used by author's permission