GEOLOGIC ROAD AND TRAIL GUIDE TO SNOW …utahgeology.org/road_logs/uga-29_first_edition/SP_guide/...Figure 4 - Geologic map of the Snow Canyon State Park area (modified from Willis

Geologic Road, Trail, and Lake Guides to Utah’s Parks and Monuments 2000 Utah Geological Association Publication 29 P.B. Anderson and D.A. Sprinkel, editors

INTRODUCTION

Snow Canyon State Park is located in the southwest corner of Utah in Washington County, northwest of St. George (figure 1). Snow Canyon is approximately 5 miles (8 km) long, with elevations in the park ranging from 3,000 feet (915 m) at the mouth of the canyon to 5,024 feet (1,532 m) at the top of a sandstone peak northwest of the canyon. Because of southwest Utah’s mild winter climate, the park is open all year. Use caution if hiking during the hot summer months. There are more than 20 miles (32 km) of hiking and biking trails, and more than 5 miles (8 km) of equestrian trails. The park is physiographically located in the transition zone between the Colorado Plateau to the east and the Basin and Range Province to the west, which also roughly coincides with the leading edge of the Sevier orogenic belt. The rocks of Snow Canyon have been uplifted and tilted northeastward as the west limb of the northward plunging St. George syncline (Higgins and Willis, 1995). This movement caused the brittle, massively bedded, Jurassic Navajo Sandstone to fracture in two intersecting sets of parallel cracks, called joints. These joint sets, along with the very uniform nature of the cross-bedded sandstone, control the weathering pattern of the rock to form the intricate landscape of the canyon (figure 2). The jagged and irregular, dark surface of Quaternary basaltic flows create a stark contrast with the rounded shapes of the red and white sandstone. Three episodes of volcanic activity document the continued uplift and subsequent downcutting of the area.

GEOLOGIC ROAD AND TRAIL GUIDE TO SNOW CANYON STATE PARK , UTAH Janice M. Higgins Utah Geological Survey Salt Lake City, Utah

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Figure 1 - Location of Snow Canyon State Park.

J.M. Higgins Geologic Road and Trail Guide to Snow Canyon State Park, Utah

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Bluff Street in St. George continues north as State Route 18. Turn west (left) onto Snow Canyon Parkway and proceed 3.9 miles (6.5 km). At the intersection with Snow Canyon Drive, turn north (right). This intersection marks the beginning of the road/trail log for the park (figure 3). Snow Canyon Drive continues through the park to intersect State Route 18.

MILEAGE DESCRIPTION

INTERVAL/CUMULATIVE 0.0 0.0 Intersection of Snow Canyon Parkway (Ivins’ Center Street) and Snow Canyon

Drive. TURN NORTH (RIGHT) ONTO SNOW CANYON DRIVE. The road to the immediate left leads to Tuacahn Amphitheater in Padre Canyon; stay straight. Note the Ivins slump to the west (left) at the front of Red Mountain where a slump block of Navajo Sandstone from the top of the mountain rests on the Kayenta Formation at the base of the mountain. The Kayenta Formation, which forms the lower, horizontally bedded portion of the surrounding mountains, is Early Jurassic in age (Imlay, 1980) and was deposited in fluvial, distal fluvial/playa, sabkha and minor lacustrine environments (Sansom, 1992). This formation contains nearly all of the known dinosaur footprints found in the St. George area; however, none are known within Snow Canyon State Park boundaries. Only the upper member of the Kayenta Formation (Jku) is exposed within Snow Canyon State Park (figure 4). It is 800 feet (244 m) thick (Willis and Higgins, 1996).

0.8 0.8 STOP 1 - PARKING LOT OF PROPOSED RED CLIFFS DESERT

RESERVE EDUCATION CENTER AND AMPHITHEATER at the mouth of the canyon. Just ahead is the fee station to gain entry into Snow Canyon State Park. A trail across the Santa Clara flow leads to the drainage nick point and beyond, to Johnson Arch. The Johnson Arch portion of the trail is currently only open November 15th through March 1st due to the presence of nesting peregrine falcons and other sensitive species. When open, hiking is allowed only on the

Figure 2 - Oblique aerial view looking north from the mouth of Snow Canyon. Padre Canyon, to the west of Snow Canyon, is along the left edge of the photo. State Route 18 is near the right edge. Weathering pattern is controlled by joints in the Navajo Sandstone.

P.B. Anderson and D.A. Sprinkel, editors 2000 Utah Geological Association Publication 29

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Figure 3 - Detailed map of field trip route and stops. Base from reduction of USGS 7 2= topographic maps.


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Figure 4 - Geologic map of the Snow Canyon State Park area (modified from Willis and Higgins, 1995 and 1996). Description of map units and cross section A-A= shown on figure 5.


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designated trail because of the delicate riparian area created by springs in the side canyon near the arch. Hiking to the top of the arch is prohibited, but the view from the base of the arch justifies the hike. The trail is 1.5 miles (2.5 km) round trip from the parking lot.

The gradational contact between the Kayenta Formation (Jku) and the Navajo

Sandstone (Jn) is near road level since the rocks are dipping seven degrees to the northeast (figure 5). The contact has previously been placed at the top of the highest playa sand, thereby including transitional strata in the Kayenta Formation (Bugden, 1992; Hintze and Hammond, 1994). This included a significant section of eolian sand in the Kayenta Formation and placed the contact high on the cliff, since the transition zone here is up to several hundred feet thick (Tuesink, 1989; Sansom, 1992). However, placing the contact at the base of the transition beds makes it easier to map and coordinates this area with mapping in the region to the east, including Zion National Park (Doelling and others, Utah Geological Survey, unpublished mapping). Also, this placement of the contact can be easily correlated on well logs and seismic lines. The contact, which is now placed at the top of the highest mudstone interval, corresponds to a slight color change with darker reddish-brown, thinner bedded, horizontal strata of the Kayenta Formation below and lighter, moderate-reddish-orange, planar to massively cross-bedded and vertically jointed Navajo Sandstone above (figure 6) (Willis and Higgins, 1996).

Figure 5 - Map units and symbols for the geologic map of Snow Canyon State Park. Unconsolidated map units are not shown on the generalized cross section A-A=.


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The Navajo Sandstone and correlative sandstones are famous as one of the world’s largest coastal and inland paleodune fields, which covered much of what is now Utah and portions of adjacent states in the Early Jurassic (Blakey and others, 1988), between 190 and 180 million years ago (Peterson and Pipiringos, 1979). Paleowinds of this Sahara-like desert blew mostly from the north and created dunes that would become the Navajo Sandstone (Sansom, 1992). The source of the sand is unknown. Peterson (1988) suggested that it is likely recycled from Paleozoic and Triassic sandstone to the north in Montana, perhaps as far away as Alberta, Canada, with minor additions from the east margin of the marine embayment to the west. However, Marzolf (1988) proposed that the ultimate source of sand was transported northwest by Kayenta streams, reworked along the shore, and blown back onshore to form dunes. Except for the basal transition beds, the 2,500-foot-thick (760 m) Navajo Sandstone consists of massively cross-bedded, fine- to medium-grained, well-rounded and frosted quartz grains that are poorly to moderately well cemented (Willis and Higgins, 1996). This highly jointed sandstone weathers to form the bold, rounded cliffs for which southern Utah is famous. The basal transition beds consist of thick cross-bedded eolian intervals, interbedded with thin, planar-bedded sandstone, muddy sandstone, and minor mudstones with crinkle bedding, tepee structures and mineral casts formed by the growth of minerals such as gypsum and halite. These transition beds represent wind-blown sand deposited on a sabkha surface (Sansom, 1992). The planar bedding formed by eolian sand blowing across the sabkha and adhering to the wet surface.

The youngest lava flow (Qbs) within Snow Canyon State Park, and the entire

region, is the Santa Clara flow (figure 6, foreground). It is geochemically classified as hawaiite by Best and Brimhall (1974). Using the classification of Le Bas and others (1986), it plots as a subalkaline basalt. It has a distinctly higher iron content than other flows in the area (Willis and Higgins, 1996). The

Figure 6 - Contact between the Jurassic Kayenta Formation, below, and the Navajo Sandstone, above, at the mouth of Snow Canyon. Note the Santa Clara flow in the foreground. View looking east.


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rock is dark brownish black, dense, and aphanitic, with abundant small olivine crystals.

The Santa Clara flow has not been radiometrically dated. Attempts to find

charcoal beneath the flow for carbon-14 dating have been fruitless so far and the flow is too young to obtain a reliable 40Ar/39Ar age. Hamblin (1963, 1987, and unpublished mapping) classified the flow as stage IV (in a modern drainage but shows little evidence of erosion or alteration) and estimated it as slightly more than 1,000 years old. However, in 1985, hikers discovered human skeletal remains along with an atlatl partially buried by 2.5 inches (6 cm) of sand inside a lava tube cave. Features of the atlatl indicate that it was probably manufactured during late Archaic to Basketmaker II times, some 1,500 to 2,500 years ago (Madsen, 1992). A similar atlatl from nearby Antelope Cave, just south of the Utah and Arizona border, dates to 1,850 + 60 years before present (Janetski, 1984). The flow must be sufficiently older than those dates to allow enough weathering time for the roof of the lava tube to collapse, permitting access to the cave. Additionally, the iridescent sheen typical of young basalt flows has been mostly weathered away, and pedogenic carbonate, which typically takes a few thousand years for noticeable amounts to accumulate (Machette, 1985; Birkeland and others, 1991), coats joints in road cuts along State Route 18 to the north. For these reasons, the flow is probably 20,000 to 10,000 years old (Willis and Higgins, 1996).

A short walk across the jagged aa surface of the Santa Clara flow on the trail

heading east allows viewing of the drainage nick point, which has a vertical drop of 40 feet (12 m). Upstream from this point, the wash bottom is on top of the flow, but downstream from that point the stream has cut a gorge along the edge of the flow in which the base of the flow is 30 to 40 feet (9-12 m) above the wash bottom. This nick point is created by headward erosion of the intermittent drainage returning to a former base level. There are no cuts across the flow that would help determine whether the wash has been cut lower than the thickest part of the flow.

The trail continues into a side canyon to Johnson Arch, a natural arch in the

Navajo Sandstone. The arch is about 200 feet (60 m) in length and formed in a narrow fin of sandstone as joints were enlarged by weathering. Undercutting of the sandstone along a spring line occurs as cementing minerals are removed from between the sand grains. This undercutting leads to spalling, which creates an incipient arch in the rock. With continuing weathering and erosion along joints and spring lines, the arch that has formed will eventually collapse.

Looking up the road, past the park entry station, provides an excellent view of

Island in the Sky (figure 7). This 600-foot-tall (182 m) monolith of Navajo Sandstone is largely separated from the rest of the mountain because of weathering along a zone where joints are highly concentrated. CONTINUE NORTH PAST THE FEE STATION.


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0.7 1.5 STOP 2 - JENNY’S CANYON TRAIL PARKING LOT. This 0.5 mile (0.8 km) round trip walk leads to a narrow slot canyon developed along closely spaced joints in the Navajo Sandstone along the southeast edge of Island in the Sky. Weathering along the joints continues to widen the canyon. Note the Santa Clara flow, which is largely covered by sand weathered from the Navajo Sandstone. Look carefully at the basal beds of the Navajo Sandstone for crinkle bedding, tepee structures and mineral casts, indicative of a sabkha environment.

0.1 1.6 To the left are poorly formed dunes composed of sand that has weathered from

the Navajo Sandstone. The sand accumulates in thin sheets and irregular hummocky mounds on gently sloping areas in Snow Canyon and in depressions and protected areas on the Navajo Sandstone throughout the park. Here, the sand piles up in the mouth of the small side canyon to depths of 50 feet (15 m) (figure 8) (Willis and Higgins, 1996). The parking lot to access the dunes is ahead. TURN LEFT INTO THE PARKING LOT.

Figure 7 - Sandstone monolith AIsland in the Sky@ is largely separated from the rest of the mountain to the east because of weathering along a zone where joints are highly concentrated. Photo looking north from the mouth of Snow Canyon.

Figure 8 - Poorly formed sand dunes at the mouth of a small side canyon near the mouth of Snow Canyon. The sand is derived from the weathering of the Navajo Sandstone. Photo looking west.


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0.3 1.9 STOP 3 - SAND DUNE AND WEST CANYON TRAIL PARKING LOT. Picnic and restroom facilities are available. The horse trail that extends through Snow Canyon begins here. The 3.5-mile-long (5.6 km) gravel road that goes north up West Canyon is maintained to provide access to St. George City water wells. Although private motorized vehicles are prohibited, mountain bikes and foot traffic are permissible. These wells, drilled to depths of 500 to 850 feet (150-258 m), produce between 200 and 700 gallons per minute from the Navajo Sandstone (Horrocks-Carollo Engineers, 1993), which is the major aquifer in the area (Willis and Higgins, 1996; Hurlow,1998).

Across the road on the nearly vertical wall of Island in the Sky, differential

erosion is evident along certain bedding planes (figure 9). Jokingly referred to as “rockpecker holes,” these small circular holes erode at a faster rate than the surrounding rock, perhaps because the sand grains that once filled them were not as well cemented together as those of the surrounding rock. CONTINUE NORTH (LEFT) ON THE MAIN ROAD. PARK ON THE LEFT SIDE OF THE ROAD AFTER CROSSING THE SMALL DRAINAGE (HACKBERRY WASH).

0.3 2.2 STOP 4 - HACKBERRY WASH PARKING LOT. A short walk up Hackberry Wash reveals downcutting as this drainage was diverted to the edge of the Santa Clara basalt flow where it now erodes the softer Navajo Sandstone (figure 10). This shows the beginning of the topographic inversion process where a stream erodes less resistant sedimentary rock along the margins of a flow. Eventually, the former valley floor down which the basalt flowed is left standing as a high sinuous ridge.

Figure 9 - ARockpecker holes@ in the Navajo Sandstone, created by differential erosion along certain bedding planes, in the nearly vertical west wall of Island in the Sky. Photo looking east.


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Across the road, again on the nearly vertical wall of Island in the Sky, is an incipient arch (figure 11). Note the horizontal bedding plane along the base of the arch where a spring line began the undercutting that allowed the arch to form. The deposits of desert varnish on the cliff face have a slightly different origin than the streaks that come down from the top of the cliff. Both are deposited as a surface stain as water carrying dissolved minerals evaporates, but desert varnish originates as water percolates through the rock while the streaks from the top of the cliff are created by surface runoff. The minerals are deposited as a laminated coating or “patina” on the surface of the rock. Desert varnish formation is aided by an organic process as the wind deposits clay particles and microorganisms (bacteria) on the sandstone. The microorganisms then extract manganese and iron oxides from the rock and clay particles to form a layer of varnish (Warneke, 1993). This process takes a very long time to form

Figure 10 - Downcutting is evident as Hackberry Wash was diverted to the edge of the Santa Clara flow, eroding the softer Navajo Sandstone. This shows the beginning of the inversion process that could eventually create an inverted valley. Photo looking north.

Figure 11 - An incipient arch on the nearly vertical north wall of Island in the Sky. Note the surface water stains and desert varnish on the cliff face.


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a thin layer. Native Americans would peck through this varnish layer to expose the lighter colored sandstone to make a petroglyph. Older petroglyphs are darker in color because the desert varnish has started to reform.

Pioneer names are carved into the sandstone along the north edge of Island in the

Sky (figure 12). The best access to that trailhead is from this parking lot. Follow the paved walking/biking trail up the canyon for 0.2 miles (0.3 km) and use the crosswalk to reach the trailhead to the right of the road. To the left of the road, across from the trailhead, note the small arches formed by differential erosion in the lower sandstone knobs, near road level.

0.5 2.7 STOP 5 - HIDDEN PINYON AND THREE PONDS TRAIL PARKING LOT. An informative kiosk at the end of this parking lot explains the basic geology of the park. A trail guide of both geology and native plants along the 1.5 mile (2.5 km) Hidden Pinyon Trail loop is available in the box at the trailhead. Three Ponds Trail, a moderate to strenuous trail through loose sand into a slot canyon with cliffs 400 feet (120 m) high, is 3 miles (5 km) round trip. A series of three natural “tanks” weathered in the Navajo Sandstone provide a catchment basin for seasonal precipitation.

Much of the Navajo Sandstone seems unusually dark in color in this area (figure

13). The Navajo Sandstone is very uniform in composition although geochemical analyzes of representative samples indicate great variability in the minerals that cement the silica sand grains together. “Typical” moderate-reddish-orange sandstone includes 5% each of sodium and aluminum and 2% iron of the total volume of the rock. By contrast, the “white” Navajo Sandstone has 3% aluminum, 1% sodium and only 0.5% iron (Willis and Higgins, 1996). Also common in the Navajo Sandstone throughout the park are some horizons

Figure 12 - Pioneer names on the sandstone along the north edge of Island in the Sky.


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where secondary enrichment of cementing minerals results in a very dark-brownish-black sandstone (ironstone) with total rock volume comprising 20% iron, 2% aluminum and manganese concentration as high as 5% (Willis and Higgins, 1996). These iron-manganese-rich horizons are more resistant to weathering and erosion and thus commonly cap stacks of rock called “hoodoos”. The iron and manganese oxides commonly form concentric rings to create ironstone concretions, nicknamed “Moki marbles,” that are more resistant to erosion than the surrounding rock (figure 14). Across the road to the south, Island in the Sky is nearly separated from the rest of the mountain by weathering, which widened a joint or joints to form a slot canyon (figure 15).

0.1 2.8 CAMPGROUND - Facilities include a 36-unit campground with hookups and

tent sites, modern rest rooms, showers, sewage disposal station, drinking water, and a group area.

Figure 13 - The unusually dark color of the Navajo Sandstone in this area and other areas throughout the park is created by a higher percentage of iron and manganese cementing the silica sand grains together than is found in the typical Ared@ sandstone. Photo looking northwest from Hidden Pinyon Trail parking lot.

Figure 14 - AMoki marbles,@ ironstone concretions that develop as iron and manganese oxides form concentric rings around a central point, weather out of the Navajo Sandstone. The concretions average 0.75 inches (2 cm) in diameter. Photo taken at Galoot Hill.


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0.3 3.1 CATTLE GUARD 0.3 3.4 STOP 6 - LOWER GALOOT HILL PARKING LOT AND PICNIC AREA. At

this point, the canyon floor has flattened out. As the stream-eroded, V-shaped canyon filled with basalt, only the taller ridges were left uncovered. The isolated sandstone monoliths are nicknamed “turtle backs.” Just how deep the canyon was, or how thick the flow is, is a matter of speculation since the base of the flow is not exposed in the canyon; however, drainage level was still controlled by and must be graded to the Santa Clara River. Note the rough aa surface of the basalt across the canyon floor and the vesicles in the basalt that are elongated in the direction of flow (figure 16).

Figure 15 - Weathering has widened joints to form a slot canyon, which has nearly separated Island in the Sky from the rest of the mountain. Photo looking southwest.

Figure 16 - Rough Aaa@ surface of the Santa Clara flow on the canyon floor. The older Snow Canyon Overlook flow caps the ridge that rims the east side of Snow Canyon. This is an excellent example of inverted topography.


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The surface of the sandstone is more deeply weathered here than it is farther

down the canyon where the walls are nearly vertical. This differential weathering accentuates the cross-bedding. Each of these cross-beds was once the front or face of a migrating sand dune, before it was buried by the next layer.

To the east, the older Snow Canyon Overlook basalt flow (Qbso) caps the

sandstone bench that forms the east rim of Snow Canyon (figure 16). This flow, sourced perhaps 5 to 10 miles (8-16 km) northeast of the park along the flank of the Pine Valley Mountains, apparently followed the course of an ancestral Snow Canyon and now forms a classic example of inverted topography. The ancestral Snow Canyon drainage subsequently shifted west of the flow and cut the modern Snow Canyon, similar to the downcutting that has started along the edge of the Santa Clara flow at Hackberry Wash (Stop 4). The Snow Canyon Overlook flow yielded an 40Ar/ 39Ar age of 1.16 + 0.03 million years (Ma). This isolated remnant is classified as a trachybasalt on the total alkali-silica (TAS) diagram of Le Bas and others (1986), and labeled hawaiite by Best and Brimhall (1974). It is dense, strongly jointed, and weathers very dark brown with abundant, small olivine phenocrysts that weather out in relief, forming a “sand-paper” texture (Willis and Higgins, 1996).

0.3 3.7 STOP 7 - UPPER GALOOT HILL PARKING LOT AND PICNIC AREA. A

short walk to the south up Galoot Hill, named after an old movie set that has since been torn down, allows for a firsthand look at “Moki marbles” and cross-bedding in the Navajo Sandstone (figure 14). Again, differential weathering accentuates the cross-bedding. Small vertical cracks in the rock give it a “checkerboard” or “elephant skin” look (figure 17). These shallow cracks are not joints, but are expansion cracks that form as load is removed and as temperature changes. The Galoot Hill “turtle back” is completely surrounded by the Santa Clara lava flow.

Figure 17 - Differential weathering accentuates cross-bedding and small vertical cracks in the Navajo Sandstone to give it a Acheckerboard@ or Aelephant skin@ look. Photo looking east at the west side of Galoot Hill. Note the Snow Canyon overlook flow capping the east rim of Snow Canyon in the background.


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The third and oldest volcanic unit within the park is visible in the cliff to the east where an old channel in the Navajo Sandstone is filled with two thick basalt flows (figure 18). The Lava Ridge (Qbl) flow fills the bottom, after cascading 600 feet (180 m) in 0.25 miles (0.4 km) to reach the channel. It is capped by the Snow Canyon Overlook flow (Willis and Higgins, 1995). The Lava Ridge flow erupted from a group of heavily weathered cinder cones on Lava Ridge, about one mile (1.6 km) east of the park. The Middleton lobe of this flow to the south and east of the park yielded an 40Ar/ 39Ar age of 1.41 + 0.01 Ma. The rock is a moderate- to dark-gray to dark-brownish-gray, quartz-bearing, basaltic trachyandesite, according to the TAS classification of Le Bas and others (1986). Euhedral plagioclase phenocrysts are prominent, and quartz, pyroxene, and olivine phenocrysts are common. The Lava Ridge flow is about 250,000 years older than the overlying Snow Canyon Overlook flow. This is the only place in the park where a younger flow is at a higher elevation than an older flow. Usually, so much downcutting occurs between flows that the younger flow lies at a lower level than the older flow. Note the basalt talus that locally covers the sandstone slope beneath the basalt-capped ridge.

0.2 3.9 STOP 8 - PETRIFIED DUNES TRAILHEAD - A trail log for this 1.5-mile (2.5 km) round trip hike to lava tube caves within the Santa Clara flow is included here. The trail crosses eolian sand, Navajo Sandstone, and the Santa Clara flow to lava tube caves within the flow, and provides a spectacular view of lava cascades into West Canyon (figure 19). This trail is highly recommended to capture the essence of the park. Take a flashlight to explore inside the caves.

Notice the level of the basalt here at the parking lot, and it’s rugged aa surface.

As the trail heads west (left), the basalt is largely covered by eolian sand weathered out of the Navajo Sandstone. Side trails that lead to monoliths of sandstone break off from the main trail, first to the left and, later, to the right; stay straight on the main trail.

Figure 18 - This channel along the east rim of Snow Canyon in the Navajo Sandstone is filled with two separate basalt flows. The oldest volcanic unit within the park, the Lava Ridge flow (1.41 + 0.01 Ma), fills the bottom of the channel and is overlain by the younger Snow Canyon Overlook flow (1.16 + 0.03 Ma). This is the only area of the park where an older flow is beneath a younger flow.


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Figure 19 - Aerial photograph of Petrified Dunes Trail to the lava tube caves. Optional hikes from the end of the trail are shown as dashed lines. Note the excellent alignment of the lava tube cave collapse features, which delineate the lava tube within the Santa Clara flow. Snow Canyon Drive is along the right (east) edge of the photo.


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A. Upon reaching the first outcrops of Navajo Sandstone, note the weathering pattern on the dip face of the cross-bedded layers (figure 20). This 120o juncture is common in nature since it creates the largest volume possible with the least amount of surface area.

B. Just ahead, weathering of the sandstone along the north edge of a large monolith has created what looks like a row of elephant trunks (figure 21). This fluted hillside formed as weathering proceeds at a faster rate along northeast-trending joints than it does between the joints. Differential weathering also locally exposes joint surfaces coated with desert varnish (figure 22). The trail turns north (right), down the sandstone into a small canyon, and follows the drainage along the base of the sheer sandstone cliff (figure 23). The cross-beds in the sandstone, which have been accentuated by weathering, are well exposed in the cliff face.

Figure 20 - Weathering pattern on the dip face of the cross-bedded Navajo Sandstone. This 120o juncture is common in nature. Polygons average one foot (0.3 m) in diameter.

Figure 21 - This fluted hillside, which looks like a row of elephant trunks, formed as weathering proceeds at a faster rate along northeast-trending joints than it does between the joints. Photo looking northwest.


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C. As the trail starts to climb out of the small drainage that formed between the sandstone and the Santa Clara flow, two cross-beds with “Moki marbles” pinch out to the south (figure 24). Just ahead and part way up the cliff face, is a very dark-brownish-black horizon in the Navajo Sandstone where the silica sand grains are well cemented by iron and manganese (figure 25). Common in the park, this “ironstone” is composed of quartz sand in which, of the total volume of the rock, about 20% is iron and 5% is manganese. A piece of this very resistant rock has fallen and cracked to form “butterfly rock” (figure 26). Beyond “butterfly rock,” the trail turns west (left) and cuts across the Santa Clara flow, which creates somewhat rough footing along its aa surface. Just past the drainage, enjoy large features in the basalt, such as pressure ridges, as well as fine features like vessicles and olivine phenocrysts.

Figure 22 - Differential weathering along joints exposed a joint surface, now partly coated with desert varnish. Photo looking southwest.

Figure 23 - The Petrified Dunes Trail to the lava tube caves follows the drainage that formed between the Santa Clara flow and the Navajo Sandstone, along the base of the sheer sandstone cliff. Photo looking north.


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Figure 24 - Two cross-beds with AMoki marbles@ in the Navajo Sandstone pinch out to the south (right).

Figure 25 - A very dark-brownish-black horizon (ironstone) in the Navajo Sandstone where silica sand grains are well cemented by iron and manganese. Photo looking east.

Figure 26 - A piece of very resistant Aironstone@ rock has fallen and cracked to form Abutterfly rock.@ Photo looking southeast.


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D. At the major trail intersection and sign, turn south (left) for a short distance.

One of the largest lava tube cave collapse structures in the park, which is 100 feet by 150 feet (30 X 45 m), is to the west (right) of the trail (figure 27). Lava tubes form as the surface of the flow cools and hardens, thus insulating the underlying molten lava. As the lava drains from the tube, a cave forms. Weathering eventually leads to a gravity collapse of the roof which allows access to the tube-shaped cave. Aerial photographs show the excellent alignment of these collapse features, which delineate the lava tube. The cave entrance is about 15 feet by 25 feet (5 X 8 m). Inside the cave, some interesting collapse and flow structures within the flow are seen by flashlight.

Several short hikes branch off from here. A trail to the west leads to a spectacular view of lava cascades into West Canyon. To the south, a short trail leads to the top of a sandstone point, which provides an excellent view in all directions (figure 28). The trail to the north from the major trail intersection leads to more collapse features and cave entrances. Follow the same trail back to the parking lot. CONTINUE NORTH (LEFT) ON THE MAIN ROAD.

Figure 27 - One of the largest lava tube cave collapse structures in the park is 100 feet by 150 feet (30 X 45 m). The tube-shaped cave is accessible because gravity has caused part of the roof of the lava tube to collapse. The cave entrance is about 15 feet by 25 feet (5 X 8 m). Photo looking northeast.

Figure 28 - View to the north, from the top of the sandstone point along the Pet-rified Dunes Trail, of the Santa Clara flow dividing around sandstone Aturtle backs@ before cascading into West Can-yon.


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0.5 4.4 STOP 9 - LAVA FLOW OVERLOOK AND WHITE ROCKS TRAILHEAD. A trail leads to more collapsed lava tube structures (0.5 miles/0.8 km), another overlook of West Canyon (0.75 miles/1.2 km), and then curves to the north and east to White Rocks, a natural white sandstone amphitheater, pioneer catchment ponds, and piles of white sand (2.5 miles/4 km). Double the mileage for a round trip.

The prominent peak directly north in the White Rocks area appears creased by a

deeply weathered vertical gash (figure 29). This crack is one of several widely spaced, heavily eroded joints that trend just west of north. Erosion along these lineaments forms Snow Canyon, West Canyon, Padre Canyon (houses Tuacahn Amphitheater to the west of the park), and almost detaches Island in the Sky to the south (figure 2). Some of these lineaments are several miles in length and have brecciated zones 5 to 30 feet (1.5-9 m) wide (Willis and Higgins, 1996). Brecciation and siliceous and calcareous recementation is generally intense along these fractures. Any significant offset is difficult to recognize on these fractures, largely because the Navajo Sandstone has few stratigraphic markers, but strata may be offset along some of these joints. Hurlow (1998) interpreted the West Canyon area as a separate structural compartment for a regional ground-water study because of assumed movement on these lineaments.

Across the canyon, in the west wall, the interfingering pattern of red and white Navajo Sandstone is truly spectacular (figure 30). South of this point, the sandstone is nearly all red, whereas to the north, it is nearly all white. Unlike the red wall of the Grand Canyon or the Altar of Sacrifice at Zion National Park where the red color of the rock is simply a surface stain, here the red cementing minerals of the Navajo Sandstone color the entire rock. The discoloration or “whitening” of the rock occurs after lithification. The subsequent loss of color is caused by fluid interaction that either dissolves the cementing minerals out from between the sand grains, leaving the sandstone very friable, or reduces the cementing minerals through chemical reactions that change the oxidation state. In other areas, such as Zion National Park, the cementing minerals have been dissolved from the upper portion of the Navajo Sandstone, leaving the sandstone quite “white.” However that color change does not interfinger like this one.

Figure 29 - The prominent peak in the White Rocks area directly north of Lava Flow Overlook appears creased by a deeply weathered vertical gash. This crack is one of several widely spaced, heavily eroded joints that trend just west of north. Erosion along these lineaments forms Snow Canyon, West Canyon, Padre Canyon to the west of the park, and almost detaches Island in the Sky to the south.


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The slope of the Santa Clara flow that fills Snow Canyon obscures the view into the bottom of West Canyon, which can be seen from the end of the Petrified Dunes trail to the lava tube caves at Stop 8. Looking south to the mouth of Snow Canyon, the “turtle backs” of Navajo Sandstone resemble stacks of pancakes and Island in the Sky seems almost to block the entrance (figure 31). This view is in line with the prominent set of joints in the Navajo Sandstone that trend northeast (figure 32). These joints don’t show evidence of micro-faulting as there is no brecciation and minor or no silification along the joints. Formed as the brittle rock is stressed by tectonic events or undergoes stress reduction because of erosional unloading, these cracks increase the surface area of the rock, thereby increasing the weathering rate of the rock. These joints are believed to have resulted from deformation associated with a minor thrust detachment of the Late Cretaceous to early Tertiary Sevier orogenic thrust belt, which is postulated in Cambrian strata about 4,000 feet (1,200 m) beneath the surface (Willis and Higgins, 1996; Davis, 1999).

Figure 30 - Across the canyon from the Lava Flow Overlook, in the west wall of Snow Canyon, is the interfingering pattern of red and white Navajo Sandstone.

Figure 31 - Looking south toward the mouth of Snow Canyon from the Lava Flow Overlook across the surface of the Santa Clara flow, Island in the Sky seems almost to block the entrance.

Figure 32 - This view to the southwest from Lava Flow Overlook is in line with the prominent set of joints in the Navajo Sandstone that trend northeast.


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CONTINUE UP THE ROAD, which has almost climbed out of Snow Canyon at this point. To the right of the road, note that mostly “white” Navajo Sandstone, partially seen beneath the talus of basalt, crops out beneath the 1.16 + 0.03 million-year-old Snow Canyon Overlook flow (figure 33). To the left of the road, the Santa Clara flow, which is estimated as only 20,000 to 10,000 years old (Willis and Higgins, 1996), was nearly at the same level before it cascaded into Snow Canyon.

0.7 5.1 CATTLE GUARD NEAR PARK ENTRY STATION. 0.1 5.2 STOP 10 - BIKE TRAIL PARKING LOT AT INTERSECTION WITH State

Route 18 (STOP SIGN). This very scenic portion of the bike trail system follows State Route 18 south about 9 miles (15 km) into St. George City.

The two small, basalt-capped hills to the left of the road mark the northernmost

extent of the Snow Canyon Overlook flow (1.16 + 0.03 Ma). The broad expanse of basalt north of these hills is the Santa Clara flow (estimated 20,000-10,000 years old). To the south, part of the Lava Ridge flow cascade is visible (1.41 + 0.01 Ma).

To the north and east along State Route 18, weathering has created a

“checkerboard” or “elephant skin” pattern on the “white” Navajo Sandstone (figure 34). Note the uneven distribution of the joints in the Navajo Sandstone (figure 35) (Hurlow, 1998). TURN NORTH (LEFT) ONTO STATE ROUTE 18.

0.8 6.0 MILE MARKER 12 ON State Route 18. 0.1 6.1 In the small side canyon to the right, a dike associated with the Santa Clara flow

protrudes out of the Navajo Sandstone (figure 36). Additional dikes are present near the ridge crest. Ahead, to the right of the road, is the closest of two cinder cones that mark the location of the vents that produced the Santa Clara flow.

Figure 33 - The east wall of Snow Canyon shows yel-lowish-white Navajo Sand-stone above that of the usual red color. Basalt talus from the Snow Can-yon Overlook flow locally covers the sandstone.


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Figure 34 - To the north and right side of the intersection of Snow Canyon Drive and State Route 18, weathering has created a Acheckerboard@ or Aelephant skin@ pattern on the Awhite@ Navajo Sandstone.

Figure 35 - Note the uneven dis-tribution of the joints in the sand-stone in the cliff to the east (right) of State Route 18, just north of the road intersection with Snow Can-yon Drive. Photo looking east.

Figure 36 - In the small side canyon to the south (right) of State Route 18 just north of mile marker 12, a dike associated with the Santa Clara flow protrudes out of the sandstone. Photo looking south.


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0.4 6.5 STOP 11 - CINDER CONES. Although there is no officially recognized trail to

the top of the cinder cone, the path is well worn (figure 37). Use caution if you attempt the climb, since cinders are sharp and unstable. Vents at the base of this cone and the one to the north, just up the road, are the sources for the Santa Clara flow. The symmetry of the cones is preserved because cinder cones typically form near the final phase of the eruptive cycle. Cinder cones that do form early in the cycle are usually torn apart by the undermining flow. Although the cones themselves are extinct and local volcanism is considered dormant, since there have been no eruptions during historical time, the extensional tectonic setting that caused these eruptions along joint systems in the Navajo Sandstone is still present (Condit and others, 1989; Sanchez, 1995; Smith and others, 1999).

The laccolith that forms the Pine Valley Mountains to the east intruded 20.9 + 0.6 million years ago as magma squeezed between two layers of rock to form a mushroom shape before cooling (McKee and others, 1995). The subsequent removal of the overlying layers exposed the laccolith, which is approximately 10 miles wide and 30 miles long (16 X 50 km). The Claron Formation, which is also exposed at Bryce Canyon National Park and Cedar Breaks National Monument, is the pink-colored formation that underlies the laccolith. Perhaps fluid associated with this intrusion was at least partially responsible for the interfingering color change of the Navajo Sandstone in this area. The sandstone is white from the interfingering area in Snow Canyon all the way to the base of the Pine Valley Mountains.

0.2 6.7 TURN RIGHT ONTO DIAMOND VALLEY DRIVE (7950 N.), AS A

SAFE WAY TO TURN AROUND AND HEAD SOUTH (LEFT) ON State Route 18 TOWARD ST. GEORGE.

1.4 8.1 INTERSECTION WITH SNOW CANYON DRIVE; STAY STRAIGHT

ON State Route 18. 1.0 9.1 TURN RIGHT ONTO DIRT ROAD TO SNOW CANYON OVERLOOK.

Figure 37 - Although there is no officially recognized trail to the top of the cinder cone, the path is well worn along the east side. This is the southernmost of two cones associated with the Santa Clara flow.


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0.3 9.4 STOP 12 - SNOW CANYON OVERLOOK. From the vantage point of the overlook, a panoramic view of the entire canyon allows for a review of the general geology.

Snow Canyon is carved into the Navajo Sandstone and, at the mouth of the

canyon, the upper member of the Kayenta Formation. Weathering accentuates the cross-beds in the sandstone that were formed during the Jurassic Period as sand dunes of the low-latitude desert migrated in response to a prevailing wind generally from the north (Sansom, 1992). During this time period, this portion of North America was only about 15 degrees north of the equator (Peterson, 1988). The color change in the Navajo Sandstone from red at the mouth of the canyon to white at the head of the canyon is well exposed across the canyon where the colors interfinger. The Navajo Sandstone has been fractured by two sets of joints: one set is relatively closely spaced fractures that trend northeast, and another set of more widely spaced fractures that trend just west of north. Erosion along this second set forms the major canyons in the area. In the bottom of Snow Canyon, rounded “turtle back” monoliths of sandstone protrude above lobes of lava that flowed around them only 20,000 to 10,000 years ago (figure 38). Originating from vents at the base of two cinder cones next to State Route 18, the Santa Clara flow cascaded into Snow Canyon and filled the V-shaped, stream-cut canyon with basalt (figure 39). Downcutting along the west edge of the flow has begun the topographic inversion process.

Figure 38 - View south from the Snow Canyon Overlook toward the mouth of Snow Canyon, showing rounded Aturtle back@ monoliths of sandstone protruding above lobes of the Santa Clara lava flow that flowed around them only 20,000 to 10,000 years ago.

Figure 39 - Originating from vents at the base of two cinder cones next to State Route 18, the Santa Clara flow cascaded into Snow Canyon and partially filled the V-shaped, stream-cut canyon with basalt. View shows only the southernmost cone and is from east of the park, looking west. The Beaver Dam Mountains are in the background.


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Three levels of lava flows document the eruptive and erosional history of Snow Canyon. The oldest, the Lava Ridge flow, came from cinder cones one mile (1.6 km) to the east of Snow Canyon State Park. It now caps the ridge to the east of Snow Canyon behind the Winchester Hills subdivision (Willis and Higgins, 1995). A small portion of this 1.41 + 0.01 million-year-old flow cascaded down a small channel and into an ancestral Snow Canyon near the present location of State Route 18. The Lava Ridge flow effectively displaced drainage and was buried by the Snow Canyon Overlook flow 1.16 + 0.03 million years ago. The Snow Canyon Overlook flow displaced the drainage to the west, which subsequently created the modern Snow Canyon. The Santa Clara flow partially filled this canyon only 20,000 to 10,000 years ago. Downcutting and widening of West Canyon is once again causing the location of Snow Canyon to shift to the west.

Note the fractures in the Snow Canyon Overlook basalt along the edge of the

overlook caused by the widening of the columnar joints as the softer sandstone is eroded from beneath the basalt. Eventually, the basalt breaks off to become talus on the sandstone slope below. Also note how vesicles in this basalt are elongated in the direction of flow. A significant stage VI pedogenic carbonate (caliche layer) has developed on the surface of the basalt (Machette, 1995; Willis and Higgins, 1996). RETURN TO STATE ROUTE 18.

0.3 9.7 INTERSECTION WITH State Route 18. TURN RIGHT ONTO State Route 18

TO RETURN TO ST. GEORGE (7 MILES). END ROAD LOG REFERENCES Best, M.G., and Brimhall, W.H., 1974, Late Cenozoic alkalic basaltic magmas in the western Colorado

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Birkeland, P.W., Machette, M.N., and Haller, K.M., 1991, Soils as a tool for applied Quaternary

geology: Utah Geological and Mineral Survey, Miscellaneous Publication 91-3, 63 p. Blakey, R.C., Peterson, Fred, and Kocurek, G., 1988, Late Paleozoic and Mesozoic eolian deposits of

western interior of the United States: Sedimentary Geology, v. 56, p. 3-125. Bugden, Miriam, 1992, The Geology of Snow Canyon State Park, Washington County, Utah: Utah

Geological Survey Public Information Series 13, 16 p. Condit, C.D., Crumpler, L.F., Aubele, J.C., and Elston, W.E., 1989, Patterns of volcanism along the

southern margins of the Colorado Plateau-Springerville field: Journal of Geophysical Research, v. 94, p. 7,975-7,986.


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Davis, G.H., 1999, Structural geology of Colorado Plateau region of southern Utah with special emphasis on deformation bands: Geological Society of America Special Paper 342, 168 p.

Hamblin, W.K., 1963, Late Cenozoic basalts of the St. George basin, Utah, in Heylmun, E.B., editor,

Guidebook to the geology of southwestern Utah: Intermountain Association of Petroleum Geologists Twelfth Annual Field Conference, p. 84-89.

---1987, Late Cenozoic volcanism in the St. George basin, Utah: Geological Society of America

Centennial Field Guide--Rocky Mountain Section, p. 291-294. Higgins, J.M., and Willis,G.C., 1995, Interim geologic map of the St. George quadrangle, Washington

County, Utah: Utah Geological Survey Open-File Report 323, 90 p., scale 1:24,000. Hintze, L.F., and Hammond, B.J., 1994, Geologic map of the Shivwits quadrangle, Washington

County, Utah: Utah Geological Survey Map 153, 21 p., scale 1:24,000. Horrocks-Carollo Engineers, 1993, Culinary water resources study: St. George City Water and Power

Department, June 1993, 128 p. Hurlow, H.A., 1998, The geology of the central Virgin River basin, southwestern Utah, and its relation

to ground-water conditions: Utah Geological Survey, Water Resources Bulletin 26, 53 p., 6 plates.

Imlay, R.W., 1980, Jurassic paleogeography of the conterminous United States in its continental

setting: U.S. Geological Survey Professional Paper 1062, 134 p. Janetski, J.C., 1984, An archaeological and geological assessment of Antelope Cave (NA5507),

Mohave County, Northwestern Arizona: Addendum to Brigham Young University Department of Anthropology Technical Series No. 83-73; Provo, Utah, Brigham Young University, M.S. thesis, 93 p.

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volcanic rocks based on the total alkali-silica diagram: Journal of Petrology, v. 27, p. 745-750. Machette, M.N., 1985, Calcic soils of the southwestern United States: Geological Society of America

Special Paper 203, p. 1-21. Madsen, D.B., 1992, An atlatl from Snow Canyon State Park, in Utah Archaeology 1992: Utah

Statewide Archaeological Society, v. 5, no. 1, p. 133-136. Marzolf, J.E., 1988, Controls on late Paleozoic and early Mesozoic on eolian deposition of the western

United States, in Kocurek, G., editor, Late Paleozoic and Mesozoic eolian deposits of the western interior of the United States: v. 56, p. 167-191.


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McKee, E.H., Blank, H.R., and Rowley, P.D., 1995, Potassium-Argon ages of Tertiary igneous rocks in the eastern Bull Valley Mountains and Pine Valley Mountains, southwestern Utah, in Maldonado, F. and Nealey, L.D., editors, Geologic studies in the Basin and Range-Colorado Plateau Transition in southeastern Nevada, southwestern Utah, and northwestern Arizona: U.S. Geological Survey Bulletin 2153, p. 241-252.

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States: Sedimentary Geology, v. 56, p. 207-260. Peterson, Fred and Pipiringos, G.N., 1979, Stratigraphic relations of the Navajo Sandstone to middle

Jurassic formations, southern Utah and northern Arizona: U.S. Geological Survey Professional Paper 1035-B, 43 p.

Sanchez, Alexander, 1995, Mafic volcanism in the Colorado Plateau/Basin and Range transition zone,

Hurricane, Utah: Las Vegas, University of Nevada, M.S. thesis, 92 p. Sansom, P.J., 1992, Sedimentology of the Navajo Sandstone, southern Utah, USA: Oxford England,

Department of Earth Sciences at Wolfson College, unpublished Ph.D. dissertation, 291 p. Smith, E.I., Sanchez, Alexander, Walker, J.D., and Wang, Kefa, 1999, Geochemistry of mafic magmas

in the Hurricane volcanic field, Utah- Implications for small- and large-scale chemical variability of the lithospheric mantle: Journal of Geology, v. 107, p. 433-448.

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Kayenta Formation and Navajo Sandstone (Jurassic) in southwestern Utah: Flagstaff, Arizona, Northern Arizona University, unpublished M.S. thesis, 189 p.

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---1996, Interim geologic map of the Santa Clara quadrangle, Washington County, Utah: Utah

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Documents

GEOLOGIC ROAD AND TRAIL GUIDE TO SNOW …utahgeology.org/road_logs/uga-29_first_edition/SP_guide/...Figure 4 - Geologic map of the Snow Canyon State Park area (modified from Willis