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Biek et al. (2000) identified 24 outstanding geologic features in Zion. These features are summarized below and located on their map, reproduced here as Figure 7.
1. The Springdale Landslide Triggered on September 2, 1992, by a magnitude 5.8 earthquake, this old landslide has probably moved many times in the past. In 1992, 14 million cubic meters (18 million yd3) of mostly Moenave Formation slid on the weak claystone of the Petrified Forest member of the Chinle Formation (Biek et al. 2000). Three homes and two water tanks were destroyed, utility lines were disrupted, and Utah Highway 9 was closed.
2. The Watchman Overlook Millions of years of geologic history are captured in the bedrock units in this view of Zion Canyon. The resistant bench of Shinarump Conglomerate at the mouth of the canyon can be followed vertically upward to the gray limestones of the Carmel Formation high above the Navajo Sandstone. Landslides, joints, pediments, debris flows, and river terraces that help define the erosional history of Zion Canyon can also be seen from this overlook.
3. Zion-Mt. Carmel Highway Tunnel This tunnel, completed in 1930, was the first milliondollar highway constructed in the U.S. The 1.8 km- long (1.1 mi) tunnel was blasted through the lower 80 m (260 ft) of the Navajo Sandstone.
4. Joints along Zion-Mt. Carmel Highway Weathering and erosion processes aggressively attack the sandstone faces exposed in near- vertical fractures, or joints, in the Navajo Sandstone and create the spectacular landscape of Zion. The orientation and alignment of side canyons in the eastern part of the park are controlled by the prominent set of north- northwesttrending joints. Excellent examples of these joints can be seen along the Zion-Mt. Carmel Highway.
5. Checkerboard Mesa Checkerboard Mesa is an example of two weathering processes, one controlled by stratigraphy and one by climate. The “checkerboard” results from the roughly perpendicular sets of grooves in the Navajo Sandstone. The nearly horizontal grooves follow layers of coarse sand that coincide with eolian bedding sets whereas the vertical grooves have been interpreted to be the result of local expansion and contraction of the rock surface due to changes in temperature and moisture (Biek et al. 2000).
6. Sand Bench Landslide About 7,000 years ago, the relatively thin wall between two closely spaced joints in the Navajo Sandstone collapsed. The resulting Sand Bench landslide blocked Zion Canyon just east of The Sentinel, creating Sentinel Lake. For thousands of years, the Virgin River has been eroding the eastern part of Sand Bench landslide. The result is the river has steepened the landslide creating unstable slopes with the potential for further landslides. Recent landslides in 1923, 1941, and 1995 have temporarily dammed the Virgin River. Prior to the initial Sand Bench landslide, the Virgin River flowed 21 m (70 ft) lower in elevation than it does today.
7. Sentinel Lake Stretching from the Court of the Patriarchs on the south upstream to the Temple of Sinawava, Sentinel Lake was 61 m (200 ft) deep in its early stages. Horizontal lake sediments that can be seen along the Emerald Pools Trail and the Sand Bench Trail indicate that the lake was probably full of water all year round.
8. Hanging Valleys During and after rainstorms, waterfalls cascade from the mouths of hanging valleys that rim the main canyons in Zion. These scalloped tributary valleys are alluvial hanging valleys. Large rivers, such as the North and East Forks of the Virgin River, have more erosive energy than the small, typically ephemeral, tributary streams that feed them. The larger rivers cut their canyons faster than streams in the side valleys. Eventually, these tributary valleys are left “hanging” above the floor of the main canyon.
9. Weeping Rock The effects of groundwater movement along a contact between the permeable Navajo Sandstone and relatively impermeable Kayenta Formation are displayed at Weeping Rock, a picturesque alcove near the base of the Navajo Sandstone, below the mouth of Echo Canyon. Downward infiltration of groundwater directly beneath Echo Canyon is impeded by the Kayenta Formation. Flow is therefore redirected laterally toward the cliff face and ultimately, groundwater seeps out of the rock. The exact path the water follows, and where it discharges, is strongly influenced by joints in the sandstone. A lush hanging garden on the ceiling of the alcove enjoys yearround moisture due to the seeping groundwater. Because the water is alkaline, tufa (calcium carbonate) structures form on the surface of Weeping Rock.
10. The Narrows of Zion Canyon Beyond the north end of Zion Canyon Scenic Drive, the North Fork of the Virgin River flows for about 16 km (10 mi) through a spectacular gorge cut into Navajo Sandstone. The gorge narrows to a 300- m (1,000- ft) slot canyon at The Narrows where the minimum width of the canyon floor is about 5 m (16 ft).
11. Crater Hill Flow and Cinder Cone Marking the vent of one of the more voluminous volcanic flows in southwestern Utah, the Crater Hill cinder cone is the largest cinder cone in the park. Flowing southward into Coalpits and Scoggins Washes, basalt from the Crater Hill flow accumulated to a depth of over 122 m (400 ft) in the ancestral Virgin River valley. Volcanic features such as pressure ridges, which form concentric rings and large rafted blocks of basalt are “frozen” in the upper surfaces of the flow. Lake Grafton formed when the flow blocked the Virgin River and Coalpits Lake formed when it blocked Coalpits and Scoggins Washes.
12. Coalpits Wash Several episodes of recent geologic history and fluvial geomorphology can be seen along Coalpits Wash. In the lower part of Coalpits Wash, basalt plugged the channel of the Virgin River exposing post- basalt gravels, a major debris flow plug, and several young terraces cut by the rapidly adjusting stream. Near the head of the basalt flow, in the upper part of Coalpits Wash, huge chaotic basalt boulders stand as evidence of undercutting and collapse of the flow into the newly formed wash.
13. Trail Canyon Lake The Pleistocene- age Grapevine Wash basalt flow and a more recent large landslide involving the Kayenta Formation combined to create a dam upstream from the confluence of the Left Fork and Right Fork of North Creek. Lake sediments overlie the landslide deposits. A variety of fossils have been uncovered from these lake sediments including snails, fish vertebrae, and a bison thoracic vertebra.
14. Basalt Stack at Left Fork North Creek Hiking a short distance east on the Grapevine Springs Trail reveals a spectacular view of 17 cooling units from the Grapevine Wash basalt flow. This flow erupted from a group of vents on the Lower Kolob Plateau at and near Spendlove and Firepit Knolls. Lava flowed southward around sandstone knobs and eventually cascaded into North Creek. The basalt plug is at least 137 m (450 ft) thick and radiometric ages taken from the top and bottom of the flow indicate that all of the flow was emplaced about 270,000 years ago. The entire stack, therefore, is the result of one or more closely spaced eruptions. Accumulation of basalt of this volume in a relatively short period of time is unusual for volcanic deposits on the Colorado Plateau.
15. Dinosaur Tracks at Left Fork North Creek About 0.8 km (0.5 mi) up Left Fork on the Subway hiking trail lies a large boulder of Kayenta Formation sandstone covered with tracks from a large bipedal tridactyl (threetoed dinosaur).
16. Subway The Subway is another classic example of differential erosion and the influence of joints on the development of the canyons in Zion. Located on the Left Fork of North Creek, this narrow canyon is wide and rounded at the bottom and narrow and steep- walled at the top because the lower transitional strata of the Navajo Sandstone are less resistant to erosion than the upper strata of the Navajo. The stream in the canyon flows along a series of joints during periods of low flow, thus illustrating the influence of joints on canyon development.
17. Firepit and Spendlove Knolls The Firepit and Spendlove Knolls are two nearly perfectly conical cinder cones located near the Kolob Road in the west- central part of the park. They mark two of the vents associated with the Grapevine Wash basalt flows. Other, small cones are exposed to the south and west of these two.
18. Old Debris-Flow Deposits Huge igneous boulders derived from the Pine Valley Mountains form much of the old debris flow deposits west of Little Creek Sinks and to the north on the Upper Kolob Plateau. The boulders are up to 7.3 m (24 ft) long, 6.7 m (22 ft) wide, and reach an estimated 3.7 m (12 ft) thick. These large blocks were transported to the east and northeast at least 16 km (10 mi) and possibly as much as 26 km (16 mi). This movement would be impossible today because of the topographic barrier provided by the Hurricane fault zone. Consequently, the debris- flows must be older than the Hurricane fault zone.
19. Hop Valley The seldom visited and enchanting Hop Valley lies in the Kolob Canyons portion of Zion (figure 7). Hop Valley “Lake” formed sometime prior to 2,640 years ago when a landslide dammed the mouth of the canyon. Though the sediment trap formed by the landslide performed much like a lake, the sandy nature of the sediments filling the valley indicates that it rarely held standing water. Contrast this with the sediments behind the Sand Bench landslide that are fine silts and clays. Hop Valley is the youngest of the large, landslide- dammed paleolakes in Zion. Sediment deposited in the lake now forms the valley floor that slopes gently north. These valley fill sediments may be as much as 107 m (350 ft) thick.
20. Kolob Arch Spanning 94.5 m (310 ft) and with a window height of 101 m (330 ft), Kolob Arch is the world’s longest natural arch (Biek et al. 2000). The arch is 24 m (80 ft) thick and formed in the middle of the massively cross- bedded Navajo Sandstone. The alignment of the arch suggests that it is related to the north- northwest- trending joint system in the park, and possibly to the exfoliation joints that parallel the East Cougar Mountain fault.
21. Finger Canyons of the Kolob Eroded into the edge of the Upper Kolob Plateau, the Finger Canyons are a series of west- trending canyons that formed along a series of west- trending joints that isolate large monoliths of Navajo Sandstone. Each canyon resembles a miniature Zion Canyon with a broad canyon mouth where the erodible, pre- Navajo strata are exposed and then taper to a slot canyon in the upper reaches of the canyons.
22. Taylor Creek Thrust Fault Zone Thrust faults (low angle reverse faults) repeat the strata in the fault zone. The Taylor Creek Thrust Fault Zone is an excellent example of this type of structural geology. The Moenave strata have been repeated along one principal and several lesser east- dipping thrust faults on the east flank of the Kanarra anticline. The thrusts are Sevier- age back thrusts that formed under a west- east compressional regime during the Late Cretaceous to early Tertiary.
23. Double Arch Alcove Formed in the massively cross- bedded Navajo Sandstone, the Double Arch Alcove illustrates two different processes of arch formation. The lower arch formed by spring sapping and lateral stream erosion and the upper arch was controlled by jointing.
24. Hurricane Fault Zone The Hurricane fault zone is a major, active, steeply westdipping normal fault that stretches at least 250 km (155 mi) from south of the Grand Canyon northward to Cedar City. Along the southern boundary of the park, tectonic displacement is about 1,098 m (3,600 ft). Zion rests on the eastern block and as this block was uplifted, the erosive power of streams draining the Kolob Terrace increased to help form the present landscape.
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