This section provides descriptions of the most prominent and distinctive geologic features and processes in Navajo National Monument.

Figure 7. Betatakin ruins in an alcove formed in the Navajo Sandstone, Betatakin Canyon, Navajo National Monument.

Alcoves
The primary geologic features of interest at Navajo National Monument are the alcoves that formed due to groundwater flow, dissolution of carbonate cement, gravity, and wind erosion (figure 7). The Navajo Sandstone is porous and permeable but the underlying Kayenta Formation is not. The Kayenta, therefore, acts as a barrier to vertical groundwater flow. Water infiltrating into the sandstone on the Shonto Plateau moves laterally at the Navajo- Kayenta interface and emerges as springs along the canyon walls.

The carbonate matrix holding the sand grains of the Kayenta Formation together slowly dissolves as a result of ground water flow. Loosened grains destroy the integrity of the rock, undermining the cliffs of Navajo Sandstone lying above. Over time, slabs of the Navajo Sandstone break away into the canyon, eventually forming alcoves. These alcoves, especially ones with a spring at the Navajo/Kayenta interface, were attractive sites for occupation by prehistoric peoples.

A textbook example of alcove formation is found at Betatakin ruins near the visitor center (figure 7). The ruins occupy an alcove that formed as the Navajo
Sandstone, undermined by erosion of the less resistant Kayenta Formation, broke away in arching slabs. The arched ceiling of Betatakin ruins offers a unique acoustic experience, as whispers from one end of the ruins can be easily understood at the other. Vibrations off the arched, cross- bedded strata probably contribute to this phenomenon.

Navajo Sandstone
The Navajo Sandstone consists mostly of large- scale cross- bedded, pale salmon- colored, eolian sandstone with minor lenses of pink clay and light gray freshwater limestone that formed in small lakes and ponds during the Jurassic (Peterson 1994). The Navajo Sandstone thickens to the northwest and may reach 1,800 feet (550 m) thick on the Navajo Reservation (Cooley et al. 1969). The Navajo Sandstone is one of the largest preserved eolian systems in the stratigraphic record, and along with its correlatives, the most widespread eolian deposit in North America (Blakey 1994; Peterson 1994). These Lower Jurassic dune sands are distributed from northwestern Wyoming and adjacent Idaho southward to southern Arizona and northern Mexico.

Much of the surface of the Navajo Sandstone is barren of vegetation so that the preserved arcs and swirls of eolian cross- bedding in the preserved dunes are fully exposed. Several sets of vertical joints in the rocks were produced by strains and stresses of deep burial and subsequent uplift. The vertical joints are deeply incised and serve as watercourses for surface flow during rainstorms. Rows of miniature pools mark these watercourses. The pools fill with rainwater and serve as habitats for small plants and animals, many of them microscopic. By- products of these animals and plants include acids that dissolve the limy cement holding the sand grains together. Lichens secrete acids that also loosen grains that are blown away by wind. The barren patches are surrounded by thin soil that is hardly more than wind- blown sand held together by roots of trees, shrubs, and grass.

A surface stain of manganese oxide and/or iron oxide known as desert varnish locally forms a brown or black coat on the bare Navajo Sandstone (figure 7). Ancestral Puebloans used this desert varnish as a backdrop for their petroglyphs. Pictographs were also drawn on the salmon- colored sandstone.

Betatakin Unit
Betatakin Canyon is a side canyon that connects to Tsegi Wash. Tsegi Wash flows into Laguna Wash near US Highway 160. Both of these streams ultimately drain into the San Juan River at Mexican Hat (Chronic 1983). Betatakin Canyon contains the Betatakin cliff dwelling, Kiva Cave cliff dwelling, and the remains of the Wetherill cabin (NPS 2003). Tree- ring dating indicates that the alcove was occupied around A.D. 1250.

Betatakin contains about 135 rooms tucked into a cliffside alcove 452 feet (138 m) high and 370 feet (113 m) wide (figure 7). Vegetation surrounds a small spring at the Navajo- Kayenta interface at the base of the alcove. The spring has likely accelerated cliff collapse in this area. The natural and cultural resources of the Betatakin Unit are protected from grazing by a boundary fence between the unit and Navajo land.

Betatakin Canyon is visible from the visitor center. Thin beds of freshwater limestone in the Navajo Sandstone cap the highest hillocks near the Visitor Center. These limestones were probably deposited in ephemeral ponds in interdune areas similar to those in modern dune fields (e.g. Great Sand Dunes National Park). There are many small alcoves on switchbacks along the Aspen Forest Trail which is now closed to the public as a result of rockfall hazards. The sandy floors of these alcoves are marked by tracks and droppings of small animals. Crossbedding in the sandstone controls alcove shape, leading to a sloping ceiling or an arched ceiling like that at Betatakin.

Figure 8. Keet Seel ruins in an alcove cut into the massive Navajo Sandstone.

Keet Seel Unit
The Keet Seel site, located up a north- south trending side canyon in the Tsegi Canyon system, contains the largest cliff- dwellings in America: the Keet Seel cliff dwelling (figure 8) and the Turkey Cave cliff dwelling. Many of the features in Betatakin Canyon are present at Keet Seel. Like Betatakin, Keet Seel has a spring at the contact between the Navajo Sandstone and Kayenta Formation. Pictographs of birds are preserved in Turkey Cave.

Pottery and tree- ring dating indicate that the ancient Puebloans lived here as early as A.D. 950. Inhabitants of Keet Seal did not come in groups, as at Betatakin, but arrived and departed randomly. As a result, Keet Seel contains more variation in room design and construction and more kivas than Betatakin.

A surge in building activity in 1272 suggests the arrival of a new group of people. Population growth apparently taxed the capacity of the alcove and people began moving out. Those who remained converted abandoned rooms into granaries; however, they also left around 1300.

Figure 9. Photo of Inscription House. From the Stuart M. Young Collection.

Inscription House Unit
Inscription House was named for an inscription on one of the walls noted in 1909 by Byron Cummings and John Wetherill. This cliff dwelling lies at the base of a higharching sandstone cliff on the north side of an arm of Nitsin Canyon. The site has been closed to the public since 1968 because of urgent stabilization needs and because the local people desire privacy.

The smallest of the three ruins, Inscription House contains about 74 living quarters, granaries, and kivas (figure 9). Other cliff dwellings at the site include Owl House and Snake House. A tree- ring date of 1274 indicates that Inscription House was occupied about the same time as Betatakin and Keet Seel.

As with the other sites, a spring is associated with the ruins. Inscription House Spring flows out of the Navajo Sandstone and into Navajo Creek, which flows northward into Lake Powell (Thomas 2003).

References:

Blakey, R. C. 1994. Paleogeographic and tectonic controls on some Lower and Middle Jurassic erg deposits, Colorado Plateau. In Mesozoic Systems of the Rocky Mountain Region, USA, edited by Mario V. Caputo, James A. Peterson, and Karen J. Franczyk. Denver: SEPM (Society for Sedimentary Geology), Rocky Mountain Section, 273- 298.

Chronic, H. 1983. Roadside Geology of Arizona. Missoula: Mountain Press Publishing Company, 291- 294.

Cooley, M.E., J.W. Harshbarger, J.P. Akers, and W.F. Hardt. 1969. Regional Hydrogeology of the Navajo and Hopi Indian Reservations, Arizona, New Mexico, and Utah. USGS Professional Paper 521- A, 61 p.

National Park Service. 2003. Final General Management Plan and Environmental Impact Statement: Tonalea: Department of Interior, National Park Service, 197 p.

Peterson, F. 1994. Sand dunes, sabkhas, stream, and shallow seas: Jurassic paleogeography in the southern part of the Western Interior Basin. In Mesozoic Systems of the Rocky Mountain Region, USA, edited by Mario V. Caputo, James A. Peterson, and Karen J. Franczyk. Denver: SEPM (Society for Sedimentary Geology), Rocky Mountain Section, 233- 272.