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This section provides descriptions of the most prominent and distinctive geologic features and processes in Navajo National Monument.
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
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 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.
Keet Seel Unit
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.
Inscription House Unit
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).
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