The Geology of Navajo National Monument, Arizona: How the Alcoves Formed

Adapted from the National Park Service Geodiversity Atlas, which draws on the Geologic Resources Inventory to describe each park's rocks, landforms, soils and geologic processes.

The alcoves

The chief geologic features of Navajo National Monument are its alcoves: great arching recesses in the cliffs, carved by groundwater, the dissolving of the natural cement in the rock, gravity and wind.

They form at the boundary between two rock layers. The Navajo Sandstone above is porous and lets water through; the Kayenta Formation below does not. Water soaking into the sandstone on the Shonto Plateau sinks until it reaches the Kayenta, then flows sideways along the contact and emerge as springs in the canyon walls.

That seeping water slowly dissolves the carbonate cement that holds the Kayenta's sand grains together. As the grains loosen, the rock weakens and undercuts the Navajo Sandstone cliffs above. Over time, slabs of sandstone break off and fall into the canyon, leaving an alcove behind. Alcoves with a spring at the base were especially attractive places for ancient peoples to live.

Betatakin, near the visitor center, is a textbook example. Its ruins sit in an alcove formed as the Navajo Sandstone, undermined by erosion of the softer Kayenta, broke away in arching slabs. The curved ceiling makes for remarkable acoustics: a whisper at one end of the ruins can be clearly understood at the other.

The Navajo Sandstone

The Navajo Sandstone is mostly pale salmon-colored sandstone laid down by wind, with large-scale cross-bedding, and small lenses of pink clay and light gray freshwater limestone that formed in lakes and ponds during the Jurassic. It thickens to the northwest and may reach 1,800 feet (550 m) on the Navajo Reservation. Together with rocks of the same age elsewhere, it is one of the largest ancient wind-blown sand deposits preserved in the rock record and the most widespread in North America, stretching from northwestern Wyoming and neighboring Idaho to southern Arizona and northern Mexico. Much of its surface is bare, so the arcs and swirls of its fossil dunes are fully exposed.

Deep burial and later uplift cracked the rock into sets of vertical joints. These become channels for water in rainstorms, marked by rows of small pools that fill with rain and shelter tiny plants and animals, many microscopic. Acids they produce dissolve the lime cement between the sand grains, and lichens release acids that loosen grains for the wind to carry away. Around the bare rock, a thin soil, little more than windblown sand held together by the roots of trees, shrubs and grass, clings on.

Desert varnish

In places the bare sandstone wears a brown or black coating of manganese oxide, iron oxide or both, called desert varnish. Ancestral Puebloans used it as a backdrop for petroglyphs, and they drew pictographs on the salmon-colored sandstone.

Part of the Geologic Resources Inventory's geologic map of the monument. National Park Service.

Regional setting and maps

Navajo National Monument lies in the Colorado Plateaus physiographic province and shares its geologic history and characteristic rock formations with a much wider region. The Park Service's Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, has mapped the monument, and a Soil Resources Inventory has also been completed; both are on the NPS DataStore.

Sources

• National Park Service, "NPS Geodiversity Atlas—Navajo National Monument, Arizona." https://www.nps.gov/articles/nps-geodiversity-atlas-navajo-national-monument-arizona.htm
• Studies cited include Peterson (1994), Blakey (1994) and Cooley et al. (1969). NPS DataStore saved searches 3029 (geology) and 3076 (soils): https://irma.nps.gov/DataStore/
• A photograph on the source page by Paul Hermans, under a Creative Commons license, is not reproduced here.