Bandelier: the rock that let people carve their houses

The Ancestral Puebloans who lived at Bandelier chose it for reasons a geologist
can explain. The rock that makes the mesas and the sheer-walled canyons is
volcanic tuff, and tuff is a peculiar material: **strong enough to hold a
vertical canyon wall, soft enough to be dug out with hand tools.**

They carved their dwellings into it.

Where the rock came from

Bandelier sits on the Pajarito Plateau, inside a volcanic complex that has
been active for more than 13 million years, at the meeting point of the Rio
Grande rift and the Jemez Mountains volcanic field.

Two explosive eruptions between 1.61 and 1.25 million years ago put out a
combined 160 cubic miles of rock, ash, pumice and tuff across Bandelier and
the country around it. The deposit they left — volcanic ash welded solid by
superheated gas — is the Bandelier Tuff, and it is a world-class example of
welded tuff.

What the tuff gave people

Three things, and the settlement needed all three:

• Shelter — rock that could be carved into dwellings
• Water — the tuff holds it, which made farming possible
• Food — the canyon-and-mesa topography the tuff weathers into supports a
  rich ecosystem

The geology did not merely surround the culture. It is a fair reading to say
it made the culture possible.

Fossils, and what packrats keep

Bandelier's fossils are not in the tuff but in what came after it. Holocene
terraces in Frijoles Canyon hold a small amount of material in place,
including detrital charcoal and a packrat midden.

The park has many packrat middens and their full age range is unknown. One in
Frijoles Canyon has been dated to between 3,680 and 3,290 years old.

All NPS fossil resources are
protected under the Paleontological Resources Preservation Act of 2009.

Three hazards, all still running

Flash flooding and debris flows. Fire has reshaped this landscape. Two
decades of it have produced extraordinary flood and debris-flow events that
have hit staff, visitors, trails, bridges and buildings.

Rockfall. Ongoing in the canyons, sometimes spontaneous, sometimes
triggered by earthquake, frost, the daily heating and cooling cycle, or rain —
and markedly worse for a period after a wildfire. Nearly every park facility
in a canyon is exposed to it.

Earthquakes. The Pajarito Fault runs through the monument. It and the
others along the western margin of the Rio Grande rift can produce magnitude
5.0 or greater, and the USGS puts the probability of one in the next century
at more than 50%. A quake that size near Bandelier would damage park
facilities and set off widespread rockfall in the canyons.

Where it fits

Bandelier belongs to the
Rocky Mountain System physiographic province
and shares its history and some of its formations with a region reaching far
beyond the monument's boundary.

Sources

Written from the National Park Service's Geodiversity Atlas entry for
Bandelier National Monument, a work of the United States government in the
public domain. Eruption volumes and welded tuff from the National Park Service
(2005); packrat midden dating from Tweet et al. (2012).