 | The architecture of Pueblo Bonito : |  |
CLIMATIC CHANGE
Bryan was not alone in believing that the climate of Chaco Canyon
had changed toward the less humid sometime after the Bonitians
settled there. Presumably these agricultural folk were drawn to the
valley by its broad, flat acres, by an abundance of water just beneath
the surface, and by pine forests on bordering mesas. But they soon
exterminated the forests and they eventually learned that the flat acres
of the valley were altogether unproductive. They may have noted
also that floodwaters bearing upcanyon silt did not always arrive
when expected and that crops failed for lack of moisture and replenished
soil.
It was possibly a period of subnormal precipitation at the end of
the 11th century rather than destruction of the Chaco forests that
initiated the erosion culminating in the third-cycle arroyo, Bryan's
"post-Bonito channel." Yet annual growth rings in many constructional
timbers from Pueblo Bonito are so uniform in thickness as to
indicate the pines had grown where moisture was fairly constant
year after year. This fact, plus the quantities of tule or bulrushes
(Scirpus actutus Muhl.) and coarse grasses utilized with those timbers,
led Bryan to the conviction that rainfall in Chaco Canyon formerly
exceeded his 1925 10-inch estimate. He offered no geological
support for this conviction, but the botanical evidence alone led
N. H. Darton, another geologist and a guest of the Expedition
at its 1922 symposium, to share Bryan's belief in a climatic shift
from the dry toward the less dry. Bulrushes grow in wet places.
Previously and for different reasons the theory of climatic change
throughout the Southwest had been advanced by Hewett, Henderson,
and Robbins (1913), Huntington (1914), Gregory (1916), and
others. But, as emphasized by Kidder (1924, p. 54), these advocates
did not always consider the fact that an Indian requires far
less potable water than a white man and is content to irrigate his
fields with the runoff following summertime showers. Together,
Darton and Bryan believed that as little as one additional inch of
precipitation annually would go far toward restoring the habitableness
of Chaco Canyon. Together, black alkali and the 12th-century
arroyo brought an end to local agriculture; floodwaters subsequently
refilled that arroyo and buried the old Bonitian farmlands under an
additional 2-5 feet of upcanyon alluvium.
| The architecture of Pueblo Bonito : | |