Collapse: How Societies Choose to Fail or Succeed (27 page)

wasteland, and why, having gone to all that work of building it, did they
then abandon it?

When Native American farmers moved into the Chaco Canyon area
around
a.d.
600, they initially lived in underground pit houses, as did other
contemporary Native Americans in the Southwest. Around
a.d.
700 the
Chaco Anasazi, out of contact with Native American societies building
structures of stone a thousand miles to the south in Mexico, independently
invented techniques of stone construction and eventually adopted rubble
cores with veneers of cut stone facing (Plate 11). Initially, those structures
were only one story high, but around
a.d.
920 what eventually became the largest Chacoan site of Pueblo Bonito went up to two stories, then over the
next two centuries rose to five or six stories with 600 rooms whose roof sup
ports were logs up to 16 feet long and weighing up to 700 pounds.

Why, out of all the Anasazi sites, was it at Chaco Canyon that construction techniques and political and societal complexity reached their apogee?
Likely reasons are some environmental advantages of Chaco Canyon, which
initially represented a favorable environmental oasis within northwestern
New Mexico. The narrow canyon caught rain runoff from many side-
channels and a large upland area, which resulted in high alluvial ground
water levels permitting farming independent of local rainfall in some areas, and also high rates of soil renewal from the runoff. The large habitable area
in the canyon and within 50 miles of it could support a relatively high
population for such a dry environment. The Chaco region has a high diversity of useful wild plant and animal species, and a relatively low elevation
that provides a long growing season for crops. At first, nearby pinyon and juniper woodlands provided the construction logs and firewood. The earliest roof beams identified by their tree rings, and still well preserved in the
Southwest's dry climate, are of locally available pinyon pines, and firewood remains in early hearths are of locally available pinyon and juniper. Anasazi
diets depended heavily on growing corn, plus some squash and beans, but
early archaeological levels also show much consumption of wild plants such
as pinyon nuts (75% protein), and much hunting of deer.

All those natural advantages of Chaco Canyon were balanced by two
major disadvantages resulting from the Southwest's environmental fragility.
One involved problems of water management. Initially, rain runoff would have been as a broad sheet over the flat canyon bottom, permitting flood-
plain agriculture watered both by the runoff and by the high alluvial
groundwater table. When the Anasazi began diverting water into channels
for irrigation, the concentration of water runoff in the channels and the

clearing of vegetation for agriculture, combined with natural processes, re
sulted around
a.d.
900 in the cutting of deep arroyos in which the water
level was below field levels, thereby making irrigation agriculture and also
agriculture based on groundwater impossible until the arroyos filled up
again. Such arroyo-cutting can develop surprisingly suddenly. For example,
at the Arizona city of Tucson in the late 1880s, American settlers excavated a
so-called intercept ditch to intercept the shallow groundwater table and divert its water downstream onto the floodplain. Unfortunately, floods from
heavy rains in the summer of 1890 cut into the head of that ditch, starting an arroyo that within a mere three days extended itself for a distance
of six miles upstream, leaving an incised and agriculturally useless flood-
plain near Tucson. Early Southwest Native American societies probably at
tempted similar intercept ditches, with similar results. The Chaco Anasazi
dealt with that problem of arroyos in the canyon in several ways: by build
ing dams inside side-canyons above the elevation of the main canyon to
store rainwater; by laying out field systems that that rainwater could irri
gate; by storing rainwater coming down over the tops of the cliffs rimming the canyon's north wall between each pair of side-canyons; and by building
a rock dam across the main canyon.

The other major environmental problem besides water management in
volved deforestation, as revealed by the method of packrat midden analysis. For those of you who (like me until some years ago) have never seen pack-
rats, don't know what their middens are, and can't possibly imagine their
relevance to Anasazi prehistory, here is a quick crash course in midden
analysis. In 1849, hungry gold miners crossing the Nevada desert noticed some glistening balls of a candy-like substance on a cliff, licked or ate the
balls, and discovered them to be sweet-tasting, but then they developed nausea. Eventually it was realized that the balls were hardened deposits
made by small rodents, called packrats, that protect themselves by building nests of sticks, plant fragments, and mammal dung gathered in the vicinity,
plus food remains, discarded bones, and their own feces. Not being toilet-trained, the rats urinate in their nests, and sugar and other substances crystallize from their urine as it dries out, cementing the midden to a brick-like consistency. In effect, the hungry gold miners were eating dried rat urine
laced with rat feces and rat garbage.

Naturally, to save themselves work and to minimize their risk of being
grabbed by a predator while out of the nest, packrats gather vegetation
within just a few dozen yards of the nest. After a few decades the rats'
progeny abandon their midden and move on to build a new nest, while the

crystallized urine prevents the material in the old midden from decaying.
By identifying the remains of the dozens of urine-encrusted plant species
in a midden, paleobotanists can reconstruct a snapshot of the vegetation
growing near the midden at the time that the rats were accumulating it,
while zoologists can reconstruct something of the fauna from the insect and
vertebrate remains. In effect, a packrat midden is a paleontologist's dream: a
time capsule preserving a sample of the local vegetation, gathered within a
few dozen yards of the spot within a period of a few decades, at a date fixed
by radiocarbon-dating the midden.

In 1975 paleoecologist Julio Betancourt happened to visit Chaco Canyon while driving through New Mexico as a tourist. Looking down on the
treeless landscape around Pueblo Bonito, he thought to himself, "This place
looks like beat-up Mongolian steppe; where did those people get their timber and firewood?" Archaeologists studying the ruins had been asking
themselves the same question. In a moment of inspiration three years later,
when a friend asked him for completely unrelated reasons to write a grant
proposal to study packrat middens, Julio recalled his first impression of
Pueblo Bonito. A quick phone call to midden expert Tom Van Devender established that Tom had already collected a few middens at the National Park
Service campground near Pueblo Bonito. Almost all of them had proved to
contain needles of pinyon pines, which don't grow anywhere within miles today but which had nevertheless somehow furnished the roof beams for
early phases of Pueblo Bonito's construction, as well as furnishing much of
the charcoal found in hearths and trash middens. Julio and Tom realized
that those must be old middens from a time when pines did grow nearby,
but they had no idea how old: they thought perhaps just a century or
so. Hence they submitted samples of those middens for radiocarbon dating. When the dates came back from the radiocarbon laboratory, Julio and Tom
were astonished to learn that many of the middens were over a thousand years old.

That serendipitous observation triggered an explosion of packrat mid
den studies. Today we know that middens decay extremely slowly in the
Southwest's dry climate. If protected from the elements under an overhang
or inside a cave, middens can last 40,000 years, far longer than anyone
would have dared to guess. As Julio showed me my first packrat midden
near the Chaco Anasazi site of Kin Kletso, I stood in awe at the thought that
that apparently fresh-looking nest might have been built at a time when mammoths, giant ground sloths, American lions, and other extinct Ice Age mammals were still living in the territory of the modern U.S.

In the Chaco Canyon area Julio went on to collect and radiocarbon-date
50 middens, whose dates turned out to encompass the entire period of the
rise and fall of Anasazi civilization, from
a.d.
600 to 1200. In this way Julio
was able to reconstruct vegetational changes in Chaco Canyon throughout the history of Anasazi occupation. Those midden studies identified defor
estation as the other one (besides water management) of the two major envi
ronmental problems caused by the growing population that had developed
in Chaco Canyon by around
a.d.
1000. Middens before that date still incor
porated pinyon pine and juniper needles, like the first midden that Julio had
analyzed, and like the midden that he showed me. Hence Chaco Anasazi set
tlements were initially constructed in a pinyon/juniper woodland unlike the
present treeless landscape but convenient for obtaining firewood and construction timber nearby. However, middens dated after
a.d.
1000 lacked
pinyon and juniper, showing that the woodland had then become com
pletely destroyed and the site had achieved its present treeless appearance.
The reason why Chaco Canyon became deforested so quickly is the same
as the reason that I discussed in Chapter 2 to explain why Easter Island and
other dry Pacific islands settled by people were more likely to end up defor
ested than were wet islands: in a dry climate, the rate of tree regrowth on
logged land may be too slow to keep up with the rate of logging.

The loss of the woodland not only eliminated pinyon nuts as a local food
supply but also forced Chaco residents to find a different timber source for
their construction needs, as shown by the complete disappearance of
pinyon beams from Chaco architecture. Chacoans coped by going far afield
to forests of ponderosa pine, spruce, and fir trees, growing in mountains up
to 50 miles away at elevations several thousand feet higher than Chaco
Canyon. With no draft animals available, about 200,000 logs weighing each
up to 700 pounds were carried down the mountains and over that distance
to Chaco Canyon by human muscle power alone.

A recent study by Julio's student Nathan English, working in collabora
tion with Julio, Jeff Dean, and Jay Quade, identified more exactly where the big spruce and fir logs came from. There are three potential sources of them
in the Chaco area, growing at high elevations on three mountain ranges
nearly equidistant from the canyon: the Chuska, San Mateo, and San Pedro Mountains. From which of those mountains did the Chaco Anasazi actually
get their conifers? Trees from the three mountain ranges belong to the same
species and look identical to each other. As a diagnostic signature, Nathan

used isotopes of strontium, an element chemically very similar to calcium and hence incorporated along with calcium into plants and animals. Stron
tium exists as alternative forms (isotopes) differing slightly in atomic weight, of which strontium-87 and strontium-86 are commonest in nature. But the strontium-87/strontium 86 ratio varies with rock age and rock rubidium content, because strontium is produced by radioactive de
cay of a rubidium isotope. It turned out that living conifers from the three
mountain ranges proved to be clearly separated by their strontium-87/ strontium-86 ratios, with no overlap at all. From six Chaco ruins, Nathan
sampled 52 conifer logs selected on the basis of their tree rings to have been
felled at dates ranging from
a.d.
974 to 1104. The result he obtained was that two-thirds of the logs could be traced by their strontium ratios to the
Chuska Mountains, one-third to the San Mateo Mountains, and none at all
to the San Pedro Mountains. In some cases a given Chaco building incorpo
rated logs from both mountain ranges in the same year, or used logs from
one mountain in one year and from the other mountain in another year,
while the same mountain furnished logs to several different buildings in the
same year. Thus, we have here unequivocal evidence of a well-organized,
long-distance supply network for the Anasazi capital of Chaco Canyon.

Despite the development of these two environmental problems that re
duced crop production and virtually eliminated timber supplies within
Chaco Canyon itself, or because of the solutions that the Anasazi found to these problems, the canyon's population continued to increase, particularly
during a big spurt of construction that began in
a.d.
1029. Such spurts went
on especially during wet decades, when more rain meant more food, more
people, and more need for buildings. A dense population is attested not
only by the famous Great Houses (such as Pueblo Bonito) spaced about a
mile apart on the north side of Chaco Canyon, but also by holes drilled into
the northern cliff face to support roof beams, indicating a continuous line
of residences at the base of the cliffs between the Great Houses, and by the
remains of hundreds of small settlements on the south side of the canyon. The size of the canyon's total population is unknown and much debated.
Many archaeologists think that it was less than 5,000, and that those enormous buildings had few permanent occupants except priests and were just
visited seasonally by peasants at the time of rituals. Other archaeologists
note that Pueblo Bonito, which is just one of the large houses at Chaco
Canyon, by itself was a building of 600 rooms, and that all those post holes
suggest dwellings for much of the length of the canyon, thus implying a
population much greater than 5,000. Such debates about estimated popula-