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Archive for the ‘Paleoclimatology’ Category

Sleeping Ute Mountain and Surrounding Landscape from Four Corners

If you stand at the Four Corners monument and look in the direction of Colorado you will see Sleeping Ute Mountain dominating the view.  From this direction you are looking at the southwest side of the mountain, and in front of it you see the southern piedmont.  On the right side of the piedmont, though not visible from this distance, is Cowboy Wash.  It’s one of several ephemeral streams running from the mountain itself across the piedmont to the San Juan River.

One thing that might strike you about the view from this perspective is that it looks like an awfully dry, desolate, uninhabitable wasteland.  And you would be correct to think that.  The southern piedmont of Sleeping Ute Mountain is an extremely arid and inhospitable environment even by the standards of the Southwest, which is saying something.  It’s only a few miles from Mesa Verde to the east and the Great Sage Plain to the north, both areas that get relatively abundant rainfall and supported large and prosperous prehistoric communities, but it is worlds away from them environmentally.  While those areas get sufficient rainfall to support dry farming, and the Great Sage Plain is commercially farmed even today, the southern piedmont does not, and any type of agriculture there would have to rely on some sort of irrigation.  Today the Ute Mountain Ute tribe has a large irrigation project in the area, using water brought in from McPhee Reservoir, 45 miles to the north, via the Towaoc Canal.  The construction of the reservoir and the canal was part of the Dolores Project, which involved substantial archaeological excavation of the inundated area that significantly improved archaeological understanding of the prehistory of the region.  This work took place from 1978 to 1985 and was known as the Dolores Archaeological Project, the largest salvage archaeology project in US history.

McPhee Reservoir, Dolores, Colorado

The creation of the irrigated fields on the piedmont resulted in further salvage excavations in the 1990s.  Among the sites excavated was 5MT10010, which contained considerable evidence of a gruesome incident of probable cannibalism around AD 1150.  It is not the only site in the area to show evidence of cannibalism during this period; in fact, three other sites in the same community, excavated slightly earlier in connection with the construction of the canal, also showed evidence of having been destroyed in an incident involving extensive processing of human remains in a way suggesting cannibalism, and there are several other sites in the area showing similar assemblages, most from the same period but at least one from a later period.  It is at 5MT10010 that the most solid evidence for actual cannibalism, as opposed to processing of bones in a way that may or may not indicate actual consumption of human flesh, in the form of a coprolite that tested positive for the presence of human muscle tissue.

There are many questions that arise from these findings, but one of the most puzzling is also one of the simplest: what were people doing living at Cowboy Wash in the first place, and how did they manage it?  After all, they weren’t building giant dams and canals of the sort involved in the Dolores Project.  In many parts of the Southwest, especially upland areas like Mesa Verde, dry farming using only rainfall was standard during this period, and water control techniques were generally used only for domestic water if a nearby spring or other reliable source was not available.  There are a few springs on the southern piedmont that probably would have supplied sufficient domestic water for the small number of people living there, but the rainfall would definitely not have been sufficient to farm with.  The only source of water at all sufficient for agriculture would have been the occasional floods, from spring snowmelt and summer thunderstorms, that would flow through Cowboy Wash itself and the other drainages on the piedmont.  None of these flows permanently today, and there is no evidence that they ever did.  As at Chaco Canyon, then, which is similarly dry, farming would have to have been based on some sort of technique for capturing the floodwater.

Flowing Chaco Wash and Cliffs below Peñasco Blanco

There are a variety of ways this might be done, including diverting the rainwater from cliffs, as was done at Chaco, planting along the sides of the drainage where the floods would regularly overflow the banks, and what is known as “ak-chin” farming, as practiced by the O’odham of southern Arizona, which involves planting right in the path of the runoff at places where the velocity of the water is relatively low, as at the mouths of tributaries to main arroyos.  There are no sheer cliffs on the southern piedmont like the ones at Chaco, so probably a mix of overbank and ak-chin farming would have been practiced at Cowboy Wash.

A paper by Gary Huckleberry and Brian Billman addresses the nature of farming at Cowboy Wash, and also addresses a related issue, which is whether periodic entrenchment of arroyos due to drought played a role in the patterns of abandonment and migration that characterize Southwestern prehistory.  It is pretty clear by now that the paleoclimatological record shows periods of drought corresponding to periods of abandonment of certain parts of the Southwest, and one proposed mechanism for how this would have worked is that drought would have led to increased erosion and/or hydrological changes in the water table that led to the entrenchment of arroyos, which would have been disastrous for populations dependent on certain types of floodwater farming (especially overbank), as the broad floodplains of the local drainages would have been replaced by deep channels that took the water away quickly instead of letting it overflow to water the crops.  Ak-chin farmers would not necessarily have been affected to the same degree, but if the side drainages they used became entrenched as well they would not have been able to use their techniques either.  Thus, drought would lead to arroyo-cutting, which would lead people to leave formerly productive areas for others that were less affected.  This theory has been proposed as an explanation for certain events at Chaco, with the idea being that some of the social changes late in the Chacoan occupation were due to degradation of the Chaco Wash and the need to change agricultural strategies.  The phenomenon of arroyo-cutting in general is richly illustrated in historic times at Chaco.  The early reports of the Chaco Wash from the nineteenth century indicate that it was a shallow, meandering drainage, much like the current condition of the Escavada Wash to the north and the “Chaco River” that is formed by the confluence of the two at the western end of the canyon and flows north to the San Juan.  By the early twentieth century, and accelerating since then, however, the Chaco Wash through the canyon has cut down significantly and there is a very deep arroyo channel apparent today.

Entrenched Arroyo at Chaco

The drought-downcutting-abandonment theory makes sense as far as it goes, but as Huckleberry and Billman point out there are some problems.  For one thing, the extent to which arroyo-cutting is actually linked to drought, rather than other factors including the specific geology of the area, is hotly debated and there is no consensus.  The idea that while drought may be one factor causing arroyo-cutting there are other factors involved as well is supported by the fact that in different drainages in the Southwest that have been studied in depth the periods of arroyo-cutting do not necessarily correspond to region-wide droughts or other climatic changes.  In some areas they do, but in other areas they don’t.  At Cowboy Wash specifically, the available evidence indicates that the wash began to entrench sometime before AD 950, and that it began to refill with sediment sometime between AD 1265 and 1400.  If abandonment does in fact correspond to arroyo-cutting, then presumably the Cowboy Wash area should have been abandoned between 950 and 1265, and possibly occupied before and after this.  If downcutting results from drought, there should also be evidence of drought during the 950 to 1265 period.

The basic upshot of the Huckleberry and Billman paper is that neither of these expectations is met.  The evidence for drought conditions at Cowboy Wash generally matches that for the rest of the region, with the major droughts in the mid-twelfth century and late thirteenth century AD and several smaller droughts at irregular intervals before then.  This doesn’t show any particular relationship to the stratigraphic evidence for arroyo-cutting, which seems to have been going on to some degree throughout the period from AD 950 to at least AD 1265.  Furthermore, the evidence for settlement doesn’t line up either.  The marginal nature of the Cowboy Wash area implies that it would probably not have been occupied for most of prehistory, and this was indeed the case.  There were a few ultimately unsuccessful attempts to colonize the southern piedmont, however, and they don’t show any particular relationship to the periods of arroyo-cutting (although they do perhaps relate to periods of drought).  The first agricultural occupation of the area came during the Basketmaker III period, when a few pithouses were apparently used seasonally as summer fieldhouses, presumably associated with nearby fields, from about AD 600 to 725.  After these were abandoned, at a time which may correspond to a drought, the area does not seem to have been occupied again for more than three hundred years.  Then, around AD 1050, a few permanent, year-round sites were built.  These seem to have been occupied for only a few years, however, as there was no significant buildup of trash associated with them.  After they were abandoned, three larger villages, including one at Cowboy Wash, were established around AD 1075.  These had extensive trash deposits and seem to have been occupied for one or two generations.  These communities were apparently abandoned, however, when the next occupation began in the 1120s by a population with apparent links to the Chuska Mountain area to the south.  This occupation at Cowboy Wash is the community that was apparently destroyed around AD 1150 (again coincident with a major drought) when its inhabitants were mutilated and cannibalized.  After this event, the area was once again abandoned until about AD 1225, when two new communities were founded, including one again at Cowboy Wash.  Within a few decades the population at Cowboy Wash appears to have aggregated at Cowboy Wash Pueblo, following a typical pattern for the region.  Also typical of the region, the whole southern piedmont seems to have been abandoned by AD 1280, at the time of the “Great Drought” that coincides with major changes throughout the Southwest.

Entrenched Chaco Wash from Cliff Top near Pueblo Bonito

So basically, all of the attempts at year-round occupation of the southern piedmont seem to have occurred during the period that Cowboy Wash was being downcut.  While these were all ultimately unsuccessful, some lasted for a few decades, so clearly they were able to grow some food at some times.  This strongly implies that at least in this case, arroyo-cutting was not particularly linked to abandoned, although drought probably was.  Huckleberry address the issue of how farming could have been done during periods of downcutting by looking at Cowboy Wash and its tributaries today.  They find that while some portions of the main wash, especially, are indeed heavily downcut, other portions are not, and they label this type of drainage a “discontinuous ephemeral stream,” which is to say, a normally dry wash with some portions that are severely downcut and others that are not.  On the uncut portions, which include much of the length of the tributaries, overbank or ak-chin farming could easily be done today, and this was presumably the case in antiquity as well.  The hydrology of the area is such that the areas of downcutting would not have been stable, and would have tended to migrate upstream, but the complexity of the system is also such that this would not have made the entire system unusable; while some parts were being newly cut, others would be filling in, and prehistoric farmers would merely have to move their fields around a bit rather than abandoning the area entirely.

All that being said, however, the question of why people were trying to settle this quite harsh and difficult area in the first place.  It is interesting to note that the attempts at settlement generally came during periods of relatively favorable environmental conditions, which would have made this area a bit less forbidding than usual, as well as during times of increased regional population, when all the good land may well have been taken and some people were forced to seek out the more marginal areas.  The violence that appears to have accompanied the drought of the twelfth century, especially, suggests that when the good times came to an end social relations got very bad very fast.  Huckleberry and Billman suggest that the reason people did end up abandoning Cowboy Wash, the times when they were not attacked, was merely drought itself, which they were unable to cope with as well as other populations, even those who also used floodwater farming techniques, because the size of the watershed was relatively small and the amount of rainfall feeding the washes was also small, so the total amount of water they had to work with was much smaller even in good times than at place like Chaco with large watersheds.  In that context, even a small decrease in annual precipitation could be devastating, leading to failed harvests and the need to move away.

Non-Entrenched Escavada Wash from New Mexico Highway 57

Indeed, there is evidence that the time of the massacre at Cowboy Wash was very difficult for the people there.  Archaeobotanical studies of pollen and other plant remains showed that there was apparently little or no maize in or around 5MT10010 at the time of abandonment, which is quite surprising for a Pueblo site.  The plant remains that were there were mostly from wild plants such as chenopod, amaranth, and tansy mustard, all of which would have been available in the spring and likely would have been intensively collected if there were no stored corn available due to a failed harvest the previous fall.  In addition to pinpointing the season in which the incident occurred, this implies that times were very tough for the inhabitants of 5MT10010, and perhaps for their attackers too.  The coprolite showed no sign of having plant material in it, which suggests that whoever left it had not just eaten some corn at home before setting out to attack 5MT10010.

Another paper associated with the project, by Patricia Lambert, suggests another problem the Cowboy Wash inhabitants apparently had: disease.  In this paper Lambert reports on analyses of ribs of individuals at 5MT10010 and other sites in the Cowboy Wash area dating to various periods of occupation that had lesions on them suggestive of those seen in modern collections of individuals known to have died of tuberculosis and (to a lesser extent) other respiratory diseases.  These lesions were found in 11 of 32 individuals from Cowboy Wash that had enough of their ribs left to examine.  One of the individuals with lesions was from 5MT10010.  This was an adult woman who was not one of the victims of the attack at site abandonment but who had instead died earlier and been formally buried.  Lambert also examined comparative collections of remains from Pueblo Bonito at Chaco and Elden Pueblo near Flagstaff Arizona.  Only 3 of the 45 individuals from Pueblo Bonito and 2 of the 20 from Eldon Pueblo had similar lesions, suggesting that this disease was much more prevalent at Cowboy Wash than at these other sites, even though it was not absent at them.  Lambert notes that tuberculosis is an opportunistic disease that tends to strike people whose systems are compromised by other problems such as hunger and stress.  The evidence for physical violence in the Cowboy Wash sample, even setting aside the cannibalism assemblages, was much greater than in the other two samples as well.  Combined with the harsh environment, this suggests strongly that Cowboy Wash was a difficult place to live for several reasons.  Farming was possible but risky, and when conditions turned bad both hunger and violence from other hungry people were constant threats.

Given this context, the occurrence of extreme events such as cannibalism incidents at Cowboy Wash starts to make some sense.  Cowboy Wash is a place of extremes.
ResearchBlogging.org
Huckleberry, G., & Billman, B. (1998). Floodwater Farming, Discontinuous Ephemeral Streams, and Puebloan Abandonment in Southwestern Colorado American Antiquity, 63 (4) DOI: 10.2307/2694110

Lambert, P. (2002). Rib lesions in a prehistoric Puebloan sample from southwestern Colorado American Journal of Physical Anthropology, 117 (4), 281-292 DOI: 10.1002/ajpa.10036

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T-Shaped Doorway at Lomaki, Wupatki National Monument

The paper by Glenn Davis Stone and Christian Downum that I mentioned in the last post, which evaluated the archaeological record of the Wupatki area of northern Arizona in the light of Ester Boserup‘s theory of agricultural intensification, was based largely on the data from an extensive archaeological survey of Wupatki National Monument done by the National Park Service in the 1980s.  This data is presented in a more complete form in an earlier paper that Downum cowrote with Alan Sullivan.  This paper looks at the previous models proposed for the settlement and abandonment of Wupatki in the context of the new data from the survey.

Cinder Cones from the Citadel, Wupatki National Monument

The most influential model for the prehistory of Wupatki has been that presented by Harold Colton of the Museum of Northern Arizona based on work done in the 1930s and 1940s.  Colton saw the extreme aridity of Wupatki as having discouraged settlement there until the eruption of Sunset Crater Volcano in AD 1064 spread a layer of volcanic ash over the area.  This ash acted as a natural mulch to retain water from the infrequent rains which would otherwise evaporate from the thin soil.  Colton looked at the large number of sites that seemed to have been built in the aftermath of the eruption and saw a “land rush” in which people from all over the local area come to Wupatki to take advantage of the improved conditions for farming from the ash fall.  Over time, however, the ash cinders began to blow away in the strong winds and the productivity of the land declined, so the people began to aggregate into the large pueblos for which the Wupatki area is best known.  Once in these aggregated villages, the poor sanitary conditions of living in such close quarters, combined with the continuing decline of agricultural conditions, forced the abandonment of the whole area some time in the thirteenth century.

Wall Abutment, Wupatki Pueblo

This is a plausible story on the face of it, but Colton’s account has been challenged more recently by other archaeologists who point out that a great many of the structures built soon after the ash fall that Colton included in calculating the population increase were small, ephemeral structures that probably served as field houses or other special-use locations rather than year-round dwellings.  This implies that Colton was double-counting both these impermanent structures and the actual permanent houses of the people who used them, thus coming up with inflated population figures on which he based his “land rush.”  The systematic nature of the survey in the 1980s provided the opportunity to determine just how many sites there really were and how many actually served as permanent dwellings.

The Citadel and Sunset Crater from Lomaki, Wupatki National Monument

As Downum and Sullivan tell it, the results basically vindicate Colton’s critics.  The vast majority of the structures found were small and relatively impermanent, with few artifacts.  In addition, a careful tabulation of sherd types at most of the sites showed that the immediate post-eruption period, far from being the land rush of Colton’s theory, was actually a time of relatively limited occupation.  There were more sites from this period than from the pre-eruption period, when the area was nearly uninhabited, but still not very many.  It was not until a few decades later, starting around AD 1130, that building began to really pick up, as indicated by both sherd types and tree-ring dates.  The high point of construction didn’t come until the 1160s, a century after the initial eruption.  (It is actually not clear how long the eruptions continued after the beginning around 1064, and there may well still have been occasional activity by the volcano this late or even later.)  Construction seems to have effectively ceased by 1220, and the area was probably abandoned not long after that.

Beam Sampled for Tree-Ring Dating, Wupatki Pueblo

The upshot of all this for Colton’s theory is that, while it does seem to be true that the ash improved the suitability of Wupatki for agriculture, people didn’t immediate act to take advantage of this.  Downum and Sullivan propose that this may have been because it took some time for the effects of the ash fall on the soil to manifest, but I think a more plausible explanation for this can be found by looking outside the immediate area to the larger region.  The decades after 1130 were a time of extensive drought throughout the northern Southwest.  This is when Chaco collapsed (or at least declined), and there were likely extensive migrations all around the region.  In this context, people may have come to Wupatki less from the “pull” factor of the beneficial effects of the volcanic ash and more from the “push” factors of drought and/or political instability elsewhere.  Of course, there were at least some people farming at Wupatki before this, so the fertility of the area may have become well known at the same time as things were deteriorating elsewhere, making both push and pull factors part of the regional dynamics.

Great Kiva at Wupatki Pueblo

In line with the arguments in the later paper by Downum and Stone, Downum and Sullivan here argue that agriculture for most of the period of occupation of Wupatki was extensive rather than intensive.  They do claim, however, that intensification came right at the end of the occupation period, after 1220, on the basis of more intensive usage of the sites from that period based on sherd counts.  This is kind of dubious, and it appears that Downum changed  his mind about it in the eight years between this paper and the later one.  Intensification at this can, however, be incorporated into the argument made in the later paper that intensification was impossible in this area due to ecological conditions.  Once people began to leave the area, perhaps spurred by increased warfare and/or continuing climatic instability, those who remained would not necessarily have been able to secure access to the large amounts of land they had had claimed earlier as part of the consolidated political groups associated with the large pueblos in the Stone and Downum model.  These few remaining farmers may then have attempted to intensify production on the smaller amounts of land available to them.  Given the aridity of the area, however, this would not have worked reliably enough to allow them to stay, so within a few decades or less they would leave as well, leaving the entire Wupatki area abandoned by 1275.  Note that this is when the famous “Great Drought” associated with the abandonment of Mesa Verde and other areas began, so the aggregation and abandonment processes associated with Wupatki may well have been different from the similar processes elsewhere in the Southwest.

Upper Walls Built on Rock Outcrop, Wupatki Pueblo

Since I’ve been taking note of the scholarly context of the papers I’ve been discussing lately, I should point out that this one is very much an archaeology paper, and a classic processual one at that, with lots of statistics and an explicit model of interactions between people and the environment.  This certainly makes it more “scientific” than, say, the later paper by Downum and Stone, which is more anthropological and not very scientific at all, but as with many such archaeological papers the scientific trappings are somewhat superficial.  This is definitely not as rigorous an attempt at quantitative social science as the economics paper on plowing and gender roles I discussed a little while ago, for instance.  I would therefore argue that this is only science in a somewhat questionable expansive sense, and not necessarily anthropology at all, despite the frequent claims of processualists to be doing “archaeology as anthropology.”  Again, however, that doesn’t mean it’s not worthwhile scholarship.  Regardless of how it’s classified, this is interesting research that can serve as a useful source of data for a variety of other studies such as the one Downum later did with Stone.
ResearchBlogging.org
Sullivan, A., & Downum, C. (1991). Aridity, activity, and volcanic ash agriculture: A study of short-term prehistoric cultural-ecological dynamics World Archaeology, 22 (3), 271-287 DOI: 10.1080/00438243.1991.9980146

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Civil Engineering Plaque at Mesa Verde

I’ve previously discussed water control technologies at Chaco, where they were particularly important given the extreme aridity of that area even by Southwestern standards.  There is abundant evidence, however, that water control was a widespread activity throughout the ancient Southwest, even in areas with more reliable water sources.  The best-studied water control systems have been the impressive large-scale canal systems built by the Hohokam in southern Arizona, but less elaborate systems are known in the northern Southwest as well.

Among the better-studied of these systems are those in the Mesa Verde area of southwestern Colorado.  In comparison to Chaco especially this area is much more suitable for agriculture.  The Mesa Verde proper in particular is high enough that it gets quite a bit of regular precipitation, and it is generally thought that the majority of agriculture on the mesa throughout its occupation was dry farming on the mesa top, depending only on direct rainfall.  Interestingly enough, however, there is extensive evidence of water control features even in this more favorable environment.  A detailed description of some of them can be found in an article by Arthur Rohn published in 1963.  He focused on two main types of soil and water control: checkdams forming small terraces, presumably agricultural, along intermittent drainages and large reservoirs, probably for domestic water.  The checkdams, which have since been discovered in other parts of the greater Mesa Verde region such as Hovenweep as well as other regions of the northern Southwest (including Chaco), consisted of small masonry walls, laid without mortar, which served to hold back water and soil which would otherwise drain right off the mesa top during rainstorms.  Some drainages had dozens of these, typically about a yard high and a few yards apart.  Most had been breached at some point after the abandonment of the area and were visible only as rock alignments of varying lengths and heights, but some apparently still held soil and water back well enough that they were covered in vegetation, preventing Rohn from observing much about them.  The agricultural function of these terraces is suggested by the frequent association with them of small structures generally interpreted as seasonal field houses.

Checkdam, Hovenweep National Monument

It is not at all clear, however, why the people on Mesa Verde would have needed to go to the effort to build all these terracing systems when they had so much fertile land right on the mesa top.  Rohn calculated that the likely extent of the terraces added only about 1% of the total area of tillable land on top of the mesa.  He suggested several potential reasons for their construction, including depletion of mesa-top soils, increasing population and subsequent need for more intensive farming, and cultivation of specialized crops of high value that made the additional effort invested in constructing the terraces worthwhile.  Ultimately, however, Rohn had insufficient data to come to any firm conclusions about the purposes of the terraces, and as far as I can tell the situation has not improved much since his time despite the much more extensive paleoclimatic data now available.

The other water control features that Rohn described were the large reservoirs associated with certain of the more densely populated areas of the mesa.  Most of these consisted of large dams, much larger than the small checkdams, across certain canyon heads, where they likely impounded water either for use right there or to soak down through the porous sandstone to feed springs underneath.  These reservoirs thus used the natural characteristics of the canyon heads and required relatively little additional effort to store water for human use.

Far View Reservoir, Mesa Verde

The best-known reservoir on Mesa Verde, however, which Rohn described in detail, was quite different.  Rohn called it Mummy Lake, which was the standard name for it in his time, but it is now often known as Far View Reservoir.  This is a large oval masonry structure, of mostly artificial construction and about 90 feet in diameter.  It is near the cluster of sites known as the Far View Group, including Far View House, which is often claimed to be an outlying Chacoan great house.  These sites mostly date to the Pueblo II period in the eleventh and early twelfth centuries AD (contemporaneous with the height of the Chaco system), which makes them earlier than the most impressive sites on the mesa, which date to the Pueblo III period (late twelfth and thirteenth centuries AD) when Mesa Verde apparently had its highest population.

Far View Reservoir Intake Channel, Mesa Verde

Far View Reservoir was apparently not used for agricultural irrigation, as it has an intake channel but no outlet.  It was fed by an elaborate canal system upstream that channeled water to down the mesa.  Rohn noted that the intake channel was of quite sophisticated design:

The feeder ditch coming from the north did not empty directly into the north side of the reservoir, but rather ran by the west uphill side until it met the mouth of the intake channel at the southwest corner. There water was diverted into the inlet around a right-angle turn and conducted in a northeasterly direction into the reservoir. Such a complicated maneuver caused the suddenly slowed water to drop its silt burden in the intake channel, which could be easily dredged, rather than in the deepest part of the reservoir, where dredging operations would be difficult and would muddy the water.

Trenching of the reservoir by Earl Morris in 1934 revealed that the original bottom lay about 12 feet below the intake.  This would give the reservoir a maximum capacity of about 76,000 cubic feet, equivalent to about 1.74 acre-feet or 568,000 gallons.  That’s a lot of water.

Since there was no outlet from the reservoir, it presumably didn’t feed a system of irrigation canals.  What, then, was this water for?  Rohn’s answer, with which most other archaeologists have agreed, was that it was used for domestic water.  Trenching of the walls of the reservoir revealed pottery of Pueblo II date, contemporaneous with the nearby Far View sites, which makes sense.  A small ditch led off from the main ditch leading to the reservoir, emptying some of the water diverted from upstream into a small drainage with a series of checkdams similar to those documented elsewhere on the mesa, which were presumably farmed by the Far View residents.  Most of the water, however, went into the reservoir, from which it could be easily extracted with pots and brought home for cooking and other daily uses.  Residents of other parts of the mesa seem to have used nearby springs (perhaps fed by canyon-head reservoirs above them) for their domestic water, but there are no springs near the Far View group, so this elaborate reservoir seems to have been built to support the community there, which as Rohn pointed out was the largest concentration of population on this part of the mesa before the Pueblo III period.

Cliff Palace, Mesa Verde

At some point in late Pueblo II or early Pueblo III a very elaborate ditch was built carrying water from the Far View area south almost to the very end of the mesa.  This ditch skirts the Far View sites, suggesting that they were still occupied when it was constructed, but it heads toward the major cliff dwellings to the south that became the major focus of occupation in late Pueblo III.  It’s not clear exactly what this ditch led to, but the fact that it heads toward the major cluster of sites including Cliff Palace, in an area with few springs but a very large population during late Pueblo III, suggests that it likely supplied domestic water for these sites, especially after the abandonment of the Far View sites allowed the intake channel to Far View Reservoir to be blocked and all of the water from the whole system to be brought south.

Rohn mentioned in his article that while Far View Reservoir is the only such reservoir known from this part of Mesa Verde proper, there are several other such facilities known from elsewhere in the region, especially in the Montezuma Valley to the northwest.  A more recent article by Rich Wilshusen, Melissa Churchill, and James Potter (from 1997) provides a valuable summary of information known on reservoirs throughout the region, as well as detailed information on one reservoir studied intensively by the Crow Canyon Archaeological Center.  This reservoir is known as Woods Canyon Reservoir after Woods Canyon Pueblo, a late Pueblo III site nearby.  Also in this general area are a Chaco-era (late Pueblo II) outlying great house known as the Albert Porter site and a site called Bass Ruin that apparently dates to the poorly understood early Pueblo III period, in between the decline of Chaco and the rise of the large aggregated pueblos and cliff dwellings in late Pueblo III.  This reservoir much less elaborate than the Far View one, consisting merely of an earthen dam built across a natural drainage, impounding the water behind it.

Excavation of both the dam and the impounded reservoir area, along with surface collection of sherds, showed that the dam was likely constructed during early Pueblo III or possibly earlier.  An innovative use of tree-ring dates from trees growing on top of the dam in the 1950s, which must have begun growing after the reservoir no longer held water, put the date of dam failure at no later than about AD 1350.  Assuming that it would have taken a century or two for the reservoir to fill with enough sediment for the dam to fail, the authors put the likely usage of the reservoir in early Pueblo III.  These two lines of evidence converge nicely.

White Ware Bowls at Chaco Visitor Center Museum

Another striking aspect of the potsherd evidence was the extraordinarily high prevalence of white wares (77%) and of jars (71%).  The predominance of white wares and the low occurrence of gray utility wares suggests that most of the sherds came from white ware jars used to carry water from the reservoir to habitation areas which broke in the process, and the lack of bowls shows that those habitation areas were not in the immediate vicinity of the reservoir.  Habitation sites usually have assemblages consisting mainly of gray ware jars, which were used for cooking, with large numbers of white ware bowls, which were used to serve food, as well.  The authors mention that previous work at Far View Reservoir (after Rohn) had shown a similar distribution of ceramic wares and forms, and the few sherds mentioned in Rohn’s article also show this distribution.  Given this, as well as the lack of nearby canals or soils suitable for farming, the authors conclude that this reservoir was likely used primarily or solely for storage of domestic water, as Rohn had argued for Far View Reservoir.  They also note that the dating was surprisingly early; these reservoirs are usually found in association with late Pueblo III aggregated sites, and there has been a frequent assumption that they served those communities.  The evidence from Woods Canyon, however, suggests that the reservoir was actually constructed well before Woods Canyon Pueblo, at a time when the local population lived at Bass Ruin or even in the Chacoan community around the Albert Porter site.

Gray Ware Jars at Chaco Visitor Center Museum

In addition to this interesting information about this one reservoir, the authors collected all the information available at the time on other reservoirs in the Mesa Verde region, including the extensive information published only in the so-called “gray literature” (i.e., reports from salvage excavations and other cultural resource management projects that are not easily available to the general scholarly community).  From this data set they find that there are two main categories of reservoirs: those built as integral parts of late Pueblo III aggregated villages and those like Wood Canyon Reservoir built near such villages but probably dating to an earlier period and associated with Chaco-era or immediate post-Chaco communities.  This implies that these large reservoirs may not have been a response to drought as climatic conditions deteriorated in the twelfth and thirteenth centuries, as is often assumed, but that they may instead have been monumental public architecture, like great houses, great kivas, and roads, associated with Chacoan communities and used to sustain the large populations of those communities.  As conditions did deteriorate, however, the existence of these communities and their dependable water sources may have encouraged others to join them, leading to the well-known process of aggregation and formation of large villages during the late Pueblo III period.

Furthermore, the creation of these large, permanent features would have required substantial labor and indicated a commitment of a community to a particular location for the long term.  This was likely a new development in the northern Southwest during Pueblo II, perhaps associated with Chacoan influence; previously, sites had been mostly occupied for quite short periods of time, and people seem to have moved very frequently.  From the eleventh century on, however, the trend is toward increasing commitment to particular localities, although the actual sites in which people lived didn’t necessarily last very long.  Multiple sites occupied one after another in a given area, with the general trend toward increased aggregation and more defensive locations, is typical throughout the Mesa Verde region in the period between AD 11oo and 1300, when the whole area was abandoned.  The role of Chaco Canyon, which is both one of the longest-occupied areas in the prehistoric Southwest and one where water control is most necessary, in all this is interesting to ponder.

Pueblo Bonito and Basin with Captured Rainwater

Finally, it’s worth noting the distinction between different uses of water here.  The largest quantities of water would have been needed for agriculture, but only at certain times of the year, and with careful planning the seasonal rains and spring runoff could be harnessed to adequately water the crops.  The amount of water needed for domestic use was much smaller, but it was needed all the time.  Springs were likely adequate for domestic use as long as populations remained small, but as larger communities developed in some areas with few springs more elaborate measures were necessary to ensure sufficient water was available at all times.  This was most obvious in very dry places like Chaco, but even better-watered areas like Mesa Verde began to have to deal with these issues as population increased and the climate changed.
ResearchBlogging.org
Rohn, A. (1963). Prehistoric Soil and Water Conservation on Chapin Mesa, Southwestern Colorado American Antiquity, 28 (4) DOI: 10.2307/278554

Wilshusen, R., Churchill, M., & Potter, J. (1997). Prehistoric Reservoirs and Water Basins in the Mesa Verde Region: Intensification of Water Collection Strategies during the Great Pueblo Period American Antiquity, 62 (4) DOI: 10.2307/281885

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Cliff Palace, Mesa Verde

Well, I said I would probably continue to do posts here while I was guest-blogging for Keith Kloor, and obviously that didn’t happen.  I did write some posts over there that would probably be of interest to my readers here, especially on the concept of “collapse” as applied to Chaco and Mesa Verde.  I’ll have some more posts here soon, but that’s what I’ve got for now.

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Looking East from the Pueblo Alto Trail

In my earlier post about Stephen Hall‘s recent paper reporting on maize pollen at Chaco Canyon dating as early as 2500 BC, I said briefly that this really shouldn’t be surprising to anyone who’s been following this kind of research closely, and also that I would discuss the context for it later.  Basically, the context is that there has been a considerable amount of evidence accumulating in the past twenty years or so firmly placing maize in the Southwest much earlier than most archaeologists had previously thought.  Much of this evidence has been gathered in a recent paper criticizing Jane Hill‘s arguments for the spread of maize having occurred as part of a migration of Uto-Aztecan speakers from Mesoamerica.  The earliest directly dated maize seems to be from the Los Pozos site in the Tucson Basin, with a date range of 2860 to 2470 BC (this represents the calibrated 95% confidence interval, as do all subsequent radiocarbon dates in this post unless noted otherwise).  According to the text of the paper, however, this date has been questioned, and the earliest uncontroversial direct date on maize comes from the aptly named Old Corn site near Zuni, with a range of 2460 to 2060 BC.  Note that the earliest date Hall found at Chaco (2567 to 2332 BC) has an upper bound slightly earlier than this, although the ranges overlap considerably.  Similarly early dates occur at some Tucson Basin sites and at Three Fir Shelter in northern Arizona, although the Three Fir Shelter date has a particularly large confidence interval.  The considerable geographic extent of these early maize dates implies that the initial introduction of maize to the Southwest may well have been quite a bit earlier than the earliest archaeological evidence for it.  It also suggests that at least in most areas, with some possible exceptions such as the Tucson Basin, maize was initially integrated into a hunter-gatherer lifestyle without causing major changes to either the subsistence system or the social structure of the groups adopting it.  This challenges many of the traditional assumptions about the effect of the introduction of agriculture on hunter-gatherer societies, including the idea that farming necessarily leads to an immediate shift to settled village life.  There is little to no evidence in most areas of any such shift for thousands of years after the earliest evidence for maize agriculture, suggesting that the impetus for such a transition (which did indeed happen in almost all parts of the Southwest eventually) must be sought elsewhere.

Una Vida from Petroglyph Area

Okay, so there’s plenty of evidence for agriculture as early as the pollen dates at Chaco in many parts of the Southwest, but Chaco ain’t Tucson.  The high, dry, harsh environment of the central San Juan Basin seems particularly unsuitable for agriculture compared to a lot of the other areas with early evidence for maize.  So surely this is a surprising finding at least for the local area, right?

Actually, no.  Evidence for the early use of maize in the general area of Chaco, though until now not within the canyon itself, has actually been out there for over twenty years, although it doesn’t seem to have gotten much attention in the subsequent literature.  It is described by Alan Simmons in an article from 1986, reporting on research done initially as a cultural resources management (CRM) project in connection with plans for coal mining in the area directly to the east of Chaco.  This project, known as ADAPT I, found many typical Archaic sites in the area, some of which were excavated as mitigation activities because of the expected impacts of mining.  (I believe this mining operation never actually happened, but I don’t know any of the details.)  These Archaic sites were typical for the San Juan Basin, which is to say that they were mostly lithic scatters with few diagnostic artifacts or datable materials, but a few of them ended up being very surprising in having more under the surface than was apparent at first, including possible hearths or ovens, some of which contained maize pollen associated with charcoal that was radiocarbon dated to the Archaic period and one of which even contained maize macrofossils that were directly dated to the late Archaic.  (The dates Simmons relates are apparently uncalibrated and are therefore not directly comparable to the calibrated date ranges given above, but they are comparable to the uncalibrated dates given by Hall for the pollen samples, including at the early end.)

Looking North from New Alto

This very unexpected result led to a new project, known as the Chaco Shelters Project, to try to get more data on the Archaic period in the Chaco area through intensive study of the type of site most likely to contain well-preserved material: rockshelters.  Two shelters were excavated, both outside of the park boundaries, one to the east (Sheep Camp Shelter) and one to the west (Ashislepah Shelter).  Neither provided much in the way of artifacts, which was something of a disappointment, but both showed evidence of Archaic use, and Sheep Camp Shelter contained macrobotanical remains of both maize and squash.  Neither contained maize (or squash) pollen.  Both maize and squash remains were directly dated to the late Archaic.  The squash results were more important, as the resulting dates were at the time the earliest evidence for the presence of squash in the Southwest.  Despite the presence of these direct dates, the earliest dates Simmons had were on charcoal associated with the maize pollen at the ADAPT I sites, and these could potentially be questioned given the known problems with this kind of indirect dating.  Now that Hall has come up with direct dates on the pollen itself which cover roughly the same timespan, however, Simmons’s indirect evidence looks more convincing than it may have at the time.

At the time Simmons was writing, there was much less direct evidence of Archaic agriculture in the Southwest than there is now, and the few reported early dates were problematic and disputed.  He therefore has a substantial and quite useful discussion of the general issue in addition to reporting his specific data.  He clearly comes down on the side advocating a gradual shift from hunting and gathering to agriculture, with maize being initially adopted by hunter-gatherers as part of a preexisting subsistence system and only becoming the primary means of subsistence considerably later.  This view was generally associated with arguments for in situ development of Anasazi culture out of the local Archaic tradition, and was opposed by those who preferred to see agriculture as being introduced later and all at once, perhaps as part of a migration of agriculturalists from the south (a position now maintained by Jane Hill, among some others).  With regard to the Chaco area specifically, Simmons accounts for the differences between the open ADAPT I sites (with maize pollen but little macrobotanical evidence) and Sheep Camp Shelter (maize and squash macrofossils but no pollen) by proposing that maize was initially integrated into a subsistence system involving seasonal mobility, with spring and summer being spent in the ADAPT I area, where the many sand dunes would have fostered the growth of wild plants, and winter being spent in the more protected rockshelters in the canyons to the west.  When maize and squash were added to the seasonal round, they would have been planted on or near the dunes in the spring, harvested in the fall, and taken back to the shelters to provide durable, storable food for the harsh and largely barren winter.  He admits that the shelter evidence, especially, is a bit weak, as it doesn’t really support the idea of extensive winter use of the shelters, but there were apparently many Archaic sites near the shelters that may provide more support for occupation of the general area during the winter.  He therefore concludes that the model he proposes is generally consistent with the data although more research is needed to confirm it.  He mentions Atlatl Cave, in the park and excavated by the Chaco Project, as one possible additional data point in favor of his model.

Pueblo Bonito from Peñasco Blanco

Hall’s new pollen data from within the canyon provides some support for Simmons’s model, especially in backing up the indirect dates with direct ones, but it also suggests some possible modifications to it.  It clearly seems to indicate that the canyon itself as well as the dunes to the east was used to grow corn during the Archaic.  This casts some doubt on Simmons’s proposal of seasonal mobility, at least in the way he frames it.  Growing corn in the canyon would imply occupation of it, perhaps in rockshelters such as Atlatl Cave, during the spring and summer in addition to (or instead of?) during the winter.  That is, Simmons’s seasonal mobility model could perhaps be “condensed” into a similar model of subsistence activities throughout the year, but with year-round residential occupation in the canyon.  People would presumably have still traveled around to gather resources in different areas at different times of year, but this would be “logistical” as opposed to “residential” mobility, with hunting and gathering parties setting out from a more permanent base camp solely to collect resources as opposed to moving the whole group to the location of the resources.  It is of course also possible that Simmons’s model should be modified in some other way, keeping the seasonal mobility but changing the role of the canyon in it, but it’s hard to see where else in the area would have been more suitable for winter occupation.

One of Simmons’s suggestions, that the advent of corn agriculture may have made the Chaco area an easier place to live by providing a fairly reliable food source in a place with limited and unreliable wild food resources, is particularly interesting.  The San Juan Basin is well-known for its considerable Archaic population, which has generally been interpreted as indicating seasonal mobility and specialized adaptations to the region’s sparse resources, but what if that population was actually supported in part by incipient maize (and squash) agriculture?  People often marvel at the apparent barrenness of the Chaco environment and the oddity of the idea that anyone would try to farm there, but this is another way to look at the harshness of the environment.  To put it differently, farming in Chaco is certainly difficult and risky, but can you imagine trying to live there without farming?
ResearchBlogging.org
Merrill, W., Hard, R., Mabry, J., Fritz, G., Adams, K., Roney, J., & MacWilliams, A. (2009). The diffusion of maize to the southwestern United States and its impact Proceedings of the National Academy of Sciences, 106 (50), 21019-21026 DOI: 10.1073/pnas.0906075106

Simmons, A. (1986). New Evidence for the Early Use of Cultigens in the American Southwest American Antiquity, 51 (1) DOI: 10.2307/280395

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Petroglyph Panel above Crevice Containing Packrat Midden CC-3

One important line of evidence in understanding the climatic history of Chaco Canyon, a subject of considerable interest given the harsh aridity of the current climate and the incongruous grandeur of the archaeological remains, has been the study of packrat middens.  These are collections made by packrats of materials found near their nesting locations, which they collect and then seal with urine.  The key thing about these middens, in addition to the preservation ensured by the protective coating of urine, is that the packrats have fairly small territories within which they collect material, so the contents of the middens, which can easily last for thousands of years, reflect the vegetation of the immediately surrounding area at the time the midden was created.  Since they generally consist of organic material that can be radiocarbon dated, the middens can potentially offer enormous insight into climate change over time.

There is some difficulty with this, however, because the proper interpretation of the contents of the middens is not always clear.  Interpretations are often based on the visible plant remains (known as “macrofossils”), but these may be biased by behavioral factors owing to the diet and habits of the packrats themselves.  Juniper, for example, tends to be overrepresented because it is one of the main components of the packrat diet.  One way to avoid this problem is to look not just at the macrofossils but also at the pollen grains contained in the middens.  Since these are too small for the packrats to be consciously choosing them, they would have to be deposited by wind or other natural processes, which may make them more reliable clues to the relative abundance of various plant species as opposed to the mere presence or absence of species.

Crevice Containing Packrat Midden CC-3

One researcher who has long been associated with this point of view is Steve Hall, who was for many years a professor of geography at the University of Texas.  He is now retired and has a consulting firm in Santa Fe specializing in the geology of Southwestern archaeological sites.  He has been involved with packrat midden studies at Chaco since the 1980s, and has recently published an important article reporting on a recent reanalysis of some of the middens that resulted in some remarkable conclusions.

What Hall found in looking at the midden samples was corn pollen.  This is not surprising in and of itself, since the Chacoans were of course an agricultural people with a corn-based subsistence strategy.  The focus of the paper, however, is on radiocarbon dating of the pollen grains, which resulted in some remarkable findings.  Conventional radiocarbon dating requires much larger samples than would be possible for pollen grains, but the relatively new accelerator mass spectrometry (AMS) technique requires only very small samples, and also tends to give more precise dates than traditional radiocarbon dating.  Hall dated pollen samples from two packrat middens in crevices near Casa Chiquita, referred to as CC-2 and CC-3.  These are both right along what is now the “Petroglyph Trail” section of the Peñasco Blanco trail, and fragments of the middens can be seen from the trail, although they are now substantially broken up from sampling for study.  CC-3 is right beneath one of the best-known petroglyph panels on the trail.  CC-2 is not as directly associated with any particular petroglyph panel, but it is in close proximity to a few.

Crevice Containing Packrat Midden CC-2

The results basically showed that the corn pollen was old.  Some of the samples were very old indeed, on a par with the oldest known maize in the Southwest.  The oldest sample, from CC-3, had a 95% probability of being from between 2567 and 2332 BC.  Another sample, from CC-2, had a 95% confidence interval of 1457 to 1254 BC, while the rest of the samples from both middens were later but still pretty old, dating to the 800s through 400s BC.  In addition to the AMS dating, Hall measured the abundance of maize pollen in the overall pollen samples from the middens and looked at the size of the pollen grains themselves.  He found that maize pollen was quite abundant, and that the grains were consistently bigger than comparison samples of modern and later Chacoan maize pollen.  Since maize pollen is pretty heavy in general, and this particular maize pollen was even heavier than usual, he concluded that it couldn’t have traveled far, and that it must have been blown into the crevices from a cornfield directly in front of them, where the packrats later incorporated it into their middens along with various other materials.  One thing he points out in the article is that twigs from the same middens dated to much later than the pollen, which casts doubt on the common practice of dating middens only by single macrofossils.  It seems that material incorporated into the middens can vary considerably in age, and direct AMS dating of pollen is a better approach to determining its age than dating of associated macrofossils.  There’s a bit more to the article, but those are the highlights.

Proposed Site of Archaic Cornfield near Casa Chiquita

This actually shouldn’t be all that surprising to anyone who has been following recent developments in the archaeology of the Archaic period in the Southwest, but I have not been, and I found it pretty surprising.  After reading this paper I looked into earlier research a bit and found that there has been quite a bit of direct dating of maize and associated materials resulting in comparable dates, including some from rockshelters near Chaco.  I knew that there was considerable evidence for early maize further south, in places like the Tucson area, but it seems that evidence for Archaic corn agriculture is pretty well established further north as well.  The implications of this for Southwestern archaeology in general are therefore fairly limited, and it mostly just adds another data point to the accumulating evidence on Archaic agriculture.

The implications for Chaco specifically, however, are huge.  This shows pretty convincingly that agriculture in the canyon goes back to long before the main flourishing of the Chaco System in the eleventh and early twelfth centuries AD, and it suggests that Chaco’s importance may go back to thousands of years before that at least on some level.  One interesting thing Hall points out is the proximity of these middens, and by implication the nearby cornfield, to Atlatl Cave, a well-known Archaic site on the mesa top about a kilometer away.  The radiocarbon dates for Atlatl Cave match up pretty closely with the later dates on the corn pollen, and Hall suggests that the people who lived there were the same ones growing the corn in the canyon below.  It’s a reasonable suggestion, and it puts a lot of interpretations of the early prehistory of Chaco in a new light.  I’ll be reading up on the Archaic period and trying to understand what this discovery means for our understanding of Chaco during that time.  I should have some more posts on the topic soon.  For now, though, I’ll just note that this may indicate more continuity of occupation or at least cultural knowledge of Chaco than is generally assumed.
ResearchBlogging.org
Hall, Stephen A. (2010). Early maize pollen from Chaco Canyon, New Mexico, USA Palynology, 34 (1), 125-137 : 10.1080/01916121003675746

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Reno, Nevada

I’ve talked a bit about Jane Hill‘s theory that agriculture was introduced to the Southwest by a migration of speakers of Uto-Aztecan languages from Mesoamerica, which she supports mostly through somewhat unconvincing linguistic evidence.  A recent paper in, yes, PNAS offers a strong set of counterarguments to Hill’s theory, and offers an alternative theory in which maize agriculture gradually diffused north from Mesoamerica through a chain of hunter-gatherer groups in western Mexico speaking Southern Uto-Aztecan languages.

"Plateau Country Trees" Sign, Pipe Spring National Monument

The authors, who include some fairly prominent Southwestern archaeologists, frame their argument explicitly in opposition to Hill’s, and they begin by pointing out the weakness of her linguistic arguments for a set of maize-related terms in Proto-Uto-Aztecan, which is a crucial part of her theory that the family originated in Mesoamerica and spread northward.  They also point to DNA evidence showing that Uto-Aztecan speakers in the Southwest and in Mesoamerica genetically resemble surrounding populations speaking other languages much more than they resemble each other, which mostly suggests a long period of interaction and intermarriage among neighboring groups on both ends, implying that whatever migration occurred probably happened far in the past and not providing any particular evidence for which way it went.  This evidence is interesting, and I had not seen it before.  I think it may be the strongest evidence against Hill’s theory, which only works with a relatively recent migration from south to north that would presumably show up in the genetic evidence.

Sign Describing Pinyon and Juniper, Pipe Spring National Monument

The other evidence they present, both on behalf of their own theory and against Hill’s, is quite a bit weaker, I think.  They argue for a Uto-Aztecan Urheimat in the Great Basin at the very opposite end of the current distribution of the family from Hill’s proposal of western Mesoamerica, and for a very early dispersal of the family to the south starting around 6900 BC.  They make these proposals based on evidence from the suite of terms for animals and plants that can be reconstructed for Proto-Uto-Aztecan.  As Hill noted in her papers, this set of flora and fauna is found throughout most of the current range of the languages, from the Southwest down far into Mexico, and so it doesn’t directly provide much evidence for the initial location of the protolanguage.  The authors of this paper, however, argue from negative evidence that it can be pinpointed to a northerly location at an early time.  Their reasoning is interesting: apparently two terms that cannot be reconstructed for Proto-Uto-Aztecan, even though the items they refer to occur almost everywhere Uto-Aztecan languages are spoken, are “pinyon” and “oak.”  They take this to mean that the protolanguguage was spoken in a context where there were no pinyons or oaks, and the only place they can find this to be the case is the west-central Great Basin between 9700 and 6900 BC.  Around 6900 BC conditions in the Great Basin became considerably drier, and the authors point to this event as the catalyst for the initial breakup of the Proto-Uto-Aztecan speech community with the ancestors of what would become the Southern Uto-Aztecan languages migrating south in search of wetter areas, eventually ending up in the southern Southwest and northwest Mexico, with the ancestors of Northern Uto-Aztecan speakers remaining in the north at least until pinyons began to spread northward around 5500 BC.  Later, when maize agriculture began to diffuse northward from Mexico, the Southern Uto-Aztecan groups were arranged in a long continuum along the Pacific coast and the adjacent mountains, and they passed the maize plant and knowledge of its cultivation steadily northward until it reached the southern Southwest and was adopted by local hunter-gatherer groups there, only some of which spoke Uto-Aztecan languages.  The earliest well-dated maize remains in the Southwest (mostly from the Tucson area) date to around 2100 BC, so the posited Southern Uto-Aztecan continuum must have been in place by then, which would definitely require a very early date for the initial breakup of the protolanguage if indeed it originated in the north.

Live Oak, Pipe Spring National Monument

This all makes sense, as far as it goes, but it’s really just as speculative as Hill’s theory, and as the authors point out what’s really needed to make further progress in understanding this issue is more archaeological data on the area in between Mesoamerica and the Southwest, i.e., the area through which maize would have to have passed on its way north.  They do point out one important strike against Hill’s theory, which is the lack of any clear similarities in material culture between the earliest agricultural sites in the Southwest and contemporary agricultural sites in Mesoamerica, which may argue for a diffusion of agriculture but could possibly also just mean that the migration of agriculturalists, whether or not they spoke Uto-Aztecan, took place much earlier.  Indeed, this paper appears to put the starting point for the spread of maize at the place and time of the earliest macrobotanical evidence for maize (that is, the earliest actual cobs found in the archaeological record), which comes from Guilá Naquitz Cave in Oaxaca around 4300 BC.  This is a very odd choice of place and time, however, since it is increasingly clear from starch grain and phytolith evidence, as well as genetics and the distribution of wild teosinte varieties, that maize was initially domesticated considerably earlier and further west, being definitely present in the Rio Balsas area of Guerrero by 6700 BC.  It’s particularly odd since they do in fact cite one of the recent papers presenting this evidence.  Since maize would certainly have had to initially diffuse from West Mexico to Oaxaca, there’s no reason to think there’s anything special about Oaxaca or 4300 BC from the point of view of the history of maize agriculture in general; this just happens to be the oldest setting with sufficiently good preservation to allow macrobotanical evidence to survive.  Instead, any diffusion of maize to the Southwest would certainly have begun in West Mexico, and could have begun as early as 6700 BC.  A migration at this point would be very difficult to detect in the archaeological record, and the presence of maize cultivation in the Southwest this early would also be very difficult to see.  The paper’s authors do acknowledge the possibility that maize cultivation had been going on in the Southwest before the earliest evidence of it so far discovered, but they don’t seem to really grapple with the implications of that for their theories.

Trucks at Rest Stop, Luning, Nevada

In general, I find the kind of arguments used in this sort of research interesting but not very conclusive.  The use of reconstructible terms for flora and fauna has a long history in historical linguistics, but it has not actually been very successful in pinpointing the original locations of protolanguages.  One reason for this is that terms change over time, and it’s not always clear if a given word, even if shared by most languages in a family, has the same meaning now that it had in the protolanguage.  Negative evidence of the sort used here, in particular, is problematic.  What if Proto-Uto-Aztecan did actually have a term for “pinyon” that just happened to be lost or used for another plant in several of the daughter languages?  This is very much a possibility, and it’s basically impossible to judge its likelihood based on available evidence.  Also, while using paleoclimatological data to give dates to linguistic events is an interesting idea, I do think this paper goes a bit too far with it.  There are a lot of assumptions behind the idea that a drought in the Great Basin around 6900 BC caused the breakup of Proto-Uto-Aztecan, and given that this date is much earlier than all other dates that have been proposed for this event, those assumptions really need to be critically examined.

Pinyon Trees, Pipe Spring National Monument

Nevertheless, for all its flaws, this is an interesting and important paper, particularly in the way it seeks to incorporate more lines of evidence into the ongoing debate over the introduction of agriculture to the Southwest.  There has been so much research on this topic lately, much of it overturning long-held assumptions, that it is good to see some attempts to tie it all together, even if they are ultimately unconvincing.
ResearchBlogging.org
Merrill, W., Hard, R., Mabry, J., Fritz, G., Adams, K., Roney, J., & MacWilliams, A. (2009). The diffusion of maize to the southwestern United States and its impact Proceedings of the National Academy of Sciences, 106 (50), 21019-21026 DOI: 10.1073/pnas.0906075106

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