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Archive for the ‘Geology’ Category

Mt. Taylor from Chaco

As I’ve discussed before, the patterns of use and importation of chipped stone at Chaco are somewhat puzzling. Unlike many other commodities, such as wood, corn, and pottery, which were imported from specific distant locations within the Chacoan sphere of influence in astonishing quantities during the height of Chaco’s regional power, chipped stone seems to have been largely a mundane, utilitarian concern. Throughout all periods of Chaco’s occupation most chipped stone was local. At Chaco’s peak of power and influence between AD 1020 and 1130 there was a slight uptick in imports of stone, particularly a distinctive pink chert from the Narbona Pass area to the west.

As I noted in the earlier post, however, obsidian follows a different pattern from the other imported stones. It is most common not at the height of Chaco’s regional power in the eleventh century but much earlier, in the Basketmaker III period between AD 500 and 750, when it is the most common nonlocal type of chipped stone. This was a time when Chaco may have seen an earlier period of regional importance, although figuring out what was going on at this time is very difficult for several reasons. By the Pueblo I period the amount of obsidian seems to drop precipitously, and it doesn’t start to recover until the very end of Chaco’s period of Pueblo occupation after AD 1120. This pattern puts obsidian decidedly out of phase with most other material culture imports to the canyon, which tend to correlate with the well-known evidence for social complexity and monumental architecture that we associate with the Chaco Phenomenon.

A recent paper by Andrew Duff, Jeremy Moss, Tom Windes, John Kantner, and Steven Shackley tries to put the obsidian evidence on a firmer footing by using geochemical sourcing to identify the source outcrops for a broad sample of obsidian found at Chaco and at various Chacoan outlier communities in the San Juan Basin. As they note, this is the latest chapter in a complicated story. Way back in the 1980s, the Chaco Project did an extensive sourcing study of obsidian found in its excavations in the canyon using X-ray fluorescence (XRF), a non-destructive sourcing technique that was then relatively new in archaeology. Their results, reported by Cathy Cameron in a number of publications, were surprising. They seemed to show that the closest source of obsidian, Mt. Taylor, provided very little of the obsidian found at Chaco (about 4%), while a distant source, Red Hill in Catron County, New Mexico, provided a very high proportion, especially in the assemblages from earlier sites. Also well-represented was obsidian from the Jemez Mountains, the second-closest source, with the proportion of Jemez obsidian increasing over time, a common pattern in the northern Southwest.

This seemed to indicate that there were substantial early ties between Chaco and the Red Hill area, far to the south but still just barely adjacent to some known Chacoan outliers. This result was mentioned in many publications on Chaco over the years, although many people didn’t really seem to know what to think of it. However, it soon began to be questioned. After this initial sourcing study had been done, Tom Windes submitted some samples of obsidian from Pueblo Alto and the Spadefoot Toad site for obsidian hydration dating, which involved a sourcing analysis as an intermediate step in the dating process. These analyses were inconclusive when it came to dating the artifacts (not uncommon in the Southwest, where obsidian hydration has a poor record as a dating technique), but the sourcing portion suggested strongly that the samples that had previously been sourced to Red Hill instead came from Mt. Taylor. Windes mentioned this anomaly in his site reports, as did Cameron in her subsequent publications on the subject, but a full published account didn’t appear until this new study.

The new study also used XRF to do the sourcing analysis, but both analytical techniques and source characterizations have improved considerably since the 1980s, so the results were quite different from the first effort. For some reason this study was unable to do a complete reanalysis of the earlier samples (although it implies that this may be possible in the future), so there was only limited overlap and the focus was mostly on recent samples collected by Windes at Basketmaker III and Pueblo I site in and around Chaco, as well as outlier sites studied by Kantner in the Red Mesa Valley near Mt. Taylor and by Duff at the southern edge of the Colorado Plateau near Red Hill.

The results were not really surprising, in that they have been known in broad outline since Windes submitted his samples for dating and reported on the sourcing anomalies, but it’s nice to see them formalized in a peer-reviewed paper. Basically, this study found that no samples from Chaco came from Red Hill, although a few came from other sources in the same general area, and that the most common source found at Chaco was Mt. Taylor. Over time there was a trend in the Chaco data showing a shift from Mt. Taylor to Jemez sources, accompanied by the well-known trend toward less obsidian in assemblages overall. The sample from the Blue J site near Mt. Taylor, in contrast, showed high proportions of Mt. Taylor obsidian increasing over time, in marked contrast to the Chaco pattern. The southern sites showed assemblages of obsidian almost entirely composed of Red Hill and other nearby sources.

Basically, the overall pattern was a classic distance-decay distribution, where the prevalence of a given source at a given site was mostly predictable by the distance between the source and the site. This is in sharp contrast to the pattern for many other imported goods at Chaco, which are present in high quantities in the source areas and at Chaco but not in between. This suggests strongly that obsidian was not part of any general Chacoan exchange system(s) but was procured by individual communities in accordance with their own needs, mostly using the closest sources. This is in keeping with the general tendency for chipped stone to be a relatively low-priority commodity in these societies.

The paper mentions the decline in overall abundance of obsidian after the Basketmaker III period at Chaco, but doesn’t spend much time discussing it beyond saying this:

The overall decrease in obsidian use noted at Chaco sites may reflect a shift in technological focus away from hunting and a subsequent focus on grinding technology as agriculture becomes the dominant subsistence strategy.

As I’ve noted before, this is almost certainly wrong; the decrease in question occurs at the end of the Basketmaker III period, at which time there is considerable evidence that Southwestern populations were already heavily dependent on agriculture. The decrease in obsidian is still odd, though. One thought I’ve had to explain it is that maybe the obsidian from after this period isn’t actually missing at all, but is at Pueblo Bonito, which had lots of obsidian but was excavated a long time ago using techniques that aren’t really comparable to the modern techniques used by the Chaco Project and later efforts that resulted in the collections being analyzed here. I would suggest that an XRF sourcing analysis of the Bonito obsidian would be interesting. As it is, there’s a huge shift in the proportions of the different sources at Chaco between Basketmaker III and Pueblo I. The earlier samples (dominated by the huge samples from the major villages of Shabik’eschee and 29SJ423) show a predominance of Mt. Taylor obsidian, while the later ones show mostly Jemez sources. The sample size is so much smaller for the later period, however, that I’m skeptical about taking this flip at face value. Including the Bonito assemblage might help to bridge this gap, or at least explain it.

Finally, it’s again noteworthy how unimportant obsidian appears to have been to the Chaco system. Even if the Pueblo Bonito evidence ends up indicating a more important role at Chaco itself, the various outlier communities appear to have used local sources and to have followed their own priorities in acquiring this commodity rather than getting it through any Chaco-controlled or -oriented system. This is one of the ways that Chaco appears to diverge from Mesoamerican societies, despite recent evidence that it may have had more contact with them than was previously believed. Obsidian was hugely important symbolically in Mexico, and control of major sources was a major source of power and wealth for various Mesoamerican polities. In the Southwest, however, nobody seems to have cared that much about controlling major obsidian sources, and obsidian seems to have been distributed as a fairly ordinary commodity without any particular symbolic importance. I think this is one of the strongest pieces of evidence suggesting that whatever influence Mesoamerican societies may have had on Chaco was indirect and mediated by Chacoan elites rather than imposed directly from Mexico, as some have argued.

In any case, while this isn’t really the most exciting paper, it’s still an important one in straightening out a part of Chacoan archaeology that had become pretty confused. Obsidian may not have been all that important at Chaco, but it’s still worth studying in part precisely because of its mundanity.
ResearchBlogging.org
Duff, Andrew I., Moss, Jeremy M., Windes, Thomas C., Kantner, John, & Shackley, M. Steven (2012). Patterning in procurement of obsidian in Chaco Canyon and in Chaco-era communities in New Mexico as revealed by X-ray fluorescence Journal of Archaeological Science, 39 (9), 2995-3007 DOI: 10.1016/j.jas.2012.04.032

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USGS Sign, Anchorage, Alaska

One hundred years ago today, one of the biggest volcanic eruptions in recorded history took place in southwestern Alaska. The volcano, known as Novarupta, is located in what is now Katmai National Park, which was established in 1918 as a direct result of the eruption and its effects on the landscape. As a result, this anniversary is a big deal for the National Park Service in Alaska, which has a special issue of its journal Alaska Park Science devoted to the eruption as well as a special webpage of information on volcanoes in the region. The main institution involved in Alaska volcanology, however, is the Alaska Volcano Observatory, a joint program of the US Geological Survey, the University of Alaska Fairbanks Geophysical Institute, and the State of Alaska Division of Geological and Geophysical Surveys. They are spearheading much of the commemoration of Novarupta and have their own website listing events and other information on it.

The eruption of Novarupta began on June 6, 1912 and continued for three days. There are various contemporary reports of the eruption from people who were in the area at the time, including one by I. M. Dailey of the US Coast and Geodetic Survey, who was stationed 150 miles away in Seldovia at the time, which was soon published in the Bulletin of the American Geographical Society and is now available free through JSTOR. Other reports come from the larger town of Kodiak, even closer to the eruption, where a foot of ash fell in the course of sixty hours of darkness. A fuller picture of what had happened at the site of the eruption itself didn’t emerge until a series of expeditions over the next few years sponsored by the National Geographic Society traveled to the vent site and undertook a variety of studies that had an enormous impact on the emerging field of volcanology. In an article in the aforementioned special issue of Alaska Park Science Judy Fierstein, a USGS volcanologist who has done extensive research at Novarupta and other volcanoes at Katmai, summarizes both the early reports and studies and more recent work that has clarified the reconstruction of exactly what happened over those three days in 1912.

Many of the early National Geographic expeditions were led by the botanist Robert Griggs, who would be quite influential in shaping interpretations of Novarupta. He discovered and named Novarupta itself, although Fierstein notes that he actually concluded erroneously that the main vent for the eruption was not there but at Mt. Katmai six miles away, which collapsed dramatically into a huge caldera during the event. Subsequent research has revealed, however, that while most of the magma that erupted was indeed stored in a chamber under Mt. Katmai, it actually erupted through Novarupta. Understanding exactly how this complicated internal “plumbing” system of the two volcanoes worked is one of the continuing challenges in ongoing research on the eruption.

Griggs also discovered and named the famous “Valley of Ten Thousand Smokes” where flows of ash from the eruption filled a large valley downslope from Novarupta. At the time Griggs visited just a few years after the eruption, this valley was filled with steaming fumaroles where the still-hot ash met various water sources, and the astonishing sight prompted the poetic name. Within a few more years, however, most of the fumaroles had stopped steaming, and they’re pretty much all dead now. Still, the name remains, as does the strikingly barren landscape with little vegetation having recovered even a hundred years later. The area is so otherworldly that it was actually used as a training ground in the 1960s for astronauts preparing to go to the moon.

Remarkably, despite the scale of Novarupta, which ejected more than three cubic miles of magma in the form of a cloud of ash thousand miles wide and 100,000 feet high that wind currents eventually carried as far as Algeria, there were no deaths from the eruption. The inhabitants of the three Native villages near the volcano had fled after a series of earthquakes in the days leading up to the eruption indicated that a major catastrophe was likely, and the few other communities in this sparsely populated area were far enough away that the levels of ash fall they received were substantial but not deadly. Even Kodiak, which was the hardest hit community due to both proximity and wind direction, managed to get through the initial period of fear and panic during which the whole population took refuge on a Coast Guard cutter docked in the harbor, and it soon recovered its earlier prosperity. The residents of the three closest villages were unable to return to their homes, of course, and were resettled in other communities elsewhere on the Alaska Peninsula, where their descendants reside to this day. All this is in striking contrast to the death toll of over 36,000 people from the eruption of Krakatoa in 1883, which was of a similar magnitude but occurred in a much more densely populated area.

Indeed, the lack of major cultural or long-term environmental effects from Novarupta (outside of the immediate area, of course) initially led one prominent archaeologist, Don Dumond of the University of Oregon, to initially discount the role of volcanic eruptions in general as a force shaping the culture history of the Alaska Peninsula. Dumond conducted long-term excavations primarily in the Brooks River area of Katmai National Park from the 1950s to the 1980s, and his interpretations based on that work have been enormously influential in shaping interpretations of the region’s prehistory. Initially he saw no particular correlation between the numerous volcanic ash layers in his Brooks River sites, which he assumed came from eruptions of the numerous local volcanoes, and the cultural changes visible in the archaeological record. More detailed geophysical work on the ash itself, however, led one volcanologist to suggest that some of the larger ash layers at Brooks had come from various known eruptions at Aniakchak, a very large volcano about 150 miles further south on the Peninsula. If eruptions had deposited ash that far away, they must have been much larger than Novarupta, which would therefore not necessarily be a good guide to their ecological or cultural effects. Dumond therefore decided to look over the record of ash deposits and culture history in other parts of the peninsula to determine if these ash layers really were as widespread as they should be if they came from Aniakchak or another very large volcano.

He published his findings in a 2004 paper, and the results were intriguing but inconclusive. Many other sites on the Peninsula did have ash layers that could potentially be from the same eruptions as the major Brooks River ones, but difficulties in precisely dating these layers and relating them to the dating of the known eruptions at Aniakchak left the issue largely open. While this particular study was largely inconclusive, I find the general idea of trying to determine the relationships between volcanoes and human history very interesting (as longtime readers will recall), so hopefully more research along these lines in this exceptionally volcanically active area will shed more light on the question, which is potentially of great importance in explaining certain cultural and linguistic phenomena.

Regardless of whether volcanic eruptions were major factors in the prehistory of Alaska, it’s clear that they are a major risk factor for us today because of a new development since 1912: the rise of air travel. We need only recall the havoc wreaked by the eruption of Eyjafjallajökull in 2010 (a vastly smaller eruption than Novarupta) to realize how big an effect a major eruption can have on aircraft, and southwestern Alaska is one of the most heavily traveled air routes in the world for both passengers and freight. Indeed, one of the main reasons the Alaska Volcano Observatory was established was to monitor Alaska’s volcanoes for their risk to air traffic. One study using simulations based on the scale of the Novarupta event found that a similar event today could potentially shut down essentially all air traffic throughout the northern hemisphere (and, in the unlikely event that ash moved into the southern hemisphere as well, even the whole world). And note that this is by no means an extremely unlikely occurrence; eruptions of this scale happen regularly, and while there hasn’t been one since Novarupta, there’s every reason to think another one will happen at some point. The only thing to do is to keep watching the volcanoes and studying their past eruptions to try to glean insights for the future. This hundredth anniversary of one of the most important such eruptions is an excellent opportunity to remember that.
ResearchBlogging.org
Dailey, I. (1912). Report of the Eruption of Katmai Volcano Bulletin of the American Geographical Society, 44 (9) DOI: 10.2307/200811

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Earthquake Park, Anchorage, Alaska

On this date in 1964, which happened to be Good Friday, the largest earthquake in US history struck Southcentral Alaska. With its epicenter in Prince William Sound and its magnitude measured as 8.6 or even higher on the Richter Scale, the Good Friday earthquake caused massive destruction throughout the region. The nearby town of Valdez was completely destroyed and later rebuilt on a different, less seismically vulnerable site. Other towns such as Seward and Whittier didn’t suffer that fate, but 13 of the 70 residents of Whittier died in the quake and the following tsunami, and port facilities both there and in Seward suffered so much damage that they have never fully recovered their economic importance.

Anchorage, which was a bit further away, suffered somewhat less, although it was still hit hard. Indeed, its port managed to stay open in the aftermath of the quake and tsunami, which was one of the factors making Anchorage become the main economic center for the region and the whole state in the following years. This is not to say that the earthquake wasn’t a major disaster for Anchorage as well, however. The city was built on sediments that turned out to be pretty vulnerable to shifting when hit with an impact of that scale, and parts of the downtown were literally torn apart. An upscale residential area known as Turnagain Heights suffered a landslide so devastating that the city, rather than attempt to rebuild, designated it as a park. It is now aptly named Earthquake Park, and while there isn’t much obvious evidence of the effects of the earthquake anymore, there are interpretive signs that tell the story quite effectively.

Sign Describing Turnagain Heights Landslide, Earthquake Park

Earthquakes are a fact of life in Alaska, which just north of the area where the Pacific Plate slides under the North American Plate (known as the Aleutian Trench). This movement is continual, but it only occasionally occurs in the form of massive slips of the sort that cause earthquakes on the scale of the one in 1964. The more usual effect is to warp and compress the land of Southcentral Alaska. This warping, along with other factors such as glaciation, leads to complicated changes in sea level over time, changes which can vary substantially in areas only a few miles apart.

And that, in turn, greatly complicates study of the archaeological record of Alaska. Rising sea level typically destroys archaeological sites (although in certain very specific circumstances it may instead preserve them in excellent condition), and changes in sea level over time may make sites from a given time period difficult to locate even if they haven’t been submerged. Furthermore, cultures of coastal Alaska have generally been oriented toward the sea, with settlements typically located fairly close to wherever the coastline was at the time. This means that sea-level fluctuations have even more profound impacts on understanding the archaeological record here than they might in some other areas.

Interpretive Signs at Earthquake Park

These factors are illustrated dramatically in a 1996 article by Aron Crowell and Daniel Mann reporting on research in Kenai Fjords and Katmai National Parks attempting to carefully document sea-level changes and their effects on the archaeological record. These two parks are not very far from each other, being about 200 miles apart and both located on the coast of the Gulf of Alaska, but their geological and cultural histories differ dramatically. Kenai Fjords, as its name implies, is dominated by coastal fjords created when rising sea level inundated glacial valleys. Its archaeological record is sparse and limited to the past few centuries.

Katmai, on the other hand, has a relatively stable coastline and an enormously rich and well-documented archaeological record dating back several thousand years. Much of what is known about the archaeology of southwestern Alaska is based on long-term research in various parts of Katmai by Don Dumond of the University of Oregon from the 1950s through the 1980s, and there has been considerable additional research since then. The cultural sequence in this area closely parallels that of the nearby Kodiak Archipelago, which was one of the most densely populated parts of Alaska when the Russians arrived in the eighteenth century. The Kenai Fjords area, on the other hand, had a much lower density at this time, as did Prince William Sound further east, with which it has many similarities. This lower density has been plausibly attributed to lower resource productivity in these areas, but Crowell and Mann point out that the extremely low density of prehistoric archaeological sites in Kenai Fjords is perhaps also due to geological processes and seal level change.

Welcome Sign, Earthquake Park

To investigate the differences between the two parks, Crowell and Mann intensively investigated certain parts of each, with intriguing results. One of the areas investigated in Kenai Fjords was a narrow sand spit with a series of beach ridges containing a total of four archaeological sites (most dating to the eighteenth and nineteenth centuries and the earliest about 500 years old) and a small tidal pond surrounded by trees killed when the 1964 earthquake caused the land to drop about one meter, exposing their roots to the seawater that permeates the sandy soil. This same seawater also fills the pond, so the sediments in and around it could be interpreted as a record of changes in relative sea level over time. Digging down in search of radiocarbon-datable material, the archaeologists found a series of buried tree stumps 1.8 meters below the present surface that seemed to have been killed in a sudden cataclysm surprisingly similar to what happened in 1964. Six radiocarbon samples taken from the bark of these stumps gave dates that were statistically identical and averaged out to about AD 1170.

This strongly suggests that there was a major earthquake at least as strong as the Good Friday one in or around 1170. This is consistent with evidence from several other parts of the region, where other studies have found less precise evidence for major geological changes around this time. The implication for the issue of archaeological site preservation is that any coastal sites in this area predating 1170 would have been destroyed by the earthquake. Since the geomorphology of Kenai Fjords means that the only usable settlement locations here are on low coastal sand spits like this one, this earthquake probably completely destroyed the previous archaeological record. This would explain why there are so few sites in the park, and why they basically all date to the late prehistoric period or later. The relatively low productivity and other drawbacks of the area may well have limited population earlier as well, but with the earthquake having destroyed any evidence of what was going on previously there’s no way to know for sure.

Sign Describing the Good Friday Earthquake, Earthquake Park

Katmai was a different story, however. Crowell and Mann discovered 22 new sites there in the course of this project (as compared to 16 sites total in Kenai Fjords documented by this and other projects), suggesting that further survey along the coast will likely reveal even more as yet unknown sites in this very productive area. Interestingly, many of the sites they found were located away from the present coastline, often in uplifted areas that may have been coastal at times of higher sea level than at present. Radiocarbon dates from these sites and from peats in the intertidal zone allowed Crowell and Mann to reconstruct a tentative sea level curve for much of the past several thousand years, with the main conclusion being that sea level has been pretty consistent over the past 4000 years, with a slight rise sometime in the past couple hundred years, possibly as a result of subsidence of land in the 1964 earthquake. (Crowell and Mann don’t discuss the possibility that global sea level rise as a result of climate change may be another explanation for this, but it strikes me as plausible.) In any case, whatever the source of this slight recent rise in sea level, it had few apparent effects on archaeological site preservation beyond contributing to the exceptional preservation of one waterlogged site. The subsidence from the 1964 earthquake was very slight in this area, and this appears to have been true for the 1170 one as well. The rich archaeological record of the Katmai coast, then, appears to be in part a function of the relative stability of sea level in this area over thousands of years, in striking contrast to areas just a few hundred miles away.

The archaeological record is always incomplete, and any conclusions drawn from it have to be tempered by knowledge of that incompleteness. As this paper demonstrates very effectively, however, the factors determining just how much of the past is left for us to study vary a lot, especially in places like Alaska where the forces of the earth are exceptionally active. The modern population of Alaska learned just how active those forces can be in 1964, and it stands to reason that the ancient population learned the same lesson in 1170.

"Living with Earthquakes" Sign, Earthquake Park

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How Far West? Plaza, Beaver Creek, Yukon Territory

On this date in 1942, US Army engineering crews working east from Delta Junction, Alaska and west from Whitehorse, Yukon met up near Beaver Creek, Yukon, completing the Alaska Highway.  For the first time, Alaska was accessible from the Lower 48 by road.  This was a remarkable achievement, especially since it was done in only a few months; construction had begun in March and accelerated in June when the Japanese invaded the Aleutian Islands, making the need for a military route to Alaska seem more pressing.  Also noteworthy about the construction effort was that about a third of the soldiers who worked on the project were black, and their impressive accomplishments apparently played a role in the total integration of the military a few years later.

Two years after the highway was completed, while it was still under US Army control, Frederick Johnson of the Peabody Foundation (now the Peabody Museum of Archaeology at Phillips Andover) conducted an archaeological survey along the route as part of a multidisciplinary expedition that had begun the year before with biological and geological studies.  Johnson published a short article in 1946 describing the survey and illustrating some of the artifacts found.  The main importance of this (apparently largely forgotten) article was just to preliminarily describe the archaeology of a large region that, because it had been so difficult to get to before the construction of the highway, was almost completely unknown.  Johnson says as much in the article, while also noting the theoretical importance of gaining an understanding of this region because of its potential role in the peopling of the New World as a migration corridor.  He doesn’t press the latter point, however, and it’s pretty clear that the artifacts he describes are much too recent to have had anything to do with the initial migrations into North America (although there’s no way he could have known this at the time and he doesn’t say anything about it).

White River, Yukon Territory

The presence of geologists in the group was useful for Johnson’s purposes, as it meant that the stratigraphy of the sites he excavated could be evaluated with an expert eye.  Most or all of the artifacts from the sites he discusses in detail, and perhaps of all the sites he found, came from what appeared to be a single layer of reddish-brown soil under a layer of volcanic ash over five inches thick in some places but much thinner in others and apparently absent entirely in some.  This ash is presumably the so-called White River Ash from the eruption(s) of Mt. Churchill, which is in this general area; these eruptions may have been pretty important in the prehistory of this region and beyond.  The artifacts therefore predate the eruption(s), but not necessarily by very long.

The artifacts themselves, all of stone and almost all chipped rather than ground, consist of projectile points, scrapers, gravers, and a variety of miscellaneous forms.  Johnson notes that the assemblages from all the excavated sites are quite similar, which is noteworthy because the area covered was so large.  This implies they probably date to about the same period (as does the similar geology of the strata in which they were found) and may have been made by people from the same cultural tradition.  The sites themselves all appear to represent the same kind of occupation, which Johnson terms a “workshop” where stone tools were made or repaired.  Only one site had a firepit, and none had anything suggesting the presence of structures.  Beyond that there isn’t much Johnson can say, since the state of knowledge of subarctic prehistory was so rudimentary at the time and the area he was discussing had essentially no prior information at all.

Kluane Lake, Yukon Territory

As it happens, the part of the Alaska Highway I drove on my way up to Anchorage is near some of the sites Johnson mentions, many of which are in the Kluane Lake area of the southwest Yukon.  It was interesting to see placenames I recognized in this article.  I’ve been reading quite a bit about Alaska archaeology recently, and it’s really quite fascinating.  I’m not sure quite where Johnson’s findings fit into the overall picture, since his article seems to be virtually forgotten by now and recent work basically never cites it.  His pictures of artifacts are probably good enough to tell where they would fall in currently understood culture sequences, however, and I have an idea of where that might be but I’ll have to look into it a bit more before discussing it further.  I’m still not sure if this blog will be the best venue for discussing Alaska stuff, but it’s what I’ve got for now and I figured I should mark the occasion of the highway’s anniversary by talking a bit about its much deeper past.
ResearchBlogging.org
Johnson, F. (1946). An Archaeological Survey along the Alaska Highway, 1944 American Antiquity, 11 (3) DOI: 10.2307/275560

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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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Sunset Crater Volcano

The effect of the eruption of Sunset Crater Volcano on the prehistoric population of northern Arizona has long been a topic of interest to archaeologists.  As I’ve mentioned recently, in the 1930s and 1940s Harold Colton of the Museum of Northern Arizona in Flagstaff came up with a theory to explain the settlement dynamics of the Wupatki area northeast of Sunset Crater.  In Colton’s view, the eruption resulted in a level of volcanic ash falling on Wupatki that acted as a mulch to retain water and make that very arid area suitable for farming for the first time, resulting in a “land rush” in which people from all over the region converged on Wupatki to farm the newly available land.  Over time, however, the ash began to blow away and the land became less productive, so people aggregated into the large pueblos for which Wupatki is best known, then left entirely when the area could no longer support its population.  Dendrochronological evidence from timbers at Wupatki Pueblo later provided a basis for dating the eruption to around AD 1064, which would put the “land rush” shortly after that.  Other evidence has shown that the abandonment of the area probably occurred some time in the thirteenth century, a time when many parts of the Southwest were being abandoned as well.

As I’ve mentioned, recent archaeological survey at Wupatki has cast doubt on some aspects of this model.  The main influx of population seems to have come after AD 1130, a few decades after the eruption, and the scale of that influx was probably quite a bit lower than Colton estimated, since many of the sites he counted to compute his population estimates were probably season field houses or other temporary structures rather than permanent habitations.  This implies that there wasn’t really a “land rush” the way Colton described it, but rather a substantial increase in population at some point after the eruption, perhaps in response to drought or other problematic conditions in other parts of the Southwest.

A few parts of Colton’s model do seem to hold up, however.  Experiments have shown that the levels of ash found at Wupatki do indeed work well as a mulch.  Without this mulch, dry farming in the area with any reasonable measure of reliability is basically impossible, since there just isn’t enough rain, and irrigation or floodwater farming isn’t possible on any substantial scale either due to the geological conditions and the lack of permanent surface water sources.  Furthermore, the Wupatki survey showed that this lack of agricultural suitability made the area essentially uninhabited before the eruption.  Of nearly a thousand datable sites recorded by the survey, only two dated to before the eruption.  The biggest influx of population came after about 1130, but there was already a fairly significant movement of people into the area in the immediate post-eruptive period.  Perhaps these people first experimented with agriculture using the ash as a mulch, and were so successful that when conditions deteriorated elsewhere others joined them.  The ash was liable to blow away in the strong winds, however, and over time the advantages it offered as a mulch would have diminished as a result of this and other factors, so it’s quite possible that it was declining agricultural productivity, perhaps exacerbated by warfare to defend land claims, that led the area to be abandoned in the thirteenth century.

Volcanic Rock in Masonry at the Citadel, Wupatki National Monument

That’s all well and good, but where did the people who moved to Wupatki after the eruption come from?  Colton saw them as coming from all over, but at least in the immediate post-eruptive period a more specific answer is tempting: perhaps they came from the area right around the volcano, which would have been rendered uninhabitable (and certainly unfarmable) by lava flows and massive ash fall.  A relatively recent paper takes a close look at the circumstances of the Sunset Crater eruption and its likely effects on local people, and basically comes to this conclusion.

From a detailed analysis of the details of the eruption, the authors of this paper found that the area of the heaviest ash fall and the largest lava flows was probably densely populated and heavily farmed before the eruption.  They cast some doubt on the tree-ring evidence pointing to an AD 1064 date for the eruption itself, but they argue on other grounds that the eruption likely took place between AD 1050 and 1100 and that it was relatively quick, lasting from a few weeks to a few years at the most.  Because the high-elevation area where the eruption took place gets more precipitation than lower-elevation Wupatki, it would have been the most favorable area for farming at the time, and a large number of homes and farms were likely buried by the lava and ash.  The amounts of ash falling right around the volcano would have been much too thick to serve as a mulch.  The ash itself is sterile, so it could only function effectively as a mulch if plants could reach their roots down through it to the soil underneath.  The few inches of ash cover at Wupatki would have allowed this, but the uplands immediately around the volcano got over a foot of ash, which would have effectively killed any agricultural potential.

Lava at Sunset Crater

Thus, the effects of Sunset Crater on local agriculturalists were two-fold: they were forced to leave a rather large and previously quite productive agricultural area around the volcano, but they were able to go to a previously unproductive area nearby that was made newly fertile by the ash.  Cinder-cone eruptions like the one that created Sunset Crater rarely cause much direct loss of life, and that would have been particularly the case in this context, since the pre-eruption populations lived in dispersed farmsteads and were probably not organized sociopolitically at any level above the household or extended family.  This would have allowed rapid reactions to the eruption, which would primarily have taken the form of migration away from the immediate area.  Since the population was so dispersed, people fleeing the ash-fall zone would likely have had relatives or friends in less affected areas to whom they could go for shelter and assistance in the immediate aftermath of the eruption.  The population movements spurred by the eruption, however, could well have resulted in groups infringing on territory claimed by others and resulting violence and loss of life.  Within this context, the relatively empty Wupatki area may have seemed particularly attractive even before its enhanced potential for farming was discovered.

Another reaction of people in the local area to the eruption, which was documented in an earlier paper by some of the same authors, was the apparent practice of placing corncobs in the path of lava and carrying the resulting “corn rocks,” with visible imprints of the cobs (which were vaporized by the heat) to rather distant settlements.  Given the amount of effort this would have required, it probably had some ritual significance, perhaps to appease the spirits of the volcano or something similar.

Mt. Trumbull from Pipe Spring National Monument

In addition to the Sunset Crater eruption, the authors of this paper also discussed a smaller and less studied eruption that likely took place about the same time at Little Springs, next to Mount Trumbull on the Arizona Strip just north of the Grand Canyon.  Here there was relatively little ash fall, so the loss of productive land and enhanced productivity of other land seen in the Sunset Crater case did not occur.  Instead, the main effect was a lava flow, with the land immediately surrounding it continuing to be largely ash-free and fertile.  The people who had lived and farmed in the immediate area covered by the lava flow would have had to leave, but people clearly continued to live and farm right around the lava, and they also built sites on top of the flow itself.  These sites have few artifacts and likely served defensive purposes, a theory that is supported by the presence of an elaborate system of trails on the lava flow that would have made it an effective refuge in times of war.  The use of defensive refuges or strongholds separate from ordinary living quarters is well-attested in the prehistoric and historic record of the Southwest.  Similar to the corn rocks at Sunset Crater, in this area there were some rocks with potsherds embedded in them, a sign of similar ritual behaviors with respect to the volcano.

These two eruptions and the different reactions to them by local populations show the effects that sudden, catastrophic events can have on human societies.  The eruption of the much larger White River Volcano a bit earlier and its effect on local Athapaskan populations in Alaska and the Yukon is another example.  Unlike many other catastrophes, volcanic eruptions are generally pretty visible in the archaeological record, which makes them a useful source of information on how societies adapt to sudden shocks.
ResearchBlogging.org
ORT, M., ELSON, M., ANDERSON, K., DUFFIELD, W., & SAMPLES, T. (2008). Variable effects of cinder-cone eruptions on prehistoric agrarian human populations in the American southwest Journal of Volcanology and Geothermal Research, 176 (3), 363-376 DOI: 10.1016/j.jvolgeores.2008.01.031

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