Archaeological Patterning across Southern Wyoming: Part 2

In a previous post, I visually examined the distributions of some artifact, feature, and site types across southern Wyoming along and East-West gradient. This inspection produced evidence for a transition between the Plains and Wyoming Basin cultural patterns in the vicinity of Rawlins. In this post, I examine the same data using a range of statistical techniques and come to the same conclusion - but with cooler charts! You will need to read the previous post first in order for this one to make sense.

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Visual inspection of east/west distribution graphs, discussed above, identified eight variables as potential differentiators between the eastern and western areas of southern Wyoming. These are stone cairns and alignments, ceramics, FCR and thermal features, ground stone, housepits, rock art, stone circles, and steatite artifacts. To assess the statistical significance of these visually-identified differences, a chi-squared analysis was conducted. The visual inspection of the graphs suggested that the boundary between the eastern and western parts of the study area should be placed somewhere between R80W and R100W. For most artifact and feature types, the change was observed between R80W and R90W. For the purposes of the chi-squared analysis, therefore, Ranges were placed into two groups: a western group, located in R85W and further westward, and an eastern group, located east of R85W.

	Stone Circles	Ceramics	Ground Stone	FCR Thermal	Cairns Alignments	Rock Art	Housepits	Steatite	Totals
East	426	59	110	1423	213	5	3	2	2241
West	390	158	636	16614	361	86	60	13	18318
Totals	816	217	746	18037	592	91	63	15	20559

Table 1: Selected feature and artifact type counts organized by east/west. East/West line placed between Ranges 84W and 85W

Table 1 contains the counts of the potentially significant feature and artifact types organized into eastern and western groups. Pearson’s chi-squared test shows the difference between these two groups to be highly significant (χ2=2083.691, df=7, p<2.2x10-16).

Figure 1: Adjusted chi-squared residuals, artifact and feature types by East vs. West. East/West line placed between Ranges 84W and 85W. From left to right: cairns/alignments (C/A), ceramics (Cer), FCR/thermal features (FCR), ground stone (GS), housepits (HP), rock art (RA), stone circles (SC), and steatite (ST).

Further information can be gleaned from an inspection of the adjusted chi-squared residuals (Figure 1). These residuals provide a measure of the importance of specific variables in distinguishing between the two groups. Values above +2 and below -2 indicate significant deviations from expected values in the contingency table, providing a convenient threshold of statistical significance. When plotted, the residuals clearly show that only five of the eight variables meet this threshold of statistical significance. These are cairns and alignments, ceramics, FCR and thermal features, ground stone, and stone circles. Of these, ground stone is the weakest differentiator and stone circles the strongest. In general terms, then, the eastern group has significantly more sites with cairns/alignments, ceramics, and stone circles, and significantly fewer sites with FCR and thermal features than does the western group. This is not to say that there is no difference between the two areas in terms of rock art, housepits, or steatite artifacts, merely that given the current sample these variables do not significantly differ between the two groups. All three of these variables involve very low site counts (Table 1).

MDS and PCA

In order to assess the degree to which the five variables identified as significant in the chi-squared analysis distinguish between the eastern and western portions of the study area, a multidimensional scaling (MDS) analysis was conducted. This technique assess the n-dimensional distance between Ranges based on the values of the five variables identified above, and attempts to approximate that relationship in a reduced number of dimensions, such that the data can be visualized on a graph. Only Ranges with more than 50 prehistoric occupations were included in the analysis. The analysis was conducted using the cmdscale function of the R statistical software package.

Figure 2: Multidimensional scaling of site assemblages, grouped by Range.

The results (Figure 2) indicate a clear separation of eastern and western areas in the sample. All of the Ranges west of R89W are located in a tight cluster in the lower right corner of the graph, while all Ranges east of R80W are clearly separated from the western cluster. The tight clustering of the western samples reflects large sample sizes and low variance, while the wider spread of eastern samples is probably produced by lower sample sizes. Interestingly, samples from R80-90W are located between the two other groups in a tight linear cluster, suggesting that this area is a kind of transition zone, consisting of a mix of the attributes that identify the western and eastern archaeological patterns. This could be produced either by a true cultural blend, or, perhaps more likely, by a cyclical shifting of the boundary between these two patterns with changes in paleoenvironmental conditions.

Figure 3: Multidimensional scaling of site assemblages, grouped by Range, labeled E/W. Ranges 80-90W excluded from analysis.

Figure 3 represents the results of a second MDS analysis, this one excluding Ranges in the transition zone (R80-90W). In this figure, Ranges are labeled with a single letter indicating membership in the western or eastern group. The separation of the groups in this graph is complete, with all of the western samples located in a tight cluster in the lower right corner. When the intermediate transition zone is excluded from the analysis, therefore, the groups are completely separated by this technique, with no overlap whatsoever.

Figure 4: Principal Components Analysis biplot of site assemblages, grouped by Range, labeled E/W. Ranges 80-90W excluded from analysis.

Figure 4 displays a biplot of the results of a principal components analysis (PCA) of Ranges using the same five variables that were employed in the MDS analysis. The analysis was conducted using the princomp function of the R statistical software package. The results are the same as the MDS analysis, but the superposition of variables on the biplot provides additional information on what differentiates these clusters. The eastern and western groups are completely separated on Principal Component 1, which is comprised primarily of the cairns/alignments, stone circles, and FCR/thermal variables. Ceramics and ground stone, though statistically significant, are the primary constituents of Principal Component 2, and are only weak differentiators of the two groups.

Summary

The results of this analysis suggest that archaeological patterns in southern Wyoming are strongly separated into distinct eastern and western patterns. The western pattern is characterized by a high frequency of FCR and thermal features, and the eastern pattern by high frequencies of stone circles, cairns, and alignments. The western pattern is predominant from R91W westward, and the eastern pattern from R79W eastward. The area in R80-90W appears to constitute a transitional zone, where elements of both patterns are combined. This could represent a cultural gradient between the two areas, the Plains to the east and the Wyoming Basin to the west. However, it seems more likely that this transition is produced by the east/west movement of the Plains/Wyoming Basin ecotone with changes in critical paleoenvironmental parameters such as temperature and precipitation. However, and adequate evaluation of this hypothesis would require analysis of the east/west pattern distinction through time, which is impossible with the available data.

Another significant factor that should be addressed but is currently intractable is the extent to which these western and eastern patterns reflect surface visibility and therefore differential site discovery and site recording in the southeastern and southwestern portions of the state. It is possible that high surface visibility in the Wyoming Basin leads to a higher discovery rate for ephemeral sites containing FCR and thermal features (and not containing stone features). It is not possible to evaluate this hypothesis at present. However, the fact that there is no significant difference between the two areas in terms of lithic artifact frequencies would suggest that differential site discovery is not the dominant factor structuring archaeological patterning at this large scale. A formal test of the influence of surface visibility is needed, though, before this possibility can be entirely discarded.

Don't get a Ph.D. Seriously.

Thanks to John Hawkes for pointing out this article in the Chronicle of Higher Education. It deserves to be widely read. Here is a choice quote:

It's hard to tell young people that universities recognize that their idealism and energy — and lack of information — are an exploitable resource. For universities, the impact of graduate programs on the lives of those students is an acceptable externality, like dumping toxins into a river. If you cannot find a tenure-track position, your university will no longer court you; it will pretend you do not exist and will act as if your unemployability is entirely your fault. It will make you feel ashamed, and you will probably just disappear, convinced it's right rather than that the game was rigged from the beginning.

These are hard truths that are seldom aired publicly. Though the article focuses on the humanities, most of what he says also applies to Anthropology.

In archaeology, things are a bit different. I would counsel anyone wanting to pursue a career in CRM to get an M.A. It is definitely useful. A Ph.D., however, while it may prove useful at times, will never pay for itself. For the past four years ACRA has put out a report on salaries in archaeology. If you look through the results you will find that for a similar position and responsibilities an archaeologist with an M.A. makes about $10k more per year than an archaeologist with a Ph.D. Add to that the years in graduate school in which you will earn no salary and gain no real useful experience in the industry, and a Ph.D. in archaeology looks like a pretty bad investment.

Unless you want to be a tenure-track professor somewhere. Good luck with that. As the author of the Chronicle piece puts is, "the minority of candidates who get tenure-track positions might as well be considered the winners of a lottery."

Archaeological Patterning across Southern Wyoming: Part 1

Recently I did a large file search, for an area spanning southern Wyoming, from Nebraska to Idaho and Utah - an 88 mile wide strip of land along the Colorado line. The study area covers about 81,994 sq km - about the size of the Czech Republic, or, as Tom Wake pointed out to me, slightly smaller than San Bernardino County. I had to develop some custom software for this, in order to be able to automate almost a thousand separate WYCRO searches and combine the results into a single dataset. The final dataset contained some 29,000 prehistoric archaeological sites, which is a nice dataset to work with even if we have only a small amount of information for each site.

Since WYCRO does not support a UTM boundary search option, the search was conducted by Public Land Survey System (PLSS) township and range units, comprising an area from Township 12N to 26N, and Range 60W to 121W: a total of 930 townships. R60W is, of course, on the far eastern edge of the state, and R121W on its extreme western margin. The point of the exercise was to look at archaeological patterning along the east-west axis, and to attempt to identify a zone in which the transition from the Plains to the Wyoming Basin actually took place, culturally speaking.

Here are some of the more interesting results.



The graph represent the number of recorded prehistoric sites along the east-west axis. The horizontal axis represents Range W, with east to the right and west to the left. All of the townships in a particular range were combined to produce a single prehistoric site count for each Range W value. These values are the gray points. The red line is a loess-fitted trend line, a kind of local average, fitted by the loess function of the R statistical package. Obviously, there are a lot more recored archaeological sites in the western part of the study area, from about R85W and westward. For reference, R85W is in the vicinity of Rawlins. This high number of recorded sites in the west is no surprise, since oil and gas development on public lands has been quite intensive in southwestern Wyoming.



The above graph represents the percentage of sites with recorded FCR (fire-cracked rock) or thermal features (hearths) along the east-west axis. The horizontal axis represents Range W, with east to the right and west to the left. All of the townships in a particular range were combined to produce a single percentage for each Range W value. FCR and thermal features are much more common in the western portion of the transect. This could in part reflect lifeway differences, with more processing of marginal resources such as tubers in the Wyoming Basin as opposed to hunting on the Plains. It could also reflect the generally higher surface visibility in the drier western portion of the transect.

FCR and thermal features are relatively common, with a mean per-Range frequency of 50.2%, with a maximum frequency of 85.9% and a minimum of 10.3%. The loess-fitted trend line suggests that FCR and thermal features are very common (>40% frequency) to the west of approximately R80W, and somewhat less common (<40% frequency) to the east of that line. The zone from R80-90W appears to be a transition zone, in which FCR and thermal feature frequency declines regularly from west to east. The pattern observed in FCR and thermal feature frequency is complementary to the observed pattern of stone circle frequency (below), with high FCR/thermal feature frequency corresponding to low stone circle frequency, and vice versa.



The above graph shows the percentage of sites with stone circle features along the east-west transect. Stone circles are considerably more common in the East, supporting their association with Plains cultures.

Stone circles are relatively common, with a mean per-Range frequency of 7.1%, with a maximum frequency of 37.7% and a minimum of 0%. The loess-fitted trend line suggests that stone circles are very common (>10% frequency) to the east of approximately R90W, and much less common (<5% frequency) to the west of that line. The zone from R80-90W appears to be a transition zone, in which stone circle frequency declines regularly from east to west. The high variance in stone circle frequency in the eastern portion of the study area likely reflects the effect of small samples in some Ranges. The change in stone circle frequency is the most robust pattern observed in the dataset.



The mean per-Range frequency of Stone cairns and alignments (above) is 4.4%, with a maximum of 35.8% and a minimum of 0%. In general, areas to the west of R90W fall below this level, while areas to the east of R90W fall above it. Cairns and alignments, therefore, appear to be more common in the eastern portion of the study area. While some of the high values in the eastern area are no doubt the result of sample effects, the overall patterns appears to be robust.



Finally, ceramics (above) are much more common in the East than in the West. Areas to the west of R80W seem to have low ceramic frequencies (<1%),while areas further to the east seem to have higher ceramics frequencies (>1%), though the eastern area of course has a high variance as would be expected given its small sample sizes.

On the whole then, we get a fairly clear picture of two cultural traditions, one associated with stone circles, cairns, alignments, and ceramics (Plains), and the other associated with FCR and thermal features (Wyoming Basin). These two traditions intersect somewhere in the vicinity of Rawlins, which, unsurprisingly, is also located along the ecotone between the wetter eastern grasslands and the scrub desert of southwestern Wyoming.

Since the aggregate radiocarbon record, which I will discuss in another post, show that the archaeological record of southern Wyoming is dominated by Late Prehistoric sites, this difference is probably produced mainly by human habitation within the last two millennia. Whether such a cultural divide existed in the Archaic period remains an open question, one that cannot be addressed with the sort of coarse-grained data that I have used here.

In a later post I will present some statistical analysis of these data that further strengthen the notion that there is a clear division between the archaeological record of southwestern and southeastern Wyoming.