The Calixtlahuaca Chronology, part 1

Most of our work in the lab this summer focuses on two themes: processing various miscellaneous artifact categories, and working on chronology. Many people think that archaeological chronology is boring, and they are probably right. But if archaeologists can’t get our time sequence straight, it’s hard to say much that is interesting about events and processes in the past. Perhaps due to some strange mental defect, I happen to think chronology is interesting and even fun (sometimes). I’ve developed a number of Postclassic chronologies in the state of Morelos; each region has needed its own sequence (Hare, and Smith 1996; Smith, and Doershuk 1991). I’ve also written on theoretical aspects of archaeological chronology (Smith 1992), arguing that the degree of effort archaeologists need to invest in chronology is related to the kinds of ancient processes they want to study.

At Calixtlahuaca, we want to know how the site was founded and grew through time, and we would like to be able to tell apart the periods before and after the site was conquered by the Triple Alliance (Aztec) Empire. This will require a relatively fine-grained chronology, comparable to those I worked out for parts of Morelos. We will use three basic ingredients for the task: radiocarbon dates, stratigraphy, and ceramics. Most of our time in the lab right now is dedicated to ceramic classification, focusing this year on contexts that will help us establish the chronology and assign dates to structures and deposits. More about this later.

Last winter we ran the first batch of C14 dates for Calixtlahuaca. The results are shown in the accompanying figure. Each horizontal line with blobs is a calibrated date. I won’t explain calibration here; there are good web pages on calibration. The important fact is that each “date” is in reality a range of probabilities, shown by the blobs in a calibration chart like this. Due to the peculiarity (perversity?) of the atmospheric carbon during Postclassic times, the calibration curve fluctuates, so that for many dates there are two different ranges of years that could represent the true age of the sample. This makes it difficult to figure out just how old some of the samples really are.

Most of our dates fall into the interval of the Late Postclassic (or Late Aztec) period (ca. AD 1300-1520, green and red on the chart). This is not surprising; our initial hypothesis, based on ceramics from García Payón’s collections and our 2006 survey, was that most of the occupation at the site was from the Late Postclassic period. We have several dates from the Middle Postclassic (or Early Aztec) period (ca. AD 1100-1300), again not surprising. Most Aztec-period urban sites were occupied in both periods (Smith 2008), with growth and expansion in the later period. The earliest dates may extend back into the Early Postclassic (or Toltec) period, AD 900-1100; to me this suggests that the Middle Postclassic occupation may have begun prior to 1100. There are few or no Early Postclassic ceramics at the site.

Unfortunately none of these dates (except possibly the bottom two) fall clearly into the second half of the Late Postclassic period (the red zone on the chart). This corresponds to the period after the Aztec conquest of the Toluca Valley. The most likely explanation is that we simply did not happen to submit carbon samples from occupations of this time period, and this will be a priority when we submit our next batch of samples. We are beginning to see some ceramic patterns in the lab that suggest that we will be able to isolate this late portion of the Late Postclassic with ceramics.

Now, an important caveat: THESE RADIOCARBON DATES PROVIDE ONLY A VERY PROVISIONAL PICTURE OF THE DATING OF THE SITE! We have a few deposits dated by 2 or more dates, but most of the dates are from a single carbon sample from an individual context. An old saying in the carbon dating world is that “One date is no date” (Aitken 1990:95). The reason for this derives from the fact that C14 dates are really probability distributions (the blobs), not single points in time. When one gets multiple dates from a single context and most of them agree, then the context is dated much more securely (and much more precisely). So until we get more dates, this is an initial look at the chronology of our deposits.

References Cited:

· Aitken, M. J. (1990) Science-Based Dating in Archaeology. Longman, New York.

· Hare, Timothy S. and Michael E. Smith (1996) A New Postclassic Chronology for Yautepec, Morelos. Ancient Mesoamerica 7:281-297.

· Smith, Michael E. (1992) Braudel's Temporal Rhythms and Chronology Theory in Archaeology. In Annales, Archaeology, and Ethnohistory, edited by A. Bernard Knapp, pp. 23-34. Cambridge University Press, New York.

· Smith, Michael E. (2008) Aztec City-State Capitals. University Press of Florida, Gainesville.

· Smith, Michael E. and John F. Doershuk (1991) Late Postclassic Chronology in Western Morelos, Mexico. Latin American Antiquity 2:291-310.

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