If necessary, review the archaeological computer module for dating to make sure you understand radiocarbon dating if necessary. Below is a brief summary for the dating methods you will use in this exercise. Before we briefly reassess the methodology used to date Hohokam remains, we should review what is basically known about Hohokam chronology.
Debates concerning the Hohokam chronology always start with and return to Emil Haury's initial formation of the Hohokam phase sequence and his subsequent reconfirmation of this sequence based on excavations at the site of Snaketown, south of Phoenix. Haury defined a sequence with four periods divided into nine phases. His sequence begins with the Vahki phase around 300 B.C. and finishes with the end of the Civano phase about A.D. 1450. Numerous variations of this sequence have been advocated. Discussions of the Hohokam phase sequence primarily center on differences and changes in ceramic types and chronometric dating. Ceramic types created by Haury that mark each phase are relatively effective in distinguishing individual phases. Other items within the material culture tend to be time sensitive as well as ceramics. Changes in the distribution of figurines, three-quarter grooved axes, and other artifact types can be used to identify particular periods of time.
Challenges to this traditional period and phase sequence have left it largely unchanged except in degree. There is evidence that phases tend to overlap in time if measured across space. The calibration of the Hohokam chronology has been a major source of challenge. Estimations for the beginning of the sequence differ as do starting points for the Colonial, Sedentary and Classic periods, and the end of the Classic period.
There are differences in the sequence between the area around Phoenix and that near Tucson. There is a general concurrence between the two areas in the nature of material culture change but there are obvious differences.
Seriation
Seriation is a relative dating
method. It involves arranging
archaeological materials into a presumed chronological sequence based on
cultural and stylistic change.
As long as items are gathered from the same cultural tradition, archaeologists
assume that stylistic change occurs relatively gradually over time. By
tracing similarities and differences in styles and by measuring the relative
popularity of these differing styles, one can reconstruct a sequence.
Hohokam ceramics have been grouped into types such as Estrella Red-on-gray, Gila Butte Red-on-buff, Santa Cruz Red-on-buff, and Sacaton Red-on-buff. These are distinctive in terms of the attributes that distinguish them and can be ordered based on changing popularity. The assignment of a feature in an archaeological site to a phase depends upon the archaeologist's ability to associate the manufacture and discard of the ceramics in the feature with the construction and use of that feature. Information you will be provided is based on the assumption that final use (floor contact) and initial filling with discard ceramics will provide a basis for determining phase association. In certain cases, this is a highly reliable measure. Feature 14, for example, had a nearly complete floor assemblage when it burned and was abandoned. This leaves us a highly reliable basis for determining it's temporal placement. Other features did not have intact floor assemblages. In these cases we have to rely on the trash found in the fill immediately above the floor. For this reason, you will be provided information from the floor and the first 10 cm. above the floor. This provided sufficient numbers of ceramics to assist archaeologists in reconstructing temporal placement.
You will be provided information about both decorated and plainware ceramics to use in a seriation. You will be provided raw counts and relative percentages. Use the percentage data to perform the seriation.
Archaeomagnetic Dating
In the early to mid 1960s, a
new technique for dating archaeological material emerged. this technique,
known as archaeomagnetism, was introduced by Dr. Robert Dubois. As applied,
archaeomagnetism relies on thermo-remnant magnetism; that is remnant of
magnetic orientation is sediments that were once heated sufficiently to
alter their magnetic orientation toward magnetic north. Clay, when heated,
acquires a remnant magnetism with a direction paralleling the earth's magnetic
field. Once the clay cools, the fired clay hold that magnetism until reheated.
By knowing the date (by some other dating method) of a feature and measuring
the direction of magnetism in clays from this feature, it is possible to
determine the ancient pole location
(called the virtual geomagnetic pole or VGP) of the earth's magnetic field
at the time this clay was last fired. When a large number of these ancient
VGPs are dated through this method, a composite curve of polar wandering
(a VGP curve) can be reconstructed. The VGP curve can be used then as a
master record against which the VGPs of samples of unknown age can be "dated".
This implies that by measuring the remnant magnetism from a clay hearth
of a pithouse from Los Hornos, we can obtain a reference point relative
to this curve and therefore chronometrically date the pithouse. (See
master curve in more detail.)
Measurement of the ancient magnetic field direction is made on pieces of fired clay collected at archaeological sites in an oriented fashion. Each of the pithouses that was excavated at Los Hornos had a clay lined hearth immediately inside the entry. These were the source for our archaeomagnetic samples. Pieces of the baked clay were carefully isolated by carefully sawing it each hearth. A nonmagnetic, cube-shaped mold (aluminum) was placed over the isolated column and filled with plaster. A record of magnetic north and the vertical and horizontal placement of the sample was then made and recorded on the sample recording sheet. Usually, eight to twelve of these specimens were collected and submitted to Colorado State University for processing. The ancient magnetic direction and age determination for a given feature was calculated on the basis of the mean direction of all specimens collected from the feature. The data were then summarized and returned to Archaeological Consulting Services for interpretation relative to other data from the site.
Radiocarbon Dating
The basis for radiocarbon dating
is magnificently simple. Carbon 14 is continuously produced in the upper
atmosphere by the action of cosmic rays, which set free neutrons that transmute
nitrogen in the air into radioactive carbon. Incorporated in carbon dioxide,
the radiocarbon moves through the atmosphere and is absorbed by plants.
animals in turn build radiocarbon into their tissues by eating the plants.
As long as they are alive, plants and animals go on ingesting radiocarbon.
When an organism dies, and ceases to take in fresh carbon, its built-in
clock begins to run down. The disintegrations of its carbon-14 atoms tick
away the seconds and the years: in 5,568 years (on the average) only half
of its original store of radiocarbon atoms is left, and in another 5,568
years only half of those, or one quarter of the original number.
Long before that time, of course, most plants and animals have decayed into dust. but when the remains of an organism are fortuitously preserved, as a house beam, a bit of charcoal, or a seed, the age of the remains can be calculated. The amount of radiocarbon the organism possessed when it was alive is known, and so is the rate of its radioactive disintegration. It is easy to compute the age of the remains by counting the amount of radioactivity that still remains.
Since many of the features excavated at Los Hornos contained traces of charcoal, there were ample materials to sample for radiocarbon dating. Samples were carefully selected according to the type of material and quantity available for processing. Priorities were given to materials that represented annual growth - seeds or plants such as arrow weed used in house construction. Second priorities were given to house beams or support posts used in construction.