INTRODUCTION TO CHRONOLOGY
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.