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Bedrocks Metates along the Chaquaqua Drainage: Building a Conceptual Frameworkof Prehistoric Landscape Knowledge
ABSTRACT: The canyons and tributaries of the Purgatoire River in Southeastern Colorado
reveal evidence of continuous and seasonal occupation of Indigenous peoples for thousands of
years. Evidence abounds for habitation by plains hunter gatherers, horticulturalists and
possibly ancestral woodland culture groups. A unique feature of this regional system is the
presence of numerous types of bedrock metates occurring at varying elevations and site
assemblages. This paper addresses a critical and understudied problem: how prehistoric
peoples in this region incorporated bedrock grinding features within their socio-cultural view
of the landscape. Photogrammetry and GIS are used to develop and explore a regional spatial
distribution of these features at the intra-site and inter-site level.
Bedrock Grinding Features and Southern Plains Archaeology
Bedrock grinding areas (generally classified as metates, mortars and slicks) are an
integral part of the prehistoric landscape in southeastern Colorado and northeastern New
Mexico (Figure 1). They are generally located among the canyon sidewalls and rock
outcrops east of the Rocky Mountains, south of the Arkansas River and north of the
Cimarron River in northern New Mexico and eastern Oklahoma. Reported within site
descriptions from this region since Renaud (1931), the densest concentration appears to be
along the canyon sidewalls of the Chacuaco, Apishapa and Purgatoire Rivers and their
tributaries (Campbell 1969; Loendorf 2008; Owens 2007). The waterways cut through the
plains and mesas in a complex maze which eventually connect with the Arkansas River
(Campbell 1969; Fenneman 1931; Gunnerson 1989). Bedrock grinding features were
generally created along the sandstone rims and ledges of the canyons, though others have
been found among the ubiquitous rock art boulders of the basalt strewn formation known
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as the Hogback near Trinidad, Colorado (Andrefsky 1990; Campbell 1969; Chomko and De
Vore 1990; Gunnerson 1989; Hartley and Vawser 2003; Loendorf 2008; Renaud 1931).
Archaeology in the region reveals a complex array of lifeways and subsistence
patterns from the remains of lithic, grinding tools, architecture and ceramic artifacts
although bedrock grinding features are difficult to place within the archaeological context
(Campbell 1969; Eighmy 1984; Zier et al 1999). There is sparse ethnographic
corroboration of their use within the region. Unlike their portable or site-fixed
counterparts, bedrock grinding features are permanent on the landscape leaving them
exposed to a variety of erosional forces. This permanence is important in creating a
perspective of landscape. At present there are few reliable methods to directly date these
surfaces or to assess functionality through pollen or starch analysis. Temporal claims are
based on association with surrounding features such as hearths, rock art and structures, or
on diagnostic projectile points found in associative context (Campbell 1969; Chomko and
De Vore 1990; Loendorf 2008). The few bedrock grinding surfaces that have been dated,
either with cation ratio techniques or by context with other diagnostics, are thought to date
from the late Archaic about 2000 RCYBP to about RCY500 BP (Campbell 1969; Chomko and
De Vore 1990; Loendorf 2008; Owens 2007). Since there is no historic record of the use of
these features, it appears that these features ceased to be used by the time Athapaskan
speakers appeared in the region at some point during the late 14th or 15th century
(Campbell 1969; Gunnerson 1989; Hammond and Rey 1940; Owens 2007; Stoffle 1984;
Zier et al 1999).
Bedrock Grinding Stations and Social Landscapes
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Bedrock grinding features, like other permanent cultural features, such as
architecture, have the potential to inform past social dynamics between people and their
environment, as well as the role this relationship may have had within their symbolic
framework. What we understand about groundstone is generally from a functional view
point (Adams 2002; Buonasera 2007; Perry 2004) or related to subsistence strategies
(Bender 1990; Diehl 1996; Wright 1994). However, bedrock grinding features are
particularly useful in viewing social landscapes; especially how peoples gathered in order
to process and prepare staple foods (Holmberg 1998; Parkman 1994; Villalobos-Boehm
1997). Grinding at fixed locales for large groups or for bulk resource processing was often
done in large family groups or were community level grinding events (Adams 1999;
McCarthy et al 1985; Parkman 1994; Scroth 1996) though grinding on bedrock surfaces
has also been recorded at the family or individual household level (O'Connell et al 1991).
Signatures of these social processes remain on the landscape long after the grinders and
their communities have disappeared.
Studies of bedrock mortars in California have provided intriguing insight into the
socioeconomic relationship between prehistoric communities and their landscape as well
as providing evidence for seasonal movement across the landscape (McCarthy et al 1985;
Morgan 2008; Parkman 1994; True et al 1979). As fixed points on the landscape, bedrock
grinding features transform physical localities into places with meaning and social
significance and provide an organizational framework of the landscape. I advance the
argument that the permanent grinding places can be viewed landscapes themselves, work
spaces that were imbued with meaning, personal and social. Unlike the features in
California and elsewhere in the world, there is little ethnographic reference to help
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decipher the meaning of the grinding places in the southern plains. However the physical
evidence that remains can be explored with current technology to offer some insight into
the prehistoric grinding landscape of the southern plains.
Working the Problem
The problem, in developing a method to explore prehistoric landscape perception
through these bedrock grinding surfaces, centers on building a linking argument between
function of grinding places and the meaning of grinding places. With no regional
ethnography or consistent recording of features within an archaeological context to assist,
the model foundation becomes the individual grinding surfaces themselves. The grinding
stations must be defined within their unique, individual context along with the natural and
archaeological landscape in which they are located. The goal is to construct grinding
landscape in the sense of a taskscape (Ingold 1993). This will establish a base relationship
between these surfaces and the environment. Thus the grinding stations become the basic
element of the socio-cultural landscape.
Bedrock grinding features are not, however, independent of their environs. How
then to model that particular landscape which may include other archaeological features
(rock art, grinding features, structures, rock shelters, etc.) as well as the nearby natural
resources. Since the both the temporal sequence and functional nature of the grinding
stations are uncertain at this time, I have decided to view each bedrock feature as
independent of site data (see Morgan 2008:248). Site boundaries are often arbitrarily
assigned or decided by natural slope or breaks in the landscape such as streams, drainages,
or elevation. This may distort inferences about feature context, obscuring the meaning
imbued in, or functional nature of, the work areas.
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Independent recording the individual features in a defined area refines our
knowledge of the actual landscape features or landscape artifacts present in any given area
across this region. The separation of the grinding features from the construct of “site”
means that I can define the nature of each grinding feature as an organized landscape, or
work space, confined to the environ of a particular boulder or exposed bedrock area. Using
photogrammetry to model this surface area, I can generate digital terrain models to
compare, statistically and visually, individual grinding areas to one another. Then, by
recording the surrounding archaeological features and artifacts within a select parameter, I
can build an area specific map that will compare the grinding features found within that
area to one another, as well as to nearby archaeological features or artifacts and to natural
resources also found within that area. If grinding use and wear are indications of
demographics, long term use and/or materials being ground then I should be able to build
a baseline model of the workspaces and how they fit into the landscape in with they are
situated (Adams 1999; McCarthy et al 1985; Morgan 2008; Nelson and Lippmeier 1993;
Parkman 1994; Schneider 1993). I will describe the preliminary and rough results of a
study which has revealed the dynamic and complex nature of the bedrock grinding work
space (Lynch et al 2007).
A Working Case Study
Clustered bedrock features along the Chaquaqua (Chacuaco), are found in many
different environmental settings but appear to be confined to the cliffs along the canyon
systems. To explore these complex site areas, we began with a simple question: do the rock
art panels found in association with bedrock metates inform the grinding workspace? We
chose a site was that had work spaces which could be quantified with photogrammetry,
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buried metates which could yield dateable artifacts, and rock art. The presence of rock art
could provide a temporal window, since it is well seriated for this region, as well as a
cultural or social context. We operated under assumption that the rock art and the grinding
workspace were contemporaneous (Lynch et al 2007).
Site 5LA11455 is a small rockshelter with a single bedrock workstation and a pecked
abstract motif along an adjacent boulder (Loendorf 2008). The rockshelter is about 10
meters below the rim and opens to the east overlooking the Chaquaqua waterway about
200 meters down slope (Figure 2). The bedrock grinding surface is on a large sandstone
boulder which dominates the shelter (Figure 3). There are four main workstations each
comprising a large, deep, oval-shaped basin oriented perpendicular to a shallower oblong
basin metate (Figure 3). Workspace was cramped. The ceiling of the shelter is about one
meter above the grinding surface. Two metates were filled with sediment and protected by
a rat midden so there was potential to extract datable and identifiable material (Loendorf
2008; Lynch et al 2007).
The physical landscape, in which these sites are located, is rough terrain; not easily
accessed from above or below. The site was confined to the shelter (rock art and bedrock
metates) and the terrace in front of the shelter (Loendorf 2008). However, while recording
the site for this study, we located another bedrock grinding station to the south of this
shelter and third grinding station along the ridge just above as well as two other areas
within twenty-five meters of the site. None of these other areas had been recorded. The
grinding areas were not in rock shelters and, though different in shape and wear pattern,
they may have been used in association with 5LA11455. This illustrates the dilemma faced
when trying to sort out use and landscape perception of these grinding features.
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The presence of rock art panels at a number of these grinding sites (and others
along the Chacuaco) raises intriguing questions about the imagery and its’ meaning or role
within the site context. But in the absence of regional specific ethnographic evidence – how
can we understand the site relationships? For the purpose of that inquiry we decided to
approach the site, panels and metates surfaces, as an integrated site component in a
“domestic context” (Quinlan and Woody 2003:375). Assuming a direct association
between the two main components: rock art panels and groundstone, it is possible to infer
that the work space had significance to the prehistoric inhabitants of the space (Bachelard
1969; Ingold 1993; Tuan 1977). It seemed likely that we could learn more about who was
the intended audience then who actually created the panels by looking at the workspace
itself (Loendorf 2008).
The results of the study provided limited but intriguing insight into the work space.
Comparing the rock art to other sites in the region indicated that the panel almost certainly
dated to the Diversification Period and may represent an extended range of occupation or
multiple visits between AD1050 – AD1450 (Loendorf 2008:72). The rock art panel is about
two meters above the ground surface and is visible from bedrock grinding station but not
so visible from the rock shelter opening. Could this mark identify the grinders? Is there a
similar pattern at other stations? Would it help determine why grinding was done here?
The use of photogrammetry to define the workspace provided information that has
enormous potential for groundstone research. I will briefly describe the procedures we
used to build a model of this grinding workspace. Using a Canon EOS camera I
photographed overlapping images along the surface area (Breithaupt et al 2004; Matthews
2008; Matthews et al 2006). This yielded hundreds of images of the surface (Figure 4).
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These images were used in a 3D measure and modeling software package for
photogrammetric analysis. Three-dimensional model of the surface and the digital image
dataset was generated. This model can be rotated to better visualize the grinding surface
(Figures 5, 6). Digital terrain data generated from the stereoscopic images were generated
to show topographic contours (Figure 7). This process can better define feature
morphology, generate data for intra-feature analysis, and cross feature analysis, to infer
functional relationships between individual grinding surfaces.
The Next Step
The process described above generates analyzable data that can be used
independently to analysis grinding areas or can be employed with Geographic Information
Systems (GIS) to build a prehistoric landscape model for the southern plains. The first step
in this process is to begin recording the bedrock grinding features, neighboring
archaeological features or artifacts and groundtruthing the local vegetation. Once these
data are entered into GIS we can begin to tease out the patterns, if any exists, of design,
locality and temporality. It is precisely the existence or absence of these archaeological
patterns that will chronicle the prehistoric social-cultural landscape of this region.
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Figures
Figure 1. Map showing the approximate location of the Chaquaqua Plateau. (Images E.Lynch)
Figure 2. Rockshelter 5LA11455 located above the Chacuaco Creek in SoutheasternColorado. (Image: Elizabeth Lynch)
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Figure 3. The workspace at 5LA11455 and the adjacent rock art panel. (Images: LarryLoendorf and Elizabeth Lynch in (Lynch et al 2007))
Figure 4. Image depicting the camera positions from which stereoscopic(overlapping) images were taken. These images were used in a 3D measureand modeling software package for photogrammetric analysis. (Image:Neffra Matthews In (Lynch et al 2007))
Figure 5. 3D model of the surface and the digital image dataset rotated tobetter visualize surface features.2007))
Figure 6. Cross section of grinding artifact. (Images: Neffra Matthews. In2007))
Figure 7. Digital terrain model of the bedrock surface. (Images: Neffra Matthews inet al 2007))
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3D model of the surface and the digital image dataset rotated tobetter visualize surface features. (Images: Neffra Matthews In (Lynch et al
Figure 6. Cross section of grinding artifact. (Images: Neffra Matthews. In
ure 7. Digital terrain model of the bedrock surface. (Images: Neffra Matthews in
3D model of the surface and the digital image dataset rotated to(Lynch et al
Figure 6. Cross section of grinding artifact. (Images: Neffra Matthews. In (Lynch et al
ure 7. Digital terrain model of the bedrock surface. (Images: Neffra Matthews in (Lynch