The Hole Story: Understanding Ground Stone Bedrock Feature Variation in the Lower Pecos Canyonlands
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Ground stone bedrock features are common at archaeological sites in the Lower Pecos Canyonlands of southwest Texas. These features are human-made holes pecked, ground, or worn into bedrock or large boulders, and were used for a variety of processing activities by the indigenous peoples. Although archaeologists in the region have informally recognized different “types” of ground stone bedrock features (e.g., slicks, grinding facets, deep mortars), there have been no dedicated studies of bedrock features. Due to their widespread occurrence in the region, bedrock features represent an untapped research avenue regarding the lifeways of Lower Pecos hunter-gatherers. Therefore, to gain a better understanding of these understudied features I mapped, documented, and analyzed 824 bedrock features at ten sites across the region.
Using Structure from Motion (SfM) photogrammetry, I collected high-resolution three-dimensional data of bedrock features. Measurements (length, width, and depth) for each feature were calculated from the 3D data in ArcGIS. These quantitative, metric data were analyzed for feature variation at individual sites, and then compared between sites to determine any differences. Results showed the metric distribution is not significantly different between the sites. Feature data were then combined into sub-regional groups (Pecos River, Devils River, Eagle Nest Canyon and 41VV75), and metric differences compared based on the geographic location. The analysis showed no significant differences between the sub-regional groups. In order to better characterize the data and understand the range of morphological variation in the entire data set, a cluster analysis was conducted. This analysis resulted in definition of four distinct clusters characterized in regards to their metric attributes.
To further analyze the four clusters and how they might differ, I analyzed the use-wear patterns within each cluster. This analysis suggested that two clusters (Clusters 1 and 3) were mostly shallow, general-use features, while the other two clusters (Clusters 2 and 4) were deep, specialized features. Both Cluster 2 (conical mortars) and Cluster 4 (cylindrical mortars) represent features that required a substantial time and energy investment to create. Further, based on the use-wear patterns, features in Clusters 2 and 4 were intentionally manufactured to a specific shape for a specific purpose. In contrast, features in Clusters 1 and 3 were initially manufactured for ease of use, and subsequent use appears to have minimally modified the surface. The findings of the use-wear analysis are supported by ethnographic accounts of how shallow and deep bedrock features were used.
This study represents an initial exploration of bedrock features in the Lower Pecos Canyonlands. More research with a larger sample of bedrock features is needed to refine and test the hypothesized clusters and functions put forth in this thesis, as well as create a formalized typology.