Obsidian: The MVP of Yellowstone’s “Stones”
by Robin Park
Obsidian is a volcanic glass formed when magma is extruded from the earth’s crust and cools very rapidly, with little moisture content or crystalline inclusions. It was generally the most popular tool stone material used by the ancestors of Native Americans in the Greater Yellowstone Ecosystem (GYE) and was prized as a tool stone material for practical (and potentially cultural) reasons. As a glass, the atomic structure of obsidian is “disordered,” which means it has no “preferred direction of fracture” (Shackley 2005). This is the reason for obsidian’s conchoidal fracture pattern and allows for easy flaking and sharp edges (up to 10 times sharper than surgical steel), qualities that make it an excellent tool stone. Obsidian was also abundantly available in the GYE, which was convenient for the highly mobile hunter-gatherers who called this land home prior to the arrival of non-Native people. In addition, some of the places where obsidian was collected or quarried were considered culturally significant to a number of tribes who historically inhabited this area, perhaps further adding to the preference for obsidian as a tool stone material (Park 2010).
Obsidian is also “prized” by archeologists, for one key reason: this material and other rhyolites can be analyzed for their chemical composition, which provides a unique signature or “fingerprint” for each source. Thus, tools made of obsidian can be traced back to their source using this fingerprint, providing insight to archeological analysis of trade and travel routes and potentially even cultural significance for tool stone collection areas.
The source affinity of cherts and other cryptocrystalline materials (locally available and frequently used tool stones in Yellowstone) can also be determined at the trace elemental level. However, cryptocrystalline materials have a high degree of intra-source variability. The process is expensive, and the results are not always of a nature useful to archeological research questions.
Obsidian, on the other hand, is an ideal tool stone for determining source affinity to a degree that is archeologically applicable. Instrumental trace element analysis of obsidian can be performed and results obtained through energy dispersive x-ray fluorescence (EDXRF), which is a relatively inexpensive, non-destructive, and highly accurate technique for mapping a source’s fingerprint. Intra-source variability in obsidian typically falls into a predictable range for those sources in the Yellowstone area.
When determining the geochemical “source” of an obsidian artifact, certain trace elements are assigned more analytical weight based on findings; elements Rubidium (Rb), Strontium (Sr), Yttrium (Y), and Zirconium (Zr) show the most consistent inter-source variability for the region (Dr. Richard Hughes, personal communication 2008). These elements are considered “diagnostic,” signifying these trace elements are well-measured by EDXRF and show high variability between sources, while maintaining low intra-source variability (Hughes 2007). These diagnostic elements are, therefore, most useful in distinguishing between different geochemical sources. The trace elements Zinc (Zn) and Gallium (Ga) are also recorded but not considered diagnostic of distinct chemical groups because they “don’t usually vary significantly across obsidian sources [in the Greater Yellowstone area]” (Hughes 2007).
Obsidian Cliff (48YE433)
Through time, several Native American cultures had knowledge of and access to different obsidian sources in the GYE. There are 19 known obsidian sources used by prehistoric peoples in the Yellowstone region spread out to the south, southwest, west, and northwest, offering choice and opportunity (Park 2011).
Perhaps the most well-known obsidian source in the Greater Yellowstone Area is Obsidian Cliff. The glassy cliff exposure of this source visible from the road rises 60 meters from the ground; the flow itself covers an area of approximately 14.5 square kilometers (Davis et al. 1995). It is a highly significant source at both the regional, national, and international levels. Artifacts made from obsidian from this source are found as widespread as Texas, Washington, southern Alberta (Brink and Dawe 1989; Reeves 2003), and Hopewellian burial mounds in Ohio (Griffin et al. 1969; Hatch et al. 1990; Hughes 1992), indicating it was a prized material extensively traded/exchanged (and directly accessed) by people for thousands of years. Large-scale archeological reconnaissance and survey of this source in the late 1980s culminated in its nomination as a National Historic Landmark (Davis et al. 1995).
Thanks to extensive sampling of the Obsidian Cliff flow, the geochemical composition of this source is known to cluster within an expected range and the geochemical integrity of the source has been well established (Hughes 1990). The Obsidian Cliff source locality is in the northwestern region of the park, to the east of the Gallatin Range and adjacent to the modern-day Mammoth to Norris section of the Grand Loop Road. This locality consists of an exposed cliff face (which is the feature popularly known as Obsidian Cliff or the Obsidian Cliff Plateau) and the flow area immediately east of the cliff face. There is evidence for both the utilization of cobbles as well as direct quarrying of the bedrock obsidian (Davis et al. 1995). Fifty-nine quarry pits/tool workshop locations were documented on the Obsidian Cliff Plateau, along with thousands of flakes and tools (Davis et al. 1995).
When visually inspected, Obsidian Cliff obsidian is glassy and smooth with few inclusions, and ranges in color from black to brown, mahogany, gray, and even green. It is typically semi-translucent in opacity, but infrequently can also be opaque. It is considered to be high-quality obsidian for tool making, particularly for knives and projectile points.
Many of the Native American tribes with ancestral or cultural associations with Yellowstone have indicated Obsidian Cliff is a spiritually and ideologically significant place, and have oral histories describing ancestral collection of obsidian from this place (Park 2010). In addition, medicinal use of obsidian from Yellowstone by more than one culture has been documented (Park 2010). This ethnographic information contributes to the overall cultural significance of what is also an exceptional archeological site.
Past and Future Research using Obsidian
There are over 45 rhyolitic flows in Yellowstone containing obsidian; however, only about 15% have obsidian with the right qualities (such as absence of flaws in the material and usable cobble size) to be made into tools (Dr. Ann Johnson, personal communication 2009). Geochemical Research Laboratory located in Portola Valley, California, (with Director Dr. Richard E. Hughes) has performed the majority of analysis of Yellowstone obsidian since 1988.
Currently a large dataset of sourced obsidian artifacts found within park boundaries exists and is continually being added to by ongoing archeological surveys. Dr. Leslie Davis pioneered the collection and use of obsidian sourcing information to gain insight to archeological questions in Yellowstone (Davis 1979). Others have made significant efforts to compile comprehensive datasets of sourcing results (Cannon and Hughes 1993), and to use these data to understand annual travel routes (Johnson et al. 2004) and analyze spatial and temporal trends in tool stone source selection (Park 2010). Recent archeological surveys by the University of Montana of the shoreline of Yellowstone Lake have significantly added to our dataset and further defined new obsidian source localities (such as the Parker Peak source, McIntyre et al. 2013). Future research will likely focus on more fine-grained spatial and statistical analysis, in addition to continually building a comprehensive dataset of source localities of obsidian artifacts.
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