Written by: Emily Moffat, Regulatory Approvals Coordinator, Archaeological Survey of Alberta
By examining the microscopic details of a lithic material, the geological history and characteristics of the rock comes into focus, which contributes to our understanding of the archaeological record. Archaeologists use this information to understand how people made tools, how they collected or traded stones, and how they moved around past landscapes. Thin sections therefore provide a different lens through which we can view human behaviour.
Thin sections are extremely fine slices of material that are viewed under a microscope to observe details not visible to the unaided eye. Petrography is the detailed description of the composition and texture of rocks and although it started in the field of geology, it has since been applied to archaeology. Petrographic analysis of thin sections has proven to be a powerful tool in better understanding archaeological materials, such as stone tools and other lithic artifacts, by furthering our knowledge of the rock types that they were made from.
To make a thin section, a small cut of rock is adhered to a glass microscope slide and polished down to a thickness of about 0.03 mm. At this point, the sample is so thin that light can pass through it. Petrographic microscopes are specifically designed to view rock thin sections because they have light polarizers that reveal unique optical properties of minerals. By viewing the rock under these polarizers (termed plane polarized and cross polarized light), the minerals within the sample can be identified and small-scale features that give clues as to how the rock formed become visible.
Within Alberta, over 50 unique lithic raw materials have been identified in the archaeological record as stone tools, flakes, and other artifacts. The majority of the lithic materials that these artifacts are made from are fine-grained, as these are the easiest to flint knap or shape into finished forms. Sometimes these rock types look very similar to each other. Differentiating and accurately identifying the rocks that artifacts are made from is a key starting point for archaeological interpretations and thin sectioning is one technique that can help with this. Thin sections provide a unique window into the geological history of a sample, which allows for a more detailed classification of the rock type. This information is important for assigning an accurate name to the material. Thin sectioning is a destructive technique so it is generally only applied to unmodified pieces of lithic materials, but the information learned from this method adds to our understanding of the rock and lays the foundation for other non-destructive methods used to assess artifacts.
Petrographic analysis of thin sections has been incorporated into a number of studies in the Alberta Lithic Reference Project, an initiative aimed at helping archaeologists identify lithic materials through the detailed documentation of their microscopic, macroscopic, and geochemical properties. Thin sections were of particular use in characterizing the following three lithic materials:
Beaver River Sandstone
Beaver River Sandstone (BRS) is a significant toolstone sourced from deposits in the oil sands region of northeastern Alberta. The Quarry of the Ancestors located near Fort McMurray, Alberta is perhaps the most well known outcrop deposit of BRS. Lithic assemblages from this area are dominated by BRS but artifacts of this material have also been recovered from archaeological sites in central Alberta and parts of Saskatchewan. BRS is described as an opaque grey to tan-coloured rock that is fine to medium-grained. In thin section, more details are apparent. The sand-sized particles are rounded to subangular quartz grains that are held together by silica cement. Hematite, tourmaline, mica, and other minerals have also been observed in small amounts. This information helps differentiate BRS from other silicified sediments, which have a different composition and texture.
Tertiary Hills Clinker
Tertiary Hills Clinker (THC) is a fascinating raw material that comes from the Northwest Territories. THC is found at archaeological sites throughout northern and central Alberta and its occurrence here yields information about past interactions between peoples inhabiting these regions. THC is a glassy, translucent to opaque rock that can range from white or light grey to light brown, red, and purple. The most unique aspect of this material are the tiny holes present throughout. These holes are at most 2mm in diameter, so some degree of magnification is required to see them clearly, and they may appear as dark speckles if they have been infilled with sediment. Thin section analysis shows that these are vesicles, or preserved gas bubbles in rock. This observation, along with results from other analytical techniques, demonstrated that THC formed from mudrock deposits that had been fused or melted through the burning of underground coal seams. This formation process means that the rock type is a clinker, rather than other similarly glassy rocks derived from volcanic eruptions.
Grizzly Ridge Opal
Grizzly Ridge Opal (GRO) is a toolstone that outcrops near Deer Mountain in north-central Alberta. An archaeological site complex was discovered in this area that contained a number of flakes and unworked pieces of the raw material. Research efforts are currently underway to characterize GRO but thin section analysis has played a large role in naming the material. GRO is a relatively distinct looking raw material with vitreous white and brown patches throughout. At first the rock was deemed to be a chert, which by definition is primarily composed of microcrystalline or cryptocrystalline quartz. Thin section analysis revealed that, in addition to quartz, the rock contains a significant amount of opal, which informs how the material originated. Interestingly, small pieces of petrified wood were also identified in some samples, which reveals the conditions and environments in which the material formed.
Kristensen, T.J., M.R. Turney, R. Woywitka, B. Tsang, M. Gingras, P. Rennie, E. Robertson, T. Jones, J. Speakman, and J.W. Ives. 2016. Beaver River Sandstone: A silicified toolstone from northeast Alberta. In: Back on the Horse: Recent Developments in Archaeological and Palaeontological Research in Alberta, edited by R. Woywitka, pp. 136-153. Occasional Paper 36. Archaeological Survey of Alberta, Edmonton, Alberta
Kristensen, T.J., T.D. Andrews, G. MacKay, S.C. Lynch, M.J.M. Duke, A.J. Locock, and J.W. Ives. 2016. Tertiary Hills Clinker in Alberta: A partially fused vesicular toolstone from the Mackenzie Basin of Northwest Territories. In: Back on the Horse: Recent Developments in Archaeological and Palaeontological Research in Alberta, edited by R. Woywitka, pp. 100-112. Occasional Paper 36. Archaeological Survey of Alberta, Edmonton, Alberta.
Thank you to Dr. Andrew Locock for microscope images of Grizzly Ridge Opal, and Dr. Britta Jensen and Jordan Harvey for their help acquiring additional microscope images.