Archaeology through a different lens: Thin section analysis of lithic materials

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.

Rock thin section and petrographic microscope. Source: Emily Moffat.
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Connecting the Continent: Stone Tools in Alberta

After a few thousand years, most of the archaeological record in Alberta has been winnowed down to pieces of rock used to make tools. Organic artifacts, structures, and other less durable things generally don’t survive thanks to erosion and decomposition. To maximize the information we can pull from those pieces of stone, the Alberta Lithic Reference Project (ALRP) was formed by a consortium of archaeological consultants, heritage managers, geologists, students, and university researchers. The goals are to accurately and consistently identify the types of raw materials that pre-contact people used to make stone tools. Why is this important? Specific types of rock were traded and moved widely across the continent and serve as valuable indicators of cultural relationships and/or human mobility patterns. Read more