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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Knife River Flint quarries and the Alberta connection

Written by: Emily Moffat, Regulatory Approvals Coordinator, Archaeological Survey of Alberta

Stone tools were central to life in pre-contact North America and the rocks that they were made of were highly valued. The archaeological record throughout vast regions of North America, including much of Alberta, contains Knife River Flint (KRF), one of the most significant and intriguing tool stones used before the arrival of Europeans. KRF gets its name from the Knife River, a tributary of the Missouri River that flows through the United States Midwest and Southeast Regions.

In an era of limited human mobility compared to modern times, KRF was regularly transported hundreds of kilometres from its source in North Dakota, where it was quarried for thousands of years. A relatively small geographical region contains the majority of quarry pits and this location is the hub of KRF’s widespread distribution.

Map showing the major source area where Knife River Flint was quarried, the area where scattered cobbles were collected, and the overall extent of Knife River Flint artifacts based on published records. White circles show the extent of archaeological sites in Alberta that have produced a Knife River Flint artifact (Source: Todd Kristensen).

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