From Macro to Micro: Characterizing Lithic Raw Materials

This is where I take archaeology and geology, smashing them together in a glorious mess.

In order to source the lithic (stone) raw materials that we recovered in our assemblages (see previous entry "My Research 101" for explanation as to why I am doing this), I am attempting to describe the stone types in a way that is based in geology but is accessible to archaeologists with little or no background in geology. I do have a limited background in geology myself, mostly a few introductory courses along with some upper level ones in sedimentation and stratigraphy, GIS mapping, and mineralogy, but I am not a geologist.

I am describing our lithic artifacts using both macroscopic and microscopic attributes. Macroscopic attributes are those visual characteristics (colour, patination, lustre, mottling, speckling, banding, texture, inclusions, etc.) that you can see via the naked eye. Microscopic characteristics are those that require the use of, obviously, a microscope (preferably a polarizing microscope). These include mineral content and relative proportions, fabric, and texture among many others. The lists of attributes I am recording are derived from those used by geologists to describe and characterize rocks within the three major rock types: igneous, metamorphic, and sedimentary.

Macroscopic analysis is frequently not adequate for distinguishing between different rock types thus necessitating the use of microscopic analysis. Further, microscopic analysis often can tell us a lot about the specific formation environment of that particular rock (i.e. it's source). Chert is a great example to further clarify this.

Chert, overall, is highly variable. Within a single formation of chert there can be a considerable amount of variation in it's visual (macroscopic) characteristics. Even if the source is visually homogeneous there may be microscopic variations between the various members of that formation. Thus, when we talk about variation in cherts we can talk about inter-source variation (that between different sources) and intra-source variation (that within a single source). However, inter-source variation can be quite low too. You can have several sources of chert that all look visually the same, so we need to look at the microscopic features of each source individually to distinguish which one is which. We also have to look at a number of samples from each source must be look at in order to account for intra-source variability. Now if we don't have descriptions of the sources, like in this case (we did a field season of survey and were not able to find any potential sources, and this is the first study of its kind to occur in this region so there is no baseline data), we can still look at the microscopic attributes of the cherts to at least determine how many (potential) sources were being used and what formation environments should we expect to find these sources.

So I start by grouping the cherts into "types" based on their macroscopic features. I then sample each "type" for thin sectioning. These thin sections are then analysed for their microscopic attributes and again grouped into "types" based on similarity or difference to each other. Hopefully the macroscopic types will correlate with their microscopic types. If not, that's ok too - what that means is that it is likely there is intra-source variation. Either way I need to then look further at the microscopic attributes of the types to try to connect them to a formation environment. Again, certain microscopic features can allow me to say that this chert was formed in this particular environment over another. The idea is to then go back to the geological maps for the region and identify possible formations that match the microscopic evidence for where the chert could have come from so we can go back into the field and check them out.

Whether I am able to find the sources or not, it is my hope that the method I am using, and the charactertization scheme I am developing, will allow archaeologists working in Tanzania to describe their assemblages in a standardized way. This will then allow us to start to look at similarities and differences in raw material selection and use within and between various regions of the country.

In the future I'll post some pictures to clarify what I've tried to explain above. Eventually, once my research is done, I am going to develop a website where anybody can see descriptions, photos, and microphotographs of all of our raw material types (not just the chert).