5.5.5 General considerations

All lithic artefact specialists should have a basic geological knowledge of the raw materials used in Scottish prehistory - where they occur and in which periods they were used. However, the acquisition of relevant information for many less common raw materials can be problematic and, in those cases, it is essential that the lithic analyst works with a geologist. It is for example difficult to distinguish between fresh Staffin baked mudstone and the tuffs of eastern Skye (such as, for example, the form recovered from Clachan Harbour on Raasay; Ballin et al. 2010b). The chalcedonic silicas of the Inner Hebrides and west coast (e.g. An Corran and Camas Daraich; Wickham-Jones and Hardy 2004, 20; Saville et al. in press) may also be difficult to deal with, and the striped lithic raw materials (mostly meta-sediments) recovered from the the Southern Hebrides and the Western Isles are notorious (in some of these cases, even the geologists seem to disagree, defining these types of rock as variously either mylonite, baked mudstone, or hornfels).

An expert geological input is also relevant in connection with acquiring an understanding of the appearances and properties of lithic raw materials. Staffin baked mudstone and Lewisian mylonite were probably both more or less unpatterned in their fresh states, and they may have acquired their striking appearances (marked dark-light banding) as part of the weathering process. It is also likely that the appearance of some raw materials (e.g. meta-sediments) as, for example, loose-textured and apparently ill-suited for flaking, is a result of weathering since deposition.  It is worth noting, however, that in many cases the size of outcrop which will spark archaeological interest may be well below that which would register as significant to a geologist.  Issues of scale in fieldwork may have to be resolved.

In cases where geological consultation does not deal with a problem in a satisfactory manner, such as the case of mylonite/baked mudstone/hornfels (above), fieldwork may provide a solution. In this instance, the possibility of the raw material being mylonite could be tested by attempting to find primary outcrops, or even quarries, along the main faultline of eastern Lewis, where mylonite occurrences have been reported (Smith and Fettes 1979, 78).