Composition of Metabasic Rocks Used for Stone Adzes and Other Implements During the Neolithic in the Three Lakes District of Switzerland

Very fine-grained meta-basic rocks have been commonly used as a material for adzes and other tools during the Neolithic by the population of the Three-Lakes Region of western Switzerland. Large numbers of artefacts and remains after dwellings were found along the lake shores of lake Morat, Bienne and Neuchâtel. Some of the raw material used for the stone tools has been locally found in the moraine left behind by the glaciers of the last ice age. Suitable rocks for the tools also have been acquired by the communities through trading. The origin of used coarse grained rocks can often be traced to specific occurrences of exposed rocks in the Alps. Particularly well-suited rock material for tools are very fine-grained meta-basic rocks of green colour designated as “greenstone” in archaeology. Petrographically the rocks include a wide range of various rocks including greenstone in its strict sense. Because the used rocks are very fine-grained its minerals cannot be determined without damaging the sample by preparing thin sections. It is however possible to analyse the bulk composition of the artefacts by newly developed XRF equipment not crashing the samples. In this communication we present total rock compositions of 14 stone artefacts from the Three-Lake area. The samples are dense fine-grained greenish metamorphic rocks. The rocks define four different compositional groups. One sample is made of nephrite. Four samples represent jadeitic compositions similar to the jade from Monviso in the Alps. Four further samples belong extremely iron-poor amphibolites of unknown origin. A group of five further samples is compositionally located between ophiolitic meta-basic rocks and jadeitite. To conclude, the material belongs to four type of source rocks from unknown locations. An origin from Bohemian basic hornfels widespread in the Neolithic of Central Europe can be excluded.