The Making of the Tanum Rock Carvings - A Geological Perspective

Abstract

The Bronze Age rock carvings in Tanum, Bohuslän, are made in glacially polished granite, a hard crystalline rock that is difficult to engrave. This raises the question: How did the Bronze Age carvers make them, and which techniques and tools were used? In this report, the purpose is to offer a geological perspective on these questions, through an investigation into one such engraved rock panel, Tanum 28 on Aspberget. The relationship between carvings and the properties and features of the rock was studied through close observation with hand lens, and it was found that most carvings on the panel were engraved in homogenous granite, but a few of them intersected local geological features such as mineral orientation, a pegmatite dike, and a fracture. Additionally, some speculative aspects on the making of rock carvings in general are discussed, such as the value and use of rock flour produced from rock carvings, and the presence of water flows on engraved panels, which could be useful when engraving rock. Further, seismic p-wave velocity was measured inside and outside the rock carvings with a Pundit 200. This was done to measure the microfracture density of the rock, and test if p-wave velocity can be correlated to a typological age chronology of ship carvings. The results indicate that such a correlation is present, which supports the idea that rock carvings may be dateable by p-wave analysis. However, the small sample size, the uncertainty of typologically dating rock carvings, and local variations in pwave velocity, call for caution in interpreting the results. Finally, an unexpected discovery was that some rock carvings had a higher p-wave velocity inside the carving than outside.