Rock carving depth reflects carvers' geological insight

Rock art serves as a remarkable window into Bronze Age societies and is usually interpreted through motif and setting. The exceptionally well-preserved art in Bohuslän is recognized as world heritage with a wide range of imagery engraved into the local Bohus granite. Here we connect the depth of these carvings with near surface structure of this granite by combining ultrasonic pulse-echo B-scans with Structure-from-Motion photogrammetry and DEM based depth modelling for five major panels. The B-scans reveal surface parallel reflective horizons at ~8-9 mm and ~18-20 mm depth (11 and 25 mm at one site) consistent with para-glacial stress-relief microfracture zones. Our investigation shows an average depth for most figures of ~0.96-1.52 mm, and even the deepest carvings lay entirely within the first weak layer. While they exceed the post-glacial depth of weathering (≤1 mm), which was previously thought to be the limiting factor for carving depth. A nearby, polished but uncarved outcrop lacks reflective layers. We conclude that carvers preferentially selected panels with paraglacial weakening and even show preferential placement on an individual panel. Thus, placement and choice of locations were based on geological necessity and its understanding by the carvers.