Rock Art Research

The markings on the stones of Terceira Island, in the form of simple or crossed grooves, resemble similar occurrences found in other parts of the world and have occasionally been defined as petroglyphs. A satisfactory explanation for such markings has been hard to find, despite multiple attempts, and has ranged from mechanical and chemical alteration to bicycle tracks. The explanation for the ‘carvings’ on regolith blocks is by gradual abrasion of large tree roots due to aeolian oscillation and agitation of the trees, which accrues the effects of frequent microearthquakes in the Azores. This hypothesis, previously suggested by R. G. Bednarik for similar grooves found in siliceous sandstones in Queensland, New South Wales (Australia) and the Pennines (U.K.), and on granite in Tasmania, seems to be the one that adjusts best to all observations and the characteristics of the grooves on the volcanic rocks of Terceira Island.