High-resolution mapping and analysis of shiny grooved rock surfaces: The case study of the Skiles Shelter, Lower Pecos, Texas

Shiny grooved rock surfaces found in archaeological sites are common across west Texas and the Southwest (U.S.). In most cases they are cursorily described, with very little in terms of detailed characterization and analysis. Interpretations regarding the meaning of the grooves include fertility rituals and functional use (e.g., bone tool or axe polishing), while the formation of the shiny surface has been attributed to lichen, foot or hand traffic, and hide or plant processing. In the Lower Pecos region (Texas), such remains are common in rockshelters. In several cases, there is only one such surface at a prominent location within the site, commonly associated with bedrock features (mortars, slicks, etc.). We used Structure from Motion (SfM) to document, characterize and analyze more than 700 elongated grooves and incisions concentrated on one shiny surface, at Skiles Shelter. All were mapped, and the larger grooves were also measured and studied using macro-lens and microscope photos. The direction and sequence of engraving were reconstructed, with evidence for most grooves and incisions being earlier than the shiny cover. The incorporation of small pits with the engravings was established. Although organic residues were retrieved from several bedrock features at the site, no distinct chemical signature was identified from the shiny surface. We could not ascertain the origin of the shiny crust, and based on other works and our observations we suggest that hand or foot traffic, hide working and plant fiber processing are the most likely explanations.