Nature vs. Culture: present-day spatial distribution and preservation of open-air rock art in the Côa and Douro River Valleys (Portugal)

Abstract

Late-glacial and Iron-Age open-air rock art of the Côa River Valley shows a similar spatial distribution, with several clusters along the Côa and Douro River tributaries that are mostly exposed to the southeast. In this report, we try to determine whether the artists of both periods deliberately chose the same natural panels for rock art or its present-day spatial distribution is imposed by formation and weathering processes, previous or subsequent to the engraving. Geological structural analysis, from regional to field scales, shows a NNE-SSW sinistral strike-slip fault system that crosses the study area, together with a set of fracture/joints with the same orientation and formed by the same tectonic stress. Direct field measurement and the description of 713 natural panels, engraved and un-engraved, reveal that the preserved rock art panels correspond to the most common tectonic fracture/joint systems (NNE-SSW) of the study area. Locally, the hydrographic network is conditioned by the same structural control. Differential weathering exists between the panels exposed on opposite margins of watercourses, with preferential degradation of the rock art panel surfaces exposed to the NW. We propose that, on the scale of the valley, the surface weathering of the rock art panels results from differential solar radiation, humidity, lichen and bryophyte colonisations. Interpretation of field observations, a frequency-probabilistic procedure, pair-wise comparison matrix and geographic information system analysis were combined to evaluate a Côa panel formation and preservation predictive model using archaeological, topographical and hydrological data. Four variables were extracted and weighted from the collected data, including topographic slope and aspect, solar radiation and cost-weighted distance to watercourses, which were used as environmental input data. The archaeological input data (rock art occurrences) were used to calculate the variable ratings and to evaluate both the Côa panel formation and preservation predictive model and external validation maps, with the results showing an agreement of 80% and 70%, respectively. Field verification revealed unknown rock art panels in areas with high and very high values. The Côa panel formation and preservation predictive model provides a useful framework to guide survey and heritage management.