a b s t r a c tThe pre-Magdalenian phase of the Côa River Valley open-air rock art is mostly distributed at the boundary between the rocky valley slopes and the floodplain that correspond to the most favourable geomorphological setting for the preservation of pecked and deeply superposed engravings of the most famous artistic phase of the area. The natural vertical panels of the engraved art located at this geomorphological interface have suffered weathering during two cold events of the Lateglacial. They were buried by several colluvial and alluvial deposits. The reconstruction of the sedimentary and archaeological context of the CôaRiver Valley engraving permits a better assessment of the preservation processes and interpretation of Palaeolithic open-air rock art.

Geoarchaeological analysis of the Middle and Upper Palaeolithic record preserved in cave, rock-shelter and open-air sites in the northern sector of the Meso-Cenozoic of the Western Iberian Peninsula margin (Portugal) reveals several disconformities (erosive unconformities), hiatuses and surface stabilization phases. A recurrent disconformity, dated to ca. 29,500–32,000 cal yr BP, in the time range of Heinrich event 3, must correspond to a main erosive event related to the impacts of climate change on the landscape, including a reduction in vegetation cover and altered precipitation patterns, with the consequent accelerated down-cutting by stream systems, slope reactivation and endokarstic reorganisation, causing the erosion of sediments and soils accumulated in cave, rock-shelter and open-air sites. These processes create a preservation bias that may explain why Early Upper Palaeolithic finds in primary deposition context remains exceptional in the carbonate areas of central-western Portugal, and possibly elsewhere in the other places of Iberia. The impact of such site formation processes must therefore be duly considered in interpretations of the current patchy and scarce archaeological record of the Middle-Upper Palaeolithic transition in south-western Iberia.

Open-air rock art preservation Landsat imagery Pair-wise comparison matrix Frequency-probabilistic procedure Predictive model a b s t r a c t 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 frequencyprobabilistic 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.