Climbing dunes in Faro de Anha Natural Monument, Viana do Castelo

The Local Natural Monument corresponds to the NE side of the Galeão massif (57 ha), located on the left bank of the mouth of the Lima River. In this area, three flattening surfaces are preserved: Vila Fria Surface (50 m), Ola Surface (75 m) and Faro de Anha Surface (100 m) (Carvalhido, 2012; Carvalhido, 2014a, 2014b, 2014c). These surfaces are regularized by sandy deposits, well calibrated, preserving interlaced stratification and cross lamination by current ripples (Carvalhido, 2012, 2014a, 2014b, 2014c, 2014d).  The Areias de Galeão are dune sands deposited at the culmination of the climate crisis of the Little Ice Age (Dalton Minimum) (radiometric dating indicates deposition between the years 1780 and 1800). The typical criss-cross stratification of this unit is clear in some sectors, although sometimes only low-angle beams are observed (Carvalhido, 2012, 2014d). The morphometry and morphoscopy data prove the aeolic origin of the deposits, consisting of grains from rolled to sub-rolled features, with perforated and shiny rounded surfaces. The low humidity conditions, associated with the strong winds that would have been felt during the Dalton Minimum, will have been responsible for the transport and subsequent wind regulation of the Galeão surfaces. Despite the carbonated content of Areias de Galeão, the recent age, the weak thickness of the deposits and the cold conditions (conducive to the dissolution of carbonates) will have prevented their consolidation. Parish memories are an asset with potential for exploring this theme (e.g. disappearance of the parish of Aster under the dune sands). The base of the Areias de Galeão unit (first 20 cm) conserves medium to fine granite clasts, of two micas and biotitic (MPS = 20 cm), subangular (Carvalhido, 2012).  Under the dune sands there is the lithostratigraphic unit Areias da Ribeira da Areia, which corresponds to the water availability conditions that precede the Maunder Minimum, which constitutes the period of full installation of the Little Ice Age in Europe. The standard deposits retain some of the sets of frontal ripples with preserved internal structure - tangential to sigmoidal frontal blades, inclined towards NE and dorsal counterflow blades. The OSL dating of the deposits revealed installation between the years 1590 to 1650 (Carvalhido, 2012, 2014c).  The available radiometric data, the stratigraphic contact in involution and the decacentimetric clasts of medium grain granite, of 2 micas, angular, in the transition between the two levels, allow to deduce the association of the deposits with climatic evolution until the installation of the Maunder Minimum: the lower level is associated with a medium with available liquid water, associated with currents (Areias da Ribeira da Areia), to degrade the wind deposits at higher levels, later transitioning to cold conditions (signs of cryoturbation in the transition between both units), strong winds but not very humid (Maunder Minimum) (Areias de Galeão) (Carvalhido, 2012).

Areia de duna do Galeão

Geoparque Viana do Castelo