Geossítio 40 Bad Stone
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The “Pedra Má” geosite is located in the middle of the Arda Ecovia.Its curious name refers to the characteristics of this so-called hornblende rock: high hardness and little alterability. Over time, this has contributed to the retention of sediment upstream, preventing it from flowing into the River Arda. In turn, the retention of sediment contributed to the existence of fertile soils in the Arouca valley (located upstream). The geosite divides the Arouca valley and the Rossas valley, located downstream from this geosite.
Looking closely at the bed of the River Arda, which forms part of the Douro River Basin, we can see circular depressions – the Marmitas de Gigante. These are formed due to the energy of the water and the abrasion that the sediment it carries causes on the rocks of the riverbed.
Informação Geológica
Downstream from the Arouca valley, which is infrastructured by the Arda Ecovia, is the “Pedra Má”, a geosite marked by the existence of corneana, a metamorphic rock formed as a result of contact metamorphism between the Arouca batholith (upstream from this geosite) and the ante-ordovician metasediments (located downstream in the Rossas valley).
The hardness and low alterability of the corneum may have led to the retention of sediments in the Arouca alveolus, contributing to the permanence of fertile agricultural soils in the areas surrounding the Monastery of Arouca. The Arouca valley where the River Arda flows is embraced by the Freita (to the south), Gamarão (to the north) and Senhora da Mó (to the east) mountains.
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