Peñas de Aia Natural Park

The Peñas de Aia Natural Park — Aiako Harria in Basque — covers 6,913 hectares at the eastern tip of Gipuzkoa, between the Bidasoa and Urumea rivers. It is the only granitic outcrop along the entire Basque Cantabrian coast, giving it a distinct geological character: the three main peaks — Irumugarrieta (834 m), Txurrumurru (821 m), and Erroilbide (837 m) — rise as Palaeozoic granite masses above a mantle of Atlantic forest. This contrast between bare rock and dense vegetation defines the park landscape from every vantage point.

The northern slopes, receiving over 2,200 mm of annual rainfall — up to 2,800 mm at the summits — hold the park's largest acidophilous pedunculate oak forest, spanning 1,217 hectares. Acidophilous beech forests cover another 619 hectares, mainly at higher elevations with northern exposure. Alder and ash galleries extend along 138 hectares of watercourses, creating corridors where constant humidity maintains a stable microclimate. Dry acidic heathlands occupy 466 hectares on exposed ridges and hilltops. Beneath the canopy, ferns and mosses carpet the ground in unbroken green, punctuated only by lichen-covered granite boulders.

The fauna has a Eurosiberian character, with over 147 recorded vertebrate species. Roe deer and wild boar are the most common mammals, while the European mink and Atlantic salmon represent the species of greatest conservation value in the park rivers. Notable birds include the griffon vulture, peregrine falcon, and middle spotted woodpecker. Streams flowing down from the peaks sustain native brown trout and white-clawed crayfish populations.

The park territory is shared among five municipalities: Oiartzun (44.7%), Errenteria (25.1%), Irun (18.2%), Hernani (9%), and San Sebastián (3%). The ancient Arditurri mines, worked since Roman times for silver, lead, and fluorite, are now a visitable site with illuminated galleries and interpretive panels. The remains of nineteenth-century mining infrastructure blend into the forested landscape, offering a contrast between historic human activity and the natural recovery of the surroundings.

From the summits of the three peaks, on cloudless days, the Cantabrian coastline is visible to the north and the first Pyrenean foothills to the south. The park trail network includes circular routes of varying difficulty, from family walks along valley floors to technical ascents of the granite ridges. The sound of water running through streams and the song of the robin accompany walkers on the lower paths, where light filters green through the oak canopy.