This area comprises a long mountain range, with an east-west direction, which includes the highest peaks of the Basque Country (Aitxuri, 1,550 m) and is the boundary between the Cantabrian and Mediterranean slopes, as well as separating the Historical Territories of Alava and Gipuzkoa.
Most of the substratum is formed by urgonian limestones whose erosion gives rise to spectacular karstic phenomena: ridges, dolines, torcas, lapiaces, ravines, etc. Among them are famous places such as the Hoya de La Lece, the San Adrián tunnel, the Urbia depression, the Arantzazu ravine, etc. In Elguea-Urkilla the substratum changes and sandstones and clays emerge, resulting in a landscape of rounded slopes with an undulating profile.