This 3500-ha forest of Pyrenean oak is the largest in Europe and is situated on the sandy soils of the Izki river basin. It is the home of large mammals such as wild boars, roebuck and wild cats, as well as its population of woodpeckers. Izki also contains a number of gigantic yew trees, the “Aginal”, near Apellániz.
This valley is edged by limestone mountains with massive rock faces such as La Muela, Soila and Arlucea, forming a huge amphitheatre at the head of the valley. In these mountains, the nature of the forests changes and the Pyrenean oak is replaced by the gall-oak on the terraces and by beech woods in high, shaded areas. Another of the characteristic uses of Izki is livestock breeding. There are extensive pastures between the peaks of San Cristóbal and San Justo, where horses and cows graze all year round.
The only village within the Park is Corres, where there is an Information Centre. This village conserves most of its medieval structure and is situated on high ground overlooking a spectacular ravine.