This Site of Community Importance lies at the north end of the province of Burgos (Castile and Leon), bordering Cantabria on the southern slope of the Eastern Mountain SCI with which it borders.

Although in a strictly geographical sense this small SCI of just over 2,000 hectares is not found in the hydrographic basin of the river Miera, in the hydrogeological sense it is fully integrated in it, since the Castro Valnera massif and adjacent mountains is actually a great subterranean developed karst connected underground, hosting some of the greatest Iberian caves system.

With a relatively isolated situation with reference to the rest of mountainous systems, the extreme humidity and the great lithological variety of the zone constrain the isolation of the vegetal communities, among which the hygro peat zones stand.

From the botanical point of view this place has been known since ancient times for the presence and the discovery of a series of isolated species around the Massif, which together with other large numbers of rare species, convey here some of their most important populations, located in its area that is its ​​limit of distribution. The importance of the site also lies in the diversity of its flora, which presents endemic and Eurosiberian species, such as Gentiana boryi Boiss.

The main habitats present in the Valnera Mountains are European dry heaths and Atlantic acidophilic beech forests with Ilex undergrowth and sometimes Taxus sp.There are also important wooded masses in the surrounding valleys, generally dominated by beech forests and other unique forests and interesting bush formations.