The Basque Country is an eminently mountainous territory, situated between the ends of the Pyrenees and the Cantabrian mountain chain, which join up within Basque territory in what has usually been called the "Basque threshold". This depression is the northwestern continuation of a great Cretassic geosynclinal that goes along the Ebro fault trough, which was later buried by the strong sediments of the tertiary period. In fact, detritic sedimentary rocks (sandstones, clays and marls) are the most frequent lithologic elements to be found in the Basque Country, appearing in mainly Cretassic-era flyschoid series and producing generally rounded reliefs.

In the conditions of rainfall and strong altitude differences in short distances, that are characteristic of the Cantabrian side of the Basque Country, hydric erosion is the most active geomorphologic agent, more active even than the action of waves in coastal cliffs zones and than karstification in limestone mountain ranges. In places where valleys broaden and slopes are less sep, reverse deposit their load of sediments forming flood plains. This Cantabrian side, with the predominance of siliceous substrata and steep slopes, usually has soils that are characterised by their acidity (pH around 4,5-5) and higher percentages of organic matter and iron. Only the alluvial soil at the bottom of valleys, deep and fertile as they are, and the lithosoils that are characteristic of limestone mountainous areas, do not follow this norm.

Lithologic Map (88 kb)

Slope Map (128 kb)

Soil Map (56 kb)