Most of the region lays on a granitic structure,with the exception of two narrow strips that cross it from NW-SE, one of silurian with carboniferous and gravestone structures and another one of archaic shist.

The soil's main origin is the granite disaggregation. Generally, it is characterised by its low depth, predominantly sandy to franco-sandy (superficial) textures, a natural high acidity and poor in phosphorus.

The fertility levels are naturally low, as one may easily conclude from the characteristics already mentioned. However and due to the nature of the agrarian systems used in the region since the old times, the soils gained a considerable fertility that allowed supporting one of the most highest population densities in the country. The secret for this fertility may be attributed to the two main types of human intervention in the natural conditions: the control of the terrain, through the construction of terraces and the intensive and persistent penetration of organic substances.