The Castelo de Vide syncline is a periclinal structure with an axis oriented in NW-SE direction (Fig. 26.1). The length of the structure along its axis is about 40 km; the maximum width perpendicular to the axis is about 10 km. The contact of the syncline with the surrounding rocks is marked by Ordovician quartzites (Arenig). Towards NE these quartzites are in direct contact with the Hercinian intrusive Nisa granites, marked by metamorphic contact rocks (hornfels), whereas toward SW they overly the Pre-Hercinian Portalegre granites, with arkoses in the basis marking the transgression of the early Ordovician rocks over the granites. Locally, rocks of the "slate and greywacke complex" are present beneath the NE limit of the Arenig quartzites. The geostructure of this syncline was described by Teixeira , Gongalves et al. , Perdigao & Fernandes , Fernandes et al. ; Perdigao , Perdigao , Perdigao  and Silva and Camarinhas .
The carbonate rocks that form the aquifer of Escusa in the centre of this pericline are predominantly dolomites  which are classified as dolostones. The effects of active karstic processes at field scale are responsible for the presence of lapiaz, swallow holes, sinking streams, flowing from the neighbouring low permeability schists, and frequent collapse of the roof of shallow dissolution cavities. The thickness of the carbonate formation is about 200 m, but the maximum depth drilled up to now is 139 m. The area corresponding to the limits presented in the map in Fig. 26.1 is of 7.9 km2.
The carbonate formation is covered in most of its extension by terra rossa and colluvium deposits resulting, respectively, from the weathering of the carbonate rocks and from the mechanical weathering of the surrounding crystalline rocks (predominantly fragments of the Ordovician quartzites).
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