The region is dominated by the Quaternary Chaves sedimentary basin, a graben aligned on a north-northeast to south-southwest orientation bounded by the Hercynian massif and metamorphic schistose formations. Mixed with the schistose complex, there are bands of graphitic slates, mainly in the southern part of the urbanized area of Chaves and south-east of Faiões. To the north, and into Galicia, the basin extends into the depression of the Verin Basin. One of the oldest formations in the region, it dates back to the Ordovician period ( between 488.3±1.7 to 443.7±1.5 million years ago), is composed of schist and graywacke deposits. During the Ordovician-Silurian geological periods quartzites and schists were formed, metamorphosed by Hercynian granitic intrusions, at the end of the Paleozoic.