The Fraga do Pena geosite results from the Mação-Penhascoso lacolith, a granitic, hypabyssal body, which follows the synclinal folding of the metasediments that constitute the Paleozoic succession, and that in the SW flank also contacts the intrusive Belver plutonite.
At the top, the granitic lacolith of Mação-Penhascoso connects intrusively with quartzite strata or concordantly with black pelite beds.
Numerous xenoliths, sometimes laminated and recrystallized, still occur in the bed and margins of the Coadouro stream.
This granite body, relatively flattened and farmed, corresponds to a very altered granite, so that observations are hampered by the degree of alteration and arenisation.