The area around the Capella di San Sebastiano exposes a major sedimentary unconformity: High-grade crystalline rocks formed during the Variscan orogeny are unconformably overlain by a coarse-grained conglomerate. Its clasts come from the same Variscan crystalline rocks below, which indicates the complete erosion of this former mountain belt and recycling of its rocks in much younger sediments.
The Capella di San Sebastiano stands on rocks of the Variscan basement. In this case, the basement rock are migmatites that are composed of alternating brighter and darker layers; the brighter mainly contain quartz and feldspar, the darker contain lots of biotite. Migmatites are the product of partial melting of a protolith rock during high-temperature metamorphism. Just uphill from the chapel, coarse-grained conglomerates of Eocene age are exposed that overly the Variscan basement along an erosional unconformity. The conglomerates contain large clasts of crystalline rocks (e.g., migmatites, granites) eroded directly from the basement below.