In this Natural Monument of Pedras Ruivas, the high lithological diversity is represented by three lithostratigraphic units – the Desejosa Formation of the Lower Cambric consisting of micaschists, micaschists with andalusite nodules (dominant lithology), nodular graphite phyllites, micaschists with tourmaline and quartz metaconglomerates; the Santa Justa Formation of Lower Ordovician (Tremadocian-Arenigian) composed of quartzites with ichnofossils of Cruziana rugosa (among others) and metapsamopelites; and lastly the Valongo Formation of do Ordovician (Lanvirnian-Landeilian) constituted by micaschists with garnet and andalusite, chiastolitic micaschists and, although without cartographic significance, staurolytic micaschists.