In this Natural Monument of Pedras Ruivas, two sedimentary units are preserved, the Silts from Cambôa do Marinheiro and the Conglomerates and Sands of Forte Rego de Fontes, which witnessed the existence of a confined swampy lagoon environment confined and away from direct marine influence, between 18000 to 11000 years ago. This lagoon lasted in the landscape for about 7 thousand years and would be supplied by spring melting waters from the snows of the local slopes.