Limestones and dolomites
Around the crystalline Argentera massif there is a wide band of sedimentary rocks right on the edge of the Park. These sedimentary rocks form the ridges between the lower Gesso Valley and the Stura and Vermenagna valleys; vast outcrops of limestone and dolomite, they are a much lighter colour than the gneiss in the Park and in many places the stratification is evident.
The solubility of limestone rocks is one of the main causes of the formation of narrow, deep gorges, such as the well-known Gorge della Reina near Entracque. In the Park, the most evident karst phenomena are the Grotte del Bandito in Valle Gesso, now a Nature Reserve, and the bare rocks incised by the abundant runoff water of the Val Grande, above Palanfrè.