In the Central Plateau, outcropping rocks that vary in age from Mesozoic to Recent include Mesozoic, volcanic-sedimentary package, sedimentary, and plutonic rocks. Mesozoic metamorphic rocks of very low grade have also been reported. The Mesozoic Era in the region is represented by the Chilitos Formation of the Guerrero Terrain, comprising andesitic volcanic rocks with intercalations of greywacke, radiolarites and lenses of limestone. The Cenozoic Era is represented by volcanic rocks, intrusive igneous rocks of acid and intermediate composition and continental conglomerates. The Quaternary includes basalts, lacustrine deposits, alluviums and occasionally layers of evaporates, travertine, sinter, and pebbles.