Cretaceous rocks are composed of basalts and interbedded argillites, sandstones and minor coals. Another name for poorly lithified argillites is "mudstone". Examining Pliocene and younger sediments reveals sands, argillites, and silts, indicating lacustrine deposition. Metamorphism of argillites produces slate, phyllite, and pelitic schist. The main rocks of the belt are migmatized greywacke and argillites. The argillites grade into shale when the fissile layering typical of shale is developed.