 Ordovician units are also present and form sediment layers up to thick.About 14.5 million years ago volcanic activity started in the region, first west of Galán but by 7 million years ago it shifted to the future caldera, forming the Cerro Colorado, Pabellon and Cerro Toconquis composite volcanoes on its future western rim.The more westerly centres are today represented by eroded volcanoes.