The metamorphic rocks of Ecuador, once thought to constitute a uniform and ancient basement, belong to several distinct units of widely differing ages, compositions, and metamorphic histories. The largest area of metamorphic rocks constitutes an unbroken belt on the eastern Andean slope from the Colombian to the Peruvian border. The bulk of the rocks in this belt are of Cretaceous age, and were metamorphosed in the greenschist facies under a barrovian facies series. Rocks in the extreme north, and those south of lat. 3° 15′S are different, and may be of Paleozoic age. A smaller terrain, on the western Andean slope in southwestern Ecuador, is yet more varied. It includes four metamorphic terrains: the polymetamorphic Piedras Group of Precambrian age; the low-pressure Tahuin Group of Paleozoic age; the high-pressure Raspas Formation of Cretaceous age; and a low-pressure terrain of uncertain age north of the La Palma fault. Several isolated outcrops of metamorphic rocks in Ecuador are of uncertain significance.