The block-faulted rocks of the Cordillera de Vilcanota and the Cordillera de Carabaya (forming the Cordillera Oriental) to Maldonado, Peru, are metamorphic schists and quartzites overlain by Ordovician slates. Upper Paleozoic brown-red shales and sandstones occur to the west, and to the east occur vast Tertiary plains. The pre-Cambrian Araza gneiss (probably Archean) underlying the Marcapata metamorphic rocks is the first recognized occurrence of gneiss on the eastern slope of the Andes.

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