From the Sierra Macarena in the north to the Sierra Divisor east of the Río Ucayali, the eastern ranges of the Andes form an amphitheater along the western border of the Upper Amazon Basin. This Andean arc was for centuries a combined physical and cultural boundary. The noman's land of the Ceja and the hostility of the Montaña with its rough relief, short lateral valleys, turbulent rivers, and with the “conservatism of the forest” represented a rigid and formidable physical barrier.
The eastern region of the Inca Empire, Anti-Suyo, certainly never reached far beyond the forest line along the eastern ranges. The deepest penetration into the Selva took place probably under Inca Túpac Yupanqui, during the wars against the Shiris, who settled around “Chachapuyas and Muyupampas.”