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 floors of the deepest valleys, villages inhabited) by primitive Indian groups who still follow the ways of their fathers and who themselves find the higher and colder parts of the cordil- lera so inhospitable that they go up into the altitudes only in sumuner to graze their flocks of sheep and llamas and thus relieve the valley floors of the pressure for forage that would necessarily follow if they all remained huddled in the narrow strips of pasture land that the uncertain streams support.

The long secondary ranges that extend forward from the main southeastern front of the Andes have diminishing eleva- tion as they penetrate the Argentine plains. In almost all cases the summits of the outlying or detached topographic units, as well as the secondary ranges along the main mountain front, have an older aspect than the ravined border of the ranges. The topography is in a far more advanced stage of development; the mountain crests or high upper slopes, as well as the spur tops, are covered with grass, and their ravined borders are sprinkled with thin woodland. The ravines have been cut recently as a result of late uplift. Their steep head- water slopes and narrow bordering declivities are youthful features in marked contrast to the lawnlike high-level slopes which they are gradually invading.

These features are well developed southeast of Salta in the Cordillera Lambrama and again on the Cuesta del Obispo and Cerro Bayo west of Rosario de Lerma. In fact, the entire mountain mass between Rosario de Lerma and Poma is of this general type, and the contrast between it and the main wall of the Andes west of Poma and extending along the western border of the Calchaqui valley is most striking.

The Puna de Atacama is the collective name given to the basins, valleys, salt-covered basin floors, mountain knots, chains, and alluvial piedmont deposits that are the chief topographic and drainage features of a belt of exceedingly high and cold country that lies between the main chain of the Andes as shown in Figure 87 and the eastern mountain wall