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 that overlooks the Calchaqui valley and the basins of Cata- marca and Fiambala.

The eastern border of the Puna de Atacama is rugged in contrast to the rounded ranges that stand east of that border. The contrast between the two groups of forms is further heightened by the fact that they face cach other across the deep Calchaqui valley, a depression only five miles wide and extending north and south for twenty-five miles. In this narrow zone a block of red sandstone has been dropped down between closely parallel faults that bound older rocks such as slates, quartzites, and schists on the east and similar rocks with a cover of volcanic material on the west. The red sandstones were probably faulted and folded at the same time. Then came a period of great volcanic activity, probably beginning in the Tertiary and running into the Pleistocene and Recent with some glaciation in the Pleistocene, heavier on the eastern border of the mountains because of the greater precipitation there. The first terminal moraine which we saw in the ravine of Penas Blancas above Poma was at 11,000 feet, and the best developed terminal moraine at 11,650 feet. Glaciation has flattened the valley floor so that there is an extensive pas- ture at 11,800 feet.

West of the Calchaqui valley are immense tracts of volcanic rock in all stages of erosion and great differences of age. Some of the youngest lava flows are on the floors of the present val- leys or along the lower slopes where the valley floor once stood, the river having since cut a new narrow valley to one side of the obstructing flow (Fig. 66). Here and there, as in the valley of Pefias Blancas, are small lava-dammed lakes or their exposed floors where the lakes have since been drained.

The contrast between the younger and higher mountains on the west of the Calchaqui valley and the lower but older moun- tains to the east of it is heightened by the character of the

Fig. 87 (opposite)—Reduced from the American Geographical Socicty’s Millionth Map of Hispanic America. Seale 1:4,500,000, Only the main divides are indicated, and the main peaks and passes. The term Puna de Atacama is applied to the tract east of the Cordillera de los Andes to the main eastern divide and northward approximately to the 23rd parallel. Ab,=abra, or pass; Port.= portezuela, or pass; Ap, = apacheta, or trail marker; S.=salar or salina; C.=cerro, hill or peak; V. and Vn, =voleano; Sa, =sierra, or chain; Ns. = Nevados, or snowy peaks; Ms.= Morros, or hills.