CLIMATOLOGY OF THE PERUVIAN ANDES 153 



courses are not directly toward the plains. Thus, one of the 

 largest valleys in Peru, the Urubamba, drops to 3,400 feet at 

 Santa Ana and to 2,000 feet at Eosalina, well within the eastern 

 scarp of the Andes. The mountains immediately about it are from 

 6,000 to 10,000 feet high. The result is a deep semi-arid pocket 

 with only a patchy forest (Fig. 54, p. 79 ). 5 In places the degree 

 of seclusion from the wind is so great that the scrub, cacti, and 

 irrigation remind one strongly of the desert on the border 

 of an oasis, only here the transition is toward forests instead 

 of barren wastes. The dense forest, or montana, grows in the 

 zone of clouds and maximum precipitation between 4,000 and 

 10,000 feet. At the lower limit it descends a thousand feet 

 farther on shady slopes than it does on sunny slopes. The 

 continuous forest is so closely restricted to the cloud belt that 

 in Fig. 99 the two limits may be seen in one photograph. All 

 these sharply defined limits and contrasts are due to the fact 

 that the broad valley, discharging through a narrow and remote 

 gorge, is really to leeward of all the mountains around it. It 

 is like a real desert basin except in a lesser degree of exclusion 

 from the rains. If it were narrow and small the rains formed on 

 the surrounding heights would be carried over into it. Eain on 

 the hills and sunshine in the valley is actually the day-by-day 

 weather of the dry season. In the wet season the sky is overcast, 

 the rains are general, though lighter in the valley pocket, and 

 plants there have then their season of most rapid growth. The 

 dry season brings plants to maturity and is the time of harvest. 

 Hence sugar and cacao plantations on a large scale, hence a 

 varied life in a restricted area, hence a distinct geographic prov- 

 ince unique in South America. 



INTEK-ANDEAN VALLEY CLIMATES 



Not all the deep Andean valleys lie on or near the eastern 

 border. Some, like the Apurimac and the Maranon, extend well 



6 A dry pocket in the Huallaga basin between 6° and 7° S. is described by Spruce 

 (Notes of a Botanist on the Amazon and Andes, 2 vols., London, 1908). Tarapoto at 

 an elevation of 1,500 feet above sea level, encircled by hills rising 2,000 to 3,000 feet 

 higher, rarely experiences heavy rain though rain falls frequently on the hills. 



