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By Root 1824 0

and crumples up like a carpet shoved into a chair-leg. Just offshore the impact pushes the plate on the bottom of the Pacific down and under the advancing coast, creating a trench almost five miles deep. Inland the impact thrusts up the two parallel mountain ranges that make up the Peruvian Andes: the high Cordillera Negra, the Black Range, to the west, and the still higher Cordillera Blanca, the White Range, to the east. (The White Range has snow; the Black rarely does. Hence the names.) In northern Peru a third range rises between them; the altiplano, a scoop of high plains some five hundred miles long, fills the gap in the south. Taken together, the cordilleras and the altiplano make up the Andes, the second-biggest chain of mountains in the world.

Sandwiched between the Andes and the Pacific, Peru’s coastland is a skinny gray-brown ribbon. From a geographer’s point of view, it is a splendid anomaly, commencing with its extreme aridity. Over most of the South American landmass, the prevailing winds come from the east, across Brazil. As the warm, wet Amazonian air hits the towering Andes, it cools and sheds its moisture in the form of snow. Almost nothing is left for the Peruvian coast, which sits in the mountains’ rain shadow. Surprisingly, the coast is also walled off from moisture on the Pacific side, where the trade winds create a second rain shadow. Blowing from the southwest, the trades push the warm surface waters northeast, pulling frigid water from the deep offshore trench to the surface. The upwelling, known as the Humboldt Current, chills the air above it. Coming from the west, the Pacific trade winds hit the cold air from the Humboldt Current and are forced upward in a classic temperature inversion of the sort common in southern California. In temperature inversions, air movement is inhibited—the cold air can’t rise and the warm air doesn’t fall—which in turn inhibits rainfall. Walled off from wet air by both the Andes and the Humboldt Current, the Peruvian littoral is astonishingly dry: the average annual precipitation is about two inches. The Atacama Desert, just south of Peru on the Chilean shore, is the driest place on earth—in some places rain has literally never been recorded. Space researchers use the Atacama as a model for the sands of Mars.

Pizarro’s pilot once explained how to navigate from Mexico’s Pacific shore to Peru: Sail south along the coast until you no longer see trees. Then you are in Peru. Yet the coast is not a classic, Sahara-style desert of sand dunes and scorching sun. It is punctuated by more than fifty rivers, which channel Andes snowmelt to the sea. The lines of vegetation along their banks are like oases, fertile places where people can farm in an otherwise almost lifeless land. For much of the year the ocean air is cold enough on winter mornings to make fog roll into the valleys a hundred feet deep. People wear sweatshirts and futilely wipe at the mist on their windshields. By noon the fog lifts, having deposited a few hundredths of an inch of moisture (summed over the year, the fog gives the desert most of its annual two inches of precipitation).

If the anti-Clovis arguments are correct, paleo-Indians walked or paddled to Peru fifteen thousand years ago or more. But Peru’s first known inhabitants appear in the archaeological record sometime before 10,000 B.C. According to two studies in Science in 1998, these people apparently lived part of the year in the foothills, gathering and hunting (for the latter, no traces of Clovis points have been found). When winter came, they hiked to the warmer coast. At Quebrada Jaguay, a dry streambed on the nation’s southern coast that was one of the two sites described in Science, they dug up wedge clams and chased schools of six-inch drumfish with nets. They carried their catch to their base, which was about five miles from the shore. (Quebrada means “ravine” and often refers to the gullies caused by flash floods.) Quebrada Tacahuay, the other Science site, was closer to the shore but even drier: its average annual rainfall is less than a