NATURAL BRIDGE in Monument Valley spans a huge hole bored by sand-bearing winds into a water-weakened sandstone butte. Beneath it, Navajo herders tend their sheep.

2
The Creation of Deserts

MEASURED against the awesome depths of geologic time, so much more easily reckoned than comprehended, the deserts of today are relatively recent formations on the face of the earth. Just how new they are is a matter of active scientific dispute. Until a few decades ago the textbooks called the deserts "earth-old," meaning that they dated from creation. Meanwhile science was outdating the textbooks and creation was being rudely pushed back. The earth is now estimated to be on the order of five billion years old. It has been supporting life for a third to half that time, and it has had deserts at least since the Permian period, which ended 230 million years ago, although no such ancient desert exists today.

Geologists, looking at the record of the rocks, have found in some contemporary desert regions signs of aridity going back 63 million years, to the beginning of Tertiary times. But paleobotanists, looking at the fossils embedded in the sedimentary rocks, say most of these deserts can be no older than the late Cenozoic, or one to five million years old. They argue that virtually all desert-adapted plant and animal species have evolved

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