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New technique dates glaze on desert rocks

From San Francisco, at a meeting of the American Geophysical Union

Scientists have developed a quick, easy, portable, and nondestructive way to determine the age of desert varnish, the mysterious dark coating that slowly develops on rocks in many arid regions of the world (see "Thin Skin," in this week's issue: Available to subscribers at Thin Skin).

Desert varnish is rich in iron oxides and manganese oxide–both possible products of biological processes–and contains varying amounts of clay, says Nicholas E. Pingitore Jr. of the University of Texas at El Paso. Researchers don't fully understand how desert varnish forms on exposed rock surfaces, but they agree that it does so exceedingly slowly. Ancient artists created petroglyphs scraping away the dark coating to reveal the lighter rocks beneath.

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