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Study: Glaciers on L.A.'s doorstep 5,000 years ago
LOS ANGELES, California (AP) -- Glaciers clung to the slopes of a mountain just 120 kilometers (75 miles) east of now-sweltering Los Angeles as recently as 5,000 years ago, according to a new study. The glaciers on San Gorgonio Mountain were likely the southwesternmost in what is now the United States during the waning of the last Ice Age, beginning 20,000 years ago. Lewis Owen, of the University of California, Riverside, said he and his colleagues found evidence that glaciers on the mountain made four advances over the last 20,000 years, the last between 9,000 and 5,000 years ago. The glaciers eventually vanished as global temperatures rose. Another scientist disputed the findings, saying the mountain's glacial past probably ended 12,000 years ago. Details appear in the August issue of the journal Geology. The existence of the small glaciers has been known for decades, but previous studies estimated they melted as long as 70,000 years ago, Owen said. The new study marks the first attempt to pin down the chronology using a relatively new scientific technique. "One of the biggest problems in geology is finding when things actually happened," Owen said. San Gorgonio Mountain, at about 3,450 meters (11,500 feet), is the highest peak in Southern California. Conflicting beliefsDuring the last Ice Age, glaciers filled valleys descending from the peak, advancing and retreating multiple times. During each advance, the glaciers pushed ahead tongues of debris, called moraines, in turn stranding them as they melted back. Owen and his colleagues dated boulders plucked from moraines by measuring the abundance of beryllium-10 found in the rocks. The radioactive isotope forms in common quartz when exposed to cosmic rays that bombard the Earth. The buildup of beryllium-10 is like the ticking of a clock and permits accurate dating of how long rocks have been exposed. For each of the three oldest moraines, the sampling gave consistent dates. Boulders found in the youngest moraine were broadly scattered in age, however. That diminishes the likelihood that the mountain harbored glaciers as recently as 5,000 years ago, said Milan Pavich, a research geologist with the U.S. Geological Survey in Reston, Virginia. Pavich, who was not connected with the research, said 12,000 years ago is more likely. "It certainly would be more consistent with what we know about regional glacial history," he said. Copyright 2003 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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