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Scraping the bottom

11:57am, December 31, 2006

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

A survey of deep waters in western Lake Superior reveals the tracks left when massive icebergs scraped the lakebed during the last ice age.

Scientists have previously seen iceberg scours on the bottom of Lake Superior, but those were found in a shallow region near Wisconsin's Apostle Islands. The newly discovered scrapes were detected with sediment-probing sonar in 200-meter-deep water far northeast of the islands, says Nigel J. Wattrus, a geophysicist at the University of Minnesota–Duluth. The tracks—some of them tens of meters across and as much as 6 m deep—are now buried by about 10 m of sediment that has accumulated on the lake floor since the icebergs plowed the region. No one has ever reported iceberg tracks that deep in Lake Superior, says Wattrus.

Because the lake's surface has never been more than 70 m below its modern level, the icebergs that formed the tracks m

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