FLOW REVERSAL. Currents driving the icebergs that scoured channels in the seafloor off South Carolina at the height of the last ice age ran almost exactly opposite to today's prevailing currents. Channel shown in inset is about 100 meters wide.Hill, et al.

Recent sonar surveys off the southeastern coast of the United States have detected dozens of broad furrows on the seafloor—trenches that were carved by icebergs during the last ice age, researchers suggest.

The channels, roughly parallel to the coast, are between 10 and 100 meters wide and typically less than 10 m deep, says Jenna C. Hill, an oceanographer at Coastal Carolina University in Conway, S.C. She and her team discovered the enigmatic features while conducting oceanographic surveys about 100 kilometers off Georgetown, S.C., in the summer of 2006. Waters in the area range between 170 and 220 m deep, she notes.

Most of the trenches run along straight paths for several kilometers, and one lengthy furrow stretches almost 20 km. Short berms alongside each groove are presumably composed of material that was plowed aside when the channels were carved, says Hill.

The seafloor features generally run in a southwest-northeast direction. However, the researchers noticed that some of the channels they discovered during a second survey last summer ended with a semicircular pit at their southwestern terminus. Suddenly, says Hill, the features made sense: Icebergs had plowed the furrows, and pits marked the sites where the ice masses became grounded and later melted.

The seafloor culs-de-sac indicate that the currents driving the icebergs flowed to the southwest, opposite to prevailing currents today. At present, warm waters of the northeast-flowing Gulf Stream bathe the region, says Hill. However, she and her colleagues suggest that an offshore shift in the Gulf Stream at the height of the last ice age—when sea levels were more than 100 m lower than they are now—would have allowed glacially fed, iceberg-rich coastal currents to penetrate this far south. Hill and her colleagues presented their findings last month in San Francisco at a meeting of the American Geophysical Union.

The team's theory "makes dynamical sense," says John M. Bane, Jr., an oceanographer at the University of North Carolina at Chapel Hill. Even today, he says, a seafloor feature about 100 km southwest of the berg-scoured region—a broad area called the Charleston Bump—can cause instabilities in the Gulf Stream that deflect the current offshore for a few weeks at a time, causing reversals in the coastal current. At the height of the last ice age, when sea levels were substantially lower, the Gulf Stream may have been more frequently, if not permanently, deflected offshore.

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