Scraping the bottom.

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 must have reached at least 130 m deep. A piece of ice that big could have towered as high above water as a four-story building. Most of the kilometers-long tracks run along a line from west-northwest to east-southest, a hint of the prevailing winds and currents at the time, says Wattrus.

Samples of lakebed sediment suggest that the tracks were formed near the end of the last ice age, about 10,000 years ago. At that time, the ice sheet that covered much of northeastern North America was retreating northward past Lake Superior, and rivers flowing into the lake were carrying distinctive reddish sediments that had eroded from nearby bedrock.--S.P.