WASHINGTON — The Potomac and Susquehanna rivers worked double-time to create gorges that remain scenic wonders today, carving through bedrock at twice the usual rate.
The Great Falls of the Potomac, just outside Washington, attracts more than a half-million visitors a year to its tumbled rock-strewn gorge.
It was cut during the same cold, stormy period when the Susquehanna was grinding out Holtwood Gorge near Harrisburg, Pa., a team of geologists led by Luke J. Reusser of the University of Vermont reports in Friday's issue of the journal Science.
The two rivers began cutting these gorges about 35,000 years ago, as the last ice age was advancing, and their unusually rapid activity eased by about 13,000 to 14,000 years ago.
Enriquetta Barrera, director of Geology and Paleontology at the National Science Foundation, said the report provides the first data on how quickly rivers of the eastern seaboard cut into rock.
"The Potomac and Susquehanna have shown they can cut nearly a meter of solid rock every thousand years," she said. "Pretty impressive for old rivers."
During the period studies the rate of rock erosion in Holtwood Gorge increased from 0.2 meter per 1,000 years to 0.6 and during the same period on the Potomac it was 0.8 meter per 1,000 years.
While the rapid cutting occurred in the last ice age, Prof. Paul R. Bierman of the University of Vermont, a co-author of the paper, noted that there were glaciers in the northern drainage area of the Susquehanna but not the Potomac.
That indicates that it wasn't the glaciers themselves that caused the increased river erosion, but related climate change that brought more storms and floods.
"You can get a real appreciation for the power of rivers, standing next to the Potomac," Bierman said.
"There are undated gorges all over the world," he said. "We want to work next on other East Coast rivers and streams to see how similar their histories are to the two major rivers sampled so far."
The researchers analyzed rock samples from the gorges for 10-beryllium, a rare isotope that forms when cosmic rays from the sun strike rocks and sediments at the earth's surface.
With that information they could estimate when the rivers exposed bare rock surfaces, known as terraces, where people climb and hike today. Once researchers knew the age of each river terrace and its height above the current river bed, they were able to calculate how quickly the rivers cut through bedrock.
They concluded that the 10- to 20-yard-deep gorges formed much more rapidly than previously thought, as a result of climate changes.
The research was funded by the National Science Foundation and the Department of Energy. Besides Reusser and Bierman, researchers working on the study were from the U.S. Geological Survey, University of Maryland and Lawrence Livermore National Laboratory.
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