In the first study to directly measure when and how quickly rivers outside of growing mountain ranges cut through rock, geologists at the University of Vermont have determined that it was about 35,000 years ago that the Susquehanna and Potomac rivers, respectively, began carving out the Great Falls of the Potomac and Holtwood Gorge. Great Falls, located about 15 miles outside of Washington, D.C., hosts hundreds of thousands of visitors each year; Holtwood Gorge lies along the Susquehanna River, near Harrisburg, Penn.
As reported in the July 23 issue of the journal Science, the geologists analyzed rock samples collected from the gorges for 10-beryllium, a very rare isotope that is produced when cosmic rays collide with rocks and sediments at the earth’s surface. These analyses helped them gauge when the rivers abandoned their ancient beds and, consequently, exposed bare rock surfaces, known as terraces, where people climb and hike today. Knowing the age of each river terrace and its height above its current river bed, they were able to calculate how quickly the rivers cut through bedrock. Their conclusions: Incision of the 10- to 20-meter-deep gorges happened at a rate far more rapid than previously thought, and was prompted more by regional climate changes tied to the last ice age than by water pouring from melting glacial ice.
“The period of incision we measured correlates with a period of cold and stormy climate during the last glacial period that is also recorded in ice cores drilled into the Greenland ice sheet,” said Luke Reusser, a graduate student of geology at the University of Vermont and lead author of “Rapid Late Pleistocene Incision of Atlantic Passive-Margin River Gorges.”
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