December Editorial      


    Water cuts through rock at surprising speed


      A few months ago, geologists from the University of Vermont published their findings on the speed at which rivers cut into bedrock. Because bedrock is very hard, it was expected that the process would be an extremely long one. However, the data collected by the scientists revealed that the process may actually be much faster than previously thought. The study, which was published in the journal Science, focused on two major US rivers, the Susquehanna river and the Potomac river.

      The Susquehanna is the nation's sixteenth largest river and is the largest river lying entirely in the United States that flows into the Atlantic Ocean. It meanders 444 miles from its origin at Otsego Lake near Cooperstown, N.Y., going through the states of New York, Pennsylvania, and Maryland until it empties into Chesapeake Bay. The other river, the Potomac is sometimes called the "Nation's River," because it flows through the US capital - Washington. It begins as a small spring at the Fairfax Stone in West Virginia, and winds through the mountains and valleys of Appalachia, past battlefields and old manufacturing towns. The river flows more than 380 miles and is more than 11 miles wide when it reaches the Chesapeake Bay at Point Lookout, Maryland.

      In the July issue of Science, the geologists reported that about 35,000 years ago the Susquehanna and Potomac rivers began carving out Holtwood Gorge and the Great Falls of the Potomac. The latter, about 15 miles from Washington DC, is now a major tourist attraction. The geologists collected rock samples from both gorges and analysed them for 10-beryllium (a very rare isotope that is produced when cosmic rays collide with rocks and sediments at the earth’s surface). This allowed them to estimate when the rivers left their ancient beds and consequently exposed the bare rock surfaces known as terraces. 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. Although 35,000 years may seem a very time, it is all needed for a river to cut through very hard bedrock which is resistant to erosion. The scientists believe that the cuts, which their studies showed to be 10 to 20 meters deep, were only possible due to extreme climatic changes which induced major floods. During the cold and stormy climate of the last glacial period, melting ice would have created the large and long-lasting floods needed for the erosion to occur.

      The Potomac and Susquehanna rivers have shown that in the right circumstances they can cut nearly a meter of solid rock every thousand years. That is a very impressive speed to get through solid stone.


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