A previous analysis of the sediments had placed their age at only one to two million years old. This would have meant that the Three Gorges, which separate the Sichuan Basin in the west from the lowlands in the east, would have had to
have been cut very rapidly. 

But the new findings, published in Geology, show that the sediments must have been deposited long after the gorges were cut. By looking at minerals in the granite that underlies the Three Gorges dam, the researchers, led by Alexander Densmore of Durham University, were able to estimate when this would have taken place.

‘We used the number of damage trails in the mineral apatite to tell us when the rocks were cooled below a particular temperature, and thus when the gorge incision began,’ Densmore explained. The team found that samples taken from near to the dam showed evidence of cooling having taken place about 45 million years ago, while samples taken from farther away showed no evidence of that cooling. According to the scientists, this meant that the cooling was caused by the gorge incision, rather than more general, regional erosion.

‘The Yangtze River is much older than previously thought and extremely high incision rates weren’t required to create the distinctive gorges,’ Densmore said. ‘It formed slowly, over a much longer time span.’

August 2010