Controversies over wonders of the world capture the public imagination from time to time. Now some Caltech scientists have dared suggest from a new technique that measures the depth of overburden on apatite crystals that there was an older canyon of similar proportions cut into Mesozoic strata above the current canyon from a river that flowed the opposite direction. Geologists have puzzled about the presence of sediments that isotopically match western rocks on the east coast and postulated a “California River”.
If you visited the Grand Canyon in search of this older canyon you would not look down, but rather up. Ironically this older canyon was cut into younger rocks that have eroded away to form the flat surface that was uplifted and cut by the Colorado River to form the current canyon after the end of the most recent glaciation. Geomorphic features are conserved and it doesn’t surprise me that an older river would follow the same path.
Here are some pictures by Wayne Ranney from his excellent review in http://earthly-musings.blogspot.com/2012/12/the-latest-big-controversy-on-age-of.html.