Globull Warming is responsible. The many many ice ages that were in effect over millions of years and the subsequent melting of the great ice sheets, over and over again probably supplied copious amounts of runoff water in very short periods of time that carved the canyons in separate eras......
Deep down somewhere in the bowels of the liberal academic ‘climate coven’ this finding is going to be the impetus to renew the cat call of urgency that Climate Change will kill us if we don’t act now, sooner in fact, because the Grand Canyon is 50 million years older than we thought....mark my words....