>>Theories about the formation of the Grand Canyon keep changing too. When I was in elementary school I was told that it was carved out very slowly over millions of years. This was presented as established fact. Now there’s no agreement. One theory holds that a big flood breaking through from a large lake carved it out.<<
I have not heard that, but it would not matter. It puts not a dent (and may clarify) the overall theory of how the Earth was formed (geology), continental drift, etc. etc.
But if you could kindly post the link to the scientific article/journal where this “Big flood” concept is described I would be interested in reading it.
Grand Canyon Gorge Is 9 Times Older Than Thought
http://www.freerepublic.com/focus/news/1999143/posts
http://www.nbcnews.com/id/50250163/#.UoEI7vnrzP0
As this article shows, scientists these days cannot come to any agreement about how the Grand Canyon formed. However the average person is under the erroneous impression that they know how it was formed.
One of the discoveries that led to the rethinking of the conventional theory is when a scientist noticed the evidence that the water flowed in the opposite direction of the Colorado when the canyon was being formed. Therefore it seemingly could not have formed it. This article discusses that.