== 1.4 or 1.8 Billion? == 

 The first paragraph mentions figures that seem contradictory, and at the very least are highly confusing: 1) that the canyon exposes rock that is 1.4 billion years old, and then 2) that the rock it exposes spans in age from 200 million to about 2 billion, a difference that works out to about 1.8 billion. That is a large disparity, both in proportion and in absolute terms. Perhaps the best answer ought to be stated once, reducing confusion.