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