MARGARET WARNER:Finally tonight: Just how old is the Grand Canyon?

The conventional wisdom holds that this natural wonder of the world was shaped by the Colorado River about five or six million years ago. But there's been a long-running debate over whether it's much older. And, yesterday, researchers published a study in the journal "Science," arguing these majestic formations were formed by two much older rivers cutting through the landscape some 70 million years ago. That was during the age of the dinosaurs.

We have NewsHour science correspondent Miles O'Brien here to tell us what the debate is all about.

So, Miles, from six million to 70 million years, is that scientifically as big a gap as it appears?

MILES O’BRIEN:Yes, it's a big deal.

And there is a big debate in the scientific community right now over it, Margaret.

When you look at the Grand Canyon, as a layperson—we have been there and see it—you would say, well, clearly, the Colorado River formed this over many millions of years. And we know the Colorado River is between five and six million years old, no scientific debate there. There is plenty of evidence on that.

So, you would say, well, the canyon must be that old. Well, this new paper which came out—Becky Flowers of the University of Colorado, along with Ken Farley at Caltech, took a series of readings on the eastern and western portions of the river, which measured a helium isotope which stops escaping at about 70 degrees. You can tell how the rocks have been rising out of center of the earth and sort of pinpoint where the depth would be.

And they came to the conclusion that there was a Grand Canyon there some 70 million years ago before the river. Well, how would that be?

Well, they say there were actually two rivers, one that flowed one direction 70 million years ago, another that flowed the other direction 50 million years ago, and that made a canyon, which now the Colorado River is using. Make sense?

MARGARET WARNER:Meaning what, the other river has disappeared?

MILES O’BRIEN:Pretty much.

MARGARET WARNER:In geologic time.

Now, there are some real debunkers of this.

MILES O’BRIEN:Yes, indeed, Karl Karlstrom of the University of New Mexico, who speaks on behalf of a large community of scientists who say, no, that is not true. They do believe that there were canyons there of that vintage, but was it a Grand Canyon?

So, in many senses, this might be kind of a semantic debate. But it shows you how interesting this place is to scientists.

MARGARET WARNER:First, before we get to that, can you explain in layman's terms how they came to different conclusions? In other words, are they testing different materials? Are they using different methods?

MILES O’BRIEN:There is not much debate that they got a good number on the age of the rocks and the depths of the rocks. The question is...

MARGARET WARNER:You mean on the side of the canyon?

MILES O’BRIEN:On the side of the canyon as they measured the helium isotope as it escaped. That is not the question.

The question is, was that part of a so-called Grand Canyon, or was this a predating, an older canyon, a paleo-canyon, which eventually morphed into the canyon that we see today?

So, in a way, it is a semantic debate. They are sort of both right, except that the paper which came out says, the Grand Canyon which we see today is 70 million years ago.

And that's where scientists on the other side are saying, wait a minute, there might have been some old canyons there, but it is not our Grand Canyon.

MARGARET WARNER:And on what do they base that?

MILES O’BRIEN:Well, they are basing it on this data that they have, a big mound of geologic and this isotope data, which tells them that basically the Colorado River was at the center of this canyon.

MARGARET WARNER:Now, I read that, if it were 70 million years old, in the age of dinosaurs, the landscape would have looked very different than we imagined.

MILES O’BRIEN:Yes.

Well, you know, if you go back 75 million years ago, think about it. You would have been at the beach there. You would have seen sharks in that place.

So what was going on in that part of the world 75 million to 70 million years ago was a very interesting formation, a very interesting time, where you saw something that was very low, at sea level, rise up because of the plates that are colliding together that give us the Rocky Mountains and all the tectonic activity there.

And so there was a lot happening there, if we had had the opportunity.

MARGARET WARNER:Well, and in the dinosaur era, we always think of them as having all this lush vegetation too.

MILES O’BRIEN:Well, they had a beach too. And they had—they could have seen some sharks swimming in what is now the Grand Canyon.

MARGARET WARNER:So, back to what—is this scientifically important or is this just fascinating because we all love the Grand Canyon?

MILES O’BRIEN:Well, yes, this is as good a place as any if you want to study the history of our planet.

You know, basically when you look at the wall of the Grand Canyon, which goes down a mile, you are reading a history book.

You just have to know how to read the rocks. And the deeper you go, the farther—further back in time go. Two billion years, that is about half the planet right there.

So geologists love the story that they can unfold there. It tells them a lot about erosion. It tells them about tectonics, volcanism, and it gives them a glimpse into the earlier days of our planet, without having to drill a deep hole. So, it's fun.

And then there's the five million of us who like to go and just go, wow.

MARGARET WARNER:And I guess the question is, are they going to have to change the lecture and the, what do they call it, the Trail of Time or something that they have at the national...

MILES O’BRIEN:The Trail of Time just got much longer, maybe.

Karl Karlstrom says no. He's going to make sure that they don't do that. So there will be further—I guess we can say stay tuned on this one.

(LAUGHTER)

MARGARET WARNER:We will stay tuned, maybe not for six million years.

(LAUGHTER)

MARGARET WARNER:Miles O'Brien, thank you.

MILES O’BRIEN:OK. Pleasure, Margaret.