Rethinking the Formation of The Grand Canyon!

Why does it cut though a Plateau?

 

 

 

Back to Geology!

Backcountry Hiking!

 

This summer more than three million tourists will venture to the rim of the Grand Canyon and are likely to feel their jaws drop as they gaze upon the world famous spectacle: a vast wilderness of rocks, deep gorges and a mighty river lost below cliffs, buttes and pinnacles festooned in pastel shades of purple, orange, pink and green.

Inevitably, some of these visitors will turn to a park ranger and ask: "How did the Grand Canyon get formed? Why did this happen here and nowhere else?"

The honest answer is that nobody knows. One hundred and thirty-one years after John Wesley Powell first mapped the Colorado River by riding its 161 rapids in heavy wooden boats, no one can prove how the canyon was formed.

But it is not for lack of trying.

Geologists are puzzle freaks who love nothing more than collecting fragmentary clues -- clumps of gravel, fossilized shells and pollens, the dates that muddy sediments were deposited in dried-up lakes or whole mountain ranges were lifted -- and then trying desperately to figure out how the modern topography before their eyes was produced.

The week of June 10, 2000 , more than 80 earth scientists will gather in Grand Canyon Village to compare notes and work toward a unified theory of how and when the Grand Canyon formed. The last time a similar group of experts met to discuss the evolution of the Colorado River was in August 1964, before modern theories of plate tectonics revolutionized geophysics with the notion that continents could drift, slam into one another and alter entire landscapes the size of the American Southwest. Not surprisingly, the favorite theories from the previous era are now on the table for serious revision.

"It's time to take a new look at the evidence,"said Dr. Richard Young, a geologist at the State University of New York at Geneseo, who organized the conference and is an expert on gravel deposits in the canyon. In the times since the last meeting, geologists have dated more rock formations, identified new deposits and are working from a more sophisticated framework about the overall geological history of the Southwest, he said.

The meeting, which brought together geologists, paleontologists and climate experts who do not get much chance to interact.

After they talk and go home to think about what they have learned, pieces of the puzzle may perhaps fit together in new ways. The problem is that many features in the Grand Canyon do not make any sense.

The modern Colorado appears to be a young river that flows out of the Rockies and hits a huge plateau, called the Kaibab Upwarp, which is 50 million to 70 million years old. Instead of being shunted away from this barrier, the river runs right through it. Moreover, when sediments from the river are examined closely, it is clear that the western end of the canyon -- where it flattens out and begins its final run to the Gulf of California -- is many millions of years younger than the eastern part of the river.

To many experts, this difference means that the Grand Canyon could have been cobbled together from ancient river basins that were created during different geologic eras. But if so, when and how were those ancient rivers formed and where did they go?

In seeking answers, geologists are faced with a difficult problem.

For critical periods of canyon formation, the geologic record is entirely missing. The rocks and fossils that researchers need in order to tell a coherent story have either washed away or been buried, presumably in places not yet discovered.

Nevertheless, experts have used the few clues they have to develop two main competing theories about the formation of the canyon. One maintains that the Colorado River is old and that it carved the Grand Canyon at least 70 million years ago. The other says that the river is young and carved the canyon within the last five million years.

The first explanation, advanced by Powell and other early geologists, holds that the river has a simple history. In the beginning of the Tertiary period, 70 million years ago, Arizona was flat and the river flowed in its present course, roughly east to west.

Then, during various periods of uplift apparently caused by collisions between gigantic slabs of the earth's crust, the Kaibab Upwarp began to rise at a rate that exactly matched the river's capacity to erode the landscape. According to this view, the canyon cutting took place gradually, with the river staying in place and the land around it rising upward.

This theory held sway for more than 50 years, Dr. Young said, but today it has few adherents because too many pieces of the puzzle do not fit. For example, as mentioned, a major part of the riverbed shows strong evidence of being younger than the Kaibab Upwarp.

One holdout for the old river explanation is Dr. Don Elston, a retired geologist with the United States Geological Survey in Flagstaff, Ariz., who has been working on canyon formation theories since the mid-1950's.

"I've attempted to solve the problem using stratigraphy and climate data from all over the Colorado Plateau," Dr. Elston said in an interview. "Some people call it geofiction, but I think I'm right."

When dinosaurs roamed the earth, the southwestern landscape was about 1,000 feet above sea level and very wet, Dr. Elston explained.

Then the region experienced three periods of uplift followed by erosion. In the first uplift, about 100 million years ago, mountains grew across northern and central Arizona and later eroded so that a blanket of gravel a few hundred feet thick poured out to the north. These older gravels can be found today on both sides of the Grand Canyon, he said.

A second uplift occurred 60 million to 75 million years ago, producing huge rivers that eventually flowed to the west and collected in enormous inland lakes. It was during this period that the Grand Canyon was carved out, Dr. Elston said. Then the uplift ceased, erosion continued and this early Grand Canyon filled up completely with gravel.

A third uplift took place five million years ago to form the Rocky Mountains and again changed regional drainage patterns, Dr. Elston said. The land in the north rose while the southwest, relatively speaking, fell.

As a result, streams began flowing westward down from the Rockies, and found the earlier drainage channel. As they flowed down the old channel, they removed all the gravel and essentially scoured out the Grand Canyon.

"The modern Colorado River did not carve the Grand Canyon in the last few million years," Dr. Elston said. "The canyon was already there." After this period, he said, the climate grew very dry, which explains why there are no older river deposits at the western end of the canyon. The Colorado River dried up, ran sporadically or ran completely underground, he said.

Dr. Elston's theory is "a nice ad hoc explanation," Dr. Young said, but most people don't buy it.

Since the 1950's, numerous expeditions to the western mouth of the Grand Canyon, in a region called the Grand Wash Cliffs, have found no evidence of an older Colorado River running through, sporadically or otherwise.

Local sediments stem from a younger geologic formation called the Basin and Range -- an area that begins where the Colorado Plateau drops off at the western edge of the Grand Canyon and the landscape stretches out for hundreds of miles, all the way to eastern California.

Everything points to the idea that the upper reaches of the Colorado River are very old and the lower reaches are very new, Dr. Young said.

If the upper Colorado River did not always flow into its present course, where did it go? Dozens of papers have been written on this, said Dr. Steve Reynolds, a geologist at Arizona State University in Tempe. Most of these theories posit two separate river systems that somehow hooked up not far from today's visitors center on the south rim.

A popular theory from the 1960's held that one of the rivers came down from Colorado or Utah, flowed south along the course of what is now the Little Colorado River and joined the Rio Grande into the Gulf of Mexico. Meanwhile, to the west, a small unnamed river flowed across the Basin and Range province and into the Gulf of California.

This younger river, or stream drainage system, was vigorous and steep, according to this theory. Through a process called headward erosion, it gradually moved eastward, gobbling up the land around its headwaters in Pac-Man fashion, until it met up with the ancestral upper Colorado.

This younger river captured the older river, Dr. Reynolds said, established its present course and then carved the Grand Canyon only four million to six million years ago.

Of course there are problems, Dr. Reynolds said. The Continental Divide would have prevented an ancient Colorado River from joining the Rio Grande. So other theories propose that the river came south, hit the Kaibab Upwarp and did a U-turn back to the north. This old river system would have emptied into inland lakes in Utah or Nevada. Unfortunately, the deposits that could prove such a
drainage system existed have not been found.

Another theory holds that the ancestral Colorado flowed from the north and into a huge lake in central Arizona, laying down the so-called Bidahochi formation.

But Dr. Michael Ort, a geologist at Northern Arizona University in Flagstaff, said he had evidence that this body of water, called Hopi Lake, was ephemeral. It would fill up occasionally and then dry out for long periods, Dr. Ort said. It does not appear that an ancient Colorado River could have drained into Hopi Lake, he said.

On the other hand, some people think that Hopi Lake could have filled from an unknown source to overflowing about five million years ago and somehow broken over a barrier that had separated it from the Colorado River. This would have created a catastrophic flood that carved the Grand Canyon very quickly, within a couple million years, Dr. Ort said. The problem with this idea is that no Bidahochi sediments have been found anywhere in the present Colorado River drainage. Again, physical proof of the theory is lacking.

Finally, several people are now saying that the existing Colorado River did carve the Grand Canyon but did so while flowing in the opposite direction of today.

The modern river is just too weird, said Andre Potochnik, a river guide who is writing his doctoral dissertation on deposits related to early Southwest river systems. It does not follow fault lines as most rivers do and it has tributaries that come in at obtuse angles, he said.

Mr. Potochnik argues that a much older river flowed west to east down the Kaibab Upwarp and carved the canyon. Later, as tectonic forces changed the land to tilt in different directions, the river changed the direction of its flow and became the modern Colorado.

Wayne Ranney, a geology instructor at Yavapai College in Prescott, Ariz., argues that the Little Colorado River probably flowed north through Marble Canyon, a stretch of the river where tributaries come in at an angle that is the opposite of what one would expect, given the way the water flows.

"The river system I envision would have flowed north into the Glen Canyon area," Mr. Ranney said. "Every time I see this landscape, I'm more convinced that at least this part of the river went the other way. The beauty of this theory is that it ties together a lot of conflicting ideas concerning evidence for an old river east of the Kaibab Upwarp and a young one west of it."

Figuring out the Grand Canyon is like being a police officer called to the scene of a four-car accident, Dr. Reynolds said.

"But by the time you get there, three of the cars have been towed away, they repaved the road and washed away the skid marks. You are left with only one piece of the puzzle."

Larry Stevens, a river guide and expert on Grand Canyon ecology, said that the Grand Canyon might be a "geological koan." "People can spend a lifetime pursuing these questions and we may never know the answers," Larry said. "Once you've been in The Canyon, everything else is just commentary"

Making Sense of Grand Canyon's Puzzles
June 6, 2000 New York Times By Sandra Blaksee



Geology of Grand Canyon, Northern Arizona with Colorado River Guides: Lees ferry to Pierce Ferry, Arizona
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