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Book Review 289: Carving Grand Canyon
July 20, 2013 - Harry Eagar
CARVING GRAND CANYON: Evidence, Theories and Mystery, 2nd edition, by Wayne Ranney. 190 pages, illustrated. Grand Canyon Association paperback, $16.95
Looking into the Grand Canyon (or even many smaller erosional surfaces), it is hard to imagine what goes on in the minds of Young Earth Creationists. It wasn’t done in a day.
But when was it done? Geologist Wayne Ranney’s little survey ought to accompany any visit to the canyon. It makes the big hole even more interesting.
Reconstructing past landscapes is not simple, and it will startle most people to learn that the Colorado River (or whatever was the major drainage of the region) used to run the other way.
As the Rockies rose (or, alternatively, as the Basin and Range province sank), the flow switched direction. An outstanding question is whether any of the old northeast drainage canyons were saved to be incorporated into today’s canyon.
Ranney has good discussions about how quickly rivers can saw through mountains -- very quickly if the gradient is steep enough. Today’s canyon falls eight feet in the mile, compared with much less than a foot in the mile for the Mississippi.
Yet the Grand Canyon is not getting any deeper, not even by fractions of a millimeter, now. The bedrock is buried in around 75 feet of gravel, cobbles and silt. It would take a mighty flood to scour the canyon so that the cutting could begin again.
The canyon probably achieved its present depth during the melt phases of the recent ice ages -- that is, no cutting in the last 10,000 years or so.
Ranney calls attention to the remarkable history of the high, rolling plateaus that surround the canyon. While not dramatic, they have their own secrets.
It diminishes the drama of the canyon itself a little to know that in the Four Corners region, erosion has completely eliminated sedimentary rocks that were once as much as 18,000 feet thick. More than three miles. The canyon is only a mile deep and while big (277 miles long by up to 10 miles wide), it is not even a fraction as large as these vanished rocks.
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