Science Puzzle
What Carved This Canyon?
Stand on the rim of the Grand Canyon and look down. It is over a mile deep and eighteen miles across. At the bottom runs the Colorado River, which from up there looks like a thread of silver.
That modest river cut this? How?
The Answer
Yes, and the secret ingredient is not power. It is time, plus a second process most people forget.
Water alone barely scratches rock. What does the cutting is the grit: sand, pebbles and boulders that the river drags along its bed, sandblasting the stone beneath, day and night, for millions of years. Remove a fraction of a millimetre a year and after six million years you have removed a mile of rock.
The forgotten half of the story is that the land was also rising. The Colorado Plateau has been pushed slowly upward by forces deep in the Earth. The river, unable to flow uphill, simply kept cutting down as fast as the land rose, like a saw held still while the timber is pushed into it. The canyon is as much a record of uplift as of erosion.
And the walls read like a book. Each band of rock is a layer of sediment laid down in order, oldest at the bottom, so a walk from rim to river is a walk back through hundreds of millions of years.
The principle: Erosion over geological time. Slow abrasion by sediment-laden water, combined with tectonic uplift, can carve enormous landforms given millions of years.