Grand Canyon National Park - Lost Eons
And still the river rushes onward, downward toward its junction with the Little Colorado -- the beginning of the Grand Canyon proper. The base of the Redwall cliff rises from the riverbank atop another, other layer of sea-packed limestone, the Muav. Between the two limestones is a 150-million-year unconformity: a blank period when the land was above water and whole pages of the record were torn away by erosion. Something must have happened during all those millennia, but in Marble Gorge the only signs that remain are a few pockets of lavender rock, called Temple Butte Limestone.
By the time the junction is reached, the Muav Limestone has itself risen, and the sloping walls of greenish-gray Bright Angel Shale, which holds fossils of crablike trilobites and other long-vanished creatures, have begun to blend into the underlying Tapeats Sandstone. Together, these three rock layers tell the story of a fishless ocean's long incursion from the west more than half a billion years ago: the sandstone was its invading beach and shallows, the shale its muddy offshore bed, and the limestone its depths. The changes from one layer to another are almost imperceptible. At the place where the Little Colorado adds its blue waters to the Colorado, the sandstone is still flaky with remnants of shale, and forms ledgy cliffs where tamarisks thrive.
If the geologists' theory of river piracy is true, then there must have been a precise moment just a few million years ago when the Little Colorado reversed its flow and, instead of filling the huge lake to the southeast, began to empty it. The process may have started with a trickle down a steep slope, but at some point the trickle became a rush -- a waterfall, perhaps, or a mighty cascade of rapids -- that was to persist for a long time. It must have been a grand spectacle, this Niagara, cutting ever downward through layer upon layer of rock, diminishing itself by its own roaring power. If so, all sign is long since ago. There are not even rapids at the junction of the rivers now, only a sandbar where mingling sediments have settled.
It was near this junction, only two miles down the Colorado, that the Hopi Indians gathered sacred salt, which oozed from canyon walls and hung like stalactites. To reach it, they entered the canyon by way of the Little Colorado reversed its flow and, instead of filling the huge lake to the southeast, began to empty it. The process may have started with a trickle down a steep slope, but at some point the trickle became a rush -- a waterfall, perhaps, or a mighty cascade of rapids -- that was to persist for a long time. It must have been a grand spectacle, this Niagara, cutting ever downward through layer upon layer of rock, diminishing itself by its own roaring power. If so, all sign is long since gone. There are not even rapids at the junction of the rivers now, only a sandbar where mingling sediments have settled.
It was near this junction, only two miles down the Colorado, that the Hopi Indians gathered sacred salt, which oozed from canyon walls and hung like stalactites. To reach it, they entered the canyon by way of the Little Colorado, past the great sipapu -- a large mineral done built up around a bubbling spring -- through which they believed their ancestors had come into the world.
Today, the once-red Colorado River runs green through Marble Gorge, about 80 percent of its silt having settled to the bottom of Lake Powell, behind the Glen Canyon Dam that closed the tap upstream in 1964. It will only be a few centuries until the artificial lake that drowned Glen Canyon, that sedate and immensely beautiful chasm through which Powell passed on his way from Canyonlands to Marble Gorge, will be filled with silt. Then, red and abrasive again, the river will overflow the dam and resume its work. Meanwhile, the Little Colorado adds a small measure of cutting power before the river turns westward, to grind its narrow channel through the vast breadth of the central canyon.
After plunging for a few miles between firm walls of hard-packed sandstone, the river cuts through another unconformity, where the rock that would tell the story of the past is missing. Known as the Great Unconformity, it represents a staggering descent in time, a gap of 1,200 million years. During this vast span of eons, a complete mountain range arose and was worn back down to a plain, a sea advanced and laid down more than 12,000 feet of sedimentary rock upon that plain, those rocks were broken by earthquakes and reshaped into a landscape of low, rolling mountains and deep valleys, and those gentle peaks were eroded away. All that remains of the final mountains is a few isolated remnants of tilted stone wedged into the sharp dividing line between the ages.
There are no fossils in the black Vishnu schist below the Great Unconformity; although the first faint sparks of life had already been struck when this fire-twisted rock was formed, all record of it is gone here, destroyed by the immense heat that gave the rock its being. For nearly 200 miles, its primordial darkness lines Granite Gorge, rising to heights of 2,000 feet above the water, broken only by occasional veins of pink Zoroaster granite that boiled up from the earth's molten center when the planet was young and fierce.