Although the Colorado River spent roughly 6 million years carving out the canyon itself, the rock into which the river has cut is far older. The mile-deep walls on both sides of the gorge reveal no fewer than twenty-seven formations whose lineages straddle eight geological periods, during which nearly 40 percent of the planet’s chronology was etched directly into the stone. By some measures, those walls showcase perhaps the finest cross-section of terrestrial time visible anywhere on the globe, a vertical concatenation of history stacked in horizontal strata, much like the pages of an immense
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