Читать книгу One Best Hike: Grand Canyon. Everything You Need to Know to Successfully Hike from the Rim to the River—and Back онлайн

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In contrast, the South Kaibab Trail is predominantly a ridge route that exploits locations where the normally steep rock layers have begun to erode, because they are outcropping along a narrow ridge. And here too, the passage through the Redwall Limestone follows a small fault.

Passive margin, 525 million to 320 million years ago: The breakup of Rodinia created a “passive margin,” or tectonically quiet region, along the then western edge of North America, the Colorado Plateau region. Along such margins the seafloor often sinks rapidly continually creating more space for sediment to accumulate and allowing thick rock strata to form. The prominent striped layers in the Grand Canyon, a series of strata 4000 thousand feet thick, were deposited during this tectonic regime with the type of sediment changing with shoreline position and water depth. (Up to 18,000 feet of sediment was deposited on the Colorado Plateau, but only the lower strata are preserved in the Grand Canyon.)

The oldest (and lowest) of the layers is the Tapeats Sandstone, formed from former beach sands. The Bright Angel Shale, formed in shallow offshore waters, follows; the mud-sized particles comprising the shale were carried a short distance out to sea before being deposited. The Muav Limestone, the third layer, is constituted of a combination of calcite that precipitated and calcite-bearing shells that were deposited. Together, these three layers represent a 20-million-year period of rising sea level: While sand-sized sediment was amassing at one location, silt was being deposited in shallow water to the west, and calcite was accumulating even farther west. As the sea level rose, each environment shifted eastward, such that the three sediment types overlay each other in the Grand Canyon.