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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The environment was similar when the next stratum, the Hermit Formation, was deposited, except that sea levels were lower and the formation is exclusively terrestrial. Large rivers continued to carry red mud and fine sand from the deserts to the east. As the continent became ever drier and the shoreline retreated farther west, large dunes spread across the Grand Canyon area. These giant sand dunes are preserved as the Coconino Sandstone (275 million years ago).

While the inland drought was continuing, the shoreline crept east again: By 273 million years ago, a very shallow sea again covered the area, leading to the deposition of intertidal deposits (the Toroweap Formation) and then deeper water where calcite accumulated (the Kaibab Formation; 265 million years ago). The Toroweap Formation contains gypsum and salt crystals, minerals formed by the evaporation of water.

This marks the end of the sequence of sedimentation that is preserved today—the 4000-foot-thick sequence of multicolored strata that makes the Grand Canyon so spectacular. However, the subsequent 250 million years are important as well: Those rocks needed to be lifted far above sea level and carved by flowing water. The remainder of the geologic description is therefore focused on the sequence of events that led to the creation of the Grand Canyon.