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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Extension to the west, 18 million to 6 million years ago: During this time period, stretching and thinning of western North America caused the Basin and Range Province in Nevada to form and the land surrounding the Colorado Plateau to decrease in elevation. Once the regions to the south and west were lower than the plateau, runoff began to etch south-flowing drainages, the direction of today’s Colorado River.

However, sediment deposits suggest that the Colorado River itself did not exist until approximately 6 million years ago. Instead, there were still large inland lakes and shorter waterways that were not connected. Water likely flowed along some of the Colorado River’s present path and through some of its major tributaries, although possibly opposite from the direction it does today. For instance, water may have flowed down the Little Colorado and then up Marble Canyon, but not through the main Inner Gorge of the Grand Canyon to the west of the confluence of these two drainages.

What caused these many rivers to coalesce into a single large drainage system is not definitively known, but two theories rise to the forefront. One is that the inland lakes suddenly joined together, possibly because of a catastrophic overflow of one of them, providing the force to cut through topographic barriers and integrate previously disconnected drainage systems. A second theory is headward erosion, whereby erosion causes waterways to move progressively upstream, cutting into slopes. Eventually this process destroys a ridge that had previously divided two drainages and connects the two stream systems. A combination of these two processes likely integrated the Colorado River system.