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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A second geologic event also created large and probably temporary knickpoints: There were large lava flows that dammed the Colorado River approximately 640,000 years ago. Although probably short-lived, these dams would have allowed enormous reservoirs to form along the Colorado River’s course. When the dams broke or were eroded upstream, the water stored behind the dam would have been instantaneously released, providing massive erosive force.

AN EVOLUTIONARY STORY

The physical conditions that create the rock strata you traverse are repeated again and again, creating multiple layers of limestone, shale, or sandstone. However, the Grand Canyon’s sedimentary strata were deposited over 500 million years, and the fossils in successive strata record much of the evolutionary history of life.

At the time the Bass Limestone was deposited, the only lifeform was single-celled colonial bacteria called stromatolites, visible in the rock as wavy bands. Multicellular, shell-bearing, aquatic animals evolved by the start of the Cambrian era, 542 million years ago; the Bright Angel Shale contains abundant trilobites, an early shell-bearing creature. Worm burrows, termed trace fossils, are also abundant. By the time the Redwall Limestone was deposited, different invertebrates dominated the seas, and crinoids (a stalklike relative of starfish) and brachiopods are abundant in this layer. Unlike at the time of the Bright Angel Shale’s deposition, plants and animals now colonized the land. Along the South Kaibab Trail, plant fossils are visible in the Hermit Formation; some specimens of ferns are on display on the west side of Cedar Ridge for all to observe. The Coconino Sandstone preserves reptile footprints. Stop and consider that these critters didn’t exist when you are just a couple of miles farther down the trail.