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Bright Angel Trail, NPS Photo |
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Exposed in the walls of the Grand Canyon, are numerous faults that document the region’s earthquake – or tectonic – history. Since faults in the Grand Canyon are not only exposed on horizontal surfaces, but also in the walls of the canyon, geologists are provided with a rare opportunity to study what faults look like thousands of feet down into the earth’s crust. Faults are seen cutting through practically every geologic layer in the canyon, from the oldest, two-billion-year-old Precambrian rocks through some of the most recent lava flows less than 10,000 years old.
The amount of movement measured on the faults varies from 15 feet to 16,000 feet. One of the most famous faults at the Grand Canyon is the Bright Angel Fault. Originating south of the canyon, it is oriented northeast and slices through Grand Canyon Village, down past Indian Gardens and Phantom Ranch, and northward up Bright Angel Canyon – which is a fault-oriented canyon – and terminates near the North Rim. The Bright Angel Trail descends steeply down the broken, shattered rocks along the fault line, which provides one of the few breaks in the massive cliff faces that generally prohibit descent into the canyon.
Monoclines seen in the Grand Canyon are another expression of the region’s faults. Monoclines are folds, or bends, in the otherwise horizontal rock layers that dominate the canyon. Folds form when a fault deep underground becomes active but doesn’t actually break the surface rocks. Instead, the surface layers bend to form a fold that is draped over the displacement along the underlying fault. The most visible example is from Desert View Watchtower, where the East Kaibab monocline traverses the canyon and has folded the rock layers seen on the north side of the canyon.
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