Sunday, January 16, 2011

Geology of the Grand Canyon: Finding Fault with the Bright Angel

In the photo, the Bright Angel Canyon and its trail lay within the gash of the fault all the way across the Grand Canyon to the opposite rim. Confirmation of the displacement perhaps can be best seen by observing the horizon-line on the far side of the canyon. Notice that the horizon is at two different levels, with the west side (left) slightly higher than the east side (right), indicating the displacement by the fault.

With each footstep you take down the Bright Angel Trail you travel an average of 60,000 years back through time. In total, the 9.7 mile descent from the rim to the river transports you through almost 2 billion years of the Earth’s history. Located on the South Rim of the Grand Canyon, the Bright Angel Trail will take you all the way down to the Colorado River, across of you keep going, and head you up to the North Rim. It’s a beautiful but difficult trail (mainly on the ascent) and easily the most popular, especially with first-time, rim-to-river backpackers. The Bright Angel has it all, many switchbacks,  several water stations (and even outhouses), considerable shade depending on the time of day, great geology, and stupendous views at every step.

 A zoomed-in view shows the gorge of the Colorado River from right to left (east to west)
crossing the fault of the Bright Angel from top to bottom (southwest to northeast).

I've added arrows to a USGS map showing the relationship and intersection
of the Bright Angel Fault (red arrows) and the Colorado River (black arrows).

We know that the original trail is not of contemporary construction by the Indian rock art and remnant habitations found along the way, the latter found alongside the Colorado River and scattered along the ascent on the opposite side. Prehistoric Indians used the trail to access the perennial water source within the canyon from the rims. The Indians got a head start from Mother Nature in establishing the trail; otherwise, they never would have been able to descend through the impassable cliffs blocking their way, primarily of the Coconino Sandstone and Redwall Limestone. In 1891, miners exploring the canyon for its purported riches improved upon the ancient path to the bottom, and with the coming of tourism, the trail assumed its present day form.

In Precambrian time, over half a billion years ago, movement within the Earth’s crust in the region opened a huge gash called the Bright Angel fault. Compressional fault movement provides an invitation for the forces of erosion to initiate, crushing rock along the fault and allowing water to percolate through the layers, chemically dissolving susceptible minerals. With the onset of the Laramide Orogeny in late Mesozoic time, the fault experienced tensional forces as the Colorado Plateau was uplifted as a whole. All these processes essentially weakened the rock and contributed to the formation of a canyon. Bright Angel Canyon traces the route of the fault from the North Rim to the South Rim of the Grand Canyon and beyond, a distance of over 60 miles. Thus, ancient Indians made advantage of the geology in constructing a trail.

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