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Tuesday, March 27, 2012

Rocks For Shots

How knowing a little geology makes you a better landscape photographer

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Sunrise illuminates pinnacles and hoodoos carved into the Claron Formation from Bryce Point, Bryce Canyon National Park, Utah.
Beginning about 225 million years ago (mya), another lofty range began to rise to the west and south of the Plateau region in what's now Nevada and Arizona. These mountains likely resembled the Andes in scale and were shoved up into the sky by the collision of the Farallon Plate with the western edge of North America. This collision started when North America began to separate from Europe as the supercontinent of Pangaea split apart to form the Atlantic Ocean. For the next 200 million years, rivers draining these mountains deposited thousands of feet of additional sediments across the Plateau region. Like the Andes today, these mountains also contained large stratovolcanoes that periodically sent clouds of volcanic ash east on the prevailing winds. These ash deposits provided the vivid colors we see today in the Chinle Formation of Arizona's Painted Desert and the Morrison Formation in southern Utah.

Beginning about 70 mya, geological events were set in motion that created the Rocky Mountains we know today. Known as the Laramide orogeny, this mountain-building episode began when the Farallon Plate, which for most of its tenure had slammed into the West coast and subducted at a steep 45º angle into the Earth's mantle, began to subduct at a much shallower angle, sliding beneath the North American continent all the way to the present location of Denver, Colo., before diving into the mantle. This "flat-slab" subduction pushed up the entire Rocky Mountain chain, from Canada to New Mexico. As part of this same process, many of the Plateau's most recognizable landforms—the monoclines (uplifts) of the Waterpocket Fold, the San Rafael Swell and the Grand Canyon's Kaibab Plateau—were also shoved into the sky. In addition to the creation of individual mountain ranges and monoclines during this period, the entire interior West, from the Sierra Nevada Mountains to western Kansas, was uplifted thousands of feet.

The Left Fork of North Creek carves through a layer of the Kayenta Formation to form a tunnel known as The Subway in Zion National Park, Utah.
By the end of the Laramide orogeny about 40 mya, the Colorado Plateau region was surrounded by mountains—the Rockies to the east and those old Andean-style mountains to the west and south. It was a high-elevation basin with no outlet to the sea. Rivers drained into this basin from the surrounding highlands to create inland lakes much larger than today's landlocked Great Salt Lake. Rivers draining into one of these lakes deposited the colorful Claron Formation of Bryce Canyon National Park. As one of the youngest layers now exposed on the Plateau, the Claron forms the top layer in the "Grand Staircase" of rock strata that steps south from the 9,000-foot rim of Bryce to the bottom of the Grand Canyon where the Colorado River laps at the 1.7-billion-year-old Vishnu Schist.

The final step in the formation of the landscape we see today involved the evolution of the Colorado River. Geologists now seem to be coalescing around the idea that the uplift caused by the Laramide orogeny, followed by the collapse of the Basin and Range Province to the south and west around 20 mya, first allowed the ancestral Colorado River to begin nibbling away at the southwestern edge of the Plateau. The full length of the Colorado River as we know it today, from its headwaters in the Rockies to its delta in the Sea of Cortez, formed as recently as 6 mya, and allowed, for the first time, rivers and streams to begin carving down into the thick sandstone layers of southern Utah and Arizona to create all the deep canyons we recognize today. Much of this canyon-carving occurred during the various Ice Ages of the last 2.5 million years as enormous glaciers formed and melted in the Rocky Mountains. One can only imagine the prodigious volume of water carried by the Colorado and Green Rivers as thousands of cubic miles of ice melted at their headwaters. This period of erosion removed an estimated 6,000 vertical feet of rock from the Plateau's surface, all of which was carried away as sediment by the Colorado River and dumped into the Sea of Cortez. If you do the math, this amounts to about 130,000 cubic miles of material!

Colorful outcrops of the Brushy Basin, Morrison Formation, near Hanksville, Utah.
How To Use Geology For Your Photography
Now that I've provided the background, I'll explain how I use this information as a tool in my photography. I first put this knowledge to work about 20 years ago in my search for beautiful, narrow slot canyons. I noticed how the most photogenic ones were always carved into sandstones that had been deposited as windblown sand dunes; the Navajo Sandstone of Antelope Canyon is a classic example. These very uniform eolian sandstone layers allow streams to slice straight down into them with very little canyon widening. It should come as no surprise, then, that the most spectacular narrow canyons on the Plateau are found in the Navajo and Wingate Sandstones, both of which are eolian deposits. If you know how to identify these layers and know where they occur across the Plateau, it's a simple matter to find these beautiful canyons. I've used this technique to locate countless photogenic slots in the Escalante Canyons, the Waterpocket Fold and the San Rafael Reef, all of which contain extensive outcrops of both the Navajo and Wingate Sandstones.


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