Field Notes: Navajo Sandstone

Much of the beauty and spectacle of the rock formations of the American Southwest is a product of the ubiquitous Navajo Sandstone. In a layer ranging from a thousand to more than two thousand feet thick, and displaying a whole palette of colors, it’s the secret sauce for places like Zion National Park, the Vermillion Cliffs and a host of other geologic wonders.

Navajo Sandstone cliffs, Zion National Park, Utah

The Navajo Sandstone is believed to have been laid down in the Jurassic period, about 200 million years ago. At that time, the western shores of North America were near the western Utah border. It may have looked much like the Namib Desert today, an ocean shoreline backed by many, many miles of vast, arid sand dunes. Episodically, storms would blow in, much like the storm surge of a hurricane today, truncating the tops of the dunes and layering on new windblown sand.

Eventually, new terranes accreted to the western coast, and the sand dunes were buried under new layers of outwash debris, inundated by island seas, and buried by still more deposits. Water percolated down through those layers into the buried sand dunes, cementing the grains of sand together. In some areas, the water dissolved the traces of iron in the sand; in other areas, the water deposited and enriched the iron, and oxidized it. Oxidized iron, of course, is mostly red, the signature color of the formation.

Beginning perhaps 15 million years ago, that vast bed of layers of lithified sand and mud began to uplift, creating what is now the Colorado Plateau. All the while, erosion continued, cutting down through the layers, creating the spectacular scenery and geology on display today.

It’s all sand, but it’s not simple; Navajo Sandstone complex, Zion National Park, Utah

When the sandstone was still dunes, wind directions changed, dunes collapsed, and seasonal streams disturbed the sands. Some dune fronts slumped, creating mini-landslides and jumbles. The result can be very messy, with the layers of sand at different angles, different orientations, lensed with finer sands; geology, as WC may have noted earlier, is messy.

It’s called “cross-bedding,” and it’s sandstone that preserves all those variations in the originating sand dunes. At it’s simplest, it can be quite elegant.

Classic cross-bedding in Kayenta Sandstone, the layer immediately under Navajo Sandstone, Grand Staircase-Escalante Nat’l Monument, Utah

The angle of the sandstone as it is exposed by erosion impacts the rate of erosion. Stacked in horizontal layers, it is more durable. With the layers tilted up or down, erosion rates are more rapid. The result is differential erosion, which is what gives us those beautiful, fluted shapes.

The Narrows, Zion Nat’l Park, Utah

Trillions of tiny grains of windblown sand, fused by water and dissolved minerals, a couple of million years of erosion and, voila! Spectacular geology.

Wire Pass Canyon, Grand Staircase-Escalante Nat’l Monument, Utah

The geology professors at the University of Oregon tried to persuade WC and his classmates that volcanology was exciting, that sandstone was boring, sedimentary rock. Obviously, they were jealous.

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