The Navajo Sandstone
Navajo Sandstone. Click for larger image.
The Navajo Sandstone is a geologic formation of the Glen Canyon Group, which is found in the Colorado Plateau. It spans from Arizona to Colorado and from Mexico to Canada. Its age is estimated to be of Late triassic to Early Jurassic and it's believed that the boundary between the two periods is somewhere within the Navajo. It can be visited in many parts of Utah ad is a prominent outcrop at may of Utah's national and state parks and monuments. Zion, Capital Reef, Glen Canyon, Grand Staircase-Escalante, Canyonlands, and the San Rafael Swell are among them.
The Navajo Sandstone was deposited in an "eolian" sedimentary environment. The word "eolian" means that it was formed by wind and is more correctly spelled "Aeolian". The Aeolian Islands are also known as the "Windy Islands" in the Tyrrhenian Sea and figure prominently in Homer's Odyssey. Sand dunes are the classic eolian sedimentary environment. A "sand sea" of dunes is often referred to as an erg and the Navajo Sandstone was formed in one of the truly great ergs of geologic history. The cross-bedding shown in the photograph above is typical of the Navajo. The steeper lines actually trace out the downwind, or "leeward" side of the dune. This is where the sand grains are deposited in a wind shadow. The more horozantal lines trace out the windward, or "stoss" side of the dune, which is an area of erosion by the wind. The sand on the stoss side is moved by the wind and deposited on the leeward side. The different groupings of cross bedding are created as one dune climbs over the stoss side and is deposited above another dune.
Click here to watch a short Youtube Video showing sand being blown by the wind on a dune in Utah's San Rafael Desert.
Most of us are familiar with the notion that pebbles being jostled around by water acquire a polished surface. This happens because the viscosity of the water tends to buffer the impacts of the grains and so, they tend to just grind upon one another. The air is much less viscous than water and so, when a wind-blown sand grain impacts another, it does so with greater velocity and force. The impact of the two grains leaves microscopic impact craters and scratches on their surfaces. Once those sand grains have been blowing around for thousands of years, they become well rounded and their surfaces grow to be completely covered with those tiny scratches and the sand grain gets what geologists refer to as "frosted" surfaces. Below is a photo of frosted and well rounded grains of sand which came from the Navajo Sandstone.
The Navajo Sandstone was deposited in an "eolian" sedimentary environment. The word "eolian" means that it was formed by wind and is more correctly spelled "Aeolian". The Aeolian Islands are also known as the "Windy Islands" in the Tyrrhenian Sea and figure prominently in Homer's Odyssey. Sand dunes are the classic eolian sedimentary environment. A "sand sea" of dunes is often referred to as an erg and the Navajo Sandstone was formed in one of the truly great ergs of geologic history. The cross-bedding shown in the photograph above is typical of the Navajo. The steeper lines actually trace out the downwind, or "leeward" side of the dune. This is where the sand grains are deposited in a wind shadow. The more horozantal lines trace out the windward, or "stoss" side of the dune, which is an area of erosion by the wind. The sand on the stoss side is moved by the wind and deposited on the leeward side. The different groupings of cross bedding are created as one dune climbs over the stoss side and is deposited above another dune.
Click here to watch a short Youtube Video showing sand being blown by the wind on a dune in Utah's San Rafael Desert.
Most of us are familiar with the notion that pebbles being jostled around by water acquire a polished surface. This happens because the viscosity of the water tends to buffer the impacts of the grains and so, they tend to just grind upon one another. The air is much less viscous than water and so, when a wind-blown sand grain impacts another, it does so with greater velocity and force. The impact of the two grains leaves microscopic impact craters and scratches on their surfaces. Once those sand grains have been blowing around for thousands of years, they become well rounded and their surfaces grow to be completely covered with those tiny scratches and the sand grain gets what geologists refer to as "frosted" surfaces. Below is a photo of frosted and well rounded grains of sand which came from the Navajo Sandstone.
Photomicrograph of well rounded and frosted sand grains from the Navajo Sandstone
Outcrop of cross-bedded Navajo Sandstone in the San Rafael Swell
Peekaboo Arch within the Navajo Sandstone
Looking upwind toward the leeward side of a small dune in the San Rafael Desert of Utah. In the background rise the Henry Mountains.
