Textbook Cross-Bedding

One stop on my final undergraduate geology trip this March included a day at Zion National Park, located in southwestern Utah.  As I mentioned with Death Valley, there is a lot of stuff to talk about here and more will follow in the future.  For now I want to talk about the perfect cross-bedding present in the park.  But first, an obligatory shot of the awe-inspiring oasis that is Zion Canyon:

Drool-inducing view at Zion National Park near Virgin Utah. Photographed 03/10.

The view above is at the bottom of the canyon, where the Virgin River continues to slowly erode away the canyon walls and floor.  An exciting white-knuckle roadway leads up towards the top of the canyon, where there is more to see.  This is where you can see some great cross-bedding:

The textbook cross-bedding in the Navajo Sandstone near the top of Zion Canyon.

Here’s a better look:

The textbook cross-bedding in the Navajo Sandstone near the top of Zion Canyon.

So how did this cross-bedding form?  The Navajo Sandstone was formed from vast sand dunes in the early Jurassic period (almost 200 million years ago).  Sand dunes have a gently-sloping side facing the prevailing wind (windward), and an opposite steep side (leeward).  As the wind moves over the dune, it increases in velocity and picks up sand grains on the gentle windward side.  As the wind reaches the crest of the dune, its velocity decreases and the wind-borne sand grains are dropped.  These grains accumulate on the steep leeward side of the dune.  The dune migrates in the direction of the wind as the sand grains accumulate in layers on the lee side.  Cross-bedding like that above is a cross-section of such deposition.  A graphic may help illustrate this process:

Sand dune deposition, migration, and formation of cross-bedding.

In the third photo above, the three distinct beds represent dune migration to the left, then the right, and then the left again as wind direction changed.

Although not conducive to the preservation of fossils, there have been fossils recovered from the Navajo Sandstone including several cool vertebrates.

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About Jeremy Sell

Science and nature nerd.
This entry was posted in Geology, National Parks, Paleoecology, Paleontology and tagged , , , , . Bookmark the permalink.

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