Textbook Alluvial Fans

Alluvial fan photographed 03/10 in Death Valley National Park, California.

Alluvial fans are common geologic features in basin and range provinces like the one in the American southwest.  They’re composed of sand and gravel that has been eroded from mountains, transported by water, and then deposited in enormous fan-like shapes on more level ground.  Some of the most well-known are in Death Valley National Park, and I got to see some of them up close on an undergraduate geology trip last winter.

Alluvial fan at the base of the Panamint Range. Photographed 03/10 in Death Valley National Park, California. Alluvial fan outlined in red.

To understand their formation, it’s essential to understand the tectonic forces involved in basin and range creation.  These regions are the result of extensional forces that stretch the earth’s crust over a wide area.  As the crust is pulled apart, it fractures and forms normal faults:

Initial crustal extension and formation of normal faults.

As the crust continues to pull apart, central blocks known as grabens drop in elevation relative to the surrounding higher blocks, known as horsts.

Formation of basin and range topography and alluvial fans.

Water from snow melt and rain storms carries sand and gravel down the sides of the horsts, slowly carving canyons into the mountains.

Looking south into Mosaic Canyon from the mouth of the canyon. Photographed 03/10 in Death Valley National Park, California.

When this sediment-laden water exits the canyons, it reaches much more level and wide terrain.

Looking north into Death Valley from the mouth of Mosaic Canyon. Photographed 03/10 in Death Valley National Park, California.

The water velocity drops, and it releases its sediment load.   Sand and gravel are deposited in layers with each rainstorm.

Quaternary period (2.6 million years ago to present) gravel deposits from erosional outwash at the mouth of Mosaic Canyon. John Parvin for scale.

Over time this repeated deposition leads to the formation of giant alluvial fans.

Alluvial fan between the Panamint Range and Badwater Basin. Photographed 03/10 in Death Valley National Park, California.

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About Jeremy Sell

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

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