Morrison Formation

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The Morrison Formation in Arches National Park.

NPS photo by Neal Herbert

 

The Morrison Formation is composed of three members (listed from youngest to oldest): Brushy Basin, Salt Wash and Tidwell. Each has different characteristics due to changes in the depositional environment. This layer does not occur in Canyonlands.

Deposited:
Late Jurassic (144 to 208 million years ago)

Depositional Environment:
Varied between river, tidal flat and shallow marine

Appearance:

  • Brushy Basin: multicolored Bentonite clays (volcanic ash)
  • Salt Wash: Light-colored, ledge-forming sandstone
  • Tidwell: red marine sandstones frequently containing chert

Example
Visible in the northern parts of Arches National Park (Salt Valley) and along Highway 191 north of Moab

Notes:
The Salt Wash Member is a source of uranium which was mined throughout the Moab area. Dinosaur tracks and fossils also appear in the Morrison Formation.

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