Morgan Formation
The banded Pennsylvanian limestone-and-sandstone ledge of the Uinta canyons — cyclic sea-floor and sand between the Madison and the Weber.
Also known as: Morgan
Pennsylvanian — the great coal swamps
The world back then
Pennsylvanian, about 315 million years ago. A shallow sea slides on and off a broad shelf, dropping gray lime one cycle and red-and-white sand the next — banded, cyclic, warm. Not dramatic, just patient: sea floor, beach, sea floor, over and over. The Uinta country as a quiet tropical shelf.
Those patient bands are the striped bench in the Uinta canyons, and you drift the record of a million-year rhythm in a single afternoon. The old shelf pulsed sea-in, sea-out for ages; you only have to watch. The quiet seat at the end of all that repetition is, once again, yours for basically nothing.
Recognize it in the field
- Typical color
- gray limestone; white to red sandstone
- Weathers as
- Ledge- and slope-forming — recessive banded limestone-and-sandstone benches relative to the Weber cliff.
- Grain
- interbedded carbonate and clastic
Look for
- interbedded gray limestone and white-to-red sandstone
- limy shale and limestone at the base
- cyclic marine carbonate-clastic shelf
- recessive relative to the overlying Weber
How this rock was made
Pennsylvanian rising-sea (transgressive)-falling-sea (regressive) cycles across the ancestral Uinta shelf.
The setting: cyclic marine to marginal-marine carbonate-clastic shelf
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Uinta Mountains (Middle Rocky Mountains province; NPS files Dinosaur NM under Colorado Plateaus)
- the Uinta canyon walls (Split Mountain, Whirlpool Canyon)
Float past it: Split Mountain · Whirlpool Canyon