Pinkerton Trail Formation
The dark-gray crinoidal limestone at the base of the Hermosa Group — the Pennsylvanian sea's first floor, mostly buried beneath canyon country.
Also known as: Pinkerton Trail
Pennsylvanian — the great coal swamps
The world back then
Pennsylvanian, about 313 million years ago. The very first Pennsylvanian sea floods in over an old weathered land surface, laying down dark, fossil-rich lime — crinoids, shells, small reefs on a shallow shelf. It's the opening act for the whole basin, thin and easy to miss. Warm, shallow water over freshly drowned dirt.
You almost never see it — thin and mostly buried — but it's the floor the whole stack stands on, quietly under your boat. The sea's opening move, hidden underground, still holds up everything you're drifting past. Real, important, and doing its job while you do nothing.
Recognize it in the field
- Typical color
- dark gray
- Weathers as
- Limestone ledge/cliff former where present, but thin and rarely exposed (it wedges out and is largely subsurface).
- Grain
- fine- to coarsely-crystalline limestone
Look for
- dark-gray crinoidal and fusulinid-bearing marine limestone
- interbedded dark-gray silty shale
- thin, wedge-shaped unit that thins/wedges out southwestward
- local reef/algal (biohermal) deposits
- sits directly on the red-brown Molas Formation
How this rock was made
Deposited during the initial Pennsylvanian marine rising sea drowning the land (transgression) over the Molas paleosol/regolith; includes reef trends.
The setting: open marine shelf carbonate
Wedge-edge to ~195 ft in the Paradox Basin; revised usage up to 500–600 ft. The lower 0–175 ft was later split off as the Lime Ridge Formation.
In the rock record
Part of the Hermosa Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau (Paradox Basin)
- Type locality on the Pinkerton Trail near Durango, Colorado (subsurface in Canyonlands — the Paradox above is the oldest exposed rock there)