Curtis Formation
The glauconite-green marine sandstone of the San Rafael Group — a brief Jurassic sea over the Entrada, capping Goblin Valley.
Also known as: Curtis
Jurassic — the golden age of dinosaurs
The world back then
Middle Jurassic, about 163 million years ago. The sea makes a quick comeback over the old dune country — a shallow green-tinged marine flooding, the seafloor dusted with glauconite, a mineral that only grows underwater. Shellfish and burrowers work the sandy bottom. It's brief, wet, and salty, a short chapter between deserts.
You drift that same footprint at your leisure now, the whole brief sea reduced to a green ledge sliding by. The ocean only visited for a moment; you get the whole thing, unhurried. There's a real luck in being the comfortable creature who arrives long after the hard part is over and just enjoys the view.
Recognize it in the field
- Typical color
- light gray to light greenish-gray (glauconitic)
- Weathers as
- Resistant ledge- and cliff-former — greenish-gray sandstone that caps the goblins of Goblin Valley and ledges out over the softer red Entrada.
- Grain
- medium- to coarse-grained, cross-bedded
Look for
- greenish-gray glauconitic sandstone (the diagnostic sea-green color)
- cross-bedded, fossiliferous, medium-to-coarse sandstone
- clay galls, carbonaceous trash, multicolored chert grains
- sharply truncates the Entrada along the J-3 unconformity
How this rock was made
A brief Middle Jurassic marine incursion (the Curtis sea) over the Entrada sand sea (erg); the glauconite confirms marine deposition.
The setting: shallow-water marine
193 ft at Curtis Point (type), 252 ft at the north end of the San Rafael Swell, 40 ft at Sand Creek (Waterpocket Fold).
In the rock record
Part of the San Rafael Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau