Morrison Formation
The Late Jurassic dinosaur-bearing badlands of the Colorado Plateau — rainbow-banded bentonitic mudstone and river sandstone, the rock of the Dinosaur quarry.
Also known as: Morrison
Jurassic — the golden age of dinosaurs
The world back then
Late Jurassic, about 150 million years ago. A hot, seasonal plain of rivers, floodplains, and drying lakes rolls east from a volcanic highland — dinosaurs everywhere, ash drifting into the ponds, the whole landscape green in the wet season and cracked in the dry. This is the golden age of giants. You would be an appetizer.
Now it's the rainbow badlands where the bones come out, and you drift past, un-hunted, doing nothing but gawking. That plain was ruled by fifty-foot animals; today the biggest danger is sunburn. Showing up 150 million years late, comfortable and off the menu, is the best-timed entrance in history.
Recognize it in the field
- Typical color
- variegated — light gray, greenish-gray, pale red/maroon
- Weathers as
- Slope- and badland-former (soft variegated bentonitic mudstones), with resistant channel-sandstone ledges (Salt Wash, Westwater Canyon).
- Grain
- clay to medium sand
Look for
- rainbow-banded (gray/green/maroon) bentonitic mudstone badlands
- fluvial sandstone lenses that host the dinosaur bone
- freshwater limestones and lacustrine beds
- the world's premier Late Jurassic terrestrial fossil record
How this rock was made
A semi-arid, seasonal river-laid (alluvial) plain east of a volcanic highland (whose ash became the Brushy Basin bentonite), on a pre-Rocky-Mountains North America.
The setting: river channels, floodplains, lakes, and swampy lowlands (fluvial-lacustrine)
Capitol Reef: Tidwell 50–100 + Salt Wash 100–500 + Brushy Basin 200–350 ft; up to ~650 ft (Wikipedia). Regional (summed members).
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
- Dinosaur National Monument (Carnegie Quarry)
- Capitol Reef (Waterpocket Fold)
- San Rafael Swell
- the Cisco / Ruby–Horsethief corridor (widespread bench)
Float past it: Cisco to Dewey Bridge · Ruby–Horsethief Canyon