Dakota Sandstone
The resistant tan Cretaceous caprock at the base of the seaway succession — coal-bearing sandstone that rims canyon country's mesas and hogbacks.
Also known as: Dakota
Cretaceous — the last of the dinosaurs, ended by an asteroid
The world back then
Mid-Cretaceous, about 97 million years ago. A great inland sea is rising and marching east across the continent, and you're on the advancing shoreline — rivers, swampy coastal plains, and beaches, dinosaurs in the marshes, the first flowers just showing up. Warm, wet, buggy, and slowly drowning as the water climbs. The whole world is turning into coastline.
You drift over that old beach now, high and dry, watching the tan caprock slide by — no rising sea coming for your camp. The world was busy drowning back then; today it holds still and lets you look. A comfortable seat above deep time is a strange and enormous piece of luck.
Recognize it in the field
- Typical color
- tan to brownish-gray (also yellow, white, red)
- Weathers as
- Ledge/cliff- and hogback-former — resistant sandstone caps that erode into small cliffs and hogbacks.
- Grain
- fine to medium; basal conglomeratic
Look for
- resistant basal-Cretaceous sandstone/conglomerate ledge
- coal, carbonaceous shale, and early flowering-plant leaf beds
- marks the sub-Dakota unconformity over Jurassic strata
- records the westward advance of the Western Interior Seaway
How this rock was made
Rivers, coastal plains, and beaches during the mid-Cretaceous rising sea drowning the land (transgression) of the Western Interior Seaway, stepping eastward as the sea rose.
The setting: river (fluvial) / coastal-plain with shallow-marine and beach deposits
Up to ~150 ft on the Colorado Plateau; thin and discontinuous.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
- Capitol Reef (caps the Waterpocket Fold section)
- San Rafael Swell rim
- Dinosaur National Monument
- the Cisco / Ruby–Horsethief corridor (thin resistant rim)
Float past it: Cisco to Dewey Bridge · Ruby–Horsethief Canyon