Mesaverde Group
The Book Cliffs made rock — a Late Cretaceous wedge of shoreline sandstone, coal-swamp shale, and black coal that built east as the Western Interior Seaway retreated.
Also known as: Mesaverde, Mesa Verde Group
Cretaceous — the last of the dinosaurs, ended by an asteroid
The world back then
Late Cretaceous, about 80 million years ago. A warm, shallow sea — the Western Interior Seaway — laps against a vast, swampy, coal-forming coastal plain, and this is the beach and the bog behind it. Rivers push new sand out into the surf, hadrosaurs browse the muggy swamps, and offshore the water is full of sharks, giant turtles, and mosasaurs. It is hot, wet, buggy, and constantly rearranging — paradise if you have gills or armor, a long bad afternoon if you don't.
Now that shoreline is the Book Cliffs — a wall of sandstone and coal running two hundred miles across Utah, and the black seams in it are that swamp, compressed. You cruise past a fossil coastline that took millions of years to build, doing nothing but watching it slide by. The comfortable animal who arrives long after the sea drains, the swamp dries, and the whole thing turns into a highway view — that's the winning ticket, again.
Recognize it in the field
- Typical color
- tan to gray sandstone, gray shale, black coal
- Weathers as
- A long banded escarpment — resistant cliff- and ledge-forming sandstones alternating with recessive gray shale slopes and black coal seams. The signature cliff-and-bench wall of the Book Cliffs.
- Grain
- clay to medium sand
Look for
- cliff-forming shoreface and deltaic sandstones (Star Point, Castlegate) over coal-bearing coastal-plain shales (Blackhawk, Neslen)
- black coal seams — the West's great Cretaceous coalfields
- an eastward-building (regressive) clastic wedge shed off the Sevier mountains
- the sandstone-into-Mancos-shale pinchouts are world-famous textbook exposures along the Book Cliffs
How this rock was made
The advancing shoreline of the Western Interior Seaway, prograding east off the rising Sevier mountain front across a swampy, coal-forming coastal plain in the Late Cretaceous.
The setting: beaches, deltas, lagoons, and coal swamps along a retreating sea, with river (fluvial) sandstones inland (shoreface / deltaic / coastal-plain / fluvial)
Thickens westward toward the Sevier source; the coal-bearing Blackhawk and the cliff-forming Castlegate are the signature Book Cliffs units.
In the rock record
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
- the Book Cliffs
- the Wasatch Plateau
- Mesa Verde (type area)
- the rim of Desolation and Gray Canyons