Formation · Cretaceous · mudstone

Cedar Mountain Formation

The overlooked Early Cretaceous bench above the Morrison — gray-green smectitic mudstone with a basal chert-pebble conglomerate, and the home of Utahraptor.

Also known as: Cedar Mountain, Cedar Mountain Shale

Cretaceous — the last of the dinosaurs, ended by an asteroid

The world back then

Early Cretaceous, about 120 million years ago. The great Jurassic dinosaur boom is over, and this is the stranger, less-famous chapter that came next — a low, muddy, seasonally-dry plain of sluggish rivers and drying ponds, draining east off a rising mountain front to the west. It is the age of Utahraptor: a 1,000-pound sickle-clawed killer the size of a grizzly, hunting armored Gastonia through the scrub. You would not last the afternoon.

Now it's a drab gray-green bench most people float or drive right past without a glance — which is the whole joke, because it's one of the great dinosaur crime scenes on the continent. You get to wander Utahraptor's hunting ground in trail runners, the most dangerous thing out here a cactus. Being the soft, slow, unarmored animal who shows up 120 million years after the sickle-claws is a spectacularly good draw.

Trail Talk

The non-geologist version — for when someone starts nerding out and you want in.

The gist: The drab greenish-gray bench between the rainbow Morrison below and the cliffs above. Easy to ignore — and full of Early Cretaceous dinosaurs.
Drop this: It marks a huge gap in time: the rivers here scoured down into the Jurassic Morrison and started over tens of millions of years later. That gritty chert-pebble bed at its base — the Buckhorn — is the reset button.
Or just say: The boring gray layer that's secretly full of Utahraptor.
Rock band: A muted jam — drab olive and gray mudstone, a purple bruise here and there, capped by a gritty gravel of polished black and honey chert pebbles. Not a flashy rock, man. But run your eye along that pebble bed and you're reading a lost world's reset button. Quiet, patient, heavy.

Recognize it in the field

Typical color: drab gray-green to pale purple, with a gray chert-pebble base
Weathers as: Low badland- and slope-former (soft smectitic mudstone), with a resistant basal chert-pebble conglomerate (the Buckhorn) that often stands as a low ledge or caprock.
Grain: clay to pebble/cobble (basal conglomerate)

Look for

drab gray-green to purple smectitic (bentonitic) mudstone that swells and cracks
a basal chert-pebble to cobble conglomerate — the Buckhorn — often forming a low ledge
resistant sandstone lenses and nodular freshwater limestones within the mudstone
one of North America's key Early Cretaceous dinosaur units (Utahraptor, Gastonia, Eolambia)

How this rock was made

A low, seasonally-dry plain draining east off the rising Sevier mountain front, laid down in the long gap after the Jurassic Morrison and before the Late Cretaceous seaway arrived.

The setting: river channels, floodplains, ponds, and soils on a low coastal plain (fluvial-lacustrine, with well-developed paleosols)

Type ~125 m (410 ft) at Cedar Mountain (Emery County); the formation thickens westward toward the Sevier thrust front and thins to the east.

In the rock record

↑ Younger — rests above

Dakota Sandstonea gap in time (unconformity) · most of the Colorado Plateau

Cedar Mountain Formation you are here

Morrison Formationa gap in time (unconformity) · most of the Colorado Plateau

↓ Older — sits beneath

See the full regional rock columns →

Where to see it

Regions: Colorado Plateau

San Rafael Swell
Cedar Mountain (Emery County, Utah — type area)
the Yellow Cat area near Arches
Mussentuchit Wash (Emery County)