Entrada Sandstone
The Middle Jurassic sandstone of the San Rafael Group — the fin-and-arch rock of Arches National Park.
Also known as: Entrada
Jurassic — the golden age of dinosaurs
The world back then
Middle Jurassic, about 164 million years ago. Coastal dunes, beaches, and salty tidal flats ring a shallow sea — wind piling salmon and white sand into the well-cemented middle that will one day make arches, softer earthy beds above and below. Hot, windy, half-desert and half-shore. Sand blowing over drying mudflats under a bright sky.
That sand is Arches National Park now, and you drift past its cousins doing nothing but pointing. It took a coastline, a desert, and 164 million years of patient erosion to carve those spans — and you just show up and look. Somebody should be a little nervous about how good a deal that is.
Recognize it in the field
- Typical color
- red to reddish-brown; white where the Moab caps
- Weathers as
- Two facies: a well-cemented cliff-forming sandstone (the Slick Rock) that erodes into fins and arches, over/under earthy, less-cemented slope-formers that 'weather like shale' and undercut it — the recipe for arches.
- Grain
- fine-grained sandstone
Look for
- smooth salmon-and-white sandstone fins and arches
- cliff-forming Slick Rock over the recessive Dewey Bridge
- large-scale eolian cross-bedding with tidal-flat and sabkha beds
- the rock of Arches National Park
How this rock was made
Dune fields, beaches, and tidal mudflats around the shallow Jurassic Sundance/Carmel sea.
The setting: coastal wind-blown dune sea (eolian erg) with tidal-flat and salty coastal flat (sabkha) margins
312 ft at Entrada Point (type), 844 ft at Muddy Point, 1,430 ft at Circle Cliffs.
In the rock record
Part of the San Rafael Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Colorado Plateau
Float past it: Cisco to Dewey Bridge · Dewey Bridge to Hittle Bottom