Summerville Formation
The thin, chocolate-brown tidal-flat rhythmite at the top of the San Rafael Group — striped mudstone-and-sandstone below the Morrison.
Also known as: Summerville
Jurassic — the golden age of dinosaurs
The world back then
Middle Jurassic, about 162 million years ago. The sea is pulling out, leaving a drying coastal flat where tides lay down mud and sand on a strict schedule — even, striped, chocolate-brown layers, over and over. Flat, salty, and metronomic, the last calm before the Morrison's rivers arrive. Nothing dramatic; just the tide keeping time.
Now it's the thin striped band below the dinosaur badlands, and you drift past a flat that once kept perfect time. It counted tides for ages; you only have to float over it once. The luck of arriving warm and unhurried, long after the tide stopped counting, is a thing worth noticing.
Recognize it in the field
- Typical color
- chocolate-brown; red near the base
- Weathers as
- Thin-bedded rhythmite that weathers into striped, chocolate-brown badland slopes; thin resistant sandstones stand out as ledges and seams.
- Grain
- silt-clay mudstone and fine sandstone, thin-bedded
Look for
- rhythmically alternating chocolate-brown gypsiferous mudstone and laminated sandstone
- thin, even bedding (a rhythmite)
- red clay near the base
- tidal-flat / sabkha banding
How this rock was made
A drying Middle Jurassic coastal flat as the Curtis sea withdrew, just before the Morrison river (fluvial) system.
The setting: tidal-flat / salty coastal flat (sabkha)
To 331 ft at Drunk Man's Point (south); thins east to 55 ft (Salt Valley) and 90 ft (near the Dolores River).
In the rock record
Part of the San Rafael Group.
↑ Younger — rests above
↓ Older — sits beneath