Bright Angel Shale
The soft green-and-red Cambrian shale that forms the Grand Canyon's Tonto Platform — the fossil-rich slope between the Tapeats and Muav cliffs.
Also known as: Bright Angel
Cambrian — the first great burst of sea life
The world back then
Middle Cambrian, about 505 million years ago. You're wading a warm, shallow sea creeping east over the drowned edge of the continent, the bottom a soft green mud stirred up by storms. Trilobites work the sea floor; nothing has figured out how to live on land yet, so there's nowhere dry to stand. Just shallow water and quiet gray-green coastline in every direction.
Now you're floating over the fossil of that sea floor, bone dry, watching the green slope drift past. Being a land animal with a boat, shade, and a cold drink is a preposterously good deal. Almost nothing that ever lived on this planet got to lie back and just look at it.
Recognize it in the field
- Typical color
- greenish (glauconite/chlorite) with reddish-brown iron-oxide beds
- Weathers as
- Slope-former — the recessive green-and-red slope that forms the broad Tonto Platform between the Tapeats and Muav cliffs.
- Grain
- fine to medium; local coarse sandstone
Look for
- glauconite-green and iron-red mudstones
- forms the Tonto Platform bench
- the most fossiliferous Tonto Group unit
- intertongues with both the Tapeats and the Muav
How this rock was made
A shallow Cambrian sea over a drowning coast during the Sauk rising sea drowning the land (transgression).
The setting: shallow-marine shelf (storm-influenced; estuary/tidal-flat settings)
Sources give ~300–450 ft in the canyon; the exact bounds are inconsistent (regional ~57–450 ft) — no single precise max encoded, pending a primary measured section.
In the rock record
Part of the Tonto Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Grand Canyon / Colorado Plateau
- the Tonto Platform, Grand Canyon
- Bright Angel Canyon (type)
Float past it: Grand Canyon