Muav Limestone
The gray Cambrian limestone at the top of the Tonto Group — the mottled cliff above the Tonto Platform, laid down by the first sea to drown the continent.
Also known as: Muav
Cambrian — the first great burst of sea life
The world back then
Middle Cambrian, about 500 million years ago. The great sea keeps deepening westward, laying gray lime over the drowned continent — offshore, quiet, and mottled, shellfish scattered across a soft carbonate floor. Above it, an enormous chunk of time will go missing. A calm gray sea over a world with nothing yet living on land.
That gray sea floor is the cliff above the green Tonto bench, and you're down in the daylight far below it. The sea drained away hundreds of millions of years ago; the river stayed to carve you a corridor. Being the comfortable creature at the tail end of the story keeps turning out to be the best place to be.
Recognize it in the field
- Typical color
- gray to grayish-white (weathers dark gray to rusty-orange)
- Weathers as
- Cliff- and ledge-former — the gray cliff capping the Tonto Platform above the Bright Angel slope.
- Grain
- fine- to medium-grained; mottled
Look for
- mottled gray limestone and dolomite
- horizontal lamination and cross-stratification
- records repeated transgressive–regressive cycles
- intertongues laterally with the Bright Angel Shale
How this rock was made
Deposited on a westward-deepening Cambrian shelf during the great marine rising sea drowning the land (transgression) that drowned the continent.
The setting: offshore marine shelf (Sauk transgression)
46 ft (eastern Grand Canyon) to 249 ft (western); to ~820 ft at Frenchman Mountain, Nevada.
In the rock record
Part of the Tonto Group.
↑ Younger — rests above
↓ Older — sits beneath
Where to see it
Regions: Grand Canyon / Colorado Plateau
- western Grand Canyon
- Muav Canyon (type section)
Float past it: Grand Canyon