[13] Recently, the undifferentiated dolomites, now known as the Frenchman Mountain Dolostone, have been restore as a formation within the Tonto Group.

In addition, along with the Frenchman Mountain Dolostone, the Sixtymile Formation is regarded to be part of the Tonto Group.

This contact is either an angular unconformity truncating tilted strata of the Grand Canyon Supergroup or a nonconformity cut into the crystalline Vishnu Basement Rocks.

[1][2] As currently defined Tonto Group consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone.

Typically, fine sandstone becomes common towards the top in its upper 12 to 15 m (39 to 49 ft), which is part of a transition zone between it and the overlying Bright Angel Shale.

It is mostly composed of fissile shale (mudstone) and siltstone with some thicker beds of brown to tan sandstones and dolostones all of which are sometimes divided into numerous members.

Sedimentary structures are abundant in the Bright Angel Shale and include current, oscillation, and interference ripples.

The Bright Angel Shale has a complex gradational and interfingering relationship with the overlying Muav Limestone and underlying Tapeats Sandstone.

However, individual fossil quarries in the Bright Angel Shale, when excavated, are just as productive as many other Cambrian formations in the Great Basin and Rocky Mountain regions.

The Muav Limestone weathers to a dark gray or rusty-orange color and forms cliffs or small ledges.

They include sponges, brachiopods, hyoliths, helcionelloids, trilobites, eocrinoids, and enigmatic invertebrates (Chancelloria, Scenella).

[2] The Frenchman Mountain Dolostone consists of white to dark gray, thin- to medium-bedded dolomite, which is separated by the underlying Muav Limestone by a disconformity throughout the Grand Canyon region.

[1][2] The Tonto Platform is a very prominent, wide bench that occurs near the bottom of the eastern Grand Canyon.

The gentle slopes of the Tonto Platform were created by rapid backwearing[clarification needed] of the Bright Angle Shale.