The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a Group, that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
Muav Limestone-(broader-based gray cliff supporting tall-reddish Redwall Limestone cliff) and Bright Angel Shale-(greenish & extensive slope-former), resting on Tapeats Sandstone-(short, dark vertical cliff on gorge rim) and the Tonto Platform, inner canyon, Granite Gorge (the two units are easily seen below the red-stained Redwall Limestone (~550 ft thick))
View of Tower of Set peak and sub-unit cliff section from Tonto Trail, Granite Gorge, north of Mohave Point, Grand Canyon Village, South Rim. The peak is behind and separated from a cliff unit (with small prominence), in front-(photo center, right, Tower of Set (peak) to its left). Vertical erosion in cliff of Redwall Limestone, upon horizontal Muav Limestone cliff. The Tapeats Sandstone sits in foreground on Granite Gorge, and is seen as thinly-bedded. The
close-up and far views of the Tonto Group formations. Note in far view the cliff-run of the Tapeats Sandstone cliff below the (whitish)-greenish Bright Angel Shale-(often dull-greenish, but even yellowish in northeast Grand Canyon).
Except where underlain by the Sixtymile Formation, the Tapeats Sandstone is the Cambrian geologic formation that is the basal geologic unit of the Tonto Group. Typically, it is also the basal geologic formation of the Phanerozoic strata exposed in the Grand Canyon, Arizona, and parts of northern Arizona, central Arizona, southeast California, southern Nevada, and southeast Utah. The Tapeats Sandstone is about 230 feet (70 m) thick, at its maximum. The lower and middle sandstone beds of the Tapeats Sandstone are well-cemented, resistant to erosion, and form brownish, vertical cliffs that rise above the underlying Precambrian strata outcropping within Granite Gorge. They form the edge of the Tonto Platform. The upper beds of the Tapeats Sandstone form the surface of the Tonto Platform. The overlying soft shales and siltstones of the Bright Angel Shale underlie drab-greenish slopes that rise from the Tonto Platform to cliffs formed by limestones of the Muav Limestone and dolomites of the Frenchman Mountain Dolostone.
Muav Limestone-(greenish, slope-forming) and Bright Angel Shale, resting on Tapeats and the Tonto Platform, inner canyon, Granite Gorge (the two units are easily seen below the red-stained Redwall Limestone) (550 feet (170 m) thick)
Tapeats Sandstone on the Great Unconformity on Vishnu Schist, covered by erosional layers masking the schist.
View of Tonto Group, in descending order: Muav Limestone, Bright Angel Shale, and Tapeats Sandstone, overlying the Great Unconformity cut into Vishnu Basement Rocks in Granite Gorge region.
view opposite Lipan Point, (at Desert View (Grand Canyon), East Rim), showing the banded Nankoweap Formation, (horizontal Tapeats at left, extending at base of prominence, in two fingers of rock)