The Frenchman Mountain Dolostone is the uppermost and youngest of five Cambrian geologic formations that comprise the Tonto Group.
It consists of beds of mottled white to gray dolomite often separated by thin seams of shale, especially in its lower part.
West into the Lake Mead region, it thickens considerably and is 370 m (1,210 ft) thick at Frenchman Mountain near Las Vegas, Nevada.
[1] In some publications, the dolomite beds comprising the Frenchman Mountain Dolostone is ignored and only the Muav Limestone is illustrated.
L. F. Noble redefined his Muav Limestone as being the "...predominantly calcareous part of the Tonto group" lying beneath either the discontinuous lenses of overlying Devonian beds or base of the Redwall Limestone and overlying the Bright Angel Shale.
[3][9] The Cambrian undifferentiated dolomites was formally named the Frenchman Mountain Dolostone and restored to the Tonto Group.
Thin layers of shale (mudstone) frequently separate dolomite beds, especially in the lower part of this formation.
These differences are why E. D. McKee and C. E. Resser recognized and mapped the Frenchman Mountain Dolostone as separate stratigraphic unit from the Muav Limestone.
[3][9] Based upon sedimentary structures and stratigraphy, the Frenchman Mountain Dolostone is interpreted as shallow subtidal to possibly intertidal in depositional environments associated with a regressing sea.