Stony Mountain Formation

The Stony Mountain Formation is a stratigraphical unit of Ashgill age in the Western Canadian Sedimentary Basin.

It takes the name from the community Stony Mountain, Manitoba, and was first described in the town quarry by D.B. Dowling in 1900.

Subdivisions
The Stony Mountain Formation is divided in the following sub-units:
 * South (Williston Basin)
 * Penitentiary Member: argillaceous dolomite
 * Gunn Member: interbedded calcareous shale and fossiliferous limestone
 * North
 * Gunton Member: crystalline dolomite
 * Lower Stony Mountain: argillaceous dolomite

Distribution
The Stony Mountain Formation occurs throughout the Williston Basin. It reaches a maximum thickness of 45 m in the sub-surface at the Canada/United States border, and thins out towards the east, north and west. In Manitoba, where it is exposed at the surface in the erosion belt, it has a thickness of 30 m.

Relationship to other units
The Stony Mountain Formation is slightly unconformably overlain by the Stonewall Formation and sharply overlays the Red River Formation or the Herald Formation.