Stony Mountain Formation
Stratigraphic range: Ashgill PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Underlies	Stonewall Formation
Overlies	Red River Formation
Thickness	up to 45 metres (150 ft)[1]
Lithology
Primary	dolomite, shale
Other	Anhydrite, limestone
Location
Coordinates	49°55′06″N 97°31′35″W / 49.9184°N 97.5265°W / 49.9184; -97.5265 (Stony Mountain Formation)
Region	WCSB Williston Basin
Country	Canada  United States
Type section
Named for	Stony Mountain, Manitoba
Named by	D.B. Dowling, 1900

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.^[2]

Lithology

Subdivisions

The Stony Mountain Formation is divided in the following sub-units: ^[1]

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.^[1] It reaches a maximum thickness of 45 metres (150 ft) 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 metres (100 ft).

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.^[1]