Big Snowy Group

The Big Snowy Group is a stratigraphical unit of Chesterian age in the Williston Basin.

It takes the name from Big Snowy Mountains in Montana, and was first described on the north slopes of the mountain by H.W. Smith in 1935.

Subdivisions
The Big Snowy Group is composed of three subdivisions, from top to base:
 * Heath Formation: black shale with sandstone lenses.
 * Otter Formation: limestone and grey to green shale
 * Kibbey Formation: shaly sandstone

Distribution
The Big Snowy Group reaches a maximum thickness of 135 m in the Williston Basin. It is exposed in outcrop in the Big Snowy Mountains, Little Belt Mountains, Castle Mountains and Lombard Hills of central Montana. It occurs in the sub-surface throughout the central part of the Williston Basin and into a limited area of south-central Saskatchewan.

Relationship to other units
The Big Snowy Group is unconformably overlain by the Tyler Formation in Montana, and by the Watrous Formation in Saskatchewan; It disconformably overlays the Madison Group.