There are four Sub-units of the Three Forks Group, a stratigraphical unit of the Famennian age in the Williston Basin between Canada and the United States.
The Three Forks Group is a stratigraphical unit of Famennian age in the Williston Basin. Geologic group Western Canadian Sedimentary Basin, Williston Basin Canada
United States 
Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. The Famennian is the latter of two faunal stages in the Late Devonian epoch. 
The Williston Basin is a large intracratonic sedimentary basin in eastern Montana, western North Dakota, South Dakota, and southern Saskatchewan, that is known for its rich deposits of petroleum and potash. The basin is a geologic structural basin but not a topographic depression; it is transected by the Missouri River. 
