Book excerpt: The Jefferson Formation (upper Devonian) comprises a thick lower member and the Birdbear Member. The overlying Three Forks Formation (Devonian and Mississippian) comprises the Logan Gulch, Trident, and Sappington Members. These members are described at the classic type section of the Jefferson and Three Forks Formations near Logan, Mont., and their distribution in southern Montana and northern Wyoming is shown. The members are correlated with stratigraphic units adjoining parts of Montana. Idaho, Wyoming, North Dakota, and Canada.