Geologic mapping in the Disturbed Belt of northern Lewis and Clark County, Montana, has revealed major northeast-southwest trending facies changes within the Upper Cretaceous Montana Group. Typically, from oldest to youngest, the Group comprises the Telegraph Creek, Virgelle, Two Medicine, Bearpaw, Horsethief, St. Mary River, and Willow Creek Formations, aggregating about 5,000 feet in thickness.

The basal formations of the Montana Group, the Telegraph Creek and Virgelle, are regressive shallow-marine and lagoonal feldspathic sandstones and shales rising steeply across time lines north and east. Overlying them with gradational contact is a lagoonal facies of sandstone and shale of the Two Medicine Formation, about 550 feet thick and equivalent to the upper Eagle Sandstone.

The herein-defined Big Skunk Formation, 2,100 feet of volcanic-rich, arkosic sedimentary rock, disconformably overlies the Eagle equivalents of the Two Medicine and fills the upper two-thirds of the Two Medicine stratigraphic interval along the western margin of the Disturbed Belt. The Big Skunk includes from bottom to top, members A through E, which are, respectively: a basal sequence of grayish red, volcanic-rich, sedimentary breccia and mudstone; an ash-flow tuff 121 feet thick; variegated, volcanic-rich mudstones, shales, and sandstones; gray to black mudstones and conglomeratic sandstones; and gray-green mudstones and sandstones interstratified with tuffs. Stream deposition in a piedmont environment predominated; the source vents were probably a few miles westward. Northeastward he Big Skunk thins to a tongue of volcanic-rich strata, underlain by 1,125 feet and overlain by 125 feet of floodplain-deposited Two Medicine shale and mudstone. These overlying Two Medicine strata pinch out southwestward.

As the southern edge of the Bearpaw Shale lies north of Lewis and Clark County, the marine Horsethief Formation disconformably overlies Two Medicine along the eastern margin of the Disturbed Belt. Marine Horsethief sandstones grade southwestward to a brackish-water facies and then to lagoonal mudstone and shale composing the basal St. Mary River Formation. Reworking of a volcanic-conglomerate facies formed magnetite-rich sandstones capping the Horsethief.

Undifferentiated strata of the St. Mary River and Willow Creek Formations overlie the Horsethief Formation along the eastern edge of the Disturbed Belt and the Big Skunk Formation along the western edge. This undifferentiated unit includes a basal lagoonal facies, a mudstone-shale floodplain facies, and an upper sandstone facies all aggregating at least 3,730 feet in thickness and all rising northeastward across time lines.

The Montana Group in northern Lewis and Clark County, Montana, is essentially a large clastic wedge, 6,550 feet thick and regressive north and east. The sedimentary detritus was eroded largely from volcanic source terranes. The active period of vulcanism represented by the Big Skunk Formation is time-correlated with the Elkhorn Mountains vulcanism of central Montana, and should be distinguished from the nearby Adel Mountain vulcanism, which is post-Horsethief and pre-Eocene thrust faulting in age.