The Ford Creek area is located in the Sawtooth Range, 20 mi southwest of Augusta, Montana, and covers an area of about 38 sq mi. Exposed sedimentary formations total about 5,860 ft in thickness and are mainly Precambrian to Cretaceous in age. Igneous rocks of probable pre-Laramide age are represented by two sills of quartz-dioritic composition.

The oldest rocks, thin-bedded red argillites at the top of the Belt Supergroup, have an exposed thickness of about 400 ft. Approximately 1,800 ft of Middle and Upper Cambrian rocks, consisting of a basal sand, shale and thin- to massively bedded carbonates disconformably overlie the red argillites. The Devonian System consists of about 1,000 ft of carbonate rocks that disconformably overlie the Cambrian System. The Madison Group of Mississippian age is about 1,300 ft thick and consists of a lower limestone unit and an upper dolomite unit. Three hundred and twenty feet of marine Middle and Upper Jurassic rocks of the Ellis Group lie disconformably on the Mississippian rocks and are overlain by 195 ft of non-marine Upper Jurassic rocks of the Morrison Formation and 900 to 1,000 ft of non-marine Lower Cretaceous rocks of the Kootenai Formation. Quaternary glacial and stream deposits occur in valleys occupied by major drainages.

Normal and thrust faults are the major geologic structures in the map area. All faults trend northwest and dip to the southwest. The southwestern area is characterized by broad, gentle and nearly symmetrical folds in Precambrian and Paleozoic rocks, and by three thrust faults. The central closely folded area is characterized by normal and thrust faults, and by tight, overturned and doubly plunging folds in Cretaceous and Jurassic rocks: four thrust faults and two normal faults occur in this area. The northeastern area is characterized by two thrust faults and by gentle to overturned folds in Mesozoic rocks.

Three phases of Laramide deformation are recognized in the map area: (1) an early compressional phase which produced folding, followed by imbricate thrusting, (2) a phase of relaxation of compressive forces which resulted in normal faulting, and (3) a renewed compressional phase which produced thrusts that overrode earlier developed structures. A later phase of large-displacement normal faulting recognized to the west and northwest is not represented in the map area.

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