Great Falls is underlain by the Lower Cretaceous Kootenai Formation (Aptian), a series of red sandstones and shales which is considered to have been deposited in a "mostly" nonmarine environment. The city is on a crest of a broad northwest-plunging anticline called the South Arch, which lies just east of the Montana Thrust Belt. As we drive southwestward towards the Montanta Thrust Belt today, we will travel off the northwest flank of the South Arch. This will take us gradually from Lower Cretaceous to Upper Cretaceous outcrops until we reach the edge of the mountain front.
Key Lithologic Features
Cretaceous sedimentary rocks (Kootenai and Blackleaf Formations)
Tertiary Adel Mountains volcanic field
Square Butte, Shaw Butte, Crown Butte, and Cascade Butte: Tertiary igneous sills composed of layered syenogabbro
mafic dikes associated with the Adel Mountains cutting Two Medicine Formation
dikes
Three Sisters volcanic stock and associated dikes
volcanic breccias: trachybasalts, trachyandesites, and latites
Mississippian (Madison Fm. and Snowy Group), Jurassic (Ellis Fm.), and Cretaceous (Kootenai Fm.) sedimentary rocks associated with the Craig Thrust fault
Structures
Craig Thrust with drag folds
Craig anticlinorium
Landforms
Square Butte, Shaw Butte, Crown Butte, and Cascade Butte
right border caption]Click for route map