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5.10 GEOLOGY
5.10.1 Regional Geology
The Bear Lodge Mountains of north-eastern Wyoming are part of the Black Hills Uplift, formed
during the Late Cretaceous-Tertiary Laramide Orogeny. The uplift has a north-westerly
orientation and extends from the western South Dakota – Nebraska border through north-
eastern Wyoming into south-eastern Montana. The exposed basement consists of Precambrian
schist, gneiss, and granite overlain by Paleozoic and Mesozoic clastic and carbonate
sedimentary rocks that were subsequently eroded from higher elevations. The Paleozoic and
Mesozoic rocks were subjected to large-scale monoclinal folding that encircles the Black Hills
Uplift. Younger Oligocene, Miocene, and Pliocene sediments disconformably overlie the older
sedimentary and igneous rocks at lower elevations of the uplift (see Figure 5.10-1).
Figure 5.10-1
Geologic Setting and General Geology of Bear Lodge Mountains
(From Roche, 2012)
Tertiary alkaline intrusive bodies in the northern Black Hills lie within a N70-80W trending belt
that extends from Bear Butte in South Dakota, through the Bear Lodge Mountains, to Devil’s
Tower and Missouri Buttes in north-eastern Wyoming (see Figure 5.10-1). The alkaline igneous
rocks generally transition in composition from silica-saturated to silica-undersaturated from east
