The Tongue River Member of the Tertiary Fort Union Formation is the primary source of groundwater in the Northern Cheyenne Indian Reservation in southeastern Montana. Coal beds within this formation generally contain the most laterally extensive aquifers
Tuck, L.K.,Pearson, Daniel K.,Cannon, M.R.,et al. Estimates of the volume of water in five coal aquifers, Northern Cheyenne Indian Reservation, southeastern Montana. 2013-01-01.