Upper Cretaceous aquifer

The major aquifers of the Northern Great Plains aquifer system are sandstones of Tertiary and Cretaceous age and carbonate rocks of Paleozoic age. These aquifers, along with regional confining units that separate some of them, form one of the largest confined aquifer systems in the United States. The Upper Cretaceous aquifers extend throughout much of the area, but are principally in the subsurface, where they contain mostly saline water.