The water table in North Dakota ranges between 15 and 50 feet deep, or 100 feet in upland areas. Fully wet areas are recognized in drill cores by unoxidized dark material. The lignite and sandstone of the Fort Union Group in the west is the top groundwater unit and a common source of water for farms and ranches. The western two-thirds of the state is underlain by an extensive sandstone aquifer, mainly in the Fox Hills Formation and the lower Hell Creek Formation. Although it is close to the surface in the east, it slopes to up to 2000 feet below the ground surface in south-central North Dakota. Below this unit is fine-grained Cretaceous shale and sandstone that produce almost no water. The underlying Kara Formation limestones are beneath almost the entire state. Known as the Dakota aquifer, it is commonly tapped for artesian wells.
