The Madison Group of Mississippian age comprises, from oldest to youngest, the Lodgepole and Mission Canyon Limestones and the Charles Formation. The Madison crops out in the Bighorn and Pryor Mountains and in the mountains west of the study area. These rocks consist of cyclically deposited normal-marine carbonates and restricted-marine carbonates and evaporites. The Madison ranges in thickness from about 700 feet (210 metres) in the Bighorn Mountains to about 2,000 feet (610 metres) in the Williston basin. The top of the Madison ranges in depth from land surface in the mountains to about 10,000 feet (3,000 metres) below land surface along the MontanaWyoming border. The potentiometric surface of water in the Madison Group slopes northeastward from the outcrops. Potentiometric lows occur in the Cat Creek anticline-Porcupine dome area along the Bighorn River and at the north end of Powder River basin. Potentiometric highs occur in outcrop areas, on the interstream divide between the Bighorn, Tongue, and Yellowstone Rivers, and along the Cedar Creek anticline. The potentiometric surface ranges from 1,200 feet (370 metres) below land surface in the topographically high areas in the southern part of the area to 1,000 feet (300 metres) above land surface along the Yellowstone River. Yields from wells range from about 50 gallons per minute (3 litres per second) at several places to a reported 1,400 gallons per minute (88 litres per second) from a flowing well on the north side of the Porcupine dome. Yields estimated or reported from drill-stem tests range from about 1 to 157 gallons per minute {0.1 to 9.9 litres per second). Water from the Madison generally contains less than 3,000 milligrams per litre dissolved solids south of T. 1 N., and from 3,000 to 10,000 milligrams per litre in most of the remaining area; in the Williston basin the concentration increases to more than 100,000 milligrams per litre. In the southeastern and southwestern parts of the area, calcium, magnesium, and sulfate ions constitute more than 75 percent of the dissolved constituents, in milliequivalents per litre. North of about T. 8 N., sodium, potassium, and chloride ions constitute more than 50 percent of the dissolved constituents; in the Williston basin, sodium, potassium, and chloride ions constitute more than 75 percent of the total.