The central part of Apache County, Ariz., includes an area of about 3,300 square miles between the Navajo Indian Reservation to the north and U.S. Highway 60 to the south. Sedimentary rocks in the area range from Pennsylvanian to Quaternary in age and from 2,000 to more than 6,000 feet in thickness. The strata were tilted to the northeast, and part of the Upper Triassic and all the Jurassic and Lower Cretaceous rocks were eroded away before strata of Late Cretaceous age were deposited. Basaltic lava flows and cinder cones, representing four general periods of eruption in late Miocene to Quaternary time, are widespread in the southern part of the area. Pennsylvanian and Permian rocks overlie basement rocks of granite and diorite and include the Supai Formation, the Coconino Sandstone, and the Kaibab Limestone. The Supai Formation is 1,000 to 2,000 feet thick and consists of interbedded red and brown mudstone, siltstone, sandstone, limestone, and evaporites. It contains water of very poor quality outside Apache County. The Coconino Sandstone is 200 to 250 feet thick and consists of light-gray fine- to medium-grained sandstone. It contains water suitable for domestic use in the south and water unsuitable for most purposes in the north. The Coconino Sandstone underlies all Central Apache County in the subsurface. The yellowish-gray to dark-gray Kaibab Limestone is present in the southern two-thirds of the area and is 0 to 350 feet thick. It contains water where it is fractured and combines with the Coconino Sandstone to form a single hydrologic unit that yields from 6 to 74 gpm (gallons per minute) of water per foot of drawdown. An unconformity Heparates the Permian rocks from the overlying Triassic rocks, which comprise the Moenkopi and Chinle Formations and the Wingate Sandstone. The Moenkopi Formation is 35 to 250 feet thick and consists of intercalated brownish-red siltstone, sandstone, and conglomerate. It contains salty water in some areas but is dry in most. The Chinle Formation is 0 to 1,550 feet thick and unconformably overlies the Moenkopi. The Chiule consists of multicolored claystone, mudstone, siltstone, sandstone, and conglomerate. Some of the sandstone units yield small amounts of water, usually of a quality unsuitable for domestic use. The Wingate Sandstone is about 250 feet thick and is present only in the extreme northeastern corner of the area. It consists of intercalated, reddish-brown sandstone and siltstone and does not contain water. The Upper Cretaceous rocks comprise the Dakota Sandstone, from 50 to 115 feet thick; the Mancos Shale, about 150 feet thick; and the Mesaverde Group, as much as 200 feet thick. These rocks consist of yellowish-gray, light-green, and reddish-brown sandstone and carbonaceous siltstone. Some of the sandstone units contain water of suitable quality for domestic use, and wells in these units yield from 10 to 1,000 gpm. Sedimentary rocks of Eocene(?) age are about 800 feet thick and unconformably overlie Cretaceous rocks. They consist of light-brown and medium-red conglomerate, sandstone, and siltstone. These sedimentary rocks contain small amounts of water suitable for domestic use and yield from 10 to 25 gpm in the Springerville area. The Datil Formation of Miocene(?) Tertiary age consists of more than 800 feet of sedimentary rocks, which are composed largely of volcanic fragments. The Datil Formation does not contain water in the one small area where it crops out. The Bidahochi Formation of Pliocene age consists of 0 to 800 feet of white, green, and brown claystone, mudstone, and sandstone. Locally it yields from 10 to 50 gpm of water suitable for domestic use. Quaternary rocks consist of as much as 500 feet of alluvium, sand, gravel, travertine, cinders, and lava. The alluvium along the large drainages contains water that differs in quality from place to place. In most areas where it occurs, the lava