The Ninemile Formation was named for exposures along Ninemile Canyon on the west side of the northern Antelope Range (Nolan and others, 1956).

The upper contact of the Ninemile with the overlying Antelope Valley Limestone is diachronous and gradational. The Antelope Valley is considered late Early Ordovician (Arenigian) in age at its type section, and Early to Middle Ordovician based upon conodonts, graptolites, trilobites, brachiopods and bryozoans in the northern Toquima Range (McKee, 1976b).

The Ninemile Formation is composed of fine-grained to porcellaneous, grey to olive-greenish-grey limestones, with gray to green interbedded calcareous shale, argillaceous to sandy limestones, and fine-grained calcareous quartz sandstones several inches in thickness (Nolan and others, 1956; Merriam, 1960). Brachiopods and trilobites are abundant. Both the upper and lower contacts of the Ninemile are gradational over an interval of about 50 feet. The formation commonly weathers to yellowish and brownish slopes with local low ledges and cliffs in the more calcareous lithologies (Kleinhampl and Ziony, 1985).

In the Toquima Range, McKee (1976b) considered the Stoneberger Shale of Kay and Crawford (1964) to be the Ninemile Formation. It is composed of contorted and sheared, olive-green to brown siltstone with shaly partings, and a few thin, buff, trilobite-bearing limestones, up to 4 inches in thickness, that are broken by faulting. In the Toquima Range, the Ninemile is often poorly exposed and marked by a soft brown soil horizon (Kay and Crawford, 1964; McKee, 1976b).

In the Hot Creek Range the Ninemile Formation contains an upper portion of gray-brown siltstone with nodular to lenticular limestone, and layers of very fine-grained, laminated to thick bedded, silty limestone (Quinlivan and Rogers, 1974). This silty upper portion grades downward into dark-gray to greenish-gray fissile shale, with thin layers of thin-bedded, silty and laminated limestone, and calcareous quartzose siltstone (Quinlivan and Rogers, 1974).

In the southern Grant Range, the Ninemile is composed of thin to medium-bedded, silty and argillaceous, aphanitic to coarsely crystalline, blue to black, yellow, orange and red-weathering limestone with sporadic chert nodules and stringers (Cebull, 1967). Interbedded with the limestone are dark gray to greenish fissile shale and gray calcareous siltstone.

Thicknesses in the Ninemile vary from 195 to 550 feet within the Monitor, Hot Creek, and Antelope Ranges, reflecting structural deformation (Kleinhampl and Ziony, 1985; Merriam, 1963). 500 to 700 feet of the Ninemile have been measured in the northern Toquima Range (McKee, 1976b). 1,400 feet are present in the Grant Range east of Railroad Valley, and 1,644 feet have been measured in the Horse Range, where the boundaries of the various formations of the Pogonip Group are interpretive and have apparently been chosen differently (Cebull, 1967).

The Ninemile Formation can be broken out within the Antelope, Monitor, Hot Creek, Toquima, Grant, and Horse Ranges.

Although detailed biostratigraphic analysis is lacking, the Ninemile appears to at least in part represent deposition in an embayment along the submerged shallow Ordovician shelf. This quiet and relatively deep water embayment, occupied the area from the Toquima Range on the west, to at least as far east as the Monitor and Antelope Ranges (Ross, 1977).