The Plympton Formation was named by Hose and Repenning (1959) for rocks originally assigned to the Phosphoria Formation at Plympton Ridge in the central Confusion Range, about 22 miles north of U.S. Highway 6.

The Plympton is Permian in age and probably contains the Leonard-Guadalupe boundary (Collinson, 1968). At its type locality, it conformably overlies the Kaibab and is overlain by the Gerster Formation. In the Pequop, Cherry Creek and Ferguson Spring Mountains it overlays the Loray Formation and is overlain by the Phosphoria Formation, and in the Butte Mountains the Plympton rests on the Loray and underlies the Gerster Formation (Steele, 1960). According to B. Wardlaw and J. Collinson ( in Coats, 1985), the Plympton and Gerster Formations may intertoungue with one another over a broad area which includes the Butte Mountains, Medicine and Cherry Creek Ranges, and Spruce Mountain Quadrangle.

The Plympton Formation is dominantly composed of faintly laminated, yellowish-gray to rust brown weathering, fine-grained dolomite, and light milky gray, brownish weathering chert in beds or nodules, with thin interbeds of light yellowish-gray siltstone (Steele, 1960; Hose and Blake, 1976). Light yellow and reddish fine-grained sandstones are reported in the upper portion of some sections in White Pine County (Hose and Blake, 1976). Most of the Plympton is unfossiliferous and is lithologically diverse on a regional scale, although the five lithologic units divided at the type section by Hose and Repenning (1959) can often be distinguished.

In northeastern Nevada, two lithologic units which form the basal third of the formation can be regionally recognized (Coats, 1985). The basal unit is composed of dark-gray and brown, phosphatic chert and mudstone that may be equivalent to the Meade Peak Phosphatic Shale Member of the Phosphoria Formation. Overlying the basal unit is a sequence of massive, fine-grained, silty, light-gray crystalline dolomite, and interbedded dark-gray microcrystalline dolomite and minor dolomitic limestone (Coats, 1985). Chert nodules are common near the top of this unit. The upper two-thirds of the Plympton are lithologically variable from section to section, with massive bedded chert and yellow-gray dolomite interbedded with sandstone, siltstone, breccias, gypsum, and chert-granule conglomerate (Hose and Repenning, 1959; Coats, 1985).

In the Butte Mountains, Steele (1960) divided the Plympton into two members. The lower member is about 184 feet of light brown-gray to light yellowish-orange, silty limestone and calcareous siltstone which contain abundant well-rounded gray, green, red and black chert fragments. Steele (1960) correlated this lower member with the second member of the Garden Valley Formation as described by Nolan and others (1956). Unfortunately Steele (1960) did not provide a description of the upper member of the Plympton. Bissell (1964) described the Plympton in the Robbers Roost area of the Butte Mountains as several hundred feet of aphanitic to sucrosic, light tan-gray dolomite, calcareous dolomite, and dolomitized encrinal limestone. Sides (1966) described the Plympton in the central Butte Mountains as about one-third chert, and the remainder of dolomite and dolomitic limestone. The basal 110 feet of the unit are interbedded yellow, sucrosic dolomitic limestone, and thin to medium beds of chert. This is overlain by about 60 feet of medium-bedded to massive, coarsely crystalline, yellow limestone with nodular chert. The upper 340 feet of the formation are yellow to gray, thin to thick-bedded, fine to coarse-grained limestone and dolomitic limestone, and interbedded thin to medium-bedded stringers, nodules and irregular patches of gray to brown chert (Sides, 1966).

In the Medicine Range, Collinson (1968) described the Plympton as containing four units. The lower 75 feet is poorly exposed, silty, yellow weathering chert, overlain by about 115 feet of light-gray, calcareous dolomite with an upper 2 foot bed composed of chert granule conglomerate. The third unit is 190 feet of chert and interbedded phosphatic dolomite which may correlate with the Meade Peak Member of the Phosphoria Formation. The uppermost unit is composed of 125 feet of interbedded poorly exposed dolomite and chert.

In the Maverick Springs Range, the Plympton consists of light gray and brown, coarsely crystalline to locally micritic, cherty dolomite, and thin-bedded argillaceous limestone and medium to thick-bedded very cherty calcareous dolomite (Bissell, 1964).

At Phalen Butte northwest of Currie, the Plympton is about 300 feet of sandy and calcareous, brown-gray, aphanitic to coarsely crystalline dolomite, with a basal 20 foot unit composed of calcareous and dolomitic siltstone (Bissell, 1964). In the northern Cherry Creek Range, the Plympton is composed of 115 feet of thick-bedded to massive, red-brown and gray, bedded chert and cherty siliceous limestone and dolomite, overlain by 192 feet of light gray to pink, aphanitic dolomite and coarse-grained cherty dolomite (Bissell, 1964).

In the southern Pequop Mountains, the Plympton is composed of light gray, thin to thick-bedded dolomite, bedded gray and brown chert, and interbedded yellowish-gray dolomitic mudstone and reddish-yellow-weathering siltstone (Yochelson and Fraser, 1973). Bissell (1964) described several hundred feet of Plympton between Indian and Brush Creeks in the southern Pequop Mountains as light gray to brown, aphanitic to coarsely crystalline dolomite, sandy dolomite, and dolomitic limestone with abundant gray and brown chert nodules stringers and lenses. Snow (1964) described the Plympton in the Dolly Varden Mountains as about 50 feet of tan to milky-white, medium-bedded chert.

In the Ferguson Mountain area near the southern end of the Goshute Mountains, Berge (1960) has mapped the Plympton as light gray, coarse-grained and crystalline dolomite which forms massive cliffs. In the Spruce Mountain Quadrangle, the Plympton is described as light gray, fine-grained cherty dolomite with thin beds of chert-granule conglomerate (Hose, 1972; Coats, 1985). In the southern Schell Creek Range (T. 12 N., R. 65 E.) the Plympton is less than 100 feet of white to light gray, aphanitic and sucrosic dolomite and calcareous dolomite (Bissell, 1964).

Steele (1960) measured about 983 feet of Plympton in the Butte Mountains, and Bissell (1964) reports thicknesses of 438 feet near Summit Springs, and 190 feet in the Robbers Roost area of the Butte Mountains. 515 feet were measured by Sides (1966) in the central Butte Mountains. Collinson (1968) measured about 510 feet in the Medicine Range, and Bissell (1964) measured two sections of 273 and 435 feet in Medicine Range. It is 270 feet thick in the Maverick Springs Range (Bissell, 1964), 307 feet in the Cherry Creek Range (Bissell, 1964), 330 feet at Phalen Butte northwest of Currie (Bissell, 1964), 307 feet in the northern Cherry Creek Range (Bissell, 1964), 238 feet at Ferguson Mountain (Berge, 1960), and over 500 feet in the Spruce Mountain Quadrangle (Hose, 1972). It is 1,000 to 1,200 feet in the southern Pequop Mountains according to Yochelson and Frazer (1973), and about 444 feet between Brush and Indian Creeks according to Bissell (1964). About 74 feet of Plympton Formation are present in the southern Schell Creek Range in T. 12 N., R. 65 E. (Bissell, 1964).

The Plympton has been identified at Phalen Butte northwest of Currie, in the Maverick Springs, Medicine, northern Cherry Creek, and southern Schell Creek Ranges, southern Pequop and Butte Mountains, Ferguson Mountain, and the Spruce Mountain Quadrangle. It has roughly the same distribution as the underlying Kaibab Formation.

Yochelson and Fraser (1973) interpreted the Plympton within the southern Pequop Mountains to be shallow-marine outer shelf sediments which formed peripheral to low-lying islands, and included beach and tidal flat deposits. Wardlaw (1975) suggested that the Plympton was deposited in essentially a supratidal sabkha-like environment with occasional intertidal and subtidal depositional conditions.