The Shwin Formation was named by Gilluly and Gates (1965) for a metamorphosed and structurally complicated sequence of rocks exposed in the Goat window area of the Roberts Mountains thrust near Shwin Ranch, about 4 miles southwest of Mount Lewis in the Shoshone Range.

Inarticulate brachiopod and "agnostid" trilobite faunas suggest a Middle Cambrian age for the Shwin Formation. Gilluly and Gates (1965) suggest a correlation of the Shwin Formation with the Secret Canyon Shale in the Eureka District on the basis of both faunal and lithologic similarity.

The Shwin is composed of chloritic argillite and phyllite, massive diabase and greenstone, black calcareous slate and mudstone, mottled shaly limestone, and calc-phyllite (Gilluly and Gates, 1965). The lithologic sequence is poorly understood. The formation is isoclinally folded and complexly faulted with beds sheared out or structurally interlayered along thrusts. Gilluly and Gates (1965) report that greenstone and chloritic phyllite make up much of the lower portion of the formation, and gray thin-bedded calcareous mudstone and shale, mottled shaly limestone and shale-pebble conglomerates are in the upper portion of the formation.

Gilluly and Gates (1965) estimated a thickness of 1,500-2,000 feet for the Shwin Formation although all formational contacts are faults with the possible exception of the contact of the Shwin with the Eldorado Dolomite which although sheared, may be conformable.

The Shwin is only exposed within the central Shoshone Range.

The Shwin is a transitional unit representing intimate interlayering of original diabasic volcanics now metamorphically recrystallized as argillites and phyllites, and carbonate-shale-conglomerate lithologies deposited in a continental slope setting. The preservation of laminae in siltstone, mudstone and shale units as well as abundant organic matter and authigenic pyrite, and preserved "agnostid" trilobites suggest relatively deep water deposition, while the mottled texture in the limestones suggests the presence of a burrowing infauna (Gilluly and Gates, 1965).