The Sevy dolomite (500-1,600 feet thick) unconformably overlies Laketown dolomite (Middle Silurian) and is tentatively correlated with the Lone Mountain formation of central Nevada. It is aphanic and homogeneous and represents very slow deposition near sea-level. Halysites is found near the base. In a belt marginal to the miogeosyncline during Sevy time, Silurian and Ordovician dolomites and quartzites were eroded. The widespread sandy member at the top of the Sevy is derived from erosion of the Eureka (Middle Ordovician) quartzite.
The Simonson dolomite (500-1,600 feet thick) is characterized by heterogeneous types of dolomite strata, pronounced lamination, conspicuous mottling, and erosional disconformities. It contains stromatoporoid biostromes and bioherms, tubular corals and bryozoans, and Middle Devonian brachiopods including Stringocephalus. Periodically the sea floor was exposed and subjected to erosion and reworking of the sediments. Toward the west limestone becomes prominent, and the Simonson is correlated with the Nevada formation. The upper contact of the Simonson is a narrow zone of interbedded dolomite and limestone which passes upward into a thick limestone section.
The evidence of increasing depth of water from the Sevy to the limestone above the Simonson is believed to represent eastward transgression of the Devonian sea.
In eastern White Pine County the miogeosyncline subsided less than in the Confusion basin at the south and southeast or the Roberts Mountains basin at the west. This is known as the North Snake Range “positive area.” Simonson sediments are proved to converge over this area which persisted during Sevy and Simonson time.