The Sevy is typically fine crystalline to microcrystalline tan and gray dolomite which weathers with a distinctive very light gray surface. It occurs in beds from a few inches to 2 feet thick and is usually finely laminated. Sedimentary breccias, and soft-sediment deformations are common. Floating quartz sand grains occur throughout the formation but are usually concentrated near the top. Carbonate sand grains are associated with the quartz grains.
The lithogenic unit is remarkable in being recognizable through an area of 100,000 square miles in California, Nevada, Utah, and Idaho. Its average thickness is 500 feet. Throughout most of its extent, which corresponds with the Cordilleran miogeosyncline, the Sevy overlies Silurian dolomite and is overlain by middle Devonian dolomite.
It is proposed that the Sevy originated as a primary evaporitic dolomite with redeposited dolomite debris and was formed on extensive mud flats at or near sea-level. These flats were periodically flooded by the issue of streams from the east, and depressions on the flats were filled with water.
Marine invasions from the sea on the west were blocked or impeded by the possible existence of a barrier inherited from a Silurian barrier-reef trend in central Nevada.
The quartz sand is believed to have been derived from the erosion of middle Ordovician sandstones along the eastern margin of the depositional area. Streams carried the sand to the edge of the mud flats. Later, the sand grains were probably blown across the periodically emergent flats and entrapped in shallow pools or areas of unhardened sediment.