The Gilmore Gulch Formation was named for sediments exposed along Gilmore Gulch, a tributary of Tybo Canyon in the Hot Creek Range (Ferguson, 1933).

The Gilmore Gulch Formation is less than 30 Ma in age and is assigned an Oligocene age (Kleinhampl and Ziony, 1985).

The Gilmore Gulch Formation is composed of lenticular units of platy to blocky, grayish-brown to yellowish-brown, tuffaceous sandstone and siltstone, dark-gray shale, medium-gray thin-bedded to locally laminated and crenulated limestone, grayish-orange phenocryst-poor and shard-rich, partially welded, devitrified tuff, and minor conglomerate. Local dacitic to andesitic tuffaceous sandstone, and pebble and cobble conglomerate and bedded tuffs are present within the Gilmore Gulch (Kleinhampl and Ziony, 1985). These sediments are up to 2,000 feet in thickness and contain an 800 foot thick member of yellowish-gray devitrified and flow layered dacite in the Tybo Quadrangle (Quinlivan and Rogers, 1974).

The Gilmore Gulch Formation is 400 to 500 feet thick at the type section along Tybo Canyon (Ferguson, 1933), about 200 feet in the Moores Station area (Ekren and others, 1973), and are locally 2,000 feet thick in the Tybo Quadrangle in the Hot Creek Range (Quinlivan and Rogers, 1974).

The Gilmore Gulch has only been mapped within the Hot Creek Range.

The depositional setting of the Gilmore Gulch is poorly understood and documented. These sediments probably represent intertonguing fluviatile and lacustrine units deposited with water-lain and ash-flow tuffs, very similar to the Miocene Humboldt Formation.

In the Carlin-Pinon Range and Elko areas, the Oligocene sedimentary section is represented by the Indian Well Formation, which is composed of tuffaceous sedimentary rocks interbedded with tuff and ash-flow tuffs. The Indian Well Formation is described below.