The Hay Ranch Formation was named for exposures at the Hay Ranch in sec. 16 and 20, T. 28 N., R. 52 E., on the eastern flank of Pine Valley (Regnier, 1960).

The Hay Ranch Formation is Middle Pliocene to Middle Pleistocene in age. Fission-track ages on pearlite in the formation are 0.6 +/- 1 Ma to 1.9 +/- 0.1 Ma (Smith and Ketner, 1976).

The Hay Ranch Formation is composed of white to gray vitric ash and tuff, clay, minor limestone, tuffaceous siltstone and sandstone which interfinger with coarse pebble to boulder conglomerates and fanglomerates nearer to the range front. The ash and tuff are commonly altered to zeolites such as heulandite and erionite (Regnier, 1960). The clays in the Hay Ranch are in massive green beds a few feet to 25 feet thick. They are locally silty, gypsiferous or zeolite-rich (Regnier, 1960). The limestones in the Hay Ranch Formation are massive, dense, light gray to white, and are locally gastropod-rich. The conglomerates are graded channel-fill units with pebbles and cobbles of limestone, and gray, black and green chert. Fanglomerates with boulders up to 6 feet across are locally present in the formation along the front of the Pinon Range (Smith and Ketner, 1976). The siltstones in the Hay Ranch are brown to light gray, fine to medium-grained, tuffaceous and locally cross-bedded.

The Hay Ranch Formation is at least 1,300 feet thick in Pine Valley and always dips less than 15 degrees (Smith and Ketner, 1976).

The Hay Ranch Formation is present in Pine Valley and in a small exposure to the east near Cedar Ridge along the eastern flank of the Pinon Range (Smith and Ketner, 1976; Regnier, 1960). Thin and poorly exposed portions of the Hay Ranch may have been locally mapped with Quaternary alluvium in Elko County (Coats, 1985).

The Hay Ranch Formation represents a fluviatile and lacustrine basin-fill sequence. The limestones in the Hay Ranch represent lacustrine deposition with periodic influx of water laid tuff and ash, and fluvial influx of gravel, silts and sands.