The Harmony Hills Tuff is named for exposures in the Mount Stoddard Quadrangle of the Harmony Hills, Iron County, southwestern Utah (Mackin, 1960).

The Harmony Hills Tuff is Miocene in age. Radiometric age dating gives an age of 20 to 24 Ma (Ekren and others, 1977).

The Harmony Hills Tuff is a pink to purple or grayish-red lightly to moderately welded dacitic or quartz latitic crytal-rich tuff, which is often composed of two or three separate ash-flow tuff sheets (Cook, 1965). Grayish-pink pumice lapilli and lumps are obvious within the tuff, and locally are as large as footballs (Ekren and others, 1977). Biotite books 4 mm in diameter and 3 mm thick are abundant and characteristic of the Harmony Hills unit. Phenocrysts typically make up about 30 to 35 percent of the tuff and are composed of 70 to 74 percent plagioclase, 15 percent biotite, 2 to 10 percent quartz, 1 to 8 percent clinopyroxene, 2 percent orthopyroxene, a trace to 5 percent hornblende, and a trace of alkali feldspar (Ekren and others, 1977).

Cook (1965) estimated the average thickness of the Harmony Hills Tuff to be about 250 feet, and Ekren and others (1977) report a range in thickness from 100 to 300 feet for various exposures in Lincoln County.

The Harmony Hills Tuff is exposed primarily to the south and west of the evaluation area. It is present in the Pioche Hills, and Wilson Creek Range.