Named derived from Heppsie Mountain, Jackson Co, OR, where andesite is well exposed. Consists of andesitic flows that are generally thick-bedded, massive though locally slightly platy, gray to blue-gray porphyritic rocks with microcrystalline groundmass. Occurs above red agglomeratic andesitic horizon that delimits top of tuff of Wasson formation (new) (Little Butte volcanic series). Flows extend out of quad for many mi to base of basalts of Pliocene and Pleistocene age; within quad they are typical "andesites of the Western Cascades" of other writers. Age is Miocene(?) [on basis of stratigraphic position].

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Mapped with Rhododendron formation, Sardine series of Thayer (1936), Fern Ridge formation of Thayer (1933) on basis of lithology in western OR. Map give age for all units as middle(?) and late Miocene.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Pg. 21. Heppsie Andesite. Mehama Volcanics, Wasson Formation, and Heppsie Andesite are included in Little Butte volcanic series and make up the bulk of Western Cascades south of McKenzie River, in southern Oregon. Heppsie Andesite [of Wells (1956, USGS Geol. Quad. Map GQ-89)] occurs in vicinity of Medford. Is occasionally considered as a southward extension of Sardine Formation of Peck and others (1964, USGS Prof. Paper 449). It is a thick-bedded, locally platy, gray, porphyritic andesite with phenocrysts of pyroxene, hornblende, and feldspar. Unconformably(?) overlies the red, agglomeritic andesite horizon of the Wasson Formation (upper Little Butte Volcanics); underlies Pliocene basalts. Age is Miocene, based on stratigraphic position.

Source: Publication; supplemental information from GNU records (USGS DDS-6; Menlo GNULEX).

Heppsie Andesite conformably overlies Wasson Formation. Is thick sequence of andesite and basalt flows, interbedded pyroclastic and water-laid tuff breccia similar to Roxy Formation. Basal flows in its type area are about 22 Ma.

Source: GNU records (USGS DDS-6; Menlo GNULEX).