Named for Harts Pass, sec.7 T37N R18E [48 deg 43'15"N, 120 deg 40'08"W, a 6198-ft pass in Cascade crest on Whatcom and Okanogan Cos line, Slate Peak 7.5' quad, WA]. Type area designated as terrane occupied by formation within 3 mi northwest and southeast of Harts Pass. Divided into 3 members: lower is massive bedded arkose with minor shale interbeds; middle is equal parts of arkose and black shale; upper is massive bedded arkose with minor black shale. Type section constructed from different exposures within type area yields thickness of members in ft: upper, 2200 ft, middle 2500 ft, and lower, 3200 ft; total of 7900 ft. Is weakly to nonmetamorphosed. From its southeastern termination by a cross fault on east flank of Silver Star Mountain, unit extends northwest 18 mi to northern limit of geologic map of Methow Valley area. Conformably overlies Panther Creek Formation (new) and is unconformably overlain by Virginian Ridge Formation. Tentatively correlated with upper part of Jackass Mountain Group in Manning Park area, British Columbia. Ammonites from upper member are Aptian, probably late Aptian [Early Cretaceous}.

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