Unit is named the Nahahum Canyon Member of the Chumstick Formation (upper part). Replaces the shaly sandstone and shale of Whetten and Laravie (1976). Is exposed on the northeast side and northwest end of the Chiwaukum graben in the central Cascade Range of WA. Consists of fine-grained shaly sandstone and shale containing thin laminae of biotite and organic matter, suggestive of quiet-water deposition in a shallow lake environment. Conformably overlies the lower part of the Chumstick Formation; grades laterally into sandstone and conglomerate of the Chumstick Formation near the Entiat fault and near the core of the Eagle Creek anticline; unconformably underlies the Wenatchee Formation (new). Thickness is about 2000 m. Age is middle(?) Eocene based on a radiometric (zircon) age of about 45 Ma for tuff in the lower part of the Chumstick Formation and an age of 34 Ma for volcanic rocks in the Wenatchee Formation.

Source: US geologic names lexicon (USGS Bull. 1565, p. 216); GNU records (USGS DDS-6; Menlo GNULEX).