Nathlie Mountain olivine basaltic andesite flows and fragmental rocks (Pleistocene). Upper part consists of sequence of agglomerate, lapilli tuff, hyaloclastite, pillow breccia, and both pillowed and nonpillowed lava flows. Both tuff and matrix of fragmental rocks are generally palagonitized. Lower part consists chiefly of sequence of medium-thickness to thick (10-90 m), dark-gray to greenish-black flows that locally exhibit highly developed columnar joints. Single flow of hornblende dacite is present low in section. Lavas from both upper and lower parts of unit are chiefly aphyric to strikingly porphyritic olivine basaltic andesite. * * * Source of dacite flow is apparently local; olivine basaltic andesite lavas may all have originated at Mount Sanford.
Geologic map of the Gulkana A-1 Quadrangle, south-central Alaska