North of Government Peak, a high angle east–west fault passing through
Hatcher Pass separates plutonic rocks on the north side of the fault from
schist south of the fault. The Hatcher Pass schist consists mainly of metamorphosed and deformed sedimentary rocks, of Late Cretaceous to Paleocene age. Both deformed and undeformed small felsic dikes occur in the schist. Several bodies of serpentinite are contained within the schist. West of Government Peak, the Arkose Ridge Formation lies to the south of the schist and the contact is a low-angle detachment fault. The Arkose Ridge Formation is unmetamorphosed, tilted and slightly folded, and consists of
Tertiary sedimentary
arkose, shale, sandstone, and conglomerate rocks that are clearly derived from the schists and intrusive rocks to the north. Locally, quartz diorite or gneiss bodies occur on the arkose-schist contact west of Government Peak. Minor basalt flows occur within the Arkose Ridge Formation. On Government Peak, a 90 Ma (million years old) rock unit variously mapped as gneiss, or as intricately intermixed amphibolite and quartz diorite, or as a migmatite, and most recently as a mafic unit of gabbro and pyroxenite with local foliation and mylonitic textures structurally underlies the Arkose Ridge Formation and outcrops in between the Hatcher Pass schist and the arkose. South-dipping low angle detachment faults separate the units. East of Government Peak and the Little Susitna River the mafic rock unit is in contact with the Arkose Ridge Formation to the south and the felsic plutons to the north. ==See also==