The main divide is granitic batholith. Northeast-trending belts of downfaulted and eroded Tertiary rocks lie on the southeast side and make up the Trinity Islands. With the occurrence of moraines and ice margin ridges, it appears that ice from the Alaska Peninsula Section banked up against the western shore of the West Kodiak Island Section. Dominant soils are Typic Haplocryands and Typic Vitricryands. Most of the glacial deposits are mantled with ejecta from the 1912 eruption of Mount Katmai and Novarupta.
Soils are shallow on mountain peaks, rock escarpments, and colluvial slopes.
Some depressions are filled with fibrous peat. Potential Natural Vegetation. The vegetation is primarily alpine tundra heath meadows and barrens with moist and wet sedge meadows occuring at lower elevations. Shrub thickets occur along some drainages. Saltwater bays provide habitat for Aleutian terns and harlequin ducks.