The Aleutian Mountains consist of segments of the peninsular terrane, which includes mildly folded and faulted late Mesozoic sedimentary rocks, locally intruded by granitic rocks and surmounted at intervals by volcanic piles dating from the late Tertiary to the recent age. Many volcanoes have calderas. A major fault extends along the north side of the eastern part of the range, separating the sedimentary rocks from the granitic batholith on the north. Dominant soils are Typic Haplocryands and Typic Vitricryands. Most of the glacial deposits are mantled with volcanic ash and scoria and are very easily eroded.
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 willow and alder occurring at lower elevations and along drainages. The Alaska Peninsula provides significant staging and migration habitat for waterfowl in the spring and fall. Numerous Steller sea lion rookeries and hauling grounds occur along the coast.