The ecoregion includes the steep footslopes, alluvial fans, floodplains, outwash plains, scattered moraines, river terraces, and river deltas of the Pacific coastal mountains. Geologic formations are Lower Tertiary interbedded sedimentary, volcanogenic, and volcanic rocks in the Prince William Sound area, Upper Cretaceous sandstone and slate on Kodiak Island, and Mesozoic volcanic and intrusive rock along with Mesozoic and Paleozoic sediments in the southeastern portion of the ecoregion. More recent formations are dunes of aeolian sand that border some of the floodplains throughout the ecoregion. Nearly all of the ecoregion was subject to late glacial advances during the Pleistocene epoch, although no glaciers remain. The region is generally free from permafrost.
Lakes are plentiful in some areas and absent in others.
Soils Dominant soils are Terric Cryohemists, Andic Cryaquods, Andic Humicryods, Lithic Humicryods, and Typic Humicryods. Soils near the mountains formed in gravelly and stony moraine deposits, or in a mantle of volcanic ash over the morainal deposits. Soils of river deltas, terraces, alluvial fans, and floodplains formed in waterlain silts and clays. Poorly drained depressions are filled with fibrous peat. Many parts of the ecoregion are susceptible to flooding or to inundation by tidewater.