The Willamette Valley has nearly level to gently sloping floodplains bordered by dissected high terraces and hills. The Puget Sound Valley is a moderately dissected tableland covered by glacial till, glacial outwash, and lacustrine deposits. This province includes isolated hills and low mountains. Coastal mountains are responsible for the drier and less muted climate.
Fog partially compensates for the summer drought.
Principal trees are western redcedar, western hemlock, and Douglas-fir. In interior valleys, the coniferous forest is less dense than along the coast and often contains deciduous trees, such as big-leaf maple, Oregon ash, and black cottonwood. Poorly drained sites with swamp or bog communities are abundant. Inceptisols, and Ultisols are the principal soil orders. Inceptisols dominate in Puget Sound Valley.