 Historically, much of the area was an open oak-studded prairie supported by somewhat excessively drained gravelly sandy loam soil, though agriculture and development of the Dungeness valley have changed this ecosystem.Most soils under Sequim have been placed in a series that is named after the city.This "Sequim series" is one of the few Mollisols in western Washington and its high base saturation, a characteristic of the Mollisol order, is attributed to the minimal leaching of bases caused by low annual rainfall.