The western part of Washington County, underlain by strongly folded clastic sedimentary rocks, is steeper and more forested than most of the eastern part, where limestone and dolomite predominate. The eastern edge of the county is underlain by metamorphic rocks.
Ground water occurs predominantly under unconfined, secondary-permeability conditions.
Unconsolidated sediments and overburden generally are not sufficiently permeable or are of insufficient saturated thickness to be used directly for water supplies. Ground-water levels range from near land surface to more than 150 feet below and fluctuate from a few feet to tens of feet. Highest water levels occur during February through April, and lowest levels occur in October. Long-term trends are of lesser magnitude than seasonal variations. Reported well yields range from 0 to 385 gallons per minute, and specific capacities range from 0.00 to 86 gallons per minute per foot.