 Washington County is surrounded by Benton County to the north, rural Madison County to the east, the rural Crawford County to the south, and Adair County, Oklahoma to the west.Washington County sits on a basement of Precambrian granite and rhyolite, as most of the continental interior of the United States does.Much of the county's geologic history must be inferred from nearby Oklahoma and Missouri research, due to the steepness of the more recently formed mountains that did not form in the neighboring states.