 It is bounded on the north by the Potomac River; across the river are Frederick, Washington and Montgomery counties in Maryland; it is bounded on the south by Prince William and Fauquier counties, on the west by watershed of the Blue Ridge Mountain across which are Jefferson County, West Virginia and Clarke County, and on the east by Fairfax County.The Bull Run Mountains and Catoctin Mountain bisect the county.To the west of the range is the Loudoun Valley.