Most of the county lies in the Piedmont region, with rolling hills and deep stream valleys such as Difficult Run and its tributaries. West of Route 28, the hills give way to a flat, gentle valley which stretches west to the Bull Run Mountains in Loudoun County. Elevations in the county range from near sea level along the tidal sections of the Potomac River in the southeast portion of the county to more than 500 feet (150 m) in the Tysons Corner area.