Fairfax County is bounded on the north and southeast by the Potomac River. Across the river to the northeast is Washington, D.C., across the river to the north is Montgomery County, Maryland, and across the river to the southeast are Prince George's County, Maryland and Charles County, Maryland. The county is partially bounded on the north and east by Arlington County and the independent cities of Alexandria and Falls Church. It is bounded on the west by Loudoun County, and on the south by Prince William County. 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. The Piedmont hills in the central county are made up of ancient metamorphic rocks such as schist, the roots of several ancestral ranges of the Appalachians. The western valley is floored with more recent shale and sandstone. This geology is similar to adjacent bands of rocks in Maryland and further south in Virginia along the eastern front of the Appalachians. An area of of the county is known to be underlain with natural asbestos. Much of the asbestos is known to emanate from fibrous tremolite or actinolite. The threat was discovered in 1987, prompting the county to establish laws to monitor air quality at construction sites, control soil taken from affected areas, and require freshly developed sites to lay of clean, stable material over the ground. For instance, during the construction of Centreville High School a large amount of asbestos-laden soil was removed and then trucked to Vienna for the construction of the I-66/Nutley Street interchange. Fill dirt then had to be trucked in to make the site level. Marine clays can be found in widespread areas of the county east of Interstate 95, mostly in the Lee and Mount Vernon districts. These clays contribute to soil instability, leading to significant construction challenges for builders. Fairfax County uses the urban county executive form of government, which county voters approved in a 1966 referendum. Under the urban county executive plan, the county is governed by the 10-member Fairfax County Board of Supervisors with the day-to-day running of the county tasked to the appointed Fairfax County executive.