US 340 reduces to two lanes and begins its traversal of the Potomac Water Gap, where that river passes through two ridges. The highway descends to and crosses over the Shenandoah Subdivision and the Shenandoah River, then it follows the east bank of the river to the river's confluence with the Potomac River. US 340 follows the south bank of the Potomac River as the river passes between Blue Ridge Mountain to the south and Elk Ridge, also known as Maryland Heights. At Blue Ridge Mountain, the U.S. Highway enters Loudoun County. US 340 has its sole intersection in its second run through Virginia with SR 671 (Harpers Ferry Road) in Loudoun Heights. The route then crosses the Potomac River into Washington County, Maryland, on a southwest–northeast angle on a two-lane steel continuous deck truss bridge that passes above Chesapeake and Ohio Canal National Historical Park, CSX's Cumberland Subdivision, and Sandy Hook Road. US 340 expands to a four-lane freeway at the community of Sandy Hook, curves east through a trumpet interchange with MD 67 (Rohrersville Road), and crosses Israel Creek at Weverton. The highway, the canal, and the railroad parallel the Potomac River through the second ridge, which comprises South Mountain in Maryland and Short Hill Mountain in Virginia.