The Shenandoah Valley can be defined many ways. One possibility is the area:
That definition would include land in the James River watershed, south of Steeles Tavern. Natural Bridge is a landmark feature of the Valley and Ridge physiographic province - but is it in the Shenandoah Valley? Similarly, Lexington is often paired with staunton as two historic sites in the valley... but which valley? The Maury River, which runs through the northern edge of Lexington, is a tributary of the James River.
Using a more-restrictive definition, the Shenandoah Valley could be limited to just the drainage of the Shenandoah River, excluding Natural Bridge, Lexington, and Rockbridge County. The watershed divide between the Shenandoah and James rivers is close to the Augusta/Rockbridge county line. Just south of Staunton, the Steeles Tavern exit on Interstate 81 identifies a high point on the watershed divide between the Potomac and James river watersheds.
The Shenandoah Valley is a subunit of the Valley and Ridge physiographic province. If you keep going south on the highway from the James River, you'll reach the watershed divide between the Roanoke and James rivers - where a "Leaving the Chesapeake Bay watershed" sign is posted on I-81 - and enter the Roanoke Valley. One way to lump all the names - refer to the "Great Valley." That term can include valleys north of the Shenandoah River, especially the Hagerstown/Frederick Valley in Maryland and the Cumberland Valley in Pennsylvania.
There is no obvious physical difference in the landscape or barrier to travel on the southern end of the valley until you reach the James River, and there are no obvious cultural differences between residents of Augusta and Rockbridge counties. The limestone soils in the Great Valley enabled Augusta and Rockingham to become the dominant agricultural producers in Virginia.
The famous McCormick reaper was developed on a farm near Steeles Tavern; wheat farming was common throughout the valley, south to Lexington and north to Winchester, in the 1850's. The region was the "Granary of the Confederacy" in the Civil War, until the Burning Raid by General Sheridan in 1864 destroyed many barns and limited the agricultural capacity of the valley.
The alignment of the Blue Ridge mountains and the valley shaped military strategy in the Civil War. As one prominent historian described the region:1
the two forks of the Shenandoah River flow north, divided by the two limbs of Massanutten Mountain (with Fort Valley between them) until the forks unite at Front Royal and the main stem joins the Potomac River at Harpers Ferry
Source: National Aeronautics and Space Administration (NASA), Earth Observatory, The Sinuous Shenandoah
Up the Valley: In most American elementary schools, maps have been hung vertically on the classroom wall, with north at the top - and many students unconsciously assume that "north" means "up." The Shenandoah River runs north, and water flows runs downhill the whole way - even though it flows north. The mouth of the Shenandoah River is at Harpers Ferry, where the Shenandoah joins the Potomac River. Someone who floats downstream on the Shenandoah River is floating north towards the Potomac River.
If you hear a long-term resident refer to a place that is "up the valley," they mean uphill - south towards Staunton and the divide at Steele's Tavern on the Augusta/Rockbridge County border. The now-defunct Upper Valley Regional Park Authority, which used to manage Grand Caverns in Grottoes, was near Waynesboro. Compared to Luray and Front Royal and Harrisonburg, Grottoes is up the valley - not downstream near Harpers Ferry...