Nearly 500 million years ago, the rocks that now comprise the Shenandoah Valley lithified from Cambrian and Ordovician tidal flat sediments into vast, horizontally bedded sequences of limestone and dolomites. Subsequent collision with an offshore land mass in the late Ordovician and early Silurian period, or roughly 400 million years ago, followed by another continental impact between 320 and 230 million years ago.
Subsequent erosion peeled away weaker overlying rocks. The spectacular Blue Ridge emerged from the fold's core to become the Valley's eastern boundary. The complex layering of horizontal lying limestones, shales, and sandstones to the west buckled and folded into vertical pleats as sharply creases on a closed accordion. Subsequent erosion reduced this surface. The softer limestones and shales became valleys and the hardest sandstones and quartzite emerged as ridges.
The pirate of a tributary, intercepting different streams and redirecting them to the Potomac River, grew to become the Shenandoah River. It was the first of the tributaries in western Virginia to reach the soft limestone layer that's now the base of the Shenandoah Valley. That tributary intercepted stream after stream west of the Blue Ridge, beheading first the Occoquan, next the Rappahonock and finally the tributaries of the York. After each capture, the Shenandoah and the Potomac grew larger.
The Shenandoah River carved out the Shenandoah Valley, dissolving the limestone and carrying the sediments north to the Potomac River. The Shenandoah River joined the Potomac River west of the hard Blue Ridge barrier. The Potomac kept cutting through the volcanic bedrock of the Blue Ridge, with the energy provided by the extra water from other streams.
Today the Shenandoah Valley is defined by the surrounding mountain ranges. In the east are the Blue Ridge. To the west are the Alleghenies. Bisecting the Valley is the Massanutten Mountain range, running approximately 50 miles.
Due to the Shenandoah River running from south to north, the Valley residents utilize the ancient natives tongue for direction. Since the river flows downhill, the northern end of the Shenandoah Valley is considered the "lower" Shenandoah Valley. While heading south, one is heading upriver, therefore heading into the "upper" Shenandoah Valley.