The eastern American Great Valley a geologic region characterized by easily eroded shale or limestone strata and not associated with a single river, as the word valley implies. Portions of its length are indeed river valleys, such as the Shenandoah Valley within the eastern American Great Valley region. The Great Valley was a main route for settlement and commerce in the United States along the Great Wagon Road from Philadelphia, Pennsylvania. The Wilderness Road into Kentucky and Tennessee branched from the Great Wagon Road at what is now known as Roanoke, Virginia. Today, the main thoroughfare within most of the Great Valley is Interstate 81.