[Footnote: Official Records, vol.ii.
p.

912.] It will be well to understand the topography of the Virginia mountains and their western slope, if we would reach the reasons which determined the lines of advance chosen by the Confederates and the counter moves of McClellan.

The Alleghany range passing out of Pennsylvania and running southwest through the whole length of Virginia, consists of several parallel lines of mountains enclosing narrow valleys.

The Potomac River breaks through at the common boundary of Virginia and Maryland, and along its valley runs the National Road as well as the Chesapeake and Ohio Canal.

The Baltimore and Ohio Railroad also follows this natural highway, which is thus indicated as the most important line of communication between Washington and the Ohio valley, though a high mountain summit must be passed, even by this route, before the tributaries of the Ohio can be reached.