Catoctin Mountain, along with the geologically associated Bull Run Mountains, forms the easternmost mountain ridge of the Blue Ridge Mountains, which are in turn a part of the Appalachian Mountains range. The ridge runs northeast/southwest for about departing from South Mountain near Emmitsburg, Maryland, and running south past Leesburg, Virginia, where it disappears into the Piedmont in a series of low-lying hills near Aldie, Virginia. The ridge forms the eastern rampart of the Loudoun and Middletown valleys.