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 50 miles 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 New Baltimore, Virginia.