This is an image of an American Civil War era cannon situated on the edge of Lookout Mountain overlooking the city of Chattanooga, Tennessee on the far side of the river below.
The description states simply:
CANNON ON LOOKOUT MOUNTAIN, TENNESSEE
–The cannon is a reminder of the Civil War era Battle of Lookout Mountain, fought here in 1863.
The card was used and is dated 21 June 1961.