The 1,125-acre Caledonia State Park is in Adams and Franklin counties, midway between Chambersburg and Gettysburg on US 30. Caledonia is in the northernmost section of the Blue Ridge Mountains known locally as South Mountain. Blue Mountain, the easternmost ridge of the Allegheny Mountains, is to the west and northwest of the park across a large low, rolling, fertile valley known as the Great Valley. South Mountain is mostly composed of a hard rock called quartzite. The valleys on either side are underlined with limestone and shale. The soil is ideal for fruit production, proven by the abundance of orchards in the surrounding area.