The Baltimore and Frederick-Town Turnpike created the towns of Parrsville and Ridgeville as it moved west, and both are now part of Mount Airy. Parrsville, to the east, was named for a nearby spring that forms the headwaters of the Patapsco River, which flows east into Baltimore Harbor. Parr’s Spring was an important landmark for early surveyors and marks the four corners of Carroll, Montgomery, Howard, and Frederick Counties. Just west, Ridgeville stands on an 830-foot ridge, the highest point on the National Pike between Baltimore and Braddock Heights. After 1831, the Baltimore and Ohio Railroad used horses to pull cars up two inclines on the east side of the ridge, while brakemen guided trains down two inclines on the west side. After a tunnel was constructed in 1901, rail travel eclipsed road travel. At the crossroads of the National Road and old Route 27, the Eagle/Nelson/Ridgeville Hotel was a landmark for a century, the old Ridgeville Garage served automobile travelers on the National Road and pointed toward both Frederick and Baltimore, and residents of Parrsville, Ridgeville, and Mount Airy worked for the Baltimore and Ohio Railroad for generations.