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Maclay's West Twin Bridge


Twin Bridges facing east.JPG

The East Bridge of the Twin Bridges is in background

Photo taken by Jodi Christman


BH Photo #176334



For more info: http://memory.loc.gov/pnp/habshaer/pa/pa3500/pa3575/data/pa3... The Maclay's Mill Twin Bridges are outstanding examples of surviving stone arch bridges constructed by a nineteenth-century designer-craftsperson. Before the appearance of professional engineers and architects during the middle of the nineteenth century, local craftspeople such as Silas Harry were responsible for the design and construction of vernacular bridges such as the Maclay's Mill Twin Bridges. No fewer than six Franklin County stone arch bridges have been attributed to Harry. An active stonemason throughout much ofthe first half of the nineteenth century, Harry also was responsible for the 1842 reconstruction of the Franklin County courthouse. The Maclay's Mill Twin Bridges were listed in the National Register of Historic Places in 1988. The bridges are also significant for their association with the Maclay family. David Maclay, the owner of Maclay's Mill at the time the bridges were built, was a prominent Franklin County landowner and former Pennsylvania legislator. Also, Maclay•a second generation owner of the mill•was the nephew of William Maclay, Peimsylvania's first U.S. Senator.

Maclay's Mill, Twin Bridge (West) is a two-span, camelback, closed-spandrel stone arch bridge. Constructed of semi-coursed local limestone rubble and blocks, the bridge is 85'-3" long and an average of 20'-6" wide with a single 16'-10" travel lane. The bridge spans the former tailrace of Maclay's Mill on a perpendicular alignment and rests on a single 6'-0"-wide conical pier.

Its approaches have abutments with wing walls that expand to widths of 39'-0" on the west and 25'-6" on the east. Like its twin, the west bridge is constructed of weathered Cambro-Ordovician limestone. Its two segmental arches have spans, measured from east to west, of 20'-2-l/2" and 20'-3-3/4" long. The arches rise, again from east to west, 3'-l 1-3/4" and 4'-l".

The bridge's two segmental arches are defined by dressed squared voussoirs and dressed springers. Keystones in each arch ring are not accentuated. Each arch ring was constructed with regular coursed ashlar blocks. The intrados of each arch barrel has been clad in cement. The masonry joints have been repointed with a hard cement mortar throughout much of the structure in a raised, irregular pattern. Unlike the east bridge, this structure has no date stone. The bridge's parapets have a single 2"-thick limestone coping course that is clad in cement. The north parapet has a low elevation above the road bed: measured at the west end, mid-span, and east end (the west end being datum), it rises V~& 3'-3", and -0'-3".


Stone arch bridge over Conodoguinet Creek on PA 4018
Franklin County, Pennsylvania
Open to traffic
Built 1827 by Silas Harry
- Silas Harry
Stone arch
Length of largest span: 30.8 ft.
Total length: 102.0 ft.
Deck width: 16.4 ft.
Posted to the National Register of Historic Places on June 22, 1988
Approximate latitude, longitude
+40.09694, -77.57028   (decimal degrees)
40°05'49" N, 77°34'13" W   (degrees°minutes'seconds")
Approximate UTM coordinates
18/280899/4441683 (zone/easting/northing)
Quadrangle map:
Average daily traffic (as of 2018)
Inventory numbers
PA 28 4018 0070 0000 (Pennsylvania Bridge Management System number)
NRHP 88000779 (National Register of Historic Places reference number)
PANBI 17610 (Pennsylvania BRKEY bridge number on the 2011 NBI)
BH 30943 (Bridgehunter.com ID)
Inspection report (as of April 2018)
Overall condition: Poor
Superstructure condition rating: Poor (4 out of 9)
Substructure condition rating: Fair (5 out of 9)
Sufficiency rating: 18.6 (out of 100)
View more at BridgeReports.com


19th Century (7,673)
Arch (12,249)
Built 1827 (10)
Built during 1820s (97)
Deck arch (11,489)
Flared (403)
Franklin County, Pennsylvania (122)
NR-listed (2,963)
Open (40,030)
Owned by state (16,110)
Pennsylvania (5,356)
Silas Harry (10)
Span length 25-50 feet (15,741)
Stone arch (3,101)
Structurally deficient (13,987)
Total length 100-125 feet (5,199)

Update Log 

  • January 13, 2015: HAER photos posted by Dave King
  • March 4, 2014: Photo imported by Dave King
  • February 19, 2011: New photos from Jodi Christman
  • November 28, 2010: New photos from Jodi Christman
  • November 22, 2010: Updated by Jodi Christman: Bridge was struck by a car 11/12/10. It's closed right now. http://dailyme.com/story/2010111500003250/bridges-maclays-mill-closed-car-crash.html
  • October 30, 2010: Updated by Jodi Christman: Added more info. in description
  • September 6, 2010: New photo from Jodi Christman


  • Jodi Christman - masterofchaos [at] outlook [dot] com
  • HAER PA-457 - Maclay's Mill Twin Bridge (East & West), Spanning Conodoguinet Creek at Maclay's Mill Road (State Route 4018), Mowersville, Franklin County, PA