Lock 20 is located just south of Riegelsville on the present-day Delaware and Lehigh Canal. Traffic there consists mainly of hikers and dog-walkers taking advantage of the well-maintained towpath, along with motorists coasting into the parking lot from Routes 611 or 212 for a leisurely picnic on the sleepy, scenic grounds. But 175 years ago, the scene was very different.

In 1827, the Commonwealth of Pennsylvania began building the Delaware Divison Canal, spanning the sixty miles between Easton and Bristol. Two years later, the “Narrowsville Lock” was built on the canal in front of a gristmill owned and operated by Nockamixon Township businessman Samuel Rufe. The mill’s great buhrstones, powered by a wing dam on the Delaware River, had by then moved from primarily grinding grain to pulverizing the “clinker”—small lumps of limestone, shale, iron ore and sand that have been calcined, or heated to a partially molten state—from the Holland Township cement kilns three miles upriver on the New Jersey side.

Once ground to a fine powder, this material was considered natural hydraulic cement, meaning that it both slaked—transformed chemically—when mixed with water, and also set up underwater to form a hard product that became impervious to the liquid. Discovered in England in 1796, this material was critical to the building of canals, locks, dams, bridges and other construction that required constant submersion in water without loss of structural integrity.

From Narrowsville, the hydraulic cement was shipped in barrels that held approximately three bushels, and sold for about sixty cents apiece, further down the canal to create the rest of the sidewalls and other canal structures. H.G. Sargeant, general engineer of the project, reported in 1829 to his superintendent, Thomas Kennedy, that the Narrowsville Plant manufactured “most of the hydraulic cement used on the line.”

The remainder of the cement was produced at a similar operation in Solebury Township. The Asher Ely limestone quarries, located about two miles north of New Hope and a mile west of the river, had its own limekilns and a gristmill on the same property. This mill was situated on Primrose Run, which powered its millstones and still discharges into the river at the now well-known Phillips Mill.

Because the Delaware River valley is rich in deposits of the kind of limestone needed for the process, our area still contributes to the manufacture of hydraulic— or Portland—cement through several manufacturers in New Jersey and Pennsylvania.