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Monocacy Aqueduct

Library of Congress, Historic American Buildings Survey

As the Chesapeake & Ohio(C&O) Canal wound its way west, following the Potomac River, it was necessary to bridge the numerous creeks and rivers that emptied into the Potomac.  Creating aqueducts became an engineering challenge for the builders of the canal.  Over the 184-mile course of the C&O Canal from Georgetown to Cumberland, Md., 11 stone aqueducts and over 150 stone culverts were built to allow the canal boats an unimpeded journey.

The aqueducts were marvels of engineering skill and masonry craftsmanship. The structures were built not only to carry the weight of water and canal boats, but also to withstand the flow of the rivers or creeks over which they stood.  Stonecutting and the laying of the blocks had to be exact and precise to keep the canal’s waters from leaking.

The longest and arguably the most impressive of the 11 aqueducts is the Monocacy Aqueduct, built over the mouth of the Monocacy River in Frederick County. The Monocacy River is the largest of the rivers and creeks emptying into the Potomac River along the route of the canal, and the sheer size of the mouth of the Potomac posed major obstacles for the canal’s engineers.

Also known as the C&O Canal Aqueduct No. 2, the completed structure was 438’ in length (and over 500’ from “wing to wing”) and contained seven arches (none of the other aqueducts had more than 3 arches).  Each of the arches has a span of 54’ and the piers between them are 10’ thick. The side of the aqueduct that held the towpath (the towpath was where the mules that pulled the canal boats walked) was 8’ thick, while the upstream side was 6’ thick.

The Monocacy Aqueduct was built with pink and white quartzite, giving the structure an interesting and slightly colorful pinkish hue.  The stone was quarried from Sugarloaf Mountain, several miles to the north.  In order to help carry the blocks of stone from quarry to worksite, a narrow-gauge, wooden-railed railroad was constructed.

Work on the aqueduct began in 1829 and during the four years of construction, three different contractors were used. Completed in 1833, the aqueduct cost over $127,900 to build.  A stone tablet in the middle of the span is engraved with the names of the directors of the canal company, the aqueduct’s engineer, and the builders of the structure.

When it was completed, the Monocacy Aqueduct was noted as the jewel of the C&O Canal, and it has been reported that likenesses of it were engraved on silver platters showing the “Seven Wonders” of the State of Maryland during the nineteenth century.

During the Civil War, the C&O Canal was an important transportation and communication link for the Federal Government.  Union troops were charged with keeping the canal open.  Confederate troops tried several times to destroy the aqueduct but were unsuccessful on each attempt, because the soldiers could not make deep enough holes in the stone to allow the explosives to be effective.

Today the Monocacy Aqueduct is showing the wear and tear of over 170 years of facing the natural elements.  Several major stabilization projects have kept the structure intact and standing, allowing the Monocacy Aqueduct to still be considered one of the finest canal features in the United States.

—Thomas Robertson

Community College of Baltimore County