Seneca Aqueduct, the first of eleven aqueducts erected along the C&O Canal, was constructed between 1828-32 at a cost of $24,340.25. The face of the aqueduct is cut Seneca red sandstone. It is 113 ft. long between abutments and has three segmental arches, each with 33 feet span and a 7½-foot rise.
Mason's marks are on the berm side at the west end of the trunk and on the berm side at the east end of the trunk. In a raid by General J.E.B. Stuart, June 27, 1863, a boat was burnt in the aqueduct. A timber trunk was placed in the aqueduct in 1873 because the masonry walls were 9 inches out of plumb and the arch was supported by the inner liner only. The aqueduct was taken down and rebuilt in 1873-74 with iron braces placed to retain the stonework. The west arch fell September 1971, being carried out by water backed-up in a flood of Seneca Creek. The berm parapet and coping remained intact over west arch but were removed when temporary repairs were made.
During the operating years of the canal, Seneca was the first important transshipment point above Georgetown. At times it boasted stores and a granary on its large basin, and there were the quarries for the much-desired red Seneca sandstone just above the aqueduct.
Read a more detailed History of Seneca from Along The Towpath.
Lock 24 - Rileys Lock
Constructed in 1830-31 with an 8½-foot lift as an integral part of Seneca Aqueduct. It also was faced with cut Seneca red sandstone. The upper recess connects with the trunk of the Seneca Aqueduct.
The lockhouse on the berm side was constructed between 1829-30 at a cost of $893.25. The 1½-story house is built of cut and coursed ruble Seneca red sandstone.
"Riley Family History" provides information on the family whose name became associated with the lock.
The quarries occupy a large swampy area west of the aqueduct extending west to Mile 23.13 and is a former canal basin. Quarries on the north side of the basin were opened about 1774 and the stone was used in the Potomac Company canal locks on the Virginia side of Great Falls in 1797. While there are six major quarries along the canal, the John P. C. Peter Quarry, at the margin of the canal near the mouth of Seneca Creek was the source for stone for the Seneca Aqueduct, many locks and culverts on the C&O and the Alexandria Canals. This sandstone was soft, easily cut and carved in quarrying but hardened after exposure. In the years after the Civil War, the stone cutting mill employed up to a hundred men at times.