Seneca Quarry is a historic site located at Seneca, Montgomery County, Maryland. It is located along the Chesapeake and Ohio Canal on the north bank of the Potomac River, just west of Seneca Creek. The quarry was the source of stone for two Potomac River canals: the Patowmack Canal on the Virginia side of Great Falls; and the C&O Canal, having supplied red sandstone for the latter for locks 9, 11, 15 - 27, and 30, the accompanying lock houses, and Aqueduct No. 1, better known as Seneca Aqueduct, constructed from 1828 to 1833.