Along the C&O Canal, upstream from the Seneca Aqueduct, are a series of abandoned quarries where an unusual red sandstone was cut. Look for stone bluffs, sometimes with tool marks. Behind the turning pond are the ruins of a stonecutting mill, and traces of a narrow-gauge railway used to transport stone. Nearby are old fence foundations that had stood in DC’s Lafayette Park and were returned to their source. If you hike to the Bull Run Culvert, and follow Bull Run upstream into a gulch, you will be in the remains of the Bull Run Quarry, the source of the red sandstone used in the construction of the Smithsonian Castle and many other buildings in Washington and Baltimore. Sharp explorers may spot places were houses or buildings once stood, and a cemetery for quarry workers.

Vagrarian said:

    The best time to explore is in late fall, winter, and early spring; in warm months the area is overgrown with creepers and brambles and is impassable. Further information can be found in the book "The Smithsonian Castle and the Seneca Quarry" by Garrett Peck.