This is the steepest terrain, the deepest gorge and the most dramatic scenery in the Rock Run valley. Today I found a route that does not require rock climbing equipment or extrordinary fitness. Again the fire hydrant marks a reasonable place to enter the woods. Follow an animal trail straight down to the stream level then make your way downstream until you can see the historic arch that allows the stream to pass under the aqueduct bringing water from intakes above Great Falls down to the City of Washington.
Staying on the bank of the stream will require crawling over and under large and smaller fallen trees. In very warm weather, walking in the stream or taking advantage of a pool along the way could be fun. There are a number of large rocks excellent for sitting and taking in the view.
Even on a 90F day like this one, I choose to wear heavy weight jeans that repel thorns and trail shoes that reduce slipping.
If you sit on a large rock near the arch and look back up the stream, the cache should be in plain sight and high enough to avoid being swept away. Take it down before trying to lift the lid. It will hold trade items but the outer container leaks some which is why it was retired from its previous function.
Congratulations to RDip for being the First to Find.