During the creation of this cache it is revealed that:
1. The bluffs are so aptly named because the CSA used this location (or close to) to fire at the Union troops while working on the Washington Aqueduct Dam.
2. The structures you see were not used for artillery but to have been used to construct the dam.
To find the Letterbox:
Standing on the highest platform, turn away from the river and walk 30 paces, at the base of the holly tree you turn left and walk 10 paces and there you will find the container you seek.