Downstream in Ancient Rome or, Signs of the (Ancient) Times

At a roman archaeological dig you find many occasions where stones have been reused.  Why carve a new one when you can just pick up a nice specimen from a nearby ruin?  Altars, tombstones, dedicatory inscriptions, all end up being built into new walls.

Sometimes it is hard to tell.....

The Vindolanda site has some water issues, so there are drains everywhere.  The above stone was built into the side of one.

Was this an example of reuse?  Or did it indicate that "this way is downstream"?