Weren't those called "limes"?
Boy, you had me stumped until I did a little searching. Of course you are correct. Our word "limit" is derived from the same Latin root, the walls representing the outer limit of Roman authority. How did you end up at this posting from two years ago?
Besides earthen fortifications which have decayed, there is also the example of the enormous crater created by an explosion in the Battle of the Crater. The crater is still visible but the remaining hole is not large at all.