How can I get to the Sinking Tombs?

In Chapter 2 of Möbius Final Fantasy, there are several paths between battles that are blocked off. In most cases, I can either find a key or go around the long way and get to the blocked area.

However, the portion of the map labeled Sinking Tombs does not appear to have a back path. As you can see, I've completely encircled it, but the only road in is blocked. Unlike unlockable roadblocks, where a key or later action will open up access, this road displays an X. It does not tell me what key I need to find, as the locks do, it simply says "you cannot go this way. You must find another route."