The point at which the old city of Khazad-dûm started to be known as Moria the Black Chasm is not definitively established. According to The Silmarillion, it acquired the name Moria '...afterwards in the days of its darkness...' (Quenta Silmarillion 10, Of the Sindar). It would be natural to read this as a reference to the coming of Durin's Bane and the exile of the Dwarves, and that may very well be the intended meaning.

As a possible counterpoint, however, it is worth noting a comment in Appendix B to The Lord of the Rings, which tells us that, even before the coming of the Balrog, the city's '...numbers had dwindled until many of its vast mansions became dark and empty.' On this basis, we might see the 'days of its darkness' to have actually begun earlier, sometime before the awakening of Durin's Bane. On this alternative reading, Khazad-dûm might already have been known as Moria earlier in the Third Age, while there were still dwindling numbers of Dwarves occupying the darkening city.

The exact configuration of the Mountains of Moria is not shown on any map, so the relative positions shown here are derived from descriptions in The Lord of the Rings (though given the detail of those descriptions, there is little margin for error). In addition, though it seems inevitable that the Sirannon must have been a tributary of the Glanduin, it is not known precisely how the two rivers joined.