Understanding verse 30 requires recalling the final statement of verse 29: “on the north, even until they came to the land which they called Bountiful.” Nephite lands were bounded on the north by the land called Bountiful. On the other side, farther north, was the land called Desolation. Although Mormon is ostensibly outlining Lamanite and Nephite lands, he has another purpose in mind. The land Desolation plays no part in this earlier history of the Nephites, but it will be important later. The name, however, is one that Mormon has already used in connection with Ammonihah, which he called the “desolation of the Nehors” (see Alma 16:11).
The important part of the desolation of the Nehors was the destruction of Ammonihah, and that “their lands remained desolate” (Alma 16:11). The important definition of the land Desolation was “the land which had been peopled and been destroyed.” Mormon associates the land northward not only with the Jaredites, but specifically with destroyed Jaredites. This verse is here for Mormon’s literary purposes, not to further the information about the story of the sons of Mosiah in Lamanite lands.
Verse 31 tells modern readers that the desolation of destroyed Jaredites is contrasted with the bountiful nature of the land under the righteous Nephites. In Mormon times, the Nephites will physically, and symbolically, move into the norther lands, into Desolation. It will be a time when they will have left behind their covenant to follow God, and thus they will move from God’s Bounty to the Jaredite Desolation, where the Nephites will also be destroyed.