Mormon sees three lands that might be conceptually seen as layers, although the geography itself would not be so neatly represented. At the far south was the land of the Lamanites, including the land of Nephi, which was Nephite territory until Mosiah, the father of King Benjamin, left for the land of Zarahemla.
The next layer is the Nephite lands, bounded on the south by the narrow wilderness, and on the north to the line in the small neck which was also called the land Bountiful. It is interesting that in verse 33, Mormon notes that the Lamanites “should have no more possession on the north, that they might not overrun the land northward.” In this case, “the land northward” is only a relative direction. It is not the land of Desolation, but rather the Nephite lands themselves, those lands being northward of the Lamanite lands.
The final layer was the land Desolation. Note that in verse 34, the Nephites are protecting the land Bountiful so “that they might have a country whither they might flee.” The Nephites do not see heading south as any kind of salvation. Indeed, when forced to flee, they flee northward from their lands into Desolation, above the small neck.
Our current Chapters 21 and 22 were a single chapter in 1830. That chapter ends at this point. Even though there is a chapter break, the story of the sons of Mosiah among the Lamanites is far from ended.