Geographical: The previous verse tells us that these people went to “the land northward.” One of the possibilities is the area previously inhabited by the Jaredites. When Mormon describes the hill Cumorah, it is in a “land of many waters, rivers and fountains” (Mormon 6:4). Nevertheless, this particular reference probably refers to a land even further north than the land of the Jaredites in which the hill Cumorah (known as Ramah to the Jaredites) was located.
The essential elements that allow us to make a tentative identification of this area are:
Northward
Many waters
Desolate of trees
Buildings of cement
From perhaps 100 BC to 600 AD there is one area in Mesoamerica that fits all of these descriptions, and that it Teotihuacan. It is north of the Nephite (and Jaredite) lands. It is near the lake that used to occupy the current site of Mexico City. The lack of trees and the environmental imbalance created by denuding the land of trees is hypothesized as a major factor in creating the downfall of Teotihuacan. One of the reasons for the lack of trees was the creation of the cement with which their buildings were built (George C. Vaillant. Aztecs of Mexico. Penguin Books, 1966, pp. 78-9O). There is only one area in Mesoamerica that fits this description well, and that is Teotihuacan.
Historical/ Historiographical: In spite of the accuracy of this description of Teotihuacan, there is nevertheless a major problem with the fit. That problem is one of timing. The particular condition of the area being devoid of trees does not describe the Teotihuacan of 49 BC, but rather the Teotihuacan of 350AD and later.
What we have in this case is Mormon describing the Teotihuacan that he knows in his own time, and assuming that it was the same at the time of this part of his history. When Mormon describes this people who leave, he is describing a group of people who do not return, and of whom Mormon would have no record. Mormon therefore has no historical document before him that would accurately describe the land northward to which these people emigrated. Nevertheless Mormon describes that land in great detail.
If Mormon has no historical record to tell him where the people went, and has no historical record to tell him what that land was like, how does he describe it so thoroughly? It is Mormon who makes the association between this group of people and the city of Teotihuacan for his own purposes. He describes that city and land from the perspective of his own time, because that it what he knows.
Based on the probable historical records that Mormon would have had, it is even possible that these people never arrived at Teotihuacan. Mormon’s description of this land and connection are important to Mormon’s story, not because of historical accuracy, but because of his desire to tie the Gadiantons of this time period to the Gadiantons who plague Mormon in his day. By having this party leave for the land northward, and eventually cover the whole face of the land (verse 8), Mormon links the later Gadiantons to the previous through apostate Nephites. For Mormon’s purposes, the downfall of the Nephites will always be seen as caused, or at least enabled, by the internal contentions and apostasy of the Nephites. This includes the destructive influence of the latter Gadiantons.