Mormon had retrieved the Nephite archive from the hill Shim so that he would have access to it, and so it would not be lost to the rapidly advancing Lamanite armies. That condition was being repeated, but in a much worse way, in Cumorah.
Mormon knew that they were involved in “the last struggle of [his] people.” Therefore, he was again concerned for the safety of the Nephite archive. There was nowhere to take it, therefore, he “hid up in the hill Cumorah all the records which had been entrusted to [him] by the hand of the Lord.” His desperate hope was that they would be safe, “for the Lamanites would destroy them.”
The important part of this act of preservation is the final phrase where he gave “a few plates” to his son Moroni. Those “few plates” were those containing The Book of Mormon. They were not buried in Cumorah, but were given into Moroni’s care. They were not immediately handed over, however, as Mormon continued to write in them after the final battle. Then, Moroni wrote additions to Mormon’s record, consisting of Mormon 8-9, the book of Ether, and the book of Moroni. Clearly, Moroni did as he was asked and kept the plates. They certainly were not among those already buried in Cumorah. In his own book, Moroni notes that he was continuing to write long after the end of the Nephites at Cumorah (Moroni 1:1,4).
Thus, there is no record that Mormon’s plates were ever in the Nephite Hill Cumorah. Moroni possessed them in his wanderings some thirty-six years later (Moroni 10:1). While it is possible that he returned from wherever he had found safety, to bury the plates in a hill now occupied by an enemy that had sworn to kill him, it does not seem likely. The plates were certainly preserved in a hill, one to which Joseph Smith had easy access when the Lord’s time was right for the record to come forth.