When Joseph Smith and Oliver Cowdery wrote Letter VII, they were responding to critics who claimed the Book of Mormon was fiction, derived from an unpublished manuscript written by Solomon Spaulding.
They knew the best way to respond to critics was by citing facts. They made sure there was no question about which hill in New York was Cumorah. They also made sure there was no question about the location of the final battles of the Nephites and Jaredites, or the location of Mormon’s depository.
They explained at length that there is one Hill Cumorah and it is in New York.
Here are excerpts from Letter VII.
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You are acquainted with the mail road from Palmyra, Wayne Co. to Canandaigua, Ontario Co. N. Y. and also, as you pass from the former to the latter place, before arriving at the little village of Manchester, say from three to four, or about four miles from Palmyra, you pass a large hill on the east side of the road. Why I say large, is, because it is as large perhaps, as any in that country. To a person acquainted with this road, a description would be unnecessary, as it is the largest and rises the highest of any on that route. The north end rises quite sudden until it assumes a level with the more southerly extremity, and I think I may say an elevation higher than at the south a short distance, say half or three fourths of a mile. As you pass toward Canandaigua it lessens gradually until the surface assumes its common level, or is broken by other smaller hills or ridges, water courses and ravines. I think I am justified in saying that this is the highest hill for some distance round, and I am certain that its appearance, as it rises so suddenly from a plain on the north, must attract the notice of the traveller as he passes by.
At about one mile west rises another ridge of less height, running parallel with the former, leaving a beautiful vale between. The soil is of the first quality for the country, and under a state of cultivation, which gives a prospect at once imposing, when one reflects on the fact, that here, between these hills, the entire power and national strength of both the Jaredites and Nephites were destroyed.
By turning to the 529th and 530th pages of the Book of Mormon, you will read Mormon’s account of the last great struggle of his people, as they were encamped round this hill Cumorah. (It is printed Camorah, which is an error.) In this valley fell the remaining strength and pride of a once powerful people, the Nephites—once so highly favored of the Lord, but at that time in darkness, doomed to suffer extermination by the hand of their barbarous and uncivilized brethren. From the top of this hill, Mormon, with a few others, after the battle, gazed with horror upon the mangled remains of those who, the day before, were filled with anxiety, hope, or doubt. A few had fled to the South, who were hunted down by the victorious party, and all who would not deny the Savior and his religion, were put to death. Mormon himself, according to the record of his son Moroni, was also slain.
But a long time previous to this national disaster it appears from his own account, he foresaw approaching destruction. In fact, if he perused the records of his fathers, which were in his possession, he could have learned that such would be the case. Alma, who lived before the coming of the Messiah, prophesies this. He however, by Divine appointment, abridged from those records, in his own style and language, a short account of the more important and prominent items, from the days of Lehi to his own time, after which he deposited, as he says, on the 529th page, all the records in this same hill, Cumorah, and after gave his small record to his son Moroni, who, as appears from the same, finished it, after witnessing the extinction of his people as a nation….
This hill, by the Jaredites, was called Ramah: by it, or around it, pitched the famous army of Coriantumr their tent. Coriantumr was the last king of the Jaredites. The opposing army were to the west, and in this same valley, and near by. From day to day, did that mighty race spill their blood, in wrath, contending as it were, brother against brother, and father against son. In this same spot, in full view from the top of this same hill, one may gaze with astonishment upon the ground which was twice covered with the dead and dying of our fellowmen….
In this vale lie commingled, in one mass of ruin, the ashes of thousands, and in this vale were destined to be consumed the fair forms and vigorous systems of tens of thousands of the human race—blood mixed with blood, flesh with flesh, bones with bones, and dust with dust!
Source: Letter VII