"the plains of the Nephites" in Ohio, Indiana, and Illinois

On the way to Missouri during the Zion's Camp march, Joseph and his companions stopped at the banks of the Mississippi river. Joseph wrote a letter to Emma on June 4, 1834. He told her how they found "proof of the divine authenticity" of the Book of Mormon.

Here's a map from BYU that shows the route they took and their locations on the indicated dates.

(click to enlarge)

https://mappingmormonism.byu.edu/the-restoration-1820-1845

Excerpt from Joseph's letter to Emma.

The whole of our journey, in the midst of so large a company of social honest men and sincere men, wandering over the plains of the 

Nephites

, recounting [p. 57]

occasionaly the history of the Book of Mormon, roving over the mounds of that once beloved people of the Lord, picking up their skulls & their bones, as a proof of its divine authenticity,

14

 and gazing upon a country the fertility, the splendour and the goodness so indescribable, all serves to pass away time unnoticed, and in short were it not at every now and then our thoughts linger with inexpressible anxiety for our wives and our children our kindred according to the flesh who are entwined around our hearts; And also our brethren and friends; our whole journey would be as a dream, and this would be the happiest period of all our lives.

https://www.josephsmithpapers.org/paper-summary/letter-to-emma-smith-4-june-1834/3

Note 14: On 3 June, the Camp of Israel passed through the vicinity of what is now Valley City, Illinois, where several members of the camp climbed a large mound. At the top, they uncovered the skeletal remains of an individual JS reportedly identified as Zelph, a “white Lamanite.” Archeologists have since identified the mound as Naples–Russell Mound #8 and have classified it as a Hopewell burial mound of the Middle Woodland period of the North American pre-Columbian era (roughly 50 BC to AD 250). (Godfrey, “The Zelph Story,” 31, 34; Farnsworth, “Lamanitish Arrows,” 25–48.)  

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Comprehensive Works Cited

Faulring, Scott H. “Early Marriages Performed by the Latter-day Saint Elders in Jackson County, Missouri, 1832–1834.” Mormon Historical Studies 2 (Fall 2001): 197–210.Godfrey, Matthew C. “‘Seeking after Monarchal Power and Authority’: Joseph Smith and Leadership in the Church of Christ, 1831–1832.” Mormon Historical Studies 13 (Spring/Fall 2012): 15–37.

Farnsworth, Kenneth W. “Lamanitish Arrows and Eagles with Lead Eyes: Tales of the First Recorded Explorations in an Illinois Valley Hopewell Mound.” Illinois Archaeology 22 (2010): 25–48.