In the spring of 1849 Gunnison was assigned as second in command of the Howard Stansbury expedition to explore and survey the valley of the Great Salt Lake. That winter was particularly heavy and the expedition was unable to leave the valley. Gunnison took the opportunity to befriend some Mormons and study the Church of Jesus Christ of Latter-day Saints. When he finally returned to Washington, DC, he wrote a book titled "The Mormons or Latter-Day Saints, in the Valley of the Great Salt Lake: A History of Their Rise and Progress, Peculiar Doctrines, Present Condition."