Seek: Reports from the Edges of America & Beyond by Denis Johnson:
But Powell was listening to local legend. In truth it’s doubtful that the Ashley who carved his name onto a rock and then wrecked his party in the Green River ever saw the temple, or even its foundation, in Salt Lake City. A William Henry Ashley, who eventually became a congressman for Missouri, is credited with having navigated the Green River in 1825 and with establishing fur trade routes in that country that made him a rich man; and in 1826 he led an expedition that reached the vicinity of the Great Salt Lake. He died in 1838. The Mormons, on the other hand, didn’t establish their colony in Salt Lake until decades later: In the late 1840s, without a map, a stream of some twelve thousand apostles of Brigham Young—a New Englander with twenty-seven wives and scores of children—crossed the world from Nauvoo, Illinois. The first of them arrived in the summer, exhausted and starved, near the endless burning white flats of the Great Salt Lake, leaving in their wake four relay colonies and more than two thousand graves.
At that time Utah was a part of Mexico, forsaken by the American people and also, according to the mountain men who had first reached her, forsaken by God. Here, in the Salt Lake Valley, a full thousand miles beyond what had been, until then, the fathest American frontier, the Mormons settled down to build their temple and lay out a celestial city and await, as they continue to do, the destruction of the world by fire.