The third and last frontier advance carried migrants across the remaining reaches of the continent to the Pacific Ocean and then turned back to fill in the areas passed over in the first forward drive. It began around 1840 and lasted to 1890 and beyond, when the federal census announced the end of the frontier era. This is usually referred to as the trans-Mississippi frontier.
The region faced by the American pioneers in this last advance was different in character from anything that they had met before, and the frontiers that emerged were more varied and more colourful than any earlier environments had produced. Beyond the first stretches of prairie lands lay the dry, treeless plains where rainfall was too scant for the accustomed agricultural crops and methods. Its nutritious grasses, however, had supported immense herds of bison, and the settlers, after pushing old crops well beyond their limits, gave way to the cattlemen with their picturesque ranch houses, their cow towns, and their cowboys. It was a new kind of frontier, where capital and colour went together to stir the imagination of those whose lives were cast in more prosaic times and places.
Beyond the plains were the Rocky Mountains, with inland basins sometimes as large as an eastern state and with fur and mineral riches that brought the trapping frontier and the mining frontier to their fullest development. California and Oregon, on the Pacific Coast, with their mines, their trading posts, their missions, and their international complications, brought an end to the westward journey and turned the frontier dweller back to the half-finished continent.
These last frontiers were unique in many ways. Vast distances and greater physical difficulties to be overcome created problems that the individual alone could not solve. Group action, government aid, and outside capital were required to carry forward the necessary irrigation projects, to build the transcontinental railroads, and to develop the mines and the timber resources. Everything was on a larger scale; everything was exaggerated. Movement and settlement often came by rushes, not by a slow, steady advance. One of the largest of these mass migrations was the 1,300-mile (2,100-km) exodus carried out by thousands of followers of the Church of Jesus Christ of Latter-day Saints along a route that came to be called the Mormon Trail. Hardships were greater—the tragic outcome of the Donner party served as a reminder of this—and rewards were in proportion. All that any frontier had meant in the past to American life in terms of optimism, waste, lawlessness, abundance, and progress were there magnified at the very time that the physical frontier was coming to an end.