The Oregon Trail stretched roughly 2,000 miles across the American West, but no one actually built it as a road.

Its beginnings were far older. Indigenous peoples had traveled portions of the route for generations before fur traders began following existing paths and natural passages through the mountains in the early 1800s. South Pass in present-day Wyoming proved particularly important: its broad, gradual crossing of the Continental Divide made it possible for wagons to get through the Rocky Mountains.

Then came the wagons.

In 1836, missionaries Marcus and Narcissa Whitman traveled west with wagons, although they eventually had to leave them behind. In 1843, nearly 1,000 emigrants set out from Independence, Missouri, in what became known as the Great Migration. They demonstrated that a large wagon train could make it all the way to Oregon, and thousands followed.

Between 1840 and 1860 alone, an estimated 300,000 to 400,000 people traveled the overland route toward Oregon, California, Utah and other western destinations. The journey could take five or six months, with wagons typically covering only about 10 to 20 miles a day.

All that traffic changed the ground beneath it.

Wagon wheels, oxen, horses and thousands of people repeatedly crossed the same stretches of earth. Where there was room, wagon trains could spread out and create broad swales. But mountains, rivers and other obstacles sometimes squeezed them back into the same narrow passage, where the tracks became deeper and deeper.

Near Guernsey, Wyoming, geography forced practically every westbound wagon over the same ridge of soft sandstone. Thousands of iron-rimmed wheels and animal hooves gradually carved channels into the rock as much as five feet deep.

Those ruts are still there.

More than 175 years after the great wagon migrations began, you can stand in Wyoming and walk through grooves worn into stone by people heading west.