By A.R. Williams 
Using new techniques to gauge how long rocks have been exposed to sunlight, researchers have significantly narrowed the period in which [the surreal-sized pictographs in Horseshoe Canyon, Canyonlands National Park, Utah] must have been painted.
Their reconstruction of events: 2,000 years ago a sheet of rock fell from the cliff. Artists then used the fresh surface as their canvas. About 900 years ago another sheet fell, taking a few painted figures with it.
Steven Simms, a Utah State University archaeologist involved in the research, thinks the paintings may have been made within a few hundred years of the first rockfall, during a time of major transformation as corn farmers from the south moved into a region peopled by hunter-gatherers.
In Simm’s scenario “the farmers come in large numbers. they take over the land, hunt all the game. The hunter-gatherers are pushed to the margins.”
Note: The Horseshoe Canyon pictographs are located in an adjunct to Canyonlands National Park.