The illustration above comes from an outstanding book - Hard Road West, by Keith Heyer Meldahl - that I'll review in more detail later. It shows the technique used at a location eventually dubbed "Roller Pass."
At Roller Pass, the final 400 feet from a 30-degree slope of jagged boulders, "as steep as the roof of a house"... Oxen could not pull wagons directly up such a grade. Instead, the animals were unhitched from the wagons and driven to the top, where they could heave along the flatter ground at the pass. Some emigrants placed log rollers at the top of the slope. They yoked the oxen to hundreds of feet of chain, and then passed the chain over the rollers and down the slope to a waiting wagon. Then, under shouts and cracking whips, as many as 12 yoke of oxen hove to on each wagon..."
Small image credit here.