The road starts in the village of Virgin and almost immediately does something unusual and spectacular. It climbs right up the middle of an inverted stream, a place where a lava flow filled a river canyon only to be attacked by erosion. The flow was resistant so new canyons formed on both sides, leaving the former bottom of a river as a ridge. It's a little tricky to see, but note how the land falls away on both sides of the road in the picture above. Some of the basalt crops out on the right. The rocks are the siltstone and shales of the Triassic Moenkopi Formation, one of the oldest layers exposed in Zion National Park
Why the volcanism? We are near the edge of the Colorado Plateau, at the faulted boundary with the Basin and Range Province of western Utah and Nevada. The stretching of the crust and subsequent fracturing of the rocks has provided an avenue for the molten magma to reach the surface.
Where did all the sand come from? Much of it probably came from the remnants of the Ancestral Rockies, or the incipient highlands related to subduction off to the west, but recent research suggests that some of the sand was derived from rivers with headwaters in the vast Appalachian-Caledonian mountain system in what is today the southeastern United States.