It seems that arches and bridges form when there is ample water flow. There are more arches up Escalante Canyon and other canyons around Lake Powell. The nps website has an interesting section on the history of Rainbow Bridge. It is located at www.nps.gov/rabr/adhi/adhi1.htm I was also wondering if carbon dioxide flows on Mars could produce a similar effect. I read an article in Sky and Telescope a few months back about carbon dioxide flows and the canals on Mars. I've been trying to locate it but can't seem to find the right issue. Debbie