Arch evolution can occur over thousands of years, but La Dame Blanche (The White Lady) in France consisted of a crumbly clay-like rock and evolved from youth to maturity to collapse in the course of just a few years.
The photo below taken by Albert Zwinkels in 2009 shows a small, young opening.
The photo below by NABS member Guilain Debossens taken in June 2014 shows a mature span of 20 feet.
The photo below by Raphael Rodon was taken in April 2015, showing that the arch had collapsed.
La Dame Blanche (NABSQNO 31T-663186-4933509) was located in the vicinity of Dieulefit village in the Drôme department of France, about 100 miles south of Lyon and 120 miles north of Marseille.