The promised Arches Explanation:
This graphic (and a lot of this explanation) is credited to the NPS at this website, but we wanted to put an explanation on our blog since I explained it so poorly in the video! Essentially, multiple layers of harder rock and porous sandstone were deposited on a salt bed. The salt bed bulged in different places because of tectonic plate shifts, making the rock curve upwards and crack in parallel lines. Then, wind and water erosion (water + sandstone = calcite,) wore away at the layers. The ‘fins’ are generally the first part of forming an arch, but we saw some ‘arches’ that just looked like shallow caves hacked into the side of a HUGE rock. To make matters even more confusing, the right amount of rain is needed for this process, otherwise the rocks would a) never erode or b) erode too fast and collapse before the arch is formed. See why I had trouble explaining it on the spot in the video?
There are over 2,000 arches in the park but occasionally one collapses- at least 42 collapses have been recorded including this one in 2008.