The Following Resource is taken from the National Park Service Website:

How are Arches Formed?

1. As the Earth upwarped here, deep cracks penetrated to the buried sandstone layer.
    
2. Erosion wore away exposed rock layers and enlarged the surface cracks, isolating narrow sandstone walls, or fins.
    
3. Alternating frosts and thawing caused crumbling and flaking of the porous sandstone and eventually cut through some of the fins.
    
4. The resulting holes were enlarged to arch proportions by rockfalls and weathering. Arches eventually collapse, leaving only buttresses that in time will erode. Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. (Natural Bridges National Monument is a superb place to view these bridges). Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.