here.
It is an interesting process, arch formation, which is different from a natural bridge, or the shoreline processes that can create a stone arch. Delicate Arch, as shown in Marcia's post, is a beautiful example of a rather gradual process of differential erosion and chemical weathering due to ground water seepage, differing rock types, jointing, wind ablation, and freeze/thaw cycles. Natural arches are the result of moving water cutting through a meander neck or undercutting a rock fin. Also, there are other arches, such as the arch I visited in a volcanic dike that resulted from the differential weathering of a radial pattern of columnar jointing.
I'll look for some better links to explain the process. Or maybe Arrell still has his notes from the field trip to Arches N.P.?
Space/Science » in reply to Only Water?
A cool little video of arch formation: