As the Earth upwarped here, deep cracks penetrated to the buried
Erosion wore away exposed rock layers and enlarged the surface
cracks, isolating narrow sandstone walls, or fins.
Alternating frosts and thawing caused crumbling and flaking of the porous sandstone and eventually cut
through some of the fins.
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